CRETACEOUS CLIMATE-OCEAN DYNAMICS: FUTURE DIRECTIONS
FOR IODP
A JOI/USSSP AND NSF SPONSORED WORKSHOP
CONVENED BY
Karen L. Bice, Timothy J. Bralower, Robert A. Duncan, Brian T. Huber,
R. Mark Leckie, and Bradley B. Sageman
The Nature Place, Florissant, Colorado
July 14-17, 2002
SESSION CONVENORS
1. Cretaceous Climate Record as Revealed by Stable Isotopes - Paul Wilson, Ken MacLeod
2. Biotic Records of Global Change during the Cretaceous - Mark Leckie, Brad Sageman,
and Jochen Erbacher
3. Oceanic Anoxic Events: Causes and Consequences - Hugh Jenkyns, Lisa Pratt, Wolfgang
Kuhnt
4. Sea Level Record and Mechanisms for Global Eustatic Change - Brian Huber, Mark
Leckie, Lonnie Leithold
5. Atmosphere and Ocean Circulation in a Greenhouse World - Karen Bice, Chris Poulsen
6. Environmental and Biotic Consequences of Large Igneous Provinces - Bob Duncan, Tim
Bralower
INTRODUCTION
In recent years, a surge in the amount and quality of paleoclimatic data has renewed
interest in Cretaceous climate and ocean dynamics. New data have provided a more precise
picture of paleotemperatures and climate variations, and research on Cretaceous climate has
entered an exciting, multidisciplinary phase in which geological, geochemical, geophysical and
paleontological data can be integrated between marine and terrestrial realms and into modeling
studies, with the goal of better constraining the controls on climate change during intervals of
overall warmth. In July, 2002, the JOI-USSSP/NSF-sponsored Workshop on Cretaceous Climate
and Ocean Dynamics brought together a mulitnational group of 90 scientists with diverse
research interests and expertise (Appendix 1). The conference objective was to summarize the
current state-of-the-art in our understanding of Cretaceous paleoclimate and to discuss future
priorities. Ocean drilling has been crucial in our advances to date and is critical for the future of
research in this field.
Social Relevance of Cretaceous Climate Research
Ultimately, our interest in Cretaceous climate stems from the current concern over
modern global warming. Extreme warmth in the middle part of the Cretaceous represents one of
the best examples of "greenhouse" climate conditions in the geological record (Barron, 1983).
Substantial evidence for this warmth includes upper ocean isotopic paleotemperatures of 2228° C at southern high latitude sites (Huber et al., 1995), bathyal temperatures reaching 20° C in
the subtropical North Atlantic (Norris and Wilson, 1998; Fassell and Bralower, 1999; Huber et
al., 1999), and a large champsosaurid reptile that was intolerant of freezing conditions discovered
at 78° N (Tarduno et al., 1998). These data suggest that globally averaged surface temperatures
in the mid Cretaceous were more than 10ºC higher than today.
Some of the most important earth science questions of our time relate to understanding
how human activities may be modifying current and future climates. Will Earth enter another
warm climate state due to rising atmospheric greenhouse gas concentrations? If so, what kinds of
biota are likely to adapt/evolve and which face extinction? Will a future warm Earth system
exhibit climatic and biotic stability or abrupt change and extreme states? Will Earth enter a
"permanent" El Niño or will the system exhibit variability that allows for periods of
increased/decreased marine productivity? From the standpoint of marine productivity, is
upwelling an effective process during a warm climate interval? If not, what is? How much above
modern values can tropical sea surface temperatures rise? How accurately do climate models
predict the effects of increasing greenhouse gas concentrations? Cretaceous climate studies may
be the best key we have to answering these questions.
One of the most critical questions involves how the biosphere will (or will not) adapt to
global warming. Because sea level was high, there is a rich terrestrial record for the Cretaceous.
This paleontological record serves as a "natural laboratory," an archive of biotic responses to
climate change in terms of migration, extinction, adaptation and diversification. Cretaceous
research allows us to assess the biotic responses to factors such as warming/cooling,
humidity/aridity, and sea level change. The Western Interior Seaway is a particularly useful
tableau in which these changes played out because continental impacts are so evident, thereby
providing an accessible, high-resolution record with a large signal to uncertainty ratio. The
Cretaceous Western Interior Seaway and other epicontinental seas also serve as important
archives because the geography of these regions approximates the coastal plain-maritime
interface that is today so densely populated and exhibits the fastest rate of population increase.
Of all the past warm climate periods, the Cretaceous may be the one most richly
described with respect to its terrestrial fossil and marine chemical records. These sediments
therefore present a special opportunity to investigate how terrestrial and marine environments are
coupled on a warmer Earth. Because Cretaceous climate was far from stable, with evident
precessional and sub-precessional cycles in both terrestrial and marine records, we have the
opportunity to examine climate change within a warm world on several time scales. Cretaceous
climate data can therefore help to inform the public about both near- and long-term possible
effects of anthropogenic climate changes. This perspective is simply not available in modern and
historical records. Although the Cretaceous can not serve as a direct analog for a future
greenhouse Earth (because of the very different land-sea configuration), it is clear that
Cretaceous sediments may hold the best record with which to improve our understanding of
climate variability and biotic responses to change on an overall warm Earth.
Challenges in Cretaceous Climate Research
One of the major challenges facing researchers is that most coupled and uncoupled
ocean-atmosphere models underestimate the polar warmth that is indicated by mid Cretaceous
fossil and geochemical data (e.g., DeConto et al., 2000), especially for Cenomanian-Turonian
time when the latitudinal thermal gradient may have been at its lowest. This failure suggests
deficiencies in our ability to properly interpret the warm climate data record, our understanding
of greenhouse climate dynamics, and/or deficiencies in the models. In cases where the latter is
true, the immediate lesson relevant to future climate change is that these models--the same ones
used in much modern climate research--are dramatically underestimating potential future polar
warming. Where different models exhibit different greenhouse gas sensitivities, it is only by
comparing these models to paleoclimate data that we can begin to say which extreme in CO2
sensitivity may be more reasonable than another. Model-data discrepancies have also highlighted
the importance of accuracy in regional modeling and helped us to improve our ability to model
the effects of elevation and vegetation on local and regional climates. While the paleoclimate
community is challenged by model-data discrepancies, it is also uniquely positioned to evaluate
the reliability of models used in future climate research when atmospheric conditions differ from
the modern.
While the mid Cretaceous had an extreme warm climate, other times during the
Cretaceous, such as the Aptian and Maastrichtian, were characterized by cooler deep water
temperatures and possible ice-sheets in East Antarctica. Evidence for marine regression suggests
glacio-eustatic control of sea level during the mid-Maastrichtian (Miller et al., 1999).
Geochemical evidence has also been cited for continental glaciation during parts of the Early and
Late Cretaceous (Stoll and Schrag, 1996, 2000). The recognition that the Cretaceous was not a
long interval of stable, "equable," warm climate allows us to investigate climate variability on a
variety of timescales using Cretaceous records. The long term overall warming trend from the
Aptian to the mid Cretaceous extreme warmth, followed by an overall cooling trend to the
Maastrichtian is the longest timescale variation to be explained. On shorter time scales,
considerable effort is devoted to understanding what drove periods of dysoxic and anoxic marine
conditions or oceanic anoxic events (OAEs)--stagnant intervals that corresponded to pulses of
extinction and evolution of marine nekton and plankton (e.g., Fischer and Arthur, 1977;
Bralower and Thierstein, 1984; Elder, 1991; Erba, 1994; Bralower et al., 1994). On even shorter
timescales, we need to explain dramatic variations in subtropical oceans at the precessional scale
(Wilson and Norris, 2001).
The Cretaceous was a time of unusually high rates of production of oceanic crust both at
spreading centers and through the eruption of Large Igneous Provinces (LIPs) (e.g., Larson 1991;
Tarduno et al., 1991; Coffin and Eldholm, 1994) such as Ontong Java and Kerguelen plateaus.
LIPs represent exceedingly large (> 105 km3) outpourings of predominately basaltic magma. The
Ontong Java Plateau, for example, represents more than 50 million km3 of basaltic magma
extruded onto the seafloor to form a 30 km thick plateau encompassing an area equal to one third
of the contiguous United States. Events of this magnitude are unknown to human experience,
but the consequences must have been dramatic. The release of gases (e.g., CO2, SO2, Cl, F,
H2O) from Earth’s interior accompanying such great eruptions likely had significant
consequences for the composition of the ocean and atmosphere, affected the evolution and
extinction of terrestrial and marine biota (Larson, 1991; Larson and Erba, 1999; Tarduno et al.,
1998; Kerr, 1998), and may have played a primary role in oceanic anoxic events. The
mechanisms linking volcanic activity, extinction/evolution and OAEs are, however, yet to be
determined. Recent and future drilling efforts will lead to a firmer understanding of the age and
evolution of several LIPs, and a clearer picture of the emplacement mechanism and potential
environmental perturbations.
The most significant remaining questions regarding the dynamics and forcing of
Cretaceous climate and oceans include: (1) obtaining an accurate reconstruction of Cretaceous
climate from proxy records, (2) understanding the mechanics of turnover in biotic communities,
(3) determining mechanisms for abrupt and gradual changes in oceanic circulation and triggers
for oceanic anoxic events, (4) understanding the controls on eustatic sea level change, and (5)
understanding the mechanisms by which oceanic magmatism, especially LIP events, affect ocean
and atmospheric chemistry, and thus life history.
Workshop Structure
The challenges and questions outlined above formed the basis of six plenary sessions and
keynote addresses (Appendix 2) that served as an overview of our current understanding of
Cretaceous climate. On the third day of the meeting, the group went into the field to view
evidence of Cenomanian-Campanian tectonic-eustatic marine cycles exposed in the Rock
Canyon Anticline near Pueblo, Colorado. The final day of the meeting included break-out group
discussions designed to identify the critical questions in four areas and future research directions
needed to address these questions. The following report is organized by thematic plenary
sessions with a summary of the presentations followed by an overview of remaining questions
and avenues for future research. Finally, we provide a list of drilling targets that will help
achieve these goals. Where sources are cited with no publication year given, the reference is to a
workshop abstract (2002, Available on the World Wide Web:
http://cis.whoi.edu/science/GG/ccod/searchAbstracts.cfm). Another outcome of the meeting is a
community web site, which continues to be updated in order to facilitate communication
regarding meetings and future drilling. The original workshop web page for the Florissant
meeting can be accessed through this site.
SCIENTIFIC OBJECTIVES FOR POST-2003 DRILLING
Cretaceous Climate Record as Revealed by Stable Isotopes
Stable Isotopes Figures with Captions
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Overview
Estimating paleotemperatures continues to be a fundamental aspect of Cretaceous
isotopic studies. Analyses of exquisitely preserved foraminifera (those that exhibit "glassy"
preservation) have invigorated these efforts (Norris et al., Figure 1-1). Glassy Turonian
foraminifera from Demerara Rise in the western tropical Atlantic yield the warmest (up to
36°C) sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic
(Wilson et al., 2002). These findings support the hypothesised "Cretaceous greenhouse"
and strengthen the case for a Turonian Cretaceous thermal maximum (KTM), thereby
highlighting a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth
(and sea level) and peak inferred tectonic CO2 production. This mismatch most likely
represents either an artefact of an as yet unidentified Turonian pulse in global ocean-crust
cycling, or the influence of other factors, such as CaCO3 subduction (Wilson et al., 2002)
or ocean gateway change (Poulsen et al., 2003). These results from the tropics have
prompted Bice et al. (in review) to re-examine material used to estimate contemporaneous
high-latitude SSTs (e.g. DSDP Site 511, Falkland Plateau). In the absence of pronounced
regional perturbation from mean Cretaceous sea water δ18O (assumed –1‰ SMOW), δ18O
data from planktonic foraminiferal calcite at this site are indicative of extreme high latitude
warmth during the Turonian (up to 32°C, Figure 1-2). Such extreme high latitude warmth
seems difficult to explain in the absence of extreme greenhouse forcing (pCO2, 4500 to
7500 ppmv; Bice and Norris, 2003). Yet textural observations, interspecific isotopic
offsets, pore water chemistry, and burial history all argue against a diagenetic explanation.
Salinites as low as 27 psu are required to explain the data as a result of freshwater input,
but such brackish conditions are inconsistent with the microfossil assemblages.
MacLeod and Huber show that the concept of globally similar climate responses is
not borne out by a δ18O time series from three sites on Blake Nose in the western North
Atlantic. Although the 5-6 million year long Maastrichtian stage is generally thought to
have been a time of widespread cooling, there was 4-6°C of apparent warming at Blake
Nose (Figure 1-3). Theoretical models and empirical data show that these estimates are
conservative because diagenesis was acting in the opposite direction to the trend observed.
Further, low-resolution data from other sites suggests that Maastrichtian warming can be
seen throughout the North Atlantic and perhaps extended into Tethys (but not into the
Pacific). Thus, ocean basin scale factors that influence regional heat distribution must have
played an important role during the Cretaceous. Such regional controls are clearly
important today but have yet to be included in Cretaceous studies. If we are going to
address these complexities, though, we need data with better temporal and spatial
resolution, as well as an improved ability to separate temperature, seawater d18O (δw), and
diagenetic effects on carbonate δ18O. Clarke and Schouten et al. discuss efforts to separate
temperature and δw effects on δ18O values using preliminary Mg/Ca data and a new
organic-based paleothermometer, respectively.
Another area of Cretaceous isotope studies that has undergone recent re-birth is the
quest to improve our understanding of the nature and cause of oceanic anoxic events.
Jenkyns and Tsilkos showed that both δ15N of bulk samples and total organic carbon
exhibit positive shifts in Cenomanian-Turonian black shales. A similar correlation is
observed in Quaternary upwelling areas and in Toarcian (Jurassic) black shales and point
to elevated productivity and denitrification in the water column over the oceanic anoxic
event. Further, examination of Cenomanian/Turonian boundary sections globally indicates
that the original concept of oceanic anoxic events (OAEs), involving global
stratigraphically synchronous black shales, breaks down at high-resolution (Figure 1-4).
Therefore, we need to move from a lithostratigraphic definition of OAEs to a
chemostratigraphic one. δ13C records continue to both provide the lynch-pin in this regard
and to be interpreted in terms of global partitioning of carbon between reduced and
oxidized reservoirs. Pronounced δ13C excursions appear to provide reliable markers even
though various factors (e.g. palaeoecology, diagenesis and local carbon cycling) can tend
to obscure such chemostratigraphic signals. Examining the Cenomanian-Turonian OAE2
in platform carbonates of southern Mexico, Elrick and Molina showed an abrupt -3 ‰
excursion in δ13C that immediately preceded a more gradual 3-4‰ positive that is
consistently expressed in multiple sections (Figure 1-5). The higher sediment accumulation
rates of platform carbonates permits previously unattainable high-resolution data on the
relationship between geochemical and physical processes occurring in pelagic and/or hemipelagic systems (anoxia, black shale deposition) and those occurring in onshore, benthicdominated systems. Ludvigson et al. show that OAEs are also associated with
environmental changes on land. They used δ13C chemostratigraphy to correlate lacustrine
carbonates with the marine Aptian-Albian sections. Systematic differences in δ13C (~1.5
‰) between the sections studied are attribute to floral response to moisture stress in the
site immediately leeward of the Sevier mountains. δ18O curves for the two study sites
diverge for the time interval corresponding to OAE1b suggesting a local intensification of
aridity associated with oceanic anoxia.
A series of presentations focused on isotopic constraints on the hydrological and
carbon cycles in the terrestrial realm. Fricke measured phosphate δ18O of dinosaur tooth
enamel to infer the δ18O of water ingested by the animal and concluded that, relative to the
present, the Late Cretaceous of North America was typified by higher temperatures and
increased vapour transport to higher latitudes. Parallel δ13C measurements on trace
carbonate within the tooth apatite suggest herbivorous dinosaur taxa partitioned available
food resources. Both Gonzales et al. and White et al. reported analyses of δ18O of
pedogenic siderite that confirm enhanced Cretaceous hydrologic cycling and provided
quantitative estimates of the increase relative the present and model predictions. Pratt et al.
discussed how δ13C studies of lacustrine organic matter could provide better estimates of
pCO2 during times when temporal trends in δ13C or carbonate and organic matter are decoupled. One important recent finding is that we have no convincing evidence of C4 plant
adaptation by Cretaceous time, even under extreme water stress conditions. A second
important recent discovery is the existence of strata with significant concentrations of
biomarkers indicative of forest fires. Mass balance considerations indicate that biomass
burning is an under-explored explanation for pronounced negative excursions in
Cretaceous δ13C records. Finally, Finkelstein et al. examined the abundance of fusinite and
polycyclic aromatic hydrocarbons (PAHs) in lacustrine samples from the Late Cretaceous
Fort Crittenden Formation. Both are indicative of fire and multiple abundance peaks
through the section suggests repeated wildfires or peat fires in the watershed. Assuming
the hydrologic cycle was more vigorous, a seasonally wet paleoclimate could have resulted
in biomass build-up and hotter fires, enhanced landscape denudation, and accelerated
erosion in the watershed post fires, thus, explaining the correlation between turbidites and
indicators of fire.
Future Directions
Worshop participants identified the following as critical needs in the areas of δ18O
paleothermometry and δ13C records:
δ18O Paleothermometry:
1) To fully evaluate the roles played by taphonomy, palaeoecology and non-equilibrium
behaviour in Cretaceous palaeothermometry.
2) To increase stratigraphic and geographic resolution of our records and thereby assess the
issue of the stability of warm climates
3) To determine the full magnitude of high-latitude warmth during peak greenhouse forcing
4) To integrate δ18O data with additional palaeo-temperature proxies (e.g. Mg/Ca)
5) To further examine the issue of δ18Osw in the context of sensitivity to short-term changes
in continental ice volume and long-term changes in hydrothermal weathering regimes.
6) To improve our understanding of the paleohydrological cycle.
δ13C Records:
1) To generate more multi-substrate records (e.g. benthic-planktic foraminiferal pairs, CO3Corg) and thereby assess competing hypotheses for carbon cycle perturbations
2) To improve the stratigraphic resolution and reduce errors associated with pCO2 estimates
3) To improve our understanding and records of new isotope proxies for pCO2 and
weathering (e.g. δ11B; 187Os/188Os).
Because of the need to access undisturbed, expanded sections containing well-preserved
microfossils, IODP is the key platform for advancing our understanding in these areas. One
priority for future drilling is the need for a high-latitude counterpart for forthcoming ODP Leg
207. The depth-transect approach (cf. sea level section) will continue to be an important strategy.
Biotic Records of Global Change during the Cretaceous
Biotic Records Figures with Captions
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Overview
The patterns of how life responds to the Earth's changing environments can provide
important insights into the processes and rates of global change, and biotic records provide
an independent and complementary proxy to geochemical tools. Tectonic forcing of climate
and ocean fertility had a profound impact on terrestrial and marine ecosystems and the
evolution of life during the Cretaceous. At times, greater emissions of CO2 led to increased
weathering of the continents and greater delivery and availability of nutrients in the marine
realm. In addition, there were possible linkages between increased hydrothermal activity on
the seafloor and enhanced productivity in the surface waters (e.g., late Valanginian and midCretaceous Oceanic Anoxic Events). Climate fluctuations due to greenhouse warming or
reverse greenhouse cooling as a feedback to excessive burial of organic carbon, were further
modulated by eustatic sea level fluctuations and albedo effects. Changing Cretaceous
paleogeography created new opportunities for many groups of organisms. However, other
ecosystems were stressed with the opening and closing oceanic gateways, redistribution of
heat by the ocean-climate system, and the partitioning of nutrients between land and sea, and
surface and deep ocean. A number of themes emerged from the diverse array of abstracts
submitted; these can be grouped by major ecosystems and by key intervals or events during
the Cretaceous.
Pelagic Ecosystems: Since the Late Triassic, when the nannoplankton began to produce
calcareous platelets thereby causing a major shift in carbonate deposition from the shelves
into the deep oceans, this diverse and abundant group of unicellular, planktonic marine algae
have been intimately linked to global change. In modern oceans, it has been demonstrated
that the distributions of certain sensitive nannoplankton taxa mirror discrete water-masses,
the existence of which is a function of ocean circulation and climate. Thus, global climate
exerts, and must have exerted in the past, a major influence over calcareous nannoplankton
and their distributions (Lees).
Lees delimited nannofossil palaeobiogeographic zones (PBZs) for the Late Cretaceous
Indian and Pacific Oceans. These latitudinally-distributed PBZs are interpreted as indicating
discrete water-masses, possessing differing temperature and nutrient properties. Movements of
the fronts separating these PBZs through time can be used as proxies to primarily indicate
warming or cooling trends (Figure 2-1). Comparison of available oxygen isotope sea-surface
temperatures (SSTs) with the PBZ-derived warming and cooling trends shows a good correlation
between the two proxies, underlining the utility of nannofossils as proxies for Mesozoic climate
change.
Bown analyzed coccolithophorid and other calcareous nannoplankton evolution and
diversity patterns through the Cretaceous. The data reveal no clear relationship between longterm diversity trends and previously recognized major global environmental change events in
the Cretaceous. Oceanic Anoxic Events 1a (early Aptian) and 2 (latest Cenomanian) occurred
well within nannoplankton diversity minima and were followed in both cases by periods of
protracted diversity increase. However, the most rapid increases in diversity broadly correlate
with cooler climate intervals. In particular, Bown suggests that enhanced species diversity in
the Campanian was related to the onset of late Mesozoic climatic cooling with much of the
diversification the result of greater palaeobiogeographic differentiation including the
evolution of high-latitude-restricted phytoplankton.
The first radiation of planktonic foraminifera was thought to begin close to the base of
the Aptian. However, Premoli Silva demonstrates that this group started to diversify much
earlier. The first diversification occurred in the early Valanginian with the appearance of the first
hedbergellids. This small fauna in both species number and size seems to persist without
apparent changes through most of the Hauterivian, the end of which is characterized by an abrupt
short flourishing of the gorbachikellids. The earliest Barremian coincided with a remarkable
increase both in abundance and number of planktonic species and genera that continued through
the Barremian accompanied by an overall increase in size (Figure 2-2). This trend continued
through the early and mid-Aptian with a further increase in number of species and overall
abundance and size of specimens after the early Aptian Oceanic Anoxic Event 1a (OAE1a). This
increase was accompanied by the progressive appearance of more ornamented morphotypes.
Premoli Silva found no turnover in planktonic foraminifera coinciding with OAE1a. The OAE1a
represented only a temporary paleoenvironmental perturbation characterized in the upper water
column by alternating low oxygen levels (proliferation of leupoldinids), to slightly richer oxygen
levels (clavate morphotypes) or to better oxygenated waters (round-chambered taxa) (Figure 23). The effects of the perturbation related to the OAE1a appear to have terminated only about
one million years after the event.
Tropical Ecosystems: Changes in the environment have modified the ecological role of reef
builders through geologic time, and major extinctions have reset evolutionary patterns. In
Cretaceous reefs, the relative ecologic role of scleractinian corals and rudist bivalves shifted
during the mid-Cretaceous, and rudist bivalves took over as the dominant skeletal organisms
for the remainder of the period (Johnson). The role of bivalves as proxies for reefal
conditions remains questionable. Johnson statistically analyzed the paleobiogeographic
distributions of scleractinian corals of the Caribbean and circum-Caribbean region for all
stages of the Cretaceous (Figure 2-4). Dispersion patterns for corals and rudists infer that the
geographic extent of the Cretaceous tropics was greater than that of our present-day
interglacial period. Knowledge of the paleolatitudinal extent of the ancient tropics is essential
for analyzing the role of the tropics in ocean and atmospheric heat transport.
Terrestrial Ecosystems: The statistical relationship between leaf physiognomy and climate for
modern plants is widely used by paleobotanists to reconstruct Late Cretaceous climate. Methods
such as simple linear regression (SLR) and multiple regression (MR) are used to derive
leaf/climate relationships for modern floras, which then are used to estimate temperature and
precipitation from paleofloras. When applied to modern test sites, physiognomic methods
provide good "ballpark estimates" of mean annual temperature (MAT) and mean annual
precipitation (MAP) but can show significant discrepancies between estimated and measured
climate. The magnitude of the discrepancy (up to 5°C for MAT) is determined by the choice of
equation used to quantify the leaf/climate relationship, referred to here as the physiognomic
transfer function (Scherer).
To determine the extent to which estimates of Cretaceous climate are biased by the
choice of physiognomic transfer function, Scherer applied a variety of published physiognomic
transfer functions to a well-preserved leaf megaflora from the latest Cretaceous (Maastrichtian)
Jose Creek Member of the McRae Formation, southern New Mexico. For the Jose Creek
assemblage, calculated MAT is 16-25°C and calculated MAP is 600-1200 mm/yr, a range of
over 8°C for MAT and 600 mm/year for MAP. Variation in calculated temperature and
precipitation, relative to the mean of all estimates, can exceed + 35%. However, the range of
values is congruent with more qualitative estimates of climate derived from paleosols and plant
life form, which indicate a warm subhumid subtropical climate for the Jose Creek Member. This
implies that physiognomic transfer functions are probably accurate to the level of climate zone.
Terrestrial climates near the time of the end-Cretaceous mass extinctions are poorly
understood. Wilf et al. estimated and correlated paleotemperatures for the terminal Cretaceous
(~66.7 to 65.5 Ma), using megafloral data from North Dakota and foraminiferal data from four
middle- and high-latitude sites. Both plants and foraminifera indicate warming near 66.0 Ma, a
warming peak from ~65.8 to 65.6 Ma, and cooling near 65.6 Ma to pre-warming temperatures,
which lasted into the early Paleocene (Figure 2-5). At similar temperatures, Cretaceous floras
from North Dakota were rich, but Paleocene floras were impoverished. Climatic and facies
changes are insufficient explanations for plant extinctions in this area, which we attribute to the
effects of bolide impact (Wilf et al.).
High Arctic: The Cretaceous of the Canadian High Arctic is represented by sedimentary and
volcanic rocks of the Sverdrup Basin that are exceptionally well exposed on Axel Heiberg and
Ellesmere Islands. These sequences contain evidence for short-term episodes of extreme warmth
and cooling during the Late and Early Cretaceous, respectively (Tarduno et al.). A Turonian to
Coniacian (~92 to 86 Ma) vertebrate assemblage from a site with a paleolatitude of
approximately 71˚N implies that polar climates were warm (mean annual temperature exceeding
14˚C; Tarduno et al., 1998). This episode may correlate with evidence for extreme high-latitude
warmth from oxygen isotope paleotemperature estimates from the Southern Ocean (Huber,
1998). The Arctic assemblage includes large (>2.4 m) champsosaurs, which are extinct
crocodile-like reptiles. The vertebrate fossils overlie subaerially erupted flood basalts of the
Cretaceous Strand Fiord Formation. These lavas are part of a large magmatic pulse (or large
igneous province) that may include large parts of Ellesmere Island and the Arctic Ocean basin.
This magmatism, coupled with coeval volcanism at six other large igneous provinces, suggests
that volcanic carbon dioxide emissions may have helped cause the peak in global warmth. In
contrast to the extreme warmth of the Late Cretaceous, Arctic sedimentary rocks of mid- and
Early Cretaceous age show evidence of cool conditions in the form of glendonite horizons.
Glendonites (pseudomorphs after the hydrated calcium-carbonate mineral ikaite), are associated
with glacial marine sediments of Permian to recent age (Figure 2-6). Preliminary
biostratigraphic, magnetostratigraphic and geochemistry data of Tarduno et al. suggest that
Valanginian age glendonite horizons represent relatively short (<100-400 kyr) episodes at a time
of low eustatic sea level.
Epicontinental Seas: Fisher applied an innovative technique to reconstruct the temperature
and circulation of ancient surface water masses from the Cretaceous Western Interior Sea.
Foraminifera construct more porous shells in warmer waters and less saline waters. Modern
planktic foraminifera are less diverse at lower salinities, so shell porosity of diverse ancient
planktic foraminiferal assemblages must be primarily in response to paleotemperature. Both
properties are largely responsible for water density, and therefore porosity may be a proxy
for relative water mass density, which is useful in interpreting paleo-ocean circulation.
Specimens of the widely distributed mid-Cretaceous species Hedbergella delrioensis have
been temporally studied through several sections and spatially studied along time-lines from
many geographic localities across the Cenomanian-Turonian Greenhorn Sea. Stratigraphic
studies show that porosity increases from the upper Cenomanian into the lower Turonian.
The porosity increase is coincident with previously proposed sea level rise. The porosity
increase is interpreted as an increase in the influence of warm water that entered from the
Tethyan Ocean (Figure 2-7). Geographic studies along time-lines reveal high porosities in the
central seaway, with porosity decreasing shoreward and northward. The distributions are
interpreted as a stratified seaway, consisting of a warm central near-surface watermass
underlain by deeper waters that surfaced shoreward and northward. As sea level rose the
warmer central core expanded north and shoreward (Figure 2-8). If porosities are used as
relative density proxy data, contour maps of porosity can be used to reconstruct geostrophic
flow.
Late Valanginian Productivity Event: Using quantitative analyses of calcareous nannofloras
and geochemistry, Tremolada et al. showed that the calcareous nannoplankton group
nannoconids experienced a sharp decline across a globally documented positive carbon isotope
excursion in the Berrisian-Hauterivian in northern Italy. The nannofossil assemblages of the late
Valanginian are dominated by W. barnesae, with relative increase of Diazomatolithus spp.,
probably indicating higher fertility conditions as further supported by the decline in the
oligotrophic nannoconids. Rising Sr/Ca ratios in the upper Valanginian carbonates supports the
interpretation of enhanced productivity. This increase slightly leads the carbon isotope excursion,
and is coeval with the onset of the nannoconid decline. According to Tremolada et al., the
nannoconid decline and productivity increase probably were induced by changes in nutrient
content caused by the Early Cretaceous volcanic activity connected with the emplacement of the
Paranà Plateau and the "pulse" in the seafloor production. These volcanic and tectonic events
provoked excessive CO2 levels in the atmosphere favoring warm and humid conditions that
induced an accelerated transfer of nutrients from the continents to the oceans increasing the
fertility of surface waters. Also, a nutrification event might have been directly caused by
hydrothermal processes connected with the igneous activity (Figure 2-9).
Biotic Response to Oceanic Anoxic Events: Leckie et al. (2002) documented the
evolutionary patterns of calcareous plankton (planktic foraminifera and calcareous
nannoplankton) during the mid-Cretaceous (Barremian-Turonian stages, ~124-90 Ma). When
combined with the radiolarian record through the same interval (Erbacher et al., 1996), there
is compelling evidence that the highest rates of evolutionary turnover, i.e., speciation plus
extinction, occur at or near the major Oceanic Anoxic Events. These episodes of widespread
organic carbon burial occur in the early Aptian (~120.5 Ma; OAE1a), across the
Aptian/Albian boundary (~113-109 Ma; OAE1b), in the latest Albian (~99.5 Ma; OAE1d),
and across the Cenomanian/Turonian boundary (~93.5 Ma; OAE2). Strontium isotopic
evidence suggests a possible link to times of rapid oceanic plateau formation and/or
increased rates of ridge crest volcanism (Bralower et al., 1997; Jones and Jenkyns, 2001;
Leckie et al., 2002). The association of plankton turnover and carbon isotopic excursions
with each of the major OAEs suggests widespread changes in the ocean-climate system
(Figure 2-10). The episodes of accelerated evolutionary activity in the plankton have been
attributed to changing upper water column structure, increased delivery of nutrients including
dissolved iron, greater oceanic productivity, and carbonate dissolution (Erba, 1994; Erbacher
et al., 1996; Leckie et al., 2002). Leckie et al. (2002) concluded that plankton evolution was
tectonically-forced, particularly by increased submarine volcanism and hydrothermal
activity, due to tectonic influences on global climate, ocean chemistry, nutrient availability,
ocean circulation, and water column structure.
According to Erba, OAE1a (late Early Aptian) and OAE2 (latest Cenomanian)
correlate with the onset and end of the mid-Cretaceous greenhouse climate, interrupted by
some brief cooling episodes in the Aptian to Cenomanian interval alternating with extremely
warm conditions leading to black shale deposition during OAEs. High-resolution stratigraphy
indicates that biotic events correlate with major igneous/tectonic events. Both OAE1a and
OAE2 are interpreted as high productivity episodes probably induced by high concentrations
of dissolved and particulate biolimiting metals in the oceans and increased CO2 in the
atmosphere during submarine volcanism (Ontong Java and Manihiki Plateaus, and Caribbean
plate, respectively). Mid-Cretaceous OAEs and cooling episodes seem correlatable with
subaerial volcanism related to the formation of Kerguelen Plateau and Broken Ridge (Figure
2-11).
Changes in the nature of oceanic surface water masses during Oceanic Anoxic Events
(OAE) were significant drivers of phytoplankton evolution. As an example, Watkins
analyzed well-preserved nannofossil assemblages in upper Albian and lower Cenomanian
hemipelagic sections from Ocean Drilling Program (ODP) Leg 171b, which record the early
history of the establishment and adaptive radiation of the genus Eiffellithus. Seven distinct
taxa evolved during the relatively short interval from 101.5 to 100.0 Ma. Newly evolved
species tended to remain at low abundance levels until a significant disruption in the pelagic
realm resulted in the precipitous decline of the dominant species. These major disruptions
correspond to significant changes or shifts in the sedimentological and carbon isotopic
records associated with late Albian OAE1d. The close correspondence of species
originations, changes in dominance, and species extinctions to changes in sediment TOC and
significant carbon isotope shifts indicates that the variability in the surface water mass during
OAE1d was the principle environmental forcing mechanism behind the adaptive radiation of
this genus during the late Albian (Watkins).
Holbourn and Kuhnt examined records of well preserved benthic foraminiferal
assemblages across Cretaceous OAE1a, OAE1b, OAE1c and OAE2 from North Atlantic bathyal
and abyssal DSDP/ODP Sites and from bathyal to neritic onshore sections in Morocco, Spain,
southeast France and northern Germany (Figure 2-12). Diversity, taxonomic composition and
preservation potential of benthic foraminiferal assemblages at shelf localities are strongly
influenced by changes in paleo-water depth, particularly at times of major sea level change.
Thus, most of the observed faunal changes at shelf sites express environmental change and
taphonomic bias, rather than true extinction and radiation events. They found no evidence for a
major benthic foraminiferal turnover during OAE1 and OAE2 at middle and upper bathyal sites
(Figure 2-13). Most taxa recorded at these locations have stratigraphic ranges extending across
oceanic anoxic events. The changes in biofacies recorded across the OAEs coincide with changes
in hydrography and sedimentological facies. By contrast, abyssal benthic foraminifers underwent
a marked radiation after the latest Cenomanian OAE2 in the Atlantic Ocean and the
Mediterranean Tethys, which may be triggered by a general change towards better oxygenated
deep-water.
With ammonite/inoceramid species loss of up to 85% in the Western Interior basin,
the Cenomanian-Turonian boundary event represents a significant biotic crisis (Meyers and
Sageman). Hypotheses proposed for the driving mechanism of species turnover include
mainly changes in benthic and water column oxygen levels associated with OAE2, and
changes in substrate consistency associated with limestone-marlstone alternation. Meyers and
Sageman examined the main hypotheses for biotic turnover in light of new sediment and
geochemical accumulation rates, as well as recalculated evolutionary rates across the stage
boundary. These new rates are based on a high-resolution time scale developed through
cyclostratigraphic analysis of the Cenomanian-Turonian Bridge Creek Limestone Member.
The new high-resolution time scale facilitates an independent quantitative assessment of the
rates of accumulation of environmentally sensitive geochemical proxies, and calculation of
rates of evolutionary change in the C-T boundary interval. A series of major modal switches
in sedimentation are identified. These modal switches may represent fundamental changes in
the ocean-climate-sediment transport system. Interestingly, the highest rates of extinction do
not correspond to intervals with the greatest indication of sulfidic conditions (Figure 2-14).
Further comparisons of paleoenvironmental data with molluscan evolutionary rate allow
evaluation of alternate biotic controls, such as substrate consistency, turbidity, and
nature/frequency of environmental disturbance (Meyers and Sageman).
Future Directions
Based on data obtained from continental records and ocean drilling, considerable
progress has been made in the last two decades toward describing and explaining Cretaceous
biosphere changes. However, important unsolved problems remain in our understanding of
the ocean-climate system and its impact on the biosphere. How is widespread marine
biological productivity sustained for 104-105 years? How important was 400 kyr and 100 kyr
cyclicity in controlling sedimentary and biotic patterns? Did metals or other trace elements
play a major role in the Oceanic Anoxic Events of the Early Cretaceous and mid-Cretaceous?
What was the relative role of greenhouse warming vs. reverse greenhouse cooling in
controlling the availability of nutrients, burial of organic matter, water mass production, and
biotic evolution during the mid-Cretaceous? What is the relative importance of upwelling vs.
continental runoff as a nutrient supply mechanism and how might this have varied through
different climate events? What were the rates of climate change during the OAEs? What was
the frequency and extent of wildfires, and what was their impact on the carbon cycle?
The rich Cretaceous terrestrial record, increasingly well-characterized marine records,
and an increasing number of proxies and model approaches will allow us to better understand
terrestrial and marine responses to internal and external climate change. Workshop
participants emphsized the need to better integrate terrestrial and marine records using
transects across the land-sea transitions. Lacustrine deposits were identified as a possibly
under-utilized source of information. The increasing use of organic biomarkers for molecular
"fingerprinting" should yield important information about community makeup and
environments. Given the vast amount of hydrocarbons sourced from Cretaceous sediments,
the portion of our community applying biogeochemical approaches might especially benefit
from greater collaboration with the petroleum industry.
Oceanic Anoxic Events: Causes and Consequences
OAE Figures with Captions
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Overview
The discovery of black carbon-rich shales in deep-sea drilling sites from the Atlantic,
Indian and Pacific Oceans (from DSDP Leg 1 onwards) led in the mid-1970s to the concept
of Oceanic Anoxic Events. Such events, whatever their exact nature and cause, were
hypothesized to foster deposition of coeval carbon-rich sediments across environments
ranging from deep oceans to shelf seas. The original concept was primarily stratigraphic in
nature, being based on the implicit assumption that the world ocean underwent a fundamental
chemical and/or biological change during such events: enhanced productivity of organicwalled microfossils and bacteria and/or enhanced preservation of organic matter were both
suggested as likely causes. Among the plethora of black-shale horizons identified both on
land and in the oceans, two major organic-rich horizons, one dated as early Aptian (Selli
Event: OAE1a), the other at the Cenomanian-Turonian boundary (Bonarelli Event: OAE2),
have proven to be of global distribution. Both of these, for example, are recorded from
submarine plateaus in the Pacific Super-Ocean, most recently the black shale of the Selli
Event drilled on Shatsky Rise during ODP Leg 198. This extremely carbon-rich horizon
(~35% TOC) contains biomarkers for cyanobacteria that could have utilized atmospheric
elemental dinitrogen - as happens in some extreme upwelling environments today - if nitrate
levels in the photic zone became vanishingly low because of utilization by plankton
(Brassell, Figure 3-1). This discovery points to upwelling and high productivity as a major
forcing function behind the causality of Oceanic Anoxic Events.
Nitrogen-isotope data from Cenomanian/Turonian black shales from Italy and
Morocco similarly are consistent with sedimentation below a water column that had
undergone denitrification (Jenkyns and Tsikos), as is the case today in zones of vigorous
upwelling, intense oxygen minima and high fluxes of planktonic carbon to the sea floor.
Other events, particularly the Paquier Event (OAE1b) are more local, being confined - as far
as we know at present - to the Atlantic-Tethyan domain.
Phil Meyers showed that low δ15N values in black shales, values that falsely mimic
land plant compositions, may reflect a combination of processes leading to elevated
production and improved preservation of marine organic matter. Marine productivity is
usually limited by availability of dissolved nitrate, but if a mid-water anoxic zone expands
upward and enhances recycling of phosphorus into the photic zone, then nitrogen-fixing
cyanobacteria can flourish. These organisms photosynthetically produce organic matter
having a δ15N value close to atmospheric nitrogen (0‰). So, while the δ15N values of most
Mesozoic and Cenozoic marine sediments usually range between 4 and 8 ‰, Rau et al.
(1987) reported δ15N values of –2 to 3 ‰ in black shales from DSDP Sites 367, 530, and
603 (Figure 3-2).
From a study of isotopes and palynology of this Oceanic Anoxic Event from three
sections in the Central Atlantic and western Tethys, Erbacher and Herrle conclude: 1) that the
average duration of OAE1b was 45 ky; 2) a strong monsoonal circulation resulted in a
dominance of the precessional signal (fertility); 3) fertility changes were superimposed on
longer-term temperature cycles corresponding to the eccentricity signal; and 4) that OAE1b
falls into an extremely warm and humid phase of an eccentricity cycle (Figure 3-3). This
event was probably a factor of 10 times shorter in duration than was OAE2. OAE3
(Coniacian-Santonian) again seems a more parochial affair (Wagner et al., Figure 3-4) with a
record dominantly deriving from the Atlantic. The question must be asked: is it actually
useful to consider these organic-rich strata as being produced by an Oceanic Anoxic Event?
Is some minimum geographic extent of a black-shale unit required in order to qualify as the
product of an OAE?
The advent of carbon-isotope stratigraphy in the nineteen eighties, with the
recognition of a characteristic positive excursions related to excess global carbon burial,
offered a proxy for the partitioning of the global carbon pool between oxidized and reduced
reservoirs. Recent carbon- isotope studies from England, Japan, Italy, Morocco and
elsewhere now demonstrate that the both the Aptian and the Cenomanian/Turonian black
shales are associated with such a positive carbon- isotope excursion in marine pelagic and
shallow-water carbonate, marine organic matter and terrestrial higher-plant material (Jahren,
Hasegawa, Heimhofer et al.; Figures 3-5, 3-6, 3-7). The Aptian event, paradoxically, is also
associated with a pronounced negative carbon-isotope excursion, recently interpreted as due
to dissociation of methane hydrates, a theme explored by Hope Jahren in her presentation.
Because these characteristic carbon-isotope excursions, both positive and negative, can be
found in deep-marine, shallow-marine and non-marine facies, they offer a novel means of
correlation between sediments deposited in the oceans and on the continents. Palaeoclimatic
data from continental interiors may now be integrated with the wealth of data from deep-sea
cores to produce a potentially global view of climate change during critical events in earth
history. The Aptian negative isotope excursion is not unique in the Cretaceous: a similar
phenomenon has been found in Atlantic and Tethyan sections predating the Paquier Event
(Gröcke). The question then has to asked: what is the exact relationship, if any, between
OAEs and negative carbon-isotope excursions, when excess global carbon burial must
produce a positive excursion? Perhaps both phenomena - OAEs and methane-dissociation
events - are responding to increases in CO2-forced global temperature, the former being
related to fundamental changes in the hydrological cycle, nutrient inputs, oceanic recycling,
the latter to changes in the stability field of methane hydrate dependent on bottom-water
temperatures. CO2 could have been added to the atmosphere from volcanogenic sources as
well as from oxidation of methane: and in one sense OAEs can be seen as the means by
which the ocean-atmosphere returns to a pre-existing equilibrium. Massive sequestration of
organic carbon during the OAE would draw down CO2, return the atmosphere to pre-OAE
conditions and produce global cooling.
In detail the exact relationship between the isotope curve and the total-organic-carbon
stratigraphy of any one section is complex. The black shale or, more particularly, its most
carbon-rich portions, is not fixed in exactly the same position with respect to carbon-isotope
curve: in some localities the onset of the positive excursion coincides with the beginning of
black-shale deposition, in others it does not. This mismatch is seen, for example with the
Cenomanian/Turonian in the Western Interior (Sageman and Meyers; Figure 3-8) where
organic-carbon accumulation rates actually seem to have increased after the OAE - the OAE
being defined by the duration of the carbon-isotope excursion. In Italy, however, the carbonisotope excursion begins abruptly at the base of the Bonarelli Level itself. Hence, at these
high levels of resolution, the "stratigraphic" concept of the Oceanic Anoxic Event breaks
down. But more importantly this is telling us that the most significant carbon sinks during
the Cenomanian/Turonian OAE have yet to be identified, although there is some evidence
that a (maybe the) major locus of organic-matter deposition was the proto-South Atlantic
(Forster et al., Figure 3-9).
The Late Cenomanian oceanic anoxic event (OAE2) is the most extensive of the MidCretaceous OAEs. The duration of OAE2 was originally estimated based on biostratigraphic
evidence and interpolation of geological timescale data points between 0.5 - 0.8 m.y. (Arthur
et al., 1988) and 0.4 m.y. (Caron et al., 1999). Recently, orbital cyclicity has been used to
estimate the duration of the event. These estimates range from 720 k.y. in Colorado (Meyers
et al., 2001), approx. 400 k.y. (Kuhnt et al., 1997) in the Tarfaya Basin (Morocco) to 320
K.y. in western Canada (Prokoph et al., 2001). The OAE2 is characterised by a large
positive global carbon-isotope excursion in both carbonate and organic matter, caused by a
major perturbation of the global carbon budget, probably due to the extensive burial of
organic matter in black shales (Schlanger & Jenkyns 1976; Arthur & Schlanger 1979;
Jenkyns 1980; Scholle & Arthur 1980; Herbin et al. 1986; Arthur & Schlanger 1987; Arthur
et al. 1988; Weissert & Lini 1991; Jenkyns et al. 1994). This period of extensive carbon
burial was coeval to a significant extinction event in the marine plankton, i.e. the extinction
of the first clade of keeled planktonic foraminifers, the genus Rotalipora.
The OAE2 represents a major turn in earth climate history (Jenkyns et al. 1994;
Norris & Wilson 1998). Decreased ocean-volume due to enhanced spreading rates and larger
mid-ocean ridge volume had led to a eustatic sea level rise of 130-350 m since the early
Aptian (Kominz 1984; Larson 1991a,b) and the worldwide formation of shallow, warm,
epicontinental seas (Brass et al. 1982). Atmospheric CO2-partial pressure was 3-12 times
higher than today due to volcanic outgassing (Larson 1991b; Berner 1991) and many
terrestrial and marine paleoclimate-proxies suggest that the mid-Cretaceous was the warmest
period in the last 200 m.y. (Frakes et al. 1992; Barron et al. 1995; Huber et al. 1995). The
high surface temperatures resulted in enhanced evaporation within shallow, warm
epicontinental seas and an intensification of the water-cycle and increased nutrient fluxes
(Föllmi et al. 1994). Recent evidence indicates that OAE2 was accompanied by a 40-80%
reduction in CO2 levels (Kuypers et al., 1999). This was suggested to be due to a rapid
increase in accumulation rates of organic matter in the subtropical Atlantic Ocean and
Tethyan marginal basins (Arthur et al. 1988; Kuhnt et al., 1990; Jenkyns et al. 1994; Kuypers
et al. 1999). However, due to generally low sedimentation rates or even hiatuses in the late
Cenomanian, a precise succession of events (carbon burial, reduction of pCO2, carbon
isotope excursion, biotic extinctions and sea surface temperature changes) has never been
established, leaving wide space for speculation about causal relationships.
Future Directions
Future research on OAEs must continue to characterize the environmental conditions
that led to the deposition of organic-rich sediments. Proxies that can provide direct
information on nutrients in surface waters, atmospheric CO2 levels, and the original
composition of biotic populations from microbes to invertebrates, are desperately needed.
Multidisiplinary investigations must focus on events that perturbed the global ocean such as
the OAEs in the early Aptian (OAE1) and the late Cenomanian (OAE2), as well as those that
appear to be more regional in scale such as OAEs 1b, 1c, and 1d. There are a number of
important questions that can best be addressed through ocean drilling:
•
What is the geographic and oceanographic distribution of these events?
•
What controls their spatial distribution and the rate of changes (chemical, physical,
biological) during the event?
•
What preconditioning (tectonic, climatic, orbital) determined the time scales of the
different OAEs and sustained them?
•
What are the triggering mechanisms and their manifestation, e.g. is there a role for
methane?
•
What is the relationship between OAEs, ocean temperature changes and links to sea
level?
•
What happened to the water column (physically, chemically, biologically) and the
upper sedimentary column?
•
How did the organic matter (sources, preservation, productivity), nutrients and ocean
structure change? How was that affected/stimulated by oxygen minima, euxinia,
dysoxia?
•
What is the impact on the carbon cycle and its budgets, reservoirs and fluxes through
the various OAEs?
•
What is the expression of these events on the continents (e.g. in lakes) versus shelf
areas and the deep ocean?
Drilling Priorities
To address many of these questions in a rigorous fashion will require sections with a
broader geographic distribution than those currently available. Dedicated drilling legs are
required to obtain multiple core records of these critical intervals. Moreover, there must be a
concerted effort to identify records of OAE in expanded, immature sections that will allow
high-resolution studies at a millennial scale. Drilling priorities include the high latitudes
(especially Arctic) transects from continent to the deep sea, and progressively "downstream"
from Large Igneous Province eruptions.
Sea Level Record and Mechanisms for Global Eustatic Change
Sea Level Figures with Captions
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Overview
One of the long-standing controversies about Cretaceous climate regards how short-term,
global variations in sea level could have occurred during a "greenhouse" climate that is assumed
to have been ice-free. The assumption of a warm, ice-free earth during the Cretaceous has been
based on a variety of geochemical, sedimentologic, and paleontological evidence that oceanic
and continental temperatures were much warmer than at present in the north and south polar
regions. On the other hand, correlations between oxygen isotopic data, strontium concentrations,
sedimentologic observations, and interpreted sea level variations have been proposed as evidence
that polar ice-sheets did exist at various times during the Cretaceous. The "Cretaceous Sea level
Record and Mechanisms for Global Eustatic Change" session was organized to evaluate
evidence from both sides of this controversy and determine what approaches are needed to
reconcile the divergent interpretations. The talks and posters presented in the session revealed
tantalizing evidence for correlations between changes in sea level and geochemical shifts that are
consistent with ice volume variation at several intervals during the Cretaceous. However, more
evidence for global synchroneity of these records is needed before an ice-sheet forcing
mechanism will be widely accepted.
The session presentations can be divided into the approaches that were used to examine the
possibility of glacio-eustasy during the Cretaceous:
•
Development of new experimental tools for determining the temperature of Cretaceous
water masses
•
Analysis of inorganic geochemical proxies that may have been influenced by ice-sheet
growth and decay
•
Sedimentologic and biotic evidence for sea level variations at Milankovitch time scales
•
Climate modeling
Experimental Paleothermometers: Linear correlation between seawater temperature and a
successively increasing number of cyclopentane rings in tetraether membrane lipids that are
biosynthesized by a group of marine bacteria (crenarchaeota) has been demonstrated in field and
laboratory studies. This "TEX86" proxy may serve as an alternative to established sea-surface
temperature (SST) proxies (such as UK37 or δ18O of carbonates) for sediments dating to the
Albian, which is when the membrane lipids of marine crenarchaeota are known to have first
appeared. Initial testing of this method on ancient sediments suggests that early diagenesis and
sediment maturity do not influence the paleotemperature signal, but further testing is needed to
determine the possibility of biasing paleoenvironmental factors. Assuming that biasing effects
were negligible, application of the TEX86 proxy to Cenomanian-Turonian sediments at DSDP
Site 367 reveals that tropical SSTs may have been up to 6°C warmer than at present (Figure 4-1).
Analysis of Mg/Ca ratios in marine carbonate is another method that is beginning to be
used to estimate Cretaceous marine paleotemperatures. This approach was applied to sediments
that span the Aptian/Albian boundary at ODP Hole 766A. Preliminary results suggest that the
apparent oxygen isotope warming across this boundary interval may actually reflect a massive
input of 16O into the global oceans as a result of a major deglaciation (Figure 4-2).
Inorganic Geochemical Proxies: The concept of long-term stability and equability of the
Cretaceous greenhouse climate is giving way to recognition of much greater variability of the
global climate on both short and long time scales. This improved understanding of Cretaceous
climate comes largely as a result of increasingly detailed stable isotope records from wellpreserved deep-sea sediments.
Two of the most complete and best-preserved long-term Cretaceous δ18O records come from the
subtropical North Atlantic and the southern South Atlantic (Figure 4-3). In the benthic
foraminifer δ18O curve from Blake Nose, six short-term, positive shifts of more than 0.3‰ are
recorded during the Albian through Maastrichtian. It is unlikely that the late Albian, midCenomanian, and mid-Campanian benthic δ18O shifts were glacially forced since (1) they are not
accompanied by positive shifts in the planktonic δ18O record and (2) the maximum benthic δ18O
values are still below 1‰, which is considered by some authors (e.g., Barrera and Savin, 1999)
as the ice/ice-free threshold based on benthic δ18O records that span the late Eocene growth of
the polar ice cap. On the other hand, coupled positive planktonic and benthic δ18O shifts and cool
deep-water paleotemperatures are recorded in the early Albian and early and late Maastrichtian
at Sites 1050, suggesting the possibility of polar ice at these times.
Comparison of additional late Campanian-Maastrichtian δ18O records from the North and
South Atlantic and the Pacific (Figure 4-4) reveals correlative positive shifts in some, but not all,
intervals that span the 70.5 Ma and 66.5 Ma cooling events. The 70.5 Ma event corresponds to
an early Maastrichtian sea level fall recognized on the coastal margin of New Jersey (Miller et
al., 1999). However, the New Jersey margin sections and other sea level curves do not show a
significant sea level fall at 66.5 Ma. Both intervals clearly warrant further investigation.
New δ18O data from well-preserved terebratulid and rhynchonellid brachiopods and
belemnites from sections in southern England (Southerham, Eastbourne) exhibit the first highresolution paleotemperature record spanning the early Cenomanian to early Turonian. The longterm record of brachiopod δ18O values shows a continuous trend of climate warming from 14.4
°C in the middle Cenomanian to 20.3 °C in the lower Turonian (Figure 4-5). Superimposed on
this long-term trend are periods of rapid cooling that are correlative to middle and late
Cenomanian positive δ13C events. The middle Cenomanian δ13C increase is predated by a rapid
δ18O increase (-1.5 to -0.2‰) that corresponds to a sea level fall within the C. inerme Subzone.
Maximum δ18O values (0.5‰) are reached with the onset of transgression in the basal T. costatus
Subzone and the occurrence of P. primus. A similar pattern is visible during the upper
Cenomanian δ13C positive excursion. δ18O values of brachiopods and belemnites indicate rapid
cooling during a sea level fall followed by subsequent warming after transgression and the brief
occurrence of P. plenus. Within both Cenomanian events, the decrease of seafloor temperatures
has a similar magnitude, and is 4-8°C if no changes in δ18O of seawater are considered. The
linkage of rapid δ18O increases with sea level falls and with the southward spread of boreal taxa
suggests that climate cooling could have been coupled to the build-up of ice-sheets and changes
in ocean circulation, and could have initiated perturbations within the global carbon cycle.
Analyses of a number of δ13C records from Cretaceous pelagic sediments reveal
widespread, synchronous shifts that are attributed to changes in the relative burial fluxes of
organic carbon and carbonate carbon resulting from variations in sea level. This proposed
relationship is supported by comparison of Campanian carbon stable isotope and sedimentologic
profiles from Tunisia, France, England, Germany and Kazakhstan, which all show remarkably
similar patterns (Figures 4-6, 4-7). Positive δ13C excursions ranging from +0.2 to +0.3‰ are
recorded across the Santonian/Campanian boundary (~83.7 Ma) and in the mid-Campanian
(~78.7 Ma), and a negative δ13C excursion of -0.4‰ in the upper Campanian (~74.8 Ma) with
durations of 600 - 750 kyr. The long-term carbon isotope trend in the Campanian pelagic
sections broadly follows first-order eustatic sea level with relatively stable high δ13C values in
the lower Campanian reflecting stable, high eustatic sea level and with decreasing δ13C values in
the upper Campanian reflecting falling eustatic sea levels.
Sedimentologic and Biotic Evidence for Cretaceous Eustasy: In order to demonstrate that sea
level is truly eustatic it is necessary to identify synchronous sea level changes on a global scale
using high-resolution bio-, chemo- and/or cyclostratigraphy. Although this is difficult to achieve
particularly in nearshore, clastic facies, several examples of eustatic sea level change during the
mid-Cretaceous are gaining acceptance based on increasingly detailed field and laboratory
studies. For example, Gale et al. (2002) identified six mid- and late Cenomanian sea level cycles
with frequencies of 400 kyr that can be correlated from the coastal plain of SE India to 11
individual sections in northwest Europe. Analysis of facies shifts and fluvial incision indicates
that rapid falls of sea level took place at the sequence boundaries, with downcutting of 10-25 m.
These are minimum values, depending on the quality and extent of outcrop. The major falls
(mid-Cenomanian, C. inerme Zone, and late Cenomanian, M. geslinianum Zone) are coincident
with positive δ18O events of up to 2 to 2.5‰ in NW European chalk successions and correspond
with the southerly migration of Boreal nektic and benthic taxa in NW Europe. Stratigraphic and
sedimentologic observations from the Western Interior basin (such as identification of regional
bioclastic accumulations that correlate westward to progradational wedges, some with incised
valleys fills: Sageman, 1996), as well as biotic data (e.g., fluctuations in macrofaunal diversity
and abundance, trace fossil content, and planktic:benthic foraminiferal ratios) indicate that the
Cenomanian sea level events identified in India and NW Europe may also be expressed within
the North American epicontinental stratigraphic record (Figures 4-8, 4-9, 4-10; Tibert et al.,
2002). Although the duration of these mid- to late Cenomanian events may be consistent with a
long eccentricity modulator (400 kyr), there has also been speculation that the rhythmic
limestone-shale deposits of the Cenomanian-Turonian Bridge Creek Limestone in the Western
Interior basin may reflect a high frequency eustatic signal (Elder et al., 1994; Sageman, 1998;
Laurin and Sageman, 2001). Sea level changes of this magnitude and rate (tens of meters within
20 to 400 kyr intervals) are strongly suggestive of a glacio-eustatic control.
The importance of accurate stratigraphy to correlating third-order sea level cycles is
demonstrated by comparison of radiometrically interpolated and biostratigraphically correlated
versions of the Haq et al. (1987) sea level curve with the sea level curve developed by Hancock
(1990) for northwest Europe (Figure 4-11). Significant mismatch in the relative timing of the
transgressive peaks and regressive troughs is caused by differences in the biozonal schemes,
stage boundary definitions, and time scales that are used for each curve. Hancock (1993) noted
that there is no simple right or wrong answer to how sea level curves should be plotted, but he
emphasized that comparison of different sea level curves is only possible if their construction is
based on identical stratigraphic criteria.
Climate Modelling: An isotope-capable version of the GENESIS atmospheric general
circulation model (Mathieu et al., 2002) was used to examine the likely isotopic composition of
an ice-sheet that formed during the Cretaceous and the size of the ice-sheet that would have been
required to produce a significant shift in seawater isotopic composition. Preliminary results
(Figures 4-12, 4-13) show that the latitudinal gradient in precipitation δ18O is much less steep
than the modern. The heavier-than-modern freshwater δ18O values predicted for mid to high
latitude North America are in agreement with those inferred from Cretaceous fossil tooth enamel
studies (Fricke). The lower δ18O gradient and 18O-enriched high latitude precipitation has at least
two important ramifications for interpreting warm Cretaceous climate records: 1) If the mean
isotopic composition of ice that formed at high latitudes was no less than -20‰, then an ice
volume equivalent to 50% of the modern Antarctic ice-sheet would produce less than a 0.2‰
shift in the mean ocean δw value. This means that a substantial volume of ice could form or
decay without being reliably detected in benthic δ18O records. In the sequence of events reflected
in the record during ice growth, eustatic sea level fall may therefore be the only "given" and is,
indeed, generally accepted as having occurred throughout the Cretaceous; and 2) A lower
latitudinal gradient in freshwater δ18O could result in a lower gradient in upper ocean δw, relative
to the modern. In that case, use of the latitudinal correction for δw based on modern GEOSECS
data (Zachos et al., 1994) would consistently overestimate tropical upper ocean temperatures and
underestimate high latitude temperatures. This seems to exacerbate the problem of Cretaceous
polar ice growth.
Future Directions
Large and rapid changes in Cretaceous sea level that have been recorded during times of
global warmth pose a paradoxical problem to geologists and climatologists as they cannot be
explained by any other known mechanism but glaciation (Pitman and Golovchenko, 1983). In
the absence of direct geological evidence, the case for Cretaceous polar ice-sheets requires
demonstration that proxy signals of sea level and climatic change are globally synchronous and
consistent in their magnitude and direction of change. Thus far, few such records exist. Although
the Exxon sea level curve has been promoted as representing a suite of eustatic cycles that can be
globally correlated (Vail et al, 1977; Haq et al., 1987), some authors have argued that none of the
Exxon cycles have been proven to be globally synchronous, the magnitude of the sea level
changes have been considerably over estimated, and the use of the Exxon cycle chart should be
abandoned (Miall, 1992; Rowley and Markwick, 1992). The recent surge of studies that postulate
ice-sheet growth and decay as the cause for Cretaceous sea level variations and geochemical
shifts (e.g., Stoll and Schrag, 1996; 2000; Miller et al., 1999; Gale et al., 2002) demonstrates the
need to delineate which sea level changes were truly eustatic.
In order to resolve the Cretaceous ice-sheet debate, field and laboratory data that can be
correlated at Milankovitch time scales must be obtained from a global array of sites and
depositional settings. The following target intervals are identified as potentially yielding the
greatest insight to the question of Cretaceous glacio-eustasy: (1) late Maastrichtian (~66.5 Ma);
(2) early Maastrichtian (~70.5 Ma); (3) upper Campanian (~74.5 Ma); (4) Coniacian-Santonian
(~89.0-83.5 Ma); (5) mid-Turonian (~92-91 Ma); (6) Cenomanian/Turonian boundary interval
(~94-93 Ma), (7) mid-Cenomanian (~95.5 Ma); and (8) Aptian/Albian boundary interval (~114112 Ma). Impermeable and low porosity clay-rich sediments from low and high latitudes should
be sought to obtain the best preserved and most climatically informative oxygen isotope records
across these intervals (e.g., Pearson et al., 2001). More detailed and higher resolution records
from continental margin, carbonate platform, and transitional terrestrial-marine environments
that have high-resolution (e.g., Milankovitch scale) age control will be essential to reconstructing
a reliable Cretaceous eustatic curve. Key questions include:
•
What were the magnitudes and rates of Cretaceous sea level changes?
•
Do Cretaceous sea level fluctuations demonstrate a Milankovitch frequency? If so,
how did Milankovitch cycles affect Cretaceous sea level?
•
Which sea level changes can be proven to be globally synchronous?
•
Which, if any, Cretaceous sea level changes were glacially forced?
In addition to the many questions related to ice-sheet growth and sea level change, there
are Cretaceous sea level change issues that have important relevance to current global warming
and environmental problems (such as coral bleaching events). We need to better understand the
timing, extent and causes of carbonate platform drowning. What is the mechanistic relationship,
if any, between sea level rise and the OAEs? In future drilling and field studies, progress in sea
level studies will require that we:
•
Identify localities that have minimal regional tectonic influences.
•
Locate terrestrial sections that record incisement during marine incursions in order to
provide a quantitative estimate of sea level regression.
•
Use every environmental proxy available in stratigraphically complete, well-dated
sedimentary sequences.
•
Accurately correlate between epicontinental and deep-sea records using orbitallytuned sections, radiometrically-dated horizons and unique isotope events.
Drilling Priorities
1) Principle target ages that could constrain the timing, rate, duration, and magnitude of
climatic and/or sea level events and potentially provide evidence for glacio-eustasy
•
•
•
Maastrichtian: The proposed 66.5 and 70.5 Ma glacio-eustatic events can be tested by
increasing the number of high-resolution, well-preserved records from deep-sea and
continental margin sequences. Important sites would include:
Late Campanian (~74.5 Ma): Can the negative δ13C excursion identified in pelagic
carbonate sequences across Europe and western Asia be identified globally, and does it
correlate with a positive oxygen isotope shift and sedimentologic evidence for a rapid sea
level fall?
Coniacian-Santonian (89.0-83.5 Ma): Can this interval, which contains one of the longest
relatively continuous records of orbitally influenced sedimentation in North America (as
well as the comparatively poorly studies OAE3 event), be used as a test case to better
•
•
•
•
understand orbital forcing of sedimentation and organic carbon burial processes and their
effect on climate?
Mid-Turonian (~91 Ma): Can the major sea level fall identified across the U.S. Western
Interior be identified globally at the same time? Could this event have been glacially
forced at a time when Earth experienced some of the warmest global temperatures of the
past 140 m.y.?
Cenomanian-Turonian boundary interval (~93.5 Ma): how rapidly did sea level change
across this interval and how did that influence OAE2?
Mid-Cenomanian (~95.5 Ma): Did a rapid cooling in deep waters and at high latitudes
occur at precisely the same time as the sea level fall identified in SE India and European
sections? Can this sea level fall be identified in carbonate platform and continental
margin sequences globally?
Aptian-Albian boundary interval (~120 Ma): Relatively positive high latitude
temperature estimates for this time suggest that the polar regions may have been cool
enough for the presence of a continental ice-sheet in the south polar region.
2) High priority drilling locations
•
•
•
Passive continental margin:
o Petroleum industry borehole logs and cores: These are considered a major
untapped resource that could significantly improve understanding of
Cretaceous global sea level change.
o Southeast India: potential to correlate nearshore clastic sequences with
deepwater water clays at Milankovitch frequencies to better constrain the
timing and magnitude of mid-Cretaceous sea level changes in that could be
compared with other high-resolution records (e.g., U.S. Western Interior)
o Brazil margin
o J-Anomaly Ridge
o Northwest Australia
Low latitudes
o Demerara Rise: Provides critical insight to tropical climate change from an
expanded Cenomanian through Turonian hemipelagic deepwater sequence; midCretaceous foraminiferal preservation is known to be excellent based on DSDP Site
144A drill cores
o Manihiki Plateau
o Hess Rise
Southern, high latitude targets: could provide critical information on fluctuations in polar
climates coincident with sea level rise/fall events (you need to tie this into sea level
change)
o Agulhas and Crozet Plateaus: Lamont-Doherty piston core records reveal the
presence of well-preserved mid-Cretaceous foraminifera.
o Naturaliste Plateau: Presence of well-preserved Cenomanian foraminifera
documented at DSDP Site 258, but that record is incomplete because of poor
drilling recovery and presence of unconformities
o Weddell Sea
o Mozambique/S. Madagascar Ridges
o Falkland Plateau
•
Arctic: Any climatic records from the Arctic are highly desirable as they could provide an
indication of the coolest temperatures on Earth available from the geologic record.
o Arctic/Lomonosov
o Hudsons Bay
o Bering Sea
Atmospheric and Ocean Circulation in a Greenhouse World
Circulation Figures with Captions
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Overview
Since the original efforts of Eric Barron and co-workers in the early 1980s,
considerable progress has been made towards understanding the Cretaceous greenhouse
world. This progress was forged through collaborations between the data-gatherers and the
climate modelers. In this spirit, the working session, Atmosphere and Ocean Circulation in a
Greenhouse World, brought together similarly disparate specialists to report on the current
state-of-affairs in Cretaceous climate science. The session served as a forum to report and
discuss many novel and exciting ideas, and to reassert the need for continued, and closer,
collaboration between climate modelers and geologists. Contributions to the Atmosphere and
Ocean Circulation session could be forcibly divided into three categories, each of which is
individually summarized below, with the following objectives:
•
To understand general circulation during the Cretaceous
•
To document and understand climate change through the Cretaceous
•
To develop critical data sets as boundary conditions and validation of numerical
models of the Cretaceous.
One of the most important conclusions reached in the session is that there is no longer
a single paradigm for understanding the Cretaceous general circulation. New paradigms
depart significantly from the classic view of the Cretaceous as a warm, equable world forced
by moderately high (4-6 x present-day levels) atmospheric carbon dioxide levels and a
reversed thermohaline circulation. Rather, the following views were expressed:
•
In a Cretaceous greenhouse world, the westerly winds developed only seasonally. In
the absence of persistent westerlies, the subtropical ocean gyres would have
weakened leading to an ocean circulation dominated by eddies. As a result, the
pycnocline would have been more diffuse than today, contributing to enhanced
thermohaline circulation and global oceanic heat transport (Figure 5-1) (Hay).
•
In order to match the warmest tropical paleotemperature estimates, general circulation
models of the Cretaceous indicate that atmospheric pCO2 must have been much
higher than previously thought (~12 x present-day levels) (Figure 5-2) (Bice et al.).
•
High atmospheric pCO2 levels are not required to achieve a very warm Cretaceous
climate. With moderate pCO2 levels (~3 x present-day levels), the Hadley coupled
ocean atmosphere model predicts a hot Cretaceous world dominated by latent heat.
The difference between the Hadley model and other climate models that require
higher pCO2 is probably in the climate sensitivity to clouds (Figure 5-3) (Valdes and
Markwick).
•
Warm Cretaceous oceans can be explained by the sinking of warm, saline highlatitude waters. With moderate pCO2 levels (~3 x present-day levels), the NCAR
Climate System Model predicts deep-ocean temperatures of 9-11 °C, and surface
temperatures that are 3-4 and 6-14 °C warmer than modern at low and high latitudes,
respectively. Ocean heat transports in the model are diminished in the Northern
Hemisphere and enhanced in the Southern Hemisphere (Figure 5-4) (Otto-Bliesner et
al.).
In contrast to the older paradigm of low latitude bottom water formation, it is notable
that all the numerical models presented in which thermohaline overturning occurred showed
that the warm Cretaceous deep oceans can be explained by the sinking of warm, saline highlatitude waters.
These views of the Cretaceous general circulation are a sign not only of the progress
that has been made in the last 20 years, but also a representation of the scientific debates that
will be played out in the next decade. As an example of the relevance of Cretaceous climate
science, many of the topics discussed in the working session mirror discussions that are being
held regarding future global climate change, including the sensitivity of the climate to pCO2,
the presence or absence of a "tropical thermostat," the role of clouds and water vapor in
climate change, and the response of ocean thermohaline circulation to polar warmth and an
increased hydrologic cycle.
Another critical issue highlighted in this session was the very large differences in the
response of the general circulation models to Cretaceous boundary conditions. For example,
in comparison to the NCAR models, the Hadley Centre model indicates a much stronger CO2
sensitivity via clouds and hydrologic cycle strength. The same spectrum of climate models
used in Cretaceous is being used to predict future climates, but it is only through comparison
against the Cretaceous data record that we can begin to quantitatively address the question of
whether the models’ sensitivity to increased greenhouse gas concentrations is too large or too
small. If the low subpolar salinities predicted by the Hadley model can be verified as
consistent with the existence of abundant and diverse Cretaceous planktonic foraminifera at
these latitudes, then there is good reason to suspect that less sensitive models used to predict
anthropogenic change may be substantially under-estimating future polar temperatures.
Another sign of the maturity of Cretaceous climate science is awareness that the
Cretaceous climate displayed considerable variability over its 79 million years. In this
working session (and others), climate variability was documented and analyzed. This
variability includes changes in the geochemical state of the oceans, long-term climate
warming and cooling, and millennial-scale changes in the hydrological cycle. Cretaceous
marine sediments from the Tethys record fundamental changes in the global carbon reservoir.
Carbon burial occurred as inorganic carbon in pelagic and shallow water carbonates during
the early Cretaceous, organic carbon in deep ocean and margin basins during the middle
Cretaceous, and inorganic and organic carbon on shelves during the Late Cretaceous (Jansa
et al.). Possible causes of Cretaceous climate variability discussed in this session include:
•
Large, rapid shifts in atmospheric pCO2. General circulation models indicate that
large shifts in pCO2 are required to explain tropical sea-surface temperature variations
in the mid-Cretaceous leading into the Cretaceous Thermal Maximum. These pCO2
large shifts fall within the range of atmospheric CO2 estimates that have been
calculated through proxy methods (Bice et al.).
•
Precessional forcing. Results from a general circulation model demonstrate that the
hydrological cycle in western North America is sensitive to precessional forcing. The
formation of bedding couplets is controlled by changes in the rate of mechanical
erosion, which is a function of surface runoff (Figure 5-5) (Floegel et al.).
•
Paleogeographic changes. Results from a coupled ocean-atmosphere model indicate
that the opening of the Atlantic Equatorial Gateway could have caused a large-scale
reorganization of the tropical climate, and regional warming. This mechanism was
considered as a cause of the Cretaceous Thermal Maximum (Figure 5-6) (Poulsen et
al., 2003).
As demonstrated by the findings summarized above, numerical models of the
Cretaceous climate can provide insights into the working of a greenhouse world, as well as
validation of the sophisticated models used to forecast future climate change. However,
Cretaceous simulations are only as good as the boundary conditions incorporated into the
models. Likewise, validation of the models necessitates accurate and detailed descriptions of
critical aspects of the Cretaceous environment. Progress has been made in both of these
areas, and include:
•
The production of high-resolution paleotopographic and paleobathymetric elevation
models for the Late Maastrichtian (70 Ma), early Campanian (80 Ma), C/T boundary
(90 Ma), Albian (110-100 Ma), early Aptian (120 Ma), and Valanginian (130-140
Ma) (Figure 5-7) (Scotese).
•
The construction of detailed paleobathymetric maps with accurate description of
gateway openings and block movements (Figure 5-8) (Lawver and Gahagan).
•
The development of detailed global vegetation maps, and an analysis of areas of
agreement and disagreement between different reconstructions (Figure 5-9)
(Upchurch et al.).
•
The reconstruction of Cretaceous climate using statistical relationships between leaf
form and climate. Much of our knowledge of terrestrial climate stems from these
statistical models, however the choice of regression model can introduce a strong
climate bias. Differences between regression models can cause climate interpretations
that vary by 7°C in mean annual temperature and 600 mm in mean annual
precipitation (Figure 5-10) (Scherer et al.).
•
The reconstruction and variability of detailed current systems, such as the middle
Cretaceous contour current system along the continental rise of the deep southeastern
Gulf of Mexico (Figure 5-11) (Buffler).
The need for detailed data sets to use as boundary conditions and to validate the
climate models will only increase as the model resolution and sophistication increase. In
addition, targeting specific time intervals of interest could facilitate model intercomparison
and validation. Such an endeavor would require support from the entire Cretaceous scientific
community.
Future Directions
The study of climate variability in the Cretaceous requires continuous time-series of
environmental change. As more continuous, high-resolution (millennial scale) archives are
developed, the picture of Cretaceous climate variability will expand. Obtaining these records
and identifying the causes of climate variability in a greenhouse world, particularly those that
can cause "surprises" such as rapid pCO2 fluctuations or tropical climate shifts, should be a
major focus of the Cretaceous scientific community.
The working session on Cretaceous atmosphere and ocean circulation demonstrated
that significant progress has been made in defining and understanding the complexities of
Cretaceous climate. Old paradigms have been replaced by a variety of new paradigms (some
speculative) that will be the focus of the community for the near future. Many of the
remaining critical questions in Cretaceous climate science are shared by the future global
change community, including the roles of clouds, water vapor, tropical climate, and CO2 in
driving global climate. The Cretaceous community is uniquely positioned to contribute
significantly to these questions. To facilitate this endeavor, we must encourage and support
more cross-disciplinary collaborations between climate scientists and those who gather
Cretaceous climate proxy data.
Environmental and Biotic Consequences of Large Igneous Provinces
Consequences of LIPs Figures with Captions
Return to top of Report
Overview
Large Igneous Provinces (LIPs) were constructed during voluminous magmatic
events that took place over geologically brief (<3 m.y.) time intervals (e.g., Duncan and
Richards, 1991; Tarduno et al., 1991; Coffin and Eldholm, 1994). These events are thought
to be associated with massive thermal anomalies in the mantle known as "superplumes"
(Larson, 1991). The volume of crust produced by LIPs in the mid-Cretaceous was almost
three times greater than in prior and subsequent time periods (Larson, 1991). In particular
three prominent LIPs were formed during this interval. The Ontong Java Plateau (OJP) and
Manihiki Plateau in the Pacific formed ~123 Ma in the late Barremian, the Kerguelen Plateau
in the Indian Ocean was erupted in the late Aptian and early Albian, and the Caribbean
Plateau is thought to have formed near the Galapagos hotspot between 93 and 95 Ma
(Duncan and Hargraves, 1984).
Increased rates of volcanism associated with oceanic plateau formation and sea
floor spreading in mid-Cretaceous time probably caused a variety of biological and
geochemical responses. A current popular model for the sporadic occurrence of oceanic
anoxic events (OAEs) in the Cretaceous ties hydrothermally-induced changes in ocean
chemistry to increased surface productivity, followed by mid-to-deep water oxygen
depletion and accumulation of organic-rich sediments (e.g., Vogt, 1989; Leckie et al.,
2002). In addition, CO2 outgassing during volcanism may have increased global
temperatures and altered deep water circulation patterns leading to increased anoxia in
warm deep waters (e.g., Arthur et al., 1985). However, small but possibly significant
mismatches between radiometrically dated oceanic basalts and stratigraphically dated
geological responses make it difficult to establish cause and effect relationships with
certainty (Figure 6-1, Larson et al.).
Metal anomalies that arise from hydrothermal activity during ridge-crest and midplate volcanism can be advected for significant distances by plumes (e.g., Baker, 1994),
especially where the water column is reducing. Megaplume activity can enrich Fe, Mn, and
trace metals (e.g., Cr, V, Ni, Zn, Cu, Ba) in proximal sediments by several orders of
magnitude (Figure 6-2; Duncan and Rubin, 1997). This signature in sedimentary sections has
great potential for precise stratigraphical correlation between hydrothermal activity and its
potential environmental responses including OAEs and plankton turnover. Preliminary metal
data from the Cismon section, for example, appear to record Ontong Java Plateau (OJP)
volcanism in strata within and directly below the Selli Level (Figure 6-3, Turgeon et al.;
Duncan and Huard, 1997) providing an enticing causal scenario for OAE1a. However, a far
more comprehensive set of drill sites is required to test this relationship, and connections
between LIP eruptions and other OAEs. In addition, the sensitivity of phytoplankton to biolimiting and toxic trace metals and the difference between the hydrothermal inputs from sea
floor spreading centers and ocean plateaus is not completely understood. Precise
paleotemperature estimates are still needed for the Barremian-early Albian interval to assess
the potential effect of this huge volcanic episode on paleoclimate.
The relationship between LIP formation and OAEs derived from radiometric
estimates is less straight forward. Radiometrically dated basalts from the OJP and Manihiki
Plateau average 123 Ma, while the Selli black shale (OAE1a) and associated isotopic ratio
anomalies occurred 2-3 m.y. later. Basalts from the Kerguelen Plateau suggest major
volcanic activity, much of which was subaerial, beginning at about 118-119 Ma, which
possibly continued at decreased rates and contributed to OAE1b and elevated
paleotemperatures in the Albian. Basalts from the Caribbean Plateau, which were previously
determined to have radiometric ages of about 88-91 Ma (Sinton and Duncan, 1997), have
been redated and with one exception now have radiometric ages of about 92-95 Ma (Duncan,
unpulished data). Thus, the case is considerably stronger, but not yet certain, that Caribbean
Plateau volcanism contributed to OAE2 and associated isotopic anomalies near the
Cenomanian/Turonian boundary at 93.5 Ma. These new age 40Ar-39Ar whole rock age
determinations are based on incremental heating experiments that, with one exception,
produce Ar-recoil patterns from which only total fusion ages can be extracted. These ages are
more precise than previous estimates, but still produce the same estimated duration of
volcanic activity of about 3 m.y. Additional improvements in radiometric dating will rely on
finding samples from which feldspar can be separated.
Ocean drilling can make a major contribution to investigating the linkage between
LIP volcanism and environmental change. High-resolution studies have and will continue to
reveal the time scale and nature of biostratigraphic, lithologic and chemical changes at
OAEs. An array of drilling sites designed to recover continuous sections for targeted OAEs,
will document "near-field" and "far-field" effects, trace element abundance patterns, and
Cretaceous ocean circulation. Advances in analytical methods (ICP-MS) open the way for
detection of a wide range of potentially diagnostic elements from large numbers of samples.
Finally, other tracers with short residence times are required to trace hydrothermal activity
regionally and globally. Os isotope ratios, specifically 187Os/188Os, are limited to analysis of
sediments deposited in oxic conditions, but hold significant potential to meet this goal
(Ravizza et al., 2001). For example, based on analyses of metalliferous sediments from
Cyprus and Oman, there is an excursion close to the Cenomanian/Turonian boundary that
may be associated with LIP emplacement in the Caribbean. Os isotopes have significant
potential to further our understanding of the linkage between LIP activity and OAEs (Figure
6-4, Ravizza et al.).
Future Directions
A number of key questions can only be addressed through ocean drilling.
•
What was the effect of the world-wide, mid-Cretaceous volcanic pulse on the
deposition of Corg and the timing of OAEs? We need to determine whether the timing
and duration of LIPs is correlative with OAE events via improved radiometric dating
of LIPs; improved dating of OAEs (GPTS, Os-dating of black shales); sites proximal
to an ocean plateau that records both volcanic history and OAE events (e.g., Nauru
Basin and OJP).
•
Which OAEs are LIP-related and which have other causes? We need high-resolution,
multiple-proxy studies of "regional" OAEs, for which no LIP has been identified.
•
What was the effect of mid-Cretaceous volcanism on paleotemperature and pCO2?
Needed: updated calculations of ocean heating by massive lava flows in appropriate
Cretaceous ocean conditions; high-resolution GCM of Cretaceous ocean with LIP
inserted to model altered flow and circulation downstream from LIP effluent; better
estimates of degassing and hydrothermal trace element "fingerprints"; understanding
of gas hydrate distribution and stability in early and mid-Cretaceous oceans; better
information about primary trace metal contents of LIPs and alteration effects; which
metals are bio-limiting (or toxic) and which phytoplankton are important in OAEs.
Was the volcanic pulse a direct cause of "greenhouse" climate conditions? Or is
methane dissociation a more likely trigger?
•
Do microplankton and microbenthos respond to the physical, chemical and biological
oceanographical changes associated with OAEs? What are the causes of apparent
extinction and evolution in these time intervals?
•
What are the different effects of predominantly submarine LIPs vs. predominantly
subaerial LIPs? Needed: high-resolution, multiple-proxy studies of K/T (Deccan) or
Valanginian (Parana) vs. early Aptian (OJP) or C/T (Caribbean) events.
Drilling Priorities
Drilling priorities for OAE1a in the Pacific include Magellan Plateau, Nauru Basin, and
seafloor sufficiently young to be above the CCD at OJP time (i.e., M1-M3 age). In the
Atlantic and Indian Oceans, the Weddell Sea and Maud Rise are high priorities. For OAE2,
future drilling in the Pacific on Magellan Rise, Manihiki Plateau, Hess Rise, and
Cenomanian-Turonian atolls is needed.
DRILLING TARGET SUMMARY
LOCATION
PROPOSAL #
PROPONENTS
PACIFIC RISES
Manihiki Plateau
Roger Larson and Elisabetta Erba
Magellan Rise
Roger Larson and Elisabetta Erba
Hess Rise
Roger Larson and Elisabetta Erba
HIGH LATITUDE SOUTH
Weddell Sea
503 (full)
Woody Wise, W. Jokat, et al.
Naturaliste Plateau
Darren Gröcke
Agulhas/Crochet Plateaus
Brian Huber
Mozambique/S. Madagascar Ridges
Dick Norris and Karen Bice
Falkland Plateau
Karen Bice, Woody Wise
HIGH LATITUDE NORTH
Arctic/Lomonosov
533 (full)
Jan Backman
Arctic-Atlantic Gateway
588 (pre)
F. Gradstein, S. Ren, et al.
Hudsons Bay
617 (pre)
Tim White
Bering Sea
Tim Bralower
CONTINENTAL MARGINS
Carolina Transect
616 (pre)
Tim Bralower, S. Culver, et al.
J-Anomaly Ridge
562 (full)
Dick Norris, Karen Bice, et al.
Cape Verde Basin
Tom Wagner
Brazil Margin
Lisa Pratt, Simon Brassell, Karen Bice
Northwest Australia
Paul Bown, Leon Clarke
NE Japan
608 (pre)
Takashi Hasagawa
OTHER CRETACEOUS OBJECTIVES
Scott Plateau
513 (full)
B. Opdyke, H. Stagg, G. C. H. Chaproniere
Hikurangi LIP
542 (pre)
N. Mortimer, R. Wood, et al.
Chicxulub Impact Crater
548 (full)
J. Morgan, R. Buffler, et al.
Caribbean LIP
561 (full)
Bob Duncan
S. Rockall-Hatton Plume
596 (pre)
Tim Morrissey, P. Shannon, et al.
Somali Basin
606 (pre)
Hiroshi Nishi, Hisatake Okada, Brian Huber
Ontong Java Plateau
623 (pre)
C. Neal, M. Coffin, et al.
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Appendix 1
Workshop Participants
Thierry Adatte
University of Neuchâtel
Michael A. Arthur
Penn State University
Enriqueta Barrera
National Science Foundation
Britta Beckmann
University of Bremen
Karen L. Bice
Woods Hole Oceanographic Institution
Paul R. Bown
University College London
Timothy J. Bralower
University of North Carolina-Chapel Hill
Simon C. Brassell
Indiana University
Richard T. Buffler
University of Texas
Pat R. Castillo
Scripps Institution of Oceanography
Leon J. Clarke
University of Wales, Bangor
Linda M. de Romero
University of North Carolina-Chapel Hill
Walter Dean
USGS, Denver
My Le Ducharme
Smithsonian Institution
Robert A. Duncan
Oregon State University
Maya B. Elrick
University of New Mexico
Elisabetta Erba
University of Milan
Jochen Erbacher
BGR
David B. Finkelstein
Indiana University
Alfred G. Fischer
University of Southern California
Cynthia G. Fisher
West Chester University of Pennsylvania
Sascha Floegel
GEOMAR
Karl B. Föllmi
University of Neuchâtel
Astrid Forster
Netherlands Institute for Sea Research
Henry Fricke
Colorado College
Andrew S. Gale
University of Greenwich
Luis A. Gonzalez
University of Iowa
Darren Gröcke
Royal Holloway College
Jake M. Hancock
Imperial College of Science, Technology and Medicine
Takashi Hasegawa
Kanazawa University
Noralynn Hassold
University of Michigan
William W. Hay
GEOMAR
Ulrich Heimhofer
ETH-Zurich
Achim Hermann
Penn State University
Peter M. Hofmann
University of Cologne
Ann Holbourn
Christian Albrechts University
Brian T. Huber
Smithsonian Institution
Hope Jahren
Johns Hopkins University
Luba F. Jansa
Dalhousie University
Ian Jarvis
Kingston University
Hugh C. Jenkyns
Oxford University
Claudia C. Johnson
Indiana University
Kirk Johnson
Denver Museum of Nature and Science
Erle G. Kauffman
Indiana University
Gerta Keller
Princeton University
Wolfgang Kuhnt
Christian Albrechts University
Roger L. Larson
University of Rhode Island
Jiri Laurin
Academy of Sciences of the Czech Republic
Lawrence A. Lawver
University of Texas
R. Mark Leckie
University of Massachusetts
Jackie A. Lees
University College London
Elana Leithold
North Carolina State University
Greg A. Ludvigson
University of Iowa
Kenneth G. MacLeod
University of Missouri
Adam McConnell
University of Massachusetts
Cheryl L. Metz
Texas A&M University
Philip A. Meyers
University of Michigan
Stephen R. Meyers
Northwestern University
Isabel P. Montanez
University of California-Davis
Dragana D. Nebrigic
University of Texas at Dallas
Richard D. Norris
Woods Hole Oceanographic Institution
David Osleger
University of California-Davis
Bette L. Otto-Bliesner
National Center for Atmospheric Research
Desiree Polyak
University of Massachusetts
Christopher Poulsen
University of Southern California
Lisa M. Pratt
Indiana University
Isabella Premoli Silva
University of Milan
Greg Ravizza
Woods Hole Oceanographic Institution
Shelley Rios
National Science Foundation
Bradley Sageman
Northwestern University
Sarah Santee
University of California-Davis
Christopher R. Scotese
PALEOMAP Project
Paul J. Sikora
University of Utah
Laura J. Snow
Oregon State University
Erica M. Sterzinar
University of Massachusetts
John A. Tarduno
University of Rochester
Steven C. Turgeon
ICBM - Universität Oldenburg
Garland R. Upchurch
Southwest Texas State University
Paul J. Valdes
University of Reading
Silke Voigt
University of Cologne
Hu Xiumian
Chengdu University of Technology
Thomas Wagner
University of Bremen
David K. Watkins
University of Nebraska
Timothy S. White
Penn State University
Peter Wilf
Penn State University
Paul A.Wilson
University of Southampton
Sherwood W. Wise
Florida State University
Appendix 2
Download workshop program (pdf).
Return to top of Report
Figures with captions - Cretaceous Climate Record as Revealed by Stable Isotopes
Figure 1-1 (Norris et al.) Compilation of paleotemperature estimates from oxygen stable isotope
ratios for clay-hosted planktonic foraminifera (red symbols), chalk-hosted planktonic
foraminifera (blue symbols), and benthic foraminifera (black symbols).
Figure 1-2 (Bice et al.) Estimated paleotemperatures for DSDP Site 511 calculated assuming
that ambient surface waters were as depleted as the lightest water in the GEOSECS dataset
poleward of 60 degrees latitude (-0.5 per mil SMOW) and adjusted by -1.0 per mil to correct for
ice-free world; δw of bottom waters was assumed to be -1.0 per mil SMOW.
Figure 1-3 (MacLeod and Huber) Paleogeographic map of the mid-Maastrichtian (Ziegler et al.,
1997) showing the approximate location of selected sites for which Maastrichtian ocean
temperatures have been estimated. Estimated paleotemperature from unpublished data and
compilations of Barrera and Savin (1999) and Frank and Arthur, (1999).
Figure 1-4 (Jenkyns and Tsikos) Chemostratigraphic correlations between the Black Band (S.
Ferriby, England) and the Cenomanian/Turonian of Olini (Columbia) and Pueblo (Colorado,
U.S.) sections.
Figure 1-5 (Elrick and Molina) Proposed carbon isotopic correlations between the lacustrine
carbonates of the Cedar Mountain Formation and carbon isotope profiles presented by
Bralower et al. (1999, Jour. Foram. Res. 29:4:418-437). Uppermost strata of the Cedar
Mountain Formation contain a 98.4 Ma volcanic ash bed straddling the Albian-Cenomanian
boundary (Cifelli et al., 1997, Proc. Natl. Acad. Sci. USA 94:11163-11167). Note the
systematic δ13C offset between the Price River (proximal foreland) and Dinosaur National
Monument (distal foreland) sections.
Return to Stable Isotopes text
Figure with captions - Biotic Records of Global Change during the Cretaceous
Figure 2-1 (Lees) Nannofossil palaeobiogeographic data can be used to indicate broad-scale
ocean-climate. Four Southern Hemisphere Late Cretaceous nannofossil palaeobiogeographic
zones (PBZs) can be determined using semiquantitative data - Subantarctic, Austral, Temperate,
Tropical. These are broadly equivalent to marine-climatic belts. Latitude-related movements in
the water-mass fronts bounding nannofossil PBZs can be used to determine ocean-climatechange trends. Such trends correlate well with published δ18O SST data. The Campanian
nannoplankton diversity maximum appears to be the result of cooling (but in a warm, greenhouse
world). Understanding the biogeographic distribution and ecological niche of each Late
Cretaceous nannofossil taxon, and the effects of climate upon these, is essential for greater
(wider) biostratigraphic effectiveness. Knowledge of the potential ecological strategy of each
Late Cretaceous nannofossil taxon could help to elucidate climate-related features of a watermass, e.g. potentially r-selected (meso-/eutrophic) taxa indicating nutrient cycling, turbulence,
upwelling and seasonality, and putative K-selected (oligotrophic) taxa suggesting water-column
stratification and relative stagnation.
Figure 2-2 (Premoli Silva) Summary of planktic foraminiferal diversity trends during the
Early Cretaceous.
Figure 2-3 (Premoli Silva) Planktic foraminiferal abundance and diversity through the lower
Aptian Selli Event (OAE1a) in Italy.
Figure 2-4 (Johnson) Paleobiogeographic distribution of scleractinian corals. The dispersion
pattern for colonial corals infers that the geographic extent of the Cretaceous tropics was
greater than that of our present-day interglacial period. Base map of Scotese and Golonka
(1992).
Figure 2-5 (Wilf et al.) Terminal Cretaceous paleotemperatures estimated from oxygen
isotope data from tests of benthic (filled symbols) and planktic (open symbols) foraminifera
at (A) middle- and (B) high-latitude drilling sites, with (C) the most complete data from (A)
and (B) plotted against temperatures derived from North Dakota leaves (this study). Large
arrowheads indicate the Bass River size and abundance acme (A) (Olsson et al., 2001) and
Antarctic appearance datum (B) (Huber and Watkins, 1992) for Pseudotextularia elegans, a
thermophilic foraminifer. Paleotemperatures were calculated using the equation of Erez and
Luz (1983), assuming an ice-free world with an average δ18O value for ocean water of -1.2
‰, Peedee belemnite standard (Shackleton and Kennett, 1975). Benthic data are based on
combined δ18O values for Gavelinella beccariiformis and Nuttallides trumpeyi, which
generally plot within 0.2‰ of each other. None of the data has been corrected for variations
in seawater salinity or vital effects, and normal marine salinity is assumed for all sites. The
Bass River borehole is in the New Jersey coastal plain, 40° N paleolatitude, 100 m
paleodepth, data from Olsson et al. (2001); Deep Sea Drilling Project Site 525A is in the
South Atlantic, 35° S, 1000 m (Li and Keller, 1998); Ocean Drilling Program (ODP) Site
1050C is in the western North Atlantic, 30° N, 1500 m; and ODP Site 690C is in the Weddell
Sea, 65° S, 1500 m (Barrera and Savin, 1999). All data for Site 1050C and some data for Site
690C are new. K = Cretaceous; P = Paleogene; C29r, C30n = magnetic polarity Chrons 29
reversed and 30 normal, respectively. Limitations on graphic presentation cause floral data
from < 1 m above and foraminiferal data from just below the K-P to appear within the gray
line representing the K-P.
Figure 2-6 (Tarduno) A glendonite from the Early Cretaceous of the high Arctic.
Figure 2-7 (Fisher) Contour map of planktic foraminiferal (Hedbergella delrioensis) porosity
data from localities across the Western Interior Sea during the latest Cenomanian
Neocardioceras Biozone time.
Figure 2-8 (Fisher) Profile of latest Cenomanian planktic foraminiferal (Hedbergella
delrioensis) porosity data from the southern part of the Western Interior Sea.
Figure 2-9 (Erba) Integrated bio-, magneto- and chemo-stratigraphy of the late Valanginian
"Weissert event" (Bartolini & Erba, in prep.). Time scale after Channell et al. (1995); δ13C
curve after Weissert et al. (1998); Igneous events after Renne et al. (2001).
Figure 2-10 (Leckie et al.) - Summary of the major geochemical, tectonic, sea level, and
plankton evolutionary events associated with the mid-Cretaceous Oceanic Anoxic Events.
Note the concentration of evolutionary turnover events (speciation plus extinction) with the
OAEs. Also note that OAE1a, 1b, and 2 are temporally associated with increased submarine
volcanic activity as indicated by the lower 87Sr/86Sr ratios (Bralower et al., 1997), and all
three are linked to increased burial of marine organic matter (e.g., Arthur et al., 1987;
Erbacher et al., 1996; Larson and Erba, 1999). We hypothesize that submarine volcanism and
hydrothermal activity at the spreading centers helped to fuel the elevated levels of marine
productivity during the OAEs by way of iron fertilization of the water column. In addition to
submarine volcanism, the strontium isotope record is also influenced by continental
weathering and runoff (e.g., Jones and Jenkyns, 2001), and the rise in 87Sr/86Sr ratios during
the Albian may in large measure record increased weathering rates with rising sea level and
global warming, despite the faster spreading rates that sustained high global sea level through
much of the Late Cretaceous. The high productivity associated with OAE1d may have been
facilitated by an ocean already preconditioned by dissolved iron (from Leckie et al., 2002).
Figure 2-11 (Erba) Mid-Cretaceous integrated stratigraphy, nannofossil evolutionary events
and evidence of cooling episodes interrupting the greenhouse conditions. The main igneous
events related to the formation of the Ontong Java - Manihiki and Kerguelen LIPs closely
correlate with OAEs, isotopic excursions and biotic changes. Timescale: Gradstein et al.
(1995). Planktonic foraminiferal zones: Premoli Silva & Sliter (1999). Nannofossil zones and
events: Bralower et al. (1997), Walsworth-Bell & Erba (in prep.), Erba (1986, 1992,
unpublished). Oxygen and carbon isotopic curves: Menegatti et al. (1998), Weissert et al.
(1998), Jenkyns et al. (1994), Herrle (2001), Wilson & Norris (2001). Igneous events: Larson
& Erba (1999), Duncan (2002).
Figure 2-12 (Holbourn and Kuhnt) Benthic foraminiferal biofacies for Cretaceous oceanic
anoxic events 1a-1d, 2 and 3.
Figure 2-13 (Holbourn and Kuhnt) Our data do not support a simple model of mass
extinctions caused by global anoxia, followed by prolific radiations during the recovery
period at the end of OAEs. We suggest an alternative model of accelerated faunal turnover
within reduced or isolated populations during and following periods of anoxia. This new
model implies that the extent, depth and intensity of oxygen minima within ocean basins
varied considerably with time during OAEs. In the case of OAE2, the most significant faunal
turnover was in deep sea assemblages, whereas faunal turnovers in shelf and bathyal
assemblages mainly reflect sea level change and taphonomic bias.
Figure 2-14 (Meyers and Sageman) The rate of first appearance, last appearance, and
turnover of bivalves (FA-LA) is compared to geochemical proxy fluxes (titanium, carbonate,
molybdenum) from the Bridge Creek Limestone Member. The accumulation rate data is from
the # 1 Portland core (Meyers et al., 2001). Evolutionary rates were calculated using the time
scale of Meyers et al. (2001) and taxa range data compiled by Harries and Little (1999). Note
the following: 1. Apparent evolutionary rates of molluscan fauna of the Western Interior
Seaway represent a balance between faunal migration, local environmental conditions, and
extra-basinal forcing. 2. Migration of Tethyan fauna into the seaway is most prominent at the
base of the Bridge Creek Limestone (S. gracile biozone), with rates of bivalve first
appearance approaching 30 spp./1000 years. 3. An elevated rate of bivalve last appearance
occurs in the upper S. gracile biozone, and is correlative with enhanced detrital flux into the
seaway. Species loss may have been forced by changes in substrate firmness and/or clogging
of the feeding apparatus as suggested by Elder (1989). 4. The major turnover of molluscan
species prior to the C/T boundary, which reflects some extinction but mainly very low
origination rates, is not correlative with evidence of the largest changes in redox state or
detrital flux, as recorded in the geochemical proxy data. This may indicate that the turnover
mechanism was extra-basinal.
Return to Biotic Records text
Figures with captions – Oceanic Anoxic Events: Causes and Consequences
Figure 3-1 (Brassell) Elemental and molecular characteristics of OAE 1a from Shatsky Rise,
depicting: 50cm organic-rich interval corresponding to OAE 1a at Hole 1207B, results from
Rock-Eval pyrolysis of organic-rich intervals in the West Pacific, and illustrative distributions of
hydrocarbon and ketone biomarkers labelled according to their biological origins.
Figure 3-2 (Meyers) Compilation of δ15N:δ13Corg values for various samples.
Figure 3-3 (Erbacher and Herrle) Cyclicity and orbital control of OAE 1b in France.
Figure 3-4 (Wagner et al.) Temporal relationship of continental supply, development of deepwater anoxia and organic carbon burial.
Figure 3-5 (Jahren) Carbon release from methane hydrate (Gt) versus time.
Figure 3-6 (Hasegawa) Based on a comparison of carbon isotopes from terrestrial organic
carbon, marine carbonate carbon and marine organic carbon, relationships between the long-term
climatic optimum during the Middle Cenomanian-Early Turonian and isotope signals are
proposed: A greenhouse-induced warm and humid climate gradually prevailed over northeastern
Asia. Under these conditions, a strengthened canopy and humidity effects drew the δ13C value of
plants in a negative direction. A gradual increase in pCO2 drove an intensified greenhouse effect
that led to the climatic optimum. It caused a long-term negative excursion in marine organic
carbon δ13C that is comparable to that for terrestrial organic carbon.
Figure 3-7 (Heimhofer et al.) Correlation of Aptian shallow-water successions from the Algarve
and Lusitanian Basins, Portugal. Stratigraphy of the Algarve Basin is based on
chemostratigraphy ( δ13C of terrestrial organic matter) in combination with palynological data.
The Lusitanian succession is dated with palynomorphs and benthic foraminifera. D2 and D3
correspond to major sedimentary discontinuities, LSE represents a near-shore equivalent of the
"Livello Selli" horizon. Colors refer to different depositional environments. Major events in the
sedimentary evolution of the two basins are marked with an arrow.
Figure 3-8 (Sageman and Meyers) Geochemical fluxes (organic carbon, carbonate,
molybdenum), and percent lamination for the Bridge Creek Limestone Member (#1 Portland
core). Accumulation rate data is from Meyers et al., 2001. Percent lamination was calculated by
applying a 2 meter moving window to high-resolution ichnologic data (oxygen-related
ichnocoenoses from Savrda, 1998), and determining the relative frequency of laminated intervals
(# laminated samples/total # samples) within each 2 meter window.
Figure 3-9 (Forster et al.) Compilation of isotopic data (from left to right: bulk organic carbon,
compound specific, and bulk carbonate), a quantification of the decrease in the carbon isotopic
fractionation factor of marine phytoplankton (ep) and organic mater mass accumulation rates
covering the OAE 2 at different sites under investigation (paleogeographic map gives location by
small numbers) in comparison with published data.
Return to Oceanic Anoxic Events text
Figures with captions - Sea Level Record and Mechanisms for Global Eustatic Change
Figure 4-1 (Schouten et al.) Estimation of the mean annual sea surface temperatures for today's
equatorial oceans and from Cenomanian-Turonian sediments at DSDP Site 367 based on analysis
of membrane lipids from marine crenarchaeota, which are nannoplankton that have thrived in
present and Cretaceous oceans. Further tests of this "TEX86" proxy should demonstrate its
reliability as a new organic paleothermometer.
Figure 4-2 (Clarke) Preliminary benthonic foraminiferal Mg/Ca data are used to investigate the
cause of a large negative shift in oxygen-isotope ratios (δ18O) that occurs across the Aptian–
Albian boundary in ODP Hole 766A (cf. Clarke and Jenkyns, 1999). Because δ18O values reflect
both the temperature of calcification and the oxygen-isotope composition of contemporaneous
seawater, it is necessary to utilize an independent paleotemperature proxy, i.e. foraminiferal
Mg/Ca ratios, in order to deconvolute the ice-volume and temperature components of the δ18O
record. The recorded Aptian–Albian δ18O shift is of a comparable magnitude (~1.5‰) to the
change of 1.2‰ in benthic δ18O values at the Eocene–Oligocene boundary that relates to the
onset of major Antarctic glaciation during the Cenozoic (cf. Zachos et al. 2001). Consequently, it
is possible that the high δ18O values during the mid–late Aptian can be explained in terms of an
ice-volume effect that resulted in seawater being depleted in 16O, rather than solely being due to
cool paleotemperatures. Statistical analyses (t-Test and F-test) of these initial data indicate that
mean Mg/Ca ratios are significantly different (0.05 confidence level) below and above the δ18O
shift, and are indicative of a cooling across the Aptian–Albian transition. As such, the Mg/Ca
data strongly suggest that the recorded δ18O change reflects variations in the isotopic
composition of seawater, due to the input of 16O from a shrinking cryosphere, rather than an
increase in paleotemperatures. Currently, no correction is made for long-term changes in the
Mg/Ca composition of the mid-Cretaceous oceans. Such an interpretation of a major midCretaceous deglaciation event will help to reconcile the controversial dropstone and glendonite
evidence for a cool/sub-freezing late Aptian climate during the assumed peak greenhouse climate
conditions of the mid-Cretaceous.
Figure 4-3 (Huber et al.) Paleotemperature estimates based on δ18O analyses of planktic and
benthic foraminifer species from the subtropical North Atlantic (Blake Plateau ODP Sites 1049
and1050) and sub-Antarctic DSDP Site 511 and ODP Site 690 (modified after Huber et al., 1995,
2002). Temperatures are estimated using the Erez and Luz (1983) paleotemperature equation and
assuming that the isotopic composition of Cretaceous seawater was –1.0‰SMOW. Planktonic data
are adjusted using the Zachos et al. (1994) correction for paleolatitude. Note that the coolest
deepwater temperatures in the Blake Plateau record occurred during the late Albian and early and
late Maastrichtian. Brief benthic cooling events are shown at ~100 and 95.5 Ma, but correlative
cooling is not observed in the planktonic record from Blake Plateau or in benthic and planktonic
records from southern high latitudes. Higher resolution records and more accurate correlation of
the across these "cooling events" to test whether they can be identified globally.
Figure 4-4 (Huber et al.) Late Campanian-Maastrichtian δ18O records from Atlantic, Pacific, and
Southern Ocean deep-sea drilling sites. Diamonds = benthic foraminifera; triangles = deeper
dwelling planktonic foraminifera; circles = near surface planktonic foraminifera. Note that the
positive (cooling) shifts in planktonic and benthic δ18O at ~66.5 Ma and ~70.5 Ma are not
apparent in several of the other δ18O curves. Such inconsistencies are an impediment to
establishing links to global climatic forcing mechanisms.
Figure 4-5 (Voigt and Gale) δ13C and δ18O values of Cenomanian articulate brachiopods and
belemnites from sections in southern England. Note the long-term warming trend and the
coincidence of temperature decreases with sea level falls in the earliest Middle and Late
Cenomanian. Abbreviations: L. Tur.- Lower Turonian, T. cost. - Turrilites costatus, A.
rhotomag. - Acanthoceras rhotomagense, A. juk.-br. - Acantoceras jukesbrownei, M. geslin. Metoicoceras geslineanum, W. dev. - Watinoceras devonense, M. nodos. - Mammites nodosoides,
SB - sequence boundary.
Figure 4-6 (Jarvis et al.) Schematic illustration of major processes affecting the carbon isotopic
composition of Campanian seawater and their relationship to sea level rise. Thick lines and red
arrows indicate the dominant processes controlling the δ13C composition of the oceans and the
atmosphere, and recorded in the marine carbonate record; thinner lines and yellow arrows are
subsidiary processes. Carbon isotope excursions are caused principally by changes in the
partitioning of carbon between the organic (Corg) and carbonate (Ccarb) sinks, driven by eustatic
sea level change. Positive isotope excursions are linked to sea level rise and higher Corg burial
fluxes; declining δ13C following transgression is controlled principally by increased Ccarb burial
at constant or decreasing Corg burial rates. SW = seawater, CO2 = atmospheric carbon dioxide.
Figure 4-7 (Jarvis et al.) Sea level change versus carbon isotope stratigraphy. The carbon isotope
curve is a five-point moving average of data from the Trunch borehole, eastern England (data
from Jenkyns et al., 1994). The δ13C curve (thick grey line) is plotted against the time scale of
Gradstein et al. (1995), calibrated using the base Campanian (83.5 Ma) and base Maastrichtian
(71.3 Ma) and assuming a constant sedimentation rate at Trunch. Isotope events are indicated by
colored bands. The relative positions of the NW European macrofossil and Tethyan planktonic
foraminifera biostratigraphies are based on a carbon isotope correlation with El Kef, Tunisia
(Jarvis et al., in press). The transgressive and regressive events (green and red polygonal boxes,
respectively) recognized in Germany (Niebuhr, 1995; Niebuhr et al., 2000) are shown for
comparison. Peaks and troughs on the regional sea level curves have been placed relative to the
appropriate biostratigraphy. The 'eustatic' curve (Haq et al., 1987) has been re-calibrated by
placing the base of TST3.4 at the bottom of the Santonian - Campanian Boundary Event, the
base of TST4.1 at the base of the mid-Campanian Event, and the base of TST4.4 at the top of
the Upper Campanian Event, and scaling the remainder of the curve accordingly. HST = highstand systems tract; LST = low-stand systems tract; TST = transgressive systems tract.
Figure 4-8 (Leckie et al.) West to east transect of clastic-dominated stratigraphic sections from
southern Utah, northeastern Arizona, and southwestern Colorado (stipple pattern: coastal plain
and shallow marine sandstone; brick pattern: limestone and/or calcarenite-rich intervals; no
pattern: marine non-calcareous and calcareous shale and mudstone). These sections represent the
western side of the Cretaceous Western Interior Sea and record the third-order transgressiveregressive Greenhorn Cyclothem. Peak transgression occurred during the early Turonian and a
major regression at the end of the middle Turonian caps the Greenhorn Cycle. Fourth-order
carbonate cycles (Leithold, 1994) and intermediate stratigraphic cycles (Gardner, 1995) illustrate
the occurrence of higher frequency sea level cycles some of which may represent 400 kyr longeccentricity cycles (from Tibert et al., 2002).
Figure 4-9 (Sageman and Meyers) Diagrammatic W-E cross section showing regional
stratigraphic relations within the Greenhorn marine cycle: stacking patterns of marginal marine
strata on the western margin (faded patterns are inferred) are correlated to the distal offshore
stratigraphic section at Rock Canyon, CO, based on biostratigraphic and lithostratigraphic data.
An interpreted relative sea level curve is shown at right (Approximate scale shows oscillations
on the order of tens of meters). Sea level fall events are based on correlation of prograding
shoreface deposits on the western margin to regionally tracable skeletal carbonate accumulations
and changes in biofacies in the central and eastern parts of the basin. Deepening events are based
on retrograding shoreface intervals correlated basinward to intervals of enhanced TOC content
and laminated shale deposition (wt. % TOC compiled from SEPM Guidebook No 4 and
unpublished data). Using the Obradovich (1993) time scale to constrain the duration of the
Cenomanian interval that contains up to six possible sea level fall events (2.53 Myr) yields an
average duration of 0.42 myr. Although this timing is consistent with a long eccentricity
modulator, it is difficult to confirm that these events are periodic. However, they appear to be
correlative with six Cenomanian sea level fall events identified by Gale et al. (2002). [Figure
modified from Sageman, 1991].
Figure 4-10 (Forster et al.) Interpretation of middle Turonian third- and fourth-order sea level
fluctuations and watermass interaction in the central Western Interior Seaway based on analysis
of small-scale cycles in the Carlile Shale from the Amoco Rebecca Bounds Core (Kansas). The
biomarker and δ18O data indicate a strong but fluctuating freshwater influence.
Figure 4-11 (Hancock) Comparison of radiometrically interpolated and biostratigraphically
correlated versions of the Haq et al. (1987) sea level curve with the Hancock (1990) sea level
curves for northwest Europe the Campanian-Maastrichtian. Differences in the timing of the
transgressive peaks and regressive troughs is caused by differences in the biozonal schemes,
stage boundary definitions, and time scales that are used for each curve.
Figure 4-12 (Bice et al.) Preliminary mid-Cretaceous experiments have been performed with an
atmospheric general circulation model that includes water isotopes in the hydrologic cycle.
These indicate that warm climate high latitude freshwater would have been much less depleted in
18
O than in the modern. Winter (July) precipitation over Antarctica in the mid-Cretaceous
experiments is -10 to -30‰ (SMOW), compared to the modern -20 to -60‰.
Figure 4-13 (Bice et al.) This graph allows estimation of the shift in mean ocean δ18O (vertical
axis) given growth or decay of an ice-sheet of a given average composition (horizontal axis) and
volume (diagonal lines). A mid-Cretaceous ice-sheet 50% the volume of the modern Antarctic
ice with an average composition of -20 per mil or heavier could produce a shift in mean ocean
δÿ18O of no more than 0.2‰. If mid-Cretaceous ice was so much less depleted than modern ice,
the model suggests that a substantial volume could have existed without producing a shift larger
than the interspecies isotopic offsets among many benthic foraminifera.
Return to Sea Level Record text
Figures with captions - Atmospheric and Ocean Circulation in a Greenhouse World
Figure 5-1 (Hay) A) A classic view of Cretaceous atmospheric and oceanic circulation, based on
analogy with the present ocean but showing here the reversal of the thermohaline circulation that
has been proposed by several authors. B) A new view of a possible Cretaceous atmospheric and
ocean circulation, showing the effect of unstable westerly winds producing eddies which replace
the general gyral circulation of today.
Figure 5-2 (Bice et al.) The colored curves show January sea surface temperature profiles
predicted by the GENESIS model using various CO2 concentrations (1120-7500 ppm) and heat
transport factors (Q=2-20). Solar luminosity was set to 99% of the modern for all runs except the
1120 ppm case, which used 99.2%. Symbols indicate upper ocean temperatures calculated from
δ18O of well-preserved planktonic foraminifera. Local water δ18O is assumed to be -1.0 per mil
(SMOW) with an adjustment for latitudinal variations in surface water (Zachos et al., 1994). The
model shows that, in order to match the warmest tropical temperature estimates, 4500 ppm or
more CO2 is required. If the lightest Late Turonian foram data at DSDP Site 511 indicate
tropical-like summer (January) temperatures (Huber et al., 1995), then the model requires 6500
ppm CO2 or higher. However, the meaning of the very light isotopic event at Site 511 is as yet
undetermined (Bice et al., this meeting).
Figure 5-3 (Valdes and Markwick) Zonal average Maastrichtian sea-surface temperature
simulated with the Hadley coupled ocean atmosphere model. With similar CO2 concentrations,
the Hadley coupled ocean atmosphere model generates much warmer Cretaceous temperature
than the NCAR CSM.
Figure 5-4 (Otto-Bliesner et al.) Vertical sections of simulated ocean temperature and salinity at
30W longitude for present day and Cretaceous 1680 ppm CO2 coupled model experiments.
Figure 5-5 (Floegel et al.) GCM modeling of one complete precessional cycle, with emphasis on
Western North America during the late Cretaceous (Cenomanian/Turonian boundary).
Figure 5-6 (Poulsen et al.) Mean annual upper ocean temperature and salinity difference resulting
from the rifting of the Equatorial Atlantic Gateway during the mid-Cretaceous. Black shading
represents the Cretaceous continents; gray shading marks the grid cells that were depressed in the
opening of the gateway.
Figure 5-7 (Scotese) Cretaceous paleogeographic reconstructions for 65 Ma (K/T boundary), 80
Ma (early Campanian, 2 views), and 120 Ma (early Albian).
Figure 5-8 (Lawver) Detailed tectonic reconstruction of the Albian (105 Ma).
Figure 5-9 (Upchurch et al.) Geologic data and climate model predictions of latest Cretaceous
vegetation.
Figure 5-10 (Scherer et al.) Mean annual temperature estimates for the Jose Creek leaf megaflora
using different regression functions.
Figure 5-11 (Buffler) Mid Cretaceous deposition and erosion in the deep eastern Gulf of
Mexico.
Return to Atmosphere and Ocean Circulation text
Figures with captions – Consequences of Large Igneous Provinces
Figure 6-1 (Larson et al.) Timing comparison of the mid-Cretaceous igneous events proposed as
causes for the two most prominent geological responses. Ontong Java and Manihiki Plateau
formation proposed as cause for OAE 1a (Selli Event) and Caribbean Plateau formation
proposed as cause for OAE 2 (Bonarelli Event).
Figure 6-2 (Duncan) Cartoon depicting a hydrothermal “event” plume produced by eruption of
large volume (~1000 km3) lava flow during construction of an ocean plateau. High
concentrations of certain metals (relative to trace concentrations in sea water) are carried to the
ocean surface in the thermally buoyant plume. Many of these metals are normally bio-limiting,
so their sudden availability could spur brief periods of high productivity. The metals are removed
from the surface at differing rates and fractionated (near field and far field patterns), but
ultimately transported to the sea floor on particles.
Figure 6-3 (Turgeon et al.) Potential geochemical paleoproductivity proxies within the Bonarelli
horizon of the Gubbio Core, taken from central Italy. Dashed vertical lines represent "Average
shale" composition of Wedepohl (1971, 1991). Darker shaded area represent times of increased
P/Al ratios.
Figure 6-4 (Ravizza et al.) High temporal resolution records of seawater Osmium (Os) isotope
variation have the potential to better constrain the timing LIP emplacement relative to important
stratigraphic horizons, such as OAEs and episodes of faunal change. Two major OAEs occur
near the initiation of major negative excursions in the marine Sr isotope record. The long marine
residence time of Sr and the similar Sr isotope signatures of LIP and MORB volcanism preclude
the possiblity of using Sr data to test the hypothesis that LIP emplacement played a causative
role in these Cretaceous OAEs. The marine residence time of Os is much shorter than that of Sr
allowing this system to respond more quickly to rapid perturbations in oceanic inputs. Moreover,
the Os isotope signature associated with LIP emplacement may be enhanced relative to Sr
because the alleged source region of LIPs is the core-mantle boundary, a domain in the deep
Earth the is very enriched in siderophile elements like Os. Although there are only very few data
constraining the Cretaceous seawater Os isotope record, low temporal resolution, preliminary
data from a sediment sequences in Oman (Ravizza, unpublished) and in Cyprus (Ravizza et al.
2001) suggest an excursion toward more mantle-like 187Os/188Os ratios may occur close to the CT OAE.
Return to Large Igneous Provinces text
Return to top of Report
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