12.710 Intoduction to Marine Geology and Geophysics
11/1
Mid Term
Sediments, Processes, and the Sedimentary Record
11/6
11/8
(McManus)
(McManus)
Deep-sea sediments: composition, distribution
Biological, chemical, and physical abyssal processes
11/13
11/15
(McManus)
(McManus)
Dating methods and the sedimentary record
Paleo-environmental proxies
11/20
(McManus)
Deep water chemistry and atmospheric p(CO2)
11/22
Thanksgiving
11/27
11/29
(Hoffmann)
(Thompson)
Paleothermometry
Pleistocene ice-age cycles
12/4
(McManus)
Sedimentary records of abrupt climate change
12/6
Final Exam
j
ökulhlaups
Catastrophic flooding associated with sub-glacial volcanic eruptions
Past climate changes
I
II
Milankovitch and the “Astronomical” or
“Orbital” theory of the ice ages.
Successes and potential failures of Milankovitch.
III
IV
V
Abrupt climate change.
External mechanisms of rapid climate change:
Non-linear orbital forcing.
Variable solar luminosity.
Aliased seasonal cycle.
Internal mechanisms of rapid climate change:
Binge-purge ice sheet instability.
Salt oscillator ocean circulation.
Tropical ocean-atmospheric dynamics.
Long-term decline into glaciation
Oxygen isotopes in benthic foraminifera from deep-sea sediments
reveal a cooling trend on Earth over tens of millions of years.
Cold (even)
18Oforams
Warm (odd)
MARINE ISOTOPE STAGES (MIS)
Milutin Milankovitch
Serbian mathematician who painstakingly calculated the
insolation effects of changes in the Earth’s orbital parameters.
Eccentricity
Variations in the shape of the Earth’s orbit around the sun,
from circular to more elliptical, occur over ~100,000 years.
Obliquity
Variations in the tilt of the Earth’s axis of rotation, driving
contrasts in seasonality, occur over ~40,000 years.
Precession
Variations in the orientation of the Earth’s tilted axis of rotation,
moving seasons along the eccentric orbit, occur over ~20,000 years.
Orbital variations in received insolation
Each orbital influence has its own distinctive pattern of influence
through time. They combine to vary sunlight on Earth.
Specmap
Study of isotope and other paleo-climate records from deepsea sediments around the world reveals Milankovitch signal.
Period (1/frequency) in thousands of years.
Devil’s Hole
Well dated speleothem record not consistent with Milankovitch?
Should it? Remains an unsolved puzzle.
Abrupt climate change
Evidence from ice cores and deep-sea sediments reveals
climate changes that seem too abrupt to be explained orbitally.
Dansgaard-Oeschger Cycles discovered in Greenland ice
cold
18Oice
warm
cooler
warmer
N. Atlantic SSTs track Greenland Air Temperatures
H-events occurred during extreme cold phases of D-O
cycles in Greenland ice
THE YOUNGER DRYAS IN THE SANTA BARBARA BASIN
• Increased bioturbation
suggesting higher O2
levels (D-O interstadials
show the opposite with
reduced bioturbation)
• Increased 18Obenthic
• Increased Pacific
ventilation?
GLOBAL SIGNATURE OF D/O EVENTS: CARIACO BASIN
ANTI-PHASING BETWEEN HEMISPHERES
Antarctica warms
during the Younger
Dryas
THE DISCOVERY OF HEINRICH EVENTS….
…. IN MARINE SEDIMENT ARCHIVES
Transport of ice-rafted debris (IRD)
Site with
ice-rafted
debris
Site without
ice-rafted
debris
“IRD Belt” - based on
network of N. Atlantic
marine sediment records
MECHANISMS BEHIND MILLENNIAL VARIABILITY
EXTERNAL FORCING
• Suborbital (Combination tones)
• Solar Variability
INTERNAL FORCING
• Ice sheets (e.g., “Binge-Purge” theory)
• Thermohaline circulation (e.g., Broecker’s “Salt Oscillator”)
• Tropical Dynamics (e.g., Eastern tropical Pacific - ENSO
dynamics)
ALTERNATIVE
• Aliasing
SUBORBITAL FORCING: MILANKOVITCH COMBINATION TONES
Tropical Atlantic coccolith abundance records:
Variations in tropical winds/upwelling cause ice rafting events?
F. profunda (upwelling indicator) maxima coeval with Heinrich events
Spectral Analysis:
-spectral power at 8.4 ky
(7.6 ky 14C), a
combination tone of
precession and
eccentricity
The mechanism: Easterly winds diminish, warm water propagates northward
via the Gulf Stream, delivery of heat causes ice sheet melting and subsequent
iceberg discharge
McIntyre and Molfino (1996)
SOLAR VARIABILITY: IRD and Cosmogenic Nuclides in the Holocene
10Be
14C
Similar pacing of ice rafting events during last interglacial (MIS-5) Bond et al. (2001)
ICE SHEET STABILITY: THE BINGE-PURGE THEORY
MacAyeal’s explanation for Heinrich Events
~7 ky pacing
Laurentide Ice Sheet (LIS) Model:
Binge (Growth) phase - basal sediment
frozen, ice sheet immobile; slow growth of
LIS
Purge phase - geothermal flux gradually
increases basal temperature, thawing basal
sediments; rapid discharge of icebergs;
(meltwater input reduces THC, cool North
Atlantic, allowing ice growth again)
~7 ky pacing
THERMOHALINE CIRCULATION: THE SALT OSCILLATOR
Conveyor On: Northward heat
transport initiates ice melting, which
reduces density of upper ocean,
reducing deep water formation
Conveyor Off: Cooling of North
Atlantic (less melt water input) and
reduced salt export gradually increase
density (and thus deep water
formation); return to Conveyor On
mode
Three Modes of North Atlantic Deep Ocean Circulation
Oc/Atm
Atm
TROPICAL DYNAMICS: EASTERN TROPICAL PACIFIC
The Idea: El Niño/Southern
Oscillation (ENSO) system in the
eastern tropical Pacific affects tropical
SST distribution, which controls
convection/atmospheric circulation
patterns - very strong teleconnections,
which could explain near-global
synchroneity of millennial events
EVENT FREQUENCY
EVENT AMPLITUDE
Experiment: Coupled oceanatmosphere model of the tropical
Pacific is run for 150,000 years, both
with and without orbital (precession)
forcing
Result: Millennial increases or
decreases in the frequency of ENSO
warm or cold events and changes in
their amplitudes
Clement et al. (1999)
THE PROBLEM OF ALIASING
D-O events show up in major ion chemistry of Greenland ice
Polar Circulation Index (PCI)
• Record developed from the analysis of
GISP2 major ion chemistry
• Proxy for relative measure of the average
size and intensity of polar atmospheric
circulation
• Generally increase (e.g., more
continental dusts and marine
contributions) during stadials and decrease
during interstadials
SPECTRAL ANALYSIS OF THE PCI
Dominant periodicity in millennial band is 1450 years
GLOBAL SIGNATURE OF D/O EVENTS: ARABIAN SEA
Greenland 18Oice
Increased prod.,
lower O2
bioturbated
laminated
Total organic carbon
(TOC) records from
two marine sediment
cores off Pakistan
showing millennialscale variability of
monsoonal surface
water productivity
and bottom-water
oxygen
D/O temperature
oscillations
also occur in
Antarctica (Vostok),
but with generally
smaller amplitudes
HEINRICH EVENTS
• Helmut Heinrich discovered a cyclic pattern of ice-rafted
debris over the last 130 ky in a series of sediment records
from the North Atlantic
• Broecker and Bond later identified six of these events
between 70 and 14 ky B.P., and named them Heinrich
events (H1-H6)
• Heinrich events occur every ~7-10 ky, and are associated
with pronounced cooling in the Greenland ice core records
and North Atlantic region
• Bond et al. (1993) noted that Heinrich events appear to
occur towards the end of increasingly colder D-O cycles
SEDIMENTARY CHARACTERISTICS OF A HEINRICH LAYER
OTHER TYPES OF IRD FOUND IN THE NORTH ATLANTIC
Hematite-stained grains
Basaltic glass, Iceland
HEINRICH EVENT CHRONOLOGY
IRD peaks
Bond et al. (1992)
BOND IDENTIFIES MORE IRD PEAKS IN N. ATLANTIC SEDIMENTS
In addition to
Heinrich events
(Huge IRD
peaks, cold
Greenland air
temp. and cold
N. Atl. SST)…
Cold phases of
D-O cycles smaller
amplitude IRD
peaks; smaller
SST signal
COLDER
Bond and Lotti (1995)
COLDER