fic drilling in the Great Rift Valley: The 2005 Lake... Scienti — An overview of the past 145,000 years of climate...
Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Scienti
fi
c drilling in the Great Rift Valley: The 2005 Lake Malawi Scienti
fi
c Drilling
Project — An overview of the past 145,000 years of climate variability in Southern
Hemisphere East Africa
C.A. Scholz
,
, E.T. Brown
a Department of Earth Sciences, Syracuse University, Syracuse NY, 13244, USA b
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c
Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812, USA d
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA e
Department of Earth Science, University of Bergen, N-5007 Bergen, Norway a r t i c l e i n f o
Article history:
Received 18 March 2010
Received in revised form 17 October 2010
Accepted 20 October 2010
Available online 9 November 2010
Keywords:
Lake Malawi
East African rift
Paleoclimatology
Scienti fi c drilling
Lake level change a b s t r a c t
The recovery of detailed and continuous paleoclimate records from the interior of the African continent has long been of interest for understanding climate dynamics of the tropics, and also for constraining the environmental backdrop to the evolution and spread of early Homo sapiens . In 2005 an international team of scientists collected a series of scienti fi c drill cores from Lake Malawi, the fi rst long and continuous, highfi delity records of tropical climate change from interior East Africa. The paleoclimate records, which include lithostratigraphic, geochemical, geophysical and paleobiological observations documented in this special issue of Palaeo
3
, indicate an interval of high-amplitude climate variability between 145,000 and ~ 60,000 years ago, when several severe arid intervals reduced Lake Malawi's volume by more than 95%. These intervals of pronounced tropical African aridity in the early Late Pleistocene around Lake Malawi were much more severe than the Last Glacial Maximum (LGM), a well-documented period of drought in equatorial and Northern
Hemisphere tropical east Africa. After 70,000 years ago climate shifted to more humid conditions and lake levels rose. During this latter interval however, wind patterns shifted rapidly, and perhaps synchronously with high-latitude shifts and changes in thermohaline circulation. This transition to wetter, more stable conditions coincided with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In 2005 an international team of scientists set out to recover a long and continuous record of past climatic changes from the African interior, through scienti fi c drilling and sampling of the sediments of
Lake Malawi. Lake Malawi is one of the largest, deepest and oldest lakes in the world, and as one of the Great Lakes of Africa, is considered among the natural wonders of the world. Also referred to as Lake Nyasa, it is situated at the southern end of the western branch of the East African Rift System (EARS), between ~ 9° and ~ 14° South latitude (
). Malawi and the other great lakes of the region
(Tanganyika, Victoria, Edward, Albert, Kivu and Turkana) are famous for their large numbers of fi sh and invertebrate species (in particular the cichlid fi shes) (e.g.
). Taken together Lakes
⁎ Corresponding author. Department of Earth Sciences, 204 Heroy Geology
Laboratory, Syracuse University, Syracuse, NY 13244, USA. Tel.: +1 315 443 4673; fax: +1 315 443 3363.
E-mail address: cascholz@syr.edu
(C.A. Scholz).
0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi: 10.1016/j.palaeo.2010.10.030
Malawi and Tanganyika contain more than 80% of the surface water found on the African continent, and are a critical resource for riparian populations. Pioneering geophysical studies undertaken by B.R.
Rosendahl and colleagues in the 1980s proved the remarkable antiquity of these lakes through reconnaissance geophysical studies
(e.g.
Rosendahl, 1987 ), and following those studies proposals rapidly
emerged for scienti fi c drilling in the deep lake waters of Africa's Great
Rift Valley (
).
Initial scienti fi c exploration of the Great Rift Valley ensued shortly after the European settlement, with seminal publications by
(1891), de Martonne (1897) and Gregory (1896)
proposing that either tensile forces or active (vertical) motions were responsible for producing the distinctive, block-fault topography of East Africa. These
early studies ( Oldham, 1922; Suess, 1891 ) contributed signi
fi cantly to emerging concepts of continental drift and plate tectonics. The remarkable depths, evident antiquity, and peculiar faunas of the
Great Lakes instigated numerous scholarly publications and debate
(e.g.
“ The Tanganyika Problem ” — Moore, 1903 ) at the start of the
20th century. The remarkable hydrological variability of Lake Malawi
4 C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Fig. 1.
A) Regional digital elevation model of east Africa generated using the GTOPO data set, showing locations of major lakes. Inset shows maximum January and July position of the intertropical convergence zone (ITCZ). B) High-resolution digital elevation model (SRTM data set) and bathymetry of the Lake Malawi Rift and catchment. Numbers indicate locations of two drill sites.
over geologic time was also described in early records, as observed through raised beaches and lacustrine sequences especially on the north shore of the lake (
Dixey, 1926 ). The importance of the African
Great Lakes for understanding climatic changes in the Pleistocene has
been noted for decades ( Livingstone, 1965
).
The early seismic imaging and sediment sampling studies in Lake
Malawi established that the basin's thick accumulations of fi ne-grained, and commonly laminated sediments contain a rich and unique record of climatic, evolutionary and tectonic change in tropical east Africa, which warranted deep sampling through extensive coring and scienti fi c drilling (e.g.
Crossley, 1984; Rosendahl and Livingstone, 1983
).
Following extensive basin framework and short core sampling studies
(e.g.
Owen and Crossley, 1989; Scholz et al., 1990; Scott et al., 1991 ), an
international collaboration of scientists organized a scienti fi c drilling program in March and April 2005. This special issue of Palaeogeography,
Palaeoclimatology, Palaeoecology is devoted to the results of detailed studies from Lake Malawi and af fi liated sites, covering the time interval of the middle – Late Pleistocene through Holocene, mainly focusing on analyses and results from the 2005 scienti fi c drill core 1C, as well as cores from Site 2. These papers include a review of basin framework (
), and studies of lithostratigraphy and geochemistry
(e.g.
other sediment cores from Lakes Malawi and Tanganyika and spanning the late Pleistocene and Holocene are also presented in this volume (e.g.
Burnett et al., 2011-this issue; Castañeda et al., 2011-this issue; Powers et al., 2011-this issue
).
2. Geological background
The fi rst reports of the geology of the Lake Malawi (Nyasa) region date from the early part of the 20th century (e.g.
) and provide details of the basement rocks and sedimentary sequences surrounding the basin. Much of the catchment of the lake is underlain by Precambrian and early Paleozoic crystalline rocks associated with
Pan-African mobile belts ( Daly et al., 1989
) (
basement of the area is composed of greenschist – amphibolite grade rocks, and is af fi liated with granites and syenites emplaced during the
Ubendian and Irumide orogenies. This crystalline rock terrane underlies many of the largest river drainages which empty into Lake
Malawi, and it is the source of most of the detrital siliciclastic material observed in deep-water Lake Malawi sediment cores.
On the western side of the North Basin are sedimentary sequences
of widely varying age ( Fig. 2
). Permo-Triassic Karoo sandstone, shale and coal-bearing intervals 2 – 3 km in thickness are observed to extend across Malawi and western Tanzania near the Ruhuhu River (e.g.
Kreuser, 1990; Yemane et al., 1989
). Terrestrial sedimentary sequences of Cretaceous age bearing vertebrate fossils are also observed outcropping on the northwest shore of the lake (e.g.
Roberts et al., 2004 ). These are overlain by Neogene and Quaternary
sediments, including fossiliferous limestones. Less than 40 km north of the northern shoreline of the lake is the Rungwe volcanic complex, composed primarily of basalt and nephelinite (e.g.
), which is one of the three late-Cenozoic volcanic centers located in the western branch of the EARS (
).
Dating of these volcanic rocks suggest an initiation of rifting in the late
Miocene ( Ebinger et al., 1989
). Because these are restricted to a single major river drainage in the catchment, volcanogenic sediments
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 5
Fig. 2.
Bedrock geology and fault map of the Lake Malawi Rift (from
entering the lake are mainly limited to the northern basin of Lake
Malawi.
The East African rift is separated into the magmatically active eastern branch, which is comprised of the Ethiopian, Turkana, Kenya, and Gregory Rifts, and the largely amagmatic western branch, dominated by large freshwater lakes. Numerous studies undertaken throughout the rift system reveal that the Ethiopian system is much older, with volcanism initiating prior to 30 ma, whereas much of the western branch of the system may be late Miocene or younger; the initiation of rifting is observed to be progressively younger from north to south (e.g.,
Ebinger and Sleep, 1998; Tiercelin and Lezzar, 2002 ). In
the 1980s, geophysical studies from the Great Lakes of East Africa led to the recognition of the pronounced segmentation and crosssectional asymmetry of rift systems, and the importance of
6 C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Fig. 3.
Perspective view of the Malawi rift generated using NASA software, illustrating pronounced rift valley segmentation. Dashed lines denote main rift segments.
lithospheric thermomechanical properties in the rifting process
(
Morley, 1988; Rosendahl, 1987 ). In particular, seismic re
fl ection studies in Lakes Tanganyika and Malawi led to the observation of pronounced cross-rift asymmetry and consistent rift segment dimensions along the full axis of the system (
). For instance the Lake
Malawi rift is comprised of three main linked half-graben basins, which alternate in polarity along axis (
(
Figs. 2 and 3 ). This pattern is observed along the 2000 km-long
western branch of the rift, and discrete segments are also observed along the Tanganyika, Kivu, Edward, and Albert Rift zones (
) (
Fig. 1 ). In Lake Malawi, deep-basin
subsidence is accommodated by slip on a few primary border faults, which in the north basin is observed on the northeastern margin, on the western margin in the central basin, and on the east side of the lake in the case of the South Basin (
Fig. 2 ). The coastlines of the border
fault margins are characterized by high mountains in the central and north basins, which comprise the footwall (e.g.
1989 ). These mountains commonly rise 1000
– 1500 m above the adjacent lake surface, and de fi ne bold, unforgiving shorelines (e.g.
Figs. 1 and 3 ). Along the length of the western branch of the rift, each
half-graben basin averages 80 – 200 km in length, and 30 – 60 km across, and the most deeply subsided points within each basin are commonly observed adjacent to the border fault and the zone of maximum footwall uplift (
).
Studies of individual border fault systems reveal evidence for dextral oblique-slip deformation along the Livingstone Mountains
border fault and the Rukwa Border Fault ( Wheeler and Karson, 1989
),
as well as in southern Lake Tanganyika ( Klerkx et al., 1998 ). Similar
outcrop-scale studies have not been carried out on the central basin border fault system in Lake Malawi, but studies of intrabasinal fault structures observed in seismic re fl ection data also suggest some amount of oblique-slip deformation in that area (
2007; Scott et al., 1994; Specht and Rosendahl, 1989 ). Detailed
observations of border fault structures in the central basin of Lake
Malawi show that several main border fault strands accommodate subsidence there (
), each of which likely evolved from the propagation or coalescence of several much smaller faults early in the history of the rift (e.g.
Schlische, 1995; Mortimer et al.,
2007 ). Intrabasinal faults within the northern and central basins of
Lake Malawi are secondary features relative to the main border faults, at least in terms of total displacement (
).
However in several localities these basement-involved fault systems produce relief on the modern lake fl oor, and for much of the history of the basin, have played critical roles in determining the sediment pathways into the most deeply subsided parts of the basins. The current morphometry of these two deep basins, with very steep margins on at least one side, gives rise to signi fi cant down-slope sediment transport systems, facilitating gravity fl ows, and especially turbidity fl ows, over broad areas of the basin (e.g.
Scholz, 1995; Soreghan et al., 1999
).
3. Climate and hydrology
As in most areas of the tropics, seasonal climate variability in the
Malawi rift valley is dominated by changes in precipitation rather than temperature, and in East Africa rainfall is strongly in fl uenced by the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ), north and south of the equator (
Fig. 1 ). Convection associated with
the passage of the ITCZ gives rise to heavy rains on the landscape. The region around the Malawi Rift is dominated during the austral summer by a single rainy season that extends from ~December to
March, although in some years it begins in October and extends through early May. Rainfall within the rift also varies by elevation and latitude, with higher terrain generally wetter, the southwest coast receiving as little as 80 cm/yr, and areas to the north of the lake
averaging more than 200 cm/yr ( Malawi Department of Surveys,
1983 ). Moisture is derived from both the Atlantic and Indian Oceans,
although East African rainfall variability has been shown to be broadly linked to sea surface temperatures of the Indian Ocean (e.g.
1994; Goddard and Graham, 1999; Marchant et al., 2007
). Sedimentation in Lake Malawi is markedly in fl uenced by the seasonal cycle, with most terrigenous material introduced into the lake margins during the intense rainy season. During the austral winters, strong prevailing winds from the south set up an oscillation of the internal strati fi
cation in the lake ( Patterson and Kachinjika, 1995 ) resulting in
pronounced upwelling and algal blooms, particularly at the north and south ends. This seasonal cycle results in annually laminated
sedimentary couplets deposited in many areas of the basin ( Pilskaln,
2004; Pilskaln and Johnson, 1991 ).
Modern Lake Malawi is hydrologically open. Seven large river
systems comprise 70% of the lake's catchment area ( Bootsma and
Hecky, 1999; Wells et al., 1994
), and the Shire River is the lake's sole outlet. Although there is nearly continuous out fl ow from the Shire
River, ~ 90% of the annual water loss is via evaporation ( Drayton,
1984 ). This condition is very different from most high-latitude lake
basins, and results in seasonal fl uctuations in water level of 1 – 2 m.
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
The lake's tenuous outlet discharge was interrupted during historical times (
Owen et al., 1990a,b ), and probably frequently in the recent
geological past. Modeling studies of past lake levels have helped quantify the response in this highly sensitive system, which is delicately balanced between precipitation and evaporation (e.g.
Owen et al., 1990a,b ), and where lake level drops of several hundred
meters can occur in a few thousand years or less.
4. Signi fi cance for paleoenvironmental reconstruction
Long-term records of tropical climatic change, and especially the timing and rate of dramatic changes in climate relative to the modern climatology described above, are essential for understanding global scale climate shifts. Much of the incident solar radiation striking the earth hits the tropics and subtropics, and the resultant heat energy is exported to the high latitudes by the oceans, and to a lesser extent by the atmosphere. Sediment records from the offshore of North Africa show that African climate responds to insolation change on orbital
(Milankovitch) time scales (
deMenocal et al., 2000 ). Those far-
fi eld records are important for providing low-resolution information about past climate of the tropical continents, but determining spatial and temporal variability of past regional climate requires new records from the continental interiors themselves. The new drill cores from
Lake Malawi sample an important region of the Southern Hemisphere continental tropics on a scale of decades to centuries, which has not been previously sampled, other than from short cores, low-resolution data sets, or through studies of punctuated sedimentary sequences preserved in outcrops. Particularly lacking are records from the
Southern Hemisphere continental tropics that are suitable for direct comparison to longer marine records.
Key science issues addressed by this project include 1) determining the direction, magnitude and timing of effective moisture, wind, and temperature change on a millennial scale, during the past several glacial – interglacial cycles; 2) assessing if observed climate shifts coincide with
SST variability in the tropical oceans, or perhaps more closely with changes in North Atlantic thermohaline circulation; 3) constraining the lake level history of Malawi, and comparing it to records of methane concentration in the polar ice cores (interpreted to be a globally averaged measure of tropical moisture on the continents); 4) determining if the observed evidence for abrupt climate change in Lake Malawi and other parts of East Africa coincides with known events from other regions on
Earth, such as Heinrich or Dansgaard – Oeschger events; and, 5) assessing if the climate of this Southern Hemisphere site responded only to changes in low latitude precessional insolation (23, 19 kyr) or also to high-latitude ice volume (100 kyr and 41 kyr) forcing in the Pleistocene.
All these issues are helpful in constraining the environmental background to early modern human evolution and migration, and to understanding species evolution in lakes.
7
). These data provide information on the distribution of the main half-graben basins, as well as on the geometry of intrabasinal fault families (e.g.
Mortimer et al., 2007 ). Some of these structures generate
10s of meters of relief on the lake fl oor (e.g.
therefore impact down-slope sediment transport processes into the basin, as well as the fi nal position of associated deposits. High-density grids of high-resolution seismic re fl ection data were acquired using small airguns as the seismic source, and these grids, nested within the sparsely spaced multichannel seismic re fl ection data, were most
suitable for locating the drill sites ( Lyons et al., 2011-this issue
)
).
The Lake Malawi sediment record is a proven, high-sensitivity
offer abundant evidence for repeated episodes of profound hydrologic drawdown of the lake, and marked lateral shifts of the lake shoreline over distances of many tens of kilometers (e.g.
1991; Scholz and Rosendahl, 1988 ). Part of the evidence for these
major lake level shifts comes from major unconformities that are observed on the shoaling, or fl exural margins of half-graben basins in
Lake Malawi (e.g.
). Accordingly, a key criteria for site selection for the drilling program included localities where the seismic data indicated that the stratigraphic section was relatively complete, with no major time gaps. Because of the ubiquitous unconformities around the basin margins, the recovery of a long and continuous stratigraphic section required drilling near the basin center in water depths N 500 m (
Seasonal wind variations and pronounced upwelling of nutrientrich deep waters at the north end of the lake during the austral winter help preserve heightened signals of paleoclimate change in the form of laminated sediments (e.g.
Johnson et al., 2002; Pilskaln, 2004
).
5. Basin framework and site selection
Studies of the sediments of the large lakes of east Africa have shown that the stratigraphy and the depositional framework of these basins are complex (
Scholz et al., 1990; Tiercelin et al., 1992
). Halfgraben sub-basins comprise the large rift-lakes, and their steep faulted margins are prone to gravity fl ows, mass wasting events, and the construction of major sublacustrine fan complexes that can extend across the fl oors of the basins for several 10s of kilometers (e.g.
Soreghan et al., 1999; Tiercelin et al., 1992
). Zones of enhanced turbidite accumulation are problematic for reconstructing detailed records of past climatic and limnological conditions, and accordingly it was imperative during the Lake Malawi drilling project to site the drilling locations away from areas where turbidites and other gravity fl ow deposits have accumulated.
The basin scale structure of the rift was assessed through regional multichannel seismic re fl ection studies (e.g.
Fig. 4.
Representative seismic re fl ection data from Lake Malawi. A) Regional multichannel seismic re fl ection pro fi le showing full sedimentary section, pre-rift basement, and location of scienti fi c drill core. B) High-resolution single-channel airgun seismic pro fi le showing ancient progradational delta in ~ − 200 of water off of the
Songwe River. See Lyons et al., for full treatment of the basin framework seismic re fl ection data.
8 C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Because of this sensitivity, a high-priority for drilling Lake Malawi also included acquiring a high-resolution record of the past ~100 ka at the northern end of the lake.
Tropical lakes are known to accumulate sediments enriched in organic matter, due to high biological productivity in the surface waters, and to enhance preservation of organic remains in the anoxic zone that persists in deeper waters. Accordingly, over long spans of geological time ( N 2 million years), such sediments are prone to organic matter diagenesis and thermogenic maturation, and can generate signi fi cant and even economic accumulations of hydrocarbons, for example as in
Lake Albert, Uganda ( Smith and Rose, 2002 ). Scienti
fi c drilling systems such as those used by the Integrated Ocean Drilling Program (IODP) drill ship JOIDES Resolution, and the Lake Malawi Scienti fi c Drilling Project generally do not return circulating drilling fl uids to the drilling vessel, and accordingly are not suitable for drilling into formations with overpressurized zones, or strata containing oil or gas. Accordingly, it was imperative to only drill in areas of Lake Malawi with no evidence or potential for hydrocarbon accumulation. This was achieved by carefully examining high-resolution and deep-basin multichannel and singlechannel seismic re fl ection data for signs of seismic amplitude anomalies that might suggest subsurface gas or fl uids. The proposed drill sites were also positioned to avoid any potential hydrocarbon traps, such as are commonly found on structural anticlines or on the crests of tilted fault blocks.
Drill Site 1 was chosen to achieve the primary science objective of a long-term and continuous record of climate change in the Southern
Hemisphere tropics of East Africa (
Figs. 1 and 4 ). The drill site is
situated in the central basin, southeast of the deep depocenter.
Continuous hemipelagic sediments comprise the stratigraphic section
of this area ( Fig. 4 ). These sediments accumulated at this site because
of its isolation from the main down-slope transport pathways present on the western and northeastern margins of the basin, and because of its relatively deep water. Additionally, no erosional unconformities are observed at this locality. Details of the drill site and cores recovered are presented in
Table 1 . Drill Site 2 was selected to recover a high-
resolution signal of upwelling, river sediment discharge, and aeolian processes known from the northern end of the lake. The drill site was positioned in an area of hemipelagic sediment deposition, and the total projected depth of the cores at this site was 40 m (
).
except every few days. The riparian countries of Malawi, Tanzania and
Mozambique have limited infrastructure and manufacturing, so most equipment and supplies required for the drilling operation were procured from outside of Africa. The project chartered the 160 ′ fuel barge Viphya from Malawi Lake Services to serve as a drilling platform
(
). The Viphya was redesigned (Lengeek Engineering Ltd.,
Halifax) to accommodate the 100 ton geotechnical drilling rig
(Seacore Ltd., Cornwall), accommodations, galley, toilet/shower, and workshop containers, drilling moon pool, and portable dynamic positioning system, which was used to maintain the position of the drilling barge for the duration of each hole (
). A dynamic positioning system with Nautronix© controllers was purchased by
DOSECC Inc. for the project. DOSECC drilling tools initially designed for the GLAD 800 drilling system were deployed within a 5 ″ API drill string on loan from the IODP. Engineering and planning required about three years of effort prior to drilling. Barge re fi t work was completed in the ship yard in Monkey Bay, Malawi, and the fi nal mobilization of the Seacore drill rig aboard the Viphya was completed on the jetty in the port of Chipoka, Malawi.
Mobilization for fi eld operations began in December 2004 with sea trials of the portable dynamic positioning system, and preparations for the drilling effort continued into February 2005. Drilling ensued in late February 2005, after extensive testing and modi fi cation of equipment. The drilling operations required a team of 26 people on the drilling barge Viphya , including nine drillers, and a team of about thirty people onshore and on support boats as logistical support staff.
The shore-based team also carried out extensive outreach efforts, visiting many Malawian secondary schools and government of fi ces
(
) ( http://malawidrilling.syr.edu/photos/Outreach%20Program/ index.html
). Technical challenges early in the fi eld operations included initial dif fi culty tuning the dynamic positioning system, and the operation of the DOSECC tools within the API drill string, but these operational issues were ultimately overcome by the resourceful staff of engineers and technicians. Routine drilling operations began on 9 March 2005, and continued for ten days at deep drill Site 1, located at a water depth of approximately 592 m in the central basin of Lake Malawi. Following the completion of four holes to a maximum
sub-bottom depth of 380 m ( Table 1 ), the
Viphya proceeded to the northern site and completed three holes at that locality.
6. Drilling engineering and fi eld operations
Lake Malawi is landlocked and there are no navigable waterways between the lake and ports on the Indian Ocean coast of East Africa.
Shipping and port operations exist on the lakeshore, but because much of the lake is bounded by faulted coastlines with rocky headlands and escarpments, there are only a few sheltered harbors along the full 560 km-length of the lake. Because of the size of the lake and the distance between these drill sites and the shoreline and sheltered harbors, it was necessary to organize a drilling operation capable of running 24 hours per day from a stand-alone drilling vessel, without the need to refuel, resupply, or shift crews to shore,
Table 1
Core locations and core details.
Hole Latitude Longitude Water depth
1A
1B
1C
1D
2A
2B
2C
11 17.6387 S
11 17.6814 S
11 17.6575 S
11 17.6183 S
10 01.0597 S
10 01.0567 S
10 01.0532 S
34 26.2331 E
34 26.1793 E
34 26.1462 E
34 26.1469 E
34 11.1607 E
34 11.1527 E
34 11.2067 E a
Corrected for initial over-penetration by 6.51 m.
592
592
592
592
359
359
359
Total depth
(mblf)
47.6
380.7
21.0
41.1
40.1
37.0
7. Methods of sediment drill core analyses
7.1. Logging, core processing and initial core descriptions
Logging in the fi eld of Site 1 holes included down-hole gamma logging following drilling, and whole core logging using a GEOTEK multisensory track logging system at a shore-based site. Measurements made with the fi eld GEOTEK instrument included GRAPE density (gamma ray attenuation porosity evaluator), magnetic susceptibility, and P-wave velocity. Following the completion of the drilling program, all cores were shipped to the National Lacustrine
Core Repository (LacCore) in Minneapolis, Minnesota. Replicate whole core GEOTEK logging was then carried out at high-resolution
on all cores recovered from sites 1 and 2 ( Table 1 ). Natural gamma
logging of whole cores was also carried out at the LacCore facility.
Following logging, cores were split and described according to standard paleolimnological procedures (
2003 ). Immediately following splitting, cores were scraped clean
and high-resolution color scans were acquired of each core, using a
DMT CoreScan digital linescan camera or a Geotek Geoscan-III digital linescan camera. Smear slides were made at selected intervals and examined in parallel with the completion of the visual core descriptions. Discrete subsamples of core were acquired for other analyses at this time. Following the splitting, description and subsampling work, cores were scanned in an ITRAX core scanner at the
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 9
Fig. 5.
Photographs of the drilling barge on Lake Malawi (A,C,D), and (B) outreach effort by drilling team scienti fi c staff.
Large Lakes Observatory at the University of Minnesota-Duluth for major and minor element abundances.
7.2. Age dating
The upper parts of all cores were age-dated using radiocarbon accelerator mass spectrometry at the Accelerator Mass Spectrometry
Laboratory at the University of Arizona. Previous studies of Late-
Pleistocene and Holocene cores from Lake Malawi have demonstrated the ef fi
cacy of dating of organic-rich bulk samples ( Johnson et al.,
), and accordingly all radiocarbon subsamples analyzed from the drilling project cores at sites 1 and 2 were bulk sediment samples.
Samples for radiocarbon dating were acquired at 50 cm intervals in the uppermost parts of each hole. Because modern Lake Malawi waters are undersaturated with respect to calcium carbonate, and the catchment contains limited amounts of carbonate bedrock, no reservoir corrections were made to radiocarbon dates. The resulting
14 C ages were calibrated using the Cologne Radiocarbon Calibration
and Radiocarbon Research Package (CAL-PAL) ( Weninger et al., 2005
).
Subsamples deeper in the cores were dated using Optically
Stimulated Luminescence (OSL), and in the case of MAL-1C, also paleomagnetic inclination and paleointensity. Paleointensity was determined using the ratio of natural remnant magnetization to anhysteretic remnant magnetization (NRM/ARM) (a measure of magnetic fi eld intensity), and 10 Be, and 10 Be/ 9 Be in core Mal-1C, were correlated to a previously published paleointensity record from
the Somali Basin ( McHargue et al., 2011-this issue; Meynadier et al.,
). Magnetic measurements of anhysteretic remanent magnetization (ARM) and natural remanent magnetization (NRM) before and after step-wise increasing alternating fi eld demagnetization steps were done on U-channel samples using a 2-G Enterprises automated
U-channel magnetometer system located at the University of Rhode
Island. These data were used to construct composite estimated relative paleointensity cores (NRM/ARM) for Lake Malawi drill sites 1 and 2 using the “ Splicer ” software program developed by the Ocean Drilling
Program. Splicer builds composite sections by using an optimized cross-correlation approach. Multiple parameters are used simultaneously to achieve the optimal match, and fi lls core gaps while avoiding stretching or compressing the depth scale. The data sets used in Splicer were the GRAPE density, susceptibility, NRM20/ARM20mT, and characteristic Inclination. Holes A and B were composited for Site
2, whereas holes C and D were composited for Site 1.
7.3. Paleoclimate indicators
A primary paleoclimate objective of the Lake Malawi Drilling Project is to assess a long-term record of effective moisture and lake level in the catchment. A variety of fundamental observations contribute to our understanding of these key measures of past climatic conditions. Primary among these are the lithostratigraphy of the core, and the sequence stratigraphy of the basin surrounding the drill sites. Lithostratigraphic observations were completed as part of the Initial Core Descriptions completed at the University of Minnesota, and lithology was further quanti fi ed through analyses of color data extracted from core images, as well as through analyses of physical properties measured on cores. Details of the seismic stratigraphy and basin framework of the Lake Malawi section are presented in
Lyons et al. (2011-this issue)
and the details of the lithostratigraphy and smear slide character of the key lithologies are
10 presented in
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Scholz et al. (2011-this issue) . Key geochemical measures of
the core (e.g., total organic carbon, total organic nitrogen, Ca abundance, and δ 13 C of organic matter) that contribute to our understanding of past limnological conditions are presented in
Scholz et al. (2011-this issue)
.
Elemental analyses of sediments using scanning X-ray fl uorescence methods have been presented in
Brown et al. (2007), Brown (2011-this issue) and Scholz et al. (2011-this issue)
. A record of past lake productivity from records of biogenic silica is presented by
Johnson et al. (2011-this issue)
. Records of landscape moisture from pollen records are presented in
Beuning et al. (2011-this issue) , and a high-resolution perspective of
water column geochemistry from diatom records is presented in
Stone et al. (2011-this issue) . Studies of ostracod assemblages are particularly
helpful in constraining lake levels, CaCO
3 saturation and water column dynamics in the system (e.g.
Cohen et al., 2007; Park and Cohen, 2011-this issue)
. An analysis of the faunal remains of fi sh and the possible impact of major hydrologic changes on fi sh paleoecology is presented in
Reinthal et al. (2011-this issue)
.
Records of past temperature from lake sediments have been recovered in the past using chironomid remains, but molecular methods using tetra ethers (TEX
86
) can now quantify paleotemperatures in tropical lakes as well, where chironomid-based methods are not useful
for paleotemperature analysis ( Powers et al., 2005; Tierney et al., 2008
).
In this volume,
Powers et al. (2011-this issue)
demonstrate further re fi nement of this method, presenting results of TEX
86 analyses of samples from short cores from northern Lake Malawi spanning the past
700 years. In a parallel study from the same short core,
describe biomarker evidence for recent changes in primary productivity from the same 700 year record.
present a 74,000 year Tex
86 sediments, subsampled from Malawi core 2A.
record from Lake Malawi
Another key set of parameters describing past climate in the continental interior comes from analyses of past wind regimes. The seasonal wind pattern associated with the migration of the ITCZ is an important aspect of the regional climate dynamics. In Lake Malawi changes in past wind regimes are assessed from sediment cores through studies of windblown material (e.g.
), upwelling as seen in laminated sediments signals of lake productivity
(e.g.
Castañeda et al., 2011-this issue ) and pollen transport ( Beuning et al., 2011-this issue
).
8. Results
8.1. Age dating
A summary of 14 C, paleointensity, OSL and inclination age dates is presented in
. In the interval 0 – 52 ka the radiocarbon ages were used to de fi ne the age – depth relationship, and a 3-term polynomial curve was used to characterize this interval. For the interval 52 – 145 ka a linear regression was used based upon paleointensity, inclination, and two OSL-based ages (
). Because the Mal-1C core initially over-penetrated to a depth of 6.5 m, we used dates from an adjacent piston core (M98-13P) to provide the age – depth relationship in the upper 6.5 m of the sediment section at this site.
8.2. Sequence stratigraphic framework
Basin-scale multichannel seismic pro fi les provide the regional and deep stratigraphic context for evaluating dense grids of singlechannel high-resolution seismic re fl ection data that are tied to the drill cores and the detailed paleoclimate measurements (
(
). Important elements of the stratigraphic section observed in high-resolution seismic re fl ection data include stacked progradational deposits, clearly identi fi ed as deltaic deposits associated with much lower stages of Lake Malawi (e.g.
Lyons et al., 2011-this issue; Scholz, 1995
). Other evidence of stratigraphic variability in the high-resolution Lake Malawi seismic records includes packages of facies couplets, which alternate between high-amplitude and continuous seismic facies and discontinuous and low-amplitude facies (see
).
These variations are observed in the seismic data at the same scale of variation as the profound changes in lithology observed in the sediment drill cores. The fi rm linkage of the drill core and seismic observations is provided through a detailed evaluation of the drill
Table 2
Age Data for Lake Malawi Drilling Project Hole 1C.
Hole Depth below lake fl oor
Lake Malawi 1C/13P
Lake Malawi 1C/13P
Lake Malawi 1C/13P
Lake Malawi 1C/13P
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
Lake Malawi 1C
15.806
21.053
17.5
21
32
39.5
46
72.5
84.5
67.25
0.555
1.7
3.705
5.46
6.705
7.5075
7.9
8.5075
9.4565
9.5565
10.5455
11.5235
12.4575
14.0895
Type of date
14 C
14
C
14
C
14
C
14
C
14 C
14 C
14
C
14 C
14 C
14
C
14
C
14
C
14
C
14 C
14 C
Paleointensity
Paleointensity
Paleointensity
Paleointensity
Paleointensity
Paleointensity
Paleointensity
Inclination
Lab number
AA34424
AA34425
AA34426
AA34427
AA71824
AA71820
AA34428
AA71821
AA65692
AA71823
AA65008
AA71822
AA65693
AA65694
AA65009
AA65010
14
C age
875
1830
3845
6235
9649
10,635
11,425
12,392
15,196
15,507
18,000
20,070
21,990
26,230
31,040
46,900
±Error
62
92
95
110
120
150
240
440
2800
45
45
55
55
52
59
75
Calendar age
(kyr BP)
36.139
50.457
39
53
67
80
83
114.5
135
122.5
0.816
1.772
4.273
7.141
11.009
12.628
13.329
14.565
18.451
18.799
21.528
23.973
26.59
30.89
±Error
(kyr BP)
0.313
0.222
0.18
0.47
0.267
0.427
0.214
0.423
3.243
0.067
0.05
0.095
0.09
0.138
0.079
0.145
15
7
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 11
Fig. 6.
A) Age – depth relationship for Site 1C drill hole. See text and Table 2 for details of geochronology. B) Paleointensity pro fi les correlating sites 1 and 2 with the Somalia Basin record of
Meynadier et al. (1992) , core 85
– 674.
12 C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Fig. 7.
Paleoclimate proxy records from sites 1 and 2 in Lake Malawi. From left to right: lithology; normalized core imagery (
Scholz et al., 2011-this issue ); red values (extracted from
imagery); saturated bulk density (from GRAPE, or Gamma Ray Attenuation Porosity Evaluator); total organic carbon (TOC); Principal Component 1 from diatom and other palaeoecological analyses (PC-1) (
); total pollen accumulation rate (PAR) ( Beuning et al., 2011-this issue
); Lake Malawi TEX
86
); Ostracode concentration ( Cohen et al., 2007; Park and Cohen, 2011-this issue ).
core velocity and density parameters, which were used to generate a synthetic seismogram for the drill core (
).
The detailed stratigraphic framework built-out from the sediment drill cores allows us to further constrain the history of water level variation in the basin over the past 145 kyr.
8.3. Paleoclimate inferences
Sediment lithology in the Malawi Drilling Project 1C drill core shows considerable variability, as initially characterized by visual examination of the split sediment cores (
). The quanti fi cation of lithology was then completed using sediment physical properties, core imagery, and geochemical methods.
Sediment density was quanti fi ed at high-resolution in the sediment cores using the GEOTEK logging system, and these data combined with color (RGB) data extracted from core images show pronounced variability, especially at depths in the core below 28 m depth (~60 ka). In the zone between the base of the core at ~ 90 m sub-bottom (145 ka) and 32 m there are marked and cyclical changes from low-density dark-brown homogeneous and laminated mud, to high bulk density, grey, massive and mottled mud (
). Bulk organic matter measurements show that the low-density sections are characterized by high values of organic carbon, whereas the highdensity zones are characterized by very low values of TOC, but signi fi cant enrichment in calcium carbonate, as measured by Ca
abundance in scanning XRF data ( Fig. 8 ). Analyses of bulk organic
matter and major elements (using high-resolution XRF core scanning) also permit the quanti fi cation of lithologic variations. At deep-water
Site 1C in the central basin, total organic carbon shows an inverse relationship with Ca abundance, and at depth this alternation occurs about every 15 –
20 m below about 32 m in the core (~ 60 ka) ( Fig. 7
).
Detailed results of bulk organic matter analyses are presented in the study of
Scholz et al. (2011-this issue)
which describes the proportions of terrestrial and aquatic organic matter deposited into
Lake Malawi over the past 145,000 years.
Paleoecological records provide a wealth of information on water column conditions over the length of the core (
Stone et al. (2011-this issue)
reconstruct a record of lake level and paleolimnology from principle component analyses of diatom paleoecology and sieved fossil and mineral residues (
).
Many of the key components of the early fossil diatom record observed in Hole 1C sediments are in general not observed in the open waters of the central basin today. The diatom record over the past 60,000 years at the deep central basin site is re fl ective of dilute, deep waters, and dysaerobic bottom conditions, similar to the modern system and comparable to what has been described for the lake for the Holocene,
Last Glacial Maximum, and deglacial intervals. Prior to 70 ka however, the diatom records as well as ancillary fossil and minerogenic residues suggest that this area and the lake in general was much shallower, alkaline and at least mildly saline. During intervals described by
Scholz et al. (2007) and Cohen et al. (2007)
as megadroughts, species are dominated by Aulacoseira taxa that are today mainly found in the southern shallow basin of the lake. The dominance of saline/alkaline plankton such as Aulacoseira ambigua during these megadrought intervals suggests a shallower closed basin, which would have had drastically different mixing processes and nutrient inputs relative to the modern system (
).
A study of ostracod assemblages from Hole 1C shows occurrences of seven genera, and these varying assemblages, in combination with other taphonomic variables such as valve breakage, and carbonate and
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 13 less than 3000 grains/cm 2 /yr, with a switch to dominantly grass pollen
( Beuning et al., 2011-this issue
). Coincident with severe drought intervals, total charcoal abundance was also dramatically reduced (e.g.
Cohen et al., 2007 ), implying a dry land climate with very limited fuel
available for brush fi res. According to
Beuning et al. (2011-this issue)
such pollen spectra are indicative of climate regimes with b 800 mm/ year rainfall availability. Following this severe megadrought interval, woodland taxa rose signi fi cantly, suggesting an increase in average rainfall in the catchment to 1100 – 1200 mm/year. The interval 75 – 30 ka was an interval of instability and variability in the pollen records, with a signi fi
cant peak in Pollen Accumulation Rate (PAR) ( Fig. 7 ) and
Podocarpus centered around 60 ka (
Beuning et al., 2011-this issue
), and then a gradual decline in total pollen production. The interval 30 –
15 ka is characterized by relatively low PAR, and stands in contrast to other areas of East Africa to the north which show a more dramatic response to presumably Last Glacial Maximum conditions.
Lithostratigraphy and sequence stratigraphy, elemental and bulk organic matter chemostratigraphy, and paleoecological analyses, especially of diatoms and ostracods provide detailed constraints on the history of lake level and effective moisture in the catchment over the past 145,000 years. Pollen records are particularly indicative of conditions of catchment vegetation during this interval. Other key records of past climate change in the basin include measures of surface water temperatures as derived from the TEX
86 paleothermometer, based on Crenarchaeota picoplankton (
Woltering et al., 2011this issue
). Reinthal et al. analyzed remains of fi sh scales, bones and teeth, and their δ 13 C isotopic values of fi sh bones, all of which provide insights to the presence of inshore versus pelagic faunas at the drill site over the past 145,000 years (
Fig. 8.
Core images from Hole 1C. A) Well-laminated, organic-rich mud from upper section of hole, typical of lake high stand deposits. B) Massive, dense grey-blue mud from middle part of hole typical of lake lowstand deposits that accumulated during megadrought intervals.
9. Synthesis of Malawi lake levels and climate variability of the past 145,000 years oxidized coatings provide additional constraints on lake levels and
paleolimnological conditions over the past 145,000 years ( Park and
) (
Figs. 8 – 10 ). Few ostracod occurrences are
noted in the upper ~ 30 m of core at the deep site, suggesting a strati fi ed lake during this time, and indicating that the deep site was continuously bathed by anoxic bottom waters for the past
~ 50,000 years ( Cohen et al., 2007; Park and Cohen, 2011-this issue
).
A Limnocythere -dominated shallow, saline/alkaline assemblage (133 –
130 ka) and a deeper water Cypridopsine assemblage (118 – 90 ka) that lived in waters 10s to 100s meters deep are observed during two older intervals (
). The latter assemblage also dominated during lake level transitions at 136 – 133 ka, 129 – 128 ka and 86 – 63 ka
( Park and Cohen, 2011-this issue ). Notably monospeci
fi c assemblages of Limnocythere spp. are typical of littoral environments in highly
alkaline and saline African lakes such as Lake Turkana ( Cohen et al.,
). These Limnocythere assemblages are also characterized by high adult/juvenile ratios, limited decalci fi cation, carbonate coatings, and valve abrasion, indicating shallow water and calcium carbonate supersaturated conditions (
Pollen pro fi les acquired from Hole 1C reveal a history of signi fi cant and sometimes rapid changes in catchment vegetation over the past
145,000 years ( Beuning et al., 2011-this issue
). The most dramatic excursions from the modern catchment fl oral assemblage are observed between 135 and 127 ka, and from 110 to105 ka, when production and abundance of Podocarpus increased and abundances of as high as 38% are observed. This indicates a dramatic expansion of montane forests to much lower elevations, and implies a cooler and drier climate during these intervals. The latter interval of severe droughts extended further, and during the period 105 – 90 ka total pollen accumulation dropped to
Lake Malawi climate variability over the past 145 ka is best separated into two main intervals, pre- and post-60,000 years before
). From 145 to 60 ka we observe evidence for remarkable variability in lake levels, mixing regime, and trophic state, which, given the size and latitudinal extent of the Malawi catchment, likely re fl ect continental scale variability in the climate system. During the last 60 ka, the lake appears to have behaved much like the modern water body, with only minor variations in water volume and water depth (e.g. within a few percent of modern values), and nothing as extreme as in earlier times.
9.1. Paleohydrology and paleoclimate variability 145 – 60 ka
9.1.1. 135 – 145 ka
Lithological, paleoecological and geochemical indicators from this
interval of the core ( Fig. 10 ) suggest a lake system considerably different
from modern, but representing relatively deep-water conditions at this site. Diatom assemblages indicate that Site 1 (presently nearly 600 m depth) had a setting comparable to the intermediate to deeper waters of the modern lake. Organic-rich sediments, with measurable but low carbonate content suggest a deep, strati fi ed lake environment in this locality, and maximum water depths on the order of 350 – 550 m. As the oldest section in Hole 1C, below major unconformities, we are unable to directly tie this interval to shoreline indicators observed in seismic re fl ection data; accordingly there is great uncertainty for the lake level estimate for this interval.
9.1.2. 124 – 135 ka
Sediments deposited during this interval are characterized as massive and dense, light-grey, carbonate-rich mud, with low TOC values. Bulk organic matter geochemical data suggest a mixture of algal and C3-pathway organic matter accumulating in the basin and at
the drill site during this interval ( Scholz et al., 2011-this issue
).
14 C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Fig. 9.
Images of key core constituents. A) Mica-rich siliciclastic sample. Q — quartz; M — muscovite; Ch — chlorite; Bt — biotite (photo courtesy M.R. Talbot). B) Charred graminoid epidermis (photo courtesy M.R. Talbot) C) Common diatom from Hole 1C (Stephanodiscus muelleri — see
). D) ostracode valve: right valve of
Cypridopsine sp. R from MAL05-1C-24E3 361 58.1
– 59.1 cm (see
Park and Cohen, 2011-this issue
). E) Phytolith from an unidenti fi ed plant (photo courtesy M.R. Talbot) F) Fish vertebrae from sample 5-H-2 (93.4
– 94.4 cm), 20.19 mblf, approximately 48,000 years old (see
Reinthal et al., 2011-this issue
).
Diatom assemblages indicate a period of highly variable lake level, and several other paleoecological indicators suggest saline and alkaline conditions. Lake levels are interpreted to be severely reduced during this time, perhaps 550 m or more below modern levels. This estimate is further constrained by a stratigraphic tie to a set of prograding clinoform re fl
9.1.3. 117 – 124 ka
This interval is characterized by several indicators suggesting intermediate to high lake levels, including organic-rich laminatedhomogenous mud with low CaCO
3 content. Vivianite crystals are
observed in the wet-sieved fraction ( Cohen et al., 2007 ), indicating
strati fi cation and bottom water anoxia. We estimate that water depths were relatively deep at drill Site 1 during this time, and probably
0 – 200 m below modern levels. Because there are no de fi nitive stratigraphic ties to seismically-identi fi ed shoreline indicators, there is considerably greater uncertainty for our paleowater depth estimates during this period. The water column was undersaturated with respect to calcium carbonate, suggesting hydrologically open conditions during this interval, and water depths comparable to modern conditions.
9.1.4. 85 – 117 ka
This period is marked by a very severe lake level drawdown to levels of at least 500 – 550 m below that of the modern lake. Sediment lithology consists of blue-grey, mostly massive carbonate-rich mud, with an interval of mediumfi ne sand. Diatom assemblages indicate elevated salinity in this interval, and Limnocythere ostracods, and carbonate coated grains support the interpretation of littoral conditions at the core site during this interval. The most severe low lake stage occurred ~109 –
92 ka, based in part on the dominance of shallow-water Aulacoseira
species diatoms ( Stone et al., 2011-this issue
). The presence of carbonate nodules in this interval, along with sediment textures resembling gleyed paleosols, even indicates the possibility of subaerial exposure at this site during this time period. A stratigraphic tie is also made to a major lowstand delta clinoform package observed in highresolution seismic re fl ection data in the north basin of the lake. Taken together, all these indicators suggest a major low lake stage and where water volumes were reduced to ~2% of the modern levels. Charcoal abundances during this interval are also dramatically reduced, indicating limited vegetation in the catchment and possibly a semidesert environment in the lowland areas.
9.1.5. 85 – 71 ka
This interval is characterized by fl uctuating lake levels, which are mainly much higher than those of the megadroughts centered at ~100 and 130 ka. A spike in ostracod and calcium carbonate abundance at
75 ka is tied stratigraphically to another deltaic deposit identi fi ed at
− 350 m below modern lake level (e.g.
).
Abundant uncoated juvenile cypridopsine ostracods indicate somewhat deeper water and less calcium carbonate-rich conditions during this time relative to the megadrought interval. Some intervals of this zone are dominated by carbonate-poor, organic-rich sediments with evidence of vivianite, suggesting that there were periods when the lake was deeper and strati fi ed.
9.1.6. 61 – 72 ka
The sediments deposited between ~ 61 and 72 ka are more regularly laminated and darker in color than the overlying sediments and display some of the highest TOC values of any material recovered
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 15
Fig. 10.
Summary of key paleoclimate indicators and other tropical climate information. A) Mean insolation at the start of the single rainy season in the Malawi catchment
(1 October – 1 December, 10°S). B) Interpreted Lake Malawi water levels over the past 145,000 years (blue) and orbital eccentricity (dotted black). Lake level interpretation developed from synthesis of sediment core paleoclimate indicators with observations from seismic re fl ection records (e.g.
Lyons et al., 2011-this issue ). Note prolonged intervals of
extremely low lake levels centered at 130 and 100 kyr BP. Intervals 1 – 8 are described in detail in the text. C) Ostracod abundance. Site one modern water depths = 592 m, and waters below ~ 250 m are anoxic. Accordingly Late-Pleistocene section is devoid of any benthic invertebrates. Single * indicates core intervals dominated by profundal ostracod taxa; double
** indicates core intervals dominated by littoral zone ostracod taxa when lake shoreline was very close to drill site. D) Total organic carbon (weight %). Intervals of elevated TOC are commonly fi nely-laminated, and indicative of intervals of high lake level, water column strati fi cation, and bottom water anoxia. E) Principal component (PC-1) of diatoms and other paleoecological indicators (from
abundance from scanning X-ray fl uorescence. Peaks in elevated Ca indicate periods of Ca saturation in the water column. Vertical grey bars indicate periods of severe low lake levels, marked aridity and prolonged drought in the Lake Malawi catchment. These occur during times of high eccentricity, when the system responded to extremes in orbital precession.
Periods of severe aridity are broadly, although not perfectly aligned with diminished mean insolation at the start of the rainy season in the Malawi catchment. See text for further discussion.
in Hole 1C, generally indicating high productivity, and strati fi cation with bottom water anoxia. Because this interval follows a period of much lower lake levels (see above), the highly varying organic matter enrichment may also in part be due to the fertilization effects of remobilized material following the earlier low lake stages (e.g.
Talbot et al., 2006 ). This interval is also marked by the last signi
fi cant occurrence of carbonate in the section, indicating a brief, ~ 2000-year period of carbonate saturation in the water column, centered at about
62 ka. It is the youngest of the high CaCO
3
– high δ 13 C and low C/N – low
TOC periods described above, and is stratigraphically correlated to a series of major lowstand delta deposits observed at 200 m below modern lake level, and adjacent to many of the modern river systems in the lake (
Lyons et al., 2011-this issue ).
9.2. Paleohydrology and paleoclimate variability 60 – 0 ka
Following the period of severely fl uctuating lake levels and climate changes between 145,000 and 60,000 years ago, the lake rose to much higher levels, and the amplitude of environmental change was much diminished compared to the earlier period. Although lake levels are
16 relatively high for the duration of the period 0 – 60 ka, the frequency of shifts in wind regimes and water column dynamics is very rapid (e.g.
Brown et al., 2007 ), and paleoclimate variations are observed to occur
on time frames of hundreds years. These high-frequency changes are well-constrained, as the age-dating of this interval is far more precise than that developed for earlier intervals.
9.2.1. 60 – 32 ka
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Lake levels during this interval are generally high, with subtle
variability as observed on the sediment core diatom record ( et al., 2011-this issue
). Ostracods are lacking through virtually the entire interval, but occasional occurrences of diatoms that tolerate elevated alkalinity suggest fl uctuating levels, however brie fl y.
9.2.2. 31 – 16 ka
Stone et al. (2011-this issue)
observe over this interval a gradual transition to diatom assemblages that bear strong resemblance to the modern fl ora, with increasing concentrations of vivianite suggesting a stable water column and bottom water anoxia.
Water levels were within 100 m of the modern levels for the entire interval, and at the deep site in the central basin relatively consistent lithologic and chemostratigraphic proxy records are observed. There is some evidence of fl uctuating lake levels in the seismic re fl ection data from the Dwangwa delta, suggesting lake level lowering of
~ 100 m around the time of the LGM. However the environmental conditions at the Site 1 drill site were relatively insensitive to water level variability of this magnitude.
Paleoclimate indicators at Site 2 near the northern tip of the lake reveal subtle climate signals that are more pronounced than at Site 1.
Core Site 2 is more sensitive to climate shifts which are detectable due to its proximity to volcanic terrane north of the lake, and to changes in lake upwelling behavior in this part of the lake basin.
report on millennial-scale variability in bulk elemental ratios of Zr:Ti, which re fl ects windblown volcanogenic dust from the
Rungwe Volcano. This millennial-scale structure of the lake sediment record bears strong resemblance to signatures of rapid climate change observed from the Greenland and Antarctic ice cores, and demonstrates strong teleconnections with high-latitude processes. Interestingly the thermal structure of the lake over this time interval does not
show that same type of abrupt response ( Woltering et al., 2011-this issue
), although their TEX
86 data set indicates a general trend
consistent with that observed in Lake Tanganyika ( Tierney et al.,
2008 ) that, on an orbital time scale, tracks Northern Hemisphere
summer insolation over the past 60 ka.
10. Discussion and on-going research
10.1. Paleoclimate record of the past 145,000 years
The response of tropical Africa to high-latitude cooling at the Last
Glacial Maximum is well-documented in many sites in east Africa, and the very limited hydrological response in Lake Malawi during this time is somewhat surprising. Surface temperatures were lower by
several degrees ( Bard et al., 1997; Gasse, 2000; Stute and Talma,
1998 ), effective moisture was reduced, and water levels were lower in
many large lakes of tropical Africa, including Lake Victoria, which was
desiccated ( Johnson et al., 1996
); Lake Tanganyika, which was ~ 250 m lower (
Gasse et al., 1989 ); Lake Albert, which was hydrologically-
closed and possibly desiccated ( Beuning et al., 1997 ); and Lake
Edward ( − 37 m,
). However in past studies of Lake Malawi sediment cores, the paleohydrological response during the LGM has proven equivocal or relatively muted, and a similar result is observed in the new Lake Malawi scienti fi c drill cores.
reported a low lake stage in the early Holocene based on analyses of sediment cores from the southern basin. Evidence for a lower lake stage in Lake Malawi within the past ~ 20,000 years is observed in 1) the diatom record from the north basin generated by
F. Gasse, which shows evidence for lower lake levels from the LGM until about 16 ka (
), and 2) in high-resolution seismic re fl ection data documented by
Lyons et al. (2011-this issue)
, where a paleo-delta of the Dwangwa river is interpreted as evidence of a comparatively minor − 100 m drop in lake level. Although there appears to be comparatively limited change in effective moisture in the Lake Malawi catchment when northern tropical East African sites clearly experienced signi fi cant impacts at the LGM, there are discernable shifts in temperature of about 3 – 4 °C, determined from
TEX
86 analyses (
). Shifts in wind strength and prevailing direction are interpreted through variability in airborne dust contributions to the lake (
Brown et al., 2007 ), and in north basin
upwelling, as seen in biogenic silica pro fi les (
2011-this issue ). Responses similar to those observed in the Lake
Malawi drill cores are also observed in Lake Tanganyika, in a
90,000 year-old condensed section recovered from the Kavala Island
Ridge (
Burnett et al., 2011-this issue
). The Lake Tanganyika surface water temperature response as measured by the TEX
86 paleothermometer is somewhat stronger, with a N 5 °C change over the past
60,000 years (
) compared to ~3 – 4° in Lake Malawi
(
Woltering et al., 2011-this issue
).
suggested that the more pronounced Lake Tanganyika signal may imply a north-tosouth gradient in the effectiveness of Northern Hemisphere climate forcing during the LGM. Paleoecological climate proxy records from the LGM, including pollen (
Beuning et al., 2011-this issue
) and
diatoms ( Stone et al., 2011-this issue
) also yield no pronounced LGM signals in the Lake Malawi drill cores.
The record from Lake Malawi drill cores from the last 60,000 years suggests a tropical environment broadly comparable to the rift valley today. The lower elevations were predominantly woodland with some grassland areas, and afromontane forests dominated the very highest elevations. Abrupt, millennial-scale changes are clearly documented during this time interval, but the primary variability was in wind intensity and direction, and lake water column dynamics and the amplitude of change of effective moisture and temperature were damped in comparison. At various times prior to 60,000 years before present however, climate was dramatically different, and megadroughts prevailed, producing cool, dry semi-desert landscapes with markedly reduced rainfall. The evidence for these extreme shifts in the hydrological regime is documented in many sediment core climate proxy records from the drill cores, as well as in geophysical data sets from the basin. Full records of paleotemperature from core
1C await further analytical work (
Johnson and Berke, 2009 ), but
results of TEX
86
60,0000 years before present. The largest temperature variability observed in the Site 2 TEX
86 records of
Woltering et al. (2011-this issue)
is between 80,000 and 60,000 years ago, coincident with the highest variability in indicators of lake level and effective moisture.
Hydrological variability in Lake Malawi over the past 145,000 years is characterized by high-amplitude variability on a 10 – 20 kyr cycle prior
to about 70,000 years ago ( Lyons, 2009
), and relatively high lake levels with subtle millennial-scale climate shifts from 60,000 years until the
). This transition has been interpreted as due to a relaxation of eccentricity modulation of precessional forcing of tropical
African climate ( Scholz et al., 2007
). During intervals of increased insolation, atmospheric convection and tropical convergence are enhanced, which leads to an increase in precipitation (e.g.
2006; Kutzbach and Street-Perrott, 1985
).
referred to a climate model focused on tropical precipitation to assess the role of orbital precession versus zonal and meridional heating gradients as drivers of local hydrological cycles (e.g.
). When precessional forcing is weak during intervals of low eccentricity, such as during the period 0 – 60,000 years before present, global teleconnections may be enhanced, and high-latitude in fl uences on tropical climate may
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19 prevail (
). The megadrought intervals interpreted from our records correspond to zones of elevated Ca and diminished
TOC, which generally occur during insolation minima (
interpret the interval between 145 and ~60 kyr ago as a period of enhanced precession-scale variability in the hydrologic cycle, dominated by periods of extreme drought conditions, due primarily to a peak in orbital eccentricity which enhanced the amplitude of precession
( Fig. 10 ). Similar signals are observed in many other tropical and
subtropical sites in Africa, including marine cores from both the Atlantic and Indian Oceans, outcroppings of lacustrine sediments deposited
during lake highstands in the Central Kenya Rift ( Trauth et al., 2003;
Deino et al., 2006; Kingston et al., 2007 ), as well as in the Pretoria Salt
Pan in South Africa ( Partridge et al., 1993
), although the phasing of wet and dry intervals varies latitudinally between different parts of the
African continent. The intensi fi cation of the African monsoon at approximately precessional-scale intervals has been documented at a number of sites (
Rossignol-Strick, 1985; McDougall et al., 2005; Trauth et al., 2003
) and records that are located close to the equator show evidence of wet climates every ~11 kyr, corresponding to the halfprecessional cycle (e.g.
Bergner et al., 2003; Short et al., 1991; Trauth et al., 2003, 2007
). It is possible that some paleoclimate proxy signatures from the Malawi cores also show this half-precessional signature
The dramatic rise in water level in Lake Malawi as well as in other
lakes in Africa after 70 kyr ago ( Scholz et al., 2007
) is evidence for increased effective moisture across a wide swath of the African tropics. This effect is most dramatically observed in extant lakes (e.g.
), as the discontinuous records preserved at outcrop localities generally do not preserve the full climate response.
The relaxation in eccentricity explains the diminished precessionscale climate variability since 70,000 years ago, but it does not account for the long-term shift to overall higher lake level. A possible explanation for this phenomenon comes from climate modeling results.
suggest that the southward shift of the austral summer Hadley cell during the LGM produced an increase in latitudinal temperature gradients, which led to dry intervals at equatorial and northern tropical latitudes, but an increase in precipitation in the Southern Hemisphere, including in the Lake
Malawi catchment. Accordingly the onset of glacial conditions in the
Northern Hemisphere over the past 70 ka may have produced a similar effect, resulting in higher lake levels. The climate modeling studies support the idea that the high-eccentricity interval from
~ 145 – 70 kyr ago is responsible for generating the high precessionscale variability in Malawi lake levels in that time frame especially.
These results combined with observations from a number of other sites suggest a mode switch to high-latitude forcing and overall wetter, more stable conditions around 70 kyr ago. The severity of the observed lowstands, especially those centered at ~100 and ~ 135 kyr
B.P. during the period of enhanced eccentricity, strongly suggest a precessional control on tropical African climate during this interval, when glacial in fl uence was relatively minor.
10.2. Further research
The sediment drill cores collected from Lake Malawi in 2005 will no doubt undergo extensive additional analyses in the years to come.
Among the key records yet to be generated are the detailed records of paleotemperature from TEX
86
, detailed biomarker studies that assess the origin of organic matter in the lake and catchment, highresolution records of vegetation change in the basin from pollen studies, and much longer records of past climate from core 1B, which extended more than 380 m below the bottom of the lake at Site 1.
Ultimately scienti fi c drill core records from other sites around the
African continent will be required in order to fully characterize
Quaternary climate changes in the region, but in parallel with these observational studies, global and regional-scale climate modeling work will also be required in order to fully constrain and quantify past climate boundary conditions in the continental tropics.
Acknowledgements
17
Many individuals and organizations contributed to the successful planning and execution of the fi eld program, as well as the analyses and support for analytical phases of the project. Especially, we thank the people and government of Malawi for permission to conduct this research, and in particular the Geological Survey of Malawi for local assistance and participation. Numerous individuals from key contractors worked tirelessly in order to complete the program, including the following: Lengeek Vessel Engineering; ADPS dynamic positioning and ship's crew; the drilling team from Seacore Ltd; DOSECC, and LacCore for assistance with core analysis and archiving. We thank the US-NSF Earth
System History and Paleoclimate programs, and the International
Continental Scienti fi c Drilling program for fi nancial support.
References
Bard, E., Rostek, F., Sonsogni, C., 1997. Interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry. Nature 385, 707 – 710.
Bergner, A.G.N., Trauth, M.H., Bookhagen, B., 2003. Paleoprecipitation estimates for the
Lake Naivasha basin (Kenya) during the last 175 k.y. using a lake-balance model.
Global and Planetary Change 36, 117 – 136.
Beuning, K.R.M., Talbot, M.R., Kelts, K., Livingstone, D.A., 1997. A revised 30,000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 259 – 279.
Beuning, K.R.M., Zimmerman, K.A., Ivory, J.J., Cohen, A.S., 2011. Vegetation response to glacial – interglacial climate variability near Lake Malawi in the southern African tropics. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 81 – 92 (this issue).
Bootsma, H.A., Hecky, R.E., 1999. Water Quality Report. Lake Malawi/Nyasa Biodiversity
Conservation Project, Senga Bay, Malawi.
Brown, E.T., 2011. Lake Malawi's response to “ megadrought ” terminations: sedimentary records of fl ooding, weathering and erosion. Palaeogeography, Palaeoclimatology,
Palaeoecology 303, 120 – 125 (this issue).
Brown, E.T., Johnson, T.C., Scholz, C.A., Cohen, A.S., King, J.W., 2007. Abrupt change in tropical African climate linked to the bipolar seesaw over the past 55,000 years.
Geophysical Research Letters 34, 20702 – 20707.
Burnett, A.P., Soreghan, M.J., Scholz, C.A., Brown, E.T., 2011e. Tropical East African climate change and its relation to global climate: a record from Lake Tanganyika,
Tropical East Africa, over the past 90+ kyr. Palaeogeography, Palaeoclimatology,
Palaeoecology 303, 155 – 167 (this issue).
Cane, M.A., Eshel, G., Buckland, R.W., 1994. Nature 370, 204 – 205.
Castañeda, I.S., Werne, J.P, Johnson, T.C., Oberem, L.P., 2011. Organic geochemical records from Lake Malawi (East Africa) of the last 700 years, part II: Biomarker evidence for recent changes in primary productivity. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 140 – 154 (this issue).
Chiang, J.C.H., Biasutti, M., Battisti, D.S., 2003. Sensitivity of the Atlantic Intertropical
Convergence Zone to Last Glacial Maximum boundary conditions. Paleoceanography 18.
doi:10.1029/2003PA000916 .
Clement, A.C., Hall, A., Broccoli, A.J., 2004. The importance of precessional signals in the tropical climate. Climate Dynamics 22, 327 – 341.
Cohen, A.S., Dussinger, R., Richardson, J., 1983. Lacustrine paleochemical interpretations based on Eastern and Southern African ostracodes. Palaeogeography, Palaeoclimatology, Palaeoecology 43, 129 – 151.
Cohen, A.S., Stone, J., Beuning, K., Park, L., Reinthal, P., Dettman, D., Scholz, C.A., Johnson,
T., King, J.W., Talbot, M., Brown, E., Ivory, S., 2007. Ecological Consequences of Early
Late – Pleistocene Megadroughts in Tropical Africa. Proceedings of the National
Academy of Sciences 104, 16422 – 16427.
Crossley, R., 1984. Controls of sedimentation in the Malawi Rift Valley, Central Africa.
Sedimentary Geology 40, 33 – 50.
Daly, M.C., Chorowicz, J., Fairhead, J.D., 1989. Rift basin evolution in Africa: the in fl uence of reactivated steep basement shear zones. Geological Society, London, Special
Publications — Inversion in Other Geological Environments 44, 309 – 334.
de Martonne, E., 1897. Die Hydrographie des oberen Nilbeckers. Z. Ges. Erdkd 32, 315.
Deino, A.L., Kingston, J.D., Glen, J.M., Edgar, R.K., Hill, A., 2006. Precessional forcing of lacustrine sedimentation in the late Cenozoic Chemeron Basin, Central Kenya Rift, and calibration of the Gauss/Matuyama boundary. Earth and Planetary Science
Letters 247, 41 – 60.
deMenocal, P.B., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky,
M., 2000. Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quaternary Science Reviews 19, 347 – 361.
Dixey, F., 1926. The Nyasaland section of the Great Rift Valley. The Geographical Journal
68, 117 – 137.
Drayton, R.S., 1984. Variations in the level of Lake Malawi. Hydrological Sciences 29,
1 – 12.
Ebinger, C.J., 1989. Tectonic development of the western branch of the East African rift system. GSA Bulletin 101, 885 – 903.
18
Ebinger, C.J., Sleep, N.H., 1998. Cenozoic magmatism throughout east Africa resulting from impact of a single plume. Nature 385, 788 – 791.
Ebinger, C.J., Deino, A.L., Drake, R.E., Tesha, A.L., 1989. Chronology of volcanism and rift basin propagation: Rungwe Volcanic Province, East Africa. J. Geophys. Res. 94,
15,785 – 15,803.
Ebinger, C., Petit, C., Burov, E., 2002. Causes and consequences of the lithospheric extension: the ups and downs of continental rifts. In: Renaut, R.W., Ashley, G.
(Eds.), Sedimentation in Continental Rifts: SEPM Special Publication, 73, pp. 11 – 23.
Finney, B.P., Johnson, T.C., 1991. Sedimentation in Lake Malawi (East Africa) during the past 10,000 years: a continuous paleoclimate record from the southern tropics.
Palaeogeography, Palaeoclimatology, Palaeoecology 85, 351 – 366.
Furman, T., 1995. Melting of metasomatized subcontinental lithosphere: undersaturated ma fi c lavas from Rungwe, Tanzania. Contributions to Mineralogy and
Petrology 122, 97 – 115.
Gasse, F., 2000. Hydrological changes in the African tropics since the Last Glacial
Maximum. Quaternary Science Reviews 19, 189 – 211.
Gasse, F., Lédée, V., Massault, M., Fontes, J.-C., 1989. Water-level fl uctuations of Lake
Tanganyika in phase with oceanic changes during the last glaciation and deglaciation. Nature 342, 57 – 59.
Goddard, L., Graham, N.E., 1999. Importance of the Indian Ocean for simulating rainfall anomalies over eastern and southern Africa. Journal of Geophysical Research 104,
19099 – 19116.
Gregory, J.W., 1896. The Great Rift Valley. John Murray, London.
Harkin, D.A., 1960. The Rungwe Volcanics at the Northern End of Lake Nyasa. Printer,
Tanzania Govt.
Johnson, T.C., Berke, M.A., 2009. A 150,000-year record of temperature from Lake
Malawi, East Africa. Eos Trans. AGU 90 (52) Fall Meet. Suppl., Abstract PP14A-05.
Johnson, T.C., Scholz, C.A., Talbot, M.R., Kelts, K., Ricketts, R., Ngobi, G., Beuning, K.,
Ssemmanda, I., McGill, J.A., 1996. Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fi shes. Science 273, 1091 – 1093.
Johnson, T.C., Brown, E.T., McManus, J., Barry, S., Barker, P., Gasse, F., 2002. A highresolution paleoclimate record spanning the past 25,000 years in southern East
Africa. Science 296, 113 – 116.
Johnson, T.C., Brown, E.T., Shi, J., 2011. Biogenic silica deposition in Lake Malawi, East
Africa over the past 150, 000 years, Palaeo-3. Palaeogeography, Palaeoclimatology,
Palaeoecology 303, 103 – 109 (this issue).
Kingston, J.D., Deino, A.L., Edgar, R.K., Hill, A., 2007. Astronomically forced climate change in the Kenyan Rift Valley 2.7
– 2.55 Ma: implications for the evolution of early hominin ecosystems. Journal of Human Evolution 53, 487 – 503.
Klerkx, J., Theunissen, K., Delvaux, D., 1998. Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift. Journal of
African Earth Sciences 26, 347 – 361.
Kreuser, T., 1990. Depositional evolution of Permo-Triassic Karoo basins in Tanzania with reference to their economic potential. Journal of African Earth Sciences 10, 151 – 167.
Kutzbach, J.E., Street-Perrott, A., 1985. Milankovitch forcing of fl uctuations in the level of tropical lakes from 18 to 0 kyr BP. Nature 317, 130 – 134.
Lewin, R., 1981. Lake bottoms linked with human origins. Science 211, 564 – 566.
Livingstone, D.A., 1965. Sedimentation and the history of water level change in Lake
Tanganyika. Limnology and Oceanography 10, 249 – 280.
Lyons, R.P., 2009. Stratigraphic and hydrologic responses to tropical climate variability: scienti fi c drilling in Lake Malawi, East Africa. Syracuse University. 228 pp.
Lyons, R.P., Scholz, C.A., Buoniconti, M.R., Martin, M.R., 2011. Late Quaternary stratigraphic analysis of the Lake Malawi Rift, East Africa: an integration of drill-core and seismicre fl ection data. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 20 – 37
(this issue).
Lyons, R.P., Kroll, C.N., Scholz, C.A., 2010. An energy-balance hydrologic model for the
Lake Malawi Rift Basin, East Africa. Global and Planetary Change 75, 83 – 97.
Malawi Department of Surveys, 1983. National Atlas of Malawi, Blantyre.
Marchant, R., Mumbi, C., Behera, S., Yamagata, T., 2007. The Indian Ocean dipole — the unsung driver of climatic variability in East Africa. Afr. J. Ecol. 45, 4 – 16.
McDougall, I., Brown, F.H., Fleagle, J.G., 2005. Stratigraphic placement and age of modern humans from Kibish, Ethiopia. Nature 433, 733 – 736.
McGlue, M.M., Scholz, C.A., Karp, T., Lezzar, K.E., Ongodia, B., 2006. Facies architecture of fl exural margin lowstand delta deposits in Lake Edward, East African Rift: constraints from seismic re fl ection imaging. Journal of Sedimentary Research 76, 942 – 958.
McHargue, L.R., Jull, T., Cohen, A., 2011. Measurement of 10 Be from Lake Malawi (Africa) drill core sediments and implications for geochronology. Palaeogeography,
Palaeoclimatology, Palaeoecology 303, 110 – 119 (this issue).
Meynadier, L., Valet, J.-P., Weeks, T., Shackleton, N.J., Hagee, V.L., 1992. Relative geomagnetic intensity of the fi eld during the last 140 ka. Earth and Planetary
Science Letters 114, 39 – 57.
Moernaut, J., Verschuren, D., Charlet, F., Kristen, I., Fagot, M., DeBatist, M., 2010. The seismic – stratigraphic record of lake-level fl uctuations in Lake Challa: hydrological stability and change in equatorial East Africa over the last 140 kyr. Earth and
Planetary Science Letters 290, 214 – 223.
Moore, J.E.S., 1903. The “ Tanganyika Problem ” . The Geographical Journal. The Royal
Geographical Society (with the Institute of British Geographers).
Morley, C.K., 1988. Variable extension in Lake Tanganyika. Tectonics 7, 785 – 801.
Mortimer, E., Paton, D., Scholz, C.A., Strecker, M., Blisniuk, P., 2007. Orthogonal to oblique rifting: effect of rift basin orientation in the evolution of the North Basin,
Malawi Rift, East Africa. Basin Research 19, 393 – 407.
Ng'ang'a, P., 1993. Deltaic sedimentation in a lacustrine environment Lake Malawi,
Africa. Journal of African Earth Sciences 16, 253 – 264.
Oldham, R.D., 1922. The African Rift Valleys. The Geographical Journal 59, 151.
Owen, B., Crossley, R., 1989. Rift structures and facies distributions in Lake Malawi.
Journal of African Earth Sciences 8, 415 – 427.
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Owen, R.B., Crossley, R., Johnson, T.C., Tweddle, D., Korn fi eld, I., Davison, S., Eccles, D.H.,
Engstrom, D.E., 1990a. Major low lake levels of Lake Malawi and their implications for speciation rates in cichlid fi shes. Proceedings of the Royal Society of London,
Series B, Biological Sciences B240, 519 – 553.
Owen, R.B., Crossley, R., Johnson, T.C., Tweddle, D., Korn fi eld, I., Davison, S., Eccles, D.H.,
Engstrom, D.E., 1990b. Major low lake levels of Lake Malawi and their implications for speciation rates in cichlid fi shes. Proceedings of the Royal Society of London.
Series B, Biological Sciences B 240, 519 – 553.
Park, L.E., Cohen, A.S., 2011. Paleoecological response of ostracods to Early – Late
Pleistocene lake level changes in Lake Malawi, East Africa. Palaeogeography,
Palaeoclimatology, Palaeoecology 303, 71 – 80 (this issue).
Partridge, T.C., Kerr, S.J., Metcalfe, S.E., Scott, L., Talma, A.S., Vogel, J.C., 1993. The Pretoria
Saltpan: a 200,000 year Southern African lacustrine sequence. Palaeogeography,
Palaeoclimatology, Palaeoecology 101, 317 – 337.
Patterson, G., Kachinjika, O., 1995. Limnology and phytoplankton ecology. In: Menz, A.
(Ed.), Natural Resources Inst., Chatham, UK, pp. 1 – 67.
Pilskaln, C.H., 2004. Seasonal and interannual particle export in an African rift valley lake: a 5-year record from Lake Malawi, southern East Africa. Limnology and
Oceanography 49, 964 – 977.
Pilskaln, C.H., Johnson, T.C., 1991. Seasonal signals in Lake Malawi sediments.
Limnology and Oceanography 36, 544 – 557.
Powers, L.A., Johnson, T.C., Werne, J.P., Castañeda, I.S., 2005. Large temperature variability in the southern African tropics since the Last Glacial Maximum.
Geophysical Research Letters 32, L08706.
doi:10.1029/2004GL022014 .
Powers, L.A., Werner, J.B., Castañeda, I.S., Johnson, T.C., Hopmans, E.C., Sinninghe Damstéc, J.S.,
Schouten, S., 2011. Organic geochemical records of environmental variability in Lake
Malawi during the last 700 years, part I: The Tex
86
Temperature Record. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 133 – 139 (this issue).
Reinthal, P., Cohen, A.S., Dettman, D.L., 2011. Fish fossils as paleo-indicators of ichthyofauna composition and climatic change in Lake Malawi, Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 303, 126 – 132 (this issue).
Roberts, E.M., O'Connor, P.M., Gottfried, M.D., Stevens, N., Kapalimad, S., Ngasalad, S.,
2004. Revised stratigraphy and age of the Red Sandstone Group in the Rukwa Rift
Basin, Tanzania. Cretaceous Research 25, 749 – 759.
Rosendahl, B.R., 1987. Architecture of continental rifts with special reference to East
Africa. In: Wetherill, G.W. (Ed.), Ann. Rev. Earth Planet. Sci., 15, pp. 445 – 503.
Rosendahl, B.R., Livingstone, D.A., 1983. Rift Lakes of East Africa: New Seismic Data and
Implications for Future Research. , pp. 14 – 19.
Rossignol-Strick, M., 1985. Mediterranean Quaternary sapropels, an immediate response of the African monsoon to variation of insolation. Palaeogeography,
Palaeoclimatology, Palaeoecology 49, 237 – 263.
Schlische, R.W., 1995. Geometry and origin of fault-related folds in extensional settings.
AAPG Bulletin 79, 1661 – 1678.
Schnurrenberger, D., Russell, J., Kelts, K., 2003. Classi fi cation of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29, 141 – 154.
Scholz, C.A., 1995. Deltas of the Lake Malawi Rift, East Africa: seismic expression and exploration implications. AAPG Bulletin 79, 1679 – 1697.
Scholz, C.A., Rosendahl, B.R., 1988. Low lake stands in Lakes Malawi and Tanganyika,
East Africa, delineated with multifold seismic data. Science 240, 1645 – 1648.
Scholz, C.A., Rosendahl, B.R., Scott, D.L., 1990. Development of coarse-grained facies in lacustrine rift systems: examples from East Africa. Geology 18, 140 – 144.
Scholz, C.A., Johnson, T.C., Cohen, A.S., King, J.W., Peck, J., Overpeck, J.T., Talbot, M.R.,
Brown, E.T., Kalindekafe, L., Amoako, P.Y.O., Lyons, R.P., Shanahan, T.M., Castaneda, I.S.,
Heil, C.W., Forman, S.L., McHargue, L.R., Beuning, K.R., Gomez, J., Pierson, J., 2007. East
African megadroughts between 135 – 75 kyr ago and bearing on early-modern human origins. Proceedings of the National Academy of Sciences 104, 16416 – 16421.
Scholz, C.A., Talbot, M.R., Brown, E.T., Lyons, R.P., 2011. Lithostratigraphy, physical properties and organic matter variability in Lake Malawi Drillcore sediments over the past 145, 000 years. Palaeogeography, Palaeoclimatology,
Palaeoecology 303, 38 – 50 (this issue).
Scott, D.L., Ng'ang'a, P., Johnson, T.C., Rosendahl, B.R., 1991. High-resolution acoustic character of Lake Malawi (Nyasa), east Africa and its relationship to sedimentary processes. In: Anadon, P., Cabrera, L., Kelts, K. (Eds.), Lacustrine Facies Analysis. Sp.
Pub. of IAS, Blackwell, Oxford, pp. 129 – 146.
Scott, D.L., Braun, J., Etheridge, M.A., 1994. Dip analysis as a tool for estimating regional kinematics in extensional terranes. Journal of Structural Geology 16,
393 – 401.
Short, D.A., Mengel, J.G., Crowley, T.J., Hyde, W.T., North, G.R., 1991. Filtering of
Milankovitch cycles by Earth's geography. Quaternary Research 35, 157 – 173.
Smith, B., Rose, J., 2002. Uganda's Albert graben due fi rst serious exploration test. Oil &
Gas Journal 100, 42 – 48.
Soreghan, M.J., Scholz, C.A., Wells, J.T., 1999. Coarse-grained deep-water sedimentation along a border fault margin of Lake Malawi, Africa: seismic stratigraphic analysis.
Journal of Sedimentary Research 69, 832 – 846.
Specht, T.D., Rosendahl, B.R., 1989. Architecture of the Lake Malawi Rift, East Africa.
Journal of African Earth Sciences 8, 355 – 382.
Stone, J.R., Westover, K.S., Cohen, A.S., 2011. Late Pleistocene paleohydrography and diatom paleoecology of the central basin of Lake Malawi, Africa. Palaeogeography,
Palaeoclimatology, Palaeoecology 303, 51 – 70 (this issue).
Stute, M., Talma, S., 1998. Glacial temperatures and moisture transport regimes reconstructed from noble gas and δ 18O, Stampriet aquifer, Namibia. Isotope
Techniques in the Study of Past and Current Environmental Changes in the
Hydrosphere and the Atmosphere: Proceedings of Vienna Symposium 1997, IAEA,
Vienna, SM-349/53, pp. 307 – 328.
Suess, E., 1891. Die Brueche de ostlichen Afrikas, Denkschr. Akad, Wiss, Wien 553,
1 – 580.
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3 – 19
Talbot, M., Jensen, N., Laerdal, T., Filippi, M., 2006. Geochemical responses to a major transgression in giant African Lakes. J. Paleolimnol. 35, 467
East Africa: an updated broad review. In: Odada, E.O., Olago, D.O. (Eds.), The East
African Great Lakes: Limnology, Paleolimnology and Biodiversity. Kluwer,
Dordrecht, pp. 3 – 60.
Tiercelin, J.J., Soreghan, M., Cohen, A.S., Lezzar, K.E., Bouroullec, J.L., 1992. Sedimentation in large rift lakes: example from the middle Pleistocene — modern deposits of the
Tanganyika Trough, East African Rift system. Bull. Centres Rech. Explor.-Prod. Elf
Aquitaine 16, 83 – 111.
Tierney, J.E., Russell, J.M., Huang, Y., Sinninghe Damsté, J.S., Hopmans, E.C., Cohen, A.S.,
2008. Northern Hemisphere controls on Tropical Southeast Africa climate during the past 60,000 years. Science 322, 252 – 255.
Trauth, M.H., Deino, A., Bergner, A.G.N., Strecker, M.R., 2003. Earth Planet. Sci. Lett. 206,
297 – 313.
Trauth, M.H., Maslin, M.A., Deino, A., Strecker, M.R., Bergner, A.G.N., Dühnforth, M.,
2007. High- and low-latitude forcing of Plio-Pleistocene African climate and human evolution. Journal of Human Evolution 53, 475 – 486.
– 489.
Tiercelin, J.-J., Lezzar, K.E., 2002. A 300 million years history of rift lakes in Central and
19
Verheyen, E., Salzburger, W., Snoeks, J., Meyer, A., 2003. Origin of the super fl ock of cichlid fi shes from Lake Victoria, East Africa. Science 300, 325 – 329.
Wells, J.T., Scholz, C.A., Johnson, T.C., 1994. Highstand deltas of Lake Malawi, East Africa: environments of deposition and processes of sedimentation. In: Lomando, A.J.,
Schreiber, B.C., Harris, P.M. (Eds.), Lacustrine Reservoirs and Depositional Systems:
SEPM Core Workshop No. 19, pp. 1 – 35.
Weninger, B., Jöris, O., Danzeglocke, U., 2005. CalPal-2007. Cologne Radiocarbon
Calibration & Palaeoclimate Research Package.
http://www.calpal.de/ . accessed 2005.
Wheeler, W., Karson, J.A., 1989. Structure and kinematics of the Livingstone Mountains border fault zone, Nyasa (Malawi) Rift, southwestern Tanzania. Journal of African
Earth Sciences 8, 393 – 413.
Woltering, M., Johnson, T.C., Werne, J.P., Schouten, S., Sinninghe Damsté, J.S., 2011. Late
Pleistocene temperature history of Southeast Africa: a Tex
86 temperature record from Lake Malawi. Palaeogeography, Palaeoclimatology, Palaeoecology 303,
93 – 102 (this issue).
Yemane, K., Siegenthaler, C., Kelts, K., 1989. Lacustrine environment during Lower
Beaufort (Upper Permian) Karoo deposition in northern Malawi. Palaeogeography,
Palaeoclimatology, Palaeoecology 70, 165 – 178.
