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
Scientific drilling in the Great Rift Valley: The 2005 Lake Malawi Scientific Drilling
Project — An overview of the past 145,000 years of climate variability in Southern
Hemisphere East Africa
C.A. Scholz a,⁎, A.S. Cohen b, T.C. Johnson c, J. King d, M.R. Talbot e, E.T. Brown c
a
Department of Earth Sciences, Syracuse University, Syracuse NY, 13244, USA
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
b
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
Scientific 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 scientific drill cores from Lake Malawi, the first long and continuous, highfidelity 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 Palaeo3, 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 scientific 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 (Fig. 1). Malawi and the other great lakes of the region
(Tanganyika, Victoria, Edward, Albert, Kivu and Turkana) are famous
for their large numbers of fish and invertebrate species (in particular
the cichlid fishes) (e.g. Verheyen et al., 2003). 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: [email protected] (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 scientific drilling in the deep lake waters of Africa's Great
Rift Valley (Lewin, 1981).
Initial scientific exploration of the Great Rift Valley ensued shortly
after the European settlement, with seminal publications by Suess
(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 significantly 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
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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 fine-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 scientific
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 scientific 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 affiliated sites, covering the time interval
of the middle–Late Pleistocene through Holocene, mainly focusing on
analyses and results from the 2005 scientific drill core 1C, as well as cores
from Site 2. These papers include a review of basin framework (Lyons et
al., 2011-this issue), and studies of lithostratigraphy and geochemistry
(e.g. Brown, 2011-this issue; Johnson et al., 2011-this issue; McHargue et
al., 2011-this issue; Scholz et al., 2011-this issue; Woltering et al., 2011this issue) and paleobiology (Beuning et al., 2011-this issue; Park and
Cohen, 2011-this issue; Reinthal et al., 2011-this issue; Stone et al., 2011this issue) from cores from holes 1C, 2A and 2B. Companion studies from
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 first reports of the geology of the Lake Malawi (Nyasa) region
date from the early part of the 20th century (e.g. Dixey, 1926) 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) (Fig. 2). The metamorphic
basement of the area is composed of greenschist–amphibolite grade
rocks, and is affiliated 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. Furman, 1995;
Harkin, 1960), which is one of the three late-Cenozoic volcanic centers
located in the western branch of the EARS (Ebinger, 1989) (Fig. 2).
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 Lyons, 2009).
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
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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 reflection
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 (Fig. 3). For instance the Lake
Malawi rift is comprised of three main linked half-graben basins,
which alternate in polarity along axis (Specht and Rosendahl, 1989)
(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 (Ebinger,
1989; Rosendahl, 1987) (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. Wheeler and Karson,
1989). These mountains commonly rise 1000–1500 m above the
adjacent lake surface, and define 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 (Ebinger et al., 2002).
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 reflection data also suggest some
amount of oblique-slip deformation in that area (Mortimer et al.,
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 (Soreghan et al., 1999), 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 (Specht and Rosendahl, 1989).
However in several localities these basement-involved fault systems
produce relief on the modern lake floor, 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 significant down-slope
sediment transport systems, facilitating gravity flows, and especially
turbidity flows, over broad areas of the basin (e.g. Ng'ang'a, 1993;
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 influenced 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. Cane et al.,
1994; Goddard and Graham, 1999; Marchant et al., 2007). Sedimentation in Lake Malawi is markedly influenced 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
stratification 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 outflow 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 fluctuations 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. Lyons et al., 2010;
Owen et al., 1990a,b), and where lake level drops of several hundred
meters can occur in a few thousand years or less.
4. Significance 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-field
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
1989). 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 floor (e.g. Scott et al., 1991), and
therefore impact down-slope sediment transport processes into the
basin, as well as the final position of associated deposits. High-density
grids of high-resolution seismic reflection data were acquired using
small airguns as the seismic source, and these grids, nested within the
sparsely spaced multichannel seismic reflection data, were most
suitable for locating the drill sites (Lyons et al., 2011-this issue)
(Fig. 4).
The Lake Malawi sediment record is a proven, high-sensitivity
archive of subtle shifts in climate (Finney and Johnson, 1991; Johnson
et al., 2002; Owen et al., 1990a,b). Sediment core and geophysical data
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. Finney and Johnson,
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 flexural margins of half-graben basins in
Lake Malawi (e.g. Scholz and Rosendahl, 1988). 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 N500 m (Fig. 1).
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 flows, mass wasting events, and
the construction of major sublacustrine fan complexes that can extend
across the floors 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
flow deposits have accumulated.
The basin scale structure of the rift was assessed through regional
multichannel seismic reflection studies (e.g. Specht and Rosendahl,
Fig. 4. Representative seismic reflection data from Lake Malawi. A) Regional
multichannel seismic reflection profile showing full sedimentary section, pre-rift
basement, and location of scientific drill core. B) High-resolution single-channel airgun
seismic profile 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
reflection data.
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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 (N2 million years), such sediments are prone to organic matter
diagenesis and thermogenic maturation, and can generate significant
and even economic accumulations of hydrocarbons, for example as in
Lake Albert, Uganda (Smith and Rose, 2002). Scientific drilling systems
such as those used by the Integrated Ocean Drilling Program (IODP) drill
ship JOIDES Resolution, and the Lake Malawi Scientific Drilling Project
generally do not return circulating drilling fluids 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 reflection data for signs of seismic amplitude anomalies
that might suggest subsurface gas or fluids. 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 highresolution 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 (Figs. 1 and 4).
6. Drilling engineering and field operations
7. Methods of sediment drill core analyses
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
1A
1B
1C
1D
2A
2B
2C
11
11
11
11
10
10
10
34
34
34
34
34
34
34
a
17.6387 S
17.6814 S
17.6575 S
17.6183 S
01.0597 S
01.0567 S
01.0532 S
26.2331
26.1793
26.1462
26.1469
11.1607
11.1527
11.2067
E
E
E
E
E
E
E
Corrected for initial over-penetration by 6.51 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
(Fig. 5). 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 (Fig. 5). 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 refit work was
completed in the ship yard in Monkey Bay, Malawi, and the final
mobilization of the Seacore drill rig aboard the Viphya was completed
on the jetty in the port of Chipoka, Malawi.
Mobilization for field 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 modification 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 offices
(Fig. 5) (http://malawidrilling.syr.edu/photos/Outreach%20Program/
index.html). Technical challenges early in the field operations
included initial difficulty 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.
Water depth
Total depth
(mblf)
592
592
592
592
359
359
359
47.6
380.7
89.9a
21.0
41.1
40.1
37.0
7.1. Logging, core processing and initial core descriptions
Logging in the field 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 field 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 (Schnurrenberger et al.,
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 scientific 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 LatePleistocene and Holocene cores from Lake Malawi have demonstrated
the efficacy of dating of organic-rich bulk samples (Johnson et al.,
2002), 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 field intensity), and 10Be, and 10Be/9Be 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.,
1992). Magnetic measurements of anhysteretic remanent magnetization (ARM) and natural remanent magnetization (NRM) before and
after step-wise increasing alternating field 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 fills 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
quantified 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
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3–19
presented in Scholz et al. (2011-this issue). Key geochemical measures of
the core (e.g., total organic carbon, total organic nitrogen, Ca abundance,
and δ13C 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 fluorescence
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, CaCO3 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 fish and the possible impact of
major hydrologic changes on fish 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 (TEX86) 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
refinement of this method, presenting results of TEX86 analyses of
samples from short cores from northern Lake Malawi spanning the past
700 years. In a parallel study from the same short core, Castañeda et al.
(2011-this issue) describe biomarker evidence for recent changes in
primary productivity from the same 700 year record. Woltering et al.
(2011-this issue) present a 74,000 year Tex86 record from Lake Malawi
sediments, subsampled from Malawi core 2A.
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. Brown et al., 2007),
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 14C, paleointensity, OSL and inclination age dates is
presented in Table 2. In the interval 0–52 ka the radiocarbon ages
were used to define 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 (Fig. 6) (Scholz
et al., 2007). 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 profiles provide the regional
and deep stratigraphic context for evaluating dense grids of singlechannel high-resolution seismic reflection data that are tied to the
drill cores and the detailed paleoclimate measurements (Fig. 4)
(Lyons et al., 2011-this issue). Important elements of the stratigraphic section observed in high-resolution seismic reflection data
include stacked progradational deposits, clearly identified as deltaic
deposits associated with much lower stages of Lake Malawi (e.g.
Lyons et al., 2011-this issue; Scholz, 1995) (Fig. 4). 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 Lyons et al., 2011-this issue).
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 firm 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
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Lake
Malawi 1C/13P
Malawi 1C/13P
Malawi 1C/13P
Malawi 1C/13P
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Malawi 1C
Depth below lake floor
Type of date
Lab number
14
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
15.806
21.053
17.5
21
32
39.5
46
72.5
84.5
67.25
14
AA34424
AA34425
AA34426
AA34427
AA71824
AA71820
AA34428
AA71821
AA65692
AA71823
AA65008
AA71822
AA65693
AA65694
AA65009
AA65010
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
C
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
14
C age
±Error
Calendar age
(kyr BP)
45
45
55
55
52
59
75
62
92
95
110
120
150
240
440
2800
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
36.139
50.457
39
53
67
80
83
114.5
135
122.5
±Error
(kyr BP)
0.067
0.05
0.095
0.09
0.138
0.079
0.145
0.313
0.222
0.18
0.47
0.267
0.427
0.214
0.423
3.243
15
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11
Fig. 6. A) Age–depth relationship for Site 1C drill hole. See text and Table 2 for details of geochronology. B) Paleointensity profiles correlating sites 1 and 2 with the Somalia Basin
record of Meynadier et al. (1992), core 85–674.
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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) (Stone et al., 2011-this issue); total pollen accumulation rate (PAR) (Beuning et al., 2011-this issue); Lake Malawi TEX86 (Site 2) (Woltering et al.,
2011-this issue); 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 (Lyons et al., 2011-this issue).
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 (Fig. 7). The quantification of
lithology was then completed using sediment physical properties,
core imagery, and geochemical methods.
Sediment density was quantified 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 (Figs. 7 and 8). 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
significant 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 quantification 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 (Figs. 7 and 9). 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 (Figs. 7, 9, and 10).
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 reflective 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 (Stone et al., 2011-this issue).
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/cm2/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 brushfires. According to Beuning et al. (2011-this issue)
such pollen spectra are indicative of climate regimes with b800 mm/
year rainfall availability. Following this severe megadrought interval,
woodland taxa rose significantly, 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
significant 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 TEX86 paleothermometer, based on Crenarchaeota picoplankton (Woltering et al., 2011this issue). Reinthal et al. analyzed remains of fish scales, bones and
teeth, and their δ13C isotopic values of fish 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. 9).
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.
oxidized coatings provide additional constraints on lake levels and
paleolimnological conditions over the past 145,000 years (Park and
Cohen, 2011-this issue) (Figs. 8–10). Few ostracod occurrences are
noted in the upper ~ 30 m of core at the deep site, suggesting a
stratified 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 (Fig. 9 and 10). 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 monospecific assemblages
of Limnocythere spp. are typical of littoral environments in highly
alkaline and saline African lakes such as Lake Turkana (Cohen et al.,
1983, 2007). These Limnocythere assemblages are also characterized
by high adult/juvenile ratios, limited decalcification, carbonate coatings, and valve abrasion, indicating shallow water and calcium
carbonate supersaturated conditions (Cohen et al., 2007; Park and
Cohen, 2011-this issue) (Fig. 9).
Pollen profiles acquired from Hole 1C reveal a history of significant
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 floral 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
9. Synthesis of Malawi lake levels and climate variability of the
past 145,000 years
Lake Malawi climate variability over the past 145 ka is best
separated into two main intervals, pre- and post-60,000 years before
present (Fig. 10). 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 reflect 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, stratified 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
reflection 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).
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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 Stone et al., 2011-this issue). 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 unidentified 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 reflections observed in the North Basin (Lyons et al., this
issue), which provides an indication of shoreline position.
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 CaCO3 content. Vivianite crystals are
observed in the wet-sieved fraction (Cohen et al., 2007), indicating
stratification 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 definitive
stratigraphic ties to seismically-identified 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 medium-fine 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 reflection 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 fluctuating 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 identified at
−350 m below modern lake level (e.g. Lyons et al., 2011-this issue).
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 stratified.
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 reflection 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 finely-laminated, and indicative of intervals of high lake level, water column stratification, and bottom water anoxia. E) Principal component (PC-1) of diatoms and other
paleoecological indicators (from Stone et al., 2011-this issue). Elevated PC-1 indicates periods when the lake was alkaline, saline to mildly saline, with much lower water levels. F) Ca
abundance from scanning X-ray fluorescence. 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 stratification
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 significant
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 CaCO3–high δ13C 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 fluctuating 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
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C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3–19
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
Lake levels during this interval are generally high, with subtle
variability as observed on the sediment core diatom record (Stone
et al., 2011-this issue). Ostracods are lacking through virtually the
entire interval, but occasional occurrences of diatoms that tolerate
elevated alkalinity suggest fluctuating levels, however briefly.
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 flora, 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 fluctuating lake levels in the seismic reflection
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. Brown et al.
(2007) report on millennial-scale variability in bulk elemental ratios
of Zr:Ti, which reflects 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 TEX86 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 hydrologicallyclosed and possibly desiccated (Beuning et al., 1997); and Lake
Edward (− 37 m, McGlue et al., 2006) (Fig. 1). 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 scientific drill cores. Finney and
Johnson (1991) 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 (Johnson et al., 2002), and 2) in high-resolution
seismic reflection 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 significant impacts at the LGM, there are
discernable shifts in temperature of about 3–4 °C, determined from
TEX86 analyses (Powers et al., 2005). 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 profiles (Johnson et al., 2002,
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 TEX86 paleothermometer is somewhat stronger, with a N5 °C change over the past
60,000 years (Tierney et al., 2008) compared to ~3–4° in Lake Malawi
(Woltering et al., 2011-this issue). Tierney et al. (2008) 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 TEX86 analyses from Site 2 in the north basin (Woltering et
al., 2011-this issue) hint at pronounced variability in period before
60,0000 years before present. The largest temperature variability
observed in the Site 2 TEX86 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
present (Fig. 10). 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. Deino et al.,
2006; Kutzbach and Street-Perrott, 1985). Scholz et al. (2007) 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. Clement et al., 2004). 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 influences on tropical climate may
C.A. Scholz et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 3–19
prevail (Chiang et al., 2003). The megadrought intervals interpreted
from our records correspond to zones of elevated Ca and diminished
TOC, which generally occur during insolation minima (Fig. 10). We
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 intensification 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
(Fig. 10).
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.
Moernaut et al., 2010), 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. Clement et al. (2004) 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 influence 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 TEX86, 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 scientific 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
17
work will also be required in order to fully constrain and quantify past
climate boundary conditions in the continental tropics.
Acknowledgements
Many individuals and organizations contributed to the successful
planning and execution of the field 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 Scientific Drilling program for financial support.
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