An NSF/ICDP Workshop on
October 10-16, 1999
Scientific Drilling on Lakes Malawi and Tanganyika
Club Makakola, on the southwestern lakeshore of Lake Malawi
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During the week of October 10, 1999, a group of 47 scientists, engineers, local institutional
representatives, and funding agency administrators met at Club Makakola, Malawi, on the
shore of Lake Malawi, to review the prospects for scientific drilling on Lakes Malawi
and Tanganyika. The meeting was opened by the Hon. Harry Thomson, Minister for Natural
Resources and Environmental Affairs of Malawi, and was followed by an open discussion of
the natural resources of Lake Malawi. Participants included representatives from Malawi,
Tanzania, the United States, and 8 other nations in Africa and Europe. Support for the
workshop was provided by the U.S. National Science Foundation (Paleoclimate, Continental
Dynamics, and Geology and Paleontology programs), and from the International Continental
Scientific Drilling Program. Major topics considered during the meeting included reviews
of the major scientific themes to be addressed through drilling, roles of local
institutions, engineering and logistical concerns, and the funding environments within
likely target funding agencies.
Many of the workshop discussions were centered around the current proposal submitted to
the ICDP and NSF for the GLAD800 drilling rig, a lightweight coring system designed for
sampling lake basins to a total drilling depth of 800-1200 meters. Administrative, financial
and engineering infrastructure of the GLAD800 concept were considered during this opening
session.Following brief summaries of the existing science programs, several speakers
presented science reviews in the areas of crustal structure and rift basin evolution,
paleoclimatology, environmental background to human origins, paleoecology and evolutionary
biology, and geochronology and paleomagnetic studies in lacustrine basins. Several breakout
sessions provided the opportunity for international groups to consider key questions to be
addressed during scientific drilling. Summary white papers on the four main scientific
themes are presented following this summary. A 1-day field trip to the Lake Malawi port
facility and shipyard at Monkey Bay, the container port at Chipoka, and to the Limnological
Field Station at Senga Bay provided participants with a sense of local infrastructure
around the lakeshore.
Following the science reviews, there were several plenary discussions covering the
following topics: Roles for Local Institutions, Reviews of Seismic Reflection Site Survey
Data, Engineering and Logistical Issues, and Funding Conditions within Target Funding
Roles for Local Institutions
Developing opportunities for capacity building within the geosciences community in East
Africa is a high priority objective of the local institutions that were represented at the
workshop. The drilling program should develop a plan for expanding educational opportunities
at various levels, including training of graduate students at local universities and
abroad, and enhancing professional growth of the present staff of professionals in
University and government positions. In addition to training, it is important for the
project to explore opportunities for improving the equipment and instrumentation
infrastructure at Universities and within government-supported laboratories.
In addition to capacity building, a top priority for local governments is to expand the
understanding of the regional geology of Lakes Malawi and Tanganyika. In order for
countries such as Malawi and Tanzania to best manage their natural resources, it is
critical to improve the state of knowledge of the subsurface structure, stratigraphy
and lithofacies variability of the lakes of the western branch of the rift valley.
Reviews of Seismic Site Survey Data
Considerable amounts of seismic reflection data of different type and vintage have been
acquired on Lakes Malawi and Tanganyika. In the mid-1980's Project PROBE acquired about
4500 km of 24-fold seismic reflection data on the two lakes, in reconnaissance grids with
line spacing s of 8-16 km. These data show that the lakes are each underlain with upward
of 5-6 km of syn-rift lacustrine sediment. In the case of Lake Malawi, which has the
greatest concentration of data, it appears that the sedimentary section thins from a
maximum of 5-6 km in the northern part of the rift valley to a few hundred m thickness at
the southernmost part of the lake. Since 1992, several medium-resolution single-channel
seismic surveys have been undertaken over selected areas on both lakes. These seismic
images provide information on the seismic stratigraphy of the lakes to 1-2 km sub-bottom,
at a vertical resolution of 1-2 m. These data reveal the complex facies geometries
evidently inherent on such tropical lakes, and reveal that several unconformities exist in
the stratigraphic section. In the case of Lake Malawi, shallow unconformities produced by
dramatic drops in lake level are observed in water depths of more than 500 m, suggesting
that it will be necessary to drill in water depths >500 m in order to obtain a continuous
stratigraphic record in that lake.
Engineering, logistics, and drilling strategies
Several engineering presentations covered the proposed GLAD 800 lake drilling rig,
Ocean Drilling Project- type drilling tools, and deployment of this equipment on a modular
barge system with dynamic positioning. The GLAD 800 concept was originally proposed for
small-medium lakes, but is also adaptable for large lake drilling, including extending the
drill string to 1200 m from 800 m, through use of a narrower-diameter drill string. Thus
the GLAD rig appears to be adequate for subsurface sampling to depths of several hundred
meters in both Malawi and Tanganyika. ODP drilling/sampling tools, including the Advanced
Piston Corers, are effectively off-the-shelf technology and are easily fit to the GLAD rig.
Adapting local vessels for drilling operations is probably the most economical means of
providing a drilling platform for a single drilling operation. However it was demonstrated
that over the lifespan of several large lake projects it may be cost effective to acquire
a modular (@C-Float@) barge system to serve as a drilling platform on successive lake
drilling projects. Options for positioning the drilling platform included an anchor
mooring system, dynamic positioning system, or hybrid system involving a combination of
dynamic positioning and anchoring technologies. Whereas anchoring/mooring systems are
simple and robust, they are slow to deploy and require a large anchor-handling tug for
mooring deployment. Dynamic Positioning Systems involve large up-front costs and require
careful maintenance and oversight during operations, but offer the greatest flexibility
and rapid deployment capability. Given the rapidly changing sea states on the lakes,
this may be the most advisable choice of positioning technology, according to the
engineering team present at the workshop.
Several strategies were discussed for initial drilling, which will likely be proposed
first for Lake Malawi, on account of its favorable infrastructure and thorough site
survey geophysical data sets already in-hand. Since distances across Lake Malawi are
significant, it is likely that 2 support vessels will be required for servicing the
drilling platform. Additionally, drilling operations will likely be carried out on a
24 hour per day, 7 day per week basis during the favorable weather window (December-March).
It is unlikely that the GLAD drilling rig will be able to operate in sea states greater
than 1-2 m, therefore it will be necessary to factor in weather delays when developing
drilling operation time lines.
Reviews of funding environments at NSF and ICDP
Several presentations considered the state of funding of the Earth System History Program
at the U.S. National Science Foundation, and at the International Continental Scientific
Drilling Program. These presentations placed the proposed scientific effort into a
realistic fiscal framework.The ICDP will fund up to 1/3 of total operational costs
for any given drilling program, with the balance of the funds to be derived from national
science agencies. In addition, ICDP is able to provide additional funding for ancillary
tasks such as downhole geophysical logging. It is expected that the total drilling budget
for a 4-site project will be in the vicinity of $2M, USD.
Excursions to scientific staging sites
October 14, we ran an excursion to the potential engineering staging sites at Monkey Bay,
Chipoka, and to the Lake Malawi Biodiversity Project lakeside facility at Senga Bay, a
possible site for core handling and conducting preliminary whole-core analyses. All three
sites were observed to be highly suitable. In Monkey Bay, local engineers provided a tour
of the local shipyard with its machine ships, slipway, two main piers, and dry-dock facility
. Three possible support vessels were in port in Monkey Bay, and seen by the participating
scientists. These vessels included the S/V Timba (Dept. of Surveys), the R/V Usipa (Lake
Malawi Biodiversity Project) and the R/V Ndunduma (Dept. of Fisheries). In addition, two
other vessels were inspected as possible drilling platforms: the barge Viphya (52 m), and
the container ship Katundu (~70 m), operated by Malawi Lake Services. The initial impression
of the engineer from Seacore Ltd., was that the barge Viphya, with modifications, would be
an adequate drilling platform for Lake Malawi.
- Several general recommendations to the scientific community
arose out of the workshop.
- An initial drilling program be proposed for Lake Malawi, given its
favorable local infrastructure and extensive seismic and sediment core datasets.
- Engineering scoping of possible platforms should continue. Cost/benefit analyses should
be carried out on platform options including
- modification of the existing barge Vipyha
on Lake Malawi and
- acquiring a modular C-Float type barge. This should be completed
within the context of a single Lake Malawi program, and alternatively, using a scenario
that includes drilling both Lake Malawi and Lake Tanganyika.
A local planning committee be established for prioritizing local needs.
- A European scientific participant committee be establish to explore avenues
of additional support from European science agencies.
- Likely scenarios for a Lake Malawi drilling program include proposals for 4 or 8 site
drilling program. A four-site drilling program would include
- a southern basin site that
would sample the southern lake/ southern latitude response to paleoclimate change, provide
nearshore sequences for evolutionary biology studies and also serve as an initial test site
for coring operations;
- a central basin site that will penetrate through the
Bruhnes-Matayama paleomagnetic reversal and provide an undisputed chronology for the
existing stratigraphic framework, and
- two northern basin sites that sample the rich
upwelling regions of the Livingstone Basin, along an offset drilling transect to extend
the record back beyond the Pleistocene. In an 8 site scenario, the offset transect would
be extended to include an additional 3-4 sites to provide a record of continuous
stratigraphic section back through the Pliocene.
- Following final agency decisions on the GLAD 800 proposal to ICDP/NSF, consult with
agency program managers and DOSECC on the best proposal framework for the project.
- Explore funding options for additional site survey activity on Lake Tanganyika. Because of
the greater depth and volume of Tanganyika, it was likely to have been a much more
substantial lake than Malawi during the severe arid intervals of the Pleistocene.
Thematic Science Summaries
The following pages contain four separate thematic summaries covering areas of Paleoclimate Studies,
Basin Evolution Studies,Evolutionary Biology, and Environmental Background to Human Origins
The large lakes of the East African Rift Valley are unique among the large lakes of the
world in terms of their sensitivity to climate change and their long, continuous,
high-resolution records of past climate change in the tropics. The paleoclimate group
identified three main questions to be addressed by deep drilling in Lake Malawi:
What is the climatic linkage between tropical Africa and the high latitudes at orbital and
Did tropical African climate predominantly respond to changes in low-latitude precessional
insolation (23-19 kyr) or high-latitude ice volume (100 kyr and 41 kyr) forcing?
- Has Lake Malawi always responded to southern hemisphere insolation forcing, as data for
the period since the last 40 ka suggest?
- Are high-frequency climate variations (analogous to Dansgaard-Oeschger or Heinrich events)
superimposed on glacial-interglacial timescale variations in wet and dry conditions, and
how have these varied over time?
- How has interannual African climate variability changed in association with longer-term climate variations?
- What are the dominant interannual modes of variability (ENSO, NAO)?
- How have these modes changes in association with changes in African climate?
- What is the long-term evolution of tropical East African climate?
- What is the dominant Milankovitch frequency back through time? i.e. do we see a shift from the present day 100 ka dominance to 41 ka dominance to 21 ka dominance, as observed in the marine record?
- In this region of tropical Africa, do we see a significant change in vegetation as the Earth shifted from a 41-ka world to a 100 -ka world?
What is the climatic linkage between tropical Africa and the high-latitudes?
Terrestrial and marine records of subtropical African paleoclimate variability during the
late Pleistocene document the dual but separate influences of high- and low-latitude
processes (deMenocal et al., 1993; deMenocal, 1995; Clemens et al., 1996). The evolution of
African climate over the last five million years reflects changes in the relative influence
of these end-member climate forcing factors. Prior to the onset of Northern Hemisphere
glacial cycles near 2.8 Ma, African climate evidently responded primarily to variations
in monsoonal circulation due to orbital changes in low-latitude insolation. Following the
growth of Northern Hemisphere glacial cycles after 2.8 Ma, African climate was evidently
subjected to periodic and large amplitude cool, dry cycles which were in-phase with
high-latitude glacial maxima. The coupling between high- and low-latitude climate increased
toward the present, with significant increases in the amplitude of glacial arid cycles in
subtropical African occurring at ~1.7 Ma and ~1.0 Ma (deMenocal, 1995). These observations
are based on ODP cores from the deep sea off Africa, and reflect a broad-scale integration
of signals from across much of the African continent. However there are regional
differences in climate evolution within east Africa on orbital time scales. One example of
this is the out-of-phase relationships in lake level between Malawi and the large lakes to
the north (Finney and Johnson, 1991; Finney et al., 1996; Johnson, 1996). Such regional
variability can only be detected by drilling the individual lakes across a latitudinal
gradient of the African continent.
African paleoclimate variability during the late Pleistocene also exhibited strong
millennial-scale variability, also associated with changes in high-latitude temperature
changes. In the North Atlantic this variability consists of ~1.5 ka Dansgaard-Oeschger
cycles, which are bundled into longer, larger amplitude Heinrich events, which recur
every 5-10 ka (Bond et al., 1993). These same millennial-scale variations are also observed
in subtropical Africa (e.g. Younger Dryas in Lake Albert – Beuning et al., 1998 and in Lake
Magadi – Roberts et al., 1994), and in marine sediments from the Atlantic Ocean and Arabian
Sea, again documenting the close linkage between high- and low-latitude climates, although
the causal mechanisms of this linkage remain obscure.
Finally, well-dated, detailed reconstructions of African paleoclimate variability indicate
that the climate transitions themselves are extremely rapid, with large transitions
occurring within decades to centuries (Gasse and VanCampo, 1992; Johnson et al., 1996;
deMenocal et al., 1999; Street-Perrott and Perrott, 1990).
Are high-frequency climate variations (analogous to
Dansgaard-Oeschger or Heinrich events) superimposed on glacial-interglacial timescale
variations in wet and dry conditions?
Grounds for anticipating centennial- to millennial-scale variations are provided by records
of the transition from LGM and Holocene conditions in Lake Malawi and other lakes of tropical
Africa. Geomorphological evidence (strandlines, deltas, incised fluvial channels, etc.)
indicate generally low-lake or dry basin conditions at the LGM and rising levels from 15 to
12 ka in equatorial Africa. This rising trend was interrupted by a number of short-lived
reversals, which are also reflected in palynological records and several geochemical and
biological palaeolimnological proxies preserved in lacustrine sediments. These indicate
regional excursions in temperature, P/E balance or wind-driven vertical mixing of the water
bodies. Interglacial conditions, as reflected in Holocene proxy records, also contain
evidence of short-term climatic reversals. Some of these began and ended abruptly. The
transition to drier conditions that led to early Holocene low levels in L. Malawi seems
to have occurred within a few hundred years around 9.8 14C ka (Ricketts and Johnson, 1996),
while the period of particularly high early to mid-Holocene levels in most north African
lakes ended equally abruptly around 4.0 14C ka. Some events seem to be local equivalents of
high-latitude climatic excursions, such as the Younger Dryas and "8.2 ka event" (Alley et
al., 1997), but others have as yet no recognized equivalents outside Africa.
How does interannual African climate variability change in
association with longer-term climate variations?
Laminated sediments recovered in box cores and multi-cores from northern Lake Malawi
consist of alternating bands of diatom ooze (light laminations) and silty clays (dark
laminations), representing the dry, windy season (June-September) and the warm, rainy
season (December-March), respectively. Pb-210 dating of several cores has demonstrated
that the laminations are varves. Similar annually-laminated sediments occur in deep water
cores from Lake Tanganyika. The thickness of the light layers and dark layers can be
measured quite precisely using computerized image enhancement and analysis tools.
Presumably, the thickness of the light layers is linked to upwelling intensity and diatom
productivity, whereas the thickness of the dark layers is a proxy for annual rainfall
intensity. Spectral analysis of lamination thickness variability over the past 300 years
reveals significant cyclicity at ENSO frequencies (Barry, in prep).
The sediments in the north basin of Lake Malawi are almost continuously varved from the
present back to about 2000 ybp, and between 6500 and 10000 C-14 ybp. Within these time
windows, varve thickness analyses will provide insight into the existence of climate cycles
on an interannual scale. Will the ENSO-scale cycles also dominate the early Holocene
record or will the early Holocene climate record be devoid of such variability, as appears
to the case in the Peruvian Andes (Rodbell et al., 1999). And with deep drilling in this
part of Lake Malawi, will we find similar inter-annual variability in the varved record
further back in time?
While short, discrete intervals of piston cores from sites further south in Lake Malawi
are laminated, the sediments are mostly homogeneous, even though they accumulated in anoxic
deep waters. This is probably due to long sediment transport pathways from the major rivers
in the north, resulting in a blurring of seasonal variability as the sediments settle, get
re-suspended and ultimately arrive at their final burial site.
What is the long-term evolution of tropical East African climate?
Evolution of the Milankovitch climate spectrum has been observed in both the marine
record (deMenocal and Bloementhal, 1996) and the record of continental lakes (Williams et
al., 1998) during the last three million years. The first major change in both records
occurs between 3.0 and 2.5 Ma, when power in the obliquity band (41 ka) increases at the
expense of power in the precessional band (21 ka). A second major change occurs at about
one million years ago when power in the eccentricity band (100 ka) increases at the expense
of the obliquity band. Pronounced depositional cyclicity, produced by high-amplitude
changes in lake level, is observed in both the Lake Malawi and Lake Tanganyika seismic
reflection records over the last half of the Pleistocene (Scholz, 1995; Lezzar et al., 1996)
. These acoustic facies couplets consist of hemipelagic high-stand deposits and
coarse-grained lowstand deposits, and are accumulating in both lake basins with a frequency
of about 100 kyr (Figure 1). Drilling in Lakes Malawi and Tanganyika will test this
depositional model and constrain both the timing and phasing of these coupled
highstand/lowstand packages. We will determine if changes in the cyclicity of the
sedimentary record of these lakes are similar to those changes observed in the marine
Figure 1. Acoustic Facies Couplets observed on central L. Malawi seismic profile,
suggesting 100 kyr lake level and depositional cyclicity.
We are confident that the suite of geochronometers available in Lake Malawi
sediments will provide sufficient control to develop a rigorous age-depth model.
Magnetostratigraphic and low-field-susceptibility measurements will provide the backbone
of our geochronological work. These techniques have been applied successfully in other
ancient lakes (i.e. Lake Baikal). This record will be calibrated using a series of
horizons dated by a range of independent techniques. Reliable radiocarbon chronologies
using woody material and charcoal exist for the last 20 ka. We anticipate that this
approach will provide age control to 40 ka. In addition, there are a series of
carbonate-rich horizons deposited during periods of evaporative concentration of lake
waters. These have the potential to be dated by uranium series techniques (isochron method,
e.g. Bischoff and Fitzpatrick, 1991), developing chronologies for the last 400 ka. Finally
, the numerous volcanic ash layers present, particularly in the northern basin, provide
opportunities for Ar-Ar dating. This should be possible for sediments older than 25 ka
given the abundance of crystalline material in some of the young ash layers in the northern
basin. In addition, trace element fingerprinting of these ash layers will provide a means
of developing stratigraphic linkages among cores
Alley, R.B., P.A. Mayewski, T. Sowers, M. Stuiver, K.C. Taylor, and P.U. Clark, Holocene
climatic instability: A prominent, widespread event 8200 yr ago, Geology, 25 (6), 483-486,
Barry, S., in prep. The high resolution stratigraphy of sediment cores from the northern
basin of Lake Malawi: varves, turbidites and tephras. M.S. thesis, Water Resource Sciences,
University of Minnesota
Bond, G., W.S. Broecker, S. Johnsen, J. McManus, L. Labeyrie, J. Jouzel, and G. Bonani,
Correlations between climate records from North Atlantic sediments and Greenland ice,
Nature, 365, 143-147, 1993.
- deMenocal, P.B., Plio-Pleistocene African Climate, Science, 270, 53-59, 1995.
- deMenocal, P.B., W.F. Ruddiman, and E.M. Pokras, Influences of high- and low-latitude
processes on African climate: Pleistocene eolian records from equatorial Atlantic Ocean
Drilling Program Site 663, Paleoceano., 8 (2), 209-242, 1993.
- deMenocal, P.B., and J. Bloemendal, Plio-Pleistocene subtropical African climate variability
and the paleoenvironment of hominid evolution: A combined data-model approach, in
Paleoclimate and Evolution With Emphasis on Human Origins, edited by E. Vrba, G. Denton, L.
Burckle, and T. Partridge, pp. 262-288, Yale University Press, New Haven, 1995.
- Finney, B. P. and Johnson, T. C., 1991. Sedimentation in Lake Malawi (East Africa) during
the past 10,000 years: a continuous paleoclimate recod from the southern tropics. Palaeo-3,
v. 85, 351-366.
- Finney, B. P., Scholz, C. A., Johnson, T. C., Trumbore, S. and Southon, J., 1996. Late
Quaternary lake level changes of Lake Malawi. In Johnson, T. C. and Odada, E. O. (eds.),
The Limnology, Climatology and Paleoclimatology of the East African Lakes: Gordon and Breach
, Amsterdam, 495-508.
- Gasse, F., V. Lédée, M. Massault, and J.C. Fontes, Water level fluctuations of Lake
Tanganyika in phase with oceanic changes during the last glaciation and deglaciation,
Nature, 342, 57-59, 1989.
- Gasse, F., R. Téhet, A. Durand, E. Gibert, and J.C. Fontes, The arid-humid transition in
the Sahara and Sahel during the last deglaciation, Nature, 346, 141-146, 1990.
- Gasse, F., and E. Van Campo, Abrupt post-glacial climate events in West Asia and North
Africa monsoon domains, Earth and Planet. Sci. Lett., 126, 435-456, 1994
- Johnson, T. C., 1996. Sedimentary processes and signals of past climatic change in the
large lakes of the East African Rift Valley. In Johnson, T. C. and Odada, E. O. (eds.),
The Limnology, Climatology and Paleoclimatology of the East African Lakes: Gordon and Breach
, Amsterdam, 367-412.
- Lezzar, K.E., Tiercelin, J.-J., De Baptist, M., Cohen, A.S., Bandora, T., Van Rensbergen,
P., Le Turdu, C., Mifundi, W., and Klerx, J., 1996, New seismic stratigraphy and
late-tertiary history of the north Tanganyika basin, East African Rift system, deduced
from multichannel and high-resolution reflection seismic data and piston core evidence,
Basin Research 8: 1-28.
- Ricketts, R. D. and Johnson, T. C., 1996. Early Holocene changes in lake level and
productivity in Lake Malawi as interpreted from oxygen and carbon isotopic measurements
of authigenic carbonates. In Johnson, T. C. and Odada, E. O. (eds.), The Limnology,
Climatology and Paleoclimatology of the East African Lakes: Gordon and Breach, Amsterdam,
- Rodbell, D.T., G.O. Seltzer, D.M. Anderson, M.B. Abbot, D.B. Enfield, and J.H. Newman, An
~15,000-year record of El Niño-driven alluviation in southwestern Equador, Science, 283,
- Scholz, C.A., 1995, Deltas of the Lake Malawi Rift, East Africa: Seismic Expression and
Exploration Implications, AAPG BULLETIN, v. 79, p. 1679-1697.
- Street-Perrott, F.A., and R.A. Perrott, Abrupt climate fluctuations in the tropics: the
influence of Atlantic Ocean circulation, Nature, 343, 607-612, 1990.
Basin Evolution Studies
Lakes Malawi and Tanganyika are among the largest Aclosed@ sedimentary systems on earth,
and hence are ideal sites for evaluating processes of basin evolution. Their stratigraphic
record contains a rich history of interplay between surface, near-surface, and crustal
processes ranging from climatic forcing of sediment loadings (e.g. Soreghan et al., 1999)
to deep-crustal control of extensional deformation and associated vertical movements that
impact regional climate. There are three main sets of questions to be considered under the
framework of Basin Evolution: Chronology and Active Tectonics, Lithofacies Calibration,
and Thermal Structure.
I. Chronology and Active Tectonics
Using dated drill cores, extensive seismic reflection grids (e.g. Rosendahl, 1987; Scholz,
1995), and information on footwall uplift and denudation and catchment evolution (e.g. van
der Beek et al.,1998), we can generate well-constrained models of sediment mass and flux
rates unavailable from other sedimentary systems. In order to accurately study tropical
paleoclimate history and evolutionary biological records on a time scale of 10-1000 kyr it
is necessary to determine the morphotectonic boundary conditions for the rift basins.
Additionally, a well-constrained basin chronostratigraphy will allow us to assess with high
temporal precision the evolution of linked fault systems (e.g. Anders and Schlische, 1994)
within the basin at different scales in space and time
The main question:-
What are the rates of fundamental basin-forming and basin-filling processes (e.g. subsidence,
heat flow, extension, margin uplift, sediment supply, lake level change, sediment
compaction) in continental rifts and how episodic or continuous are these processes?
In order answer this question, we set the following goals:
Establish the chronostratigraphy of the sedimentary section. Place the current sequence
stratigraphic framework, determined from seismic reflection data, into a rigorous
chronostratigraphic context. (i.e. expand the chronology determined from the cores into
the seismic reflection grid.)
- Quantify rates of footwall uplift and associated subsidence in the vicinity of the drill
core, and assess the episodicity of these rates and their sedimentary response.
- Active tectonics controls the development of the watershed of the basin, at the regional
and local scale. We intend to quantify denudation rates and the response time of the
uplift/denudation/sedimentation system (e.g. Foster and Gleadow, 1993).
- Changes in the basin drainage networks will be characterized through sediment provenance
studies, and reflect forcing by tectonic and climatic processes on the scale of 10 kyr to
- Improve models of sediment flux and mass balance in continental extensional basins.
- Assess the development of topographic highs and determine their impact on local
climatic and depositional conditions.
A stratigraphic test of the major part of sedimentary section (through the Pliocene) in
these lakes is the highest priority for local institutions, and will provide the principal
basis for renewed capacity-building in the East African regional geoscience community.
Radiocarbon, paleomagnetics, tephrachronology (Ar-Ar),
and U-Th as our first-order dating techniques.
- Physical properties measurements of continuous core and down-hole geophysical data
to correlate drill cores and seismic reflection data sets.
- Expand basin evolution studies beyond the borehole using the following techniques:
- Fission track thermochronology (e.g. Foster and Gleadow, 1993)
- U-Th-He thermochronology
- Cosmogenic exposure age-dating for estimation of denudation rates and sediment transport times
- GPS campaigns for estimating instantaneous extension rates
- Volumetric modeling of basin infill using DEM=s and seismic reflection data
- Construction of temporally calibrated balanced-cross sections, to determine rates of extension and subsidence
II. Lithofacies calibration, Sedimentology and Sedimentary Geochemistry
The first-order indicator of environmental change in the Lake Malawi and Lake Tanganyika
rift basins is sediment lithology (texture and composition) (Soreghan and Cohen, 1996;
Soreghan et al., 1999; Wells et al, 1999). High-amplitude and high-frequency shifts in lake
water levels exert extreme forcing on sediment character (Scholz and Rosendahl, 1988;
Scholz et al., 1990); this is accomplished through changes in base level, sediment supply,
catchment area conditions, and limnological and biotic boundary conditions. Rapid facies
variations, both laterally and vertically are the paradigm in these basins.
The main question:- What are the principal controls on the
deposition of the main sedimentary facies in tropical rift basins, what are the first-order
characteristics of these lithofacies, and how do they accumulate in space and time?
Deriving the three-dimensional variability of depositional facies in rift basins is
fundamental for extracting paleoclimate and deformational histories. This 3-D geometry is
well-established from existing seismic reflections data. However the seismic facies
framework must be calibrated using continuous sediment cores, and down-hole geophysical
Principal goals are to :
Calibrate and refine existing lithofacies and sequence stratigraphic models for lacustrine
rift basins based on continuous sediment cores, and down-hole geophysical data.
- Characterize and calibrate the distinctive acoustic facies couplets observed in
reflection records and determine the timing of the observed cyclicity.
- Quantify organic matter content both down-core and spatially, around the basin, and
determine the origin of the organic matter in space and time.
- Geochemical characterization of the sediments, with respect to provenance
and diagenetic processes
- Assess the diagenetic history of the sediments as a function of lake water
chemistry and geothermal conditions.
III. Thermal Structure of the Rift Basin
The main question:-
What is the nature of heat flow across the Lake Malawi rift basin, and how has the thermal
history impacted the evolution of extensional faults in the rift basin.
In this project, we propose to use new heat flow observations obtained from the drill holes
in Lake Malawi to critically evaluate models for the structural development of rift faults.
Although there is a large volume of scientific literature on continental rifting,
substantial gaps still remain in our understanding of how continental rifts form and evolve
structurally. In particular, little is known about fault growth during the earliest stages
of rifting, in part because fault growth is directly linked to the thermal and mechanical
structure of the lithosphere (Cowie, 1998; Jackson and Blenkinsop, 1997; Hayward and
Ebinger, 1996; Ebinger et al., 1999; Scholz and Contreras, 1998), for which we have few
constraints. Heat flow observations provide a first?order constraint on the thermal
structure of the lithosphere, and without such constraints it is not easy to assess the
mechanical state of the lithosphere at the time of rifting.
While heat flow measurements have been made in other areas of East Africa, they either come
from unrifted parts of the East African Plateau (Nyblade et al., 1990, Nyblade, 1997), or
else from regions of the Kenya rift where the crustal thermal regime has been
hydrologically disturbed (Whieldon et al., 1997). Marine type heat flow measurements
were made several decades ago in Lakes Malawi, Tanganyika, and Kivu (Von Herzen and
Vacquier, 1967; Degens et al., 1971; Degens et al., 1973), but the uncertainties associated with these measurements
are so large (>50%) that the data provide only weak constraints the thermal structure of
the rifted lithosphere. Nonetheless, they suggest that crustal temperatures beneath the
rift are elevated, while most fault models assume that the crust is cold and brittle
(Cowie, 1998; Jackson and Blenkinsop, 1997; Hayward and Ebinger, 1996; Ebinger et
al., 1999; Scholz and Contreras, 1998). The proposed drill holes in Lake Malawi would
afford us an opportunity to make high quality conventional heat flow determinations that
would indicate to what extent the rifted crust is thermally modified.
Heat flow observations can be made in the drill holes by logging temperatures several times
after drilling is complete. From the multiple measurements, the disturbance to the thermal
field around the drill hole can be determined and an estimate of the equilibrium temperature
s can thus be obtained. In addition to temperature measurements in the drill holes, core
samples will be needed to measure thermal conductivities.
Anders, M. H. and Schlische, R.W., Overlapping faults, intrabasinal highs, and the growth
of normal faults, Journal of Geology, v.102, 165-179, 1994.
- Cowie, P.A., A healing-reloading feedback control on the growth rate of seismogenic faults,
J. Struct. Geol., 20, 1075-1087, 1998.
- Degens, E.T., R.P. Von Herzen, and H.K. Wong, Lake Tanganyika: water chemistry, sediments
and geologic structures, Naturwissenschaften, 58, 229-240, 1971.
- Degens, E.T., R.P. Von Herzen, H.K. Wong, W.G. Denser, and H. W. Jannasch, Lake Kivu: structure,
chemistry and biology of an East African rift lake, Geol. Runsch., 62, 245-277, 1973.
- Ebinger, C.J., J.A. Jackson, A. N. Foster, and N.J. Hayward, Extensional basin geometry and
the elastic lithosphere, Phil. Trans. R. Soc. Lond. A, 357, 741-765, 1999.
- Foster, D.A.and Gleadow, A.W., Episodic denudation in East Africa; a legacy of intracontinental
tectonism, Geophysical Research Letters, v. 20, n. 21, p. 2395?2398, 1993.
- Hayward, H., and C. Ebinger, Variations in the along-axis segmentation of the Afar rift system,
Tectonics, 15, 244-257, 1996.
- Jackson, J. and T. Blenkinsop, The Bilila-Mtakataka fault in Malawi: an active, 100 km long
normal fault segment in thick seismogenic crust, Tectonics, 16, 137-150, 1997.
- Nyblade, A.A., H.N. Pollack, D.L. Jones, F. Podmore, and M. Mushayandebvu, Terrestrial heat
flow in east and southern Africa, J. Geophys. Res., 95, 17371-17384, 1990.
- Nyblade. A.A., Heat flow across the East African Plateau, Geophys. Res. Lett., 24, 2083-2086, 1997.
- Rosendahl, B.R., Architecture of continental rifts with special reference to East Africa, Reviews
of Earth and Planetary Sciences, 15, 445-503, 1987.
- Scholz, C.A., Deltas of the Lake Malawi Rift, East Africa: Seismic Expression and Exploration
Implications, AAPG BULLETIN, v. 79, p. 1679-1697, 1995
- Scholz, C.A., Rosendahl, B.R. and Scott, D.L. Development of Coarse-grained Facies in
Lacustrine Rift Systems: Examples from East Africa. GEOLOGY, v. 18, pp. 140-144, 1990.
- Scholz, C.A. and Rosendahl, B.R., Low Lake Stands in Lakes Malawi and Tanganyika, East
Africa, Delineated with Multifold Seismic Data. SCIENCE, V. 240, pp. 1645-1648, 1988.
- Scholz, C.H., and J.C. Contreras, Mechanics of continental rift architecture, Geology, 26, 967-970, 1998.
- Soreghan M.J., and Cohen, A.S., Textural and compositional variability across littoral
segments of Lake Tanganyika: The effect of asymmetric basin structure on sedimentation in
large rift lakes, AAPG Bulletin, 80, 382-409, 1996.
- Soreghan, M.J., Scholz, C.A., and Wells, J.T., Coarse-grained deep-water sedimentation
along a border fault margin of Lake Malawi, Africa: Part 1- Seismic Stratigraphic Analysis,
Journal of Sedimentary Research, V 69, p 832-846.
- Wells, J.T.,, Scholz, C.A, and Soreghan, M.J. Processes of sedimentation on a Lacustrine
border fault margin: Interpretation of cores from lake Malawi, East Africa. Journal of
Sedimentary Research, 69, p. 816-831.
- Wheildon, J., P. Morgan, K.H. Williamson, T.R. Evans, and C.A.Swanberg, Heat flow in
the Kenya rift zone, Tectonophysics, 236, 131-150, 1997.
- Van der Beek, P. Mbede, E., Andriessen, P., and Delvaux, D., Denudation history of the
Malawi and Rukwa Rift Flanks, (East African Rift System) from apatite fission track
thermochronology, Jrnl. Afr. Earth Sciences, 26, 363-385, 1998.
- Von Herzen, R.P., and V. Vacquier, Terrestrial heat flow in Lake Malawi, Africa, J. Geophys.
Res., 72, 4221-4226, 1967.
Evolution of Biodiversity and Ecology in Ancient Lakes: Biological Objectives
of deep drilling in Lakes Malawi and Tanganyika
The Biological Significance of Lakes Malawi And Tanganyika
Lakes Malawi and Tanganyika are aquatic island systems of elevated endemic biodiversity,
unparalleled for their potential to test hypotheses of comparative evolution on large
scales. The sedimentary record of these lakes offers us the opportunity to resolve both
evolutionary and ecological changes in their biota at time scales of decades, over hundreds
of thousands to millions of years
Despite their long histories and geological similarities, the patterns of diversity and
genetic differentiation of the biota differ dramatically between Lakes Malawi and Tanganyika
. Both lakes were colonized by cichlid fishes, thiarid gastropods and ostracode crustaceans
, but these exemplar taxa currently have contrasting aspects in the two lakes.
Approximately 1000 fish species are estimated to have evolved within the cradle of Lake
Malawi, which is approximately 10% of all freshwater fish species in the world. Despite
their astonishing multitude, these species encompass a rather modest degree of molecular
genetic and morphological change (Kornfield, 1978; Moran et al., 1994; Parker and Kornfield
, 1997). The fishes in Tanganyika are genetically and morphologically much more diverse
than those in Malawi (Sturmbauer and Meyer, 1992, 1993)., yet total only 300 species (which
is still more than all the species in the 10s of thousands of North American lakes combined
). In Lake Tanganyika, about 240 out of 250 species of prosobranch gastropods and
ostracode crustaceans are unique to that lake, and like the cichlid fish, form numerous
distinct, divergent lineages (Michel, 1994; Park and Downing, in press). The living
prosobranch gastropod fauna of Lake Malawi has undergone only limited differentiation and
few if any endemic ostracodes are reported from this lake (Martens, 1994; Michel,1994)
Understanding the history of the Malawi and Tanganyika radiations, their similarities and
differences, represents an extraordinary opportunity for evolutionary biology.
In these lakes we have a unique opportunity to investigate the dynamics of evolutionary
and ecological change. Patterns of speciation, the origin of major morphological evolution,
and the origin of major reorganizations in community structure can all be investigated in
a comparative setting in these two lakes, in the context of high resolution, long
stratigraphic records. Paleoenvironmental, tectonic and climatic reconstructions obtained
from other components of this drilling program will provide the context for interpreting
Major Questions to be Addressed in Evolutionary Biology And PaleoEcology
For our purposes of evolutionary studies the most promising groups of fossil organisms are
the gastropod molluscs and ostracode crustaceans. In addition to their preservation
potential these animals are small (easily obtained in cores), can be identified to species
level as fossils, and provide interesting targets for evolutionary studies.
Timing of diversification of the endemic radiations in Lakes
Malawi & Tanganyika.
We propose to determine the duration and extent of evolutionary radiations in target groups
of readily preserved fossil lineages. We will test the hypothesis that the Lake Malawi
biota has undergone more frequent resetting of its evolutionary clock than the biota of
Lake Tanganyika, as a result of more frequent and profound disruption of the Lake Malawi
ecosystem by climatically-driven lake level fluctuations and perhaps salinity crises.
As a result of its lesser depth and simpler tectonic configuration (fewer basins), habitat
stability is predicted to have been much less in Lake Malawi and the lake may have
frequently dried, eliminating nascent evolutionary novelty, similar to Lake Victoria in
the Late Pleistocene (Johnson et al., 1996). The more complex basin configuration and deeper
waters of Lake Tanganyika provided more time and less severe perturbations to the evolving
biota, stimulating greater levels of morphological diversification and genetic
From cores in Lake Malawi we will document changes in the Melanoides gastropod lineage
through time, as this group currently includes 12 extant, endemic species (Brown, 1994).
Similarly, the Gomphocythere ostracode lineage will be a focal taxon. When drilling moves
to its second phase in Lake Tanganyika we will compare these records with the more diverse
Lavigeria lineage, a related thiarid gastropod, and the Gomphocythere lineage for ostracodes
(Michel in press; Park and Downing, in press). In both lakes we will compare the records of
evolutionary change with our developing understanding of lake level, climate and tectonic
history, to determine if more frequent and/or extensive desiccation and disappearance of
habitat is characteristic of Lake Malawi, and if those changes are linked to extinction
events. Furthermore in Tanganyika we predict that the evolution of multiple basins of the
lake (whose age will also be determined by drilling) is linked to episodes of
diversification (e.g. Michel et al., 1992; Meyer et al., 1994; Cohen et al., 1997).
Rates of diversification in ancient lakes
Recent phylogenetic studies of living fish and molluscs suggest a “burst-like” pattern of
initial radiation in the faunas of Lakes Malawi and Tanganyika (Meyer et al., 1994; Michel,
in press; West and Michel, in press). We will test whether benthic fossil lineages support
an interpretation of rapid morphological innovation and species divergence among molluscs
and crustaceans. We further predict that these bursts will be linked to significant
episodes of tectonic or climatic change (i.e. Verheyen et al., 1996), analogous to the
rapid speciation inferred to have occurred in Lake Victoria subsequent to its Pleistocene
desiccation, during its Holocene refilling (Johnson et al., 1996) and Lake Baikal in its
basin subdivision (Sherbakov 1999). In contrast, we predict that planktic diatoms will show
much less diversification following major tectonic or climatic events because habitat
segregation and basin subdivision to will not affect populations of these readily
transported species (focussing on the Aulacoseira complex, Cocquyt and Vyvermans, 1994).
For both benthic and planktonic organisms we will trace sequences of evolutionary change
and extinction through a series of lake level fluctuations. Core data will provide
replication and thus statistical rigor to our tests of causal basis of speciation.
Evolutionary escalation and predator/prey arms races
Present-day Lakes Malawi and Tanganyika display strikingly different patterns of
predator-prey interactions in their benthic and demersal habitats. In Lake Tanganyika,
endemic crabs and fish are specialized for effective predation, and natural selection has
resulted in heavily armored gastropods, remarkably convergent on thick-shelled, spinose
marine snail morphologies (West et al., 1991; West and Cohen, 1996). The gastropods
deposit multiple cross-lamellar layers (up to 4) of skeletal carbonate, strengthening the
shell in a fashion analogous to plywood. Furthermore, the Lake Tanganyika gastropods can
repair their shells after attack (a trait rarely seen in freshwater snails, but again
common in marine ones). None of these characteristics or interactions are well developed
in Lake Malawi (Brown, 1994).
The question of why these two similar lakes have undergone such radically different
histories of predator-prey interactions can only be answered through a detailed analysis
of the history of character acquisition for the heavily-shelled molluscs in modern
Tanganyika, and a comparison of this history with that of the fossil molluscs of Lake
Malawi. It is entirely possible that similar bouts of coevolutionary “arms-races” have
occurred in the past in either lake, only to be eliminated through periodic extinction
events. Repeated episodes of escalatory coevolution among fish species have been inferred
from the fossils of Pliocene Lake Idaho, but the timing on rates of evolution of these
complexes is poorly constrained (Smith, 1987). Viviparid gastropods exhibit anti-predatory
morphologies, coincident with increased species diversity, through several sedimentary
cycles of the Lake Edward-Albert fossil deposits, but lose both diversity and armor in
the relict living fauna (van Damme & Pickford, 1999).
We hypothesize that the modern extreme biotic differences between the two lakes reflects
different ages of their respective aquatic ecosystems, Malawi being the younger,
less co-evolved system and Tanganyika the older. We can test this hypothesis through
the acquisition of shell thickness, repair and cross lamellar count data. Knowing the
age of changes in each of these factors will allow us to determine their rates of change.
Furthermore, age data can be superimposed on existing phylogenies that map out the
acquisition of multiple cross lamellae, telling us what the pattern of evolution of
these characters has been (e.g. West and Cohen, 1996). We can then use the shell repair
data to determine long-term predation intensity, testing whether the unusual mollusc shell
characters in Tanganyika are part of a coevolved complex. Lake Tanganyika promises a
clearer documentation of actual rates of predator-prey coevolution than any other biotic
system from any environment. In Malawi our interest will be to see if earlier episodes of
escalation have occurred during periods of lake-level stability, subsequently eliminated
in the modern lake fauna.
Community response to environmental change at varying time scales
Ecologists have grappled with the question of how communities change over time, whether as
coordinated groups of organisms responding en masse through invasion and local extinction,
or through individualistic shifts of species’ relative abundances. This problem has been a
particularly thorny issue because of our inability to sample community dynamics at a wide
and continuous range of time scales. On the one hand, neo-ecological studies are generally
performed with an abundance of data but over time intervals encompassing few generations.
In contrast, paleoecological studies typically cover much longer intervals but are poorly
resolved in time. In Lake Tanganyika “long-term” ecological research on cichlid fish
communities, spanning about a decade, supports a model of community coordination and
stability, at odds with lower resolution but longer duration ostracode community data
within the same lake (Hori et al., 1993; Cohen, in press). Are these contradictions real
(resulting from differences between organismal groups or habitat) or are they a result of
sampling at very different time scales?
The sedimentary records of Lakes Malawi and Tanganyika allow sampling of ecological change
at annual to decadal resolution, over time intervals spanning hundred of thousands to
millions of years, thereby addressing this important question. Analysis of change at
varying time scales through a series of cores will allow us to see how our perceptions of
change are affected by our scale of observation, an important question for almost all of
community ecology. We can extend our analysis from the temporal to the spatial scale
through the analysis of adjacent cores (in our proposed drilling program three cores will
be routinely collected at each coring site, to provide assurance of complete core recovery)
. This is also important because much current speculation about community stability or
instability rests on an understanding of how adjacent habitat patches and populations
interact with one another over time. In Lakes Malawi and Tanganyika we have the possibility
of inferring these patch interactions over very long time periods.
We can attack the problems outlined above using ostracode, diatom and palynological records.
The former two data sets will give us records of the tempo of community change and patch
dynamics in the lakes, and through comparison between the lakes, an understanding of how
community change relates to larger-scale differences in lake history. The pollen record
will allow us to see vegetational response to shifting climatic variables, and whether these
responses differ between the subequatorial climate of the Lake Malawi basin and the
equatorial climates of the Tanganyika basin.
Regional Research Questions
In addition to the questions outlined above, all of which have broad significance for
evolutionary ecology in general, issues of more regional concern can be tested with
fossils in the Malawi and Tanganyika cores. The biogeography of dispersal among freshwater
ecosystems of the African Great Lakes can be addressed for the taxa amenable to core
analysis (primarily ostracodes and molluscs, since fish are rarely preserved well enough
to identify to species level). Some data suggests that Lake Tanganyika has acted as a
long-term crucible or refugium during periods of severe dessication in other lake systems,
and that the faunas of the other African Great Lakes are derived from within “Tanganyikan”
clades (Meyer et al. 1994; Van Damme and Pickford, 1999; Park and Downing, in press, West
and Michel, in press). This hypothesis may be testable with data to be acquired in the
combined Tanganyika/Malawi drilling program, in combination with earlier-collected outcrop
data from the Kaiso Rift and Lake Rukwa.
Another important regional question for paleobiology is the role that external forcing
mechanisms have played in constraining the phylogeny of ostracodes and molluscs. Through
close collaboration with our colleagues in paleoclimatology and tectonic interpretation of
the cores we hope to be able to provide realistic and detailed scenarios of evolution
within different sub-basins of the lakes, relating, for example, the establishment of
particular barriers to dispersal with particular diversification events.
The endemic fauna as a tool for paleoclimatic and tectonic data acquisition
Fossil organisms can be powerful tools for paleoenvironmental and chronostratigraphic
interpretation. Ostracodes, molluscs and diatoms have all been widely used to infer
composition and concentration of water masses, water depths and habitat zonation. Carbonate
in ostracode and mollusc shells can also be analyzed for stable isotopes and minor elements
, again for the purpose of paleoenvironmental and paleoclimatic interpretation. And finally,
ostracodes and molluscs have been a mainstay of chronostratigraphy in the ancient
Cretaceous rift lake basins of Brazil and West Africa, a testimony to their rapid evolution
in those systems, so similar in many respects to the large rift lakes of Africa. Although
these are not, in our view, fundamental questions of ecology and evolution, they are
nevertheless of critical importance to the drilling program as a whole.
Constraints on Drilling Targets
As we have alluded to above, not all lacustrine organisms are going to be equally amenable to
study in this project. Our best hopes for evolutionary records clearly lie with small,
benthic, shelled invertebrates. These organisms are completely restricted to the oxic zones
of the two lakes today, and will not be found therefore in deep water sediments, except
during periods of significant lake level decline. This assertion is based on considerable
coring experience of the authors in Holocene and Late Pleistocene sediments and the
observation that deep water transport of dead shell material, while a real phenomenon on
steep slopes, is unlikely to be important in the types of locations where we will drill
(flat, away from rocky highs). For this reason we strongly advocate that at least some
cores in each lakes be taken from relatively shallow, sublittoral sites, or sites that
have likely been at such depths over geologic time. We recognize that these areas may not
be ideal for other purposes, such as the highest resolution paleoclimate studies.
Nevertheless, the added information such shallow sites will provide us for evolutionary
ecology studies makes the additional effort of obtaining such records well worthwhile.
This justification is further strengthened when one considers the potential of these same,
shallow-lake organisms for providing paleoclimatic proxies such as carbonate, growth
banding (for bivalves), trace elements and stable isotope records, all of which are
unobtainable in deep water.
Our experience suggests that large structural platforms or perhaps distal deltaic
environments are the best sites for collecting the types of records we require, with
paleowater depths of 0-100m. Clearly depth will change over time, but the lakes seem to
return to similar spillway elevations repeatedly, so modern depth ranges are probably
realistic guides for locations of abundant fossils in cores.
Our prior experience in the large lake coring in these types of environments suggests that
decadal-scale resolution is quite feasible. Bioturbation exists in these water depths,
but is relatively unimportant (2-3cm mixing depths) for the likely sampling spacing we
would employ. Depositional hiatuses during lake low stands are a greater concern for our
records, but these hiatal intervals in the shallow site records would be dovetailed with
records from deeper basinal sites, so we can expect a reasonably complete record at the
millenial scale considering the lakes in their entirety.
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African Great Lakes: a comparative survey of Lakes Tanganyika, Malawi (Nyasa) and Victoria.
In Martens, K., Goddeeris, B. and Coulter, G. (eds.) Speciation In Ancient Lakes. Arch.
Hydrobiol. Beih. Ergebn. Limnol. 44:161-172.
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lake-level reconstructions of Lake Tanganyika: implications for tectonic, climatic and
biological evolution in a rift lake. Basin Research 9:107-132.
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ancient lakes: Examples from the ostracod ecology and paleoecology of Lake Tanganyika. In
Rossiter, A. (ed.) Biology of Ancient Lakes Academic Press.
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Lake Tanganyika: Irreplaceable diversity supported by intricate interactions among species.
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and rapid evolution of cichlid fishes. Science 273:1091-1093.
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species flocks of the East African great lakes inferred from molecular phylogenetic data.
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Hydrobiol. Beih. Ergebn. Limnol. 44:407-423.
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In Ancient Lakes. Arch. Hydrobiol. Beih. Ergebn. Limnol. 44:285-317.
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Impact of Lake Malawi/Tanganyika Drilling on Issues of Human Origins
Lakes Malawi and Tanganyika are centrally located in the geographic belt (Ethiopia
to S. Africa) that records the earliest human ancestors (2 to 5 million years old) and the
oldest known fossils of our own species, Homo sapiens (>100,000 years old).
- Ancient lake basins of eastern Africa preserve the longest and most precisely dated
sequences of hominid fossils and archeological evidence bearing on human origins.
- High-resolution study of African lake history is thus directly pertinent to
understanding the link between past environmental change and human evolution.
Paleoclimate research focusing on Africa has made a rich contribution to the development
of hypotheses regarding human evolutionary history.
Two examples: The turnover pulse hypothesis (advanced by E. Vrba, 1980-1995) and the
variability selection hypothesis (advanced by R. Potts, 1996-1998) were developed as a
direct result of the past two decades of research on global paleoclimates and environmental
change in ancient African lake basins.
Turnover pulse means that species origins and extinctions, including episodes involving
hominids, were initiated by dramatic climatic change (aridity and cooling) in Africa during
the late Pliocene and again in the Pleistocene. Variability selection draws attention to
the oscillation evidenced in global and regional sedimentary records. According to this
body of evidence, environmental fluctuation caused inconsistencies in the adaptive settings
of early humans and thus had a formative impact on the origin of toolmaking, brain
enlargement, and other advances in human adaptability.
So far, these ideas have mainly been tested by looking at evidence of environmental change
in terrestrial sediments (e.g., the Turkana and Olorgesailie basins) and deep-sea cores.
Terrestrial records, however, have many gaps due to erosional unconformities, while the
marine record is rather far removed from the places where hominids lived.
Recovery of high-resolution cores from Lakes Malawi and Tanganyika would provide an
unparalleled record of environmental change relevant to the time and place of early human
origin and the evolutionary history of our own species. An international contingent of
geologists, paleontologists, and archeologists are ready to dedicate themselves to comparing
the records from these African lakes to those of past lakes and associated settings
inhabited by hominids.
Cores drilled from Lakes Malawi and Tanganyika will offer a body of evidence directly
pertinent to the fact that human adaptations have evolved in association with African lakes
for over 4.4 million years. This body of evidence signifies an opportunity that is
otherwise unavailable for testing the link between human evolution and change in climate
and the biota.
Potential Case Studies
Olorgesailie, a Pleistocene lake basin in southern Kenya, provides the best calibrated
record of hominid stone tools, change in the African biota, and climatic fluctuation
between 1.2 million and 49,000 years ago. Recent research suggests that environmental
variability has escalated through the Quaternary in the Olorgesailie region. One
implication is that important changes in hominid technology, formation of the modern
suite of large mammals, and perhaps the origin of the modern human lineage, all corresponded
with a widening range of habitat perturbation. Comparison with Quaternary cores from
modern African lakes would allow us to find out if these perturbations were widespread or
local, whether fluctuations in eastern African lakes were tightly correlated, and thus
whether early human populations faced inescapable shifts in their survival conditions over
a wide geographic area. Testing these ideas would have an immediate impact on our
understanding of human evolution.
Since the early 1990s, considerable effort has been put into examining the record of human
evolution against deep-sea dust records (terrestrial material blown from Africa). The
reasoning is that if key events in human evolution occurred in response to changes in
aridity or monsoons, there should be a strong correlation between these events and
environmental markers in the dust record. Drilling of Lakes Malawi and Tanganyika is
likely to offer the missing link in this analysis – i.e., the link between the terrestrial
records (where evidence of hominids is found), the aquatic lake record on land, and records
from the deep sea. Since most hypotheses of human evolutionary history are based on
correlation between geological, biotic and anthropological data sets, high-resolution
records from modern African lakes will provide a much more sound basis than we presently
have for determining any such correlations.
Submitted by Dr. Richard Potts, Director, Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S.A. (October 1999)
Environmental background to human origins
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