Andrew Cohen | East African Paleoclimate and Ecosystem History

For over 35 years the UA’s Laboratory of Paleolimnology and our colleagues have been at the forefront of research efforts to reconstruct the history of East African lake ecosystems and climate through the study of sediment cores and outcrops taken from lake deposits. Initially we restricted our studies to the Quaternary, but increasingly we are becoming interested in even longer records extending back into the Neogene. Much of our work has been conducted at Lake Tanganyika (photo A) where we have looked at variability in complex ecosystems, and the response of ecosystems to lake level and climate change during the Holocene (e.g. Alin and Cohen (2003)Alin and Cohen (2004); Cohen et al., (2016)), and have studied high resolution, and often varved records to understand linkages between climate, watershed processes, human impacts and lacustrine productivity (Cohen et al (2006); Conaway et al., (2012); McManus et al., (2015)).We are collaborating with former Laboratory of Paleolimnology members Mike Soreghan (U. Oklahoma) and Mike McGlue (U. Kentucky) to understand the origins of extensive and remarkable shell beds, found in sublittoral areas throughout Lake Tanganyika (photo E). These shell beds make up an important habitat for the biodiversity of this lake, and one that is increasingly threatened by increased sedimentation rates and climate change (Gravina et al., 2020). These results have strong implications for management of the large African lakes through a time of increasing local human impacts at the watershed scale and climate change (Cohen et al., 2017; Busch et al., 2018).

Our lab has been at the forefront of understanding the effects of Anthropocene climate change on the Lake Tanganyika ecosystems as the lake has warmed and become more strongly stratified over the last ~150 years (O’Reilly et al., 2003; Tierney et al., 2010; Cohen et al., 2016, Kamulali et al., 2021). Understanding the combined impacts of climate change and sedimentation impacts on the lake’s extraordinary biodiversity, food webs and fisheries, in particular by using models developed from paleoecological research (e.g. Cohen, 2017) will be a major focus of our new work.

At Lake Malawi (photos B and C), the UA Laboratory of Paleolimnology has played a key role in interpreting the paleoecological and evolutionary history of that lake as inferred from the Lake Malawi Scientific Drilling Project. This drilling project recovered almost 700m of core in 2005, the longest of which (382m) provides a record of climate and environmental change for tropical Africa over the past 1.25 million years (Scholz et al., 2007; Cohen et al., 2007; Stone et al., 2011; Ivory et al., 2011). High resolution records from intervals within this core have provided evidence relevant to testing (and refuting) the hypothesis that the supereruption of Mt. Toba (in Sumatra) lead to wholesale ecosystem destruction in East Africa about 75,000 years ago, with catastrophic implications for early anatomically modern humans (Jackson et al., 2015; Yost et al., 2018).

The African Great Lakes, and in particular Lakes Tanganyika, Malawi and Victoria, are famous for their enormous numbers of endemic fish and invertebrate species, which have evolved in these lakes and their surrounding waters over their long histories. For over 30 years the UA Laboratory of Paleolimnology has been involved with studies on the biodiversity, paleobiological history and diversification of this fauna, with research primarily focused on Lake Tanganyika, its intermittently connected surrounding lakes, Rukwa and Kivu, and Lake Malawi (Cohen and Johnston, 1987; Johnston and Cohen, 1987; West et al., 1991; Michel et al, 1991; Cohen, 1994; West and Cohen, 1994; Cohen, 1995; West and Cohen, 1996Cohen, 2000; Park et al, 2000; Park et al., 2001; Park et al., 2002Michel et al., 2004; Cohen et al., 2013; Salzburger et al., 2014; Ivory et al., 2016

Our latest work related to understanding the “dep-time” evolutionary history of the Africa Great Lakes is related to efforts now underway to develop a scientific drilling project for Lake Tanganyika, to explore its ~10 million year history of evolutionary diversification (Cohen et al., 2011; Cohen and Salzburger, 2016; Russell et al., 2020.


A. A storm over the Mahale Mountains at Lake Tanganyika.

B. The Lake Malawi Scientific Drilling Project Viphya Barge with the SEACORE drill
rig mounted in port at Lake Malawi. Note the large blue dynamic positioning
thruster engines mounted on the two forward corners of the barge.

C. Retrieving the hydraulic piston corer on the Viphya deck during
drilling at Lake Malawi.

D. With Hudson Nkotagu and two Tanzanian game rangers at Katavi National Park.
We have just located the point where paleolake Rukwa overspilled into the Lake
Tanganyika Basin during the Early Holocene. Water on the left today flows to
Lake Tanganyika, whereas water on the right flows to Lake Rukwa.

Lake Tanganyika shell bed

E. Close-up photo of Lake Tanganyika shell bed in the Zambian portion
of the lake, ~20m water depth. The shell beds here, as in most locations, are
composed largely of the gastropod Neothauma tanganyicense (A. Cohen photo).