Date of Award

5-12-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth & Environmental Sciences

Advisor(s)

Christopher Scholz

Keywords

diatom;eastern Africa;hydroclimate;lake;luminescence dating;Quaternary

Abstract

Eastern Africa is identified as the single most important region for early human evolution and development, and in this region today, communities are especially vulnerable to climatic stress. To understand the relationship between our ancestors and their environment and to protect vulnerable populations, we need to understand the climatic system in eastern Africa, including the history of past changes and the mechanisms that controlled them. The most frequently discussed mechanisms are local insolation and high latitude forcing, by way of glacial-interglacial cycles, changes to global circulation, or high latitude insolation. Though sometimes proposed to work in tandem, these mechanisms are often described as competing theories. Unfortunately, because of a lack of long well-dated records from this part of the continental tropics there is no consensus on this issue. A 2005 drill core from Lake Malawi extends to ~1.4 Ma and represents the longest most continuous sediment record from the continental tropics; this core was collected with the aim of resolving this debate, and the record should have been well suited to the task. Because of Lake Malawi’s great depth, the lake never completely desiccated during dry intervals in the Quaternary, producing a record with extreme dynamic range, and because of its great size, local perturbations are muted, and its sediments record regional climate signals. Unfortunately, large chronologic uncertainties, namely a gap in absolute ages from ~74 ka to ~590 ka, has limited the utility of the record. The chapters of my dissertation improve the chronology of the Lake Malawi drill core using luminescence dating, extend the diatom record from the core, reanalyze the climatic implications of the record, and describe a new diatom species found in the record. The first chapter of my dissertation fills the gap in chronology in the Lake Malawi drill core with 31 luminescence samples collected at ~4 m resolution. Luminescence dating records the last time sediment was exposed to sunlight, and while it has been applied to other systems for decades, its use in lacustrine cores is fairly new. This work required the development of a method to calculate water content and sediment dewatering rates, which could aid in the application of luminescence dating to other archived drill records. Chapter two extends the diatom record for the core by 435 samples, creating a composite record that extends from ~10–420 ka and has a ~450-year sampling resolution. By combining the diatom dataset with existing Lake Malawi proxies and the refined age model, I hypothesize that hydroclimate in the Lake Malawi region is linked to sea surface temperatures in the Indian Ocean and that Lake Malawi has an anti-phased relationship with sites to the north and south, as Lake Malawi experiences wet intervals, sites to the north and south experience dry intervals and vice versa. The length, high sampling resolution, and improved chronology make it possible to observe this relationship at Lake Malawi when other records could not. Chapter 2 argues that this relationship is produced by changes in ocean circulation, induced by glacial-interglacial cycles, that alter the north-south temperature gradient in the Indian Ocean, which in turn influences rainfall patterns. The third chapter of my dissertation describes a new species of diatom, Encyonema larvatus, observed in the Lake Malawi record ~330 ka. E. larvatus is a benthic diatom that must have lived in shallow water or riparian habitat and been transported to the drill site. While this chapter only discusses one species, it contributes to the systematic cataloging of taxa needed to understand diatom diversity and distribution. The anti-phased nature of eastern African climate has not been recognized before to my knowledge, and it could have important implications for paleoanthropology and predictions of future climate. Researchers often try to link human dispersal, morphological development, and cultural changes to periods of extreme climate in eastern Africa. If eastern Africa can experience both exceptionally dry and exceptionally wet conditions simultaneously, we may need to question some of these existing ideas. Moreover, climate models fail to recognize this anti-phased relationship, indicating that they are not accurately reproducing the region and bringing into question modeled predictions of future climate. In total, my dissertation uses diatom paleoecology and luminescence dating to put forth a cohesive climatic mechanism for eastern Africa that helps to explain some of the disagreement between records and could alter hypotheses about human evolution and predictions of future climate in this sensitive region.

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