Reconstruction of late Quaternary climate in eastern equatorial Africa
Authors:   , Baxter Allix Jean 
Published: August 2023

Human-driven global warming is critically affecting the hydrological cycle, ecosystems, and economies globally. Despite advancements in predicting Earth's climate response to elevated CO2 levels, refining climate models to reduce uncertainties remains imperative. Limited high-quality continental sedimentary archives result in a bias toward marine and higher latitude settings, but the sediments below long-lived lakes are excellent archives of past climatic and ecological variability. This thesis centers on Lake Chala in easternmost equatorial Africa, ideally located to record the climate history of this region fully dependent on Indian Ocean moisture. The International Continental Drilling Program (ICDP) project DeepCHALLA recovered a continuous core sequence of organic-rich laminated sediments from the lake bottom spanning the last ~250,000 years (250 kyr). These sediments contain ample amounts of glycerol dialkyl glycerol tetraethers (GDGTs) and glycerol monoalkyl glycerol tetraethers (GMGTs), groups of microbial membrane lipids which are useful for paleoclimate research. However, uncertainties surrounding GDGT/GMGT-based climate proxies, including their biological sources and potential biases, complicate their application.
Part 1 of this thesis focuses on GDGT/GMGT proxy validation in contemporary settings. Settling particles, suspended particulate matter, surface sediments, and catchment soils from Lake Chala are analyzed to trace biomarker origins and dynamics. For example, the so-called “BIT index” is validated as an effective moisture balance proxy for Lake Chala, and the functioning of the popular TEX86 palaeothermometry is also assessed. New temperature proxies based on branched GMGTs in East African lake sediments and detailed analysis of the origin and drivers of these lipids in the modern lake are presented. Importantly, these modern studies support that a temperature model based on branched GDGTs in globally distributed lake surface sediments is the most appropriate choice for the Lake Chala sediments.
Part 2 of this thesis investigates the stability of the present-day climate-proxy relationships throughout Lake Chala's 250-kyr history. GDGT concentrations and proxies are compared to independent data for lake dynamics and chemistry (diatoms, seismic data and lithology). Altogether, this data suggests that during the earliest depositional phases (i.e., before ~160 ka), the degree of water column stratification was not comparable to the modern-system, resulting in weaker relationships between the GDGTs/GMGTs and regional climate variability and hindering extrapolation of the inferred climate-proxy relationships throughout the full sequence. The biomarker proxies provide trustworthy records for temperature and moisture balance back to ~160 ka.

Finally in Part 3, the records of temperature and hydrological variability for equatorial East Africa, encompassing the entire last interglacial and glacial periods, are presented and their implications for past a future climate dynamics in the region are discussed. For example, these records facilitate new discussion surrounding the causes of the so-called “African Megdaroughts” in tropical Africa during the last interglacial, and reveal and important climatic tipping point ~11.7 ka between temperature and regional hydrology which is relevant for present and future climate in this region.
This study underscores the importance of climate proxies validation. It reveals Lake Chala's potential as a reference point for Quaternary paleoclimate in the tropics, contributing to a deeper comprehension of Earth's climate evolution.