Deciphering climate information from authigenic mineral transformations in the Chew Bahir sediment cores: Towards a continuous half-million year climate record from the Southern Ethiopian Rift

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The Chew Bahir Drilling Project (CBDP) has cored duplicate sediment records from the Chew Bahir basin in southern Ethiopia to test current hypotheses on human-climate interaction during the development of anatomically modern humans and their migrations out of Africa by elucidating the climatic context of critical time periods. However, deciphering paleoclimate from lake sediments is challenging due to the complex and site-specific relationships of climate parameters and sediment composition. Determining and analysing suitable climate proxies that are preserved throughout the core is essential to establishing a high resolution, continuous record on climate change. The project proposed here aims to decipher paleoenvironmental information from authigenic mineral transformations in the Chew Bahir sediment cores in order to contribute meaningful data that could be key to developing a reliable and continuous half million-year climate record from the Southern Ethiopian Rift. In a hydrologically restricted setting, such as Chew Bahir, the composition of authigenic clay minerals and zeolites are strongly controlled by the hydrochemistry of the paleolake, which responds to fluctuations in the precipitation/evaporation ratio. In a pilot study, the Chew Bahir mineral assemblages, especially the abundant clay minerals, have been shown to react sensitively to variations in paleosalinity and alkalinity by finely adjusting their crystallographic composition. Since the ~290-m-long composite core is dominantly comprised of those highly reactive clays, the material has the potential to provide continuous paleohydrochemical and paleoclimatic information through mineralogical and geochemical analyses. This potential dataset could be critical to the success of the project as some other proxies (e.g. pollen and diatoms) are only preserved intermittently. In the first two years of the project, we propose to (1a) identify characteristic mineral assemblages for wet, dry and hyper-arid phases throughout the long core; (1b) determine the precise degree of authigenic alteration in clay minerals and zeolites necessary to derive paleohydrologic data; (2a) link mineralogy (XRD) with geochemistry (µXRF) to show the links between hydroclimatic control, and the formation and preservation of the Chew Bahir proxies; (2b) decipher climatic change on orbital to decadal timescales from previously defined depositional conditions and thresholds; (3) evaluate the impact of different types of climatic change (duration, abruptness, internal variability and magnitude) on the habitat and living conditions of early humans. In summary, the results of the proposed project will form an important contribution to the understanding of complex climate proxy formation, enable continuous paleoclimate information to be deciphered from the Chew Bahir sediment cores, and contribute data that may be essential for testing hypotheses of the impact of climatic change on human evolution and mobility.