An Evaluation of the Organic Geochemical Potential to Reconstruct Mid-Pleistocene Paleoclimate Adjacent to an Established Hominin Site: Lake Magadi, Kenya

摘要

The paleo-Lake Magadi was one of five sites selected by the Hominin Sites and Paleolakes Drilling Project (HSPDP), with the goal of establishing high resolution, multi-proxy reconstructions of climate and environmental change adjacent to established hominin sites. The overall goal of the HSPDP is to provide a context of climate and environmental change for many of the milestones in hominin adaptation and evolution, spatially adjacent to where hominins lived, and temporally resolved to evolutionary (orbital to sub-millennial) timescales. The core recovered from Lake Magadi is estimated to represent the past 1.08 Ma, during which several significant hominin adaptation and evolutionary events occurred. Based on initial chronology efforts by HSPDP collaborators, the Magadi core can be separated into four sections: 0-26 kyr, 26-165 kyr, 165-242.8 kyr, and 242.8 kyr to 1.08 Ma. We estimate the future biomarker potential for the four sections, and determine Lake Magadi to have particularly exciting promise for the 0-26 kyr and 26-165 kyr sections.udTo that end, 68 pilot samples were analyzed for leaf waxes (n-alkanes and fatty acids) and glycerol dialkyl glycerol tetraethers (GDGTs), to evaluate the potential to reconstruct vegetation regime, precipitation, and temperature from Magadi. The n-alkanes were determined to have a better potential to yield compound specific carbon and hydrogen isotope data than fatty acids due to better chemical separation, and approximately 70% of samples analyzed for n-alkanes recorded a robust terrestrial signal. The TetraEther indeX with 86 carbon atoms (TEX86) temperature proxy was established for ~90% of samples analyzed for isoprenoid GDGTs, however the Methane Index and Ring Index suggest that the TEX86 is not applicable to temperature reconstruction at Magadi. Additionally, no samples contained the complete suite of branched GDGTs necessary to construct the Methylation of Branched Tetraethers and Cyclisation of Branched Tetraethers (MBT/CBT) temperature proxy.udInitial reconstruction of Lake Magadi paleoclimate shows an oscillating vegetation regime from mid-Pleistocene to present, but a directional trend towards a cooler and drier climate. Plotted alongside 18O data from the GRIP-2 ice core, our climate data demonstrate a potentially interesting correlation between climate at Lake Magadi and ice volume in the Arctic.