A comparison of biomarker records of northeast African vegetation from lacustrine and marine sediments (ca. 3.40 Ma)

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Organic Geochemistry
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Integrated terrestrial and marine records of northeast African vegetation are needed to provide long high resolution records of environmental variability with established links to specific terrestrial environments. In this study, we compare records of terrestrial vegetation preserved in marine sediments in the Gulf of Aden [Deep Sea Drilling Project (DSDP) Site 231] and an outcrop of lacustrine sediments in the Turkana Basin, Kenya, part of the East African Rift System. We analyzed higher plant biomarkers in sediments from both deposits of known equivalent age, corresponding to a ca. 50-100 ka humid interval prior to the beta-Tulu Bor eruption ca. 3.40 Ma, when the Lokochot Lake occupied part of the Turkana Basin. Molecular abundance distributions indicate that long chain n-alkanoic acids in marine sediments are the most reliable proxy for terrestrial vegetation (Carbon Preference Index, CPI = 4.5), with more cautious interpretation needed for n-alkanes and lacustrine archives. Marine sediments record carbon isotopic variability in terrestrial biomarkers of 2-3 parts per thousand, roughly equivalent to 20% variability in the C-3/C-4 vegetation contribution. The proportion of C-4 vegetation apparently increased at times of low terrigenous dust input. Terrestrial sediments reveal much larger (2-10 parts per thousand) shifts in n-alkanoic acid delta C-13 values. However, molecular abundance and isotopic composition suggest that microbial sources may also contribute fatty acids, contaminating the lacustrine sedimentary record of terrestrial vegetation. (c) 2007 Elsevier Ltd. All rights reserved.


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DOI 10.1016/j.orggeochem.2007.06.008