Biogeochemical Proxies of Climatic and Environmental Change from the Sedimentary Record of Paleolake Olduvai ( 1.80 - 1.88 Ma)
Abstract
Sediment cores recovered by the Olduvai Gorge Coring Project (OGCP) provide a long-term record of paleolacustrine systems in the Olduvai region that extends through most of the Pleistocene. The stratigraphic interval from the Bed 1 lavas (1.877 Ma) to the Tuff 1F volcanic horizon (1.803 Ma) comprises an extensive sequence of laminated dark anoxic claystones containing pyrite that ultimately transition to light-colored sandy claystones. The laminated claystones frequently contain suites of alkenones derived from phytoplankton with distributions (C37, C39 alken-2-ones; C38, C40 alken-3-ones) typical of saline lacustrine settings except that alkadienones are dominant and alkatetrenones are absent; series of microbial C27-C32 hopanes and their 2-methylhopane homologues are also present. Biomarkers in the sandy claystones are dominated by n-alkanes maximizing at C31 or C33 and exhibiting high CPI (carbon preference index) values for C25-C35 (6.1-8.7) typical of inputs from leaf waxes. Further evidence for contributions from terrestrial organic matter (OM) is provided by the prevalence of dehydroabietic acid, and abundant series of both n-alkan-2-ones and n-alkan-3-ones likely derived from soils. Thus, the biomarker distributions document two distinct organic geochemical facies corresponding to a shift in inferred sources of OM from aquatic to terrestrial that occurs in conjunction with a change in lithofacies and depositional setting associated with shallowing of Lake Olduvai over 50 kyr prior to the Tuff 1F volcanic horizon. The carbon isotopic composition of OM (δ13CTOC) from the entire sequence varies between values representative of more forested ( -27‰) and open grassland ( -15‰) ecosystems over 21 kyr, consistent with the Milankovitch precession cycle. Biomarker profiles, including n-alkane chain-length distributions and the ratio of the plant wax n-C31 to the microbial hop-17(21)-ene, record shifts in the productivity of the lake within the laminated claystone interval that are consistent with the precession cycle observed in the δ13CTOC. The δ13CTOC also provides evidence for sub-Milankovitch changes superimposed on the precession cycle, suggesting that environments inhabited by hominins varied more than previously documented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP23A2316B
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 4239 Limnology;
- OCEANOGRAPHY: GENERALDE: 4950 Paleoecology;
- PALEOCEANOGRAPHY