Alkenones in Pleistocene Upper Bed I (1.803–1.900 Ma) sediments from Paleolake Olduvai, Tanzania

Simon C. Brassell, Devon E. Colcord, Andrea M. Shilling, Ian G. Stanistreet, Harald Stollhofen, Nicholas Toth, Kathy D. Schick, Jackson K. Njau, Katherine H. Freeman

Research output: Contribution to journalArticlepeer-review

Abstract

The Olduvai Gorge Coring Project (OGCP) recovered sequences of Pleistocene lake sediments that correlate with those known from outcrops directly associated with hominin fossil horizons. The sedimentary succession from Core 2A (02° 58′ 43′' S, 035° 19′ 25.5′' E), which targeted the ancient lake-basinal depocenter, includes stratigraphic intervals (∼1.86 Ma) sandwiched between the Bed I basalt and Tuff IF dated horizons, where remains of two hominin holotypes have been recovered. The lower part of this sequence (72.3–86.9 mbs) consists of lacustrine claystones interbedded with sandy claystones characterized by high Corg (av. 2.5%) and includes several laminated intervals. It displays shifts in terrestrial vegetation recorded by values for δ13CTOC and δ2HnC31 that reflect changes in the proportions of C3 vs C4 plants and precipitation, respectively. These temporal profiles represent higher-resolution signatures of the precession-scale wet/dry cycles previously recognized in the outcropping sedimentary succession of Olduvai Gorge. Several samples from this interval contain alkenones comprising C37 and C39 alkadien-2-ones and alkatrien-2-ones, and C38 and C40 alkadien-3-ones and alkatrien-3-ones. They are distinguished from the vast majority of alkenone distributions reported in lake sediments from polar and temperate latitudes (e.g., Greenland, N. and S. America, Europe, Russia, China, S. Africa, Antarctica) by the absence of tetraunsaturated alkenones (e.g., C37:4). The alkenone distributions vary stratigraphically with higher concentrations in the drier intervals of the precession-scale cycles that exhibit characteristics resembling those of Group II haptophytes found in contemporary saline/alkaline environments rather than those of Group I haptophytes from freshwater ecosystems. Paleolake Olduvai represents the first reported occurrence of alkenones in a low-latitude Pleistocene lake, where the absence of alkatetraenones may reflect contributions from strains of Group II haptophytes that are favored by warmer temperatures or saline conditions. Temporal variations in unsaturation indices (U37K', U38K') and the proportions of alken-2-ones and alken-3-ones likely reflect influences on alkenone production by haptophytes that are associated with changes in the lake environment primarily driven by hydrologic cycles (e.g., salinity and alkalinity) rather than lake temperatures.

Original languageEnglish (US)
Article number104437
JournalOrganic Geochemistry
Volume170
DOIs
StatePublished - Aug 2022

Bibliographical note

Funding Information:
We would like to acknowledge and thank the Stone Age Institute for organizing and funding the Olduvai Gorge Coring Project (OGCP) with grants from the Kamen Foundation, the Gordon and Ann Getty Foundation, the John Templeton Foundation, the Fred Maytag Foundation, Kay and Frank Woods, and NSF (BCS1623873). In addition, we are most grateful to the Tanzanian Commission for Science and Technology, Tanzanian Department of Antiquities, Ministry of Natural Resources & Tourism, and the Ngorongoro Conservation Area Authority (NCAA) for enabling the collection of samples and continuing support of OGCP research. Support for biomarker preparation included funds provided by the Department of Earth & Atmospheric Sciences (Indiana University). NSF grant CHE1726633 enabled purchase of the GC–QTOF-MS (Mass Spectrometry Facilities at Indiana University) utilized in this study for all biomarker analyses. We would like to thank Dr. Arndt Schimmelmann (Indiana University) for providing both an n-alkane standard mix and the squalane standard and appreciate the assistance of LacCore personnel with core handling and sample collection. We also value the helpful comments from reviewers and editors that both improved the manuscript and enabled us to address its errors and omissions.

Publisher Copyright:
© 2022 Elsevier Ltd

Keywords

  • Alkenones
  • Biomarkers
  • Hominin evolution
  • Lacustrine sediments

Continental Scientific Drilling Facility tags

  • OGDP

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