3-3 Temporal Changes in the d13Corg of Early Pleistocene Sediments from Paleolake Olduvai Reflect Enhanced Lake Alkalinity rather than Increased Contributions from C4 Plants

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources

Presenting Author:

Simon Brassell

Authors:

Doiron, Kelsey E¹, Colcord, Devon E², Shilling, Andrea M³, Stanistreet, Ian G⁴, Stollhofen, Harald⁵, Njau, Jackson K⁶, Schick, Kathy D⁷, Toth, Nick P⁸, Brassell, Simon C⁹

(1) Earth & Planetary Sciences, Harvard University, Cambridge, MA, USA, (2) Earth & Atmospheric Sciences, Indiana University, Bloomington, IN, USA; Anoka-Ramsey Community College, Coon Rapids, MN, USA, (3) Earth & Atmospheric Sciences, Indiana University, Bloomington, IN, USA, (4) Earth, Ocean & Ecological Sciences, University of Liverpool, Liverpool, United Kingdom; Stone Age Institute, Gosport, IN, USA, (5) GeoZentrum Nordbayern, Friedrich-Alexander-Universität (FAU), Erlangen, Germany, (6) Earth & Atmospheric Sciences, Indiana University, Bloomington, IN, USA, (7) Stone Age Institute, Gosport, IN, USA, (8) Stone Age Institute, Gosport, IN, USA, (9) Earth & Atmospheric Sciences, Indiana University, Bloomington, IN, USA,

Abstract:

The stratigraphic succession of outcrops and sediment cores from Olduvai Gorge record early Pleistocene wet/dry climate cycles driven by orbital forcing that affected lake levels in eastern Africa. A prominent feature of the depositional history of Upper Bed I (1.90 - 1.80 Ma) for Paleolake Olduvai during this critical interval in hominin evolution are variations in the δ¹³C composition of sedimentary organic matter (OM) aligned with wet/dry climate cycles. The C_orgcontents for wetter climatic intervals are higher than those for drier intervals (avg. 2.54 % vs. 2.02 %) and their δ¹³C composition (δ¹³C_org) is markedly more negative (averages of -24.13 ‰ versus -17.85 ‰). This difference in sedimentary δ¹³C_org, also observed in contemporaneous paleosols, has previously been attributed to an increase in C₄versus C₃ plant contributions during drier intervals leading to less negative δ¹³C_org values. However, sedimentary C/N ratios indicate that wetter and drier intervals contain different proportions of OM derived from terrestrial and aquatic sources, with the latter more pronounced during drier episodes. The distributions of source-diagnostic biomarkers echo this assessment as the abundances of components derived from algae are markedly more abundant in drier intervals. Evaluation of δ¹³C signatures for these aquatic biomarkers confirms their shift toward less negative values from wetter to drier intervals, as demonstrated by data for sterenes derived from algal sterols (-17.52 ‰ to -15.82 ‰) and for alkenones sourced by haptophyte algae (-20.33 ‰ to -14.75 ‰). The less negative δ¹³C values for these phytoplankton biomarkers can be attributed to the utilization of bicarbonate rather than CO₂ as a carbon substrate. This shift reflects a change in the equilibrium between free CO₂ and bicarbonate in the dissolved inorganic carbon (DIC) pool resulting from an increase in alkalinity associated with lake regression caused by a decline in precipitation and run-off during drier intervals. These data confirm that δ¹³C signatures of aquatic biomarkers can reflect variations in lake alkalinity linked to temporal changes in terrestrial hydroclimate. Thus, a shift toward less negative δ¹³C_org values in lacustrine sediment sequences requires caution in interpretation since it does not necessarily represent evidence for an increase in OM contributions from C₄ plants.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10476

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.