Studies of African faunal evolution suggest intensified mammalian speciation and migration during the Plio- Pleistocene (c. 1.85 Ma) was triggered by regional aridification and high-amplitude climate variability. Marine sedimentological and stable isotopic studies indicate a changing moisture balance in East Africa during this time was paced by orbital-scale variations that influence African monsoon circulation and rainfall. Coeval lithostratigraphic sections from Olduvai Gorge, Tanzania demonstrates largely precessional (c. 21 kyr) Milankovitch forcing of lake sediment facies. Pollen studies and isotopic analysis of pedogenic carbonates from the same site indicate rainfall may have increased by one-third between cycles. However, evaporative and diagenetic soil processes can significantly alter the δ18O of precipitated carbonate. Thus, isotopic records of material less subject to post-depositional alteration are pivotal for analysis of local hydrological regimes. We hypothesize isotopic analysis of lacustrine organic material can provide evidence for moisture regime change in the paleolake Olduvai catchment at a time marked by the first migrations of Homo from Africa. Here we report bulk organic δ 13C and compound-specific n-alkane δ 13C and δD data from paleolake Olduvai. Bulk carbon isotopic analyses show ~10‰ range with values as enriched as -16.9‰ and as depleted as -26‰. Furthermore, bulk δ 13C values vary in concert with calculated solar insolation between 1.5 and 2.0 Ma. Hydrogen and carbon isotopic signatures for plant- (n-C29 and n-C31) and algae-derived (n-C17 and n- C19) straight-chain alkanes also have a significant spread in isotopic values. The wide range of bulk and molecular isotopic values from Olduvai Gorge samples and strong correlation of the data with sedimentological and geochemical results from previous studies indicate stable isotopic analysis of lacustrine organic material can record hydrologic signatures. Specifically, stable carbon isotopes indicate fluctuating ecological input to the lake that occurs in tandem with insolation cycles and correlate to moisture availability. Variability of n-alkane stable hydrogen isotopic values indicate changes in local hydrology, and potentially indicate rainfall amounts. This work implies significant variability characterized the ecological and hydrological conditions faced by mammals at the Plio-Pleistocene boundary and potentially influenced natural selection processes associated with hominin evolution.
|Publication status||Published - Dec 2008|
|Event||American Geophysical Union Fall Meeting 2008 - San Francisco, United States|
Duration: 15 Dec 2008 → 19 Dec 2008
|Conference||American Geophysical Union Fall Meeting 2008|
|Period||15/12/08 → 19/12/08|
- Isotopic chemisty
Magill, C., Freeman, K., & Ashley, G. (2008). Organic matter reponse to orbital rhythms at Olduvai Gorge, c. 1.80 Ma. Poster session presented at American Geophysical Union Fall Meeting 2008, San Francisco, United States.