Aridification and orbital forcing of eastern African climate during the Plio-Pleistocene

Abstract

We developed a composite outcrop proxy record of Plio-Pleistocene climate in eastern Africa using the radiometrically-dated sediments of the Omo Group that preserve evidence for early hominin evolution. New stable oxygen isotopic measurements from pedogenic carbonate nodules (δ18OPC, n = 270) are added to the existing database for a total of 759 observations spanning a ~ 3.6 Myr interval from 4.33–0.65 Ma. Linear interpolation of sample positions between tuffaceous marker horizons produced the chronostratigraphy of the δ18OPC record that was correlated to Indo-Pacific marine core sediment records and astronomical solutions of orbital climate forcing. Rainfall/evaporation changes inferred from the δ18OPC record carry orbital eccentricity signals, and the amplitude modulation of short eccentricity (~100 kyr) predicted by the astronomical solutions is evident during some intervals. δ18OPC variations recover many of the ~100 kyr cycles between ~4.3–0.7 Ma, with chronostratigraphic offsets between the Omo Group time series and astronomical solutions usually within the resolution of the radiometric uncertainties, i.e., ≤ 50 kyr; however, missing time at ~1.2 Ma contributes to nearly 150 kyr of offset. These δ18OPC data also indicate a long-term drying trend beginning at 2.11 Ma that correlates to a thicker eastern Indian Ocean mixed layer and upwelling in the eastern Pacific, implying a simultaneous response across the tropics to strengthening Walker Circulation. Published δ13CPC values from the same pedogenic carbonate nodules indicate that C4 vegetation abundances increased beginning at ~2.0 Ma, also suggesting climatic drying. Spectral analysis of the δ13CPC time series provides weaker evidence of orbital forcing as compared to the δ18OPC time series, particularly for long orbital eccentricity (405 kyr) and climatic precession (~20 kyr), suggesting other factors, such as feeding ecology of herbivores or global greenhouse gas, may have contributed to the vegetation patterns. The climatic precession component to the δ18OPC time series increases amplitude through strengthening Walker Circulation, which may signal a change in the intensity of interannual climate events. No indications were observed to suggest that the Plio-Pleistocene Northern Hemisphere glaciation resonated as either climatic cyclicity or an abrupt, stepwise shift in the regional hydroclimate. At major junctures of hominin evolution, including the origins of different genera and the earliest stone tools, environmental variability derived from the orbital forcing of the monsoon appears to have been low. However, we propose that at 2.11–1.66 Ma, directional aridity coupled with interannual climate events presented a new adaptive context where Homo erectus and Acheulian stone tools first appeared in eastern Africa.