Ecological constraints on hydrology in early hominid environments

Paleoclimate studies increasingly apply the hydrogen isotopic composition of individual biomarkers as a proxy for the composition of environmental waters. However, the environmental, physical and ecologic influences on hydrologic signatures are complex. Here, we separate the influences of climate and physiology on the hydrogen isotopic compositions of plant and algae lipids in order to reconstruct ancient precipitation and lake waters in semi-arid East Africa using Plio-Pleistocene lake sediments from Olduvai Gorge (2°48'S, 35°06'E). We measured bulk organic δ13C and molecular δ13C and δD from perennial lacustrine sediments dated between ~1.79 and 1.95 million years ago, a time slice with recognized hominid diversification events. During this interval, bulk organic δ13C varies ~10‰ and correlates strongly with molecular δ13C signatures of alkane biomarkers derived from terrestrial plants (n-C31), which range between -20‰ and -36‰ (PDB). Molecular δD signatures of n-C31 range between ~-125‰ and -165‰ (SMOW). The δD of algal biomarkers (n-C17) range between ~-85‰ and -135‰ (SMOW). To account for physiological effects, we used the δ13C of n-C31 to estimate relative C4 monocot versus C3 dicot abundance in the Olduvai watershed, establishing a mixing line for deuterium fractionation between rainwater and plant lipids. This approach is based on models of modern ecologic succession in East Africa, where C4 monocots and C3 dicots dominate landscape biomass. In the present day, the isotopic composition of mean annual precipitation in East Africa is controlled by the ‘amount effect.’ Olduvai currently receives ~550 mm yr-1 of precipitation and δD = -10‰, with an average ‘amount effect’ of 32 mm per 7‰ change in δD, albeit based on sparse sampling. Using these constraints and assuming negligible evapotranspiration, we conservatively calculate that Olduvai experienced ~440 mm of precipitation during arid times and nearly 800 mm during wetter times - a reconstruction that is strikingly similar to regional estimations for the early Pleistocene derived from pollen spectra and pedogenic carbonates. We estimated the paleochemistry of paleolake Olduvai using lake-sediment outcrops, faunal remains and analogous modern lakes in East Africa. We used the percent total organic carbon in Olduvai sediments as a relative indication of depth within the constraints of previously published depth boundaries. Fossil remains of tilapia and catfish constrain a lower lake salinity level of 10-30‰, while the presence of trona and gaylussite indicate hypersaline conditions in a framework of modern East African alkaline lakes. We then accounted for fractionation variability in algae due to changes in salinity, calculating that δD ranged between ~+80‰ and 0‰ in paleolake Olduvai waters - values within the modern range of δD for lake waters. In summary, our results indicate that Olduvai experienced essentially complete transitions between C4 monocot and C3 dicot landscape dominance, accompanied by a doubling of mean annual rainfall. Consequent salinity changes in paleolake Olduvai resulted in algal hydrogen isotopic fractionation factors that varied by ~40‰.