Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

Abstract

The Olorgesailie Formation (1.2–0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin (Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8–9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200–16,000 μS cm− 1 and pH of 7.5–9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.

Environment and Climate of Early Human Evolution

“…Terrestrial environments, especially in rift basins, can be highly variable, which becomes important in understanding the range of the resources available to early hominins, in reconstructing biogeography, and in determining whether a paleoenvironmental record is representative of a greater area. Lateral variability has been documented in fluvial and lacustrine settings in eastern Africa on scales ranging from hundreds of meters to thousands of kilometers (Bobe et al 2007;Levin et al 2004Levin et al , 2011Owen et al 2011;Quinn et al 2007). The record from the Shungura Formation, discussed above, provides a great example of how it is possible to identify distinct environments on a landscape.…”

Episodes of environmental stability versus instability in Late Cenozoic lake records of Eastern Africa

“…The EMPT is recorded in the 80 m thick Olorgesailie Formation exposed in the Olorgesailie basin, which covers the interval between~1.25 Ma and 0.49 Ma (Deino and Potts, 1990;Behrensmeyer et al, 2002;Owen et al, 2008Owen et al, , 2011Deocampo et al, 2010). The age model is based on seven 40 Ar/ 39 Ar ages of tephra layers, of which neither the oldest date (1.20 Ma) nor the~780 ka BruhnesMatuyama magnetic reversal has an error bar (Deino and Potts, 1990;Behrensmeyer et al, 2002;Owen et al, 2008Owen et al, , 2011Deocampo et al, 2010).…”

“…The age model is based on seven 40 Ar/ 39 Ar ages of tephra layers, of which neither the oldest date (1.20 Ma) nor the~780 ka BruhnesMatuyama magnetic reversal has an error bar (Deino and Potts, 1990;Behrensmeyer et al, 2002;Owen et al, 2008Owen et al, , 2011Deocampo et al, 2010). Again, we used the new probabilistic technique for complex stratigraphic sequences by to determine the best age model of the Olorgesailie sequence.…”

“…According to the interpretation of Owen et al (2009), only~45% of the time interval between~1.25 Ma and 0.49 Ma is recorded in the sediments of the Olorgesailie formation. On the other hand, the Olorgesailie formation is the key section for the variability selection hypothesis by Potts (1996) claiming that environmental instability is a key driver of evolution.…”

“…The possible influence of environmental variability, both temporally and spatially, on human evolution and dispersal is an intensely debated topic in the scientific community (Potts, 1996;Trauth et al, 2005Trauth et al, , 2007Maslin and Christensen, 2007;Maslin and Trauth, 2009;Potts, 2013;Maslin et al, 2014). However, recent evidence indicates that episodes of stable environmental conditions play an equally important role for human evolution, dispersal, and technological innovation (e.g., Grove, 2013;Shultz and Maslin, 2013).…”