Phytoliths and Microcharcoal from East African Lake Sediments Used to Reconstruct Plio-Pleistocene Vegetation and Hydroclimate

Abstract

Global climate is changing at a rate that is unprecedented for the modern era, and atmospheric CO2 is at a level that has not been seen in ~3 million years. Recorded climate observations extend back only ~130 years. In order to understand how the Earth system may respond to present and future levels of CO2 forcing, we need to interpret ancient archives of climate and vegetation. The study of human evolution also involves studying past climate and vegetation, as it may have been influenced by climate change. Reconstructing past climate and vegetation often requires the use of robust molecular and microscopic fragments of plants. This dissertation uses microscopic plant opal silica phytoliths and microscopic charcoal from East Africa to reconstruct vegetation and hydroclimate for three important places and times in Earth and human history: (i) the Lake Malawi region after the Mount Toba supereruption at ~74 ka, (ii) the mid-Piacenzian Warm Period (MPWP) and the Plio-Pleistocene transition (3.29–2.57 Ma) in the Baringo Basin, Kenya, and (iii) the early Pleistocene (1.87–1.38 Ma) in the Turkana Basin, Kenya. These three study sites were investigated by drilling ancient lake sediments to recover soft sediment cores. From these cores, a total of 1029 samples were analyzed for phytoliths and microcharcoal. For the Mount Toba supereruption, we found no change in low elevation tree cover, or change in cool climate C3 and warm season C4 xerophytic and mesophytic grass abundance that was outside of normal variability for samples synchronous or proximal to the eruption. A spike in locally derived macrocharcoal and xerophytic C4 grasses immediately after the Toba supereruption indicated reduced precipitation and die-off of at least some afromontane vegetation, but did not signal volcanic winter conditions as had been hypothesized. For the Baringo Basin during the MPWP (3.26–3.01 Ma), which is the last time global atmospheric CO2 in a warm world was at levels similar to today, intervals with exceptionally high microcharcoal influx suggest regional turnover from woody to open habitats was driven in part by fire. After ~3.0 Ma, low elevation woody cover likely never exceeded 40% and mesic tall-grass vs. xeric short-grass savanna varied at precessional periodicities. For the Turkana Basin during the early Pleistocene, spectral analysis found that microcharcoal varied at precession (23–19 kyr), half-precession (9.6 kyr), and quarter-precession (5.1 kyr) periodicities, and linked orbital-forced peaks in precipitation with elevated fire on the landscape. Phytoliths show that C4 mesic and C4 xeric grasses varied at precession and quarter-precession periodicity. Identification of the Natoo Tuff in WTK13 allowed a direct linkage to the Nariokotome Boy Homo erectus/ergaster site. At the time that the Nariokotome Boy walked this ancient landscape, phytoliths indicate that the area was a seasonally wet and open environment dominated by short stature C4 Chloridoideae grasses, sedges, and other herbaceous plants.