A 25,000-year record of climate-induced changes in lowland vegetation of eastern equatorial Africa revealed by the stable carbon-isotopic composition of fossil plant leaf waxes

The debate of climate versus CO2 in controlling the long-term dynamics of tropical African vegetation has focused on events at the upper tree-line, since the relevant paleodata tend to be from mid-elevation sites (similar to 2000-3000 m). Less well known is the relative importance of CO2 in regulating the dynamics of tropical lowland (<1500 m) vegetation, particularly that of the dry open woodlands, bush- and grasslands covering much of eastern equatorial Africa. Here we examine the stable carbon isotopic composition of n-alkanes in the sediment record of Lake Challa, a lowland crater lake near Mt. Kilimanjaro, covering the last 25,000 years. The distributions of the n-alkanes, with dominance of the long-chain odd-carbon-numbered components, and their isotopic composition reveal a mixed origin. The C-23 and C-25 n-alkanes are depleted in C-13, with delta C-13 values between -30 and -50%.. In shallow lakes these n-alkanes are thought to derive from non-emergent aquatic plants, but this is unlikely in this steep-sided crater lake as it lacks a significant littoral habitat. The C27+ n-alkanes are predominantly derived from leaf wax lipids of terrestrial plants, brought into the lake predominantly by local soil run-off. Their delta C-13 values, in particular that of the n-C-31 alkane, reveal a marked transition in local lowland vegetation from being dominated (similar to 70-100%) by C-4 plants during the glacial period until 16.5 cal kyr BP, to a more mixed C-3/C-4 composition (similar to 30-60% C-4) during the Holocene. The start of the late-glacial trend towards a greater proportion of C-3 plants coincided with the start of increasing monsoon rainfall, similar to 1500 years after the onset of the rise in atmospheric CO2 and similar to 3500 years after the onset of post-glacial warming. The transition was interrupted during the dry Younger Dryas period (13.0-11.7 cal kyr BP), when C-4 plants again became much more prevalent, almost reaching their glacial-period abundance. Notably, the principal trend in leaf-wax delta C-13 values infers C-4 dominance during both wet and dry phases of the glacial period, and a mixed C-3/C-4 vegetation during both wet and dry phases of the Holocene. Our results indicate that long-term variation in pCO(2) exerted important control on the composition of drought-adapted savanna vegetation in this currently semi-arid lowland region of equatorial East Africa. (C) 2010 Elsevier B.V. All rights reserved.