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

摘要

The debate of climate versus CO_2 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 (~2000-3000m). Less well known is the relative importance of CO_2 in regulating the dynamics of tropical lowland (<1500m) 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 25,000years. 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 ~(13)C, with δ~(13)C 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 C_(27+) n-alkanes are predominantly derived from leaf wax lipids of terrestrial plants, brought into the lake predominantly by local soil run-off. Their δ~(13)C values, in particular that of the n-C_(31) alkane, reveal a marked transition in local lowland vegetation from being dominated (~70-100%) by C_4 plants during the glacial period until 16.5cal kyr BP, to a more mixed C_3/C_4 composition (~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, ~1500years after the onset of the rise in atmospheric CO_2 and ~3500years after the onset of post-glacial warming. The transition was interrupted during the dry Younger Dryas period (13.0-11.7cal kyr BP), when C_4 plants again became much more prevalent, almost reaching their glacial-period abundance. Notably, the principal trend in leaf-wax δ_(13)C 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.