Stable carbon isotope record of Lake Challa

The C23 and C25 n-alkanes are depleted in 13C, with d13C values between-30 and-50 per mil. 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 d13C values, in particular that of the n-C31 alkane, reveal a marked transition in local lowland vegetation from being dominated (~70-100%) by C4 plants during the glacial period until 16.5 cal kyr BP, to a more mixed C3/C4 composition (~30-60% C4) during the Holocene. The start of the late-glacial trend towards a greater proportion of C3 plants coincided with the start of increasing monsoon rainfall, ~1500 years after the onset of the rise in atmospheric CO2 and ~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 C4 plants again became much more prevalent, almost reaching their glacial-period abundance. Notably, the principal trend in leaf-wax ?13C values infers C4 dominance during both wet and dry phases of the glacial period, and a mixed C3/C4 vegetation during both wet and dry phases of the Holocene. Our results indicate that long-term variation in pCO2 exerted important control on the composition of drought-adapted savanna vegetation in this currently semi-arid lowland region of equatorial East Africa.