Description: 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 (~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 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.

Description: The distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (GDGT) lipids was studied in the sedimentary record of Lake Challa, a permanently stratified, partly anoxic crater lake on the southeastern slope of Mt. Kilimanjaro (Kenya/Tanzania), to examine if the GDGTs could be used to reconstruct past variation in regional temperature. The study material comprised 230 samples from a continuous sediment sequence spanning the last 25 ka with excellent age control based on high-resolution AMS 14C dating. The distribution of GDGTs showed large variation through time. In some time intervals (i.e., from 20.4 to 15.9 ka BP and during the Younger Dryas, 12.9?11.7 ka BP) crenarchaeol was the most abundant GDGT, whereas at other times (i.e., during the Early Holocene) branched GDGTs and GDGT-0 were the major GDGT constituents. In some intervals of the sequence the relative abundance of GDGT-0 and GDGT-2 was too high to be derived exclusively from lacustrine Thaumarchaeota, suggesting a sizable contribution from methanogens and other archaea. This severely complicated application of TEX86 palaeothermometry in this lake, and limited reliable reconstruction of lake water temperature to the time interval 25-13 ka BP, i.e. the Last Glacial Maximum and the period of post-glacial warming. The TEX86-inferred timing of this warming is similar to that recorded previously in two of the large African rift lakes, while its magnitude is slightly or much higher than that recorded at these other sites, depending on which lake-based TEX86 calibration is used. Application of calibration models based on distributions of branched GDGTs developed for lakes inferred temperatures of 15-18 °C for the Last Glacial Maximum and 19-22 °C for the Holocene. However, the MBT/CBT palaeothermometer reconstructs temperatures as low as 12 °C for a Lateglacial period centred on 15 ka BP. Variation in down-core values of the BIT index are mainly determined by the varying production rate of crenarchaeol relative to in-situ produced branched GDGTs. The apparent relationship of the BIT index with climatic moisture balance can be explained either by the direct influence of lake level and wind strength on nutrient recycling, or by influx of soil nutrients promoting aquatic productivity and nitrification. This study shows that GDGTs can aid in obtaining climatic information from lake records but that the obtained data should be interpreted with care.