Late Quaternary vegetation history of southeast Africa: The molecular isotopic record from Lake Malawi

Accurate reconstructions of past hydrological variability are essential for understanding the climate history of the tropics. In tropical Africa, the relative proportion of vegetation utilizing the C3 vs. C4 photosynthetic pathway is mainly controlled by precipitation and thus past hydrological changes can be inferred from the vegetation record. In this study, biomarkers of terrestrial plants (lignin phenols and plant leaf wax carbon isotopes) are examined from a well-dated sedimentary record from Lake Malawi to provide a vegetation (aridity) record of the past 23 cal ka from southeast Africa. We suggest that the ratio of cinammyl to vanillyl (C/V) lignin phenols in Lake Malawi sediments mainly reflects inputs of C3 trees (woody tissue) vs. C4 grasses (non-woody tissue) and find that changes in the C/V ratio generally support variability noted in the n-alkane carbon isotope record. Together, these records provide evidence for increased C4 vegetation (grasses) surrounding Lake Malawi during Last Glacial Maximum (LGM), the Younger Dryas, in the early Holocene, and from ~ 2 cal ka to the present, suggesting drier conditions at these times. Elevated inputs of C3 vegetation are noted in the intervals from ~ 17–13.6 cal ka and ~ 7.7–2 cal ka, indicating wet conditions in southeast Africa. A relationship is noted between the n-alkane average chain length (ACL) and temperature, with longer ACLs associated with higher temperatures. Higher n-alkane carbon preference index (CPI) values correlate with higher mass accumulation rates of biogenic silica and may result from periodic increased northerly winds over Lake Malawi, which enhance upwelling and diatom productivity while simultaneously increasing erosion and transport of plant leaf waxes to the lake. The molecular data produced in this study suggest that the carbon isotopic signature of bulk sediment (δ13CTOC) in Lake Malawi is primarily a reflection of terrestrial inputs (C3 vs. C4 vegetation) and may not mainly reflect changes in algal productivity, as previously thought.