Glacial/interglacial variations in carbon cycling revealed by molecular and isotope stratigraphy of Lake Nkunga, Mt. Kenya, East Africa

The lipid content (n-alkanes, n-alkanols, n-alkanoic acids) and the delta(1 3)C values of these lipids were measured in nineteen sediment samples taken from a 21.18 m long core from Lake Nkunga, a high-altitude (1820 m a.s.l.) , freshwater lake situated in the montane rain forest on Mt. Kenya. The rec ord spans the early last glacial (marine isotope stages 5a to 4) and the la te Holocene (1030 +/- 45 C-14 yr BP to the present day). The intervening hi atus spans the last glacial maximum and the early to mid-Holocene. The mole cular biogeochemical results are interpreted in terms of past changes in cl imate, vegetation and atmospheric CO2 concentration. Three different source s of organic matter input were identified from the n-alkyl lipids, namely t errestrial plants, aquatic plants and algae. Unusual distribution patterns of dominant mid-chain length n-alkanes (C23-25), n-alkanols (C22-24) and n- alkanoic acids (C22-24) are attributed to aquatic macrophytes. An 11 parts per thousand decrease in bulk-carbon isotope values between the sediments o f early glacial (-17 parts per thousand) and late Holocene age (-28 parts p er thousand) is also apparent in the individual homologues of the n-alkyl l ipids. The observed isotopic shift in terrestrial higher-plant biomarkers i s similar to that observed in nearby Sacred Lake and is consistent with an expansion of C-4 grasses or sedges during the early glacial. The correspond ing vegetation was a dry montane juniper-olive forest with a grassy underst orey, representing cold, dry conditions with lower CO2. (C) 1998 Elsevier S cience Ltd. All rights reserved.