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

Abstract

The lipid content (n-alkanes, n-alkanols, n-alkanoic acids) and the δ13C 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 record spans the early last glacial (marine isotope stages 5a to 4) and the late Holocene (1030±45 14C yr BP to the present day). The intervening hiatus spans the last glacial maximum and the early to mid-Holocene. The molecular biogeochemical results are interpreted in terms of past changes in climate, vegetation and atmospheric CO2 concentration. Three different sources of organic matter input were identified from the n-alkyl lipids, namely terrestrial 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‰ decrease in bulk-carbon isotope values between the sediments of early glacial (−17‰) and late Holocene age (−28‰) is also apparent in the individual homologues of the n-alkyl lipids. The observed isotopic shift in terrestrial higher-plant biomarkers is similar to that observed in nearby Sacred Lake and is consistent with an expansion of C4 grasses or sedges during the early glacial. The corresponding vegetation was a dry montane juniper–olive forest with a grassy understorey, representing cold, dry conditions with lower CO2.