Glacial-interglacial environmental changes inferred from molecular and compound-specific delta C-13 analyses of sediments from Sacred Lake, Mt. Kenya

Molecular Stratigraphic analyses, including lipid distributions and compoun d-specific delta(13)C measurements, have been performed at 15 levels in a s ediment core from Sacred Lake, Mt. Kenya, a high-altitude (2350 m a.s.l.) f reshwater lake with a record extending from the last glacial (>40,000 cal. yr BP) through the present interglacial; Terrestrial and aquatic organic-ma tter sources were independently assessed using source-specific biomarkers. delta(13)C values of long-chain n-alkyl lipids from terrestrial higher plan ts exhibit large glacial to interglacial shifts: those from the last glacia l maximum (LGM) (-20 to -18 parts per thousand) indicate a terrestrial vege tation dominated by C-4 grasses or sedges, whereas those from the early Hol ocene (-34 to -27 parts per thousand) reflect recolonization of the catchme nt area by C-3 plants, consistent with a rapid rise in the upper treeline. Specific algal biomarkers, including five unsaturated hydrocarbons of novel structure ascribed to the microalga Botryococcus braunii, were abundant, a s confirmed by scanning electronic microscopy (SEM). An extreme delta(13)C shift of over 25 parts per thousand is displayed by the algal biomarkers, a n elevated value of -5.1 parts per thousand at the last glacial maximum (LG M) contrasting with a minimum value of -30.3 parts per thousand at the begi nning of the Holocene. A major change in the molecular distributions of the algal biomarkers parallels this large delta(13)C shift, with acyclic isopr enoid hydrocarbons dominating the last glacial and cyclic isoprenoid hydroc arbons the Holocene. The low atmospheric partial pressure of CO2 (pCO(2)) a t the LGM would favour photosynthetic organisms possessing CO2-concentratin g mechanisms, including terrestrial C-4 grasses and freshwater green algae. Hence, glacial/interglacial changes in pCO(2), and in the CO2:O-2 ratio in particular, had a significant impact on both terrestrial and aquatic ecosy stems on Mt. Kenya, in addition to the effects of climate and local environ mental factors. Copyright (C) 1999 Elsevier Science Ltd.