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

Abstract

Molecular stratigraphic analyses, including lipid distributions and compound-specific δ 13C measurements, have been performed at 15 levels in a sediment core from Sacred Lake, Mt. Kenya, a high-altitude (2350 m a.s.l.) freshwater lake with a record extending from the last glacial (>40,000 cal. yr BP) through the present interglacial. Terrestrial and aquatic organic-matter sources were independently assessed using source-specific biomarkers. δ 13C values of long-chain n-alkyl lipids from terrestrial higher plants exhibit large glacial to interglacial shifts: those from the last glacial maximum (LGM) (−20 to −18‰) indicate a terrestrial vegetation dominated by C 4 grasses or sedges, whereas those from the early Holocene (−34 to −27‰) reflect recolonization of the catchment 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, as confirmed by scanning electronic microscopy (SEM). An extreme δ 13C shift of over 25‰ is displayed by the algal biomarkers, an elevated value of −5.1‰ at the last glacial maximum (LGM) contrasting with a minimum value of −30.3‰ at the beginning of the Holocene. A major change in the molecular distributions of the algal biomarkers parallels this large δ 13C shift, with acyclic isoprenoid hydrocarbons dominating the last glacial and cyclic isoprenoid hydrocarbons the Holocene. The low atmospheric partial pressure of CO 2 ( pCO 2) at the LGM would favour photosynthetic organisms possessing CO 2-concentrating mechanisms, including terrestrial C 4 grasses and freshwater green algae. Hence, glacial/interglacial changes in pCO 2, and in the CO 2:O 2 ratio in particular, had a significant impact on both terrestrial and aquatic ecosystems on Mt. Kenya, in addition to the effects of climate and local environmental factors.