Long chain alkenones in Greenland lake sediments: Low δ 13C values and exceptional abundance

Long chain alkenones (LCAs) are a key class of biomarkers for certain members of the algal class Prymnesiophyceae (i.e., prymnesiophytes). They are ubiquitous in ocean sediments where they are extensively used for paleotemperature reconstruction, but are rare in lake sediments. Here, we report the discovery of LCAs (C37-C39) throughout a sediment core from one lake and in surface sediments of five lakes located in west Greenland. LCA concentrations in the surface sediments are one to two orders of magnitude greater than those reported for other lacustrine surface sediments around the world. The presence of C38 methyl ketones distinguishes Greenland LCA distributions from those found in other saline lakes in cold regimes. LCAs are present in five lakes with elevated salinity, but absent from five freshwater lakes in the study region. Using the published temperature calibration for lake sediments, alkenone unsaturation indices (U37K and U37K′) in the surface sediments of the Greenland lakes appear to record late spring/early summer temperature when algal blooms occur, supporting the use of lacustrine alkenones as a paleotemperature proxy. The LCAs have exceptionally low δ 13C values, ranging from −40‰ to −42‰, and are depleted by 3-10‰ relative to short chain fatty acids and sterols in the same samples. These δ 13C values are the lowest reported for alkenones in a natural setting and have important implications for tracing the alkenone producers in lakes.