Environmental controls on long-chain alkenone occurrence and compositional patterns in lacustrine sediments, northwestern China

Abstract

Long chain alkenones occur widely in lacustrine settings and their compositions are potential proxies for reconstructing terrestrial paleoclimatological and paleoenvironmental changes. However, factors influencing their occurrence, abundance, compositional pattern and unsaturation indices are not well understood. Here we investigated the alkenones in surface sediments from 19 lakes in northwestern China as well as potential environmental factors. Of the lakes, 12 contain long chain alkenones, 3 have the isomeric C37:3 alkenone and 3 have very low abundance of alkenones. Our results show that salinity has a substantial influence on the occurrence, concentration and compositional pattern. Most of the compositions are dominated by tri-unsaturated compounds, with C38 methyl ketones absent. The predominance of C37:4 and the occurrence of a C37:3 isomer are suggested to be responses to low salinity as both are only found in the three alkenone-containing freshwater lakes. The proportion of C37:4 varies by up to 50% and shows a negative relationship with salinity within the surveyed region. An arched relationship of alkenone content and salinity is apparent, with no or very low contents of alkenones in freshwater (salinity<3g/L) and hypersaline (>100g/L) environments. The U37K′ index is suggested to be a better terrestrial temperature proxy than U37K as U37K′ values from lakes containing elevated alkenone content are all within the ranges of published U37K′-temperature calibrations, and U37K values show a weak positive relationship with salinity.