Long-chain alkenones in the lake sediments of North-Minusinsk Valley (southern Siberia): implications for paleoclimate reconstructions

Abstract

Long-chain alkenones (LCAs) are polyunsaturated methyl or ethyl ketones exclusively produced by Groups 1–3 Isochrysidales, an order of haptophyte algae. LCAs have great potential as temperature and salinity proxies for paleo-reconstructions. Alkenone producers occur extensively in lacustrine and in open marine systems. However, factors influencing LCAs occurrence, abundance, compositional pattern, and unsaturation indices are insufficiently understood in lakes of southern Siberia. Here we investigated alkenone distributions in freshwater to saline lakes of a semi-arid region of southern Siberia (n = 22). We find that total alkenone concentrations increase dramatically above a salinity of about 20 g L−1. Analysis of 18S rRNA and the alkenone composition suggest that all alkenone-containing lakes are inhabited by Group 2 haptophytes except one low-saline lake, where Group 1 representatives prevail. The C37/C38 ratio is negatively correlated with salinity, and, hence, this parameter can be used as a salinity indicator. At the same time, %C37:4 is not correlated with salinity, whereas the average chain length of alkenones increases with salinity. The taxonomic composition of haptophytes and the alkenone composition were comparable to those in the lakes in the Canadian Prairies, potentially because of the similarities in climatic factors and ionic composition of lake water in the two regions. This study shows for the first time that unsaturation indexes U40K and U40K′ are positively correlated with salinity and, thus, can be used as a salinity proxy for regional climate reconstruction.