Alkanes, compound-specific carbon isotope measures and climate variation during the last millennium from varved sediments of Lake Xiaolongwan, northeast China

Abstract

Annually laminated sediments in Lake Xiaolongwan, northeast China, contain a suite of n-alkanes (C17–C33) with a strong odd over even carbon number predominance. Biomarker n-alkane proxies (average chain length, Paq ratio, grass/tree ratio, carbon preference index and compound-specific δ13C values) were used to reconstruct climate changes that occurred over the last millennium. Compound-specific δ13C values show large differences between the distinctive chain length groups of n-alkanes that originate from algae, aquatic macrophytes and terrestrial plants. Long-chain n-alkanes (C27–C33) are predominantly derived from leaf wax lipids in the forest. Variations in long-chain n-alkane δ13C values may mainly record water-use efficiency, inasmuch as the contribution from C4 plants is negligible in the Lake Xiaolongwan catchment. Short- and middle-chain n-alkanes are mainly from algae and aquatic plants. They are strongly depleted in 13C. This feature may be linked to a methane-derived, negative δ13C pool and lake overturn, which regulates dinoflagellate blooms. Parallel fluctuations are observed in δ13C27–31 values, Paq, and the grass/tree ratio throughout the record. Variations in δ13C27–31 values and Paq are in agreement with historical documents on summer and winter climate conditions. They support earlier suggestions that δ13C values in the long-chain n-alkanes and Paq may be useful indicators of effective precipitation or drought stress in this forested area. The δ13C27–31 index and Paq show distinct decadal variations. Periods with high δ13C27–31 values and a low Paq index correspond with the warm phases of the Pacific Decadal Oscillation (PDO). Values are reversed during PDO cool phases. At the decadal timescale, summer monsoon rainfall in northeast China over the last millennium may have been regulated mainly by the PDO.