Ecological development of Lake Donggi Cona, north-eastern Tibetan Plateau, since the late glacial on basis of organic geochemical proxies and non-pollen palynomorphs

Organic geochemical proxy data from surface sediment samples and a sediment core from Lake Donggi Cona were used to infer environmental changes on the northeastern Tibetan Plateau spanning the last 18.4kyr. Long-chain n-alkanes dominate the aliphatic hydrocarbon fraction of the sediment extract from most surface sediment samples and the sediment core. Unsaturated mid-chain n-alkanes (nC _23:1 and nC _25:1) have high abundances in some samples, especially in core samples from the late glacial and early Holocene. TOC contents, organic biomarker and non-pollen-palynomorph concentrations and results from organic petrologic analysis on selected samples suggest three major episodes in the history of Lake Donggi Cona. Before ca. 12.6calka BP samples contain low amounts of organic matter due to cold and arid conditions during the late glacial. After 12.6calka BP, relatively high contents of TOC and concentrations of Botryococcus fossils, as well as enhanced concentrations of mid-chain n-alkanes and n-alkenes suggest a higher primary and macrophyte productivity than at present. This is supported by high contents of palynomorphs derived from higher plants and algae and was possibly triggered by a decrease of salinity and amelioration of climate during the early Holocene. Since 6.8calka BP Lake Donggi Cona has been an oligotrophic freshwater lake. Proxy data suggest that variations in insolation drive ecological changes in the lake, with increased aquatic productivity during the early Holocene summer insolation maximum. Short-term drops of TOC contents or biomarker concentrations (at 9.9calka BP, after 8.0 and between 3.5 and 1.7calka BP) can possibly be related to relatively cool and dry episodes reported from other sites on the north-eastern Tibetan Plateau, which are hypothesized to occur in phase with Northern Hemisphere cooling events.