Vegetation and environmental changes since the Last Glacial Maximum inferred from a lake core from Saiyong Co, central Tibetan Plateau

Pollen and grain-size records from a small lake (Saiyong Co) in the source region of the Yangtze River, central Tibetan Plateau are presented and used to reveal vegetation and lake environmental history since the Last Glacial Maximum (LGM). Results indicate that the Saiyong Co basin was a marshy environment with sparse vegetation cover from 21.3 to 6.6 cal. ka BP, before becoming a lake. Highest water level is inferred for 3.9-3.4 cal. ka BP followed by a gradual drying up until 100 years ago. Since then, the lake level has begun to rise again. Pollen spectra are dominated by Artemisia, Poaceae and Cyperaceae, and vegetation changes are mainly reflected by changes in the proportion of these three taxa, especially Artemisia and Cyperaceae. Two major events occurred at 3.9 cal. ka BP and 1.2 cal. ka BP, in response to climate change. Our results also reveal that lake-level changes affect the interpretation of the pollen assemblages due to the relationship between lake size and the pollen source area. Local pollen became the dominant component in pollen assemblages at 0.6 cal. ka BP when the lake level fell to below a certain depth. A higher percentage of Cyperaceae marks the lake's retreat and reflects the local vegetation around Saiyong Co during 0.6 to similar to 0.1 cal. ka BP. In the last 100 years, a high value of Cyperaceae is linked to a wetter climate with no obvious evidence for human activities. Our results have important implications for the interpretation of fossil pollen assemblages from small lakes as the size of the lake through its history also needs to be known to prevent erroneous interpretations of the pollen data.