Divergent patterns of Holocene hydro-climatic evolution in arid central Asia and the Asian summer monsoon margin indicated by Pediastrum records

Subfossil Pediastrum, a genus of green algae (Chlorophyceae) which is sensitive to changes in the aquatic environment, is a potential proxy indicator of the evolution of aquatic ecosystems and climate change. We synthesized subfossil Pediastrum records from arid central Asia (ACA) and the Asian summer monsoon (ASM) region in order to characterize their hydro-climatic evolution during the Holocene. In ACA, low concentrations of subfossil Pediastrum during the early Holocene resulted from the higher salinity caused by a dry climate or lower lake levels. The increasing trend of subfossil Pediastrum content during the middle-late Holocene is consistent with the occurrence of wetter regional climatic conditions. In the marginal areas of the Asian summer monsoon (MASM) region of northern China, the variations in subfossil Pediastrum concentration coincide with increased summer monsoon rainfall during the middle Holocene, but concentrations were generally lower during the late Holocene as a result of increased salinity or alkalinity associated with a weakened summer monsoon. In the Tibetan Plateau, the variations in subfossil Pediastrum abundance are more complicated due to divergent moisture/precipitation patterns influenced by both the ASM and the mid-latitude westerlies. In the Indian summer monsoon-dominated region, subfossil Pediastrum abundance was high during the early Holocene and then gradually decreased after similar to 8 cal kyr BP. The observed asynchronous changes in subfossil Pediastrum in ACA and the MASM region during the Holocene may reflect the specific moisture/precipitation patterns of the monsoon- or westerlies-dominated climatic regimes. We suggest that high evaporation in arid and semi-arid areas would have rapidly increased the salinity and/or alkalinity of lakes, while aquatic environments may have been more stable in more humid areas.