Holocene Climate Change Documented By Lake Sediments From Lake Gahai In The Monsoonal Margin, Northwest China

Lake Gahai is located at a triple junction region of the East Asian summer monsoon, the westerlies, and the winter monsoon, being sensitive to the regional climate. A lake sediment core, GHB was drilled from Lake Gaihai in2008. This research focuses on the regional Holocene paleoclimate recorded by sediments from Lake Gahai. The fine fractions were analyzed for XRD, total carbonate content, carbon and oxygen isotopes of carbonate (δ18Ocand δ13Cc), TOC, TN and δ13Corg. Twenty seven AMS14C samples were dated to provide chronological control. Based on the results of Multi-proxy analysis and comparison of GHB record with those documents from adjacent regions, the hydrological process of Lake Gahai and the regional climate variability since the Holocene are reconstructed. The primary conclusions are as follows:1. The Holocene palaeoclimate reconstruction in Lake GahaiDuring the early Holocene (11890～8600cal yr BP), the climate was clearly warm-humid in this region due to the enhanced temperature and precipitation, but it was generally stable. Meanwhile, Lake Gahai had a relatively high lake level and freshwater conditions during this period. The mid-Holocene (8600to4700cal yr BP) was characterized by a relatively Warm-dry climate condition, with high-frequency variations from8600to4700cal yr BP and three wet intervals (8200,7600and7100～6600cal yr BP). However, a warm-dry and stable climate occurred during6000～4700cal yr BP. Lake Gahai experienced a generally decreasing humidity and high biological productivity compared with the early Holocene, while the lake level was declined. The late Holocene can be roughly divided into two stages:Lake Gahai experienced maximum effective moisture between4700and3000cal yr BP, the climate changed to colder but more humid than before. Lake Gahai had the lowest salinity, the highest lake level and the largest extent, whereas the biological productivity did not improved which was attributed to relatively low temperature. After3000cal yr BP, the lake experienced a coherent decrease in moisture, a sharp lake level decline and an increasing salinity continued to the present. However, the climate in the MWP was more warm and dryer, and followed by a cool and wet LIA. 2. The results of regional comparisonsRegional comparison indicates that a wet climate during the early Holocene was attributed to a stronger summer monsoon; the climate reconstruction from Lake Gahai was more similar to the monsoon region. From8600to4700cal yr BP, a dry and variable climate occurred in Lake Gahai due to the weak of summer monsoon, but it still had humid internals associated with enhanced summer monsoon. Those studies from Qaidam Basin were supported our conclusions. After4700cal yr BP, the palaoclimate change from Lake Gahai and the westerly region displayed a similar trend, which may relate to the strengthening of the westerlies.3. Possible mechanismsThe solar insolation variation on the orbital-scales in the Northern Hemisphere is the fundamental reason for the Holocene climate change in Lake Gahai. It is likely that Lake Gahai experienced more warm-humid climate during times when the Asian summer monsoon was much stronger and extended further inland due to the enhanced low-latitude solar insolation. In contrast, the westerly dominated regions were extended associated with the gradual weakening of the Asian summer monsoon which was in response to the orbitally induced decrease in summer insolation, the climate in Lake Gahai might alternate between warm-dry and cold-humid conditions at the control of the westerly. In addition, local topography may be important in modifying the climate patterns and lead to the more variable and complex climate in Lake Gahai.