Environmental Evolution Documented By Halahu Lake Sediments At The Northern Margin Of Tibet Plateau Since4.3KaBP

The Halahu lake basin, located in the Qilian Mountains, Northern Margin of Tibet Plateau, is considered to monitor climate changes as it is located close to the present margin of Asian Summer monsoon impact and westerly-driven air masses. Several lake sediment core were drilled from Halahu lake. Core H21drilled in65m water depths and core H5from30m depth were used to reconstructed the past4.3k years hydrological and climate conditions. The age model is based on4radiocarbon dates (AMS), and the fine fractions of lake sediments were analyzed for carbonate content, carbon and oxygen isotopes of carbonate. Based on the results of Multi-proxy analysis of core H21and H5,and comparison with those documents from adjacent regions, the hydrological process of Halahu lake and the regional climate variability since the4.3kaBP are reconstructed. The primary conclusions are as follows:1. The palaeoclimate reconstruction during the4360cal yrBP in Halahu lakeHalahu lake basin sustained in a close hydrological environment in past4360years.It was on covariance relations between carbonate content and the carbonate oxygen isotopes of lake sediments,and both of them were controlled by lake water body.So the carbonate content and it oxygen isotopes can be as an indicator of environmental change, carbonate content reflect a dry or wet condition, oxygen isotopes indicates the changes in humidity effectively and the water vapor sources.The climate and Environment of Halahu lake continues to become moist Since4360cal yrBP. During4360-2100cal yrBP, the climate in Halahu region was characterized as a relatively dry climate condition, due to the high evaporation and less water supply. Meanwhile, Halahu lake had a relatively low lake level conditions during this period, and it was not stable.The most dry conditions and the lowest water level occurred during3910-3500cal yrBP.Then, went to2100cal yrBP, The late Holocene period,The climate conditions bacame relatively wet and stable, Halahu lake experienced a generally increasing humidity and high biological productivity compared with the early period. the time interval between1450-780cal yrBP can be considered as the most moist period during4360cal yrBP in this region. After the end of moist period, a relatively dry climate conditions continues to this day.2. The results of regional comparisons Regional comparison indicates that the palaoclimate change during4360cal yrBP from Halahu region and Northern Margin of Tibet Plateau as well as the westerly region shows that, Environmental Evolution in Halahu lake region different from the Qinghai lake region while similar with the westerly region,which means that the climate evolution Halahu region and Qinghai lake region showed two diverse tendencies. Low carbonate contents of Bsiteng lake sediment is accompanied with the Asian monsoon intensity.The wet and dry events of Halahu lake and Bsiteng lake shows an obvious corresponding relationship,both have a significant positive correlation relationship with the Westerly Strength indicated by Holocene precipitation and modern precipitation ratio,and there are reverse correlation with Dongge cave in Asian monsoon region.Above results suggest that after mid-holocene, the intensity of Asian monsoon began to decline, Halahu Lake was then under the control of the westerly, the summer precipitation has been reducing.This resulted in the change of water vapor source of Hala Lake region, the precipitation is given priority to snow in winter. The continuous reduction of oxygen isotope values in Haha Lake during the past4ka probably has been reflecting the the water vapor source, which was changed from Asian summer monsoon rains to winter snow and glacier melt water runoff.3. The climate evolution model in Hala Lake regionThe intensity of Asian monsoon was controlled by northern hemisphere summer solar radiation. After mid-holocene, The intensity of Asian monsoon began to decline because of the decreases in the northern hemisphere summer solar radiation,Halahu lake was under the control of westerly.