| Asian summer monsoon is one of important parts of the global climatic system;it is the main route to bring moisture and heat to the mainland,thus affecting the environmental changes,ecology safety,economic development,as well as social stability in its dominated regions.The study of the variation of ancient Asian summer monsoon,especially during the period with increasing temperature,has great realistic significance;it can not only help us understand the variation of modern monsoon,but also provide a reference to predict the variation of monsoon in future under the background of the global warming.The northeastern Tibetan Plateau is located in the limit of the modern monsoon system and it has a fragile ecosystem,therefore it is sensitive to monsoon variation.Most of the reconstructed climatic results from lake sediment from the northeastern Tibetan Plateau on the multiply time scale are conflict with each other,leading to different climate change patterns and forcing mechanisms of the Asian summer monsoon since the last glacial maximum were proposed.The conflicting can be ascribed as follows: the Asian summer monsoon increased at the beginning of the Holocene or during the last deglaciation;the highest Holocene moisture/precipitation on the northeastern Tibetan Plateau occurred during the early Holocene or the middle Holocene;the moisture evolution patterns of the basin and mountain areas are the same or not;the abrupt climate changes in the region were caused by the decline of the Asian summer monsoon or the increase of the westerly.Therefore,besides reviewing and summarizing the published climatic records,it is important to further reconstruct the climate change in this critical region from lake sediment based on robust chronology and reliable proxies with clear environmental significance.Dalianhai Lake is the terminal lake of the Shazhuyu Rive in the Gonghe Basin;it is divided from Qinghai Lake by the Qinghai Nanshan mountain.Dalianhai Lake is dried since 1994;in 2012,a 203.09 m long core DLH12A(E100°23'55.0 ",N36°14'25.2",2852 m)was obtained from its dry lake bed.Based on accelerator mass spectrometry radiocarbon(AMS14C)dating results from 11 plant remains and 14 bulkorganic matter samples,the age-depth chronology model of the up 51.63 m of the core was built for the past 23.1 ka(1ka=1000 years).By using the proxies of grain size for around 2000 samples,XRF element with 2-cm resolution,total nitrogen(TN)/organic carbon(TOC)and carbon isotope of organic matter for 300 samples,and carbonate content and its carbon and oxygen isotope for 80 samples,the variations of hydrology in the basin and the Asian summer monsoon since the last glacial maximum were constructed.In addition,the high shoreline deposits of the lake was investigated,and based on AMS14 C dating and optically stimulated luminescence(OSL)dating of proper materials,the age of the shoreline was established,which further provides evidences for hydrology variations of the lake.Based on the above work,we found that:1.Shazhuyu River was a external drainage and flowed into Yellow River before Gonghe movement.After the Gonghe movement,Dalianhai Lake was formed and it became the terminal lake of the river.The sediment of the core DLH12 A was interstratified with lacustrine sediments and sand,which was deposited on the red sandy rock.The variation of the stratigraphy between the lacustrine clay and sand represents changes in precipitation,as well as variations of the Asian summer monsoon.Besides the upper 43.34 m of the lacustrine sediment deposited since the last deglaciation,the core DLH12 A contains five more layers of lacustrine sediments,which represents the periods with increasing precipitation in the region.We hypothesize that these periods may be consistent with the formation of Yellow River terraces formed after the first erosion of Yellow River in the Gonghe Basin.2.For the period 23.1-21 ka BP(51.63-48.21 m)during the last glacial maximum(LGM),mean grain size of the sediments was huge,magnetic susceptibility was high and Fe/Mn ratio was low,indicating precipitation was low and the region was under a super dry climatic condition.During 21-16 ka BP(48.21-46.27 m),sediment became fine and magnetic susceptibility and PCA2 of element increased.All these evidences suggest that precipitation increased during this period.From 16-14.7ka BP(46.27-43.34 m),the sediment was mainly composed of sand and the grain size increased,indicating it was the regional dry climate event and which can be well compared with the global H1 event.Therefore,the Asian summer monsoon decreased during the H1 event.The sediment of Dalianhai Lake became lacustrine silty clay since the B/A event and from then on the lake was formed again.This indicates that the Asian summer monsoon increased since the beginning of the B/A event.Thevariation of the Asian summer monsoon during the late glacial can be well compared with variations of other global climatic systems,thus indicating the Asian summer monsoon was adjusted by the internal feedback,besides the forcing from the summer solar insolation in the Northern Hemisphere.3.Records from Dalianhai Lake indicate that the Holocene at the region began at 11.5 ka BP(25.40 m),as indicated from increasing carbonate oxygen isotope values and decreasing carbonate carbon isotope values.XRF-based hydrology proxies and grain size data indicate that the Asian summer monsoon increased during the Holocene.Rb/Sr ratios and the mean grain size show that the precipitation was high and the lake was deep during the early to middle Holocene(10-6 ka BP),indicating that the Asian summer monsoon was strong during this period.During the late Holocene(6-0 ka BP;the upper 17.20 m),the grain size increased and the terrestrial input of the sediment decreased,indicating low lake level and decreased precipitation.In addition,tree pollen percentages from lakes on the northeastern Tibetan Plateau show a decrease trend after 6 ka BP.All these evidences suggest that the Asian summer monsoon decreased during the late Holocene.The variation of Asian summer monsoon during the Holocene follow the variation of summer solar insolation in the high latitude of the Northern Hemisphere.However,the moisture in the Gonghe Basin was another pattern and quite different with the monsoon pattern.During the early Holocene,the basin was super dry,as indicated from low A/C ratios from Genggahai Lake,low TOC values from Dalianhai Lake,as well as widely distributed aeolian sand in the basin.However,the moisture in the Gonghe Basin increased during the late Holocene as indicated from increased terrestrial productivity and the formation of soil.This different moisture evolution pattern maybe caused by high temperature during the early Holocene.4.There are several layers of sand in the sediments of Dalianhai Lake,as indicated by the increase of mean grain size and the content of >63 ?m particles.These sand layers occurred during the periods of 11-10.2,7.6-7.4,5.5-5.4,3.9-3.8,2.2-1,and 0.4 ka BP.The increase of content of >63 ?m particles represents the decrease of the lake level,further indicating the decrease of precipitation and weakening of the Asian summer monsoon.The abrupt weakening events of the Asian summer monsoon during the Holocene can be compared with the cold events in the north high latitude;however,it has its special characteristics that most of the monsoon decreasing events occurred during thelate Holocene.5.After digging 8 sections on the east shore area of Dalianhai Lake with elevation from 2866 to 2872 m,we found that the modern aeolian sand cover the top5-25 cm of the sections.Lacustrine sediments only occurred at the lowest two sections.Other sections are composed of aeolian sand and beach sand.The AMS14 C age of the plant remains from the lower part of lacustrine layer indicate that the newest transgression of the lake occurred at around 360-350 cal yr BP,which further supports the result from the core that lake level increased during the last millennium.All of the sedimentary faces of sections and its ages suggested that the highest lake level was around 2870 m during the Holocene. |
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