Late Glacial And Holocene Lake-level Fluctuations And Climatic Change Documented By Multiple Sediment Cores From Genggahai Lake

Lake level has been widely used to reflect changes in regional precipitation and/or humidity.However,on a large timescale,with the input of detrital materials to a lake,evolutions of lakebed morphology may also lead to fluctuations in water level.Therefore,assessments on influences of the detritus-input process and the lakebed-morphology evolution on lake-level fluctuations are crucial in extracting climatic information pertinent to lake levels from the lake's history.Here we present the results of multi-core analyses at Genggahai Lake in the northeastern Qinghai-Tibetan Plateau.Three sediment cores,i.e.GGH-C,GGH-D,GGH-E,were recovered from central,east littoral and west littoral areas of the lake,respectively.Different carbon species from the lake system,including dissolved inorganic carbon?DIC?of lake water,macrophyte remains in the uppermost samples of core sediments,living P.pectinatus in the lake and the DIC of spring water in the catchment,were dated to assess the potential reservoir effect of the lake.We also conducted a continuous monitoring on the physical and chemical properties of lake water in areas with different aquatic plant communities.The results show that the modern lake has a reservoir age of 1010 ¹⁴C yr BP on average.The reservoir age of the modern lake was employed to establish age-depth models of the cores.Combined with grain size,elements and macrofossil assemblages of sediments,we reconstructed the history of lake-level fluctuations and detritus input to Genggahai Lake since the late Glacial.Climatic implications of lake level were determined by assessing the influences of the detritus-input process on the lake-level fluctuations.We further investigated the history of regional erosions,aeolian activity,and climatic changes on the northeastern Qinghai-Tibetan Plateau since the late Glacial.The main conclusions are as follows:?1?Changes in macrofossil assemblages from the three sediment cores indicate that the history of the lake-level variations can be divided into three stages:During the late Glacial?16.5-11.5 cal ka BP?,water level was low and the macrophyte community was mainly dominated by Chara spp.During the early to mid-Holocene?11.5-6 cal ka BP?,water level was overall high which exceeded the water-depth limit of submerged plants,leading to the scarcity or even absence of submerged plants within the lake.The highest lake level occurred from 11.5 to 8.2 cal ka BP.During the late Holocene?since 6 cal ka BP?,water level was low.In the central lake,the macrophyte community was mainly dominated by Chara spp.;in the littoral zone,the macrophyte community was dominated alternately by Chara spp.and littoral plants.?2?The coarse particles in the lake sediments are primarily transported by strong winds.The detrital materials transported by winds preferentially deposit in the northwestern part of Genggahai Lake,which leads to higher sedimentary rates in the northwest and north parts of the lake,and low sedimentary rate in the southeast and south of the lake.Therefore,assuming that the total amount of the lake water is constant,the input of detrital materials may lead to southeastward shifts of the depocenter and water body since the late Glacial.This process would lead to changes in lake levels and grain-size components of sediments at the different coring sites.However,except for the period from 6.3 to 3.7 cal ka BP,the filling process appears to exert minor effects on the water-level evolution since the late Glacial and water-level fluctuations are mainly determined by regional rainfall amounts.From 6.3to 3.7 cal ka BP,a great deal of debris was transported into the lake,making the depocenter and water body shift to southeast rapidly,which leads to a decrease in lake level at the drilling position of core GGH-E.Changes in grain size of core GGH-C from central lake seems to be less affected by the filling process since the late Glacial,which in turn reflect the history of aeolian activity in the lake basin.?3?During the late Glacial?16.5-11.5 cal ka BP?,aeolian activity was prevalent in the study area,and the degrees of both erosion and chemical weathering intensity were low,which was mainly related to a cold,dry climate.During the early to mid-Holocene?11.5-6 cal ka BP?,the chemical weathering intensity increased and the erosion degree was still low.The increased vegetation cover largely suppressed aeolian activity during this period.During the late Holocene?since 6 cal ka BP?,the chemical weathering intensity declined and the erosion increased.Episodic aeolian activities occurred in the lake catchment,which appear to coincide with the major ice-rafting events in North Atlantic.?4?The climatic records of lake sediments from the northeastern Qinghai-Tibetan Plateau show that a warmer and wetter climate occurred during the early to mid-Holocene,compared to the late Glacial and the late Holocene.This pattern is similar to the monsoonal records from low latitudes,such as oxygen isotopes of speleothems.Late Holocene aeolian activity,accompanied by the weak Asian summer monsoon and intensified wind regime in the study area,suggests that interactions of Asian monsoon and the Westerlies plays an important role in modulating climatic changes on the northeastern Qinghai-Tibetan Plateau.Climatic changes in the study area are sensitive to changes in the Asian summer monsoon,in response to variations of orbitally-induced summer insolation.In addition,the millennial-scale climatic changes during the late Holocene may be also assosiated with intensifications of ENSO activity.