Environmental Indicative Significance Of Organic Carbon Isotope(?¹³C_org)of Sediments In Hala Lake

Affected by ecosystem,climate and environment,and characteristics of organic carbon isotopes(?¹³C_org)in lake sediments provide evidence for regional paleoenvironment and paleoclimate changes.However,different sources of organic carbon in different types of lake sediments,coupled with the differences of lake sedimentary environment,may lead to different spatial distribution characteristics of?¹³C_org in lakes.Also,the environmental significance of?¹³C_org requires detailed researches from lakes.Hala Lake(38°11??38°25?N,97°24??97°47?E,4078 m a.s.l)in the northeast of Qinghai-Tibet Plateau is a large closed deep-water lake located in the hinterland of Qilian Mountains.The lake basin is surrounded by high mountains with less human interference and the lake is sensitive to regional climate change.The environmental significance of?¹³C_org in the lake sediment is not clear yet.In this thesis,?¹³C_org were analyzed from surface sediments at different water depths in Hala Lake and 14 sediment cores from north-south sections of the lake.Grain size,XRF element scanning,organic matter content and carbonate content were also analyzed to infer the sedimentary environment,the source of organic matter,the spatial distribution characteristics of?¹³C_org.Results show that?¹³C_org of sediments in Hala Lake is negatively correlated with water depth within 55m,which can be used to indicate the change of lake water level.It is mainly caused by the difference of photosynthesis efficiency of algae in different water depths and the different types of aquatic plants.The?¹³C_org from the sediment core HL19-07(water depth:66.8m)reflects the water level changes in Hala Lake during the late Holocene in the past 4500 years,which can be roughly divided into the five stages:an overall high lake level(4.5-3 cal ka BP),a steadily decreasing lake level(3-2.1 cal ka BP),a rapid increase lake level(2.1-1.5 cal ka BP),and the current water level(1.5-0 cal ka BP).In the past 500 years(since 0.5 cal ka BP),the water level fluctuated without obvious change trend,close to the water level of modern Hala Lake.The current warming may have caused an increase in the water level.Comparing the water level changes reflected by?¹³C_org of sediment in Hala Lake with the precipitation and air temperature curves reconstructed by tree rings,we reached the conclusion that the main factor affecting the water level changes in Hala Lake is the regional air temperature because glacial meltwater is an important source of recharge in Hala Lake,and the amount of glacial meltwater recharge is closely related to regional temperature changes.