Distribution Of Branched Glycerol Dialkyl Glycerol Tetraethers In Gonghai Lake Sediments Over The Past 15 Ka And Its Environmental Significance

In the recent decade,cyclisation ratio and methylation index proxies derived from branched glycerol dialkyl glycerol tetraether lipids?brGDGTs?in lake sediments are increasingly adopted for the past climatic?for temperature?and environmental?for pH?reconstruction,and made great progress.However,the complex origin of brGDGTs in lacustrine sediments limits our understanding of their environmental significance.It is reported that cyclisation ratio and methylation index of soil source brGDGTs in lakes record soil pH and local air temperature respectively,whereas the cyclisation ratio and methylation index of aquatic source brGDGTs record lake water pH and lake water temperature respectively.In addition,the lake water temperature and lake water pH are influenced by complex factors,which also result in the uncertainty of environmental significance based on brGDGT-related proxies records.To keep the reliability of the past climatic and environmental history based on sediments brGDGTs,the major source of brGDGTs in lake sediments and environmental significance of brGDGTs-related proxies should be carefully determined before using these proxies for paleoclimatic reconstruction.In this study,we present cyclisation ratio and methylation index of brGDGTs records for the past 15 ka from Gonghai Lake?GH?,an alpine lake in north China.A high-resolution sediment chronology and abundant paleoclimatic research achievements were available for the site.For better understanding of these records,we preformed amounts of modern process work to assure the major source of brGDGTs in core GH09C sediments and its environmental significance.Modern samples and hydrological observation in lakes were used for the modern process analysis as following:1)Shanxi GH:surface sediments?20 samples?,suspended particular matter?SPM??8 samples?,and surface soils from catchment?30 samples?and surface soils from dust source region?30 samples?.2)Inner Mongolia Chagan Nur:lake surface sediments?11 samples?,river surface sediments?11 samples?and surface soils?42 samples?in the catchment of Chagan Nur?CG,a fresh water lake;‘Nur'means lake?in northern China.3)Shanxi Tianchi lakes:surface sediments?18 samples?and surface soils?22 samples?from catchment.4)Shanxi Tianchi lakes:hydrological observation in lakes water.Based on the above-mentioned works,the following conclusions have been summarized.?1?The average concentration of brGDGTs in GH surface sediments is twice than that in surface soils from GH catchment,and about two orders of magnitude higher than that in surface soils from dust regions.The average concentration of brGDGTs in CG surface sediments is 30times more than that in surface soils from CG catchment,and five times more than that in river surface sediments.The average concentration of brGDGTs in lake surface sediments from Tianchi region is 10 times more than in surface soils from lakes catchments.The distribution of tetra-,penta-and hexa-methyl brGDGTs in GH and CG surface sediments are plotted in significantly different region compared to the surface soils from both lake catchments.The distribution of tetra-,penta-and hexa-methyl brGDGTs in core GH09C samples are plotted in significantly different region compared to the surface soils from different climatic zones in China.The significant difference between concentration and distribution of brGDGTs in sediments and soils,indicate brGDGTs in GH surface sediments,CG surface sediments,Tianchi surface sediments and sediments in core GH09C mainly sourced from aquatic production.Considering the major source of brGDGTs in lake sediments are aquatic production,the cyclisation ratio and methylation index of brGDGTs are responded to lake water pH and lake water temperature.?2?The CBT-derived pH for core GH09C were relatively higher during 15-11 ka BP,and exhibited an overall decreasing trend from 11-8 ka BP.The CBT-derived pH values were generally lower and stable during 8-5 ka BP,and exhibited an overall increasing trend from 5-0 ka BP.Compared CBT-derived lake water pH record with carbonate content record,pollen-based precipitation record and compound specific?¹³C record of long chain n-alkanes records from GH,the high precipitation and low carbonate content corresponded to low water pH,and vice versa which indicated that lake water pH was controlled by the change of local humidity.Under more humid conditions such as 8-5 ka BP,C₃ forest vegetation occurred in GH catchment,and the greater precipitation amount would have resulted in the freshening of the lake water and therefore to a lower concentration of basic ions,such as CO₃²-,resulting lower water pH and ultimately recorded by CBT proxy.Considering that the precipitation amount and degree of humidity of the local climate at the site of GH is significantly controlled by the intensity of the EASM,the pH record can therefore be regarded as an indicator of EASM intensity.?3?For GH,the temperature in deep water is lower than that in shallow water,and the values of methylation index of brGDGTs(MBT/CBT and MBT'_5ME)in surface sediments from deep water is also lower than that surface sediments from shallow water.For lakes in Tianchi region,the average lake water temperature is negatively correlated with lake water depth,and the values of methylation index of brGDGTs is also significant negatively correlated with lake water depth.Under similar air temperature,the average lake water temperature decreased with lake depth increasing,and MBT/CBT and MBT'_5MEME from lake surface sediments can record water temperature,and then reflect lake water depth.The values of temperature based on MBT/CBT and MBT'_5MEME are reconstructed in core GH09C over the past 15 ka.It presented relatively higher values during 15-11 ka BP,and exhibited relatively lower values from 11-4 ka BP,and then gradually increased from 4-0 ka BP.Compared with other paleoclimatic records in GH,the relative amplitude of solar radiation per unit area in GH is below 8%since the last deglaciation,thus the input energy into GH is relatively small during the past 15 ka.During that time,the precipitation increased?decreased?,and the lake water become deep?shallow?,corresponding to relatively lower?higher?values of MBT/CBT and MBT'_5ME.It indicated MBT/CBT and MBT'_5MEME in core GH09C can record lake water depth in Gonghai Lake.Combined the modern process and paleoclimatic results,it indicated that MBT/CBT and MBT'_5MEME can record water temperature,and then can indirectly reflect lake water depth under relatively small change of per unit area energy into lake water.?4?Based on our research results of GH and CG,and comprehensive analysis of other different latitude lakes published results and aquatic source brGDGTs data,the MBT/CBT is positively correlated with mean annual air temperature?MAAT?and mean warm air temperature?MWAT?.From low-latitude to mid-latitude regions,the offset of MBT/CBT calculated temperature and MAAT is increasing,whereas offset of MBT/CBT calculated temperature and MWT is almost stable.These results indicate MBT/CBT calculated temperature bias towards MWAT in mid-latitude lakes.It can be rationally explained by the relationship between lake water temperature?LWT?and air temperature?LAT?.Mechanism can be generally concluded as:in low-latitude region,LWT can trace LAT all year round and is close to LAT,recorded by MBT/CBT proxies in lake surface sediments,MBT/CBT-calculated LWT therefore close to MAAT.In mid-latitude region,surface water is frozen and LWT are significantly high than LAT in winter,thus average LWT is higher than MAAT,close to MWAT,recorded by MBT/CBT proxies in lake surface sediments,MBT/CBT-calculated LWT therefore is higher than MAAT,close to MWAT.Our results emphasize the importance of determining the origin of the brGDGTs and environmental significance of brGDGTs-related proxies in lake sediment studies.Only were the origin of the brGDGTs and environmental significance of brGDGTs-related proxies defined,the paleoclimatic records based on brGDGTs could be reasonably explained.The modern process also needs to be tested by paleo-environmental reconstruction.Combining these two aspects can promote the study of paleo-environmental reconstruction.