Hydroclimate Change And Carbon Dynamic Response Of Wetland In The Middle Reaches Of The Yangtze River Since The Last 160 Ka BP

Peatland plays an important role in the global carbon cycle,as a terrestrial ecosystem with the largest carbon storage and highest carbon density per unit area.Modern process studies show that the peatland carbon cycling is closely related to climate change,and water level is a decisive factor in regulating peatland greenhouse gas emissions.Compared with modern process studies,the understanding of the response of carbon cycling to climate change in peatlands during geological history is still relatively weak.Peatlands are long-term accumulations and contain various scenarios in geological records,including some extreme climates.Therefore,exploring the relationship between carbon dynamics and climate change in peatlands under different climatic backgrounds during geological history has important scientific and social values.This can provide reference significance for how to enhance the carbon sequestration function of peatlands,under the background of climate change and intensified human activities in the future.The existing peatlands are mainly developed since the last deglaciation period.Previous studies show that peatlands in high latitudes of the north have made significant contributions to the increase in global methane concentration during the early Holocene.However,during the overall cold period of the ice age,the northern high-latitude regions were covered by large ice sheets,and their role was very limited.Based on theδ¹⁸O records of stalagmites indicating monsoon intensity in mid-low latitude regions,some scholars have proposed that tropical/subtropical land ecosystems in mid-low latitude regions may have played a more important role in global carbon cycling during the last glacial period.However,there is currently a lack of direct evidence from wetlands,and the relationship between carbon dynamics and climate change in wetlands is not yet clear.Therefore,this study conducted lipid molecular composition and isotopic composition analysis in Dajiuhu peatland located in the middle reaches of the Yangtze River to explore the relationship between carbon dynamics and climate change in wetlands during the late Pleistocene（covering glacial-interglacial cycles）.Through these studies,several main understandings were obtained:（1）When reconstructing paleohydrological information using the hydrogen isotope composition（δ²H）of sedimentary leaf wax lipids,differences between different compounds must be considered.Modern monitoring results show thatδ²H of two n-alkyl compounds（leaf wax n-alkanes and fatty acids）derived from the same precursor may record different seasonal biases.In subtropical regions dominated by deciduous tree species,leaf wax n-alkaneδ²H values may mainly record signals from late spring to early summer.In addition,in the sedimentary record of Dajiuhu peatland,theδ²H values of leaf wax n-alkanes show non-synergistic changes with theδ¹⁸O values of nearby Sanbao/Hulu cave stalagmites,which also inherit isotopic signals of atmospheric precipitation.This is not what was expected.The reason for the decoupling of the two may be seasonal differences,and further combines interannual variations of modern rainfall and precipitation isotopic composition to infer that the difference between the two may record information on seasonal rainfall（spring and autumn rainfall）.（2）Based on the hydrogen isotope difference(Δδ²H_C31-25)between long-chain and mid-chain alkanes,theδ¹³C value of long-chain C₃₁ alkanes from terrestrial plants and the flux of hopanoids from aerobic bacteria in sedimentary sequences,this study reconstructed changes of relative humidity in the middle reaches of the Yangtze River since 160 ka BP.The reconstructed results show that during MIS 6（154-128 ka BP）,the middle reaches of the Yangtze River gradually were relatively dry.During MIS 5（128-72 ka BP）,the middle reaches of the Yangtze River were relatively humid.During MIS 4（72-59 ka BP）,the middle reaches of the Yangtze River were generally dry.During MIS3（59-27 ka BP）,the middle reaches of the Yangtze River were generally in a humid period with large fluctuations,higher than MIS 5.Furthermore,by comparing with other paleoclimatic records in the eastern monsoon region,it was found that paleohydrological changes in the middle reaches of the Yangtze River were out of phase with those in southern China and in phase with those in central and western of northern China since160 ka BP.This spatial distribution pattern may be related to El Ni（?）o-La Ni（?）a-like climate states.（3）The response of carbon burial to climate change of wetlands in the middle reaches of the Yangtze River which located the subtropical monsoon region differ significantly from those of peatlands in high-latitude regions of north.Previous studies have shown that peatlands in high-latitude regions north of 40°N accumulated a large amount of organic carbon during warmer periods.However,this study shows that for wetlands in the middle reaches of the Yangtze River which is in the subtropical monsoon region,during glacial periods（154-128 ka BP,72-29 ka BP）compared to interglacial periods（128-72 ka BP）,it was more conducive to the development of marsh vegetation and there was a significant increase in organic matter input.Moreover,in glacial periods,during the relatively cold and dry MIS 4 period（72-59 ka BP）,the vegetation was dominated by C₃ herbaceous plants.The carbon preference index（CPI）value of n-alkanes showed that the degradation of organic matter was weaker during this period,which was more conducive to organic matter preservation and increased the carbon burial potential of peatlands.However,during MIS 3（59-27 ka BP）,which was relatively warm and humid,although terrestrial vegetation was widely developed,n-alkanes showed low CPI value,indicating that strong degradation occurred due to climate conditions and poor preservation ability of organic matter.（4）The methane metabolism intensity from wetlands was revealed using geological lipid records(including iso GDGT-0,archaeol flux and hopaneδ¹³C)in the middle reaches of the Yangtze River in the subtropical monsoon region since 160 ka BP.The results show that the methane activity intensity from wetlands in the middle reaches of the Yangtze River is controlled by organic matter accumulation,and it plays a more important role in glacial periods.During the entire MIS 5 period（128-72 ka BP）,the fluxes of archaeol and iso GDGT-0 were at very low levels,and there was almost no methane production activity.This is related to less organic matter accumulation and insufficient substrates during this period.During relatively cold glacial periods（MIS 6,MIS 3-4）,marsh vegetation proliferated,organic carbon accumulation increased significantly,and methane emissions also occurred.The peak of methane cycling occurred during MIS 3（59-27 ka BP）,and it is inferred that the humid climate enhanced the methane cycling activity of peatlands.During MIS 6（154-128 ka BP）and MIS 4（72-59 ka BP）,the climate was relatively dry.Despite sufficient substrate,the intensity of methane activity was still weak.