C-and H-isotopic Compositions Of Long Chain N-alkanes From Peat Deposits In Southern China Since 13ka BP And Their Paleoenvironmental Implications

Views are divided on the spatio-temporal patterns of Holocene hydrological changes in east China.Some studies showed nearly synchronous Holocene moisture optimum which occurred during the early and middle Holocene.Other studies displayed differences in the timing and duration of Holocene moisture optimum.These inconsistences might be partly attributed to the climate significance of various proxies and unevenness of proxy record distributions.At present,records from the middle and lower reaches of the Yangtze River and south China are relatively few.However,these areas are the key areas to capture the spatio-temporal patterns of hydrological changes in east China,as they directly affect the spatial patterns of hydrological changes.To achieve a better understanding of the spatio-temporal patterns of moisture changes in east China,multiple-proxy records with high resolution and precise dating are needed,especially in south China.Peat deposits serve as a good archive for paleoclimate research in late Quaternary.In China,peat deposits scatteredly distribute in the middle and lower reaches of the Yangtze River and south China.Their continuous sedimentary history could span the entire Holocene.In this study,analyses of n-alkanes distributions and compound-specific carbon and hydrogen isotope compositions(?¹³C and ?D),hopanoid flux,total organic carbon content?TOC?,bulk organic carbon ?¹³C values(?¹³CTOC),C/N ratios,element Ti content,Rb/Sr ratios,plant microfossils and humification degrees were combined in Shiwangutian peat deposits from Chengbu,Hunan province and Shuizhuyang peat deposits from Pingnan,Fujian Province,south China.In addition,analyses of n-alkanes distributions and ?¹³C and ?D were performed on the aboveground parts and underground parts of modern plants in Xianshanmuchang and Tianhushan wetlands from Pingnan,Fujian Province.Together,we aim to reconstruct the paleoenvironmental changes in Shiwangutian and Shuizhuyang on regional scales,explore the paleoenvironmental implications of n-alkane ?¹³C and ?D values and deepen our understanding of the spatio-temporal patterns of hydrological changes in east China.The main conclusions are as followed:1.In all peat samples from Shiwangutian peat core,C23^C33 n-alkanes were present,dominated by C29 n-alkane.n-Alkanes showed a strong odd over even predominance.They revealed a predominant contribution of higher plants to the peat.n-Alkane ?¹³C values were indicative of a major input from C3 plants.In all peat samples from Shuizhuyang,C23-C33 n-alkanes were present with a weak odd over even predominance.The most abundant homologue was C25 n-alkane followed by C24 and C29 n-alkanes.C23^C26 n-alkanes shared a common source,while C27^C33 n-alkanes had a common source.Plant microfossil analysis revealed a major input of cyperaceous plants and a minor contribution from woods and none input from Sphagnum.n-Alkane ?¹³C values revealed a major input from C3 plants.2.n-Alkanes of aboveground parts from the modern plants from Xianshanmuchang and Tianhushan wetlands generally had higher total concentrations of n-alkanes,longer carbon chain distributions and stronger odd over even predominance compared to the underground parts within the same plants.Moreover,n-alkane ?D values were generally less negative in the underground parts which might be owing to the differences in the H-NADPH biosynthetic origin in different plant organs.No such trend was observed in n-alkane ?¹³C values.3.The climate evolution over the last 9ka in Shiwangutian peatland can be divided into three stages.The first stage?9⁵ ka BP?was a period of warm and wet climate,which is regarded as the Holocene optimum.The second stage?5¹.8 ka BP?witnessed a shift to relatively cool and dry climate,which was a transitional period.The third stage?1.8⁰.3 ka BP?experienced an abrupt change to cold and dry conditions.Four stages of moisture changes were identified in Shuizhuyang peat core: 13.1¹0.7 ka BP,a humid period;10.7⁸.6 ka BP,a dry period;8.6⁴.1ka BP: Holocene moisture optimum;4.1².5ka BP: a dry period.4.In Shiwangutian peat deposits,?¹³C values of long-chain n-alkanes mainly reflected changes in relative humidity,with less negative values under dry climate and more negative values under humid climate.?D values of long-chain n-alkanes primarily recorded the precipitation ?D signal.In Shuizhuyang peat deposits,C25 n-alkane ?¹³C values were mainly controlled by plant source changes.A small fraction of input from aquatic plants or woody plants from surrounding areas under humid conditions could result in less negative C25 n-alkane ?¹³C values.The C25 n-alkane ?D values were not sensitive to the local humidity changes and vegetation composition changes.Instead,they well corresponded to the cold events in the North Atlantic.Such a teleconnective response of low altitudes to the North Atlantic cooling revealed the important role of climatic processes occurring over northern high latitudes in regulating the East Asian monsoon circulation.5.The spatial moisture pattern during the warm mid-Holocene differed from that during cold Younger Dryas,showing different controlling factors dominating under different climatic background.During the Younger Dryas,inland monsoonal areas were primarily controlled by the sea surface temperature in the western Pacific warm pool and the position of the western Pacific subtropical high with a “dry-wet-dry” mode from north to central to south China.For the coastal areas,they were largely influenced by temperature-induced evaporation and displayed a uniformly wet condition.During the mid-Holocene,moisture conditions in east China were primarily controlled by the sea surface temperature in the western Pacific warm pool and the position of the western Pacific subtropical high and displayed a “wet-dry-wet” mode from north to central to south China on the whole.Northeast China seemed to be less sensitive to the zonal SST gradient anomaly in Pacific Ocean and was mostly influenced by the SSTs in Japan Sea and Okhotsk Sea.Notably,the hydrological changes in east China also did not correspond to each other precisely in timing.