8.0ka BP以来长江中下游南漪湖沉积记录的正构烷烃及其单体碳同位素组成特征和古气候意义

摘要

通过分析长江中下游南漪湖柱状沉积物中正构烷烃分布特征及其单体碳同位素组成,重建了该地区8.0 ka BP以来的植被变化,进而反演古气候变迁;同时与其他地质记录的相关指标进行对比,探讨了长江中下游地区中全新世以来东亚夏季风的变化.8.0～4.8 ka BP,长链正构烷烃nC27-nC33含量占绝对优势,Paq值偏低,正构烷烃以陆生高等植物输入为主;相对较低的nC27C31比值及较高的ACL27-33值表明这一阶段以草本植物相对发育;长链正构烷烃δ13C整体偏负(-34.7‰ ～-32.9‰),C3植物为主;这一时期气候温暖湿润,是东亚夏季风最强盛时期.4.8～2.4 ka BP,长链正构烷烃nC27-nC33含量降低,Paq值偏高,陆生高等植物对正构烷烃的贡献相对减少;nC27C31比值升高,ACL27-33值降低,木本植物占优势,草本植物减少;长链正构烷烃δ13c相对正偏(-33.5‰～-29.1‰),Ca植物有一定的增加,但依然以C3植物为主;这一时期东亚夏季风减弱,气候朝冷干转变.2.4 ka BP以来,长链正构烷烃nC27-nC33含量升高,Paq值逐渐降低,陆源高等植物的贡献逐渐增加;逐渐降低的nC27C31比值以及逐渐升高的ACL27-33值表明草本植物重新占相对发育,木本植物含量逐渐减少;长链正构烷烃δ13C波动剧烈,呈现逐渐偏负的趋势,C3植物继续占据绝对优势,C4植物呈现逐渐减少的变化趋势;气候由寒冷干燥逐渐向温暖湿润过度.同时,这一时期各指标呈现剧烈波动的变化趋势,推测可能与长江流域人类活动逐渐增加,显著影响了自然植被变化的过程.这些指标记录的东亚夏季风随着北半球夏季太阳辐射量的减少而持续减弱,同时记录了多次冷暖/干湿交替事件,存在6次冷干事件(夏季风减弱事件),其中5次与Bond et al.提出的北大西洋冰筏漂流事件一一对应.%The composition of n-alkanes and the compound-specific stable carbon isotopes in a sediment core taken from Lake Nanyi in the Middle-Lower Yangtze region were investigated.The chronology of the sediment core was established according to the 14C-AMS data of the TOC (total organic carbon) and terrestrial leaves within the core sediments.The present study reconstructed the paleovegetation from 8.0 ka BP to present,and discussed the paleoclimate,especially the East Asian summer monsoon.The results showed that from 8.0 to 4.8 ka BP,the compositions of n-alkanes consisted mainly of long-chain n-alkane (nC27-nC33),and the low Paq values suggested that the n-alkanes were mainly derived from terrestrial higher plants.The higher average chain length (ACL) of n-alkanes and the lower ratios of n-alkanes C27/C31 indicated that grass vegetation dominated during this stage.Depleted δ13C values of long-chain n-alkanes indicated vegetation was dominated by C3 plants,suggesting that the climate was more humid and warmer in the Middle-Lower Yangtze area and the East Asian summer monsoon was strong.From 4.8 to 2.4 ka BP,high Paq values and declining long-chain n-alkane contents suggested that the contribution of terrestrial plants decreased.The nC27C31 ratios increased and the ACL decreased,indicating that woody vegetation dominated.The δ13C values of long-chain n-alkanes were between-33.5‰ and-29.1‰,indicating that C3 plants dominated,however,contributions of C4 increased.This phenomenon suggests that the East Asian summer monsoon weakened.After 2.4 ka BP,the decreased Paq values and the slightly increased contents of long-chain n-alkane suggested that the contribution of terrestrial plants increased.The decreased nC27C31 ratios and increased ACL values indicated that grass vegetation dominated in this region.The δ13C values of long-chain n-alkanes were between-33.9‰ and-28.0‰,and changed rapidly,suggesting that the climate tended to be warm and humid.The rapid change of these indexes may be related to the gradual increase of human activity in the Yangtze River Basin since 2.4 ka BP.All these indexes showed that the East Asian summer monsoon weakened when the summer solar radiation in the northern hemisphere decreased.Our multi-proxy records revealed six cold/dry climatic fluctuations that included five Bond events.