贵州草海近21 ka以来有机地球化学记录反映的古环境变化*

草海位于黔滇交界,主要受印度夏季风的影响,其湖沼沉积物记录了中—晚第四纪的环境变化历史,是研究第四纪环境和季风演变的良好载体。本研究选取的南屯NT03钻孔,岩心长405 cm,在放射性碳同位素(AMS¹⁴C)测年基础上建立了年代序列,并进行了沉积物的色差值(L^*)、烧失量(LOI550)、总有机碳(TOC)、碳氮比(C/N)和有机稳定碳同位素(δ13Corg)等古环境替代指标的测试。实验结果显示,L^*、LOI550和TOC等3个有机碳含量指标同步变化,共同指示该钻孔序列经历了3个沉积环境演变阶段,即末次盛冰期晚期(21—15 cal ka BP)河流—冲积相沉积阶段、冰消期至早中全新世(15—4 cal ka BP)高湖面湖泊—沼泽相沉积阶段和晚全新世(4 cal ka BP以来)洪冲积相沉积阶段。近2万年以来草海湖沼面积经历的收缩→扩张→收缩演变过程与石笋氧同位素变化基本吻合,表明季风降雨是引起草海湖泊水体和周边生态系统变化的主要因素。δ13Corg指标变化范围为-29.28‰~-24.19‰,表明草海盆地周围植被组成在末次盛冰期以来C4草本虽然略有增加,但均以C3植物为主。同时,TOC和δ13Corg指标证明了B/A冰消期暖事件对盆地边缘区的湿地泥炭富集起到关键作用,而多指标揭示了近4 ka以来水域变浅并发展成为冲积环境的过程,也可能与石笋记录的夏季风减弱所带来的降水量减少有关。

Palaeoenvironment changes during the past 21 ka inferred from organic geochemical records of Caohai Lake,Guizhou Province

Caohai Lake is located in the boundary area between Guizhou and Yunnan provinces where the climate is dominated by Indian summer monsoon(ISM). The continuous lacustrine bog sediments provide ideal material for revealing middle and late Quaternary environmental changes. In this paper,a core of 405 cm length(NT03)was studied. The age model was established based on 7 AMS¹⁴C ages covering the last 21 ka. Here we present the results of sediment chromatic aberration value(CIE-L^*),loss on ignition at 550 ℃(LOI550),total organic carbon(TOC),C/N ratio and stable carbon isotope(δ13Corg). The environment proxies(L^*、TOC and LOI550)varied synchronously,indicating three distinct intervals of lake level since the Last Glacial Maximum(LGM),i.e. riverine alluvial facies,lake-wetland facies and shallow water alluvial facies respectively from 21~15 ka BP,15~4 cal ka BP and after 4 cal ka BP. This change pattern is similar to the variation of speleothem δ¹⁸O records from the south China. The δ13Corg values,varying between-29.28‰ and-24.19‰,suggest C3 plant domination in regional vegetation,despite a possible moderate C4 plant increase during the LGM. The TOC and δ13Corg records show an important change at B/A event implying the local ecosystem and lake level are sensitive to the deglacial warming. There was an abrupt change in all proxy records at around 4 ka BP which may indicate a weakening summer monsoon indicated by the speleothem δ¹⁸O records.