中晚全新世湘西莲花洞石笋旋回沉积速率变化及其古气候意义

摘要

石笋旋回沉积速率是按照石笋沉积旋回测年得到的沉积速率,是石笋沉积过程的重要参数和反演古气候变化的有效代用指标.旋回沉积速率的波动可能反应了地表温度和降雨量的变化,与夏季风强度存在关联.文章以湖南湘西莲花洞LH2石笋为材料,利用42个230Th年龄获得的石笋旋回沉积速率序列重建了8.6 ka BP以来湖南湘西地区温度和降雨雨量的变化历史:8.6-4.2 kaBP期间,气候以高温多雨为特征,夏季风强,石笋生长率高,但气候并不稳定,存在一系列千年-百年尺度的波动;4.2-0.05 ka BP期间,气候整体偏冷干,旋回沉积速率偏低.总体上,在轨道尺度上,LH2石笋的旋回沉积速率与该石笋的δ18O值之间存在负相关关系.旋回沉积速率的重复性检验证明了旋回沉积速率古气候记录的有效性.石笋旋回沉积速率变化主要受北半球夏季太阳辐射量的控制和ITCZ的直接影响,在百年尺度上,旋回沉积速率的波动可能与中晚全新世的弱季风事件存在关联.%Deposition rate of sedimentary cycle of stalagmite,which is calculated based on 230 Th age dating according to sedimentary cycle of stalagmite,is an important parameter of stalagmite formation and an effective paleoclimatic index to reconstruct paleoclimate.Variations in deposition rate of sedimentary cycle may reflect the change of surface temperature and rainfall,which immediately relates to intensity of summer monsoon.Making use of stalagmite LH2,collected from Lianhua Cave,Hunan Province,China,the paper reconstructs the history of surface temperature and rainfall since 8.6 ka BP based on 42 230Th ages.From 8.6 ka BPto 4.2 ka BP,the climate is characterized by high temperature and rainy,the summer monsoon is strong,deposition rate of sedimentary cycle is high.But the climate is not stable and there is a series of millennial to centennial fluctuations.From 4.2 ka BP to now,the climate is cool and dry,deposition rate of sedimentary cycle is low.Generally speaking,in orbital timescale,there is a negative correlation between deposition rate of sedimentary cycle and δ18 O record from stalagmite LH2.The deposition rate of sedimentary cycle stalagmite D4 from Dongge Cave duplicates that of stalagmite LH2,which confirms the validity of deposition rate of sedimentary cycle to record climate changes.Basically,changes of deposition rate of stalagmite are controlled by the summer solar radiation in Northern Hemisphere and ITCZ exerts directly influence on it.In centenniarl timescale,deposition rate of sedimentary cycle of stalagmite LH2 may relate to weak monsoon events occuring in the mid-late Holocene.