近千年气候环境快速变化的长江中游石笋δ^(13)C记录

Rapidity of the Climatic and Environmental Changes During the Past Millennium Inferred from Stalagmiteδ^(13)C Records in the Middle Reaches of the Yangtze River

【目的】研究亚洲季风区中世纪气候异常期(Medieval Climate Anomaly,MCA)和小冰期(Little Ice Age,LIA)的生态水文过程及其两个阶段的转变特征,有助于进一步理解区域水文生态环境变化与季风气候之间的联系。【方法】基于长江中游湖北省永兴洞两支高分辨率石笋的δ^(13)C记录,采用ISCAM(Intra-Site Correlation Age Modelling)程序对这两条记录进行客观拼接,重建了该地区1 044~1 954 A.D.期间局域地表水文环境的变化历史。【结果】δ^(13)C在MCA和LIA时期表现为完全不同的两个状态,δ^(13)C在MCA整体减小,而在LIA整体增大。此变化特征与西南以及长江中游地区众多石笋δ^(13)C一致,表明MCA到LIA局域地表植被呼吸活动和降水发生了从强到弱的转变。永兴洞δ^(13)C显示由MCA向LIA的转变过程迅速,Rampfit方法分析得出此转变过程发生在1 434~1 460 A.D.,持续时间为26年。永兴洞记录支持MCA向LIA的转变可能由气候变化和人类活动共同影响所致的认识。通过与其他记录对比,发现永兴洞δ^(13)C与太阳总辐射量、热带辐合带、我国东部温度、厄尔尼诺—南方涛动在MCA和LIA事件尺度上存在对应关系,表明过去千年长江中游地表生态水文环境和全球气候变化有着动力联系;人类活动对地表植被的影响可能与区域或全球气候变化的背景相关联。【结论】通过对过去千年永兴洞石笋δ^(13)C序列的研究,发现MCA与LIA的生态水文特征差异显著,其变化可能与全球气候变化和人类活动有关。该研究不仅清晰地确立了MCA与LIA在长江中游地区的时间界限,而且加深了对过去千年长江中游地区两个时期的生态水文环境变化特征及其原因的理解。

[Objective]This study examined the ecological and hydrological variability and regime shift during the Medieval Climate Anomaly(MCA)and Little Ice Age(LIA)in the middle reaches of the Yangtze River.This region is climatically governed by the Asian monsoon,with increased precipitation in summer and decreased in winter.Our study helps deepen our understanding of the history of the regional ecological and hydrological changes and their connections to the monsoon climate.[Methods]Based on two high-resolution stalagmite(YX262 and YX275)δ^(13)C records from Yongxing cave,Hubei province in the middle reaches of the Yangtze River,we reconstructed the history of the local paleoclimate and environment from 1044-1954 A.D.by creating a composite of the two records using the iscam program.[Results]Theδ^(13)C records show two completely different states during the MCA and LIA,with the lowerδ^(13)C values characterizing the MCA period,and the higherδ^(13)C values the LIA.During the transition from MCA to LIA,δ^(13)C shows an abrupt change.This phenomenon indicates a regime shift from a strong to weak state for the vegetation respiration activity and precipitation variation.This variation is consistent with many stalagmiteδ^(13)C records in southwest China and the middle reaches of the Yangtze River.Here,our stalagmiteδ^(13)C record shows that the shift of the vegetation cover was rapid from the MCA to LIA.A Rampfit analysis shows that the stalagmiteδ^(13)C shift occurred between 1434-1460 A.D.,lasting 26 years,which is more rapid and significantly longer than the stalagmiteδ^(18)O shift.This variation is attributed to both climate deterioration and human activity influences,leading to a reduction of vegetation cover during this period.On the one hand,the rapid transformation of the stalagmiteδ^(13)C record in Yongxing cave may be related to the great migration event in the early Ming Dynasty.A large number of migrants swarmed into Hubei province,inevitably leading to large-scale deforestation and land reclamation and changing the ecological environment of the surface soil vegetation.Those behaviors cause positive stalagmiteδ^(13)C.On the other hand,the Yongxingδ^(13)C record is correlated with changes in the total solar radiation,intertropical convergence zone(ITCZ),temperature in eastern China,and El Niño-Southern Oscillation(ENSO)on the MCA and LIA event scales.During the MCA period,the total solar irradiance was larger,and the ITCZ was farther north,with a high frequency of El Nino states.However,during the LIA period,the total solar irradiance was relatively small,and the position of ITCZ was more southerly,in the low frequency period of El Nino states.This correlation indicates that the changes of hydrological circulation and vegetation cover in the middle reaches of the Yangtze River had a dynamic relationship with the regional and global climate change during the past millennium,and the impacts of human activities on surface vegetation may be related to the background of regional or global climate change.[Conclusions]By studying theδ^(13)C record of stalagmites in Yongxing cave in the past millennium,the eco-hydrological characteristics of MCA and LIA were found to be significantly different,which may be related to global climate change and human activities.The study not only clearly establishes the time boundary between MCA and LIA in the middle reaches of the Yangtze River,but also deepens our understanding of the characteristics and causes of eco-hydrological environment changes in the region during the two periods.