Rapidity of the Climatic and Environmental Changes During the Past Millennium Inferred from Stalagmite δ13C Records in the Middle Reaches of the Yangtze River
[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 con-nections to the monsoon climate.[Methods]Based on two high-resolution stalagmite(YX262 and YX275)δ13C re-cords from Yongxing cave,Hubei province in the middle reaches of the Yangtze River,we reconstructed the history of the local paleoclimate and environment from 1 044-1 954 A.D.by creating a composite of the two records using the iscam program.[Results]The δ13C records show two completely different states during the MCA and LIA,with the lower δ13C values characterizing the MCA period,and the higher δ13C values the LIA.During the transition from MCA to LIA,δ13C 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 stalagmite δ13C records in southwest China and the middle reaches of the Yangtze River.Here,our stalagmite δ13C record shows that the shift of the vegetation cover was rapid from the MCA to LIA.A Rampfit analysis shows that the stalagmite δ13C shift occurred between 1 434-1 460 A.D.,lasting 26 years,which is more rapid and significantly longer than the stalagmite δ18O 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 stalagmite δ13C 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 stalagmite δ13C.On the other hand,the Yongxing δ13C record is correlated with changes in the total solar radiation,intertropical conver-gence 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 activi-ties on surface vegetation may be related to the background of regional or global climate change.[Conclusions]By studying the δ13C 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 environ-ment changes in the region during the two periods.
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