Temporal And Spatial Evolution Of Precipitation Stable Isotopes In Western Hubei:Implications For Palaeoclimate And Paleoaltimetry Reconstruction

Stable isotopes of hydrogen（δD）and oxygen(δ¹⁸O)in precipitation exist widely in water and are sensitive to climate and environmental changes.Therefore,the study of stable isotopes in precipitation can help us understand the global climate evolution process.Since stable isotope signals of precipitation can be recorded by geological carriers（such as ice core,deep-sea sediments,loess,tree rings,lake sediments,stalagmites,etc.）,they are widely used in the reconstruction of palaeoclimate and paleoaltimetry.The temporal evolution of stable isotopes is often used to represent climate changes.For instance,it is widely accepted that Chinese speleothemδ¹⁸O records reflect the Asian monsoon history,but the specific relationships with different monsoon subsystems,such as the East Asian Monsoon（EAM）and the Indian Monsoon（IM）,remains controversial.Furthermore,it also remains unclear whether the relationship between theδ¹⁸O and Asian monsoon indices remains stable over different timescales.The spatial variation of stable isotopes（elevation effect）is widely used to reconstruct paleoaltimetry quantitatively.However,reconstruct results are inconformity by different methods.The study of the temporal and spatial evolution of stable isotopes in modern precipitation is a bridge of paleoclimate and paleoaltimetry in the geological period.While there is no quantitative relationship between the precipitation stable isotopes and the Asian monsoon,as well as no report about the elevation effect of stable isotopes and their response factors in the low altitude region of East Asia monsoon area.In this study,we take the precipitation stable isotope compositions which are collected from different elevation in western Hubei as the research objects,focusing on the temporal and spatial variation ofδD andδ¹⁸O in modern atmospheric precipitation,combining meteorological observation data,to explore the climate significance of precipitationδ¹⁸O and its elevation effects.Furthermore,our study combines researches of stalagmiteδ¹⁸O and stable isotope altimeters,providing new understandings for the implication of palaeoclimatic and paleoaltimetric reconstruction.Specifically,the following conclusions can be drawn:1.Temporal variation of precipitation stable isotopes in western Hubei mainly reflect the change of tropical monsoon in the northern hemisphere.Based on the precipitation stable isotopes in the HS site,which is minted from 2011,we explored the impact of local air temperature,precipitation amount and different monsoon indices.The results indicate that the changes of atmospheric circulation patterns instead of local climate conditions are the main factor determining the precipitationδ¹⁸O.On the seasonal timescales,the precipitationδ¹⁸O andδD show inverse temperature effect,with the correlation coefficients of-0.41 and-0.47,respectively.They also display the weak precipitation amount effect（r=-0.42）.Moreover,the strong correlations between precipitationδ¹⁸O and all nine monsoon indices,particularly the IM indices（MHI,SASSI,SAWSI and WYI,with the correlation coefficients of-0.46,-0.59,-0.52 and-0.54）and WNPM indices（WNPM,r=-0.60）,suggest that precipitationδ¹⁸O in the EAM region is mainly affected by upstream convective activities.On the interannual timescales,WNPM and IM indices（defined by zonal wind）are most strongly correlated withδD andδ¹⁸O（SAWSI,WYI and WNPM,with the correlation coefficients of-0.83,-0.96 and-0.86）,which indicates El Nino-Southern Oscillation（ENSO）controls the precipitationδ¹⁸O in East Asia（r=0.89）.Mechanically speaking,when El Ni（?）o happens,the weaker convection in the western Pacific,the weakened cloud-top effect,and thus the higherδD andδ¹⁸O in precipitation.Along the water vapor transport path,the changes of circulation caused by El Ni（?）o subdued the WNPM and IM activities.Hence,the attenuated upstream effect led to the positive precipitationδ¹⁸O anomaly in the eastern Asian region.As the driving force of seasonal and interannual monsoon changes can be compared to variations on orbital and suborbital scales respectively,we propose that neither local climate nor the EAM intensity is the main controlling factor of Chinese speleothemδ¹⁸O.Speleothemδ¹⁸O is determined by upstream processes and reflects the change of tropical monsoon in the Northern Hemisphere.Moreover,we approve that both external forcing and internal variability influence the long timescales stalagmiteδ¹⁸O records,solar radiation is dominant in the orbital timescales,while ocean-atmosphere interactions may play an indispensable role in monsoon variation on the suborbital timescales.2.Vertical spatial variation of precipitation stable isotopes in western Hubei is precarious.Based on the continuous monthly monitoring of precipitation stable isotopes that were carried out from 13 stations within 3000m of elevation,we summarized the influence of elevation on precipitationδ¹⁸O composition.The results show that the decreasing rates ofδ¹⁸O andδD in western Hubei are-0.17±0.05‰/100m and-1.20±0.35‰/100m,respectively,which are in line with the decreasing rates of stable isotopes in global precipitation.On the seasonal timescales,the correlation betweenδ¹⁸O and elevation is unstable（r:-0.97～0.79）,as well as the decreasing rate is not constant（k:-0.09～-0.25）.The diversity of isotopic elevation effects caused by multiple drives.Temperature is the main factor that controlling the relationship between isotopes and elevation.Meanwhile,the influences of precipitation amount and sub-cloud evaporation also complicate the elevation effect.On the interannual timescales,the elevation effect of precipitation stable isotopes is significant in western Hubei（r>-0.89,p<0.01）,which is still determined by temperature.Moreover,interannual variation of monsoon circulation also play an important role in the isotopic elevation effect.As the increase of precipitation amount,extreme rainstorm events,and the rainfall inhomogeneity in El Ni（?）o decline years will interfere with the stable isotopes elevation effect,result in the decreasing rate ofδD andδ¹⁸O getting lower.The observation results have important implications for palaeoclimate and paleoaltimetry reconstruction.In terms of the palaeoclimate reconstruction,we can extract the regional different climate signals more accurately by eliminating theδ¹⁸O values that caused by the elevation effect between different records.In addition,we should take the using conditions of stable isotope altimeters,such as the limitations of medium and low altitudes,restriction of arid and humid climate,as well as the effect of modern monsoon circulation into considerations.It is necessary to use different gradients for different climatic conditions to improve the accuracy of paleoaltimetry reconstruction.