Climate Response Of Carbon And Oxygen Isotopic Compositions Of Land Snails In The East Asian Monsoon Region

Understanding the variations and forcing mechanisms of East Asian monsoon（EAM）on various timescales have been the focus of the current palaeoclimate research.The most important geological archives for recording the EAM changes in the terrestrial system comes from the cave sediments in southern China and the eolian loess deposits in northern China.However,variations in EAM intensity recorded by speleothemsδ¹⁸O were dominated by orbitally-paced variations in Northern Hemisphere summer insolation,which is believed to be mainly driven by precession cycles of（23-kyr）;while records of the EAM reconstructed from Chinese loess deposits are dominated by 100-kyr cycles following the mid-Pleistocene transition.These discrepancies may result from an ambiguity in the climatic interpretation of speleothemδ¹⁸O values and an inconsistent relationship between precipitationδ¹⁸O and monsoonal rainfall amount,on the orbital scale.Therefore,it is important to obtain a new archive that record both rainfall amount and precipitationδ¹⁸O.Stable isotopic compositions of land snail shell have the potential to reflect palaeoecology and palaeohydrology conditions.Specifically,shellδ¹³C(δ¹³C_shell)is often used to investigate changes in vegetation composition（i.e.the ratio of C3/C4plants,or C3 plants with different water use efficiency）,thus reflecting the aridity or humidity of an environment;Shellδ¹⁸O(δ¹⁸O_shell)can generally be used to reflect theδ¹⁸O of local meteoric waters.However,the climatic significance ofδ¹³C_shell andδ¹⁸O_shell is still not fully understood,especially for snails in the EAM region,which makes it difficult to reconstruct palaeoenvironment and palaeoclimate using fossil snails.Moreover,modern snail data from the Mediterranean and the Americas show that bothδ¹³C_shell andδ¹⁸O_shell may be affected by multiple environmental factors and can be complicated by region-dependent and species-dependent behavior of snails.In addition,the empirical calibrations between the stable isotope compositions of shells and the environmental factors obtained from different regions are also different.Therefore,before attempting to use fossil snail shells in loess-paleosol sequences in northern China for past monsoon changes reconstruction,it is particularly necessary to strengthen the research on the modern process ofδ¹³C andδ¹⁸O of land snails.Based on this purpose,this paper analyzed the stable isotopic composition of snails and the corresponding plants and surface soil from the East Asian monsoon core areas（including the entire latitudinal region of the EAM region in China and a precipitation gradient in the mid-latitude China）.Our aim was to evaluate the factors and the mechanisms controllingδ¹³C_shell andδ¹⁸O_shell under different environmental background;to elucidate the climatic significance ofδ¹³C_shell andδ¹⁸O_shell proxies and to establish the quantitative relationship between the stable isotope composition of snail shells(especially theδ¹³C)and the climatic and geographic factors（especially the precipitation and altitude）.Finally,on the basis of modern process research,theδ¹³C_shellandδ¹⁸O_shell indexes were applied to the reconstruction of past climate change by using snail fossils since L5 layer（～470 ka）at Xifeng section in the south-central Chinese Loess Plateau.The main conclusions and understandings of this paper are as follows:In the EAM core areas,snail shells gradually become depleted in ¹³C traveling from the semi-arid north to the warm-wet south,in accordance with the route of the East Asian monsoon.A significant correlation was obtained betweenδ¹³C_shell and mean annual precipitation（MAP）from 74 sampling sites in the EAM core regions(δ¹³C_shell=-0.0030（±0.0003）MAP-8.80（±0.32）;r=-0.76,p<0.001)),signifying thatδ¹³C_shell can be regarded as an effective proxy for the monsoonal precipitation.A hypothetical model is proposed to explain this correlation:1)as precipitation increases,theδ¹³C of C3 plants becomes more negative and eventually induces more negativeδ¹³C_shell when a snail ingests the plants;2)increased rainfall will increase snail activity time and then metabolism,and hence tend to decrease theδ¹³C of bicarbonate in the body fluid,and finally result in more negativeδ¹³C_shell;3)increased rainfall promotes soil carbonate leaching which indirectly reduces a snail’s ingestion of carbonate.Moreover,a detailed analysis of individual species provides evidence indicating thatδ¹³C_shell as a climate proxy is species specific,that is Acusta.ravida and Bradybaena.similaris exhibits robust and consistentδ¹³C_shell–MAT andδ¹³C_shell–MAP relations in both linearity and sensitivity,but the third one,Cathaica fasciola does not show convincing applicability as a climate proxy.Such species-dependent phenomenon can be explained by carbonate ingestion,difference in eco-physiological factors and resultant metabolic rates among different snail species in different areas.Along the precipitation gradients in the mid-latitude China,monsoon rainfall determines the patterns of co-variation ofδ¹³C of snail shells,body tissue and plants.Different snail species live in different ecological environments and record significantly differentδ¹³C andδ¹⁸O:that is,higher values occur in grassland snails from the Chinese Loess Plateau,and lower values occur in snails in the woodland ecosystem of the eastern plain.From the perspective of the whole monsoon core area,it is found thatδ¹⁸O_shell has no correlation with environmental parameters,suggesting the diversity and complexity of factors affecting theδ¹⁸O_shell on a large regional scale.However,theδ¹⁸O_shell values are largely controlled by the precipitationδ¹⁸O along the precipitation gradients,which may lead to a significant relationship betweenδ¹⁸O_shell and precipitation because of the rainfall amount effect.Snails collected in the Helan Mountains across an altitudinal gradient of 1733–2851m a.s.l.Linear regression analysis show thatδ¹³C_shell is positively correlated with altitude on both slopes,with similar rates of altitudinal variation;in contrast,δ¹⁸O_shell is negatively correlated with altitude on the western slopes,but this effect is absent on the eastern slopes.The variations in the integratedδ¹³C signals of living vegetation and the flux of absorbed atmospheric CO₂ attributed to changes in metabolic rates are likely responsible for the altitudinal effect onδ¹³C_shell above 2000 m a.s.l.,while changes in vegetation type and/or carbonate ingestion by snails may be important in inducing the positive shift inδ¹³C_shell below 2000 m a.s.l.δ¹⁸O_shell is determined by the rainwaterδ¹⁸O and by the fractionation processes between shell carbonate and snail body water,and both are controlled by the temperature of the alpine environment.In addition,evaporative enrichment of the body fluid or of the environmental water prior to ingestion by snails exerts an important influence onδ¹⁸O_shell.Notably,bothδ¹³C_shell andδ¹⁸O_shell in Helan Mountains respond more sensitively to climate than in the monsoon core area.The rate of change ofδ¹³C_shell in response to temperature and precipitation above the altitude of 2000 m a.s.l.are-0.87‰/℃and+2.76‰/100mm,while the corresponding rates forδ¹⁸O_shell on the western slopes are+0.41‰/℃and-1.29‰/100mm.These results expand the range of application ofδ¹³C_shell andδ¹⁸O_shellas climatic indicators,and emphasize the differences in the climatic sensitivity of these proxies between the monsoon margin and core areas.Snail fossils in the Xifeng section show that theδ¹³C_shell values during glacial periods（L layers）generally more positive than that during interglacial periods（S layers）,which is inconsistent with the changes in the proportion of C3 and C4 vegetation recorded by theδ¹³C of soil organic matter,and thus confirms the conclusion obtained from the modern process that the shellδ¹³C tends to a precipitation proxy rather than a proxy reflecting the proportion of C3 and C4 vegetation.On the orbital timescales,the variation trend ofδ¹³C_shell of snail fossils is consistent with the variation in the mean grain size,magnetic susceptibility andδ¹⁸O of benthic foraminifera,showing an evident glacial-interglacial variation dominated by a 100-kyr cycles.These indicate that East Asian climate system in the mid-low latitude may controlled by the high-latitude ice volume.Theδ¹⁸O_shell data fluctuates frequently and has a large vibration range and does not show any cyclical change.However,to some extent,δ¹⁸O_shell data is similar to the variation in speleothemδ¹⁸O of southern China,and tends to reflect the low-latitude hydrologic process.Therefore,it seems that the relationship between precipitation(reflected byδ¹³C_shell)and precipitationδ¹⁸O(reflected byδ¹⁸O_shell)is not always be consistent on the orbital scale.In addition,bothδ¹³C_shell andδ¹⁸O_shell values fluctuated greatly within the loess/palaeosoil layer and even in the 10-20 cm depth intervals,implying the significant environment and climate change within the glacial or interglacial period and even within the millennial-to centennial timescales.The meanδ¹³C_shell values for each loess/palaeosol layer demonstrate that the vegetation in the central part of the Chinese Loess Plateau are dominated by C3 plants for the last 470kyr.Theδ¹³C_shell values exhibit a certain degree of difference among the loess/palaeosoil layers,while the modernδ¹³C_shell is significantly negative than the fossilδ¹³C_shell from each layers.This pattern of change inδ¹³C_shell can be attributed to the orbital-scale variations in theδ¹³C of C3 plants,and the atmospheric CO₂concentration and theirδ¹³C values.