Altitude effect on δ18O in stalagmites from Mt. Shennongjia, central China

Proxy data δ18O reflecting the oxygen isotope composition of meteoric precipitation are widely used in reconstructions of continental pale climate. Nevertheless, there has been a general debate that precipitation amount or temperature dominates the speleothemδ18O signal. To assist interpretation of speloethem δ18O time series, it is necessary to understand the impacts of main oxygen isotope fractionation during moisture transfer. Here we present the δ18O records of stalagmites from three caves(Heilong, Qingtian and Sanbao caves)along a transect of progressively increasing altitudes(1250～2250 m)in Shengnongjia(31.5～31.7° N,110.2～110.6°E),central China. We confirm the effect of altitude on stalagmite δ18O that mean stalagmiteδ18O values from two different same time period decrease with increasing cave altitude at a rate of about -0.1‰/100 meters, indicating that the oxygen isotope compositions of precipitation undergoes fractionation during the moisture transfer from low to high altitude. In fact, modern meteorologic precipitation data show that altitude effect has an effect on the δ18O change at a rate of -0.2‰/100 meters, which exceeds more the speleothem result. Consequently, in addition to the isotopic influence of precipitation on stalagmite δ18O, temperature differences between altitudes contribute greatly to δ18O variation. In combining with annual averaging temperature in caves, an average dδ18O p/dT of approximately -0.25‰/℃ was calculated, which agrees with the result of isotopic equilibrium fractionation by O’Neil.