THE ALTITUDE EFFECT ON OXYGEN ISOTOPE COMPOSITION OF STALAGMITE

Oxygen isotope ratios (d18O) of stalagmites from low-latitude locations may be a good proxy for local precipitation changes. To correlate the temporal and spatial variations of precipitation using d18O of stalagmites from different areas, however, it is necessary to understand the impacts of oxygen isotope fractionation during moisture transfer on the d18O of precipitation. Here we present the d18O records of stalagmites from three caves (Tiane, Sanbao and Xiniu caves) along a transect of progressively increasing altitudes (1700 to 2100 m) in Shengnongjia (N31.5-31.7, E110.2-110.6), central China. Together with calculated modern mean annual d18O values of precipitation at different altitudes (from the GNIP database, http://ecophys.biology.utah.edu/LabFolks/gbowen/pages/OIPC_Main.html), we confirm the effect of altitude on stalagmite d18O. We find that mean stalagmite d18O values from the same time period decrease with increasing cave altitude at a rate of about -0.2‰/100 meters, indicating that the oxygen isotope compositions of precipitation undergoes fractionation during the moisture transfer from low to high altitudes, i.e. the altitude effect. The average differences in d18O of stalagmites from different caves show a linear correlation to the differences in the calculatedd18O of modern precipitation at the different cave altitudes. Therefore, stalagmite d18O from different areas may have significantly different d18O in terms of absolute values. This is mainly a result of the differences in cave altitude in the study area. However, the patterns of the d18O temporal variations recorded in stalagmites from different caves are similar, and appear to indicate the Asian Monsoon changes in comparison to other Asian Monsoon records. Studying temporal variability of stalagmite d18O in combining with the altitude effect may enable a more accurate reconstruction of the history of spatial precipitation changes.