Effect of in-stream physicochemical processes on the seasonal variations in δ13C and δ18O values in laminated travertine deposits in a mountain stream channel

Travertines are potential archives of continental paleoclimate. Records of stable carbon and oxygen isotopic composition (δ13C and δ18O) in laminated travertine deposits from endogene spring waters show regular cyclic patterns which may be due to seasonal change in climate determinants such as temperature and rainfall. In this study, δ13C and δ18O measurements of three travertine specimens that grew naturally over the eight years, 2004-2011, at upstream, middle and downstream sites in a canal at Baishuitai, SW China, are presented. They exhibit clear seasonal variations that generally correlate with biannual laminations. Specifically, δ13C and δ18O values show significant positive correlation with each other for the three travertine specimens, with the correlation coefficients increasing downstream along the canal. To reveal the factors governing the seasonal and spatial variations in δ13C and δ18O values, newly formed travertines precipitated on Plexiglas substrates are also examined. Both δ13C and δ18O of the substrate travertines are low in the summer/rainy season and high in the winter/dry season, showing a great consistency with the patterns in the natural travertines. Spatially, isotope values increase downstream in both seasons, with higher increase rates in winter that are related to removal of larger fractions of dissolved inorganic carbon (DIC) from the solution and stronger kinetic isotopic fractionation in winter. Due to in-stream physicochemical processes, including CaCO3 precipitation and the associated degassing of CO2, seasonal changes in δ13C and δ18O in the travertines are amplified by two times between the upstream and downstream sites: this is opposite to trends for epigene (meteogene) tufas whose seasonal changes in stable isotope compositions are reduced downstream. We suggest in-stream physicochemical processes are a potential reason for underestimation of annual temperature ranges that are inferred from epigene tufa δ18O data.