THICKNESS AND STABLE ISOTOPIC CHARACTERISTICS OF MODERN SEASONAL CLIMATE-CONTROLLED SUB-ANNUAL TRAVERTINE LAMINAS IN A TRAVERTINE-DEPOSITING STREAM AT BAISHUITAI, SW CHINA: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTION

Abstract

A continuous high-resolution (monthly) record of stable isotopes (δ13C and δ18O) in a well-laminated freshwater travertine deposited at Baishuitai, SW China from May 1998 to November 2001 was presented. The travertine exhibits clear annual bands with coupled brown/white color laminations. Throughout field investigation, it was found that the thin (1.5–2.2 mm), brown porous lamina was formed in the monsoonal rainy season from April to September, whereas the thick (5–8 mm), dense white lamina was formed in the dry season from October to March. The comparisons of lamina thickness and stable isotope signals in the travertine with the meteorological records allow us to constrain the relevant geochemical processes in the travertine formation under different climate conditions and to relate climate variables to their physicochemical proxies in the travertine record. Sympathetic variations in lamina thickness, δ13C and δ18O along the sampled profile reflect changes in hydrogeochemistry, showing that thin lamina and low δ13C and δ18O values occur in warm and rainy seasons. The decreased amount of calcite precipitation and low δ13C values during the warm and rainy seasons is caused by dilution of overland flow after rainfall. The low δ18O values are believed to be related to the rainfall amount effect in subtropical monsoonal regions. This process is thought to be markedly subdued whenever the amount of rainfall is lower than a given threshold. Accordingly, distinct minima in lamina thickness, δ13C and δ18O are interpreted to reflect events with above-average rainfall, possibly heavy floods, and vice versa. This study demonstrates the potential of freshwater travertine to provide valuable information on seasonal or even monthly rainfall variations.