Equilibrium vs. kinetic fractionation of oxygen isotopes in two low-temperature travertine-depositing systems with differing hydrodynamic conditions at Baishuitai, Yunnan, SW China

Abstract

In order to understand the behavior of oxygen isotopic fractionation and obtain more reliable paleoclimatic data from surficial travertine deposits, the hydrochemistry, carbonate precipitation rates, and the temporal and spatial variations in δ18O values of the modern travertine in two low-temperature travertine-depositing systems (canal and pool) with differing hydrodynamic conditions at Baishuitai, Yunnan, SW China, were investigated during the warm rainy season (May 24 to November 2) in 2010. It was found that the oxygen isotopic composition of travertine in the two systems showed distinct controlling mechanisms mainly due to different carbonate precipitation rates and water residence times. For the canal system with fast flow, large deviations between the calculated and measured temperatures indicated that travertine precipitated out of isotopic equilibrium. This was due to rapid carbonate precipitation with intense CO2 degassing. δ18Otravertine values and water temperature increased downstream (∼1‰ and 4°C, respectively) while δ18Owater values remained relatively stable. It was shown that the δ18O values of travertine in the canal system recorded the δ18O values of dissolved carbonates (HCO3−) which also increased along the canal, controlled by Rayleigh-distillation effects. In contrast, in the pools system with slow flow, oxygen isotopic equilibrium between dissolved carbonates (HCO3−) and H2O was achieved and the δ18Otravertine values did not display spatial variations. Calculated water temperatures using the equilibrium fractionation factors of Coplen (2007) were identical to the measured ones, which indicates that travertine in such pools system is suitable for the reconstruction of paleo-precipitating temperatures. Finally, by comparing these two systems we have obtained a simple criterion to determine the suitability of travertine to estimate temperatures – the δ18O value of BaCO3 precipitated from water samples is compared with the theoretical δ18O value of dissolved carbonate calculated using water temperature and δ18Owater, to check if the oxygen isotopes of the dissolved carbonate are in equilibrium with the water.