Biannually laminated freshwater carbonate deposits (tufas and travertines) are potential sources of high-resolution terrestrial paleoclimate data. Because of their high precipitation rates (mm to cm/year), travertine and tufa have stood out in recording high-resolution (up to seasonal, monthly or even daily) paleoclimate information.  Therefore, they have unique advantages in reconstructing some short-lived but important climatic and environmental events, as well as the yearly- to centennial-scale climate cycles.

Among the proxies in travertine records, oxygen isotope ratio (δ¹⁸O) may be the most important one. Oxygen isotopic fractionation between calcite and parent water (Δ¹⁸O_{calcite-water}) is negatively correlated to precipitation temperature in isotopic equilibrium, which provides a theoretical basis for numerous studies to estimate the paleo-temperature. However, to use this correlation, there is a prerequisite- oxygen isotopic equilibrium in the CaCO₃-HCO₃-H₂O system must be achieved. But a growing number of studies have shown that oxygen isotopic equilibrium is rarely maintained during calcite precipitation under natural conditions. In this case, the temperature calculated by fractionation factor of equilibrium may differ significantly from the measured temperature. Sometimes, the difference between the calculated and measured temperatures could be more than ten degree Celsius. Thus, in order to obtain more reliable paleoclimatological and paleoenvironmental data from the travertines, a better understanding of the processes governing their precipitation and stable isotopic equilibrium is needed.

The travertine research group led by Prof. LIU Zaihua at the State Key Laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) has made discoveries while investigating isotopic fractionation of oxygen during travertine precipitation. The scientists examined 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 during the warm rainy season (May 24 to November 2) in 2010. It was found that travertine precipitation rates play a key role in governing the level of isotopic exchange, and then the isotopic fractionation. For the canal system with fast flow and high calcite precipitation rates, the large deviations between the calculated and measured temperatures indicated that travertine precipitated out of isotopic equilibrium. Because travertine precipitation rates were higher than the exchange rate of oxygen between HCO₃^- and H₂O, δ¹⁸O_DIC values increased along the canal controlled by the effect of Rayleigh distillation. For the pools system, oxygen isotopic equilibrium between dissolved carbonates (HCO₃^-) and H₂O was achieved. This is reasonable due to the relatively long residence time of HCO₃^- in H₂O and the low precipitation rates, which are both influenced by the prevailing hydrodynamic conditions. Calculation of the temperature of calcite precipitation in the pool P5 using isotopic fractionation factors quoted by Coplen (2007) gives temperatures that fit well with the measured ones. The above results indicate that, compared with the canal system, travertine in the pools system is more suitable for reconstruction of the precipitating temperature. Finally, the scientists proposed a simple criterion for travertine suitability for paleo-temperature estimation: check if the oxygen isotopes of dissolved carbonate are in equilibrium with those of the water.

The above study was published in Geochimica et Cosmochimica Acta (Geochimica et Cosmochimica Acta 95 (2012) 63–78). “This study provides one of the first assessments of kinetic fraction of isotopes in surface terrestrial carbonate systems. It is interesting and valuable work, potentially providing a major ‘spanner in the works’ for palaeoclimate research on these deposits”– as commented by a reviewer of Geochimica et Cosmochimica Acta.

(By YAN Hao)