Like speleothems, travertine/tufa deposited at the surface in karst areas can also be an important geological archive for terrestrial paleoclimatic studies. In particular, because of the high depositional rates (mm to cm per year), high-resolution (up to seasonal, monthly or even daily) paleoclimatic information can be obtained. Before using travertines or tufas to reconstruct climate changes, however, one must understand the cause of variations in the geochemical proxies (chiefly stable oxygen and carbon isotope compositions) in these precipitates, and the quantitative relationships between these proxies and climatic and environmental factors. The climatic interpretation of proxies in travertine/tufa and their controlling mechanisms are generally complicated and often site-specific. Direct transfer of the relationships between proxies and climatic factors from one site to another, as reported in some previous studies, may result in incorrect interpretation due to the complexity of the mechanisms controlling these proxies.

Huanglong Ravine, Sichuan, SW China, is internationally known for its unusual and diversified landscapes (e.g., travertine pools, travertine falls and flowstones) and was listed by UNESCO in 1992 in the World’s Nature Heritage. Travertine here has been studied more than 20 years. Most of the previous studies focused on the variation of hydrochemistry and precipitation mechanism of travertine. However, the potential paleoclimatic significance of the stable oxygen and carbon isotopic composition of the travertine in Huanglong Ravine has not been studied by far.

The travertine research group led by Prof. LIU Zaihua from the State Key Laboratory of Environmental Geochemistry （SKLEG）, Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) has obtained new findings in investigating seasonal variation in isotopic compositions of oxygen and carbon in modern travertine deposited under differing hydrodynamic conditions.

At regular ~10 day intervals from early May to early November in 2010, the researchers collected the water samples and modern endogenic (thermogene) travertine calcite deposited on plexiglass substrates in travertine pools and a ramp stream along the Huanglong Ravine, Sichuan, SW China, including both wet and dry conditions. They found that δ¹³C and δ¹⁸O of travertine formed in the ramp stream were low in the warm rainy season and high in the cold dry season. The positive correlation was mainly due to dilution and rainfall seasonal effects on δ¹³C and δ¹⁸O values, respectively, i.e., low δ¹³C values were caused by dilution by overland flow with depleted δ¹³C values and reduced CO2-degassing in the warm rainy season while low δ¹⁸O values of travertine were because of low δ¹⁸O values of water induced by seasonal variation in oxygen isotopic ratios of rainwater. In contrast, the δ¹³C and δ¹⁸O values of the pool travertines displayed a converse behavior which was caused mainly by the temperature effect. Low δ¹⁸O values and high δ¹³C values in the warm rainy season were corresponding with the higher water temperatures. Therefore, the δ¹³C and δ¹⁸O values of the travertine there may be used for paleo-rainfall or paleotemperature reconstruction respectively.

Their study demonstrates that endogenic travertine, like epigenic (meteogene) tufa, may be a suitable candidate for high-resolution paleoclimatic and paleoenvironmental reconstructions. However, since travertines deposited under differing hydrodynamic conditions (e.g., pools with still water contrasted to fast flow streams) have different climatic responses, it is necessary to check the depositional facies of fossil travertine samples before they can be used for palaeoclimate (temperature and/or rainfall) reconstruction.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41172232) and the 973 Program of China (2013CB956703).
Web site of the paper：http://www.sciencedirect.com/science/article/pii/S0016703713005516
(By YAN Hao)