Contrasts in variations of the carbon and oxygen isotopic composition of travertines formed in pools and a ramp stream at Huanglong Ravine, China: Implications for paleoclimatic interpretations

Abstract

Water samples and modern endogenic (thermogene) travertine calcite deposited on plexiglass substrates in travertine pools and a ramp stream were collected along the Huanglong Ravine, Sichuan, SW China at regular ∼10day intervals from early May to early November in 2010, including both wet and dry conditions. Temporal and spatial variations in the δ13C and δ18O values of the modern travertine were examined to understand their potential for paleoclimatic and paleoenvironmental interpretations. It was 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. Their 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. Meanwhile, kinetic effect on oxygen isotopic fractionation during ramp travertine deposition existed and reduced this positive correlation. 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 correlated chiefly with the higher water temperatures. Therefore, the δ13C and δ18O values of the travertine may be used for paleo-rainfall or paleotemperature reconstruction respectively. This 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.