Wet–dry seasonal and spatial variations in the d13C and d18O values of the modern endogenic travertine at Baishuitai, Yunnan, SW China and their paleoclimatic and paleoenvironmental implications

摘要

Seasonal and spatial variations in the d13C and d18O values of the modern endogenic (thermogene) travertine deposited in a calcite-depositing canal at Baishuitai, Yunnan, SW China were examined to understand their potential for paleoclimatic and paleoenvironmental implications. The sampling sites were set in the upstream, middle reach and downstream of the canal, and the modern endogenic travertine samples were collected semimonthly to measure their d13C and d18O values. It was found that both d13C and d18O values of the endogenic travertine were low in the warm rainy season and high in the cold dry season, and correlated with each other. The low d18O values in warm rainy season were mainly related to the higher water temperature and the lower d18O values of rainwater, and the low d13C values are caused by the dilution effect of overland flow with low d13C values in the warm rainy season and the reduced CO2-degassing of canal-water caused by the dilution effect of the overland flow. The linear negative correlation between the travertine d18O (or d13C) values and rainfall amount may be used for paleorainfall reconstruction if one knows the d18O (or d13C) values of the fossil endogenic travertine at Baishuitai though the reconstruction was not straightforward. It was also found that there was a progressive downstream increase of the d18O and d13C values of the travertine along the canal, the former being mainly due to the preferential evaporation of H2 16O to the atmosphere and the latter to the preferential release of 12CO2 to the atmosphere during CO2-degassing. However, the downstream increase of the travertine d18O and d13C values was less intensive in rainy season because of the reduced evaporation and CO2- degassing during the rainy season. To conclude, the downstream travertine sites could be more favorable for the paleo-rainfall reconstruction while the upstream travertine sites are more favorable for the paleo-temperature reconstruction. So, this study demonstrates that endogenic travertine, like epigenic (meteogene) tufa, could also be a good candidate for high-resolution paleoclimatic and paleoenvironmental reconstruction.