Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction

摘要

A method of combining hydrochemical data logging and in situ titrating with measurement of stable carbon and oxygen isotopes was used to reveal the hydrochemical and isotopic characteristics in the Baishuitai travertine scenic area of SW China. It was found that the travertine-forming springs have a very high concentration of calcium and bicarbonate, and accordingly very high CO_2 partial pressures, which are not likely to be produced by biological activity in soil alone. Further analysis of the s table carbon isotopes of the springs shows that the high pressure of CO_2 is mainly related to an endogenic CO_2 source. That means the Baishuitai travertine is endogenic in origin. This is contrast to the commonly accepted saying that the travertine deposition in this study simply is a product of warm and humid conditions in a karst ecological environment. Rapid CO_2 degassing from the water is triggered by the much higher partial pressures in water than that of the surrounding air. Consequently, as the waters flow downstream of the spring the pH increases, the waters become supersaturated with respect to calcite, and travertine is deposited. The preferential release of ~(12)CO_2 to the atmosphere results in a progressive increase of travertine δ~(13)C downstream. This is concluded with a preliminary discussion of variation in travertine-forming water temperatures, according to differences in stable oxygen isotopic compositions of the travertine formed in different epochs at Baishuitai. It was found that the change in water temperature is as high as 13 ℃, i.e., from 23 ℃ at about 2500 years B.P., to 10 ℃ at present. This may mainly reflect that the effect of geothermal source on water temperature is decreasing. The problems involved in paleoenvironmental reconstruction with endogene travertine are also discussed. They are the impacts of "dead carbon" in radiocarbon dating and the enrichment in ~(13)C of travertine by endogenic CO_2 and degassing of CO_2 from water, which has to be considered in paleovegetation reconstruction when using δ~(13)C data of the endogene carbonate deposits.