云南白水台现代内生钙华微层的特征及其古气候重建意义

Thickness and Stable Isotopic Characteristics of Modern Seasonal Climate-controlled Sub-annual Travertine Laminas in a Travertine-depositing Stream at Baishuitai, Southwest China: Implications for Paleoclimate Reconstruction

在云南白水台采集一现代内生钙华样品,该样品形成于1998年5月至2001年11月.切片观察,钙华中可见薄的褐色疏松微层和厚的白色致密微层,且交替出现.结合钙华样品的高分辨率碳氧稳定同位素测试,发现薄的褐色疏松微层(1.5～2.2 mm)在每年的雨季(4～9月)形成,而厚的白色致密微层(5～8 mm)在旱季(10～3月)形成.通过与气象记录的对比,建立起了这些钙华亚年层厚度和稳定同位素特征与气候变化的关系.结果发现:薄的微层及其低δ13C和δ18O值形成于温暖湿润的雨季.在雨季,雨水的稀释作用导致了内生钙华沉积的减慢和低的13C含量,而钙华的低18O值则主要与亚热带季风地区的雨量效应有关.因此,钙华微层厚度,以及δ13C和δ18O的显著降低反映了高的降雨条件,反之,则反映干旱的气候条件.本研究说明钙华可以提供有价值的年际甚至季节尺度的气候变化信息.

A continuous high-resolution monthly record of stable isotopes (δ~ 13C and δ~ 18O) in a well laminated freshwater travertine deposited at Baishuitai of Southwest China from May 1998 to November 2001 is presented. The travertine exhibits clear annual bands with coupled brown、white color laminations. Throughout the field investigation, it was found that the thin (1.5～2.2 mm) brown porous lamina was formed in the monsoonal rainy season from April to September, whereas the thick (5～8 mm) dense white lamina was formed in the dry season from October to March. The comparisons of lamina thickness and stable isotope signals in the travertine with the meteorological records allow us to constrain the relevant geochemical processes in the travertine formation under different climate conditions and to relate climate variables to their physico-chemical proxies in the travertine record. Sympathetic variations in lamina thickness, δ~ 13C and δ~ 18O along the sampled profile reflect changes in hydrogeochemistry, showing that thin lamina and low δ~ 13C and δ~ 18O values occur in warm and rainy seasons. The decreased amount of calcite precipitation and low δ~ 13C values during the warm and rainy seasons are caused by dilution of overland flow after rainfall. The low δ~ 18O values are believed to be related to the rainfall amount effect in subtropical monsoonal regions. This process is thought to be markedly subdued whenever the amount of rainfall is lower than a given threshold. Accordingly, distinct minima in lamina thickness, δ~ 13C and δ~ 18O are interpreted to reflect events with above-mentioned average rainfall, possibly heavy floods, and vice versa. This study demonstrates the potential of freshwater travertine to provide valuable information on seasonal or even monthly rainfall variations.