摘要
对四川黄龙沟钙华景区的水化学测试发现,形成黄龙沟钙华的泉水具有很高的Ca2+和HCO-3离子浓度,相应地,泉水的CO2分压显著高于大气和土壤生物成因所能产生的CO2分压。结合泉水出露的地质条件和泉口CO2气体碳稳定同位素组成(δ13C=-6.8‰)的分析,进一步发现,高CO2分压与深部成因的CO2有关。可见,黄龙沟钙华属于热成因类钙华,而非原来普遍认为的“是气候岩溶作用的产物”。此外,黄龙沟钙华的大量出现与水中方解石的迅速沉积、Ca2+和HCO-3浓度的大量降低有关。随着地下水自泉口出露,由于水的CO2分压远高于大气,水中CO2大量释放于大气,结果水的pH值迅速升高,方解石饱和指数由泉口的负值很快转变为高的正值,为方解石的沉积奠定了必要的物理化学基础。放置于水中的大理岩石片观测表明,流速较快的边石坝处的方解石沉积速率是其附近水池内的2～5倍,这清楚地显示了水动力条件对沉积速率的控制。进一步根据DBL理论模型分析发现,水动力条件对方解石沉积速率的控制在于其对固液界面间扩散边界层(DBL)厚度的影响,流速愈快,DBL厚度愈薄。且DBL厚度最终制约着沉积表面的化学组成浓度,即厚度愈小,表面H+浓度愈低(或pH愈高)、方解石饱和指数愈高,进而方解石沉积愈快。
It is found that the travertine forming springs at Huanglong Ravine have a very high concentration of calcium and bicarbonate, and accordingly very high CO2 partial pressures, which are not likely produced by the biological activity in soil alone. Further analysis on the stable carbon isotopic composition of CO2 gas in the springs (δ13C=-6.8‰) shows that the high pressure of CO2 is mainly related to a deep seated CO2 source. That means the Huanglong travertine is thermogenic in origin, which is contrast to the commonly accepted sayingthat the travertine deposition simply is a product of warm and humid climate conditions in a karst ecological environment. Moreover, the huge travertine deposits in Huanglong Ravine are related to the quick precipitation of calcite from water, which leads to a sharp decrease in concentrations of <IMG SRC='IMAGE/03130031.JPG' HEIGHT=11 WIDTH=29>Ca2+and . Rapid CO2 degassing from the water is triggered by the much higher CO2 partial pressures 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. Experiments employed marble tablets show that the deposition rate of calcite at rimstone dam, where water flows more quickly, is higher than that in the nearby travertine pool, with a factor of 2 to 5. This gives clear evidence for the influence of hydrodynamic conditions on calcite deposition rates. By applying the DBL theoretical model, it is found that higher flowing rate produces thinnerdiffusion boundary layer between solid surface and bulk solution, and the latter controls the concentrations of chemical components at the solid surface. The thinner the boundary layer, the lower the surface concentration of H+, the higher the saturation index of calcite, and thus the more quickly the calcite deposition.
出处
《地球化学》
CAS
CSCD
北大核心
2003年第1期1-10,共10页
Geochimica
基金
国家自然科学基金(40073026)
国家重大基础研究前期研究专项(2002CCA05200)
国土资源部"百人计划"项目(9806)
科学技术部社会公益研究专项资金项目(2000-164)
广西自然科学基金(桂科回0144010)
关键词
四川
黄龙沟钙华景区
水化学
起源
形成机理
大气
土壤生物
热成因
碳同位素
travertine
hydrochemistry
stable carbon isotope
deep seated CO2 sources
diffusion boundary layer
saturation index
deposition rate
Huanglong
Sichuan Province
