物理剥蚀影响硫酸风化的小流域水化学研究及其对碳循环的启示意义

THE EFFECT OF PHYSICAL EROSION ON SULPHURIC ACID WEATHERING BASED ON WATER CHEMISTRY OF SMALL CATCHMENTS AND ITS IMPLICATION FOR CARBON CYCLE

硫酸风化通过与碳酸盐类地层和海洋沉积物中的有机质相互作用影响碳循环。一般认为，黄铁矿氧化速度快，硫酸风化的通量主要受黄铁矿的供应控制，与物理剥蚀有关。新生代构造抬升伴随的物理剥蚀增强可能加速了硫酸风化，从而影响大气CO2浓度。为解析物理剥蚀对硫酸风化的影响，本研究在青藏高原东部地区采集了36个具有基于10Be浓度剥蚀速率数据并以结晶岩为主的小流域河水样品。结果表明河水的[SO42-]＊/[HCO3-]比值与物理剥蚀速率呈良好的正相关关系，表明高物理剥蚀地区硫酸风化的相对效能高于碳酸风化。在千万年时间尺度内，高原抬升导致沉积地层硫酸风化增强，可能成为一种碳源。这可能是维持新生代早期高大气CO2浓度的原因之一。当硫酸风化作用于硅酸盐时，黄铁矿风化在大于千万年的时间尺度是重要的碳汇，结晶岩硫酸风化增加可能是新生代高原抬升驱动大气CO2下降的途径之一。

Sulphuric acid weathering has great impact on carbon cycle through its interaction with the carbonate strata and the organic matter in marine sediment. It is generally accepted that pyrite can be oxidized rapidly so that the flux of sulphuric acid weathering is mainly controlled by the supply of pyrite, which is finally modulated by physical erosion. The increasing physical erosion caused by tectonic uplift in the Cenozoic may have accelerated the sulphuric acid weathering, and thus influenced the partial pressure of atmospheric CO2 （pCO2）. To deciphering the control of physical erosion on sulphuric acid weathering, this study collected 36 water samples from 36 small crystalline-bedrock dominated catchments in the eastern Tibetan Plateau, where the erosion rates have been determined previously by 10Be concentration. The results show that the contribution of sulphuric acid weathering relative to the carbonic acid weathering correlates well with the physical erosion rates. High rates of erosion are generally associated with higher contribution of sulphuric acid weathering. We propose that on the timescale less than 10 million years, the increasing sulphuric acid weathering of sedimentary strata in response to Tibetan uplift are an important source of atmospheric CO2, which may help to explain the high pCO2 in the Early Cenozoic. On the timescale larger than 10 million year, the sulphuric acid weathering of silicate might be a significant sink of atmospheric CO2. Thus, the increasing of sulphuric acid weathering of crystalline rock in response to tectonic uplift might be an alternative drives for the decreasing pCO2 of the Cenozoic Era.