硅酸盐化学风化强度评估的沉积物指标与方法研究进展*

风化作用是源-汇沉积体系中的重要环节,气候、构造、地形、植被和岩性在不同程度上控制着地表硅酸盐化学风化,量化硅酸盐化学风化强度有助于开展全球性的实例研究对比。本研究归纳总结了基于碎屑沉积物的化学风化强度指标,包括岩石学和矿物学指标、元素地球化学指标和非传统稳定同位素指标,并指出了指标在应用中存在的问题。这些指标中:砂质沉积物骨架颗粒组成、粉砂级碎屑的矿物组成和矿物表面结构特征从矿物组成和结构上直观地反映了化学风化强度,运用不多但值得关注;黏土矿物组合和主微量元素指标如CIA、Rb/Sr、α^AlE等在实例研究中运用广泛,同时也显现出易受物源和水动力分选影响等弊端;新近开发的利用锂、硼、钾、镁、硅等同位素示踪化学风化强度的方法具有广阔的应用前景,同位素的分馏机理有待完善。源-汇体系中的其他过程如物源供给、水动力分选、成岩作用、再旋回作用、成土作用、物理淋滤和生物利用等会影响化学风化指标的使用效果,通过多指标的综合运用和相互验证,可以有效提升化学风化强度评估的准确性。

Research progress of sediment indicators and methods for evaluation of silicate chemical weathering intensity

Chemical weathering is a key process in source-to-sink depositional systems,which is controlled by climate,tectonics,topography,vegetation and lithology. Quantifying the intensity of silicate chemical weathering is beneficial to carry out global case study comparison. This paper aims to summarize the sediment chemical weathering indicators,including petrological and mineralogical,element geochemical and non-traditional stable isotope proxies,and point out the potential problems in application. Among these proxies,the composition of sandy sediment framework grains,mineral composition of silty sediments and textural characteristics of mineral surface can clearly indicate chemical weathering intensity,which have been overlooked in most studies and deserve more attention. Clay mineral assemblages and element geochemical indicators,such as CIA,Rb/Sr and α^AlE,are most widely used for quantitative analysis of chemical weathering intensity. However,these proxies are easily affected by the sediment source and hydrodynamic sorting. The newly developed indexes of Li,B,K,Mg and Si isotopes show high potentials to evaluate chemical weathering intensity and need further study on their fractionation mechanism. It's important to note that all indicators can be affected by other geological processes from source to sink,e.g., source,hydrodynamic sorting,diagenesis,sediment recycling,pedogenesis,leaching and biological utilization. It is highly suggested to comprehensively use multiple indexes for evaluating silicate chemical weathering intensity,which can effectively improve the accuracy of the analytic results.