A MORE ADVANCED WEATHERING PRODUCT IN SOILS

The physical, chemical and biological processes controlling soil weathering and formation play critical roles in ecosystem functions and in terrestrial biogeochemical cycles. Soil-forming factors (climate, vegetation, parent material, topography and human influence) define soil distribution in the landscape, as well their weathering degree, their productivity and their role in the Earth’s Critical Zone for the functioning of atmosphere and hydrosphere. Soil-forming processes progressively modify parent rock material and control the pathways of primary mineral weathering and secondary mineral formation. The rapid modification of clay mineralogy is a well-documented soil-forming process, but the fate of elements released in soil solution after clay modification has not yet been studied, even though it is of crucial importance for identifying the sinks and sources controlling the geochemical balance of minor and major elements in soils and sediments. Here, the evolution of silicon (Si) isotope signature in the clay fraction of a podzolic soil chronosequence (0 to 335 years; Cox Bay, Vancouver Island) provide new insight into pedogenic processes involved in clay neo(trans-)formation, enabling to trace the source of Si in secondary aluminosilicates. The mass balance approach demonstrates an increasing depletion over time in light ²⁸Si in pedogenic clay minerals in topsoil (δ³⁰Si values increasing from -0.39 to +0.64‰ in c.a. 200 years). A part of light ²⁸Si released accounts for the relative enrichment in light ²⁸Si in pedogenic clay minerals in subsoil (δ³⁰Si = -2.31‰), compared to the original “unweathered” secondary minerals in BC horizon (δ³⁰Si = -1.40‰). The recording of Si isotopic ratios in the clay fraction as a function of the age of soil formation therefore highlights that Si released from the dissolution of secondary clay minerals in topsoil contributes to the neoformation of “tertiary” clay minerals in subsoil. This more advanced weathering process has important implications as the dissolution and re-precipitation of pedogenic clay minerals would play a key role not only in soil chemical and physical properties, but also in the preservation of organic carbon from microbial decomposition, nutrient cycling and the transfer of elements and pollutants from land to ocean.