RECONSTRUCTION THE WEATHERING INTENSITY DURING THE EARLY EVOLUTION OF VASCULAR LAND PLANTS

Continental weathering is the key nexus between the terrestrial and marine systems, because riverine influx is the major source of nutrient (such as phosphorus) that controls the marine primary productivity and biogeochemical cycles in the ocean. Biological weathering greatly enhanced silicate dissolution by using organic acids instead of carbonic acid after the greening of continents. It is reasonable to postulate that the intensity of continental weathering might have changed after the diversification of vascular land plants in the middle Paleozoic. However, it is unclear whether the weathering intensity in Devonian and Carboniferous was stronger, weaker, or similar to that of present day, because of the difference of flora between present day and the past. In order to reconstruct the weathering intensity in the early evolution of vascular plants, here we measured the Mg isotopic compositions of siliciclastic components of carbonate samples of the Wuzhishan Formation from two sections (Duli and Xiada) in South China, bracketing the interval between the conodont upper expansa and sulcata zones across the D/C boundary. Mg isotopic composition (δ²⁶Mg) of siliciclasts may record the intensity of chemical weathering, because δ²⁶Mg of weathering residues becomes heavier during weathering. δ²⁶Mg ranges from +0.26‰ to +0.52‰ (mean =+0.43‰, n=12) and from +0.47‰ to +0.79‰ (mean=+0.61‰, n=14) for the Duli and Xiada sections, respectively. These values are in the upper range of δ²⁶Mg of siliciclastic sediments/rocks, and are comparable to δ²⁶Mg of the most weathered saprolite in modern sub-tropical regions, suggesting that the weathering intensity was already high in early Carboniferous. Furthermore, δ²⁶Mg profiles of the two sections show a correlatable trend, but δ²⁶Mg of the Xiada section is systematically 0.1-0.2‰ heavier than that of the Duli section. Such isotopic difference is reflected by the variation of mineral compositions of siliciclastic components. These observations suggest that Mg isotope of siliciclastic components can be used as a reliable tracer of weathering intensity in deep time, but the interpretation of Mg isotope data could be complicated by other factors, such as the rate of physical erosion.