The intensity of chemical weathering: Geochemical constraints from marine detrital sediments of Triassic age in South China

摘要

A geochemical study of major-trace elements in detrital sediment and carbon-oxygen isotopes in carbonate was carried out for a marine stratigraphic profile of Early Triassic that is composed of argillaceous limestone and calcareous mudstone in the Lower Yangtze basin, South China. The results place constraints on the geochemical behaviors of various elements in the detrital sediment that was deposited in the residual Paleotethyan seawater. This leads to establishment of new geochemical proxies for chemical weathering of continental crust. In terms of the correlations between element concentrations and their variations in the profile, the elements are categorized into four groups with respect to the difference in their geochemical behaviors. The first group is composed of Al, Th, Sc, Be, In, Ga, K, Rb and Cs that are tightly correlated due to their immobility during chemical weathering. The second group is composed of Ca and Na that show opposite variation trends with Th and Sc, on account of their mobile behavior in the weathering profile. The third group is composed of high field strength elements such as Ti, Nb, Ta, Zr and Hf that are closely correlated with each other because they were primarily taken up by heavy minerals from sedimentary provenance. The fourth group is composed of redox sensitive elements such as Co, Cu, Fe, Mn and Ni that are correlated with S and thus mainly hosted by sulfides. Th, Sc, Ca and Na were not amenable to changes in sedimentary provenance, and thus are selected to establish the new proxies for chemical weathering. These are composed of logarithmic parameters such as log(Th/Ca), log(Sc/Ca), log[Th/(Na/5 + Ca)] and log[Sc/(Na/5 + Ca)]. They exhibit synchronous increases at the Permian-Triassic boundary, the middle Griesbachian and the early Smithian, indicating the enhancements of chemical weathering. High proxy values approaching the values for the extremely weathered product of granodiorite occurred in the middle to late Griesbachian and early Smithian, demonstrating the occurrences of extreme chemical weathering and very warm paleoclimate in those periods. These paleoclimatic changes are concordant with results from geochemical studies elsewhere in the world. Therefore, the intensity of chemical weathering can be indicated by the new geochemical proxies for the different properties of elements in marine detrital sediments. (C) 2014 Elsevier B.V. All rights reserved.