Marine carbonate records of terrigenous input into Paleotethyan seawater: Geochemical constraints from Carboniferous limestones

A combined study of trace elements, carbon and oxygen isotopes was carried out for Carboniferous marine limestones from the Lower Yangtze platform in South China. The results provide insights into the influence of terrigenous input to Paleotethyan seawater, from which pure and impure carbonates precipitated. In terms of the correlations between the concentrations of elements with different properties, variable extents of two-component mixing are evident between seawater and terrigenous detritus. This is quantitatively dictated by such parameters as the calcite-seawater partition coefficients (D-X) and the seawater-continental upper crust partition coefficients (K-X). Water-soluble elements with high and medium K-X values, such as Na, Mg, P, V, Cr, Ni, Cu, Zn, Sr, Mo and U, are susceptible to incorporation into carbonate lattices, and thus their concentrations represent the geochemical composition of paleoseawater. In contrast, water-insoluble elements with low K-X values, such as Be, Al, Sc, Co, Ga, Cs, REE, Hf and Th, cannot be incorporated into carbonate lattices, thus providing a proxy for contributions from terrigenous material. The Carboniferous marine limestones from a stratigraphic profile exhibit three positive and three negative delta C-13 excursions from the late Visean to Gzhelian. These delta C-13 excursions generally match those of contemporaneous carbonates elsewhere in the world, which can be linked to the alternative occurrences of cold and warm paleoclimates in the Carboniferous. The lowest delta C-13 values in the profile are generally associated with the lowest values of Y/Ho and the highest values of (Nd/Yb)(PAAS), Th, Sc and insoluble residue. This indicates a significant contribution from the terrigenous material to the negative delta C-13 excursions. The effect from the warm paleoclimate is also indicated by high chemical indices of alteration for the insoluble residue from the three intervals of high terrigenous input. On the other hand, the positive delta C-13 excursions are associated with the decreases of terrigenous input. Therefore, the incorporation of terrigenous material into the epicontinental seawater is temporally linked to the paleoclimatic change on continental margins adjacent to the Paleotethyan seawater. (C) 2014 Elsevier Ltd. All rights reserved.