Geochemical evidence from marine carbonate for enhanced terrigenous input into seawater during the Ediacaran-Cambrian transition in South China

Abstract

High-resolution carbon and oxygen analyses, along with residue contents, major and trace element concentrations were carried out for marine dolostone from a profile through the Upper Ediacaran to the Lower Cambrian strata in South China. The results provide geochemical evidence for enhanced input of terrigenous material into seawater in this period. Four positive and four negative carbon isotope excursions are identified in this profile. The first remarkably negative δ13C excursion exhibits the lowest value down to −2‰ at the Ediacaran to Cambrian boundary (ECB), in agreement with the global δ13C and stratigraphy correlation. The second remarkably negative δ13Ccarb excursion is as low as −1.81‰ subsequent to a positive δ13Ccarb excursion as high as 1.74‰ in the Lower Cambrian. The ECB position in the stratigraphic profile is constrained by carbon isotope correlations. The decreases of dolostone δ18O and δ13C values are synchronous across the ECB, suggesting a change of the seawater composition at that time. The increase of residue contents is concordant with decreases of Y/Ho ratios and δ18O values for the dolostone across the ECB, suggesting the increase of terrigenous input in this period. The relatively smooth REE+Y patterns of dolostone suggest its deposition in the environment of continental platform. An integrated interpretation of these geochemical data, including the major and trace element concentrations, Mn/Sr and Mg/Ca ratios, and the well preserved dolostone δ13C and δ18O values, indicates that the terrigenous material from the continental weathering is rich in nutrients and 13C-depleted organic matter, which would have contributed to the negative δ13Ccarb excursion of dolostone at the stratigraphic boundaries. The strong positive δ13Ccarb excursion in the Lower Cambrian may be linked to the biological radiation with lots of organic matter buried from the seawater in this period. The fall and rise of sea level during different periods in the same region may result in differential input of terrigenous materials (including freshwater) into marine carbonates, recording the coupling between regional tectonism and paleoclimatic change on the continental platform.