Constraining the terminal Ediacaran seawater chemistry by Mg isotopes in dolostones from the Yangtze Platform, South China

Abstract

The advent of animal biomineralization at the terminal Ediacaran remarkably changed the marine ecological system. Changed seawater Mg/Ca ratio is considered as an important external trigger for this transition, but the intrinsic relationship between seawater Mg/Ca variation and animal biomineralization is not well constrained due to the lack of reliable archives. To constrain the coeval seawater chemistry and explore its role in the origin of metazoan biomineralization, we systematically conducted petrographical and geochemical investigations on dolostones from the terminal Ediacaran (∼551 Ma) Dengying Formation of the Yangtze Block in South China. The well-preserved sedimentary fabrics, growth zones as well as cathodoluminescent bands in microbialites and fibrous dolomite cements in the Algal Dolomite Member of the Dengying Formation indicate these dolomites were formed during syn-sedimentary diagenesis. The distribution patterns of rare earth elements in these dolomite components documented relatively anoxic shallow water conditions. Furthermore, negative correlations between O and Mg isotope compositions in well-preserved dolomite components indicate the variable mixture of freshwater with seawater in the shallow platform environment. The pervasive influence of freshwater on seawater indicates an enhanced riverine input, and it should be dominated by silicate weathering flux due to the heavier Mg isotopic compositions in freshwater endmember. Enhanced silicate weathering flux at the basal Dengying Formation may be related to the abundant erosions and exposures caused by the significant changes in Earth’s surface tectonic and climatic conditions at the late Ediacaran. Consequently, the increased silicate weathering flux would deliver more Mg cations and net alkalinity to the oceans than the mixed weathering flux, which probably facilitated the subsequent skeletonization of early animals.