Evaporite deposition in the mid-Neoproterozoic as a driver for changes in seawater chemistry and the biogeochemical cycle of sulfur

摘要

We utilized a novel approach to modeling the oceanic sulfur cycle by combining delta S-34 and Delta S-33 curves from sulfate evaporite minerals in order to investigate redox conditions during the mid-Neoproterozoic. This technique allowed us to estimate the oxidized and reduced proportions of the total oceanic sulfur sink. Isotopic data from the mid-Neoproterozoic Minto Inlet Formation (Victoria Island, Northwest Territories, Canada; ca.850 Ma) show a limited range (16.8 parts per thousand +/- 1.4 parts per thousand) in delta S-34 of seawater sulfate and a sulfur cycle that is strongly shifted toward the sulfate sink (pyrite burial fraction, f(p), = 0.2), suggesting oxidizing conditions in the ocean and atmosphere at the time of deposition. These evaporites and others, which were deposited contemporaneously within a huge intracontinental basin, acted as a chemical pump, removing sulfate from the oceans and oxygen from the atmosphere to be buried as sulfate evaporites.