Sustained low marine sulfate concentrations from the Neoproterozoic to the Cambrian: Insights from carbonates of northwestern Mexico and eastern California

Stratigraphic carbonate-carbon isotope trends are similar for correlative Ediacaran and Cambrian carbonates of Sonora, Mexico, and Death Valley, California. In contrast, the sulfur isotope compositions of both carbonate-associated sulfate (CAS) and pyrite in the two regions exhibit unique trends with high degrees of stratigraphic variability. We have established that the sulfur records are coeval using delta C-13 chemostratigraphy and biostratigraphic markers, where present. Over short stratigraphic intervals, delta S-34(CAS) variability is consistent with regionally low marine sulfate concentrations during this period. Values of Delta S-34(delta S-34(CAS)-delta S-34(pyr)) range from -5.8 parts per thousand to +27.1 parts per thousand and average similar to+11 parts per thousand, consistent with limited net fractionation during bacterial sulfate reduction, which is additional evidence for low sulfate concentrations. Modeling based on these regional sulfur isotope trends suggests sustained low sulfate conditions throughout the Neoproterozoic and well into the Cambrian, with concentrations of similar to 2 mM or lower. When all of the available sulfate proxy data from our work and previously published studies are considered, most Neoproterozoic and Cambrian successions exhibit trends consistent with low seawater sulfate. The persistent and complete disagreement in delta S-34(sulfate) among multiple basins was briefly interrupted similar to 580 million years ago, coincident with the onset of the Wonoka-Shuram carbon isotope anomaly and again near the termination of Series 3 of the Cambrian characteristics generally unrecognized in older rock units. During these two intervals, similar stratigraphic trends in delta S-34(CAS) are recorded globally, whereas absolute values remain distinct among individual basins. However, these periods of broad isotopic trend agreement coincide with large-magnitude sulfur isotope excursions, which also point to low seawater sulfate concentrations. Therefore, although brief intervals of isotopic homogeneity exist, the Neoproterozoic and Cambrian ocean must have been dominated by low sulfate throughout. Ultimately, the recognition of persistently low sulfate well into the Paleozoic raises questions about the relationships between sulfate concentration in seawater and its primary controls, including ocean oxygenation and its influence on metazoan evolution. (C) 2012 Elsevier B.V. All rights reserved.