Local environmental variation obscures the interpretation of pyrite sulfur isotope records

Theoretically, within the long-term marine sulfur cycle, pyrite sulfur isotopes (delta S-34(py)) are determined by the degree of pyrite burial that is controlled by the marine redox conditions. Thus, the secular variation of delta S-34(py) records the history of marine and atmosphere redox evolution. As such, large magnitude change in delta S-34(py) in the Proterozoic has been attributed to widespread oceanic anoxia related to generally low atmospheric pO(2) levels. However, noisy delta S-34(py) signals in the past 250 million years cannot be explained by the frequent oscillation of seawater redox, because the global ocean was already fully oxidized. Here, by developing a numerical model, we show that delta S-34(py) of syndepositional pyrite is significantly affected by local factors, such as organic matter content in sediment, sedimentation rate, and seafloor redox. Thus, the noisy delta S-34(py) records may reflect local environmental variations rather than the oscillation of global ocean redox. Our study also suggests that delta S-34(py) alone cannot provide a robust constraint on the global or local ocean redox condition. We suggest that, combining with the pyrite content data and delta S-34(py) can help quantify local regional biogeochemical cycles. (C) 2020 Elsevier B.V. All rights reserved.