Local environmental variation obscures the interpretation of pyrite sulfur isotope records

Theoretically, within the long-term marine sulfur cycle, pyrite sulfur isotopes (δ³⁴S_py) are determined by the degree of pyrite burial that is controlled by the marine redox conditions. Thus, the secular variation of δ³⁴S_py records the history of marine and atmosphere redox evolution. As such, large magnitude change in δ³⁴S_py in the Proterozoic has been attributed to widespread oceanic anoxia related to generally low atmospheric pO₂ levels. However, noisy δ³⁴S_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 δ³⁴S_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 δ³⁴S_py records may reflect local environmental variations rather than the oscillation of global ocean redox. Our study also suggests that δ³⁴S_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 δ³⁴S_py can help quantify local/regional biogeochemical cycles.