Controls on pyrite sulfur isotopes during early diagenesis in marine sediments of the South China Sea

Understanding the controls on the sulfur isotope compositions of syndepositional pyrite (delta S-34(py)) during early diagenesis is critical for environmental reconstructions, especially for the reconstruction of the marine sulfur cycle. Commonly, delta S-34(py) values increase with depth in marine sediments; this phenomenon has been extensively studied, fostering the reconstruction of both global and regional depositional environments. However, more recently a decrease of delta S-34(py) values with depth has been increasingly observed, but the corresponding controls on sulfur stable isotopes remain poorly constrained. This study investigated a gravity core collected from the northern continental slope of the South China Sea. Morphology, content, and delta S-34 values of pyrite, concentration and sulfur isotopic composition of porewater sulfate (delta S-34(SO4)), carbon, nitrogen, and sulfur contents, and grain sizes of terrigenous sediments were analyzed and AMS(14)C dating of planktonic foraminifera was conducted. The delta S-34(py) values were found to decrease with depth from -16.8 parts per thousand to -42.8 parts per thousand, indicating low sulfur isotope fractionation (<38 parts per thousand) during microbial sulfate reduction in the uppermost sediments and a larger fractionation (>64 parts per thousand) at greater depth. Significant contributions of relatively S-34-enriched pyrite formed at shallow depth leads to overall delta S-34(py) variations >26 parts per thousand, whereby low delta S-34(py) values coincide with higher sedimentation rates and higher organic matter supply. This study highlights the potential significance for relatively S-34-enriched, shallow-depth pyrite for delta S-34(py) records, which may affect the reconstruction of depositional conditions and the global sulfur cycle.