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 (δ³⁴S_py) during early diagenesis is critical for environmental reconstructions, especially for the reconstruction of the marine sulfur cycle. Commonly, δ³⁴S_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 δ³⁴S_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 δ³⁴S values of pyrite, concentration and sulfur isotopic composition of porewater sulfate (δ³⁴S_SO4), carbon, nitrogen, and sulfur contents, and grain sizes of terrigenous sediments were analyzed and AMS¹⁴C dating of planktonic foraminifera was conducted. The δ³⁴S_py values were found to decrease with depth from −16.8‰ to −42.8‰, indicating low sulfur isotope fractionation (<38‰) during microbial sulfate reduction in the uppermost sediments and a larger fractionation (>64‰) at greater depth. Significant contributions of relatively ³⁴S-enriched pyrite formed at shallow depth leads to overall δ³⁴S_py variations >26‰, whereby low δ³⁴S_py values coincide with higher sedimentation rates and higher organic matter supply. This study highlights the potential significance for relatively ³⁴S-enriched, shallow-depth pyrite for δ³⁴S_py records, which may affect the reconstruction of depositional conditions and the global sulfur cycle.