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

Abstract

Understanding the controls on the sulfur isotope compositions of syndepositional pyrite (δ34Spy) during early diagenesis is critical for environmental reconstructions, especially for the reconstruction of the marine sulfur cycle. Commonly, δ34Spy 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 δ34Spy 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 (δ34SSO4), carbon, nitrogen, and sulfur contents, and grain sizes of terrigenous sediments were analyzed and AMS14C dating of planktonic foraminifera was conducted. The δ34Spy 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 δ34Spy variations >26‰, whereby low δ34Spy 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 δ34Spy records, which may affect the reconstruction of depositional conditions and the global sulfur cycle.