Enhanced sulfidization in a sedimentary turbidite layer from the Nansha Trough in the southern South China Sea

Abstract

Microbial sulfate reduction occurs in diverse depositional sedimentary environments, especially in fine-grained sedimentary turbidites. However, the extent to which turbidites impact the mechanism of sulfidization and the carbon–sulfur–iron cycle is poorly understood. Here, a sediment gravity core (10GC) with 50-cm thick turbidites was collected from the Nansha Trough of the southwestern South China Sea (SSCS) to unravel the biogeochemistry and sulfidization processes. One accelerator mass spectrometric 14C date in the planktonic foraminifera indicates that the turbidite layer was deposited at or immediately before 16 ka. Using a suite of elemental (Fe, K, Si Al, and Zr) and sediment grain size proxies suggests that the sediments within the turbidites were derived from the Sunda Shelf and the Malay Peninsula. The TOC/TN values (9–20) of the turbidites indicate deposition of terrestrial organic matter with a mixture of C3 and C4 types of vegetation. The relatively negative 13CTIC values (−7.76‰ to 2.09‰) are robust evidence for the accumulation of authigenic carbonates, which was attributed to organoclastic sulfate reduction. The rather positive δ34SCRS (CRS: chromium reducible sulfur) (−37.5‰ to −10.5‰) and high CRS contents of the turbidites compared to the hemipelagic sediments suggest that the enhanced sulfidization occurred due to a high rate of sulfate reduction in the closed system. In contrast to the turbidites, the more negative δ34SCRS values (−50.9‰ to −47.4‰) in the hemipelagic sediments reflect early diagenetic pyritization at the sediment–water interface. This study highlights the importance of turbidite sedimentation on the biogeochemical cycling of sulfur, carbon, and iron. However, further research is needed to better understand and quantify the contribution of marine turbidite sediments to the global ocean C–S–Fe cycle.