Sediment Mineralogy and Geochemistry and Their Implications for the Accumulation of Organic Matter in Gashydrate Bearing Zone of Shenhu, South China Sea

Abstract

The Total Organic Carbon (TOC) content plays a crucial role in gas hydrate exploration because a higher TOC content signifies a greater potential for buried gas hydrates. The regulatory mechanisms governing organic matter in sediment are intricate and influenced by various predominant factors unique to different regions. Notably, the Shenhu area in the South China Sea stands as a pioneering region for methane hydrate research. Despite its significance, limited research has focused on the burial patterns of TOC, resulting in an insufficient dataset to draw definitive conclusions. Consequently, a comprehensive understanding of the burial patterns and controlling factors of TOC within this area remains elusive. This study examines the pore-water characteristics, mineral composition, geochemistry, and sedimentary factors of four distinct sites within the Shenhu region of the South China Sea. The current depths of the Sulfate-Methane Interface (SMI) for sites CL54, CL56, CL57, and CL60 are identified as 28.6, 8.5, 31.9, and 8.1 m below the seafloor (mbsf), respectively. It’s noteworthy that these SMI depths align with locations known to harbor underlying gas hydrates. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses reveal that the primary sediment sources within this region encompass microbial shells (such as foraminifera and diatoms), clay, and terrestrial detritus. In addition, marine productivity exhibits a reverse correlation with TOC content, and both TOC content and Ce/Ce* ratios exhibit synchronous fluctuations with sedimentation rate. Drawing from the sedimentation rate, TOC content, as well as indicators of redox conditions (MoEF, Ce/Ce*, Mo/U) and productivity proxies (Ba/Al, P/Al) within the sampled sites, it becomes apparent that high sedimentation rate coupled with ‘anaerobic’ conditions foster favorable conditions for TOC accumulation. This comprehensive investigation not only provides valuable datasets but also offers insights into the intricate processes governing TOC accumulation.