Enrichment pattern of tungsten in sediments under methane seepage environments: Applicability as a proxy for tracing and reconstructing (paleo-)methane seepage

Abstract

The release of hydrogen sulfide by sulfate-driven anaerobic oxidation of methane (SD-AOM) during methane seepage activities has a strong impact on the evolution of marine redox conditions and the cycles of redox-sensitive elements (e.g. Mo and U). Consequently, the sedimentary enrichment or depletion of these elements is often used to trace and reconstruct (paleo-)methane seepage activity. In recent years, tungsten (W) has shown promise as an indicator of marine redox changes, but its applicability to trace methane seepage activity is unknown. This study identified significant differences in the W content of sediments in methane seepage environments compared to suboxic environments not influenced by methane seepage and oxic environments in the South China Sea. The sediments subjected to methane seepage were characterized by low W contents, similar to those of euxinic Black Sea sapropels. It is believed that the sulfidic environment resulting from SD-AOM is the primary cause of W depletion in sediments subjected to methane seepage. By examining W's interaction with its twin element, Mo, we present WMo enrichment patterns in various environments. In methane seepage environment (sulfidic environments), sediments had high MoEF (> 10) and low WEF (< 2) with a high Mo/W molar ratio (> 10); in suboxic environments not influenced by methane seepage, sediments exhibited a WMo enrichment pattern similar to the UCC (the MoEF and WEF values are predominantly below 1 and 2, respectively, with a low Mo/W molar ratio, <1.5); in oxic environments (enrichment of FeMn (oxy)hydroxides), sediments exhibited high MoEF (2 < MoEF < 10) and high WEF (> 2), with a medium Mo/W molar ratio (1.5–10). This further indicates that the Mo/W molar ratio has great potential in tracing and reconstructing (paleo-)methane seepage activities.