The methane-sulfate transition interface in offshore sediments serves as a critical boundary for abrupt transitions in sedimentary environments

Abstract

Methane is commonly observed in unconsolidated sediments prevalent coastal areas. It widely acknowledged within the scientific community that the methane-sulfate transition zone (SMTZ) forms due to the upward diffusion of methane and downward diffusion of sulfate within these sedimentary environments. However, this study presents a detailed analysis using a borehole sample from the continental shelf of the East China Sea as a representative case study, revealing distinct findings. The entire borehole was 60 m long and predominantly composed of clay with extremely fine sediment particles, conditions that are inherently unfavorable for diffusion. The SMTZ was identified at a depth of 9 m. Geochemical characteristics of the sediments showed significant differences above and below this interval. The carbon-to-nitrogen (C/N) ratios, the carbon isotopic compositions of total organic carbon (TOC), and nitrogen isotopic compositions of total nitrogen (TN), etc., all showed a peak at this specific depth. Additionally, elements such as thorium (Th), vanadium (V), cobalt (Co), and nickel (Ni), along with redox condition indicators such as the Ni/Co and Th/U ratios, demonstrated abrupt changes around the 8.5 m mark. The findings of this study suggest that the SMTZ functions as a transitional interface within the sedimentary environment, rather than merely representing the boundary where methane and sulfate converge. This insight is crucial for enhancing our comprehension of global carbon cycling in marine sediments, as prevailing estimates of methane release fluxes based on concentration gradient diffusion theory may lead to significant discrepancies between estimated and actual values.