Authigenic Minerals In The Sediments From Gas Hydrate-bearing Regions In The Northern South China Sea And Its Implication For Sulfate-methane Transition Zone

Methane hydrate is mainly distributed within the sediments along the continental margins worldwide. The upward methane flux from methane hydrate below can result in the depletion of pore water sulfate and subsequently affect the depth of the sulfate-methane transition zone (SMTZ) during the geological evolution of continental margins. Within the SMTZ, the anaerobic oxidation of methane (AOM) can leave a significant imprint on not only the interstitial water chemistry (e.g., H2S concentration and pH value), but also on the solid phase mineral compositions (e.g., pyrite, barite and carbonates). Consequently, various authigenic minerals in marine methane hydrate settings can be used as proxies for the position of past SMTZs.Several expeditions focusing on gas hydrates have been undertaken in the northern South China Sea during the past fifteen years, leading to discovery of substantial quantities of gas hydrate-bearing sediments. These sediment cores provided an excellent opportunity to study the formation of authigenic minerals within the SMTZ in marine methane hydrate settings. In this dissertation, several authigenic minerals such as pyrite, elemental sulfur and gypsum in the core sediments from nine sites located in the methane hydrate-bearing regions of the northern South China Sea are analyzed. On the basis of morphologies, concentrations and sulfur isotope data, we explain the inorganic sulfur cycle pathways and authigenic mineral formation associated with fluctuations in the sulfate-methane transition zone position (SMTZ). Further, we propose that the evolution of SMTZ in the shallow sediments could indicate the evolution of underlying methane hydrate. The main conclusions are as follows:(1) Both methods including handpicked sample analysis and the chcemical extraction can produce similar trends in pyrite concentrations and sulfur isotopic values. The paleo SMTZ is inferred from data showing anomalously high accumulation of authigenic pyrites and corresponding positive excursions of sulfur isotopic values.(2) Solid elemental sulfur (ES) mainly occurred within and near the sulfate-methane transition zone in studied site, whose formation might be related to the anaerobic oxidation of H2S. In this case, the ES formation process is linked to fluctuations in the SMTZ position.(3) Enhanced anaerobic oxidation of methane (AOM) following sufficient material supply plays a dominant role in the formation processes and unusually large size distributions of framboidal pyrite (e.g., mean size>20 μm and standard deviation>3.0 μm) within the SMTZ under conditions of methane hydrate dissociation.(4) Framboidal pyrites formed within the SMTZ exhibit obvious differences in the morphologies, size distributions and sulfur isotopic values comparing with ones formed within the sulfate reduction zone (SRZ), which cannot be an effective indicator of water-column redox conditions.(5) Athigenic gypsum is formed from SO42- ions derived in approximately equal amounts from seawater sulfate and partial oxidation of authigenic pyrite, and from Ca2+ ions likely generated through (i) carbonate dissolution linked to pyrite oxidation and porewater acidification and (ii) ion exclusion during the formation of methane hydrates; This authigenic gypsum is triggered by a decrease in the upward flux of methane related to onset of formation of methane hydrates at depth, which may have utility as a proxy for the position of the SMTZ and the development of the methane hydrate.