Gas Hydrate‐Associated Carbonates and Methane‐Venting at Hydrate Ridge: Classification, Distribution, and Origin of Authigenic Lithologies

TLDR: In this paper, the authors recognized a direct relationship between gas hydrates and sediment fracturing and the oxygen isotope composition of carbonate lithologies on Hydrate Ridge, and demonstrated carbonate-forming mechanisms of gas hydrate.

Abstract: Hydrate Ridge is part of the accretionary complex at the Cascadia margin and is an area of widespread carbonate precipitation induced by the expulsion of methane-rich fluids. All carbonates on Hydrate Ridge are related to the methanecarbon pool either through anaerobic methanotrophy or through methanogenesis. Several petrographically distinct lithologies occur in boulder fields or in massive autochtonous chemoherm complexes which include methane-associated diagenetic mudstones and venting-induced breccias. The mudstones result from methane diagenesis in different sediment horizons and geochemical environments related to very slow methane venting. Cemented bioturbation casts occur as fragments, complex framework or as clasts together with bivalve shells as part of intraformational breccias, which are restricted to chemoherm complexes. Here, fluids ascend from the sub-seafloor and support aragonite-dominated carbonate precipitation near or at the sediment surface. Voids within mudclast breccias are either aragonite-rich indicating a formation near the surface at vent sites or are cemented by dolomite, which indicates formation in deeper parts of the sediment column. Brecciation is caused by tectonic or slump processes. In addition, we recognized a direct relationship between gas hydrates and sediment fracturing as well as the oxygen isotope composition of carbonate lithologies. Such gas hydrate-associated carbonates either show layered megapores and veins as relics of the original gas hydrate fabric or consist of aragonite-cemented intraclast breccias formed by growing and decomposing gas hydrate near the sediment surface. Both rock fabrics and the enrichment of 180 in high Mg-calcite demonstrate carbonate-forming mechanisms of gas hydrate.