Signatures of geochemical changes at methane-seeps as recorded by seep carbonates

Abstract

This thesis compiles three manuscripts: (1) The first manuscript (chapter 2.1) contains petrographic and geochemical data of aragonitic seep carbonates from the Makran accretionary prism. High-resolution rare earth element (REE) analysis yield distinct total REE[aragonite] concentrations and shale-normalised REE[aragonite] patterns. The REE variations are ascribed to different pore fluid compositions and accompanied redox changes during aragonite precipitation. (2) The second manuscript (chapter 2.2) deals with early diagenetic alterations of aragonitic seep-carbonate crusts exposed at the Amsterdam and Athina mud volcanoes (eastern Mediterranean Sea). Detailed thin section analysis reveals that widespread carbonate corrosion created significant amounts of secondary porosity. In order to explain carbonate dissolution a simple model is proposed, involving the aerobic oxidation of methane and hydrogen sulphide. (3) The third manuscript (chapter 2.3) describes the distribution of REE in anoxic pore waters on Hydrate Ridge (Cascadia subduction zone, north-eastern Pacific). The pore waters are markedly enriched in REEs compared to bottom water and display variable shale-normalised patterns, remarkably with positive as well as negative Ce anomalies. Authigenic seep carbonates are suggested as efficient REE sinks at methane-seeps.