Variable methane fluxes in shallow marine systems over geologic time The composition and origin of pore waters and Authigenic carbonates on the New Jersey shelf

摘要

Carbon cycling is an important but poorly understood process on passive continental margins. In this study, we use the ionic and stable isotopic composition of interstitial waters and the petrology, mineralogy, and stable isotopic composition of authigenic carbonates collected from Ocean Drilling Program (ODP) Leg 174A (Sites 1071 and 1072) to constrain the origin of the carbonates and the evolution of methane on the outer New Jersey shelf. The pore fluids of the New Jersey continental shelf are characterized by (1) a fresh-brackish water plume, and (2) organic matter degradation reactions, which proceed through sulfate reduction. However, only minor methanogenesis occurs. The oxygen isotopic composition of the pore fluids supports a meteoric origin of the low salinity fluids. Authigenic carbonates are found in nodules, thin (approx 1 -cm) layers, and carbonate cemented pavements. Siderite is the most common authigenic carbonate, followed by dolomite and calcite. The oxygen isotopic composition of the authigenic carbonates, i.e. 1.3-6.5 per thousand PeeDee Belemnite (PDB), indicates an origin in marine pore fluids. The carbon isotopic composition of dolomite cements range from -16.4 to -8.8 per thousand PDB, consistent with formation within the zone of sulfate reduction. Siderite delta~(13)C values show a greater range (-17.67-16.4 per thousand), but are largely positive (mean = 2.8 per thousand) and are interpreted to have formed throughout the zone of methanogenesis. In contrast, calcite delta~(13)C values are highly negative (as low as -41.7 per thousand) and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation. Pore water data show that despite complete sulfate reduction, methanogenesis appears not to be an important process presently occurring in the upper 400 m of the outer New Jersey shelf. In contrast, the carbon isotopic composition of the siderites and calcites document an active methanogenic zone during their formation. The methane may have been either oxidized or vented from shelf sediments, perhaps during sea-level fluctuations. If this unaccounted and variable methane flux is an areally important process during Neogene sea-level fluctuations, then it likely plays an important role in long-term carbon cycling on passive continental margins.