GEOCHEMISTRY

Malone et al. (in press) used the ionic and stable isotopic composition of interstitial waters and the petrology, mineralogy, and stable isotopic composition of authigenic carbonates (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 freshwater to brackish water plume and (2) organic matter degradation reactions, which proceed through sulfate reduction. However, only minor methanogenesis occurs today. In contrast, the carbon isotopic composition of authigenic siderites and calcites document an active methanogenic zone during their formation. Siderite ¹³C values range from -17.67 to 16.4 Peedee Belemnite (PDB) but are largely positive (mean = 2.8) and are interpreted to have formed throughout the zone of methanogenesis. Calcite ¹³C values are highly negative (as low as -41.7) and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation. This methane may have been either oxidized or vented from shelf sediments, perhaps during sea level fluctuations. Malone et al. (in press) suggested that 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.