ASSESSING BOTTOM SEDIMENT METHANE CONCENTRATIONS AND THE EXTENT OF ANAEROBIC OXIDATION OF METHANE IN THE GREAT BAY ESTUARY, NEW HAMPSHIRE, U.S.A

Methane is produced via methanogenesis in anoxic sedimentary systems throughout the world’s oceans and freshwater terrestrial environments (wetlands, lakes, rivers). In marine and estuarine environments, anaerobic oxidation of methane (AOM) efficiently reduces porewater sulfate and oxidizes methane before it can enter the overlying water column or atmosphere. Methanogenesis and methane oxidation in brackish water systems such as estuaries, however, remains poorly constrained. In this study, headspace gas, bulk solid phase, and porewater geochemical data were used to investigate the sediment methane concentrations and extent of AOM in Great Bay Estuary, New Hampshire, USA. Our results indicate that methane production in Great Bay sediments is mostly if not completely, consumed by AOM. In 2015, 30 cores (up to 1 m recovery) were recovered from Great Bay sediments and subsequently analyzed for headspace equilibrated methane, total organic carbon (TOC), total sulfur (TS), total nitrogen (TN), and total carbon (TC). The TOC and TOC:TN provide first order assessments of the quantity and quality of organic carbon in Great Bay. Two additional cores were recovered for porewater sulfate and methane measurements during the Spring of 2016 and define the modern sulfate methane transition zone (SMTZ) at these sites. Absent or low values of methane in the anoxic sediments at most of the sites document AOM driven consumption of CH₄. Relative increases in bulk sedimentary TS track the modern and potentially paleo-SMTZs. Sediments with low TOC and lower overall marine derived organic matter (low TOC:TN) have lower methane concentrations and deeper SMTZs. Sediments with high TOC and elevated fractions of marine-derived organic matter (high TOC:TN) have higher methane concentrations and a shallower SMTZ. Spatially, the distribution of methane in the uppermost sediments of Great Bay is highest in the fine-grained tidal flats on the eastern side of Great Bay. In general, locations with low to undetectable porewater methane concentrations were nearest the river mouths. Moderate porewater methane concentrations were identified in the mud flats in the central portion of Great Bay.