ASSESSING BOTTOM SEDIMENT METHANE CONCENTRATIONS AND THE EXTENT OF ANAEROBIC OXIDATION OF METHANE IN THE GREAT BAY ESTUARY, NEW HAMPSHIRE
Methane is produced via methanogensis in anoxic sedimentary systems throughout the world’s oceans and freshwater terrestrial environments (wetlands, lakes, rivers). Methanogenesis in brackish water systems such as estuaries, however, remains poorly constrained. In this study bulk solid phase and porewater geochemical data were used to investigate the sediment methane concentrations and extent of anaerobic oxidation of methane (AOM) in Great Bay Estuary, New Hampshire. Methane produced at depth in this and in most of Earth’s surface and seafloor environments, diffuses or advects upwards into younger strata and has the potential to contribute to water column and atmospheric methane concentrations. However, the AOM is a process that efficiently reduces porewater sulfate and oxidizes methane before it can enter the overlying water column or atmosphere. In the Fall of 2015, 30 sedimentary cores were recovered from Great Bay 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. The depth where porewater sulfate and methane are depleted defines the depth where AOM occurs. Relative increases in bulk sedimentary TS track the modern and potentially paleo-sulfate-methane transition zones (SMTZs) .The results of this investigation also reveals a correlation between methane concentration and carbon quantity and quality. Sediments with low TOC and lower overall marine derived organic matter (low TOC:TN) have less abundant methane concentrations and a deeper SMTZ. 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. Several cores contained trace quantities of methane above the SMTZ, but this low level of methane is unlikely to influence the water column methane concentrations in Great Bay. Overall these results indicate methane production in Great Bay sediments is mostly if not completely, consumed by AOM.