Rapid Sediment Accumulation Results in High Methane Effluxes from Coastal Sediments

Globally, the methane (CH(4)) efflux from the ocean to the atmosphere is small, despite high rates of CH(4) production in continental shelf and slope environments. This low efflux results from the biological removal of CH(4) through anaerobic oxidation with sulfate in marine sediments. In some settings, however, pore water CH(4) is found throughout the sulfate-bearing zone, indicating an apparently inefficient oxidation barrier for CH(4). Here we demonstrate that rapid sediment accumulation can explain this limited capacity for CH(4) removal in coastal sediments. In a saline coastal reservoir (Lake Grevelingen, The Netherlands), we observed high diffusive CH(4) effluxes from the sediment into the overlying water column (0.2–0.8 mol m(-2) yr(-1)) during multiple years. Linear pore water CH(4) profiles and the absence of an isotopic enrichment commonly associated with CH(4) oxidation in a zone with high rates of sulfate reduction (50–170 nmol cm(-3) d(-1)) both suggest that CH(4) is bypassing the zone of sulfate reduction. We propose that the rapid sediment accumulation at this site (~ 13 cm yr(-1)) reduces the residence time of the CH(4) oxidizing microorganisms in the sulfate/methane transition zone (< 5 years), thus making it difficult for these slow growing methanotrophic communities to build-up sufficient biomass to efficiently remove pore water CH(4). In addition, our results indicate that the high input of organic matter (~ 91 mol C m(-2) yr(-1)) allows for the co-occurrence of different dissimilatory respiration processes, such as (acetotrophic) methanogenesis and sulfate reduction in the surface sediments by providing abundant substrate. We conclude that anthropogenic eutrophication and rapid sediment accumulation likely increase the release of CH(4) from coastal sediments.