title: Rapid Sediment Accumulation Results in High Methane Effluxes from Coastal Sediments.
abstract: Globally, the methane (CH4) efflux from the ocean to the atmosphere is small, despite high rates of CH4 production in continental shelf and slope environments. This low efflux results from the biological removal of CH4 through anaerobic oxidation with sulfate in marine sediments. In some settings, however, pore water CH4 is found throughout the sulfate-bearing zone, indicating an apparently inefficient oxidation barrier for CH4. Here we demonstrate that rapid sediment accumulation can explain this limited capacity for CH4 removal in coastal sediments. In a saline coastal reservoir (Lake Grevelingen, The Netherlands), we observed high diffusive CH4 effluxes from the sediment into the overlying water column (0.2-0.8 mol m-2 yr-1) during multiple years. Linear pore water CH4 profiles and the absence of an isotopic enrichment commonly associated with CH4 oxidation in a zone with high rates of sulfate reduction (50-170 nmol cm-3 d-1) both suggest that CH4 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 CH4 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 CH4. 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 CH4 from coastal sediments.
authors: Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
authors: Matthias
authors: http://orcid.org/0000-0002-1791-1842
authors: M
authors: Egger
authors: Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
authors: Wytze
authors: W
authors: Lenstra
authors: Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
authors: Dirk
authors: D
authors: Jong
authors: Department of Estuarine and Deltaic Studies, Royal Netherlands Institute for Sea Research, Yerseke, The Netherlands.|Department of Analytical, Environmental, and Geochemistry, Vrije Universiteit Brussel, Brussels, Belgium.
authors: Filip J R
authors: FJ
authors: Meysman
authors: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands.|Laboratoire de Glaciologie, Université Libre de Bruxelles, Brussels, Belgium.
authors: Célia J
authors: CJ
authors: Sapart
authors: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands.
authors: Carina
authors: C
authors: van der Veen
authors: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands.
authors: Thomas
authors: T
authors: Röckmann
authors: Department of Marine Microbiology and Biogeochemistry, Royal Netherlands Institute for Sea Research (NIOZ), Texel, The Netherlands.
authors: Santiago
authors: S
authors: Gonzalez
authors: Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
authors: Caroline P
authors: CP
authors: Slomp
mesh_terms: D000693:Anaerobiosis; D001272:Atmosphere; D002244:Carbon / Q000737:chemistry; D002245:Carbon Dioxide / Q000032:analysis; D004777:Environment; D005843:Geography; D019015:Geologic Sediments / Q000737:chemistry*; D000860:Hypoxia; D060106:Lakes; D008697:Methane / Q000737:chemistry*; D009426:Netherlands; D010100:Oxygen / Q000737:chemistry; D016062:Porosity; D013431:Sulfates / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D013431:Sulfates; D014867:Water; D002245:Carbon Dioxide; D002244:Carbon; D008697:Methane; D010100:Oxygen