Production of greenhouse gases in organic-rich sediments

Methane (CH4) and nitrous oxide (N2O) are greenhouse gases, which atmospheric concentrations increased since preindustrial times by ~150 and ~20%, respectively, mainly due to the increase in anthropogenic emissions. The atmospheric increase of greenhouse gases (incl. carbon dioxide (CO2), CH4 and N2O) led to various effects on the Earth’s surface and atmosphere, summarized as global climate change. In the marine environment, temperature rise, sea level rise, ocean acidification, and decreased oxygen concentrations are the most significant effects of climate change. To predict possible changes through climate change in the future, natural and anthropogenic sources and production/consumption pathways of greenhouse gases need to be determined carefully. To date, research is sparse on the sources for oceanic CH4 and N2O, thus leading to uncertainties in global ocean emission estimates. Marine, organic-rich sediments (e.g. in coastal areas) are known to be major benthic sources for CH4 and N2O, which eventually could end up in the atmosphere. Both greenhouse gases are produced by microbial processes during the degradation of organic matter in marine sediments, namely methanogenesis and denitrification (next to nitrification), respectively. However, knowledge about magnitude and environmental controls of these microbial processes is still limited. In the present study, benthic CH4 and N2O production was investigated in three different marine areas with the focus on the surface sediment (0-30 cmbsf=centimeter below surface): the upwelling region off Peru, the Eckernförde Bay in the southwestern Baltic Sea and the Gulf of Mexico. In sediments from Peru and Eckernförde Bay, the focus was set on surface methanogenesis within the sulfate-reducing zone, which has been thought to be negligible due to the successful competition of sulfate reducers for the mutual substrates hydrogen (H2) and acetate. In oil-influenced sediments from the Gulf of Mexico, the focus was set on benthic denitrification and sulfate reduction, with benthic N2O production as a side effect