The potential for Fe-mediated anaerobic oxidation of methane in coastal sediments: a model study

Abstract

Methane is an important greenhouse gas and atmospheric concentrations have been steadily increasing since the industrial revolution. Rates of methane formation in many coastal sediments are high. The methane efflux from sediments is generally very low, however, due to a variety of oxidation processes. Recently, iron-mediated anaerobic oxidation of methane (Fe-AOM) was discovered to contribute to methane oxidation in sediments. Though several studies have found geochemical evidence for this process in marine sediments, its quantitative importance remains unclear. In this study, we assess the key factors controlling rates of Fe-AOM in coastal sediments by performing a sensitivity analysis with a 1-dimensional multi-component reactive transport model. We use a coastal site in the Baltic Sea for our baseline scenario. We then focus specifically on the effects of variations in bottom water oxygen and salinity, sedimentation rate, organic carbon input, the input of iron oxyhydroxides, and the effect of biota on the rate of Fe-AOM. Our results show that high bottom-water O2 concentrations and an organic matter flux balanced against the flux of O2, iron oxy-hydroxides and sulfate into the sediment can contribute to Fe-AOM rates, but do not control high Fe-AOM rates. Further, as indicated by previous studies, we show that Fe-AOM can become a quantitatively important pathway for methane oxidation in coastal environments with low salinities and high inputs of iron oxy-hydroxides. We suggest a relationship between iron oxy-hydroxides, BW O2, BW sulfate, and organic matter in the sediment as an index to find potential locations where Fe-AOM can be studied.