Methane is a powerful greenhouse gas and its biological conversion in marine sediments, largely controlled by anaerobic oxidation of methane (AOM), is a crucial part of the global carbon cycle. However, little is known about the role of iron oxides as an oxidant for AOM. Here we provide the first field evidence for iron-dependent AOM in brackish coastal surface sediments and show that methane produced in Bothnian Sea sediments is oxidized in distinct zones of iron- and sulfate-dependent AOM. At our study site, anthropogenic eutrophication over recent decades has led to an upward migration of the sulfate/methane transition zone in the sediment. Abundant iron oxides and high dissolved ferrous iron indicate iron reduction in the methanogenic sediments below the newly established sulfate/methane transition. Laboratory incubation studies of these sediments strongly suggest that the in situ microbial community is capable of linking methane oxidation to iron oxide reduction. Eutrophication of coastal environments may therefore create geochemical conditions favorable for iron-mediated AOM and thus increase the relevance of iron-dependent methane oxidation in the future. Besides its role in mitigating methane emissions, iron-dependent AOM strongly impacts sedimentary iron cycling and related biogeochemical processes through the reduction of large quantities of iron oxides.
Large amounts of the greenhouse gas methane are released from the seabed to the water column 1 where it may be consumed by aerobic methanotrophic bacteria 2. This microbial filter is consequently the last marine sink for methane before its liberation to the atmosphere. The size and activity of methanotrophic communities, which determine the capacity of the water column methane filter, are thought to be mainly controlled by nutrient and redox dynamics 3-7 , but little is known about the effects of ocean currents. Here we show that cold bottom water at methane seeps west off Svalbard, containing a large number of aerobic methanotrophs, was rapidly displaced by warmer water with a considerably smaller methanotrophic community. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, constitutes an oceanographic switch severely reducing methanotrophic activity in the water column. Strong and fluctuating currents are widespread oceanographic features common at many methane seep systems and are thus likely to globally affect methane oxidation in the ocean water column. Large amounts of methane are stored in the subsurface of continental margins as solid gas hydrates, gaseous reservoirs or dissolved in pore water 8. At cold seeps, various physical, chemical, and geological processes force subsurface methane to ascend along pathways of structural weakness to the sea floor where a portion of this methane is utilised by anaerobic and aerobic methanotrophic microbes 1,9. On a global scale, about 0.02 Gt yr-1 (3-3.5% of the atmospheric budget 10) of methane bypasses the benthic filter system and is liberated to the ocean water column 1 where some of it is oxidised aerobically (aerobic oxidation of methane-MOx) (ref 2), or less commonly where the water column is anoxic, anaerobically (anaerobic oxidation of methane-AOM) (refs 2, 11). MOx is performed by methanotrophic bacteria (MOB) typically belonging to the Gamma-(type I) or Alphaproteobacteria (type II) (refs 12, 13): CH 4 + 2 O 2 → CO 2 + 2 H 2 O Water column MOx is consequently the final sink for methane before its release to the atmosphere, where it acts as a potent greenhouse gas. The water column MOx filter could become more
Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations. Here we present high-resolution carbon isotope data (δ(13)C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ(13)C content underwent pronounced centennial-scale variations between 100 BC and AD 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability--such as the Medieval Climate Anomaly and the Little Ice Age--and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.
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