An international team of scientists led by the geochemists Natascha Riedinger (University of California, Riverside) and Sabine Kasten (Alfred Wegener Institute) now presents first evidence for a methane-consuming process in the sediments of the western Argentine Basin in the South Atlantic that was largely unknown to occur in natural marine deposits.
One of the key questions in climate research is how much methane may enter the atmosphere as a consequence of increased global warming and in this way reinforce the greenhouse effects. Scientists suggest that in deeper sediment layers methane is oxidised by natural iron oxide minerals.
It has been known for many years now that methane forms an important basis for microbial life in the largely oxygen-free seafloor. Microorganisms utilise the plentiful supply of sulphate in seawater to oxidise methane and thus meet their energy needs. This oxidation process is so efficient that a major portion of the methane formed in the seafloor is consumed in shallower sediment layers and consequently does not reach the water column and the atmosphere.
“Sulphate concentrations, however, do decrease with increasing sediment depth ” explains Dr. Sabine Kasten from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven. “A few metres below the seafloor, therefore, methane oxidation should no longer take place.”
On an expedition off the coast of Argentina and Uruguay the international team of researchers discovered, nonetheless, that methane is consumed in deeper, sulphate-free layers of the seafloor.
“Our investigations give evidence that microbial methane oxidation takes place there making use of natural iron oxides,” adds colleague and lead author Dr. Natascha Riedinger from the Department of Earth Sciences at the University of California in Riverside. This process is shown for the first time in marine sediments. So far, it is only known from laboratory experiments and reported for lake sediments.
Since sulphate is highly abundant in the upper layers of the seafloor while iron oxides occur in lower contents the newly discovered process is presumably of minor importance for quantitative methane oxidation in the seafloor. According to Sabine Kasten, things may be different for areas near the coast and the enormous permafrost regions in the Arctic and Subarctic that cover nearly 25 percent of the land surface in the northern hemisphere. As a rule, iron oxides are widespread in coastal marine deposits and in terrestrial soils.
Press Release, February 20, 2014