Anaerobic oxidation of methane (AOM) by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere. We show geochemical evidence for AOM in deep lake sediments and demonstrate that AOM is likely driven by iron (Fe) reduction. Pore-water profiles from Lake Kinneret (Sea of Galilee, Israel) show that this sink for methane is located below the 20-cm depth in the sediment, which is well below the depths at which nitrate and sulfate are completely exhausted, as well as below the zone of methanogenesis. Iron-dependant AOM was verified by Fe(III)-amended mesocosm studies using intact sediment cores, and native iron oxides were detectable throughout the sediments. Because anaerobic Fe(III) respiration is thermodynamically more favorable than both sulfate-dependent methanotrophy and methanogenesis, its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited. Similar conditions are likely to prevail in other incompletely pyritized aquatic sediments, indicating that AOM with Fe(III) is an important global sink for methane.

Full seasonal sets of chemical and isotope profiles from the pore water of Lake Kinneret (Sea of Galilee, Israel) were produced to study methanogenesis and methanotrophy processes and the couplings between methane (CH 4 ), sulfur, and iron. Sulfate is depleted within the upper 10 cm of the sediment mainly by traditional bacterial sulfate reduction by organic matter. Maximum sulfate reduction rates calculated from sulfate concentration profiles are found at the water-sediment interface (0-1 cm 2 1.4 3 10 212 6 0.2 3 10 212 mol cm 23 s 21 ). CH 4 concentrations and modeling of dissolved inorganic carbon (DIC) and its stable carbon isotope (d 13 C DIC ) suggest that maximum methanogenesis rates of 2.5 3 10 213 6 1.5 3 10 213 mol cm 23 s 21 occur at 5-12-cm depth in the sediments, and that it ends at 20 cm. Of the produced CH 4 , 50-75% is converted to gas bubbles of CH 4 before it reaches the bottom water. Model results suggest the occurrence of anaerobic oxidation of CH 4 (AOM) in the deep sediments of the lake below the zone of methanogenesis.

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Pore water concentration profiles of sediments at a site on the Amazon Fan were investigated and simulated with the numerical model CoTReM (column transport and reaction model) to reveal the biogeochemical processes involved. The pore water profiles for gravity core GeoB 4417-7 showed a distinct sulfate-methane transition zone in which deep sulfate reduction occurs. Only a small sulfide peak could be observed at the reaction zone. Due to high amounts of iron minerals, the produced sulfide is instantaneously precipitated in form of iron sulfides. We present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate. Furthermore, sulfide, iron, pH, pE, calcium and total inorganic carbon (TIC) were included in the simulation. The program calculated mineral equilibria to mackinawite, iron sulfides (more stable than mackinawite), iron hydroxides and calcite via saturation indices (SI) by a module incorporating the program PHREEQC (Parkhurst 1995). The measured sulfide and iron profiles are obtained in the simulation output by using a constant SI (p0) for mackinawite and calcite, while a depth dependent SI distribution is applied for the PHREEQC phases "Pyrite" and "Fe(OH)3(a)", representing a composition and the kinetics of different iron sulfides and iron hydroxides. These SI distributions control the results of sulfide and iron pore water profiles, especially conserving the sulfide profile at the reaction zone during the simulation. The results suggest that phases of iron hydroxides are dissolved, mackinawite is precipitated within, and other iron sulfides are precipitated below the reaction zone. The chemical reactivity of iron hydroxides corresponds to the rate of sulfide production. The system H 2 O-CO 2 -CaCO 3 is generally successfully maintained during the simulation. Deviations to the measured pH profile suggest that further processes are active which are not included in the simulation yet.

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