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The flux of methane, a potent greenhouse gas, from the seabed is largely controlled by anaerobic oxidation of methane (AOM) coupled to sulfate reduction (S-AOM) in the sulfate methane transition (SMT). S-AOM is estimated to oxidize 90% of the methane produced in marine sediments and is mediated by a consortium of anaerobic methanotrophic archaea (ANME) and sulfate reducing bacteria. An additional methane sink, i.e., iron oxide coupled AOM (Fe-AOM), has been suggested to be active in the methanic zone of marine sediments. Geochemical signatures below the SMT such as high dissolved iron, low to undetectable sulfate and high methane concentrations, together with the presence of iron oxides are taken as prerequisites for this process. So far, Fe-AOM has neither been proven in marine sediments nor have the governing key microorganisms been identified. Here, using a multidisciplinary approach, we show that Fe-AOM occurs in iron oxide-rich methanic sediments of the Helgoland Mud Area (North Sea). When sulfate reduction was inhibited, different iron oxides facilitated AOM in longterm sediment slurry incubations but manganese oxide did not. Especially magnetite triggered substantial Fe-AOM activity and caused an enrichment of ANME-2a archaea. Methane oxidation rates of 0.095 ± 0.03 nmol cm −3 d −1 attributable to Fe-AOM were obtained in short-term radiotracer experiments. The decoupling of AOM from sulfate reduction in the methanic zone further corroborated that AOM was iron oxide-driven below the SMT. Thus, our findings prove that Fe-AOM occurs in methanic marine sediments containing mineral-bound ferric iron and is a previously overlooked but likely important component in the global methane budget. This process has the potential to sustain microbial life in the deep biosphere.

Laser-diffraction analysis (LDA) is a rapid automated method achieving highly resolved frequency distributions of particle sizes. Recently, LDA has come into use in environmental sciences. However, in the size range of silt and clay deviations from the particle-size analysis with the standard pipette method, which is regarded as the reference method for soil-texture classification, have been reported. Therefore, this study concentrates (1) on the verification of systematic relations between both methods using a series of soils of Lower Saxony (Germany) and (2) on the general applicability of the laser-diffraction method to soil-texture classification as well as (3) texture-based estimates of air capacity, available field capacity, and permanent wilting point. The comparison of LDA with the pipette method demonstrated highly significant linear correlations in each of the particle-size fractions from clay to coarse silt. The slope of regressions ranged from 0.4 with fine silt to 3.1 with clay. If the clay content derived from LDA was applied to texture classification, the resulting textural classes differed from the standard textural classes, except for purely sandy samples with a clay content of <5%. However, the linear-regression model enabled an approach of the LDA-based clay content to values produced with the standard pipette method. Using this transformation, a texture classification became practicable in many cases, but, despite of a high significance level between LDA and pipette method, still led to wrong textural classes in several cases. A comparison with regression models from other regions in Europe showed both similarities and discrepancies, even for similar substrates. Hence, the laser-diffraction analysis cannot be used for the texture classification of soil samples without verification by the standard pipette method.

Methane is abundant in marine subsurface sediments, sourced from microbial or thermocatalytic products. The relative composition of its isotopologues (12 CH4, CH4, 12 CH3D and 13 CH3D) is used to infer its sources and sinks. The anaerobic oxidation of methane (AOM) is an important methane sink reaction carried out by consortia of anaerobic methanotrophic archaea (ANME) and partner bacteria in the presence of methane and sulfate. We investigated the methane isotopologue fractionations during AOM in experiments with cultures of ANME-1 archaea and partner bacteria obtained from hydrothermally heated gas-rich sediments of the Guaymas Basin. During partial methane consumption in four sets of experiments, residual methane became enriched in 13 CH4 and 12 CH3D, following kinetic fractionations from 11.1 to 18.3 ‰ and from 117 to 180 ‰, respectively. Results from one set of experiments with D-depleted medium water (δD =-200‰, whereas the control was-55‰) are inconclusive regarding the reversibility of AOM, which would lead to equilibrium as opposed to kinetic fractionations. The value of Δ 13 CH3D (the abundance of 13 CH3D with respect to that expected from stochastic distribution) increased toward and beyond (up to 8.4‰) the value expected for isotopologue equilibrium (5.3‰ at 37 °C). The kinetic clumped isotopologue fractionation (difference between 13 CH3D/ 12 CH3D and 13 CH4/ 12 CH4 fractionations) of 4.8 to 12.8 ‰ is in contrast with our previous observation of little to no clumped isotopologue effect during aerobic methane oxidation. Our results demonstrate that AOM can contribute to nearequilibrium Δ 13 CH3D values observed in marine sediments and 13 CH3D systematics can be used to distinguish aerobic versus anaerobic methanotrophic processes in nature.