Description: Highlights • High abundance of active anaerobic methanotrophs in sediments of the blowout crater suggests adaptation to methane seepage within at most two decades. • Fast exchange processes in permeable surface sediments prevent sulfate depletion and probably methane-derived carbonate precipitation. • Methane seepage impacts isotopic and assemblage composition of benthic foraminifera. Abstract Methane emissions from marine sediments are partly controlled by microbial anaerobic oxidation of methane (AOM). AOM provides a long-term sink for carbon through precipitation of methane-derived authigenic carbonates (MDAC). Estimates on the adaptation time of this benthic methane filter as well as on the establishment of related processes and communities after an onset of methane seepage are rare. In the North Sea, considerable amounts of methane have been released since 20 years from a man-made gas blowout offering an ideal natural laboratory to study the effects of methane seepage on initially “pristine” sediment. Sediment cores were taken from the blowout crater and a reference site (50 m distance) in 2011 and 2012, respectively, to investigate porewater chemistry, the AOM community and activity, the presence of authigenic carbonates, and benthic foraminiferal assemblages. Potential AOM activity (up to 3060 nmol cm−3 sediment d−1 or 375 mmol m−2 d−1) was detected only in the blowout crater up to the maximum sampling depth of 18 cm. CARD-FISH analyzes suggest that monospecific ANME-2 aggregates were the only type of AOM organisms present, showing densities (up to 2.2*107 aggregates cm−3) similar to established methane seeps. No evidence for recent MDAC formation was found using stable isotope analyzes (δ13C and δ18O). In contrast, the carbon isotopic signature of methane was recorded by the epibenthic foraminifer Cibicides lobatulus (δ13C −0.66‰). Surprisingly, the foraminiferal assemblage in the blowout crater was dominated by Cibicides and other species commonly found in the Norwegian Channel and fjords, indicating that these organisms have responded sensitively to the specific environmental conditions at the blowout. The high activity and abundance of AOM organisms only at the blowout site suggests adaptation to a strong increase in methane flux in the order of at most two decades. High gas discharge dynamics in permeable surface sediments facilitate fast sulfate replenishing and stimulation of AOM. The accompanied prevention of total alkalinity build-up in the porewater thereby appears to inhibit the formation of substantial methane-derived authigenic carbonate at least within the given time window.

Description: Due to global warming, large volumes of methane may get released from marine sediments by melting gas hydrates, enhancing ocean acidification, bottom water hypoxia and global warming. In anoxic marine sediments, microbial induced anaerobic oxidation of methane provides a long- term sink for methane via the formation of methane derived authigenic carbonates (MDAC). In the view of destabilizing gas hydrates it is mandatory to understand, how fast AOM communities can adapt to an onset of methane flux. So far, the response of AOM consortia has been studied in theoretical models only. In laboratory experiments it was shown that AOM organisms showed extremely slow growth rates (doubling time: 4-7 months). In 1990 an oil rig caused a blow-out in the northern North Sea and formed a 20 m deep crater at the seafloor. More than 20 years later, the blow-out still releases significant amounts of methane. The site provides a unique natural laboratory to study the adaption of AOM communities in a newly formed methane-seep. A sediment core was recovered from in-side the blow-out crater, a reference core 50 m away from the crater. Both cores were sliced into three horizons (0-6, 6-12 and 12-18 cm bsf). Methane dependent sulphate reduction, which is equivalent to AOM activity, was determined by measuring total alkalinity and sulphide over 47 days in samples from the blow-out sediments incubated at 4, 13, 20, 37 and 60 °C. These long-term measurements revealed high potential rates of methanedependent sulfate reduction in all horizons (1.5 - 3.5 μmol SO42- cm-3 sediment d-1). Potential AOM rates determined by radiotracer measurements in short-term incubations (24 h, 13 °C) showed similar patterns but slightly lower rates (0.6 - 2.4 μmol CH4 cm-3 sediment d-1). The temperature optimum of AOM was between 13 and 20 °C indicating the presence of psychrophilic to mesophilic microorganisms, adapted to the relatively constant in-situ temperature (7 °C). No AOM activity was detected in incubations at 37 and 60 °C, indicating sensitivity of the AOM organisms against higher temperatures. Highest AOM activity was found in the deepest horizon. Both, long and short-term measurements of potential AOM rates show, the vast majority of sulphate reduction in the core was coupled to AOM. Neither an increase of total alkalinity nor of sulphide concentrations was detected in the sediments from the reference core incubated at 13 °C for 21 days. Samples from sediments of the blow-out crater were analyzed for their mineral composition using X-ray powder diffraction (XRD). Three out of six samples were composed of brucite (Mg(OH)2), mainly. Overall, different polymorphs of calcium carbonate such as aragonite, calcite, magnesium calcite and vaterite were the dominant minerals. Aragonite and magnesian calcite have been related to AOM. However, no indications for MDAC formation were found by stable isotope analyzes. Typically, MDAC show distinct stable isotope values (δ13C 〈-20 and δ18O 〉 0), if formed und er present day conditions in the marine environment. Some samples show negative δ13C values (-7.3 to -17.6 ‰) indicating that carbonates were, at least partly, derived from AOM. Corresponding δ18O values (between -12 and -3), clearly argue against a recent MDAC formation. All other samples showed positive δ13C which argue against microbial induced carbonate formation. Unspecific cell staining using DAPI revealed cell aggregates, which are typical for AOM consortia, in high numbers in all horizons {2.5*1012 aggregates m-2). Total cell counts were one order of magnitude higher in the blow-out core (23 - 68*108 cells ml-1), compared to the reference core (3.9-7.3*108 cells mL-1). Methane turnover and AOM aggregate density were similar to very active cold seep environments. Pore water profiles measured in a replicate core, show sufficient sulphate supply throughout the sandy and porous sediments. Therefore, it is likely that the extreme broad AOM zone exceeded the sampling depth of 18 cm. This study demonstrates that the AOM community at the North Sea blow-out is extremely active; suggesting that adaptation to strongly increasing methane fluxes might take less than 20 years. The question, whether or not, the AOM community has established within the last 20 years, cannot be answered finally. To gain certainness, it is necessary to analyze the sediment layers at reference sites 20 m bsf. lt needs to be analyzed whether or not AOM activity is there as weil.