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The Gulf of Cadiz is a tectonically active area of the European continental margin and characterised by a high abundance of mud volcanoes, diapirs, pockmarks and carbonate chimneys. During the R/V SONNE expedition ‘‘GAP–Gibraltar Arc Processes (SO-175)’’ in December 2003, several mud volcanoes were surveyed for gas seepage and associated microbial methane turnover. Pore water analyses and methane oxidation measurements on sediment cores recovered from the centres of the mud volcanoes Captain Arutyunov, Bonjardim, Ginsburg, Gemini and a newly discovered, mud volcano-like structure called ‘‘No Name’’ show that thermogenic methane and associated higher hydrocarbons rising from deeper sediment strata are completely consumed within the seabed. The presence of a distinct sulphate–methane transition zone (SMT) overlapping with high sulphide concentrations suggests that methane oxidation is mediated under anaerobic conditions with sulphate as the electron acceptor. Anaerobic oxidation of methane (AOM) and sulphate reduction (SR) rates show maxima at the SMT, which was found between 20 and 200 cm below seafloor at the different mud volcanoes. In comparison to other methane seeps, AOM activity (

The anaerobic oxidation of methane in aquatic environments is a globally significant sink for a potent greenhouse gas. Significant gaps remain in our understanding of the anaerobic oxidation of methane because data describing the distribution and abundance of putative anaerobic methanotrophs in relation to rates and patterns of anaerobic oxidation of methane activity are rare. An integrated biogeochemical, molecular ecological and organic geochemical approach was used to elucidate interactions between the anaerobic oxidation of methane, methanogenesis, and sulfate reduction in sediments from two cold seep habitats (one brine site, the other a gas hydrate site) along the continental slope in the Northern Gulf of Mexico. The results indicate decoupling of sulfate reduction from anaerobic oxidation of methane and the contemporaneous occurrence of methane production and consumption at both sites. Phylogenetic and organic geochemical evidence indicate that microbial groups previously suggested to be involved in anaerobic oxidation of methane coupled to sulfate reduction were present and active. The distribution and isotopic composition of lipid biomarkers correlated with microbial distributions, although concrete assignment of microbial function based on biomarker profiles was complicated given the observed overlap of competing microbial processes. Contemporaneous activity of anaerobic oxidation of methane and bicarbonate-based methanogenesis, the distribution of methane-oxidizing microorganisms, and lipid biomarker data suggest that the same microorganisms may be involved in both processes.

Anaerobic oxidation of methane (AOM) and sulfate reduction (SR) were investigated in sediments of the Chilean upwelling region at three stations between 800 and 3000 In water depth. Major goals of this study were to quantify and evaluate rates of AOM and SR in a coastal marine upwelling system with high organic input, to analyze the impact of AOM on the methane budget, and to determine the contribution of AOM to SR within the sulfate-methane transition zone (SMT). Furthermore, we investigated the formation of authigenic carbonates correlated with AOM. We determined the vertical distribution of AOM and SR activity, methane, sulfate, sulfide, pH, total chlorins, and a variety of other geochemical parameters. Depth-integrated rates of AOM within the SMT were between 7 and 1124 mmol m(-2) a(-1), effectively removing methane below the sediment-water interface. Single measurements revealed AOM peaks of 2 to 51 nmol cm(-3) d(-1), with highest rates at the shallowest station (800 m). The methane turnover was higher than in other diffusive systems of similar ocean depth. This higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis. SR within the SMT was mostly fuelled by methane. AOM led to the formation of isotopically light DIC (delta(13)C: -24.6 parts per thousand VPDB) and of distinct layers of authigenic carbonates (delta(13)C: - 14.6 parts per thousand VPDB). Copyright (c) 2005 Elsevier Ltd.

Gravity driven mass-flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western Argentine Basin. In this study we present geochemical data from a total of 23 gravity cores. Pore-water SO 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane (AOM). The different shapes of SO 4 profiles (concave, kink- and s-type) can be consistently explained by sedimentary slides possibly in combination with changes in the CH 4 flux from below, thus, mostly representing transient pore-water conditions. Since slides may keep their original sedimentary signature, a combined analysis and numerical modeling of geochemical, physical properties, and hydro acoustic data could be applied in order to reconstruct the sedimentary history. We present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day. The results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep-sea, depositing high amounts of reactive compounds. The high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by AOM. For the total region we estimate a SO 4 (or CH 4 ) flux of 6.6 × 10 10 moles per year into the zone of AOM. Projected to the global continental slope and rise area, this may sum up to about 2.6 × 10 12 moles per year. Provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1.2 × 10 12 moles per year as previously estimated. Thus, AOM obviously contributes very significantly to the regulation of global sulfur reservoirs, which is hitherto not sufficiently recognized. This finding may have implications for global geochemical models, as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time.