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Abstracts of oral presentations 51 Microbial activity in the south-eastern Baltic Sea (Russian sector) with special reference to the methane and sulfur cycling N. Pimenov 1 , M. Ulyanova 2 , T. Kanapasky 1 , E .Veslopolova 1 , V. Sivkov 2 1 Winogradsky Institute of Microbiology, RАS 2 Atlantic Branch of Shirshov Institute of Oceanology, RAS The geographical position and the hydrological regime of the Russian sector of the Gdansk basin of the Baltic Sea make it a zone of excessive influx of both natural allochthonous and anthropogenic OM and biogenic elements. Low depths of this area result in incomplete destruction of OM; the suspension reaching the bottom contains therefore labile OM, which is actively utilized by microorganisms of various physiological groups. Due to intensive microbial processes, oxygen is rapidly consumed; sulfate-reducing and methanogenic microorganisms are therefore activated in the upper sediment layer. Seasonal observations revealed significant oxygen limitation in the near-bottom water of the Gdansk Basin at depths over 65 m; methane concentrations increase to 90–500 nmol/l, while its usual concentration in Baltic waters does not exceed 35 nmol/l. During autumn, H2S - (up to 60 nmol/l) was revealed in the pre-bottom water of the stations deeper than 95 m. Radioisotope measurements revealed high rates of sulfate reduction (SR) in the upper sediment layers (up to 0.6 mmol dm -3 day -1 ). In autumn, SR (up to 8 µmol l -3 day -1 ) was detected also in the near-bottom water column. СН4 content in the aleuric and pelitic silts increased sharply with depth, reaching 190–900 mmol dm -3 20–30 cm below the surface. In spite of high CH4 content, intensive methanogenesis (MG) was not revealed. The highest MG rate in anaerobic surface sediment layers was 2.5 µmol dm -3 day -1 . Significant anomalies in the content of gaseous hydrocarbons in the Baltic Sea near-bottom waters revealed in early 1970s by the scientists of Shirshov Institute of Oceanology, are associated with discharge of gas-containing fluids localized in the regions with specific geomorphological structure of the sea bottom (Craters=pockmarks). We have investigated several pockmarks in the Russian sector of the Gdansk Basin. Microbiological, biogeochemical, and isotopic investigation of the near-bottom water and surface sediments of the pockmarks revealed their significant differences from other regions of the deep-water zone of the Gdansk Basin. We found out anomalies of methane concentration (up to 2.5 m from the bottom) above the crater. This zone was also characterized by increased microbial numbers, rates of dark CO2 fixation and methane oxidation. Increased 12 С content in POC was observed in the near-bottom water of the pockmark area; this finding confirms our results on increased microbial activity and suggests that in the near-bottom horizon above the pockmark, apart from methanotrophs, a sulfur-oxidizing bacterial community is formed, oxidizing the reduced sulfur compounds derived from highly reduced bottom sediments. Analysis of the data on integral SR rates in the upper 30 cm of the sediments revealed that the typical rate of formation of reduced sulfur compounds via OM decomposition was 2-3 mmol m -2 day -1 . In the pockmark sediments, where significantly higher methane content was detected, anaerobic methane oxidation (AMO) played the major role in formation of reduced sulfur compounds. SR rate there was 8.4 mmol m -2 day -1 , and AMO rate, 6.4 mmol m -2 day -1 . The origin of methane can not be determined unequivocally from the δ 13 С-СН4 values. At 10–20 cm depth, in the zone of the highest AMO rate, δ 13 С-СН4 varied from -53.2 to -56.6 ‰; in deeper layers of the sediment, it became heavier (from -40 to -45‰). Obviously apart from contemporary microbial methane, isotopically heavier methane of deep sediment layers is accumulated in the surface sediments of pockmarks. Magnetic iron-sulphides as methane proxies in southern Hydrate Ridge sediments (ODP Leg 204): Preliminary results E. Piñero 1 , J. Cruz Larrasoaña 2 , F. Martínez-Ruiz 3 , E. Gràcia 1 1 Unitat de Tecnologia Marina, Centre Mediterrani d’Investigacions Marines i Ambientals, Barcelona, Spain 2 Institut de Ciències de la Terra Jaume Almera (CSIC), Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain 3 Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, Granada, Spain Southern Hydrate Ridge (SHR) is a structural high located in the Cascadia Accretionary Complex (Oregon margin), which has been exhaustively studied since the identification of gas hydrates. During Leg 204 of the Ocean Drilling Program, nine sites were drilled in SHR sediments (Fig. 1) in order to evaluate the role of gas hydrates as a trigger of slope failures, their potential impact on climate change and its contribution as a carbon budget on the global cycle. The presence of magnetic minerals in methane-rich environments has already been related to gas hydrate formation and preservation (e.g. Larrasoaña et al., 2006). Thus, sulphides form during the early diagenesis as a consequence of the biological mediated reaction between the ascending methane and sulphate (Anaerobic Oxidation of Methane - AOM). This reaction also controls the precipitation of authigenic carbonates.
52 Abstracts of oral presentations Fig. 1. Location of Hydrate Ridge in the Cascadia Margin. a) Plate tectonic setting of the Cascadia accretionary complex. Black outlined box shows the location of Hydrate Ridge. b) Detailed bathymetric map (20 m contour intervals) of SHR. Location of ODP Leg 204 Sites 1244 to 1252, from where the samples presented in this study were collected. The main objectives in this work are: to characterize the magnetic minerals present in SHR sediments, to evaluate their possible mechanisms and environments of formation and to study the relationship between their distribution with depth and the variation of the position of the sulphate-methane transition (SMT) in the sedimentary column with time. Previous textural analyses reveal that SHR is composed by hemipelagic silty-clay sediments interbedded with numerous mass-transport deposits including turbidites and debris flows. The magnetic mineralogy of these gas hydrate-rich sediments (revealed by remanent magnetization data) is dominated by magnetite and two magnetic iron sulphides: greigite and pyrrhotite (Larrasoaña et al., 2006). Magnetite is more abundant in coarse-grained sediments and therefore it is interpreted to be detrital in origin. Greigite and pyrrhotite have been recognized as diagenetically produced during microbial reduction of sulphate in 3 zones: the sulphate zone (above SMT), the anaerobic oxidation of methane zone (SMT), and the methanic zone (below SMT). Electronic scanning microscopy analyses were performed in order to study the paragenetic sequence of the different magnetic sulphide minerals in every ambient of SHR. Results show that greigite forms above and at the SMT zone, while pyrrhotite preferentially forms below this depth (Larrasoaña et al., 2007). The formation of magnetic sulphide minerals depends on the proportion of Fe versus Fe extractable in the sediments, which controls the completion of the pyritization reaction in marine sediments. In order to study the chemical conditions in which these magnetic iron sulphides (greigite and pyrrhotite) are produced and preserved, we have measured the total Fe (TFe), reactive_Fe, total organic carbon (TOC) and total S (TS) of a set of ca. 100 samples with distinctive magnetic assemblages (magnetite+greigite, greigite-, and pyrrhotite-dominated). TOC and TS were analysed by an elemental analyser and, TFe and reactive_Fe were measured by atomic absorption spectrometry. TS contents vary between 0.2 and 0.9%. TFe values range between 4.3 and 7.3%, in which the reactive_Fe represents the 23-39%. TS/TOC ratio values are near the oxic and normal marine seawater conditions (TS/TOC = 0.36), except for two magnetite+greigite and greigite-dominated samples that show higher TS contents. All the samples show TS/reactive_Fe ratios considerably lower than of saturated pyrite (1.15), which implies a low degree of pyritization in the environment. These preliminary S and Fe results suggest that pyritization reactions at the different diagenetic zones in SHR are not driven to completion (Figure 2), and formation and preservation of greigite and pyrrhotite meta-stable minerals are favoured by a low H2S flux or by high reactive-Fe contents in their local micro-environments of formation. In this situation, the net consumption of available sulphide would be used in producing new greigite or pyrrhotite rather than evolving into pyrite. A reaction model will be applied to all the dataset in order to constrain the necessary time to form the magnetic sulphide minerals in every environment, and thus, modelling changes of the SMI depth and methane flux over time.
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