Giovanni Aloisi

Methane-related authigenic carbonates of eastern Mediterranean Sea mud volcanoes and their possible relation to gas hydrate destabilisation

Nautile submersible investigations of mud volcanoes and brine seep areas of the eastern Mediterranean Sea during the MEDINAUT cruise in November 1998 discovered extensive areas of authigenic carbonate crusts associated with methane emissions. Carbonate crusts form pavements, round slabs and circular mounds on the central, most active parts of mud volcanoes and in a fault-related valley where brines have accumulated to form a submarine brine lake. Authigenic carbonate nodules have been recovered from the same areas during the MEDINETH cruise in July 1999. Large 13C depletions of authigenic calcite, aragonite and dolomite indicate methane as a major carbon source for the carbonate. Crust pavements are formed when methane from a freshly emplaced, methane-charged mud flow is oxidised at the seafloor. In this environment, where bottom waters provide the sulphate and magnesium, aragonite is favoured versus calcite and accounts for the majority of the methane-related authigenic carbonates. Calcite, when present, contains significant amounts of Mg2+ (high-Mg calcite), and possibly other divalent ions in its crystal lattice. In areas of brine seep and accumulation, dolomitic nodules are present at shallow depth in the sediment. The 18O enrichment of the authigenic carbonates (up to 4‰ greater than calculated values for carbonates precipitating from modern eastern Mediterranean bottom waters) is interpreted as due to precipitation from 18O-rich fluids rather than as a temperature effect. The source of the 18O-rich fluids may be multiple and possibly includes the destabilisation of gas hydrates present at shallow subbottom depth, and the seepage of relic Messinian brines.

CH4-consuming microorganisms and the formation of carbonate crusts at cold-seeps

To understand the role played by microorganisms in the formation of cold seep carbonates, we conducted an integrated microbial, mineralogical and organic geochemical study of methane-related authigenic carbonate crusts formed on eastern Mediterranean mud volcanoes. We show that supersaturation with respect to carbonate minerals is induced by microbial anaerobic oxidation of methane. Combined lipid biomarker analysis and 16S rRNA gene surveys identified a highly diversified methane-consuming archaeal community possibly comprising novel species, implying that the anaerobic oxidation of methane is phylogenetically widespread and directly implicating these organisms in the process of crust precipitation. Moreover, pore-water sulphate gradients produced by co-occurring methane-based sulphate reduction exert the main control on aragonite versus magnesian calcite precipitation. We propose that this may be the dominant mode of carbonate crust formation at cold seeps world-wide, in agreement with aquatic chemistry predictions and explaining carbonate mineralogy.

Three series of non-isoprenoidal dialkyl glycerol diethers in cold-seep carbonate crusts

Three novel series of non-isoprenoidal dialkyl glycerol diethers with an inferred sn-1,2 stereochemistry were tentatively identified in carbonate crusts precipitated from methane-rich bottom-waters and pore-waters associated with Mediterranean mud volcanoes. In the first series of diethers, an 11,12-methylenehexadecyl moiety is ether-bound at the sn-2 position of the glycerol group, and in diethers from the second series, an anteiso pentadecyl group is ether-bound at the sn-2 position. In the third and minor series a pentadecyl moiety is ether-bound at the sn-2 position. In all series, ether-bound C14–C17 alkyl units (either n-alkyl, branched alkyl, alkylcyclohexyl or methylenealkyl) occur at the sn-1 position. The two major series are also distinguished by their δ13C values, which differ by 15–30‰ at the sites studied. Only the minor series has been previously reported in sediments. Members of the second and third series are similar to diethers containing C16–C18 iso, anteiso, and n-alkyl units present in the thermophilic bacteria Thermodesulfobacterium commune, Ammonifex degensii and Aquifex pyrophilus. The first series is unique, in that diethers possessing alkyl chains that contain a cyclopropane ring have never been reported.

Numerical modeling of carbonate crust formation at cold vent sites: significance for fluid and methane budgets and chemosynthetic biological communities

At many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane. Carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments. The range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear. Here, we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate. Starting with the conditions prevailing at a previously investigated reference site located on Hydrate Ridge, off Oregon, several parameters are systematically varied in a number of numerical experiments. These parameters include coefficients of bioturbation and bioirrigation, sedimentation rate, fluid flow velocity, methane concentration in the ascending vent fluids, and pH and saturation state at the sediment–water interface. The simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane (>50 mM) and if bioturbation coefficients are low ( 50 cm ka−1) inhibit crust formation. Bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust. Crusts only form over a rather narrow range of upward fluid flow velocities (20–60 cm a−1), which is somewhat enlarged (up to 90 cm a−1) if the overlying bottom waters are supersaturated with respect to calcite. At higher flow rates, methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment. The formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years (100–500 a). Crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport. Therefore, rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation. Due to the moderate flow rates and the slow diffusive transport, only very little methane escapes into the bottom water overlying carbonate-encrusted vent areas.

Integrated petrographic and geochemical record of hydrocarbon seepage on the Voring Plateau

Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the Vøring Plateau during the TTR-8 and TTR-10 marine expeditions. A petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids. All authigenic carbonates are depleted in 13C (−31.6‰ < δ13C < −52‰) indicating that methane is the primary source of the carbonate carbon. Furthermore, pyrite framboids are often associated with these samples, indicating that sulphate reduction is spatially coupled with methane oxidation and implying that the carbonates are formed through the anaerobic oxidation of methane. The oxygen stable isotope composition of the near-subsurface carbonates (3.1‰ < δ18O < 4.9‰) suggests a precipitation temperature very close to the one recorded on the sea floor (between −1 and 2 °C), which is consistent with their stratigraphic position, and a recent (Holocene?) age of formation. Carbonates sampled from greater depths (up to 5.5 m below the sea floor) are richer in 18O (4.6‰ < δ18O < 6.2‰), which is interpreted as a result of precipitation from an 18O-rich fluid. The occurrence of different carbonate mineral phases (aragonite, calcite, dolomite) is possibly related to varying dissolved sulphate concentrations in the diagenetic environment. Fluid inclusion microthermometry and Raman spectroscopy indicate the presence of an aqueous + hydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting in heterogeneous trapping.

Microbial Community Structure in Three Deep-Sea Carbonate Crusts

Carbonate crusts in marine environments can act as sinks for carbon dioxide. Therefore, understanding carbonate crust formation could be important for understanding global warming. In the present study, the microbial communities of three carbonate crust samples from deep-sea mud volcanoes in the eastern Mediterranean were characterized by sequencing 16S ribosomal RNA (rRNA) genes amplified from DNA directly retrieved from the samples. In combination with the mineralogical composition of the crusts and lipid analyses, sequence data were used to assess the possible role of prokaryotes in crust formation. Collectively, the obtained data showed the presence of highly diverse communities, which were distinct in each of the carbonate crusts studied. Bacterial 16S rRNA gene sequences were found in all crusts and the majority was classified as α-, γ-, and δ-Proteobacteria. Interestingly, sequences of Proteobacteria related to Halomonas and Halovibrio sp., which can play an active role in carbonate mineral formation, were present in all crusts. Archaeal 16S rRNA gene sequences were retrieved from two of the crusts studied. Several of those were closely related to archaeal sequences of organisms that have previously been linked to the anaerobic oxidation of methane (AOM). However, the majority of archaeal sequences were not related to sequences of organisms known to be involved in AOM. In combination with the strongly negative δ13C values of archaeal lipids, these results open the possibility that organisms with a role in AOM may be more diverse within the Archaea than previously suggested. Different communities found in the crusts could carry out similar processes that might play a role in carbonate crust formation.

Isotopic evidence of methane-related diagenesis in the mud volcanic sediments of the Barbados Accretionary Prism

Abstract Extensive sampling of chosen sectors of the Barbados Accretionary Prism during four French oceanographic cruises (1985–1993) evidenced the presence of abundant diagenetic carbonate deposits associated mostly with zones of active or ancient fluid venting and mud expulsion processes (mud volcanoes, mud diapirs, diapiric ridges). Diagenetic carbonates (low- and high-Mg calcite, aragonite and low-Mg dolomite) are found to bind together dead benthic communities associated to fluid venting and to cover extensive areas of the sea floor where complete lithification of both autochtonous deposits and mud volcanic products occurs. Extreme cases of early diagenesis occur on mud dome structures on the summit of the ridge where the expelled sediments have been entirely lithified for a thickness of several metres. Low δ 13 C values of most dolomites (down to −60.2‰) and of most calcites and aragonites (down to −53.07‰) show the major contribution of oxidized methane to the dissolved inorganic carbon in the water from which these minerals precipitated. In contrast, a number of authigenic carbonates have δ 13 C values close to 0‰ and possibly precipitated from bottom waters. Unusually high δ 18 O values of the methane-related authigenic carbonates (up to 7.66‰ in dolomites and up to 6.98‰ in calcites and aragonites) are interpreted as due to precipitation from 18 O-rich diagenetic fluids.

Evidence for the submarine weathering of silicate minerals in Black Sea sediments: Possible implications for the marine Li and B cycles

[1] The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low-temperature alteration of volcanic ash has been observed. We describe a low-temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (≈100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter-rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.