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1.
The presence of modern methane seeps at Hydrate Ridge, offshore Oregon, provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera. A series of cores were collected from the southern summit of Hydrate Ridge in 2002. Samples were preserved and stained to determine the δ13C composition of three species of live (stained) and dead benthic foraminifera: Uvigerina peregrina, Cibicidoides mckannai, and Globobulimina auriculata. Specimens were examined under light and Scanning Electron Microscopy (SEM) and exhibit no evidence of diagenesis or authigenic carbonate precipitation. Individual living foraminifera from seep sites recorded δ13C values from −0.4‰ to −21.2‰, indicating the isotopic influence of high methane concentrations. Average δ13C values (calculated from single specimens) range from −1.28 to −5.64‰ at seep sites, and −0.81 to −0.85‰ at a control (off seep) site.Two distinct seep environments, distinguished by the presence of microbial mats or clam fields, were studied to determine environmental influences on δ13C values. Individual foraminifera from microbial mat sites exhibited more depleted δ13C values than those from clam field sites. We interpret these differences as an effect of food source and/or symbiotic microbes on foraminiferal carbon isotopic values, acting to magnify the negative δ13C values recorded via the DIC pool. No statistical difference was found between δ13C values of live vs. dead specimens. This suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions. However, a few dead specimens with extremely negative δ13C composition (<-12‰) do indicate potential evidence for an authigenic influence on the recorded δ13C composition.  相似文献   

2.
Lithium concentration and isotope data (δ7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ7Li (δ7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite-illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a−1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.  相似文献   

3.
Active and inactive carbonate chimneys from the Lost City Hydrothermal Field contain up to 0.6% organic carbon with diverse lipid assemblages. The δ13C values of total organic carbon range from −21.5‰ vs. VPDB at an extinct carbonate chimney to −2.8‰ at a 70 °C, actively venting carbonate chimney. Samples collected at locations with total organic carbon with δ13C > −15‰ also contained high abundances of isoprenoidal and nonisoprenoidal diether lipids. Samples with TOC more depleted in 13C lacked or contained lower amounts of these diethers.Isoprenoidal diethers, including sn-2 hydroxyarchaeol, sn-3 hydroxyarchaeol, and putative dihydroxyarchaeol, are likely to derive from methanogenic archaea. These compounds have δ13C values ranging from −2.9 to +6.7‰ vs. VPDB. Nonisoprenoidal diethers and monoethers are presumably derived from bacteria, and have structures similar to those produced by sulfate-reducing bacteria in culture and at cold seeps. In samples that also contained abundant hydroxyarchaeols, these diethers have δ13C values between −11.8 and +3.6‰. In samples without abundant hydroxyarchaeols, the nonisoprenoidal diethers were typically more depleted in 13C, with δ13C as low as −28.7‰ in chimneys and −45‰ in fissures.The diethers at Lost City are probably derived from hydrogen-consuming methanogens and bacteria. High hydrogen concentrations favor methanogenesis over methanotrophy and allow the concurrent growth of methanogens and sulfate-reducing bacteria. The unusual enrichment of 13C in lipids can be attributed to nearly complete consumption of bioavailable carbon in vent fluids. Under carbon-limited conditions, the isotope effects that usually lead to 13C-depletion in organic material cannot be expressed. Consequently, metabolic products such as lipids and methane have δ13C values typical of abiotic carbon.  相似文献   

4.
A large collection of fluids (54 interstitial fluids and four expelled fluids) were sampled at the Manon site, at the outer edge of the Barbados accretionary complex. These warm fluids (up to 20°C) are expelled by sub-marine (5000 mbsl) mud volcanoes consisting of diapirs (unchanneled flow) and diatremes (channeled).Chlorine stable isotope ratios of these fluids were measured by IRMS with a reproducibility of ± 0.05‰ (1σ) versus SMOC (Standard Mean Ocean Chloride).A large range of δ37Cl between −5.3‰ and +0.1‰ is observed. Data from each volcanic structure describe a mixing between seawater and a low-δ37Cl fluid. The whole set of data is interpreted as the result of a mixing between two deep components and seawater. The two deep fluids are chemically distinct (e.g., in Ca, Mg, K, Li, Sr and Br contents and Br/Cl ratio). They display low and significantly different 87Sr/86Sr ratios (0.707790 and 0.707892, respectively) and δ37Cl values (−4.51 and −5.24‰, respectively).Physicochemical processes such as mineralogical transformation, diffusion, compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative δ37Cl values. Ion filtration due to sediment compaction appears to be the more likely process to explain the negative δ37Cl values observed at the Manon site. A model for the generation of these signatures is proposed where a residual negative δ37Cl fluid reservoir is created at the bottom of the prism or the sediment pile. Further compaction/fracturing and/or dewatering of the slab may flush out these fluids and focus them towards the décollement zone. Mixing between the fluids and ultimately with seawater and water released during gas hydrate destabilizations may explain the data set within the individual cores and between the different structures.  相似文献   

5.
The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δDEPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ18OEPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ18O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δDFLUID of ∼ −79‰ and −89‰, respectively, show δDEPIDOTE values of −115‰ and −125‰. In contrast, δDEPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δDEPIDOTE from well RN-10 at Reykjanes.Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote-water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ18O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δDFLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ18OFLUID values are within range of the observed fluid isotope composition. We propose that modern δDEPIDOTE and δDFLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δDROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δDFLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock-fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.  相似文献   

6.
Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with δD values from −64‰ to −25‰. All samples are enriched in water relative to fresh basalts. The δD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO2, extracted by step-heating technique, ranges between 564 and 2823 ppm with δ13C values from −14.9‰ to −26.6‰. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with δ13C = −4.5‰ and (2) an organic compound with δ13C = −26.6‰. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO2 yield after H3PO4 attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO2 degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO2 is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when “fresh” oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO2, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ∼ −4.7‰, similar to the δ13C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 × 1012 molC/yr.  相似文献   

7.
Radiocarbon analyses of bulk carbon and individual organic compounds are presented for the hydrothermal environment of the Rebecca’s Roost vent in the southern trough of the Guaymas Basin hydrothermal field. The Δ14C values of CO2 and CH4in the hottest hydrothermal fluids (317°C) are nearly “radiocarbon dead” (−944‰ and −923‰, respectively). In contrast, the Δ14C values of sediments and individual fatty acids (−418‰ to −227‰) obtained from a bacterial mat located south of the vent site are similar to values previously reported for hydrothermal petroleum in this environment and are more depleted in 14C than overlying waters. Hydrothermal fluids moving through the sediments appear to supply 14C of intermediate age to the bacteria. This carbon may take the form of, or may be supplied by processes similar to, the generation of hydrothermal petroleum. Although the bacterial mat visibly was dominated by Beggiatoa spp., such mats are known to include numerous other species. Individual compound data show that preaged carbon is being consumed by the integrated bacterial assemblage. Values of δ13C and Δ14C indicate that petroleum-derived carbon is incorporated directly into fresh bacterial biomass. Subsequently, some of this newly synthesized material also is consumed by heterotrophs, as eukaryotic sterols from the same sample also have 14C-depleted values (Δ14C = −136‰ to −110‰). Therefore, the entire system may operate as a complex consortium to transform relict carbon back into biomass. Bacterial consumption of relict carbon occurs despite the ample supply of fresh carbon delivered from the productive, overlying water column.  相似文献   

8.
Carbon isotope fractionation factors associated with the aerobic consumption of methane (C1), ethane (C2), propane (C3), and n-butane (C4) were determined from incubations of marine sediment collected from the Coal Oil Point hydrocarbon seep field, located offshore Santa Barbara, CA. Hydrogen isotope fractionation factors for C1, C2 and C3 were determined concurrently. Fresh sediment samples from two seep areas were each slurried with sea water and treated with C1, C2, C3 or C4, or with mixtures of all four gases. Triplicate samples were incubated aerobically at 15 °C, and the stable isotope composition and headspace levels of C1-C4 were monitored over the course of the experiment. Oxidation was observed for all C1-C4 gases, with an apparent preference for C3 and C4 over C1 and C2 in the mixed-gas treatments. Fractionation factors were calculated using a Rayleigh model by comparing the δ13C and δD of the residual C1-C4 gases to their headspace levels. Carbon isotope fractionation factors (reported in ε or (α-1) × 1000 notation) were consistent between seep areas and were −26.5‰ ± 3.9 for C1, −8.0‰ ± 1.7 for C2, −4.8‰ ± 0.9 for C3 and −2.9‰ ± 0.9 for C4. Fractionation factors determined from mixed gas incubations were similar to those determined from individual gas incubations, though greater variability was observed during C1 consumption. In the case of C1 and C3 consumption, carbon isotope fractionation appears to decrease as substrate becomes limiting. Hydrogen isotope fractionation factors determined from the two seep areas differed for C1 oxidation but were similar for C2 and C3. Hydrogen isotope fractionation factors ranged from −319.9‰ to −156.4‰ for C1 incubations, and averaged −61.9‰ ± 8.3 for C2 incubations and −15.1‰ ± 1.9 for C3 incubations. The fractionation factors presented here may be applied to estimate the extent of C1-C4 oxidation in natural gas samples, and should prove useful in further studying the microbial oxidation of these compounds in the natural environment.  相似文献   

9.
High-pressure, low-temperature (HP-LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500 °C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low-T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P-T-x paths.Li concentrations (?90 ppm) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP-LT rocks are significantly higher than MORB (4-6 ppm), but similar to HP-LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP-LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP-LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5-10% of the Li in these fluids is derived from sediments.Model results predict that the dehydrated bulk ocean crust is isotopically lighter (δ7Li ? +1 ± 3‰) than the depleted mantle (∼+3.5 ± 0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC-GLOSS composition over the full range of model temperatures (50-600 °C) gives an average fluid δ7Li (∼+7 ± 5‰ 1σ) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50-260 °C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid (δ7Li = +4 ± 2.3‰ 1σ, is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (∼+5 ± 3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using δ7Li alone.  相似文献   

10.
Sixteen groundwater samples collected from production wells tapping Lower Cretaceous Nubian Sandstone and fractured basement aquifers in Sinai were analyzed for their stable isotopic compositions, dissolved noble gas concentrations (recharge temperatures), tritium activities, and 14C abundances. Results define two groups of samples: Group I has older ages, lower recharge temperatures, and depleted isotopic compositions (adjusted 14C model age: 24,000–31,000 yr BP; δ18O: − 9.59‰ to − 6.53‰; δ2H: − 72.9‰ to − 42.9‰; < 1 TU; and recharge T: 17.5–22.0°C) compared to Group II (adjusted 14C model age: 700–4700 yr BP; δ18O: − 5.89‰ to − 4.84‰; δ2H: − 34.5‰ to − 24.1‰; < 1 to 2.78 TU; and recharge T: 20.6–26.2°C). Group II samples have isotopic compositions similar to those of average modern rainfall, with larger d-excess values than Group I waters, and locally measurable tritium activity (up to 2.8 TU). These observations are consistent with (1) the Nubian Aquifer being largely recharged prior to and/or during the Last Glacial Maximum (represented by Group I), possibly through the intensification of paleowesterlies; and (2) continued sporadic recharge during the relatively dry and warmer interglacial period (represented by Group II) under conditions similar to those of the present.  相似文献   

11.
This study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The 13C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca2+‐enriched and Mg2+‐depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H2S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.  相似文献   

12.
Four seep sites located within an ∼20 km2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110 °C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated.  相似文献   

13.
We experimentally determined the boron partitioning and boron isotope fractionation between coexisting liquid and vapor in the system H2O−NaCl−B2O3. Experiments were performed along the 400 and 450°C isotherms. Pressure conditions ranged from 23 to 28 MPa at 400°C and from 38 to 42 MPa at 450°C. Boron partitions preferentially into the liquid. Its overall liquid-vapor fractionation is, however, weak: Calculated boron distribution coefficients DBliquid-vapor are < 2.5 at all run conditions. With decreasing pressure (i.e. increasing opening of the solvus) DBliquid-vapor increases along the individual isotherms. Extrapolation to salt saturated conditions yields maximum boron liquid-vapor fractionations of DBliquid-vapor = 1.8 at 450°C and DBliquid-vapor = 2.7 at 400°C. 11B preferentially fractionates into the vapor. Calculated Δ11Bvapor-liquid = {[(11B/10B)vapor - (11B/10B)liquid]/(11B/10B)NBS 951}*1000 are small and range from 0.2 (± 0.7) to 0.9 (± 0.5) ‰ at 450°C and from 0.1 (± 0.6) to 0.7 (± 0.6) ‰ at 400°C. The data indicate increasing isotopic fractionation with decreasing pressure (i.e. increasing opening of the solvus). Extrapolation to salt saturated conditions yields maximum boron isotope liquid-vapor fractionations of Δ11Bvapor-liquid = 1.5 (± 0.7) ‰ at 450°C and Δ11Bvapor-liquid = 1.3 (± 0.6) ‰ at 400°C. The weak boron isotope fractionation suggests similar trigonal speciation in liquid and vapor. Although the boron and boron isotope fractionation between liquid and vapor is only weak, mass balance calculations indicate that for high degrees of fractionation liquid-vapor phase separation in an open system can significantly alter the boron and boron isotope signature of low-salinity hydrous fluids in hydrothermal systems. Comparing the model calculations with natural oceanic hydrothermal fluids, however, indicate that other processes than fluid phase separation dominate the boron geochemistry in oceanic hydrothermal fluids.  相似文献   

14.
The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ56Fe and ± 0.25 ‰ for δ58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ56Fechondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.  相似文献   

15.
The effect of dissolved barium on biogeochemical processes at cold seeps   总被引：2，自引：0，他引：2  
A numerical model was applied to investigate and quantify the biogeochemical processes fueled by the expulsion of barium and methane-rich fluids in the sediments of a giant cold-seep area in the Derugin Basin (Sea of Okhotsk). Geochemical profiles of dissolved Ba2+, Sr2+, Ca2+, SO42−, HS−, DIC, I− and of calcium carbonate (CaCO3) were fitted numerically to constrain the transport processes and the kinetics of biogeochemical reactions. The model results indicate that the anaerobic oxidation of methane (AOM) is the major process proceeding at a depth-integrated rate of 4.9 μmol cm−2 a−1, followed by calcium carbonate and strontian barite precipitation/dissolution processes having a total depth-integrated rate of 2.1 μmol cm−2 a−1. At the low seepage rate prevailing at our study site (0.14 cm a−1) all of the rising barium is consumed by precipitation of barite in the sedimentary column and no benthic barium flux is produced. Numerical experiments were run to investigate the response of this diagenetic environment to variations of hydrological and biogeochemical conditions. Our results show that relatively low rates of fluid flow (<∼5 cm a−1) promote the dispersed precipitation of up to 26 wt% of barite and calcium carbonate throughout the uppermost few meters of the sedimentary column. Distinct and persistent events (several hundreds of years long) of more vigorous fluid flow (from 20-110 cm a−1), instead, result in the formation of barite-carbonate crusts near the sediment surface. Competition between barium and methane for sulfate controls the mineralogy of these sediment precipitates such that at low dissolved methane/barium ratios (<4-11) barite precipitation dominates, while at higher methane/barium ratios sulfate availability is limited by AOM and calcium carbonate prevails. When seepage rates exceed 110 cm a−1, barite precipitation occurs at the seafloor and is so rapid that barite chimneys form in the water column. In the Derugin Basin, spectacular barite constructions up to 20 m high, which cover an area of roughly 22 km2 and contain in excess of 5 million tons of barite, are built through this process. In these conditions, our model calculates a flux of barium to the water column of at least 20 μmol cm−2 a−1. We estimate that a minimum of 0.44 × 106 mol a−1 are added to the bottom waters of the Derugin Basin by cold seep processes, likely affecting the barium cycle in the Sea of Okhotsk.  相似文献   

16.
Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or near-arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic-hydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273-285 °C) and major element compositions, low dissolved CO2 concentrations (4.4 mmol/kg) and high measured pH (4.2-4.9 at 25 °C). Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358 °C) fluid. All PACMANUS fluids are characterized by negative δDH2O values, in contrast to positive values at Vienna Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured pH (25 °C) values (∼2.6-2.7), high endmember CO2 (up to 274 mmol/kg) and negative δ34SH2S values (down to −2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from evolved magma. Dissolved CO2 at PACMANUS is more enriched in 13C (−4.1‰ to −2.3‰) than Vienna Woods (−5.2‰ to −5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li, K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of PACMANUS/NE Pual fluids has taken place as a result of seawater ingress into the upflow zone. Consistently high measured Mg concentrations as well as trends of increasingly non-conservative SO4 behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to sulfide deposition and secondary acidity production. Several low temperature (?80 °C) fluids at PACMANUS/NE Pual also show evidence for anhydrite dissolution and water-rock interaction (fixation of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from silicic magma, phase separation/segregation and seawater ingress into upflow zones.  相似文献   

17.
The mineral barite (BaSO4) accommodates calcium in its crystal lattice, providing an archive of Ca-isotopes in the highly stable sulfate mineral. Holocene marine (pelagic) barite samples from the major ocean basins are isotopically indistinguishable from each other (δ44/40Ca = −2.01 ± 0.15‰) but are different from hydrothermal and cold seep barite samples (δ44/40Ca = −4.13 to −2.72‰). Laboratory precipitated (synthetic) barite samples are more depleted in the heavy Ca-isotopes than pelagic marine barite and span a range of Ca-isotope compositions, Δ44/40Ca = −3.42 to −2.40‰. Temperature, saturation state, , and aCa2+/aBa2+ each influence the fractionation of Ca-isotopes in synthetic barite; however, the fractionation in marine barite samples is not strongly related to any measured environmental parameter. First-principles lattice dynamical modeling predicts that at equilibrium Ca-substituted barite will have much lower 44Ca/40Ca than calcite, by −9‰ at 0 °C and −8‰ at 25 °C. Based on this model, none of the measured barite samples appear to be in isotopic equilibrium with their parent solutions, although as predicted they do record lower δ44/40Ca values than seawater and calcite. Kinetic fractionation processes therefore most likely control the extent of isotopic fractionation exhibited in barite. Potential fractionation mechanisms include factors influencing Ca2+ substitution for Ba2+ in barite (e.g. ionic strength and trace element concentration of the solution, competing complexation reactions, precipitation or growth rate, temperature, pressure, and saturation state) as well as nucleation and crystal growth rates. These factors should be considered when investigating controls on isotopic fractionation of Ca2+ and other elements in inorganic and biogenic minerals.  相似文献   

18.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.  相似文献   

19.
Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present.CL photomicrographs of quartz overgrowths in the Brae Formation sandstones show three cement zones (A, B and C) which can be related to different oxygen isotope compositions: (1) the earliest, and thinnest, zone A (homogeneous CL pattern with probable δ18O values between +23‰ and +26‰—direct measurements were not possible) precipitated in the sandstones at temperatures <60 °C; (2) the second zone B (complex CL pattern and directly measured δ18O values between +15‰ and +18‰) precipitated in the sandstones most likely between 70 and 90 °C; (3) the third zone C (homogeneous CL pattern and directly measured δ18O values between +16‰ and +22‰) precipitated in the sandstones most likely at temperatures >90 °C. Calculated oxygen isotope compositions of pore waters show that zone A quartz cements, and enclosing concretionary calcite, precipitated from a meteoric-type fluid (∼−7‰) during shallow burial (<1.5 km). Zone B quartz cements precipitated from fluids which evolved in composition from a meteoric-type fluid (δ18O −7‰) to a more 18O-enriched fluid (δ18O −4‰) as burial continued to ∼3.0 km. Data from zone C quartz cements are consistent with further fluid evolution from δ18O −4‰ to basinal-type fluids with δ18O similar to the present-day formation water oxygen isotope composition (+0.6‰ at 4.0 km burial). A similar pore water evolution can be derived for all three reservoirs studied, indicating that hydrogeologic evolution was similar across sandstones of the whole Brae Formation.The quartz cement zones observed in the Brae Formation sandstones, and the pore water history derived for the area studied, is analogous to published petrographic and pore water evolution data from the nearby Brent Group reservoirs and from reservoirs located in the Haltenbanken area on the Atlantic margin offshore Norway. Considering quartz cement is a major porosity-occluding phase in many reservoir sandstones, and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites, the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally.  相似文献   

20.
The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ7Li values, partly close to those of hydrothermal vent fluids (δ7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/86Sr ratios (∼0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/86Sr ratios (∼0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.  相似文献   

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