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Martinez-Ruiz, F, Kastner M, Gallego-Torres D, Rodrigo-Gámiz M, Nieto-Moreno V, Ortega-Huertas M.  2015.  Paleoclimate and paleoceanography over the past 20,000 yr in the Mediterranean Sea Basins as indicated by sediment elemental proxies. Quaternary Science Reviews. 107:25-46.   10.1016/j.quascirev.2014.09.018   AbstractWebsite

Marine sediments record paleoenvironmental changes over time through variations in major and trace element concentrations. The main objective of this paper is to review such changes in the Mediterranean Sea basins over the last 20 ka, using the inorganic chemistry and mineralogy of marine sediment records. Elemental ratio proxies that are mostly used are discussed, and the uncertainties involved in using them for paleoclimate and paleoceanographic reconstructions are evaluated. The focus on the Mediterranean region is based on the sensitivity of this region to global climate changes due to its semi-enclosed nature. The elemental ratios that have been particularly useful for reconstructing terrigenous inputs into the Mediterranean have been Ti/Al and Zr/Al ratios as proxies for eolian dust input, and Mg/Al, K/Al and Rb/Al ratios as proxies for fluvial input. Redox sensitive elements (e.g., U, Mo, V, Co, Ni, Cr) have provided reliable reconstructions of oxygen conditions at the time of deposition. Some of these elements are also particularly susceptible to post-depositional remobilization and record diagenetic processes instead of the original environmental signatures. Regarding productivity fluctuations, most of the paleoproductivity reconstructions are based on the abundance of barite and Ba excess algorithms. The biogeochemistry of Ba is, however, not fully understood and mechanisms for barite precipitation in the water column are not yet known. Two case studies are presented: the eastern Mediterranean sapropel S1 (deposited between 10.8 and 6.1 cal ka BP) and the westernmost Mediterranean paleoclimate record over the last 20 ka.

Rose, KK, Johnson JE, Torres ME, Hong W-L, Giosan L, Solomon EA, Kastner M, Cawthern T, Long PE, Todd Schaef H.  2014.  Anomalous porosity preservation and preferential accumulation of gas hydrate in the Andaman accretionary wedge, NGHP-01 site 17A. Marine and Petroleum Geology. 58, Part A:99-116.   10.1016/j.marpetgeo.2014.04.009   AbstractWebsite

In addition to well established properties that control the presence or absence of the hydrate stability zone, such as pressure, temperature, and salinity, additional parameters appear to influence the concentration of gas hydrate in host sediments. The stratigraphic record at Site 17A in the Andaman Sea, eastern Indian Ocean, illustrates the need to better understand the role pore-scale phenomena play in the distribution and presence of marine gas hydrates in a variety of subsurface settings. In this paper we integrate field-generated datasets with newly acquired sedimentology, physical property, imaging and geochemical data with mineral saturation and ion activity products of key mineral phases such as amorphous silica and calcite, to document the presence and nature of secondary precipitates that contributed to anomalous porosity preservation at Site 17A in the Andaman Sea. This study demonstrates the importance of grain-scale subsurface heterogeneities in controlling the occurrence and distribution of concentrated gas hydrate accumulations in marine sediments, and document the importance that increased permeability and enhanced porosity play in supporting gas concentrations sufficient to support gas hydrate formation. The grain scale relationships between porosity, permeability, and gas hydrate saturation documented at Site 17A likely offer insights into what may control the occurrence and distribution of gas hydrate in other sedimentary settings.

Solomon, EA, Spivack AJ, Kastner M, Torres ME, Robertson G.  2014.  Gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the Krishna-Godavari Basin, offshore India. Marine and Petroleum Geology. 58:233-253.   10.1016/j.marpetgeo.2014.08.020   AbstractWebsite

The National Gas Hydrate Program Expedition 01 cored ten sites in the Krishna-Godavari basin, located on the southeastern margin of India. A comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin, in situ diagenetic and metabolic reactions, and fluid migration and flow pathways. Gas hydrate was present at all of the sites cored, and in situ microbial methanogenesis leads to estimates of depth-integrated average gas hydrate saturations that are typically <5%. Deep-sourced fluid and gas migration produces gas hydrate saturations up to 68% along an isolated coarser-grained stratigraphic horizon at Site 15 and up to 41% within a fractured clay-dominated system at Site 10. Our results show that the CO2 produced through net microbial methanogenesis is effectively neutralized by silicate weathering throughout the sediment column drilled at each site (similar to 100-300 m), buffering the pore water pH and generating excess alkalinity via the same reaction sequence as continental silicate weathering. Most of the excess alkalinity produced by silicate weathering in the Krishna-Godavari basin is sequestered in Caand Fe-carbonates as a result of ubiquitous calcium release from weathering detrital silicates and dissolved Fe production within the methanogenic sediments. Formation of secondary hydrous silicates (e.g. smectite) related to incongruent primary silicate dissolution acts as a significant sink for pore water Mg, K, Li, Rb, and B. The consumption of methane through anaerobic oxidation of methane, sequestration of methane in gas hydrate, and sequestration of dissolved inorganic carbon in authigenic carbonates keeps methanogenesis as a thermodynamically feasible catabolic pathway. Our results combined with previous indications of silicate weathering in anoxic sediments in the Sea of Okhotsk, suggest that silicate weathering coupled to microbial methanogenesis should be occurring in continental margins worldwide, providing a net sink of atmospheric CO2. (C) 2014 Elsevier Ltd. All rights reserved.

Solomon, EA, Spivack AJ, Kastner M, Torres ME, Robertson G.  2014.  Gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the Krishna–Godavari Basin, offshore India. Marine and Petroleum Geology. 58, Part A:233-253.   10.1016/j.marpetgeo.2014.08.020   AbstractWebsite

The National Gas Hydrate Program Expedition 01 cored ten sites in the Krishna–Godavari basin, located on the southeastern margin of India. A comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin, in situ diagenetic and metabolic reactions, and fluid migration and flow pathways. Gas hydrate was present at all of the sites cored, and in situ microbial methanogenesis leads to estimates of depth-integrated average gas hydrate saturations that are typically <5%. Deep-sourced fluid and gas migration produces gas hydrate saturations up to 68% along an isolated coarser-grained stratigraphic horizon at Site 15 and up to 41% within a fractured clay-dominated system at Site 10. Our results show that the CO2 produced through net microbial methanogenesis is effectively neutralized by silicate weathering throughout the sediment column drilled at each site (∼100–300 m), buffering the pore water pH and generating excess alkalinity via the same reaction sequence as continental silicate weathering. Most of the excess alkalinity produced by silicate weathering in the Krishna–Godavari basin is sequestered in Ca- and Fe-carbonates as a result of ubiquitous calcium release from weathering detrital silicates and dissolved Fe production within the methanogenic sediments. Formation of secondary hydrous silicates (e.g. smectite) related to incongruent primary silicate dissolution acts as a significant sink for pore water Mg, K, Li, Rb, and B. The consumption of methane through anaerobic oxidation of methane, sequestration of methane in gas hydrate, and sequestration of dissolved inorganic carbon in authigenic carbonates keeps methanogenesis as a thermodynamically feasible catabolic pathway. Our results combined with previous indications of silicate weathering in anoxic sediments in the Sea of Okhotsk, suggest that silicate weathering coupled to microbial methanogenesis should be occurring in continental margins worldwide, providing a net sink of atmospheric CO2.

Kastner, M, Solomon EA, Harris RN, Torres ME.  2014.  Fluid origins, thermal regimes, and fluid and solute fluxes in the forearc of subduction zones. Developments in Marine Geology. Volume 7( Stein R, Blackman DK, Inagaki F, Larsen H, Eds.).:671-733.: Elsevier   10.1016/B978-0-444-62617-2.00022-0   Abstract

In this paper we present an in-depth analysis and synthesis of published and newly acquired data on the chemical and isotopic composition of forearc fluids, fluid fluxes, and the associated thermal regimes in well-studied, representative erosional and accretionary subduction zone (SZ) forearcs. Evidence of large-scale fluid flow, primarily focused along faults, is manifested by widespread seafloor venting, associated biological communities, extensive authigenic carbonate formation, chemical and isotopic anomalies in pore-fluid depth-profiles, and thermal anomalies. The nature of fluid venting seems to differ at the two types of SZs. At both, fluid and gas venting sites are primarily associated with faults. The décollement and coarser-grained stratigraphic horizons are the main fluid conduits at accretionary SZs, whereas at non-accreting and erosive margins, the fluids from compaction and dehydration reactions are to a great extent partitioned between the décollement and focused conduits through the prism, respectively. The measured fluid output fluxes at seeps are high, ∼15–40 times the amount that can be produced through local steady-state compaction, suggesting that in addition, other fluid sources or non-steady-state fluid flow must be involved. Recirculation of seawater must be an important component of the overall forearc output fluid flux in SZs. The most significant chemical and isotopic characteristics of the expelled fluids relative to seawater are: Cl dilution; sulfate, Ca, and Mg depletions; and enrichments in Li, B, Si, Sr, alkalinity, and hydrocarbon concentrations, often distinctive δ18O, δD, δ7Li, δ11B, and δ37Cl values, and variable Sr isotope ratios. These characteristics provide key insights on the source of the fluid and the temperature at the source. Based on the fluid chemistry, the most often reported source temperatures reported are 120–150 °C. We estimate a residence time of the global ocean in SZs of ∼100 Myr, about five times faster than the previous estimate of ∼500 Myr by Moore and Vrolijk, similar to the residence time of ∼90 Myr for fluids in the global ridge crest estimated by Elderfield and Schultz, and ∼3 times longer than the 20–36 Myr estimate by German and von Damm and Mottl. Based on this extrapolated fluid reflux to the global ocean, subduction zones are an important source and sink for several elements and isotopic ratios, in particular an important sink for seawater sulfate, Ca and Mg, and an important source of Li and B.

Vanneste, H, Kastner M, James RH, Connelly DP, Fisher RE, Kelly-Gerreyn BA, Heeschen K, Haeckel M, Mills RA.  2012.  Authigenic carbonates from the Darwin Mud Volcano, Gulf of Cadiz: A record of palaeo-seepage of hydrocarbon bearing fluids. Chemical Geology. 300:24-39.   10.1016/j.chemgeo.2012.01.006   AbstractWebsite

Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in C-13 (delta C-13 = -8.1 to -27.9%.) as are the pore fluids (delta C-13 = 19.1 to -28.7%.), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate delta O-18 values are as high as 5.1%, and it is most likely that the crusts formed from O-18-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are <0.09 cm yr(-1)). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift. (C) 2012 Elsevier RV. All rights reserved.

Kim, JH, Torres ME, Haley BA, Kastner M, Pohlman JW, Riedel M, Lee YJ.  2012.  The effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern Cascadia accretionary margin. Chemical Geology. 291:152-165.   10.1016/j.chemgeo.2011.10.010   Abstract

Analytical challenges in obtaining high quality measurements of rare earth elements (REEs) from small pore fluid volumes have limited the application of REEs as deep fluid geochemical tracers. Using a recently developed analytical technique, we analyzed REEs from pore fluids collected from Sites U1325 and U1329, drilled on the northern Cascadia margin during the Integrated Ocean Drilling Program (IODP) Expedition 311, to investigate the REE behavior during diagenesis and their utility as tracers of deep fluid migration. These sites were selected because they represent contrasting settings on an accretionary margin: a ponded basin at the toe of the margin, and the landward Tofino Basin near the shelf's edge. REE concentrations of pore fluid in the methanogenic zone at Sites U1325 and U1329 correlate positively with concentrations of dissolved organic carbon (DOC) and alkalinity. Fractionations across the REE series are driven by preferential complexation of the heavy REEs. Simultaneous enrichment of diagenetic indicators (DOC and alkalinity) and of REEs (in particular the heavy elements Ho to Lu), suggests that the heavy REEs are released during particulate organic carbon (POC) degradation and are subsequently chelated by DOC. REE concentrations are greater at Site U1325, a site where shorter residence times of POC in sulfate-bearing redox zones may enhance REE burial efficiency within sulfidic and methanogenic sediment zones where REE release ensues. Cross-plots of La concentrations versus Cl, Li and Sr delineate a distinct field for the deep fluids (z>75 mbsf) at Site U1329, and indicate the presence of a fluid not observed at the other sites drilled on the Cascadia margin. Changes in REE patterns, the presence of a positive Eu anomaly, and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous Crescent Terrane, located east of the drilled transect. (C) 2011 Elsevier B.V. All rights reserved.

Solomon, EA, Kastner M.  2012.  Progressive barite dissolution in the Costa Rica forearc - Implications for global fluxes of Ba to the volcanic arc and mantle. Geochimica Et Cosmochimica Acta. 83:110-124.   10.1016/j.gca.2011.12.021   AbstractWebsite

Barium concentrations were measured in pore fluids and sediments from the shallow forearc of the Costa Rica subduction zone to investigate the impact of progressive barite dissolution coupled to SO4-2 depletion on the residual sediment Ba flux to the volcanic arc and mantle. At the Costa Rica subduction zone, the entire sediment section entering the trench is underthrust beneath the prism sediments of the overriding plate. Dissolved SO4-2 concentrations measured in the reference sediment section in the incoming plate at ODP Site 1039/1253 vary between 13 and 29 mM. At ODP Site 1040/1254, 1.6 km arcward from the trench, SO4-2 is depleted in the similar to 370 m of prism sediments as well as in the upper 30 m of the underthrust sediments. This suggests that, upon subduction, SO4-2 diffusion from seawater into the underthrust sediment section ceases and the available pore fluid SO4-2 at the top of the section is consumed by active microbial SO4-2 reduction. Because the only remaining source of SO4-2 is in the underthrust sediments, the depth of SO4-2 depletion in the underthrust sediments must increase with distance from the trench. Dissolved Ba2+ concentrations in the uppermost underthrust sediments at Site 1040/1254 are several orders of magnitude greater than in the reference sediment section at Site 1039/1253, indicating intense barite dissolution coupled to SO4-2 depletion. This is corroborated by a 50% decrease in the barite content within this unit. As a result of tectonic compaction, the dissolved Ba2+ released from barite dissolution is transported seaward and reprecipitated as barite when reaching SO4-2-rich fluids. As SO4-2 depletion continues arcward, greater losses of sedimentary barite must occur in the subducting sediments. If all the barite is dissolved from the subducting sediment section, 60% of the incoming bulk sediment Ba will be distilled from the sediments in the shallow forearc. Balancing the Ba output flux with this lower input flux requires a much larger sediment component recycled to the volcanic arc than previously suggested. These results indicate that diagenetic mobilization of Ba from barite can have a profound impact on the chemical composition of sediments recycled to the arc and mantle, and should be considered in the global budget of subducted sediments. (C) 2011 Elsevier Ltd. All rights reserved.

Wheat, CG, Jannasch HW, Kastner M, Hulme S, Cowen J, Edwards KJ, Orcutt BN, Glazer B.  2011.  Fluid sampling from oceanic borehole observatories: design and methods for CORK activities (1990-2010). Proceedings of the Integrated Ocean Drilling Program. 327:36.   10.2204/iodp.proc.327.109.2011   Abstract

Subseafloor borehole observatories (“CORKs”) are currently the best mechanism by which fluids from subsurface hydrologic zones can be collected to evaluate the composition, evolution, and consequence of fluid circulation in oceanic crust. The fluid-sampling capabilities of CORKs have evolved over two decades, spanning the Ocean Drilling Program and Integrated Ocean Drilling Program. The fluid-sampling system for the original CORK design consisted of a single polytetrafluoroethylene (PTFE) tube that connected to a valve at the seafloor and ended at depth in the formation. Through successes and disappointments coupled with community desires and efforts, significant iterations of CORK design and capabilities have led to the development of a range of crustal fluid-sampling systems. These iterations continue today with the development of new borehole capabilities, sensors, and samplers. This paper discusses these developments and transitions, highlighting the pros and cons of various designs, materials, and decisions. Although the evolution of CORK design has taken years because of the infrequency of CORK deployments and sample recovery operations, we as a community are now in a position to report on groundbreaking results that will enhance our understanding of subseafloor hydrogeology, crustal evolution, geochemical fluxes, microbial ecology, and biogeochemical processes, as indicated by the wealth of work referenced herein and by the complexity and flexibility of present and future designs.

Schrum, HN, Spivack AJ, Kastner M, D'Hondt S.  2009.  Sulfate-reducing ammonium oxidation: A thermodynamically feasible metabolic pathway in subseafloor sediment. Geology. 37:939-942.   10.1130/g30238a.1   AbstractWebsite

Biogeochemical fluxes and Gibbs energies in sedimentary porewaters point to the existence of sulfate-reducing ammonium oxidation. This process has not been previously inferred in natural environments. Porewater profiles in the Bay of Bengal (Indian Ocean) demonstrate that significant ammonium disappears at the ammonium-sulfate interface. Loss of ammonium at this horizon greatly exceeds possible nitrogen demand by biomass production. In situ Gibbs energies of reaction (Delta G) in Bay of Bengal and Greenwich Bay (Rhode Island) sediments indicate that sulfate-reducing ammonium oxidation is energy yielding. Relatively small and constant but consistently negative Delta G values for this reaction in both locations match the thermodynamic signature of anaerobic microbial respiration. The Delta G results and the substantial ammonium loss suggest that sulfate-reducing ammonium oxidation occurs in Bay of Bengal sediment. The Greenwich Bay Delta G results suggest that the process may also occur in anoxic sediment where the ammonium concentration profile shows no net loss of ammonium.

Solomon, EA, Kastner M, Wheat CG, Jannasch H, Robertson G, Davis EE, Morris JD.  2009.  Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone. Earth and Planetary Science Letters. 282:240-251.   10.1016/j.epsl.2009.03.022   AbstractWebsite

Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, similar to 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, similar to 0.5 km landward of the trench, in the decollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (similar to 50%) and a subduction zone fluid originating within the forearc (similar to 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the decollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of similar to 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes. (C) 2009 Elsevier B.V. All rights reserved.

Solomon, EA, Kastner M, MacDonald IR, Leifer I.  2009.  Considerable methane fluxes to the atmosphere from hydrocarbon seeps in the Gulf of Mexico. Nature Geoscience. 2:561-565.   10.1038/ngeo574   AbstractWebsite

The fluxes of the greenhouse gas methane from many individual sources to the atmosphere are not well constrained(1). Marine geological sources may be significant(2), but they are poorly quantified and are not included in the Intergovernmental Panel on Climate Change budget(1). Previous results based on traditional indirect sampling techniques and modelling suggested bubble plumes emitted from marine seeps at depths greater than 200 m do not reach the surface mixed layer because of bubble dissolution and methane oxidation(3-5). Here we report methane concentration and isotope-depth profiles from direct submersible sampling of deepwater (550-600 m) hydrocarbon plumes in the Gulf of Mexico. We show that bubble size, upwelling flows and the presence of surfactants inhibit bubble dissolution, and that methane oxidation is negligible. Consequently, methane concentrations in surface waters are up to 1,000 times saturation with respect to atmospheric equilibrium. We estimate that diffusive atmospheric methane fluxes from individual plumes are one to three orders of magnitude greater than estimates from shallow-water seeps(6-8), greatly expanding the depth range from which methane seep emissions should be considered significant. Given the widespread occurrence of deepwater seeps, we suggest that current estimates of the global oceanic methane flux to the atmosphere(1) may be too low.

Torres, ME, Kastner M.  2009.  Data Report: Clues about carbon cycling in methane-bearing sediments using stable isotopes of the dissolved inorganic carbon, IODP Expedition 311. Proceedings of the Integrated Ocean Drilling Program. 311:4.   10.2204/iodp.proc.311.206.2009   Abstract

The isotopic characterization of carbon in the dissolved inorganic carbon (DIC) pool is fundamental for a wide array of scientific studies directly related to gas hydrate research. Here we present the DIC data from pore fluid samples recovered from the northern Cascadia accretionary margin during Integrated Ocean Drilling Program (IODP) Expedition 311. Comparison of these results with data obtained from offshore central Cascadia during Ocean Drilling Program Leg 204 provides clues on carbon cycling processes that control methane inventories and fluxes.Microbial methane production preferentially incorporates the light carbon isotope. As sediment ages, more of the original CO2 is converted to methane, leaving behind an isotopically heavier residual DIC. This DIC is progressively enriched in 13C below 200 meters below seafloor with increasing distance from the deformation front.In the shallower sections, minima in downcore δ13CDIC profiles coincide with the sulfate-methane transition (SMT) zone. Here the δ13CDIC values provide information on the metabolic pathways that consume sulfate and reveal that anaerobic oxidation of methane (AOM) is not the dominant reaction at all sites drilled. There appears to be no simple correlation between the extent of AOM, depth of the SMT, and thickness of the gas hydrate occurrence zone along the transect drilled in northern Cascadia.

Kastner, M, Claypool G, Robertson G.  2008.  Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico. Marine and Petroleum Geology. 25:860-872.   10.1016/j.marpetgeo.2008.01.022   AbstractWebsite

Pore fluids from Atwater Valley (AT 13/14) and Keathley Canyon (KC 151) in the northern Gulf of Mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values, as well as hydrocarbon geochemistry, suggesting that these widely separated localities share common deep subsurface fluid origins. Seafloor mounds with focused fluid migration pathways and inferred near-seafloor gas hydrates characterize the AT 13/14 region, whereas the KC 151 region has a bottom simulating reflector (BSR) at similar to 310 mbsf, which is rather uncommon in the Gulf of Mexico (GOM). At these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission. The geochemical results, together with the pressure core data, suggest that at the AT region methane hydrate mostly occurs in near-surface sediments at mounds, consistent with focused migration pathways. In the KC region methane hydrate mostly occurs deeper in the section, in highly fractured silty-clayey sediments from similar to 220 to 300 mbsf. The pore fluids at the AT mounds and KC 151 are characterized by higher than seawater salinity. The more saline pore fluids at the AT mound and at KC151 sites, located similar to 350 km apart, are almost chemically indistinct. Ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution. Rather, this fluid is a subsurface brine derived from Jurassic or Cenozoic evaporite formation, modified by fluid-sediment reactions, and migrated to the two sites analyzed. Despite porewater salinities elevated above that of seawater, the sediment temperatures are within the range of methane hydrate stability for each of the sites. Based on Cl- dilutions the maximum gas hydrate pore volume occupancy at the AT mound sites would be similar to 9%. At KC, Cl- concentrations in pressure cores imply that in situ hydrate is unevenly distributed, with pore volume occupancy of 1-12%. Significant variations in sulfate gradients were observed, with the sulfate-to-methane transition zone (SM17) at or near the seafloor at the AT mound sites. At AT 13#2 the well-defined SMT-Z is at similar to 8 mbsf, and at KC 151#3 it is at similar to 9 mbsf. There is no coincidence between the steepness of the sulfate gradients and the presence or depth of a BSR, suggesting that the SMT-Z interfaces are measuring different aspects of the subsurface methane hydrology. At both AT and KC the delta C-13-DIC values clearly indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed. The most negative delta C-13-DIC values obtained are -46.3%. and -49.6%. at the SMTZs at AT 13#2 and KC 151#3, respectively. (C) 2008 Published by Elsevier Ltd.

Torres, ME, Trehu AM, Cespedes N, Kastner M, Wortmann UG, Kim JH, Long P, Malinverno A, Pohlman JW, Riedel M, Collett T.  2008.  Methane hydrate formation in turbidite sediments of northern Cascadia, IODP Expedition 311. Earth and Planetary Science Letters. 271:170-180.   10.1016/j.epsl.2008.03.061   AbstractWebsite

Expedition 311 of the Integrated Ocean Drilling Program (IODP) to northern Cascadia recovered gas-hydrate bearing sediments along a SW-NE transect from the first ridge of the accretionary margin to the eastward limit of gas-hydrate stability. In this study we contrast the gas gas-hydrate distribution from two sites drilled similar to 8 km apart in different tectonic settings. At Site U1325, drilled on a depositional basin with nearly horizontal sedimentary sequences, the gas-hydrate distribution shows a trend of increasing saturation toward the base of gas-hydrate stability, consistent with several model simulations in the literature. Site U1326 was drilled on an uplifted ridge characterized by faulting, which has likely experienced some mass wasting events. Here the gas hydrate does not show a clear depth-distribution trend, the highest gas-hydrate saturation occurs well within the gas-hydrate stability zone at the shallow depth of similar to 49 mbsf. Sediments at both sites are characterized by abundant coarse-grained (sand) layers up to 23 cm in thickness, and are interspaced within fine-grained (clay and silty clay) detrital sediments. The gas-hydrate distribution is punctuated by localized depth intervals of high gas-hydrate saturation, which preferentially occur in the coarse-grained horizons and occupy up to 60% of the pore space at Site U1325 and >80% at Site U1326. Detailed analyses of contiguous samples of different lithologies show that when enough methane is present, about 90% of the variance in gas-hydrate saturation can be explained by the sand (>63 mu m) content of the sediments. The variability in gas-hydrate occupancy of sandy horizons at Site U1326 reflects an insufficient methane supply to the sediment section between 190 and 245 mbsf. (C) 2008 Elsevier B.V. All rights reserved.

Wei, W, Kastner M, Spivack A.  2008.  Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean. Earth and Planetary Science Letters. 266:90-104.   10.1016/j.epsl.2007.11.009   AbstractWebsite

Chlorine stable isotopes (delta(37) Cl) and halogen concentrations (e.g. Br/Cl) in 168 pore Fluids and 23 serpentines and other solids from three subduction zones, the Nankai Trough, Costa Rica, and Mariana Forearc, provide critical information on fluid sources, flow paths, and reaction conditions. The delta Cl-37 Values of pore fluids at the Nankai and Costa Rica subduction zones, are significantly more negative (minimum -7.8 parts per thousand, 2 sigma +/- 0.3 parts per thousand) than seawater value (O parts per thousand). At Nankai Trough, the minimum delta Cl-37 value is situated below the decollement and evolves laterally from -7.8 parts per thousand at the most arcward ODP Site 808, to -7.1 parts per thousand at Site 1174, -2 km seaward from Site 808, and to -5.8 parts per thousand at the reference Site 1173. At Costa Rica, along the decollement the minimum delta Cl-37 value evolves from -5.5%. at the most arcward ODP Site 1040/1254, to -3.2 parts per thousand at Site 1043/1255, similar to 1 km seaward, and to O%o at the reference Site 1039/1253. At both subduction zones, the Br/Cl ratios are higher than the seawater value (1.5 x 10(-3)) and also show seaward evolutions. These pore fluids originate from greater depth arcward, at >= 250 degrees C, from hydrous mineral formation that preferentially incorporates Cl-37 and excludes Br. In contrast, the delta Cl-37 values in the pore fluids at the Mariana serpentine mud volcanoes are higher than the seawater value (+0.3 parts per thousand to + 1.8 parts per thousand); and the Br/Cl ratios are lower. These pore fluid values and the high Cl concentrations with positive delta Cl-37 values (+ 1.2 to + 6.0 parts per thousand) in the serpentines, support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in Cl-37, into the fluid phase. The constancy of the ocean delta Cl-37 over the past 200 Ma suggests that the isotopically fractionated chlorine in serpentinites and the Cl exchanged in subduction zones are efficiently recycled back into seawater. If the efficiency is < 100%, the residual would be transferred to the mantle, with a maximum Cl flux between 2 to 3 x 10(17) moles/Ma that would lead to an isotopic difference between the mantle and seawater over the age of the earth on the order of a few per mil. (c) 2007 Published by Elsevier B.V.

Malinverno, A, Kastner M, Torres ME, Wortmann UG.  2008.  Gas hydrate occurrence from pore water chlorinity and downhole logs in a transect across the northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311). Journal of Geophysical Research-Solid Earth. 113   10.1029/2008jb005702   AbstractWebsite

A transect of four sites drilled by Integrated Ocean Drilling Program Expedition 311 provides an ideal data set to investigate the distribution of gas hydrates across the northern Cascadia convergent margin. We quantify gas hydrate saturation ( fraction of pore space occupied by gas hydrate) in a joint interpretation of pore water chlorinity data and downhole logs of porosity and electrical resistivity. The estimated saturation profiles define a gas hydrate occurrence zone (GHOZ), the depth interval where gas hydrates are actually found. In three of the Expedition 311 transect sites (U1326, U1325, and U1327), the top of the GHOZ systematically deepens moving landward of the deformation front of the Cascadia accretionary wedge. The farthest site from the deformation front ( U1329) shows no clear evidence of gas hydrates. We apply a simple diagenetic model to explain the observed landward deepening of the GHOZ. The model computes the methane concentration in the pore fluid for a given in situ bacterial methane production, sedimentation rate, and fluid advection velocity. Model results show that lower rates of sedimentation or fluid advection result in slower increases in methane concentration with depth and deeper tops of the GHOZ. Sedimentation rates in the Expedition 311 sites decrease landward, and fluid advection rates due to the dewatering of the accretionary wedge are expected to decrease moving landward of the deformation front as well. A combination of these two mechanisms can explain the deepening of the top of the GHOZ observed in the Expedition 311 transect sites.

Solomon, EA, Kastner M, Jannasch H, Robertson G, Weinstein Y.  2008.  Dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern Gulf of Mexico slope. Earth and Planetary Science Letters. 270:95-105.   10.1016/j.epsl.2008.03.024   AbstractWebsite

Gas hydrates outcrop on the seafloor at the Bush Hill hydrocarbon seep site in the northern Gulf of Mexico. Four newly designed fluid flux meters/chemical samplers, called the MOSQUITO, were deployed for 430 days at Bush Hill to determine how dynamic subsurface fluid flow influences gas hydrate stability and to quantify the associated methane fluxes into the ocean. Three of the flux meters were deployed adjacent to an outcropping gas hydrate mound, while the fourth monitored background conditions. The flux meter measurements reveal that the subsurface hydrology in the vicinity of the mound is complex and variable with frequent changes from downward to upward flow ranging from -161 to 273 cm/yr, and with temporal variations in the horizontal component of flow. The continuous record of fluid chemistry indicates that gas hydrate actively formed in the sediments. We propose that long periods of downward flow of seawater adjacent to gas vents (up to 4 months) are driven by local sub-pressure resulting from gas ebullition through faults and fractures due to overpressure at depth. High frequency variations in flow rates (days to weeks) are likely due to temporal changes in sediment permeability and the 3-D fluid flow field as a result of active gas hydrate and authigenic carbonate precipitation, as well as the presence of free gas. Gas hydrate formation occurred as a result of long-term emanation of CH4 at focused gas vents followed by a more diffuse intergranular methane flux. The estimated CH4 flux to the water column from focused gas vents across the Bush Hill seep is similar to 5.10(6) mol/yr. This significant flux suggests that Bush Hill and similar hydrocarbon seeps in the northwestern Gulf of Mexico may be important natural sources of methane to the ocean and possibly the atmosphere. (C) 2008 Elsevier B.V. All rights reserved.

Newman, KR, Cormier MH, Weissel JK, Driscoll NW, Kastner M, Solomon EA, Robertson G, Hill JC, Singh H, Camilli R, Eustice R.  2008.  Active methane venting observed at giant pockmarks along the U.S. mid-Atlantic shelf break. Earth and Planetary Science Letters. 267:341-352.   10.1016/j.epsl.2007.11.053   AbstractWebsite

Detailed near-bottom investigation of a series of giant, kilometer scale, elongate pockmarks along the edge of the mid-Atlantic continental shelf confirms that methane is actively venting at the site. Dissolved methane concentrations, which were measured with a commercially available methane sensor (METS) designed by Franatech GmbH mounted on an Autonomous Underwater Vehicle (AUV), are as high as 100 nM. These values are well above expected background levels (1-4 nM) for the open ocean. Sediment pore water geochemistry gives further evidence of methane advection through the seafloor. Isotopically light carbon in the dissolved methane samples indicates a primarily biogenic source. The spatial distribution of the near-bottom methane anomalies (concentrations above open ocean background), combined with water column salinity and temperature vertical profiles, indicate that methane-rich water is not present across the entire width of the pockmarks, but is laterally restricted to their edges. We suggest that venting is primarily along the top of the pockmark walls with some advection and dispersion due to local currents. The highest methane concentrations observed with the METS sensor occur at a small, circular pockmark at the southern end of the study area. This observation is compatible with a scenario where the larger, elongate pockmarks evolve through coalescing smaller pockmarks. (C) 2007 Elsevier B.V. All rights reserved.

Kastner, M, Becker K, Davis EE, Fisher AT, Jannasch HW, Solomon EA, Wheat G.  2006.  New insights into The Hydrogeology of the oceanic crust through long-term monitoring. Oceanography. 19:46-57. Abstract

IMPORTANCE OF CRUSTAL HYDROGEOLOGY AND CORKS The hydrogeology of the oceanic crust influences numerous global processes and properties, including the thermal evolution of oceanic lithosphere, crustal alteration and the chemistry of crustal fluids, the nature and significance of subseafloor microbial ecosystems, tectonic and volcanic characteristics of active margins, and the creation of hydrate and ore deposits on and below the seafloor. Understanding these processes and properties has been a fundamental goal for scientific ocean drilling for over three decades, but progress has been limited in many cases by a vexing conundrum: drilling into the seafloor often causes open exchange of formation fluids and ocean bottom waters when coring penetrates through sediments and into permeable oceanic basement (e.g., Hyndman et al., 1976; Becker et al., 1983). Such open exchange strongly perturbs in situ conditions and limits validity of borehole measurements to resolve the natural state. These perturbations have greatly limited our ability to quantify fundamental physical, chemical, and microbiological parameters that control and are controlled by fluid flow within the oceanic crust.

Solomon, E, Kastner M, Robertson G.  2006.  Data Report: Barium cycling at the Costa Rica convergent margin. Proceedings of the Ocean Drilling Program, Scientific Results. 205:22.   10.2973/   Abstract

Barium concentrations were measured on 17 pore fluid and 13 sediment samples from Sites 1253 and 1254 drilled offshore Costa Rica during Ocean Drilling Program (ODP) Leg 205. An additional 83 pore fluid and 29 sediment samples were analyzed for Ba concentrations from Sites 1039 and 1040 drilled during ODP Leg 170 offshore Costa Rica. Sites 1039/1253 and 1040/1254 are part of a transect across the Middle America Trench offshore Nicoya Peninsula. The entire incoming sediment section is being underthrust beneath the margin, providing an ideal setting to examine Ba cycling in the shallow levels of the subduction zone. Results from these analyses indicate that a significant amount of Ba is liberated from the mineral barite (BaSO4) in the uppermost hemipelagic sediments arcward of the trench. The shallow distillation of Ba may impact the amount of sedimentary Ba reaching the deeper subduction zone.

Riedel, M, Collett TS, Malone MJ, Guerin G, Akiba F, Blanc-Valleron MM, Ellis M, Hashimoto Y, Heuer V, Higashi Y, Holland M, Jackson PD, Kaneko M, Kastner M, Kim JH, Kitajima H, Long PE, Malinverno A, Myers G, Palekar LD, Pohlman J, Schultheiss P, Teichert B, Torres ME, Trehu AM, Wang J, Wortmann UG, Yoshioka H.  2006.  Stages of gas-hydrate evolution on the northern Cascadia margin. Scientific Drilling. 3:18-24.   10.2204/   AbstractWebsite
Clift, PD, Chan LH, Blusztajn J, Layne GD, Kastner M, Kelly RK.  2005.  Pulsed subduction accretion and tectonic erosion reconstructed since 2.5 Ma from the tephra record offshore Costa Rica. Geochemistry Geophysics Geosystems. 6   10.1029/2005gc000963   AbstractWebsite

[1] Tephra layers recovered by Ocean Drilling Program from the forearc and trench regions offshore the Nicoya Peninsula of Costa Rica allow the temporal evolution of the volcanic arc to be reconstructed since 2.5 Ma. Major and trace element analyses by microprobe methods reveal a dominant tholeiitic character and a provenance in the Costa Rican area. The tephra show long-term coherent variability in geochemistry. One tephra dated at 1.45 Ma shows minimum values in epsilon(Nd) and maximum Li/Y consistent with very high degrees of sediment recycling at this time. However, overall Li/Y and delta(7)Li increase with SiO(2) content, suggesting addition of heavy Li through forearc tectonic erosion and crustal assimilation. Peak values in delta(7)Li starting at 1.45 Ma and lasting similar to 0.5 m. y. indicate enhanced tectonic erosion of the forearc possibly caused by subduction of a seamount at 1.45 Ma. The tephra record indicates significant temporal variability in terms of sediment subduction, reconciling the geologic evidence for long-term tectonic erosion and geochemical evidence for recent sediment accretion in the modern Central American arc.

Lanoil, BD, La Duc MT, Wright M, Kastner M, Nealson KH, Bartlett D.  2005.  Archaeal diversity in ODP legacy borehole 892B and associated seawater and sediments of the Cascadia Margin. FEMS Microbiology Ecology. 54:167-177.   10.1016/j.femsec.2005.03.015   AbstractWebsite

The Cascadia Margin is a region of active accretionary tectonics characterized by high methane flux accompanied by the formation of sedimentary gas hydrates, carbonate nodules, and carbonate pavements. Several sediment cores have been obtained from this region by the Ocean Drilling Project (ODP), and in some cases the boreholes have been sealed off, serving as sites for long-term observatories. We characterized geochemical parameters and diversity of Archaea in one such "legacy" borehole, ODP site 892b, as well as in bottom water immediately above the borehole and in two nearby sediments. The methane concentrations in the samples varied over five orders of magnitude, from approximate to 25 to 35 nM in the bottom water to approximate to 1.4 mM in one of the sediment samples. Despite these differences, the Archaeal community in all samples was dominated by gene sequences related to the methanogenic Archaea, a finding that correlates with studies of other environments characterized by high methane flux. The archaeal phylotype richness in borehole ODP 892b was limited to two phylotypes; one specifically related to Methanosaeta spp., the other to the anaerobic methane oxidizing ANME-1 group. Although some similar groups were observed in nearby sediment and seawater samples, their archaeal phylotype richness was significantly higher than in the borehole. The possible presence of a dynamic microbial community in the Cascadia Margin sub-surface and its potential roles in methanogenesis, anaerobic oxidation of methane, and authigenic precipitation of carbonate in the Cascadia Margin are discussed. (C) 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.

Valentine, DL, Kastner M, Wardlaw GD, Wang XC, Purdy A, Bartlett DH.  2005.  Biogeochemical investigations of marine methane seeps, Hydrate Ridge, Oregon. Journal of Geophysical Research-Biogeosciences. 110   10.1029/2005jg000025   AbstractWebsite

[1] A series of biogeochemical studies were conducted at the southern summit of Hydrate Ridge, offshore Oregon. Using the submersible DSV Alvin, sediment push cores were collected from two distinct seep environments characterized by the presence of clam fields (CF) or microbial mats (MM) at the sediment-water interface; samples were also collected from a nearby reference site characterized by a barren surface at the sediment-water interface. Sediment samples from each setting were analyzed for the depth distributions of total organic carbon (concentrations, delta C-13 and Delta C-14), total sedimentary nitrogen, and microbial abundance. Pore fluids were extracted and analyzed for sulfate, alkalinity, sulfide, organic carbon, and volatile organic acids. These depth distributions clearly indicate the presence of three distinctive biogeochemical settings in the surface sediments of Hydrate Ridge, and provide the basis for a comparative biogeochemical analysis. Both CF and MM sites display properties indicating enhanced microbial activity in the subsurface, compared with the reference site. MM sites display evidence of net biomass production in the subsurface; however, a loss of sediment nitrogen relative to the reference site indicates that mineralization is also enhanced. Calculations based on the removal of nitrogen indicate that greater than 30% of autochthonous organic material is lost to enhanced mineralization in the top 23 cm of one MM site. An isotope mass balance of sediment-bound organic carbon indicates a mixed source, including methane and allochthonous organic carbon dissolved in the seep fluids. The concentrations of organic carbon dissolved in seep fluids reach values of 22 mM and provide a first indication that advective transport of dissolved organic carbon from great depth may supply an important source of energy and carbon to "methane seep'' communities.