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• 1. [Article] Data report : composition of clay minerals from hemiplegic sediments at Hydrate Ridge, Cascadia subduction zone

  This report describes the results of semiquantitative analysis of clay mineral composition by X-ray diffraction. The samples consist of hemipelagic mud and mudstone cored from Hydrate Ridge during Leg ...

  Citation × Citation Citation Abstract Copy Title: Data report : composition of clay minerals from hemiplegic sediments at Hydrate Ridge, Cascadia subduction zone Author: Torres, Marta E., Underwood, Michael This report describes the results of semiquantitative analysis of clay mineral composition by X-ray diffraction. The samples consist of hemipelagic mud and mudstone cored from Hydrate Ridge during Leg 204 of the Ocean Drilling Program. We analyzed oriented aggregates of the clay-sized fractions (<2 μm) to estimate relative percentages of smectite, illite, and chlorite (+ kaolinite). For the most part, stratigraphic variations in clay mineral composition are modest and there are no significant differences among the seven sites that were included in the study. On average, early Pleistocene to Holocene trench slope and slope basin deposits contain 29% smectite, 31% illite, and 40% chlorite (+ kaolinite). Late Pliocene to early Pleistocene strata from the underlying accretionary prism contain moderately larger proportions of smectite with average values of 38% smectite, 27% illite, and 35% chlorite (+ kaolinite). There is no evidence of clay mineral diagenesis at the depths sampled. The expandability of smectite is, on average, equal to 64%, and there are no systematic variations in expandability as a function of burial depth or depositional age. The absence of clay mineral diagenesis is consistent with the relatively shallow sample depths and corresponding maximum temperatures of only 24°–33°C. Title: Data report : composition of clay minerals from hemiplegic sediments at Hydrate Ridge, Cascadia subduction zone Author: Torres, Marta E., Underwood, Michael This report describes the results of semiquantitative analysis of clay mineral composition by X-ray diffraction. The samples consist of hemipelagic mud and mudstone cored from Hydrate Ridge during Leg 204 of the Ocean Drilling Program. We analyzed oriented aggregates of the clay-sized fractions (<2 μm) to estimate relative percentages of smectite, illite, and chlorite (+ kaolinite). For the most part, stratigraphic variations in clay mineral composition are modest and there are no significant differences among the seven sites that were included in the study. On average, early Pleistocene to Holocene trench slope and slope basin deposits contain 29% smectite, 31% illite, and 40% chlorite (+ kaolinite). Late Pliocene to early Pleistocene strata from the underlying accretionary prism contain moderately larger proportions of smectite with average values of 38% smectite, 27% illite, and 35% chlorite (+ kaolinite). There is no evidence of clay mineral diagenesis at the depths sampled. The expandability of smectite is, on average, equal to 64%, and there are no systematic variations in expandability as a function of burial depth or depositional age. The absence of clay mineral diagenesis is consistent with the relatively shallow sample depths and corresponding maximum temperatures of only 24°–33°C. Close

• 2. [Article] Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance

  Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and ...

  Citation × Citation Citation Abstract Copy Title: Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance Author: Torres, Marta E., Tomaru, H., Teichert, B. M. A., Borowski, W., Tréhu, Anne Martine Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the me´lange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins. Title: Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance Author: Torres, Marta E., Tomaru, H., Teichert, B. M. A., Borowski, W., Tréhu, Anne Martine Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the me´lange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins. Close

• 3. [Article] The effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern Cascadia accretionary margin

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier ...

  Citation × Citation Citation Abstract Copy Title: The effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern Cascadia accretionary margin Author: Haley, Brian A., Pohlman, John W., Kim, Ji-Hoon, Lee, Young-Joo, Riedel, Michael, Kastner, Miriam, Torres, Marta E. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.elsevier.com/. Title: The effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern Cascadia accretionary margin Author: Haley, Brian A., Pohlman, John W., Kim, Ji-Hoon, Lee, Young-Joo, Riedel, Michael, Kastner, Miriam, Torres, Marta E. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.elsevier.com/. Close

• 4. [Article] Stable isotopes (¹⁸O/¹⁶O) and ⁸⁷Sr/⁸⁶Sr ratios in pore fluids of the Chile Triple Junction accretionary prism : implications for diagenesis and fluid migration

  Oxygen and strontium isotopic composition of sedimentary pore fluids obtained by drilling at the Chile Triple Junction have been used to obtain information on the diagenesis and transport mechanisms of ...

  Citation × Citation Citation Abstract Copy Title: Stable isotopes (¹⁸O/¹⁶O) and ⁸⁷Sr/⁸⁶Sr ratios in pore fluids of the Chile Triple Junction accretionary prism : implications for diagenesis and fluid migration Author: Dia, Aline N., Torres, Marta E., Zheng, Yan, Froelich, Philip N. Oxygen and strontium isotopic composition of sedimentary pore fluids obtained by drilling at the Chile Triple Junction have been used to obtain information on the diagenesis and transport mechanisms of fluids. Samples were collected from three sites located on an east-west transect of the pre-collision zone (Sites 859, 860, and 861) and from Site 863 located at the base of the trench slope at the point where the Chile Ridge is being subducted. Closer to the toe of the wedge (Sites 859 and 860), non-steady state depth profiles were obtained, whereas linear chemical and isotopic profiles at the upper trench slope (Site 861) are indicative of diffusion of solutes at this site. The nature of fluids from the sites located in the transect prior to ridge subduction is quite different from that observed after ridge subduction (Site 863). Even though in all cases oxygen and strontium isotopic fingerprint indicates alteration of volcanogenic sequences to form authigenic clay mineral, the reactions that modified the chemical composition at Sites 859, 860, and 861 occurred at low temperatures, while high temperature reactions are responsible for the isotopic composition of the fluids at Site 863. At this later site, the ¹⁸O of pore fluids show values as high as 0.7‰ SMOW and a ⁸⁷Sr/⁸⁶Sr ratio as low as 0.706 in the interval from 400 to 500 mbsf, a sediment section characterized by the presence of a cementation front and steep chemical gradients in the pore fluids. These data suggest that the basal fluids (>400 mbsf) at this site are likely to have been influenced by the hydrothermal reactions occurring at high temperatures. Title: Stable isotopes (¹⁸O/¹⁶O) and ⁸⁷Sr/⁸⁶Sr ratios in pore fluids of the Chile Triple Junction accretionary prism : implications for diagenesis and fluid migration Author: Dia, Aline N., Torres, Marta E., Zheng, Yan, Froelich, Philip N. Oxygen and strontium isotopic composition of sedimentary pore fluids obtained by drilling at the Chile Triple Junction have been used to obtain information on the diagenesis and transport mechanisms of fluids. Samples were collected from three sites located on an east-west transect of the pre-collision zone (Sites 859, 860, and 861) and from Site 863 located at the base of the trench slope at the point where the Chile Ridge is being subducted. Closer to the toe of the wedge (Sites 859 and 860), non-steady state depth profiles were obtained, whereas linear chemical and isotopic profiles at the upper trench slope (Site 861) are indicative of diffusion of solutes at this site. The nature of fluids from the sites located in the transect prior to ridge subduction is quite different from that observed after ridge subduction (Site 863). Even though in all cases oxygen and strontium isotopic fingerprint indicates alteration of volcanogenic sequences to form authigenic clay mineral, the reactions that modified the chemical composition at Sites 859, 860, and 861 occurred at low temperatures, while high temperature reactions are responsible for the isotopic composition of the fluids at Site 863. At this later site, the ¹⁸O of pore fluids show values as high as 0.7‰ SMOW and a ⁸⁷Sr/⁸⁶Sr ratio as low as 0.706 in the interval from 400 to 500 mbsf, a sediment section characterized by the presence of a cementation front and steep chemical gradients in the pore fluids. These data suggest that the basal fluids (>400 mbsf) at this site are likely to have been influenced by the hydrothermal reactions occurring at high temperatures. Close

• 5. [Article] Feeding methane vents and gas hydrate deposits at south Hydrate Ridge

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Feeding methane vents and gas hydrate deposits at south Hydrate Ridge Author: Torres, Marta E., Liu, Xiaoli, Riedel, Michael, Bangs, Nathan Lawrence Bailey, Johnson, Joel E., Tréhu, Anne Martine, Flemings, Peter B., Gracia, Eulalia, Liu, C. S., Chevallier, Johanna Copyrighted by American Geophysical Union. Title: Feeding methane vents and gas hydrate deposits at south Hydrate Ridge Author: Torres, Marta E., Liu, Xiaoli, Riedel, Michael, Bangs, Nathan Lawrence Bailey, Johnson, Joel E., Tréhu, Anne Martine, Flemings, Peter B., Gracia, Eulalia, Liu, C. S., Chevallier, Johanna Copyrighted by American Geophysical Union. Close

• 6. [Article] Elemental and isotopic abundance of noble gases in formation fluids recovered in situ from the Chile Triple Juction

  Sedimentary pore fluids from the Chile Triple Junction were sampled in situ, and analyzed for their noble gas composition. ³He/⁴He and ⁴He/Ne ratios were used to evaluate the fraction of helium in the ...

  Citation × Citation Citation Abstract Copy Title: Elemental and isotopic abundance of noble gases in formation fluids recovered in situ from the Chile Triple Juction Author: Torres, Marta E., Winckler, Gisela, Suckow, Axel, Bayer, Reinhold, Froelich, Philip N. Sedimentary pore fluids from the Chile Triple Junction were sampled in situ, and analyzed for their noble gas composition. ³He/⁴He and ⁴He/Ne ratios were used to evaluate the fraction of helium in the samples that originated from mantle and radiogenic sources. The results show no primordial helium in samples recovered from Sites 860 and 861. In contrast, a primordial component is present in two samples from a depth interval in Site 863 that corresponds to a zone of tectonic discontinuity. A decrease in the concentration of dissolved K, Sr, Mg, and SO₄ in this interval is consistent with the presence of a fluid which has been altered by basalt-seawater reactions. These results indicate lateral fluid migration from a hydrothermal source which is probably associated with the recently subducted Chile Ridge axis. Title: Elemental and isotopic abundance of noble gases in formation fluids recovered in situ from the Chile Triple Juction Author: Torres, Marta E., Winckler, Gisela, Suckow, Axel, Bayer, Reinhold, Froelich, Philip N. Sedimentary pore fluids from the Chile Triple Junction were sampled in situ, and analyzed for their noble gas composition. ³He/⁴He and ⁴He/Ne ratios were used to evaluate the fraction of helium in the samples that originated from mantle and radiogenic sources. The results show no primordial helium in samples recovered from Sites 860 and 861. In contrast, a primordial component is present in two samples from a depth interval in Site 863 that corresponds to a zone of tectonic discontinuity. A decrease in the concentration of dissolved K, Sr, Mg, and SO₄ in this interval is consistent with the presence of a fluid which has been altered by basalt-seawater reactions. These results indicate lateral fluid migration from a hydrothermal source which is probably associated with the recently subducted Chile Ridge axis. Close

• 7. [Article] Geological and geochemical contraints on the isotopic composition of interstitial waters from the Hydrate Ridge region, Cascadia Continental margin

  The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, ...

  Citation × Citation Citation Abstract Copy Title: Geological and geochemical contraints on the isotopic composition of interstitial waters from the Hydrate Ridge region, Cascadia Continental margin Author: Torres, Marta E., Matsumoto, R., Tomaru, A. M., Borowski, Walter S. The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, the dissolved Cl- concentrations and δD and δ¹⁸O values of the interstitial water reflect changes in the salinity and the isotopic compositions of seawater from the Last Glacial Maximum to the present. The presence of disseminated gas hydrates, which is well identified by discrete low Cl- anomalies within the gas hydrate stability zone, is accompanied by high δD and δ¹⁸O values of the freshened fluids. This is consistent with incorporation of heavy isotopes into the gas hydrate lattice, which is also apparent in the signals observed at the ridge summit. Here, massive gas hydrate formation in the upper 20 meters below seafloor leads the formation of brines with dissolved Cl- concentrations as high as 1400 mM. The interstitial waters sampled near massive gas hydrates at the ridge summit are extremely depleted in D and ¹⁸O. Clay mineral dehydration within the deep prism results in a progressive decrease in Cl¯ and δD with depth. Dehydration temperature estimates based on those data likely suggest a progressive increase in the temperature of isotopic fractionation between clay and water with distance from the prism toe. The oxygen isotope data probably reflect the combined effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement; however, there are not enough data to constrain the relative contribution of these processes to the observed signals. Title: Geological and geochemical contraints on the isotopic composition of interstitial waters from the Hydrate Ridge region, Cascadia Continental margin Author: Torres, Marta E., Matsumoto, R., Tomaru, A. M., Borowski, Walter S. The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, the dissolved Cl- concentrations and δD and δ¹⁸O values of the interstitial water reflect changes in the salinity and the isotopic compositions of seawater from the Last Glacial Maximum to the present. The presence of disseminated gas hydrates, which is well identified by discrete low Cl- anomalies within the gas hydrate stability zone, is accompanied by high δD and δ¹⁸O values of the freshened fluids. This is consistent with incorporation of heavy isotopes into the gas hydrate lattice, which is also apparent in the signals observed at the ridge summit. Here, massive gas hydrate formation in the upper 20 meters below seafloor leads the formation of brines with dissolved Cl- concentrations as high as 1400 mM. The interstitial waters sampled near massive gas hydrates at the ridge summit are extremely depleted in D and ¹⁸O. Clay mineral dehydration within the deep prism results in a progressive decrease in Cl¯ and δD with depth. Dehydration temperature estimates based on those data likely suggest a progressive increase in the temperature of isotopic fractionation between clay and water with distance from the prism toe. The oxygen isotope data probably reflect the combined effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement; however, there are not enough data to constrain the relative contribution of these processes to the observed signals. Close

• 8. [Article] Leg 204 synthesis : gas hydrate distribution and dynamics in the Central Cascadia accretionary complex

  Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was the first drilling expedition dedicated to understanding gas hydrate processes in accretionary complexes ...

  Citation × Citation Citation Abstract Copy Title: Leg 204 synthesis : gas hydrate distribution and dynamics in the Central Cascadia accretionary complex Author: Torres, Marta E., Bohrmann, G., Colwell, Frederick, Tréhu, Anne Martine Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was the first drilling expedition dedicated to understanding gas hydrate processes in accretionary complexes and provided a testbed for a number of different techniques for estimating the gas hydrate content of sediments. It was also the first time that (1) digital infrared scans of core temperature were systematically recorded for all cores from within or near the gas hydrate stability zone, facilitating identification of gas hydrate samples for further study, (2) hydrate-bearing cores were recovered and logged at in situ pressure, and (3) ODP allowed acquisition of logging-while-drilling data prior to coring at a given site, providing an initial comprehensive estimate of gas hydrate distribution that was used to design the subsequent coring program. Gas hydrate estimates based on a variety of geophysical and geochemical techniques indicate a heterogeneous distribution of gas hydrate, which results in part because of two distinct regimes for delivery of gas to the gas hydrate stability zone. In the “reaction regime,” which is pervasive throughout the study region, the average gas hydrate content of the sediments is relatively low (2%–8% of the pore space), no gas hydrate is present in the upper ~30 meters below seafloor (mbsf) because the methane content of the pore water is below saturation, and the fine-scale distribution of gas hydrate depends strongly on lithology. Superimposed on the reaction regime is a “transport-dominated regime” in which gas is focused into a stratigraphically controlled conduit and is transported as free gas to the structural summit. At the summit, high gas pressure drives free gas into and through the gas hydrate stability zone, resulting in a shallow deposit in which gas hydrate comprises ~25% of the total sediment volume to a depth of ~25 mbsf. Geochemical data indicate that most of the gas that forms the summit deposit has migrated from greater depth and has either a thermogenic or altered biogenic character, and modeling suggests that abundant free gas is needed to form gas hydrate in these conditions. Although this deposit contains only ~2% of the estimated total volume of methane trapped in gas hydrate within the study region, it may be particularly susceptible to destabilization in response to oceanographic change. Title: Leg 204 synthesis : gas hydrate distribution and dynamics in the Central Cascadia accretionary complex Author: Torres, Marta E., Bohrmann, G., Colwell, Frederick, Tréhu, Anne Martine Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was the first drilling expedition dedicated to understanding gas hydrate processes in accretionary complexes and provided a testbed for a number of different techniques for estimating the gas hydrate content of sediments. It was also the first time that (1) digital infrared scans of core temperature were systematically recorded for all cores from within or near the gas hydrate stability zone, facilitating identification of gas hydrate samples for further study, (2) hydrate-bearing cores were recovered and logged at in situ pressure, and (3) ODP allowed acquisition of logging-while-drilling data prior to coring at a given site, providing an initial comprehensive estimate of gas hydrate distribution that was used to design the subsequent coring program. Gas hydrate estimates based on a variety of geophysical and geochemical techniques indicate a heterogeneous distribution of gas hydrate, which results in part because of two distinct regimes for delivery of gas to the gas hydrate stability zone. In the “reaction regime,” which is pervasive throughout the study region, the average gas hydrate content of the sediments is relatively low (2%–8% of the pore space), no gas hydrate is present in the upper ~30 meters below seafloor (mbsf) because the methane content of the pore water is below saturation, and the fine-scale distribution of gas hydrate depends strongly on lithology. Superimposed on the reaction regime is a “transport-dominated regime” in which gas is focused into a stratigraphically controlled conduit and is transported as free gas to the structural summit. At the summit, high gas pressure drives free gas into and through the gas hydrate stability zone, resulting in a shallow deposit in which gas hydrate comprises ~25% of the total sediment volume to a depth of ~25 mbsf. Geochemical data indicate that most of the gas that forms the summit deposit has migrated from greater depth and has either a thermogenic or altered biogenic character, and modeling suggests that abundant free gas is needed to form gas hydrate in these conditions. Although this deposit contains only ~2% of the estimated total volume of methane trapped in gas hydrate within the study region, it may be particularly susceptible to destabilization in response to oceanographic change. Close