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doi:10.2204/iodp.proc.314315316.134.2009

Inorganic geochemistry

The main objectives of the geochemical program at Site C0006 are to

• Characterize in situ biogeochemical reactions;
• Constrain in situ inorganic diagenetic reactions with depth;
• Identify potential deep sourced fluids within fault zones and other permeable horizons, as well as fluid-sediment reactions at the source; and
• Constrain the subsurface hydrology, including fluid flow pathways and possible transport mechanisms.

A total of 71 whole-round samples were collected for interstitial water analyses at Site C0006 (2 samples from Hole C0006C, 2 from Hole C0006D, 46 from Hole C0006E, and 21 from Hole C0006F). Whole-round lengths ranged from 10 to 42 cm with larger subsamples collected from cores recovered deeper within the hole where the sediments were more consolidated. There was no seafloor camera or ROV deployed at this site (see “Operations”), thus the mudline was targeted blindly. The first two holes failed to target the mudline, and therefore the exact depths of the four samples collected in Holes C0006C and C0006D are unknown. Based on sulfate values of these samples (Table T10), Core 316-C0006C-1H likely overpenetrated the sediment/​water interface by >10 m and Core 316-C0006D-1H likely missed the mudline by ~8 m. The mudline was successfully recovered in Hole C0006E.

Samples were collected at a spatial resolution of two per core in the uppermost 30 m in Hole C0006E to define the sulfate-methane transition (SMT) and to identify and quantify the anaerobic oxidation of methane (AOM) magnitude at this site, as well as for future geochemical and microbiological studies. Below Core 316-C0006E-3H, one sample was taken per core when recovery was sufficient. The geochemical objectives below the SMT in Holes C0006E and C0006F were to identify the main in situ diagenetic reactions in the sediment section and to identify any exotic/​deeply sourced fluids along the fault zones. This was done through analysis of dissolved elements that reflect inorganic fluid-rock reactions or microbially mediated reactions. Elemental concentrations and δ¹⁸O isotope ratios are reported in Tables T10, T11, T12, T13, and T14 and plotted as a function of depth in Figures F37, F38, F39, F40, F41, F42, and F43. Because of the brecciated nature of some of the sedimentary material in Holes C0006E and C0006F (see “Structural geology”), some cores were extremely difficult to clean prior to processing. This is observed in elevated SO₄ concentrations in Table T12, which show that a significant number of interstitial water samples were contaminated with surface seawater during drilling operations. All of the contaminated samples were collected below the SMT where in situ SO₄ is expected to be zero. These samples were corrected for drill water contamination based on measured SO₄ concentrations using the procedure and equations outlined in the “Inorganic geochemistry” section of the “Expedition 316 methods” chapter. Uncorrected concentrations of all of the elements analyzed are presented in Tables T12, T13, and T14, and corrected values are presented in Tables T10 and T11. Only corrected values are presented in the figures.

Salinity, chloride, and sodium

Pore fluid salinity varies from 34.4 at 1.3 m CSF to 29.50 at 571.2 m CSF (Fig. F37). Salinity at 571.3 m CSF (near the base of the hole) is ~16% less than modern seawater value. The pore fluid salinity profile can be divided into two zones. (1) The first zone extends from the seafloor to the base of Subunit IIA and is characterized by a steady decrease from near-seawater value to 32.67 at the base of Unit I followed by an increase to 33.66 at ~100 m CSF (Fig. F37A). (2) Below this depth pore fluid salinity decreases with minor variations to 31.83 at 400 m CSF (base of Subunit IIC). From ~400 m CSF to TD, salinity continues to decrease but is more variable. The salinity decrease in the upper ~20 m of the sediment section reflects active sulfate reduction and subsequent precipitation of authigenic carbonates consuming dissolved SO₄, Ca, and Mg within this interval (Fig. F38A, F38C, F38D). The drop in salinity at the top of Subunit IIB at 76.4 m CSF coincides with sharp minima in both Cl and Na. The decrease in salinity from ~400 m CSF to ~603 m CSF (TD) coincides with decreasing Cl, Na, K, Rb, and Mg.

Pore fluid chloride varies, ranging from 518 to 571 mM, and the profile can be divided into three distinct zones:

1. The first zone extends from the sediment/​water interface to 100.1 m CSF. Cl in this zone initially decreases from 562 to 558 mM at 1.3 to 12.2 m CSF and then increases to 571 mM (~2.1% greater than modern seawater value) at 100.1 m CSF. Superimposed on this increase is a sharp decrease in Cl at 76.4 m CSF, coincident with a sharp decrease in Na (Fig. F37B, F37C).
2. The second zone of the Cl profile extends from 100.1 to 534.2 m CSF and is characterized by steadily decreasing Cl with local minor variations (Fig. F37B). In this zone, a Cl minimum of 534 mM at 292.0 m CSF coincides with a Na minimum. A Cl peak of 562 mM at 534.2 m CSF is coincident with a fault zone (see “Structural geology”).
3. The third zone extends from 534 m CSF to the base of the hole at 590.2 m CSF and is characterized by steadily decreasing Cl to ~7% less than modern seawater value.

The slow increase in Cl from 12.2 to 100.1 m CSF may be the result of hydrous silicate formation during volcanic ash alteration or evidence of active gas hydrate formation in the upper sediment column. This slight Cl increase could also be a manifestation of trapped interglacial seawater, which would suggest that sufficient time has not yet passed for the signal to diffuse. During core recovery, gas hydrates dissociate and release freshwater into the pore spaces, thus diluting pore fluid Cl. Two negative excursions in the Cl profile at 77 and 292 m CSF, coincident with excursions in salinity, clearly indicate gas hydrate dissociation during core recovery, with Cl values in these horizons being ~6% and 5% of seawater value, respectively.

The Na profile mimics the Cl profile and increases to ~300 m CSF, including a local minimum of 463 mM (Fig. F37C) at 77 m CSF. Another local Na minimum of 463 mM (Fig. F37C) at 292 m CSF is coincident with a Cl minimum. Below ~300 m CSF, Na decreases to TD with minor local minima and maxima. Two Na minima at 77 and 292 m CSF probably reflect localized gas hydrate dissociation during core recovery, which is also corroborated by the constant Na/Cl ratio within these intervals (Fig. F37D). The general decrease in Na from ~300 m CSF to TD also mimics the Cl profile (Fig. F37D).

Pore fluid constituents controlled by microbially mediated reactions

Sulfate and alkalinity

Sulfate decreases linearly from 25.2 mM at 1.3 m CSF to 2.14 mM at 7.9 m CSF (Fig. F38A) and is completely depleted by 12.2 m CSF. A concomitant increase in headspace methane concentration at ~8 m CSF (see “Organic geochemistry”) constrains the SMT to ~8–12 m CSF (Fig. F38F). The SMT depth is shallower at this site than at Site C0004 upslope, where the SMT is located at 16–20 m CSF. The shallower SMT at Site C0006 could be the manifestation of a higher methane flux or more intense sulfate reduction rates. Between the SMT and ~534 m CSF, sulfate remains depleted with occasional peaks reflecting drilling-induced contamination (Table T12). Results for samples from the sulfate depletion zone were corrected for drilling contamination based on measured SO₄ concentrations and are presented in Tables T10 and T11. All of these samples were highly brecciated. Below ~534 m CSF, dissolved SO₄ increases downhole (Fig. F38A). These units were highly consolidated with the exception of Core 316-C0006F-16R (534.2 m CSF), which was brecciated and from a fault zone. Thus, the outside of the interstitial water whole rounds were easy to clean, which would suggest the increase in sulfate values at the base of the hole reflects detectable in situ pore fluid sulfate. However, methane values remain elevated within this interval (see “Organic geochemistry”) and alkalinity remains constant, suggesting the increase reflects sampling artifacts associated with drill water contamination. To be conservative, SO₄ values of the deepest six samples were not used to correct pore fluid chemical data for drill water contamination.

Pore fluid alkalinity increases rapidly in Unit I and reaches a maximum of 25.8 mM at 7.9 m CSF, concomitant with an increase in headspace methane (see “Organic geochemistry”) and the SMT at this site. Alkalinity at the SMT is less than expected for sulfate reduction/​organic matter degradation and AOM, suggesting active precipitation of authigenic carbonates near the depth of the SMT. This explanation is supported by decreases in Ca, Mg, and Mn in this depth interval (Figs. F38C, F38D, F39D). Below the SMT, alkalinity decreases to 3.3 mM at 590.2 m CSF.

Ammonium, phosphate, bromide, and manganese

Ammonium monotonically increases through Unit I and Subunit IIA (Fig. F39A). Below this depth, ammonium continues to increase at a slower rate to the middle of Subunit IIB. The ammonium maximum of 6098 µM occurs at 122 m CSF, below which concentration gradually decreases in Subunit IIC. The concentration gradient steepens in Subunit IID and Unit III, decreasing to 656 µM by 590.2 m CSF. The steady ammonium increase in the upper ~72 m CSF indicates production by microbially mediated decomposition of organic matter fueled by sulfate reduction above the SMT. Ammonium remains elevated below the SMT because of a combination of ongoing organic matter degradation and high burial rates.

Phosphate increases rapidly in the upper sediment section, reaching a maximum of 89 µM at 17.4 m CSF. Below this depth, phosphate decreases until ~100 m CSF, below which it is close to detection limit to 593.8 m CSF. The increase in phosphate in the upper 17 m reflects active organic matter decomposition in the active sulfate reduction zone. Unlike dissolved NH₄, PO₄ decreases relatively sharply below the maximum at 17.4 m CSF, indicating that phosphate is controlled by mineral precipitation reactions. Thus, phosphate is likely controlled by apatite solubility, as authigenic apatite is the major sink for dissolved phosphate.

The dissolved bromide profile generally mimics the NH₄ profile. Br increases from 0.86 mM at 1.3 m CSF to 1.06 mM at 62.5 m CSF and then remains relatively constant to the middle of Subunit IIC, below which it decreases gradually to 590.2 m CSF (Br = 0.86 mM). Bromide in marine interstitial water is sensitive to organic matter diagenesis, with concentration higher than seawater value reflecting marine organic matter decomposition. Terrestrial organic matter degradation does not contribute much Br to the pore fluids. The Br profile at this site reflects a mixture between marine and terrestrial organic matter degradation, but the relatively low Br indicates that the labile organic matter is predominantly terrestrial in origin. Br at Site C0006 is lower than at Site C0004, reflecting higher rates of terrigenous sedimentation at this site.

Manganese concentrations are above seawater value in the upper 4 m of the sediment column, reflecting MnO₂ reduction during microbially mediated organic matter degradation. Manganese reaches depletion in the active sulfate reduction zone and at the SMT, likely due to precipitation of authigenic rhodochrosite at the alkalinity maximum. Manganese then increases slightly below this depth to an average of ~4.5 µM and remains relatively constant with depth to Subunit IID. The slightly elevated dissolved Mn from 17.4 to 406.0 m CSF likely reflects ongoing microbially mediated organic matter diagenesis using Mn oxyhydroxides as terminal electron acceptors. Manganese increases and is more variable in Unit III, possibly reflecting a different sediment source with a higher MnO₂ and Mn hydroxide content.

Major cations (Ca, Mg, and K)

Calcium decreases from 8.76 mM at 1.3 m CSF to 3.97 mM at 17.4 m CSF (Fig. F38C). The Ca minimum is ~7 m below the alkalinity maximum and the SMT. Magnesium rapidly decreases in the same depth range interval. Dissolved Mg decreases from 47.5 mM at 1.3 m CSF to 33.8 mM at 17.4 m CSF (Fig. F38D). This decrease is ~3 times greater than that of Ca values, suggesting precipitation of Mg and Ca in authigenic carbonates, as well as the exchange of Mg to clay minerals formed by ash alteration. Below this depth, the Ca and Mg profiles are mirror images. From 17.4 to 448.5 m CSF, Ca steadily increases to 10.75 mM, below which the slope of the profile changes. Ca reaches 17.96 mM at the base of the hole (Fig. F38C). Below the SMT, Mg continues to steadily decrease from 17.4 m CSF to 13.85 mM at the bottom of the hole (Fig. F38D).

A significant amount of ash and up to ~20% feldspar were identified in the sediment section drilled at Site C0006 (see “Lithology”). Concentrations of both of these components are highest in Subunit IIC. Alteration of the ash and feldspars could contribute to the increase in Ca (feldspar weathering) and decrease in Mg (ash alteration) from the SMT to the base of Subunit IID. In Unit III, Ca increases rapidly to 17.96 mM (~1.7 times seawater value) at the base of the hole, whereas Mg remains relatively constant. The increase in Ca in this depth interval could be the result of diffusive communication with a compositionally different fluid below the base of Hole C0006F or it could reflect more mature diagenetic reactions with depth.

Potassium decreases overall with depth. In the uppermost 17.4 m, this decrease is more rapid than below, declining from 11.8 mM at 1.3 m CSF to 10.4 mM at 17.0 m CSF (Fig. F40E). K is likely higher than seawater value as a result of pressure changes that occur during core recovery or ion exchange with NH₄ on clay minerals. Below this depth, K continues to decrease to 6.6 mM in the middle of Subunit IID at 405.0 m CSF. From 405.0 m CSF to the base of Hole C0006F, K decreases more rapidly, reaching 3.2 mM at 571.3 m CSF (~70% less than modern seawater value). The general decline in pore fluid potassium below the SMT is likely the result of in situ formation of zeolites during alteration of volcanic ash and feldspars. Authigenic zeolites, possibly phillipsite, were recognized in smear slides from selected intervals in Unit II (see “Lithology”). The rapid K decline in Unit III may reflect more mature diagenesis of volcanic ash and feldspars due to the greater age of the sediments (late Miocene; see “Biostratigraphy”) or communication with a compositionally different fluid below the base of Hole C0006F.

Minor elements (B, Li, H₄SiO₄, Sr, and Ba)

Boron decreases throughout the sediment section from 556 µM at 1.3 m CSF to 94 µM at 571.3 m CSF (~80% less than modern seawater value). B decreases most rapidly from the seafloor to the base of Subunit IIB at 163 m CSF. B gradually decreases in Subunits IIC and IID and is relatively constant, albeit with local scatter, in Unit III. Boron at Site C0006 is likely predominantly controlled by low-temperature alteration of volcanic ash with subsequent authigenic clay formation, as well as by B uptake by adsorption on clay minerals.

Lithium concentrations are above seawater value in the upper ~2 m of the sediment section, and then rapidly decrease to below seawater value from 2 to 20 m CSF. Li remains relatively low, ranging between 7.3 and 20.2 µM to ~200 m CSF; local maxima are as high as 33 µM (Fig. F40A). At ~200 m CSF, Li gradually increases to the Subunit IIC/IID boundary. From this unit boundary, Li increases from 42 µM at 406 m CSF to 136 µM at 571 m CSF.

Dissolved silica increases rapidly from 437 µM at 1.3 m CSF to 695 µM at 12.2 m CSF, below which silica varies, ranging from ~400 to 1000 µM. Throughout the section at Site C0006, diatoms and radiolarians are either absent or very rare and fragmented (see “Lithology” and “Biostratigraphy”). Thus, the initial increase in H₄SiO₄ is likely due to the dissolution of siliceous biogenic material, whereas variations in silica below Unit I likely reflect varying dominant silicate diagenetic reactions. An increase in H₄SiO₄ in Unit III, as well as more variability in concentrations, likely reflects the change in depositional environment, lithology, and age of this unit.

Strontium rapidly decreases initially in the upper 12 m and remains below modern seawater value until 43.0 m CSF. At 76.4 m CSF, Sr sharply decreases, below which Sr increases to Subunit IID. Below Subunit IID, Sr is more variable and decreases with depth to the base of Hole C0006F (Fig. F40D). The slight Sr decrease in the upper 12 m reflects minor Sr precipitation in authigenic carbonates in the sulfate-reduction zone. There was very little CaCO₃ in the sediment section at Site C0006, which is mainly dominated by clays, quartz, and feldspars (see “Lithology”). This composition is reflected in the near-seawater concentration of Sr below the SMT to ~300 m CSF. The steady increase in Sr from 300 m CSF to the middle of Subunit IID may reflect in situ volcanic ash diagenesis, as the concentration of ash and abundance of ash layers increase within this interval (see “Lithology”).

Dissolved barium increases from 0.6 µM at 1.3 m CSF to 12.7 µM at 12.0 m CSF, which is ~2 m below the base of the SMT. Ba remains remarkably constant, ranging from 9 to 19 µM from 12.0 m CSF to the middle of Subunit IID. From ~387 m CSF, Ba increases and varies through Unit III to the base of Hole C0006F. The main Ba-containing phase in marine sediments is the mineral barite (BaSO₄), with aluminosilicates and Fe and Mn oxyhydroxides being the most important secondary phases. The mineral barite is stable under oxic conditions but becomes undersaturated and dissolves when pore fluid sulfate is depleted, releasing dissolved Ba to the pore fluids. The slight increase in pore fluid Ba in the upper 8 m of the sediment section is likely due to the dissolution of Fe and Mn oxyhydroxides during organic matter diagenesis. The increase in Ba across the SMT and with depth is most likely the result of barite dissolution in the sulfate depletion zone. The near-constant dissolved Ba in Unit II is likely the manifestation of a near-uniform sedimentary barite content resulting from relatively rapid sedimentation of terrigenous material in a trench environment. The rapid increase in dissolved Ba in Subunit IID and Unit III is also likely related to the depositional environment. These sediments were deposited in an abyssal plain-type setting with less contribution of terrigenous material and a higher relative contribution of marine organic matter and biogenic barite.

Trace elements (Rb, Cs, V, Cu, Zn, Mo, Pb, and U)

The pore fluid rubidium profile mimics the K profile (Fig. F40E, F40F). Rb decreases from above seawater value to 1.06 µM at 43.0 m CSF and then remains relatively constant in Subunits IIB and IIC at ~1.0 µM. Rb decreases in Subunit IID and Unit III, reaching ~0.45 µM at the base of the hole (~30% less than modern seawater value). Rb is likely higher than seawater value in the upper 20 m of the sediment column because of cation exchange with NH₄ on clays in the zone of most intense organic matter degradation and ammonium production. The general decline in Rb with depth likely reflects uptake by authigenic zeolites formed during volcanic ash alteration. The steady decrease in Rb in Unit III may reflect more mature ash diagenesis, given the older age of sediments within this unit, or communication with a low-Rb fluid at depths below the base of Hole C0006F. Dissolved Cs remains relatively constant from the seafloor to 405 m CSF, varying from 4.4 to 8.7 µM. Cs decreases within Subunit IID from 5.3 µM at 405 m CSF to 3.2 µM at 406 m CSF and then remains relatively constant to TD.

Vanadium is above seawater value and variable from 1.3 to 243.4 m CSF (Fig. F41D). Below this depth, V remains fairly constant with depth to the base of the hole, ranging from 4.7 to 33.8 nM. V is most variable in Unit I and Subunits IIB and IIC, with concentrations ranging from 6.9 to 89.5 nM. This variability in the V profile may indicate enhanced metal mobility in certain horizons and will be the focus of postcruise research. Fe is highly variable, ranging from 0 to 51 µM. In general, Fe is much higher in Unit I and Subunits IIA and IIB, which likely reflects ongoing microbially mediated organic matter oxidation using Fe oxyhydroxides as terminal electron acceptors. Subunits IIC and IID and Unit III as a whole exhibit low dissolved Fe, except for peaks at ~200, 260, and 340 m CSF.

Both dissolved zinc and molybdenum are highly variable, exhibiting scatter in each of the lithologic units. Mo ranges from 26 to 850 µM, and dissolved Zn ranges from 30 to 1294 nM. In general, Mo and Zn are relatively lower in Unit I and Subunit IIA and higher in Subunits IIB–IID and Unit III. Copper concentration is relatively low and variable in Unit I and Subunits IIA–IID, ranging from 57 to 8466 nM. There is a sharp increase in Cu across the Subunit IID/Unit III boundary, reflecting a change in lithology. Cu is variable in Unit III, ranging from 696 to 31,893 nM.

Uranium decreases abruptly from 25.7 nM at 1.3 m CSF to 0.7 nM at 4.4 m CSF (Fig. F42). Uranium is more soluble under oxidizing conditions than reducing conditions; thus the rapid decline in U is the manifestation of the rapid depletion of dissolved oxygen in the upper part of the sediment section. U remains low with an average concentration of 1.5 nM in Unit I and Subunits IIA–IID. U is slightly higher and more variable in Unit III with a maximum concentration of 9.2 nM. Lead is relatively low in Unit I and Subunits IIA–IID, with peaks at 17.4, 137.5, 329.0, and 404.9 m CSF. Average Pb concentration increases in Unit III. The enrichment in U and Pb in Unit III reflects a change in lithology. Yttrium concentration generally decreases with depth, ranging from 6.4 pM at 12.2 m CSF to 0.15 pM at TD.

δ¹⁸O

Pore fluid δ¹⁸O varies from 0.19‰ to –4.19‰ (Fig. F43). From 1.3 to 81.9 m CSF (bottom of Subunit IIA), the δ¹⁸O profile is characterized by a steady decrease in values to –3.00‰. Below this depth, δ¹⁸O remains relatively constant to ~400 m CSF, the base of Subunit IID. δ¹⁸O is variable in Unit II, with a prominent peak of –1.60‰ at 439 m CSF.

Summary and discussion

The primary features of the pore fluid geochemical profiles collected at Site C0006 are

1. Relatively shallow SMT and high burial rates and input of terrigenous material in Unit I and Subunits IIA, IIB, and IIC;
2. Disseminated gas hydrates with localized occurrences of elevated pore space gas hydrate concentrations;
3. No detectable recent fluid flow from depth along the imbricate thrust faults above the deformation front; and
4. Distinct change in pore fluid chemistry and isotopic composition in Unit III.

Microbially mediated reactions and high influx of terrigenous sedimentary material

The pore fluid profiles in the upper ~60 m of the sediment column are dominated by microbially mediated reactions, and the SMT is reached at 8–12 m CSF. Above this interface, sulfate-reducing microbial communities utilize interstitial sulfate to oxidize sedimentary organic matter, reducing sulfate to sulfide and producing bicarbonate by Reaction 1:

Below the interface, methanogens generate methane. At the interface in some environments (especially diffusive settings), microorganisms symbiotically reduce sulfate and oxidize methane by a biogeochemical process called anaerobic oxidation of methane (AOM). The net biogeochemical process is described by Reaction 2 (Reeburgh, 1976):

Pore fluid alkalinity reaches a maximum of 25.78 mM at the depth of the SMT. Based on Reactions 1 and 2, if sulfate reduction by oxidation of organic matter is the dominant process controlling sulfate to the depth of the SMT, then two moles of alkalinity are produced for each mole of SO₄ reduced (~60 moles of alkalinity). If AOM is the dominant process controlling pore fluid sulfate, then one mole of alkalinity is produced for each mole of SO₄ reduced (~30 moles of alkalinity). Alkalinity at the SMT is less than expected from both processes because of the active formation of authigenic carbonate at this interval, which is manifested by the sharp decrease in Ca and Mg from the seafloor to slightly below the SMT. Although the linearity of the SO₄ reduction profile and relatively low alkalinity suggest that AOM is the dominant process controlling sulfate, the large decreases in Ca and Mg to the SMT indicate that ordinary organic matter oxidation is also a control. Shore-based δ¹³C-DIC analyses will aid in constraining the relative extent of AOM at this site.

Ammonium increases below the SMT, reaching a broad maximum of ~6000 µM between ~72 and 200 m CSF. This broad zone of elevated NH₄ is likely due to the combination of ongoing organic matter degradation and high burial rates (combination of high sedimentation rates and thrust faulting) at this site. Unit I has a slope association and Subunits IIA, IIB, and IIC were deposited in a trench environment where sedimentation rates are high and dominated by terrigenous material (see “Lithology”). The amount of terrigenous material and deposition rate increase upsection. Thus, the broad maximum in NH₄ is likely the manifestation of high sedimentation rates effectively burying ammonium more quickly than diffusion and advection across the sediment/​water interface could remove it. Sedimentation rates are lower and the amount of terrigenous material decreases with depth in Subunits IIC and IID where ammonium gradually decreases. Unit III is interpreted to be a deepwater marine environment with low terrigenous material input and sedimentation rates, and within this unit ammonium decreases. The greater proportion of terrigenous material in comparison to marine material is corroborated by overall low Br, which suggests degradation of terrestrial organic matter is much more prominent than degradation of marine organic matter in the upper sediment section. Br then decreases with depth into Unit III, indicating lower rates of organic matter degradation. The higher amount of terrestrial material in the upper sediment section is also observed in pore fluid Ba, which is relatively low in Unit I and Subunits IIA, IIB, and IIC, likely reflecting dilution of biogenic barite by terrigenous material. Ba increases in Unit III, which was likely deposited in a deepwater marine environment seaward of the deformation front. This type of depositional environment would have a lower contribution from terrigenous sources and thus would have higher biogenic barite concentration.

Gas hydrate occurrence

The relatively shallow SMT at Site C0006 suggests an elevated upward methane flux and the heat flow is anomalously low for this region (see “Physical properties”), both of which provide ideal conditions for gas hydrate formation in the sediment column. The thermal gradient at Site C0006 is 27°C/km, compared with >50°C/km at Sites C0004 and C0008 (see “Physical properties”), and the bottom water temperature is 2°C. Assuming equilibrium with Structure I gas hydrate, the CSMHYD program of Sloan (1998) was used to compute the gas hydrate stability field at this site. From the thermal gradient, the gas hydrate stability field for Structure I gas hydrate extends from the seafloor to ~800–850 m. Pore fluid Cl from ~100 to ~500 m CSF gradually declines from 571 to 540 mM, which likely reflects dissociation of disseminated gas hydrates within the sediment pore space during core recovery. The gradual decrease in pore fluid Cl indicates that gas hydrate, though overall very low, increases with depth. Using a background pore fluid value of 568 mM, the pore space gas hydrate ranges from negligible to <2% within the sediment column. Superimposed on the gradual decline in Cl are two Cl minima of 526 mM at 76 m CSF and 533 mM at 292 m CSF, which indicate horizons of elevated gas hydrate occurrence; however, hydrate concentrations at these two depths are still relatively low, comprising less than ~5% of the sediment pore space.

Pore fluid geochemical profiles through fault zones

The pore fluid geochemical profiles are relatively smooth through the deformation zone at Site C0006, which is characterized by a zone of imbricate thrust faults extending from ~275 to 540 m CSF (see “Structural geology”). Within this deformation zone, Ca, Sr, and Li steadily increase and Cl, Na, Mg, B, K, and Rb decrease. There are no local maxima or minima coincident with the faults at Site C0006; thus, there is a lack of evidence for recent updip flow of fluids from greater depths along these fault zones, suggesting that either recent rupture has been localized, fault permeability is not enhanced during slip events, or these faults have been inactive in the recent past.

From ~500 m CSF to the base of the hole at ~600 m CSF, there are distinct changes in many of the pore fluid chemical profiles. Within this interval, Li and Ca markedly increase and Cl, K, and Rb decrease. Sediments at the base of Hole C0006F, comprising Unit III, are late Miocene in age (see “Biostratigraphy”), and the geochemical variations could reflect more mature diagenetic reactions because of the greater potential age of pore fluids. Another possibility is that these fluids reflect diffusional communication with a fluid originating deeper than the base of Hole C0006F, potentially residing in faults and fractures associated with the frontal thrust.

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