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doi:10.2204/iodp.proc.314315316.133.2009

Inorganic geochemistry

The main objectives of the geochemical program at Site C0004 are to

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

A host of diagenetic reactions occur within the temperature range that coincides with the updip limit of the seismogenic zone (~80°–175°C). These reactions include the opal–quartz transition, the smectite–illite transition, albitization, and desorption of cations from clays, as well as cementation by carbonate, clay, and zeolite. At higher temperatures and pressures within the seismogenic zone, a suite of hydrous metamorphic minerals, such as chlorite, serpentine, and amphiboles, form and break down, releasing fluids and altering the pore fluid composition. Fluid chemistry is predictably altered with increasing temperature and pressure and, assuming fluid-mineral equilibrium at various temperatures, can facilitate estimation of the depth of fluid sources, thus constraining the role of in situ diagenetic reactions and deeper sourced reactions in fluid production within fault zones. Characterizing these in situ fluid-rock reactions and identifying fluids migrating from depth are critical to understanding the hydrogeologic behavior of the splay fault system and constraining the processes of slip along this boundary.

A total of 41 whole-round samples were collected for interstitial water analyses at Site C0004 (18 from Hole C0004C and 23 from Hole C0004D). Whole-round lengths ranged from 21.5 to 43 cm with larger subsamples collected from cores recovered deeper within the hole where sediments were more consolidated (see “Physical properties”). Samples were collected in Hole C0004C at a higher spatial resolution than in Hole C0004D to better 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. In Hole C0004C, two samples were collected per core for Cores 316-C0004-1H through 3H, which span the depths ranging from the sediment/​water interface to ~8 m below the SMT. Below this depth, one sample was taken per core. Only one sample was taken per core to TD in Hole C0004D. Because of poor core recovery in lithologic Subunits IIA and IIB, we were unable to take interstitial water samples from cores that were <1 m long. Because of the brecciated nature of the sedimentary material in Subunit IIA and Unit III (see “Structural geology”), some cores were extremely difficult to clean prior to processing. This difficulty is observed in the elevated SO₄ concentrations (Table T12), which show that a significant portion of the 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 concentrations are 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 the corrected values for the major and minor elements are presented in Tables T15 and T16. Only corrected values are presented in figures.

Salinity, chloride, and sodium

Pore fluid salinity decreases nearly monotonically from 34.5 at 2.6 m CSF to 33.3 at 29.5 m CSF (Fig. F37A). The salinity decrease in the upper ~30 m of the sediment section reflects active sulfate reduction and subsequent precipitation of authigenic carbonates consuming pore fluid SO₄, Ca, and Mg in this interval (Fig. F38A, F38B, F38C, F38D). Salinity then increases sharply to 35.9 at 114.0 m CSF near the base of Subunit IIA. Pore fluid salinity varies in Subunit IIB and Units III and IV. Salinity slightly decreases at the base of the fault zone in Unit III (Fig. F37A) and just below the fault zone at 350 m CSF, after which it steadily increases to 36.06 at 393.6 m CSF (TD).

Chloride increases steadily from near-seawater value at 2.6 m CSF to 642 mM (~15% greater than modern seawater value) at the base of Subunit IIA. The rate of increase in Cl lessens in Subunit IIB, and Cl slightly increases below the fault zone in Unit IV to 649 mM (~17% greater than seawater value) at 393.6 m CSF.

A significant amount of volcanic ash was recovered from Units I, III, and IV at this site (see “Lithology”). In these intervals, ash layers >13 cm thick were recovered, and as much as 20% ash was disseminated within both the fine-grained sediment and silty clays. Thus, the steady downhole increase in Cl at Site C0004 likely reflects in situ ash alteration where water is consumed during the formation of authigenic clays and zeolites. There was relatively little ash in Subunit IIB between 140 and 250 m CSF, which is the same interval where Cl remains relatively constant (Fig. F37B).

Dissolved sodium follows the same trend as Cl (Fig. F37C). However, the rate of increase in Na is greater than that of Cl through all three lithologic units (Na/Cl plot, Fig. F37D). This discrepancy is consistent with ash alteration, where Na is partitioned into the fluid phase during ash dissolution and authigenic mineral formation. There are four peaks in Na/Cl in Subunit IIB and Unit III at ~180, 250, 307, and 344 m CSF. Three of these peaks occur slightly above, within, and ~29 m below the main fault zone (Unit III). These maxima indicate addition of Na within these intervals.

Pore fluid constituents controlled by microbially mediated reactions

Sulfate and alkalinity

Sulfate decreases linearly from 25.31 mM at 2.7 m CSF to 17.19 mM at 5.5 m CSF (Fig. F38A, F38F). The slope of the sulfate reduction profile then increases to 9.1 m CSF. Sulfate decreases steadily from 13.9 m CSF to below the detection limit at ~20 m CSF. A concomitant increase in headspace methane at ~16 m CSF constrains the depth of the SMT at ~16–20 m CSF (Fig. F38F). Below the SMT, sulfate remains depleted except for excursions up to 10 mM in Subunit IIB and Unit III (Table T15). These units are highly brecciated, and these excursions represent contamination of the whole-round samples collected for interstitial water analyses by seawater circulating in the borehole during drilling. Sulfate concentrations from these units were used to correct the interstitial water data for drilling contamination (see “Inorganic geochemistry” in the “Expedition 316 methods” chapter) (Tables T15, T16).

Pore fluid alkalinity rapidly increases downhole in the upper part of Unit I; a maximum of 18.3 mM occurs just below the SMT at 22.9 m CSF. Below the SMT, alkalinity decreases with depth to 7.6 mM at 393.6 m CSF. Superimposed on the steady decrease in alkalinity are a maximum (15.6 mM) at 179.5 m CSF and a minimum (8.1 mM) in the fault zone at 282.2 m CSF. Alkalinity at and near the SMT is much lower than values expected from either AOM or organic matter degradation by sulfate reduction (Fig. F38B). Ca reaches a minimum at the depth of the SMT, and Mg decreases in the sulfate reduction zone (Fig. F38C, F38D). The decreases in Ca and Mg in the sulfate reduction zone indicate active precipitation of authigenic Mg-calcite, which consumes pore fluid alkalinity at and near the SMT. Dissolved Mn also decreases rapidly in this interval and becomes essentially depleted at the SMT, suggesting minor precipitation of authigenic rhodochrosite at this depth (Fig. F39D).

Ammonium, phosphate, bromide, and manganese

Dissolved ammonium increases monotonically with depth through Unit I to ~200 m CSF (Fig. F39A). Below this depth, ammonium varies, ranging from 4662 to 8343 µM. The steady ammonium increase in the upper ~180 m indicates production by microbially mediated decomposition of organic matter fueled by sulfate reduction above the SMT. Below the SMT, Mn oxides and oxyhydroxides are likely the terminal electron acceptors for organic matter oxidation based on the progressive increase in dissolved Mn through this depth interval (Fig. F39D). Ammonium concentration is variable above, within, and just below the fault zone (Unit III).

Bromide in marine interstitial waters is sensitive to organic matter diagenesis; concentrations higher than seawater reflect marine organic matter decomposition. For this reason, Br profiles are similar to dissolved ammonium profiles. Br increases from 0.89 mM at 2.7 m CSF to 1.20 mM at 126.2 m CSF, coincident with the base of Subunit IIA (Fig. F39C). Below this depth, Br increases gradually to 1.24 mM at 393.6 m CSF. There are no major variations in the Br profile in the splay fault zone (Unit III). The Br profile at this site reflects decomposition of marine organic matter with a greater rate of Br input from marine organic matter decomposition above 130 m CSF and more uniform values below this depth.

Phosphate increases rapidly in Unit I, reaching a maximum of 55.4 µM at 39.2 m CSF, below which phosphate decreases before increasing moderately at the top of the main fault zone. The phosphate increase in the upper 30 m reflects active organic matter decomposition in the active sulfate reduction zone. Manganese concentration is above seawater value in the upper 5 m of the sediment column, reflecting MnO₂ reduction during microbially mediated organic matter degradation. Mn becomes totally depleted at the SMT, which is likely due to authigenic rhodochrosite precipitation at the alkalinity maximum. Below this depth, Mn increases as a result of progressive organic matter degradation, with maxima within and near the main fault zone.

Major cations (Ca, Mg, and K)

Calcium monotonically decreases from 8.57 mM at 2.7 m CSF to 3.12 mM at 22.9 m CSF, which coincides with the depth of sulfate depletion and the alkalinity maximum (Fig. F38C). Between the seafloor and the SMT, Ca is consumed by active precipitation of authigenic carbonates, which also consumes pore fluid alkalinity. From 22.9 to 276.8 m CSF, Ca steadily increases to 8.55 mM. Below this depth, Ca remains relatively constant to TD. Increasing Ca with depth is consistent with progressive ash alteration downhole. There are no major anomalies in the Ca profile in the main fault zone at this site.

Magnesium decreases sharply from 47.8 mM at 2.7 m CSF to 38.0 mM at 22.9 m CSF (Fig. F38D). This zone of rapid Mg depletion coincides with the SO₄ reduction zone and the SMT, indicating active precipitation of Mg with authigenic carbonates in this depth interval, as well as uptake during ash alteration. Mg then steadily decreases from 22.9 m CSF to 16.5 mM at 393.6 m CSF. Superimposed on this gradual Mg decrease are three minima at 248.0, 306.8, and 344.0 m CSF. These three depths are coincident with zones of deformation above, within, and below the splay fault zone at this site. The general decrease in Mg likely indicates Mg uptake in hydrous silicate minerals (mainly clays) formed during volcanic ash alteration.

The dissolved potassium profile can be split into two zones. The first zone, from the seafloor to 153.1 m CSF, is characterized by K concentrations higher than modern seawater value in the upper ~40 m and then approximately at seawater value to 153.1 m CSF (Fig. F40E). Elevated K values in the upper 40 m are likely due to sampling artifacts related to the pressure change during core recovery or to cation exchange with NH₄ on clay mineral surfaces during the early stages of organic matter degradation and ammonium production. From 40.0 to 153.1 m CSF, K decreases with local minima, though ammonium remains elevated, indicating authigenic zeolite formation during the alteration of volcanic ash. The second zone in the K profile extends from 153.1 m CSF to TD, where K gradually decreases to 8.2 mM at 393.6 m CSF. The steady potassium decrease is due to progressive K uptake by zeolites formed during ash alteration. Superimposed on the steady K decrease are three minima at 248.0, 306.8, and 344.0 m CSF that coincide with the three Mg minima.

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

Boron decreases relatively sharply within Unit I and Subunit IIA from 498 µM at 2.7 m CSF to 228 µM at 126.0 m CSF, with local minima at the base of Unit I and in Subunit IIA (Fig. F40A). Boron decreases less rapidly with depth in Subunit IIB and Units III and IV, reaching ~193–216 µM at TD (~55%–60% less than modern seawater value). This steady B decrease with depth reflects ash alteration, mainly B uptake in authigenic clay minerals. The slope change of the B profile at the base of Subunit IIA is consistent with this interpretation, as the relative abundances of disseminated and massive ash layers decrease within this unit (see “Lithology”).

Lithium initially decreases from a near-seawater value at the top of Unit I to 21.8 µM at 48.6 m CSF. Li then increases steadily to 125 µM at 227.0 m CSF (~25 m above the fault zone). Below this depth, Li decreases to 44.4 µM at TD (393.6 m CSF). Li depletion in the uppermost 50 m of Unit I is consistent with volcanic ash alteration. The broad concentration peak above the main fault zone is not observed in any other element profiles collected at Site C0004. The source of Li at 227.0 m CSF and the shape of the Li profile remains enigmatic and will be the focus of future postcruise research.

Dissolved silica increases steadily from 625 µM at 2.7 m CSF to 966 µM at the top of Subunit IIB at 140.0 m CSF (Fig. F40B). From 140.0 to 361.2 m CSF (~50 m below the main fault zone), silica varies, ranging from 827 to 1161 µM. Three peaks in H₄SiO₄ coincide with K and Mg minima at 248.0, 306.7, and 344.0 m CSF. Below 361 m CSF, H₄SiO₄ remains rather constant at ~630 µM. Siliceous nannofossils (diatoms, radiolarians, and sponge spicules) were quite rare at Site C0004 with the exception of Unit I (see “Lithology” and “Biostratigraphy”). In Subunits IIA and IIB and Units III and IV, diatoms and radiolarians were either absent or very rare (see “Lithology” and “Biostratigraphy”). Thus, the initial increase in H₄SiO₄ is likely due to dissolution of siliceous biogenic material, whereas silica variations below Unit I reflect variations in lithology and ash alteration.

Sr is below modern seawater value in the upper ~15 m of Unit I and then increases steadily to ~118 µM at the base of Subunit IIA at 126.2 m CSF. Below this depth, Sr varies in Subunit IIB and then stabilizes and remains nearly constant from the top of Unit III to TD at 393.6 m CSF. The slight Sr decrease in the upper 15 m reflects minor Sr precipitation in authigenic carbonates in the SO₄ reduction zone. Below this interval, dissolved Sr mimics sedimentary CaCO₃ concentration (see “Organic geochemistry” and “Lithology”), indicating that Sr is primarily controlled by carbonate diagenesis.

Ba concentration was determined shipboard by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and on shore by inductively coupled plasma–mass spectrometry (ICP-MS). Ba from both methods is presented in Table T13, but only the shipboard Ba is shown in Figure F38. The main Ba-containing sediment phase in most marine sediments is barite (BaSO₄). Elevated Ba is typically found in sediments underlying high-productivity surface waters (Ganeshram et al., 2003; Dymond et al., 1992; Eagle et al., 2003); thus, Ba concentration is typically greater in carbonate-bearing sections. Under oxic conditions, the mineral barite is extremely stable. However, when SO₄ becomes depleted, barite becomes undersaturated and dissolves, releasing a significant amount of Ba²⁺ to the surrounding pore fluids. Dissolved Ba is slightly elevated with respect to bottom water from 2.7 to 13.4 m CSF. Ba markedly increases at 20 m CSF and continues to increase to the base of Subunit IIB to 285 µM (~2000 times seawater value). After a sharp drop in dissolved Ba at 101.5 m CSF, it remains relatively constant in Subunit IIB, Unit III, and the top of Unit IV. Ba increases in the fault zone to 52 µM at 290 m CSF, which is consistent with increased sediment CaCO₃. Between 352 and 400 m CSF, Ba remains relatively constant at ~50 µM. Like Sr, Ba generally mimics CaCO₃ concentration (see “Organic geochemistry” and “Lithology”). Slightly elevated Ba in the upper 13 m of Unit I likely reflects release of Ba from Fe and Mn oxides and oxyhydroxides during the early stages of organic matter diagenesis. The sharp increase in Ba below the SMT is the result of barite dissolution, and the variability in the dissolved Ba profile is likely controlled by the abundance of barite within the sediment.

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

Rb decreases relatively rapidly from 1.71 µM at 2.7 m CSF to 1.00 µM at 29.5 m CSF and then remains remarkably constant to TD at 393.6 m CSF (Fig. F40F). The decline in Rb concentration in Unit I likely reflects uptake of Rb by zeolite formation during volcanic ash alteration. The Cs profile differs from those of Rb and K and generally varies with changes in lithology. Cs increases monotonically in Unit I from 2.7 µM at 2.1 m CSF to 3.6 µM at 126.0 m CSF. An abrupt transition from Units I to II is manifested by a decrease in Cs, which remains relatively constant at ~4 µM to 306.8 m CSF. Cs increases again across the base of the fault zone into Unit IV.

Iron, copper, zinc, molybdenum, and lead display similar trends with depth, characterized by lower concentrations and variability in Unit I. Cu, Zn, V, and Pb display a peak at the base of Subunit IIA and they are relatively constant in Subunit IIB (Figs. F41, F42). All of the trace metals display higher concentrations and variability in the fault zone (Unit III) and in Unit IV. Fe increases in some intervals in Unit I and Subunit IIA as a result of ongoing microbially mediated organic matter decomposition, which uses sedimentary Fe oxides and oxyhydroxides as electron acceptors. Likewise, Zn and Mo are likely mobilized within these units because of microbially mediated reduction of both Mn and Fe oxyhydroxides.

Uranium decreases abruptly from near seawater value at 2.7 m CSF to 2.2 µM at 9.1 m CSF (Fig. F42C). Uranium is much more fluid mobile under oxic conditions than reducing conditions; thus, the rapid decline in U is the manifestation of the rapid depletion of dissolved oxygen and other complexes in the upper part of the sediment section. U remains relatively stable to 350 m CSF and then increases to 4.3 µM at TD. Vanadium decreases sharply from 110 µM at 2.7 m CSF to 11.9 µM at 9.1 m CSF. Vanadium remains relatively constant to the base of Subunit IIA with a local maximum of 94 µM. Vanadium remains near-seawater value but is variable from Subunit IIB to TD. Yttrium varies downhole with concentration ranging from 0.45 to 3.6 pM.

δ¹⁸O

Pore fluid δ¹⁸O decreases almost linearly from seawater value at 2.7 m CSF to –2.82‰ at the base of Subunit IIA (Fig. F43). Below ~120 m CSF, δ¹⁸O decreases more slowly with positive anomalies at 248, 307, and 344 m CSF. The slight increases in δ¹⁸O in these intervals likely reflect mixing with drill water in brecciated zones, as δ¹⁸O data have not been corrected for drill water contamination. Below 352 m CSF, δ¹⁸O decreases slightly to –3.98‰ at TD.

Summary and discussion

The two primary controls on the pore fluid geochemical profiles collected at Site C0004 are volcanic ash alteration and microbially mediated reactions.

Volcanic ash alteration

The dominant process controlling the pore fluid chemical profiles at Site C0004 is the alteration of volcanic ash to authigenic clay minerals and zeolites. This alteration is manifested by downhole increases in Cl and Na and decreases in Mg, B, K, and Rb. Chloride progressively increases with depth, reaching a value ~17% greater than modern seawater value at 397.6 m CSF. This progressive Cl enrichment is most likely the result of water consumption during replacement of volcanic ash by authigenic clays and zeolites. The Mg, B, K, and Rb profiles corroborate this inference because these chemical species are consumed from pore fluids during the formation of these authigenic minerals: Mg and B by clay minerals and Rb and K by zeolites. This strong signature of ash alteration was not observed at Site C0006 downslope from this site or along the Muroto transect drilled during ODP Leg 190.

Superimposed on the general concentration profiles associated with ash alteration are three anomalies associated with the fault zones at 248.0, 306.7, and 344.0 m CSF manifested by increases in H₄SiO₄, Na, and trace metals and decreases in Mg, K, and Rb. Additional shore-based analyses will be important for determining the cause of these variations.

Microbially mediated reactions

Pore fluid profiles in the upper 30 m of the sediment column are dominated by microbially mediated reactions, and the SMT is reached between 16 and 20 m CSF. Above this interface, sulfate-reducing microbial communities utilize interstitial sulfate to oxidize sedimentary organic matter, reducing sulfate to sulfide and producing bicarbonate through the following reaction:

2CH₂O + SO₄^2– → H₂S + 2HCO₃^–.

Below the interface, methanogens generate methane. At the interface in some environments (especially diffusive settings), microorganisms symbiotically reduce sulfate and oxidize methane by AOM. This net biogeochemical process is described by the following reaction (Reeburgh, 1976):

CH₄ + SO₄^2– → HCO₃^– + HS^– + H₂O.

When methane oxidation by AOM is complete, usually in diffusional settings but not necessarily in advection-dominated environments, all of the methane fluxing upward is consumed before entering the water column. AOM also drives precipitation of authigenic carbonates, affecting carbon chemistry both above and below the SMT. The δ¹³C of dissolved inorganic carbon produced by AOM is depleted with respect to that produced by oxidation of organic matter by sulfate, and shore-based samples for carbon isotopic analyses were preserved to constrain the presence and relative amount of AOM occurring at this site.

In typical continental margin sediments with low upward advection rates, the downward sulfate flux is balanced by upward methane flux. In regions of upward advection of fluid and methane, AOM is not complete and a significant amount of methane can pass across the sediment/​water interface and into the water column. These conditions are most often present within continental margin sediments that harbor gas hydrate and in subduction zones. In the case of low upward methane advection, sulfate and methane are coupled geochemical species because AOM occurring at the SMT involves the microbially mediated co-consumption of sulfate and methane. If AOM is complete, the stoichiometric ratio between sulfate and methane consumed by AOM is 1:1 and the flux of sulfate to the SMT is equivalent to the upward methane flux to the SMT. However, it is often difficult to discern between sulfate reduction by organic matter degradation and sulfate reduction by AOM from sulfate concentration-depth profiles alone. By knowing both the sulfate and alkalinity concentration-depth profile to the depth of the SMT, it is possible to discern between these two processes because for each mole of SO₄ reduced by AOM, 1 mole of alkalinity is produced. In contrast, during organic matter diagenesis, 2 moles of alkalinity are produced for each mole of sulfate reduced. In Figure F44, we plot the change in alkalinity versus the change in sulfate in the uppermost part of Unit I at Site C0004. The alkalinity data are corrected for the consumption of alkalinity by authigenic carbonate precipitation. The blue line is the trajectory expected for simple organic matter degradation, and the red line is that expected for AOM. As shown in the plot, sulfate reduction by organic matter degradation is the dominant process occurring in the upper 5 m of the sediment column, whereas AOM is the dominant process consuming sulfate from 5 m CSF to the SMT with only a minor contribution from ordinary organic matter degradation.

Below the SMT, there is evidence for ongoing microbially mediated decomposition of organic matter. This is manifested by the progressive increase in pore fluid NH₄ and Mn, as well as elevated PO₄ and Br. The similarity in the manganese and ammonium profiles indicates Mn oxyhydroxides are being utilized as terminal electron acceptors for organic matter degradation at greater depths in the sediment column. Three maxima in NH₄ at 248.0, 306.7, and 344.0 m CSF correspond to marked changes in pore fluid major, minor, and trace elements.

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