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doi:10.2204/iodp.proc.314315316.135.2009

Inorganic geochemistry

The main objectives of the geochemical program at Site C0007 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 40 whole-round samples were collected for interstitial water analyses at Site C0007 (2 from Hole C0007A, 2 from Hole C0007B, 12 from Hole C00007C, and 24 from Hole C0007D). Holes C0007A, C0007B, and C0007C actually constitute one hole, and the three different designations were a clerical error; thus, they are combined on figures designated Hole C0007C in this report. Whole-round lengths ranged from 11 to 44 cm with longer sections taken in and adjacent to fault zones. Samples were collected at a higher spatial resolution in the uppermost 30 m to constrain the dominant microbially mediated reactions occurring within the shallow sediments at this site and identify the presence or absence of anaerobic oxidation of methane (AOM) at the sulfate-methane transition (SMT). Below Core 316-C0007C-3H, one sample was taken per core if recovery was sufficient. The main geochemical objectives below the region of most intense organic matter degradation in Holes C0007C and C0007D were to identify the main in situ diagenetic reactions in the sediment section and identify any exotic/​deeper-sourced fluids along the fault zones. This was done through analysis of dissolved elements that reflect inorganic fluid-rock reactions or microbially mediated reactions.

The SMT was not recovered at Site C0007. Sulfate reduction is relatively rapid in the upper ~20 m of the sediment section of Hole C0007C; however, sulfate never reaches depletion and increases toward the bottom of Hole C0007C. Within the first cores from Hole C0007D (only ~20 m below the base of Hole C0007C), sulfate is totally depleted with localized horizons of elevated sulfate. Because we did not recover this geochemical boundary, it is difficult to ascertain whether slight variations in sulfate are the result of drill water contamination or if they reflect in situ pore water sulfate. Thus, pore fluid elemental concentrations were not corrected for drilling contamination based on dissolved SO₄ concentration at this site. Uncorrected concentrations of all of the elements analyzed are presented in Tables T12, T13, and T14, and the corresponding chemical and isotopic profiles are presented in Figures F40, F41, F42, F43, F44, F45, and F46.

Sulfate concentration ranges between 8 mM and near-seawater value in Hole C0007C. Typically, sulfate is depleted in the upper sediment section as a result of microbial reduction of sulfate by organic matter oxidation and, in some cases, methane oxidation. Enrichments in sulfate below the SMT therefore indicate pore fluid contamination during the core recovery process. Thus, in Hole C0007C it is difficult to determine if a fraction of pore fluids collected throughout the hole have been compromised by drill water contamination. All of the whole rounds collected, except Section 316-C0007-16H-2 at 149.1 m CSF, were clay-rich or silty clay lithologies and are therefore considered to have a low likelihood of contamination. All samples from Hole C0007C were collected using the HPCS. HPCS cores are typically not contaminated by drilling fluid because they are collected ahead of the drill bit. The most suspect of the pore water samples is Sample 316-C0007-16H-2, collected at 149.1 m CSF. Sample 316-C0007-16H-2 is a fine-grained sand, and many of the major and minor elements approach seawater value in this section. However, this sample was bounded by >1 m clay sections above and below, precluding significant flushing with drill water or seawater during core recovery. Furthermore, there is a lack of evidence of sulfate depletion based on the CH₄ concentration profile throughout Hole C0007C; CH₄ concentration remains near the detection limit (see “Organic geochemistry”). Because we have these reasons to believe that samples throughout Hole C0007C might be representative of in situ pore fluids, we present data in this report uncorrected; however, these data should be viewed with caution. Postcruise analyses will focus on constraining whether the elevated SO₄ is an artifact of drill water contamination or reflects in situ SO₄. As discussed below, low sulfate concentration was measured in Hole C0007D. Because these samples were collected with the RCB, it is likely the measured sulfate reflects a small amount of seawater contamination.

Because of the paucity of argon, which is used as a carrier gas for both inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and inductively coupled plasma–mass spectrometry (ICP-MS) analyses, at the time of drilling Site C0007, the ICP-MS measurement program was halted for the remainder of Expedition 316. This ensured that there was enough argon available to measure the major and minor element concentrations via ICP-AES at all sites. Trace elements (V, Cu, Zn, Rb, Mo, Cs, Pb, U, and Y) were subsequently determined via shore-based ICP-MS, and δ¹⁸O was analyzed onshore by isotope ratio mass spectrometry. Trace metal and δ¹⁸O data are presented in Table T14.

Salinity, chloride, and sodium

Pore fluid salinity generally decreases with depth from 35.33 at 1.5 m CSF to 33.23 at the base of Hole C0007D (437.0 m CSF). Superimposed on this general salinity trend are three excursions. The first is characterized by a relatively rapid decrease in salinity from 35.33 at 1.5 m CSF to 34.22 at 17.0 m CSF. This initial decrease in salinity reflects active SO₄ reduction, which consumes pore fluid SO₄ and precipitates Ca and minor Mg in authigenic carbonates (Figs. F41C, F43A, F43B). The second excursion is typified by an increase in salinity from 34.28 at 102.7 m CSF to 35.44 at 149.1 m CSF, indicating a change to more seawater-like values within this depth interval. Salinity then decreases from the base of Hole C0007C to 32.83 at the top of Hole C0007D. The third excursion in the salinity profile is manifested by a sharp decrease from 32.39 at 314.2 m CSF to 29.33 at 325.5 m CSF coincident with minima in the Cl, Na, Br, and Sr profiles.

Chloride varies from 1.5 to 33.0 m CSF in Hole C0007C, ranging from 551 to 559 mM (Fig. F40B). Chloride then remains fairly constant to the base of Hole C0007D, ranging from 540 to 564 mM. One negative excursion in Cl is at 325.5 m CSF, where Cl drops from 548 to 499 mM (~11% less than modern seawater value). This excursion is likely due to dissociation of gas hydrates within this interval during drilling and core retrieval operations. Furthermore, the slightly reduced Cl in Hole C0007D (~1%–3% less than modern seawater value) likely reflects disseminated gas hydrate within the sediment pore space that dissociated upon core recovery. Local Cl values slightly above seawater value may indicate horizons of localized formation of hydrous silicates during ash alteration. At the base of Hole C0007D, from 431.6 to 437.0 m CSF, Cl increases slightly to 557 mM, close to modern seawater value (559 mM).

In general, sodium is higher than modern seawater value at Site C0007 (Fig. F40C). Initially, Na rapidly increases from 486 mM at 1.5 m CSF to 501 mM at 55.3 m CSF with local minima. Sodium then remains relatively constant to the base of Hole C0007D. Superimposed on the elevated but relatively constant Na is a sharp minimum at 325.5 m CSF coincident with minima in pore fluid salinity and Cl and Br, also reflecting a localized occurrence of gas hydrate. Besides this inferred gas hydrate–bearing interval, Na is decoupled from Cl with localized minima and maxima (Fig. F40D, F40H), indicating that the elevated Na is not the result of water consumption during the formation of authigenic silicates but rather indicating that the elevated Na may be the result of continued ion exchange with ammonium, dissolution of volcanic ash, and/or feldspar weathering.

Pore fluid constituents controlled by microbially mediated reactions

Sulfate and alkalinity

An interesting and thought-provoking outcome of the geochemical program at Site C0007 is that the SMT was not recovered (Fig. F41B, F41F). The SMT at Sites C0004 and C0006 was reached at relatively shallow depths (<20 m CSF). At Site C0007, however, sulfate reduction is relatively rapid in the upper ~20 m but sulfate never reaches depletion in Hole C0007C (Fig. F41B) and headspace CH₄ concentration remains low (see “Organic geochemistry”). Sulfate decreases monotonically from 26.73 mM at 1.5 m CSF to 10.57 mM at 17.0 m CSF and then decreases gradually to 8.11 mM at 33.0 m CSF. Below this depth, SO₄ gradually increases to 13.71 mM at 129.9 m CSF and then increases sharply to 28.07 mM at the base of Hole C0007C (149.1 m CSF).

At the top of Hole C0007D, dissolved SO₄ is almost depleted (1.55 mM at 191.6 m CSF). This depletion constitutes a 95% decrease in sulfate over an interval of only 20 m from the base of Hole C0007C to the top of Hole C0007D. This is not entirely surprising, as the two holes are separated by ~185 m. Sulfate is relatively low throughout Hole C0007D, ranging from 0 to 8 mM. The slightly elevated sulfate likely indicates varying degrees of drill water contamination. This inference is corroborated by elevated methane between ~180 and 300 m CSF (see “Organic geochemistry”). However, because we did not recover the SMT at this site, it is difficult to discern between elevated SO₄ caused by drilling contamination and that reflecting in situ pore fluid concentrations. Thus, to be conservative we did not correct the elemental concentrations for drilling contamination based on sulfate concentration at this site, but it can be assumed that the dissolved SO₄ is low to negligible in Hole C0007D.

The alkalinity profile in Hole C0007C is a mirror image of the sulfate profile (Fig. F41C). Pore fluid alkalinity increases monotonically from 4.5 mM at 1.5 m CSF to 16.20 mM at 17.0 m CSF. Alkalinity then increases more gradually to a maximum of 18.34 mM at 33.0 m CSF. Below this depth, alkalinity decreases to 11.81 mM at 129.9 m CSF and then decreases sharply to 4.40 mM at 149.1 m CSF, which is near modern seawater value (2.325 mM). In Hole C0007D, alkalinity jumps to 15.35 mM at 191.6 m CSF, which is ~3.5 times the alkalinity measured at the base of Hole C0007C. Alkalinity steadily decreases from 191.6 m CSF to 6.47 mM at 431.6 m CSF.

Ammonium, phosphate, and bromide

Ammonium increases rapidly with depth in lithologic Unit I and reaches a maximum at 55.3 m CSF. From 55.3 to 129.9 m CSF, NH₄ remains relatively constant and then decreases sharply to 1289 µM at 149.1 m CSF (Fig. F42A). Ammonium increases abruptly between the base of Hole C0007C and the top of Hole C0007D, where NH₄ is 3867 µM at 191.6 m CSF (Fig. F42D). Below this depth, ammonium gradually decreases with depth, reaching 1403 µM at the base of Hole C0007D (437.0 m CSF). The initial rapid increase in ammonium is the result of microbially mediated decomposition of organic matter. The decline with depth in Hole C0007D likely reflects decreasing metabolic rates and thus declining ammonium production, as well as sorption on clay minerals.

Phosphate increases sharply in the upper part of the sediment section and peaks at a shallower depth than ammonium, reaching a maximum of 78 µM at 31.6 m CSF (Fig. F42B). Below this depth, phosphate decreases until 149.0 m CSF. From the base of Hole C0007C to Hole C0007D, phosphate increases to 24 µM at 191.6 m CSF. Below this depth, PO₄ varies slightly and remains close to the detection limit. The phosphate increase in the upper 30 m reflects active organic matter decomposition in the zone of most active sulfate reduction. Below this depth, dissolved PO₄ is controlled by the solubility of apatite, which is a major sink for phosphate.

Bromide in marine interstitial waters is sensitive to organic matter diagenesis, with concentration higher than seawater reflecting marine organic matter decomposition. Br initially increases from 0.83 mM at 1.5 m CSF to 0.88 mM at 55.3 m CSF. Below this depth, Br remains relatively constant to 129.7 m CSF and then drops sharply to 0.83 mM at 149.1 m CSF. Br increases abruptly from 0.83 mM at the base of Hole C0007C to 0.90 mM at the top of Hole C0007D. Below this depth, the Br profile varies with a maximum of 0.96 mM at 257.3 m CSF and a minimum of 0.82 mM occurring at 325.5 m CSF. This minimum is coincident with minima in Cl and Na and reflects gas hydrate dissociation during core recovery. Br in Hole C0007D is lower than the equivalent depths at Sites C0004 and C0006, suggesting that organic matter degradation is less intense at this site.

Major cations (Ca, Mg, and K)

Calcium decreases rapidly from 9.90 mM at 1.5 m CSF to a minimum of 4.77 mM at 31.6 m CSF (Fig. F43A), reflecting Ca consumption during authigenic carbonate formation in the zone of most intense sulfate reduction (Fig. F41B). Below this depth, Ca gradually increases to 6.41 mM at 129.9 m CSF and then increases rapidly toward modern seawater value at 149.1 m CSF. Calcium decreases abruptly from near-seawater value at the base of Hole C0007C to 5.48 mM at 191.6 m CSF in Hole C0007D. Below this depth, Ca increases gradually to the base of Unit II (Fig. F43E) and then increases in Unit III to a maximum of 13.57 mM at 402.1 m CSF. This zone of Ca increase is coincident with a fault zone extending from ~400 to 420 m CSF (see “Structural geology”). Below this depth, Ca decreases to 8.61 mM at 437.0 m CSF. The relatively sharp increase in Ca suggests that the fluid sampled in the fault zone is unique from the fluids above and below.

The magnesium profile in Hole C0007C is very similar to the Ca profile (Fig. F43A, F43B). Mg decreases gradually from 48.1 mM at 1.5 m CSF to 37.0 mM at 31.6 m CSF, reflecting the incorporation of Mg in authigenic carbonates precipitating in the zone of sulfate reduction and alkalinity production and Mg uptake during the alteration of volcanic ash. Below 31.6 m CSF, Mg remains constant to ~100 m CSF and then increases to near-seawater value (45.48 mM) at 149.1 m CSF. Mg drops abruptly from 45.48 mM at the base of Hole C0007C to 27.18 mM at 191.6 m CSF in Hole C0007D. Below this depth, Mg gradually decreases and varies to 417.9 m CSF, likely reflecting Mg uptake in authigenic clay minerals formed during the alteration of volcanic ash with depth. Below 417.9 m CSF, Mg increases slightly to higher values.

Dissolved potassium is above modern seawater value between 1.5 and 17.0 m CSF, reflecting either ion exchange with NH₄ in the interval of most intense organic matter degradation and ammonium production or a sampling artifact caused by pressure changes during core recovery (Figs. F43C, F42A). Below 17.0 m CSF, K is slightly lower than seawater value with several local minima. Potassium decreases sharply from the base of Hole C0007C to 8.5 mM at 191.6 m CSF in Hole C0007D. Below this depth, K gradually decreases to 6.7 mM at 408.2 m CSF. This gradual decline in K likely reflects the uptake of dissolved potassium in authigenic zeolites forming during the alteration of volcanic ash and feldspar in Units II and III.

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

Boron is slightly elevated with respect to modern seawater from 1.5 to 10.0 m CSF and then decreases sharply to 265 µM at 86.6 m CSF. At this depth, B increases to 407 µM at 149.1 m CSF. B is very low in Hole C0007D, ranging from 146 to 292 µM. The overall low boron at Site C0007 is likely controlled by low-temperature alteration of volcanic ash and the subsequent formation of authigenic clay minerals, as well as by B uptake by adsorption on clay minerals. It is interesting to note that B increases abruptly at 417.9 m CSF to 292 µM at total depth (TD), which is directly below a fault zone; this increase is coincident with changes in the other major and minor elements analyzed at this site.

Dissolved Li decreases sharply from seawater value at 1.5 m CSF to 7.9 µM at 14.2 m CSF. Below this depth, Li gradually increases to seawater value toward the base of Hole C0007C, reaching 23.0 µM at 149.1 m CSF. Unlike the Ca, Mg, K, and B profiles, Li does not change much between the base of Hole C0007C and the top of Hole C0007D (Fig. F44A, F44D). Lithium remains relatively constant from 191.0 to 314.0 m CSF, ranging from 15.3 to 25.4 µM. At 314.0 m CSF, Li increases steadily to 107.0 µM at 417.0 m CSF, which is coincident with chemical anomalies in other pore fluid constituents, as well as a prominent fault zone extending from 400.0 to 420.0 m CSF (see “Structural geology”). From 417.0 to 437.0 m CSF, the Li profile reverses, and Li reaches concentration observed above the fault zone at ~380 m CSF.

Dissolved silica is higher than modern seawater throughout Holes C0007C and C0007D, and values are scattered with local minima at 17, 149, 249, 305 and 352 m CSF and local maxima at 32, 92, 222, 333 and 408 m CSF.

Dissolved Sr decreases from 79 µM at 1.5 m CSF to 57 µM at 55.3 m CSF, which is consistent with the incorporation of Sr in authigenic carbonates formed in the zone of most intense sulfate reduction and alkalinity production. Below 55.3 m CSF, Sr increases sharply to a near-seawater value of 83 µM at 129.9 m CSF and remains constant to the base of Hole C0007C. This quick return to near-seawater value suggests a source of Sr below the zone of authigenic carbonate precipitation. Sr decreases sharply from 83 µM at the base of Hole C0007C to 74 µM at 191.6 m CSF in Hole C0007D and then increases linearly to 104 µM at 257.3 m CSF. Below this depth, Sr remains fairly constant to the base of the hole with a minimum of 97 µM at 325.5 m CSF and a maximum of 119 µM at 351.7 m CSF. The Sr minimum at 325.5 m CSF is coincident with sharp decreases in Cl, Na, and Br and is the result of gas hydrate dissociation during core recovery. Calcite in the sediment column at Site C0007 is low, in general less than ~8 wt% (see “Lithology” and “Organic geochemistry”); thus, variations in Sr below ~30 m CSF are not controlled by carbonate diagenesis. Variations in the Sr profile are likely the result of volcanic ash alteration.

Dissolved Ba is depleted in the upper ~10 m of Unit I. Ba then increases to 1.3 µM at 17.0 m CSF, below which Ba remains relatively constant to the base of Hole C0007C. One of the primary solid phases hosting Ba in marine sediments is the mineral barite (BaSO₄). 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 low Ba in Hole C0007C is the manifestation of the elevated sulfate, and thus the fluids are supersaturated with respect to barite. The slight Ba mobilization within Hole C0007C is likely the result of the dissolution of Fe and Mn oxyhydroxides during organic matter diagenesis. Ba increases sharply from the base of Hole C0007C to 11.9 µM at 191.6 m CSF in Hole C0007D, where SO₄ is depleted. Ba remains fairly constant to 325.5 m CSF (Fig. F41H) and then increases sharply at the Unit II/III boundary. Ba decreases sharply from 141.5 µM at 383.2 m CSF to 23.5 µM at 408.0 m CSF and then increases to values similar to those observed at the top of Unit III at the bottom of the geochemical profile at 437.0 m CSF. The minimum in dissolved Ba between 400 and 410 m CSF is coincident with increases in Ca and Li as well as a fault zone. The overall elevated Ba in Hole C0007D is consistent with sulfate depletion and the dissolution of barite with depth. Localized elevated Ba to values as high as ~140 µM occur at a change in lithology in the bottom of Unit II and in Unit III from more sand-dominated sediments above to more mud-rich sediments below (see “Lithology”). This interval is interpreted to represent a hemipelagic setting with thin turbidite sequences; biogenic phases, such as marine barite, would therefore be less diluted by terrigenous material. Thus, barite was likely higher at these depths, and the elevated Ba simply reflects the dissolution of barite because of sulfate depletion within this interval.

Dissolved Mn decreases sharply from 12.21 µM at 1.5 m CSF to 1.38 µM at 5.8 m CSF. The elevated Mn in the upper ~6 m at this site reflects MnO₂ reduction within a depth interval where MnO₂ is a favorable and important electron acceptor for microbially mediated organic matter decomposition. Below this depth, Mn is relatively constant to the base of Hole C0007C, ranging from 1.75 to 5.1 µM. Mn is scattered and variable in Hole C0007D, ranging from 1.4 to 7.3 µM. In Unit III, Mn displays considerable mobility and reaches a maximum of 17.2 µM at 402.1 m CSF, coinciding with the occurrence of a fault zone extending from 400 to 420 m CSF. Below this depth, the Mn profile reverses, approaching the same values as those above the fault zone. Dissolved Fe is more variable in Hole C0007C than in Hole C0007D. Fe is slightly elevated with respect to modern seawater value in the upper 6 m, reflecting Fe oxyhydroxide reduction during microbially mediated decomposition of organic matter. Below this depth, Fe varies from 0.55 to 16.84 µM. Dissolved Fe is elevated at the top of Hole C0007D, 4.28 µM at 191.6 m CSF, and then decreases and remains constant at ~1 µM to TD.

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

Rb concentration decreases with depth in Hole C0007C from 1.52 µM at 1.5 m CSF to 1.10 µM at 149.0 m CSF (Fig. F43B). Rb decreases between Hole C0007C and Hole C0007D to 0.92 µM. In general, Rb decreases with depth in Hole C0007D, reaching a minimum of 0.53 µM (~62% less than seawater value) at 417.9 m CSF. Cs increases from 2.9 nM at 1.5 m CSF to 7.3 nM at 102.9 m CSF (Fig. F43B). Cs then decreases to 4.1 nM at the base of Hole C0007C. Cs generally decreases in Hole C0007D, reaching a minimum of 1.9 nM between ~402 and 418 m CSF, coincident with maxima and minima in the major and minor elements, as well as a fault/​fracture zone. Below this depth to the base of the hole, there is a minor increase in Cs concentration.

Molybdenum is variable and generally above seawater value in Hole C0007C, ranging from 68 to 418 nM (Fig. F45A). Mo decreases sharply from the base of Hole C0007C to the top of Hole C0007D from 418 µM at 149.1 m CSF to 57 µM at 191.8 m CSF. Mo then increases to 444 µM at 314.2 m CSF. Below this depth, Mo is variable with concentration minima at 351.9 and 402.1 m CSF and maxima at 393.0 and 437.0 m CSF.

Copper decreases with depth in Hole C0007C from 8600 nM at 1.5 m CSF to 534 nM at 149.2 m CSF with a peak of 12,122 nM at 92.1 m CSF. Copper is more variable in Hole C0007D, ranging from 1947 to 32,983 nM. The zinc concentration profile is very similar to the copper profile, except that concentration does not decrease in Hole C0007C. Vanadium concentration decreases in Hole C0007C from 122.2 nM at 5.8 m CSF to 14.6 nM at 149.1 m CSF. V concentration increases to 27.9 nM at 304.5 m CSF, below which V decreases to 8.8 nM at TD.

Uranium decreases from 6.4 nM at 1.5 m CSF to 0.6 nM at 9.9 m CSF (Fig. F45B). Uranium is much more soluble under oxidizing conditions than reducing conditions; thus, the rapid decline in U is the manifestation of the rapid depletion of oxygen and other complexes in the upper part of the sediment section. From 9.9 to 129.9 m CSF, U remains relatively constant and then increases to near-seawater values at 149.1 m CSF. In Hole C0007D, U increases from 1.4 to 5.4 nM at 325.5 m CSF. Below this depth, U is variable, ranging from 2.7 to 4.6 nM. Lead is variable in Hole C0007C, ranging from 0.3 to 2.1 nM. Pb is relatively constant in Unit II in Hole C0007D with an average concentration of ~1.5 nM. Pb is variable in Unit III with a prominent peak of 52.6 nM at 431.6 m CSF. Yttrium decreases with depth in Hole C0007C from 1.4 pM at 1.5 m CSF to 0.63 pM at 149.1 m CSF. Y increases from the base of Hole C0007C to the top of Hole C0007D to 2.0 pM. Y concentration then decreases with depth, reaching 0.46 pM at TD.

δ¹⁸O

δ¹⁸O is variable but near-seawater value (0‰) in Hole C0007C with a minimum value of –1.78‰ and a maximum of 0.11‰ occurring near the seafloor at 1.5 m CSF. δ¹⁸O decreases abruptly between Hole C0007C and Hole C0007D to –1.63 ‰. δ¹⁸O continues to decrease to a minimum of –3.70‰ at 351.7 m CSF and then progressively increases to –1.16‰ at 437.0 m CSF.

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