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doi:10.2204/iodp.proc.331.104.2011

Geochemistry

Interstitial water

A total of 75 whole-round samples were processed at Site C0014 from four different holes (Table T8). Routine samples were collected immediately above or below whole rounds dedicated for microbiology at a frequency of about one sample per 1.5 m section. We squeezed whole rounds at the laboratory temperature of ~20°C rather than at in situ temperature. Depth profiles show discrete layering and large lateral heterogeneity within the uppermost 45 mbsf that was sampled by multiple holes. Lateral variation is especially evident within the uppermost 9 mbsf, especially for those chemical species that are most influenced by diagenetic reactions involving organic matter, such as sulfate and alkalinity, but also for those involved in hydrothermal reactions, such as K and Ca.

Layering in pore water profiles

Profiles of dissolved Mg and Na show stepwise decreases with depth in Holes C0014B and C0014D between ~9 and 13 mbsf and in Hole C0014E at ~28–31 mbsf (Fig. F19). As both Mg and Na are taken up into alteration minerals at rates that increase with temperature (e.g., You et al., 1996; You and Gieskes, 2001; Gieskes et al., 2002), we interpret these stepwise decreases as being generated by the presence of discrete less permeable layers within more permeable sediment. The stepwise decreases in Mg and Na would then result either from removal of Mg and Na into alteration minerals within the deeper more permeable layers, most likely into smectite at the prevailing temperatures of 45°–132°C, or by lateral transport of Mg- and Na-depleted hydrothermal solutions into these layers from elsewhere. The two relatively impermeable layers inferred from the pore water profiles, at ~9–13 and 28–31 mbsf, are both defined by higher density and lower porosity relative to the adjacent sediment (see “Physical properties”). Within these layers the concentrations of Mg and Na appear to be controlled by diffusion between the overlying and underlying more permeable layers. The low-permeability layer inferred at 9–13 mbsf also causes stepwise changes in K, Ca, sulfate, and ammonium that show up as steep gradients in the concentrations of these species over this interval (Fig. F19). The intervening, more permeable layers are characterized by nearly uniform Mg concentrations: the upper layer at 0–9 mbsf approximates seawater, the middle layer at 13–28 mbsf has intermediate concentrations of 14–18 mM, and the deeper layer below 31 mbsf has low concentrations of 4–11 mM. The profile for Hole C0014G is more complex, indicating that the upper low-permeability layer is laterally discontinuous, but this hole does display a steep Mg gradient roughly within the zone of the deeper less permeable layer, as does Hole C0014E (Fig. F19).

Layers defined by the pore water profiles correspond to changes in alteration intensity and mineralogy within the sediment (see “Petrology”). The uppermost zone at 0–9 mbsf, which is occupied mainly by seawater that has been more or less affected by organic matter diagenesis, lies between the bottom water temperature of 4.5° and 16°C, as measured in Hole C0014G, to 33°C, as measured in Holes C0014C, C0014D, and C0014F. Sediment in this layer is visually estimated to contain <2% alteration minerals. The next deeper layer, at 13–28 mbsf, presently lies at 45°–90°C and is altered to illite/montmorillonite, kaolinite, and pyrite. The next deepest layer, from 31 to 42 mbsf, presently lies at 99°–132°C and is altered to Mg chlorite, with disseminated pyrite. As Mg chlorite forms only above 220°–300°C (Browne, 1978; Árkai, 2002), this layer was certainly hotter in the past, possibly explaining its relative paucity of microbial life. A third less permeable layer, which is defined by its physical properties in Hole C0014B, lies at 42–44 mbsf and 132°–138°C and separates a vapor-rich permeable layer above from a brine-rich layer below, as discussed below. The present temperature gradient measured at Site C0014 increases nearly linearly from the ocean bottom water temperature at this site of 4.5°C to 145°C at 47 mbsf in Hole C0014G and then increases abruptly over only 3 m to >210°C at 50 mbsf. These higher temperatures are consistent with the observed alteration to Mg chlorite.

Within the uppermost layer at 0–9 mbsf there is great variation from hole to hole for those chemical species that are most influenced by diagenetic reactions involving organic matter, including sulfate (Fig. F20), alkalinity, phosphate, silicon (Fig. F21), ammonium (Fig. F22), and methane (Fig. F23). Holes C0014B and C0014D both show large and complementary spikes in alkalinity and sulfate within this interval, but at different depths. These profiles indicate rapid reduction of seawater sulfate, coupled with oxidation of organic matter to dissolved carbonate (alkalinity), within discrete layers. Ca, K (Fig. F19), Li, Rb, B (Fig. F24), Ba (Fig. F25) and ammonium all show a pronounced spike at 6 mbsf that likely comes from lateral intrusion of a hydrothermal solution. The effects of both reaction and injection are diluted, in turn, by lateral intrusion of seawater within this uppermost layer, especially at 9 mbsf.

Within the middle permeable layer at 12–28 mbsf are small and regular decreases in Mg, Na, and Na/Cl and increases in K, Ca (Fig. F19), Cl, Br, ammonium (Fig. F22), and Br/Cl with depth. Sulfate, alkalinity, dissolved inorganic carbon, and Si are nearly constant, as is pH, which ranges from 6.4 to 7.0 over this depth interval (Table T8). A pair of sharp methane peaks in Hole C0014B, similar to the one at 5.6 mbsf in the layer above, exactly bound this middle layer; they lie so close to its upper and lower edges that it is not obvious whether they originate within the permeable layer or within the impermeable caprocks we have inferred to bound it above and below. Hydrogen does not exceed 1 µM at depths shallower than 36 mbsf.

Below the deeper caprock at 28–31 mbsf we sampled a vapor-rich zone which extends from ~29 to 42–48 mbsf (gray shading, Fig. F22; sampling density precludes a more precise estimate of depth at the base). Chlorinity begins to decrease at the top of the lower caprock we inferred from the Mg profile. The vapor-rich zone is characterized by low chloride, bromide, sulfate, Na and Na/Cl, Mg, and relatively low alkalinity. It displays relatively high pH in the range 6.7–7.4; high K, Si, ammonium, and Br/Cl (to 1.86); and an increase in hydrogen to concentrations as high as 1 mM, which persists to the deepest sample at 127 mbsf but is likely an artifact of reaction with the aluminum core liners we used for coring at high temperatures (Fig. F26). Such vapor-rich fluids are seen throughout this region in association with high-temperature hydrothermal venting (e.g., Kawagucci et al., 2011; Suzuki et al., 2008). At Site C0014 the vapor-rich zone is weakly mineralized and the recovered cores were especially gas-rich.

Below the bottom of the vapor-rich zone at 42–48 mbsf we encountered a brine, with chlorinity up to 620 mM and elevated Br/Cl that is as high as 1.79. As noted earlier, this brine is separated from the vapor-rich zone above by a high-density, low-porosity layer at 42–44 mbsf. High alkalinity (10 ± 5 mM) and ammonium (1–1.5 mM) persist in the brine. Sulfate ranges from 4 to 21 mM and Ca from 22 to 54 mM, but both may be affected by dissolution of anhydrite during core processing, as anhydrite was detected visually and via XRD at this site at 57 mbsf and deeper. Mg is low, ranging from 3 to 12 mM. K reaches 70 mM at the high end of concentrations seen in high-temperature fluids from this region (Sakai et al., 1990; Kawagucci et al., 2011; Suzuki et al., 2008). Si is also high, as is methane, which reaches 340 mM (Fig. F23).

Minor and trace elements

The minor cations B and Li, to first order, mimic K (i.e., compare Fig. F24 with Fig. F19), consistent with the influence of high-temperature solutions at this site. Likewise, Sr mimics Ca. Ba exhibits a maximum (Fig. F25) within the zone of sulfate depletion, consistent with possible dissolution of dispersed barite within the sediment. Dissolved Mn (Fig. F25) increases abruptly at ~10 mbsf, roughly coincident with several other major and trace elements. A second, more pronounced Mn peak lies with the zone of volatile enrichment in Hole C0014G and somewhat deeper but near the boundary of that enrichment in Hole C0014B (Fig. F22). After decreasing with depth to ~40 mbsf, Mn increases again, gradually, toward the base of the profile in Hole C0014G. Iron is quite low through much of the sediment column with some increase near the base of the profile (Fig. F25).

Alkalinity, phosphate, and silicon

Although high-temperature, basalt-hosted mid-ocean-ridge hydrothermal systems tend to be a sink for alkalinity (Mottl, 1983; Wheat and Mottl, 2004), systems where sediment is present can have elevated alkalinities (e.g., Wheat and Mottl, 2000). Within the upper sediment column at Site C0014, high alkalinities (Fig. F21) are driven by relatively high rates of sulfate reduction of organic matter, which is approximated by

With increasing depth, alkalinity at this site becomes quite variable, but there appears to be a deep alkalinity minimum at the base of Hole C0014G that also seems to coincide with a minimum in Mg. This coincidence may result from addition of acidity during Mg removal (Mottl, 1983). Phosphate, like alkalinity and sulfate, may have a significant imprint of organic matter diagenesis or other microbial processes within the upper sediment column. Silicon profiles are generally consistent among the different holes, showing increasing concentrations with depth (Fig. F21). High concentrations of silicon are not unusual for high-temperature systems, which are typically saturated with quartz at temperatures above 130°–180°C (Mottl, 1983). Si concentrations at depths >90 mbsf (~4.5 mM) are consistent with equilibrium with quartz (Von Damm et al., 1991) at these temperatures, which exceed 210°C below 50 mbsf. However, Si concentrations at Site C0014 are generally lower than values for hot springs previously published for this region (Sakai et al., 1990; Kawagucci et al., 2011; Suzuki et al., 2008), which can exceed 12 mM and presumably reflect higher temperatures.

Headspace gas

Methane was detected in all samples at Site C0014 (Fig. F23). Similar trends were found for both safety gas (Table T9) and science gas (Table T10) samples. As at other sites, heated safety gas samples yielded higher estimates of pore water methane.

In Hole C0014B, spikes in methane were found in the science gas samples, which measure methane with higher depth resolution than the safety gas samples. These spikes correlate with intervals of low sulfate in the pore waters. In the other holes, methane generally remains low within the uppermost ~50 mbsf and then increases with depth. Gases sampled from void spaces in these holes at 10–25 mbsf, however, indicate that the gas content measured in headspace samples is a significant underestimate for this interval. The methane to ethane ratio in safety gas samples is high (Fig. F23), with multiple horizons throughout the sediment column showing no detectable ethane. Void gas was sampled at eight depths between 12 and 24 mbsf. This gas has an average methane to ethane ratio of 7508 ± 474 (1σ), with a maximum of 8115 at 21 mbsf. Methane content of these void gases is estimated to be as high as 44.6% by volume at 24 mbsf.

Methane concentrations were observed to be both variable (Hole C0014B) and low (Hole C0014G) within the upper part of the sediment column, indicating zones of anaerobic methanotrophy at sulfate–methane transitions. The observed high methane to ethane ratios suggest that the methane is biogenic, with production certainly occurring in the zone where void gas was observed. Higher temperatures measured at greater depth (e.g., >120°C at 38 mbsf) would seem to preclude in situ methanogenesis for the deeper peaks. Biogenic methane could be migrating downward from the biologically active zone, or the observed methane at depth (Hole C0014G) could be hydrothermal in origin.

Hydrogen concentrations (Table T11; Fig. F26) are low at depths above ~40 mbsf; at greater depths, concentrations are high but variable. Cores taken with aluminum core liners show higher hydrogen than those taken with plastic core liners, making interpretation of these data difficult. We also found that headspace samples with high hydrogen yielded an even higher concentration when analyzed again 2 days later.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO₃) content calculated from inorganic carbon concentration ranges from 33.1 wt% to below the detection limit of 0.001 wt% and is generally <0.1 wt% (Table T12; Fig. F27). High CaCO₃ contents (>1 wt%) were observed at depths shallower than 10 mbsf, where the sediment is hemipelagic silty clay or clay and the carbonate is probably biogenic, and at ~30 mbsf, where the sediment is hydrothermally altered and contains detrital sulfide and the carbonate may be authigenic. The highest concentration (33.1 wt%) was measured in a vein from 25 mbsf in Hole C0014G where there is some dolomite present. At depths below 30 mbsf, the CaCO₃ content is <0.1 wt%.

Total organic carbon (TOC) ranges from 1.20 wt% to below the detection limit of 0.001 wt% (Fig. F28). TOC is high near the seafloor and decreases with depth to concentrations <0.1 wt% below 30 mbsf, reflecting changes in lithology. Sediments from 0 to 20 mbsf are hemipelagic mud and pumice that are hydrothermally altered and contain detrital hydrothermal sulfide at 20–30 mbsf. Below 30 mbsf there are silicified volcanic sediments. Total nitrogen (TN) ranges from 0.6 wt% to below the detection limit of 0.001 wt%. Like TOC, TN is high near the seafloor and decreases with depth to ~30 mbsf. Ratios of TOC to TN (C/N) within the uppermost 30 mbsf are generally larger than the Redfield ratio of 6.6, indicating an organic origin for these elements. Some values are smaller than the Redfield ratio, however, suggesting that some portion of nitrogen may be inorganic in origin. Below 30 mbsf, TN is generally higher than TOC, indicating that most nitrogen is inorganic.

The total sulfur (TS) content is generally high, ranging from 0.66 to 15.7 wt%. The high content reflects the significant amount of sulfide and sulfate minerals that are present. TS increases with depth to a maximum at ~30 mbsf, where hydrothermally altered sediment is abundant (see “Lithostratigraphy”). The highest TS was measured in Hole C0014D within a layer of coarse sphalerite and pyrite.

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