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doi:10.2204/iodp.proc.331.103.2011

Geochemistry

Interstitial water

A total of 26 whole-round samples were processed at Site C0013 from five different holes. Routine samples were collected immediately above and/or below whole rounds dedicated for microbiology, usually at a frequency of one to two samples per 1.5 m section. We squeezed whole rounds at the laboratory temperature of ~20°C rather than at in situ temperature, as in situ temperatures varied from the ~4.5°–4.9°C of ocean bottom water in the Okinawa Trough to >150°C, as inferred from alteration mineral assemblages in the cores and the fact that the plastic core liners we used deformed and melted at depths below ~10–15 mbsf. We squeezed three samples from Hole C0013E that yielded no water (Table T6). Our deepest sample, from Core 331-C0013E-5H at 17 mbsf, was collected by piston coring without a liner; recovery in this core was only ~20%, and the rest of the core may have been washed away. Nonetheless, this sample appears to be sound based on its chemical composition.

Holes C0013B and C0013D were drilled within 3 m of each other. Hole C0013B sampled only shallow depths, yielding three samples from 0.5 to 1.3 mbsf, whereas Hole C0013D sampled a deeper interval, yielding eleven samples from 3.0 to 7.6 mbsf. For some elements, these two holes appear similar. By contrast, Hole C0013E, which lies ~8 m to the northwest, yielded four samples from 0.3 to 17 mbsf and is compositionally quite different from Holes C0013B and C0013D. Hole C0013F is more similar to Hole C0013E than to the others. Hole C0013G, for which there is only a single sample, is difficult to group (Table T6). These holes are thus compositionally quite variable. We infer that the variability stems from at least two competing processes that govern the chemistry of this region: (1) the dissolution of anhydrite during recovery and processing of the core samples and (2) the influence of hydrothermal fluid migration within the sediment column.

Major elements

Chloride, the major anion in seawater, is nearly constant with depth in Holes C0013B and C0013D at the concentration in seawater, except for two samples with concentrations as much as 5% higher at ~6 mbsf (Fig. F25). In Hole C0013E, however, Cl varies from ~34% lower than seawater to ~12% higher in the deepest sample from 17 mbsf. The deepest sample from Hole C0013F, at 5.7 mbsf, is depleted in Cl relative to seawater, as is the single sample from Hole C0013G at 8.7 mbsf. Bromide generally follows the same pattern as Cl (Fig. F25). As the major cation in seawater, Na (Fig. F26) generally follows Cl, as expected from the charge balance constraint. Na calculated from charge balance generally agrees with Na measured by ion chromatography, with a few exceptions (Table T6). The Na/Cl ratio is higher than in seawater for three shallow samples from Hole C0013B, probably because of expulsion of Na from ion exchange sites in clay minerals by other cations, but these ratios decrease steeply with depth to values lower than in seawater in all the other holes, showing that Na is being removed from the seawater-derived pore water into alteration minerals over this interval and at greater depths. The other alkali elements, K (Fig. F26), Rb, Cs (Fig. F27), and Li (Fig. F28), increase slightly with depth near the bottom of Hole C0013D and greatly in Hole C0013E to concentrations that are quite enriched for a seafloor hydrothermal system. These high concentrations are reached because felsic volcanic rocks are richer in alkali elements than are mid-ocean-ridge basalts. Boron and silicon likewise exhibit large enrichments (Fig. F28), consistent with the influence of high-temperature fluid.

Mg increases with depth in Holes C0013B and C0013D and to a lesser extent in Hole C0013F (Fig. F26), probably because of ion exchange with clays, as for Na, but decreases to only 7 mM in Hole C0013E. A decrease with depth is expected, as Mg from seawater-derived solutions in subseafloor hydrothermal systems is typically removed into smectite and mixed-layer clay minerals at low temperature and into chlorite (at ≥230°C) and amphiboles (at ≥280°C) at higher temperatures (Mottl, 1983). Ca increases irregularly with depth in all five holes (Fig. F26), to concentrations nearly five times that in seawater. Although some of this increase is typical for hydrothermal solutions, as Ca is typically leached from the host rock (Mottl, 1983), much of the increase at Site C0013 is an artifact of anhydrite, which is abundant throughout the core and which certainly dissolved during the 3–25 h between the arrival of the core on deck and the squeezing of sediment to separate pore water. Dissolution of anhydrite can also be inferred from the large increases in sulfate, which reach concentrations more than three times that in seawater. The composite sum of Ca plus Mg for all samples from this site increases nearly linearly with sulfate (Fig. F29), as would be expected if anhydrite dissolution, followed by exchange of dissolved Ca for Mg in clay minerals in the core, is the major source of the alkaline earth elements and sulfate that we measured in solution. It could be argued from Figure F29 that the relationship between these cations and sulfate is different for each hole. Such differences in slope probably result from variability in hydrothermal fluid content and in the amount of anhydrite and clay at various depths from hole to hole. These variations, in turn, would produce variable rates of anhydrite dissolution and ion exchange of dissolved Ca for Mg and Na in clay minerals. By contrast to the holes with increasing sulfate, sulfate decreases with depth in Hole C0013F to less than a third the concentration in seawater, as would be expected from anhydrite precipitation within the hydrothermal system with increasing temperature (Mottl, 1983). Refractive index, which varies with salinity, reflects the sum of all of these changes in major element concentrations.

The deepest sample obtained from this site, from Hole C0013E at 17 mbsf, is in many ways a typical high-temperature solution from a subseafloor hydrothermal system. Relative to seawater, it is slightly briny at 623 mM Cl; it has elevated Ca, K, Rb, Cs, B, and Si and depleted Mg and sulfate; and it has lost Na to the altered rocks. Although these characteristics are not surprising for hydrothermal systems (Mottl, 1983; Von Damm, 1995), both Cl and K (81 mM) concentrations are higher for this sample than those observed at the nearby Iheya North vents (e.g., Kawagucci et al., 2011). However, the Yonaguni IV system, which lies several hundred kilometers to the southwest, does contain comparable Cl and K concentrations (Suzuki et al., 2008). This deepest sample from Hole C0013E also differs from a typical mid-ocean-ridge hot spring in its high alkalinity (22 mM) and its high concentrations of the nutrient species ammonium, phosphate, and silicon, as discussed in the next section.

Alkalinity and nutrients

Alkalinity increases irregularly with depth to a concentration >10 times that in seawater (Fig. F30), probably because of oxidation of organic matter in the sediment approximated by the reaction

In basalt-hosted, sediment-free mid-ocean-ridge hydrothermal systems, alkalinity typically decreases to low and often negative values because of production of acidity accompanying Mg removal into alteration minerals (Mottl, 1983); that acidity is apparently being neutralized by reaction with sediment at Site C0013. Although we did not measure H₂S because the cores were purposely allowed to outgas for safety reasons, hydrogen sulfide was likely quite abundant in these cores, as evidenced by the high total sulfur in the sediment (Fig. F31) and the presence of sulfide minerals. Ammonium (Fig. F30) likewise increases with depth, as would be expected from the oxidation of organic matter. Phosphate is irregularly present at surprisingly high concentrations. It is possible that these nutrients have been introduced into these holes by lateral flow from Hole C0013A and subsequent holes, as these were drilled only 2–10 m away from each other and lateral flow has been inferred at Site C0013 from other data. As the holes were drilled within a few hours of each other, this lateral flow would have to have been relatively fast. Phosphate does not appear to be coupled to ammonium, and an especially phosphate-rich interval from 4.2 to 6.0 mbsf indicates that phosphate may be locally derived by mineral dissolution.

Summary

Interstitial water at Site C0013 is clearly influenced by hydrothermal activity. This influence is apparent even though the pore water we analyzed has been greatly affected by dissolution of anhydrite during recovery and processing of the cores. The deepest sample from Hole C0013E likely represents the most pristine hydrothermal fluid recovered at this site, and it is noteworthy that its high K and Cl values reside at the higher end of the end-member fluid estimates that have previously been published for the nearby vents (e.g., Kawagucci et al., 2011).

Headspace gas

Methane was detected in all samples at Site C0013 (Fig. F32). Concentrations increase with depth and reach hundreds of micromolar, peaking at ~10 mbsf before decreasing again with depth. Methane measured in safety gas (Table T7) and science gas samples (Table T8) exhibit similar trends, but concentrations from the heated safety gas vials are more scattered and about a factor of two higher. Ethane was observed in 6 of 16 safety gas samples, with concentrations typically less than 1 µM. Of these, only two showed evidence for propane and other higher hydrocarbons (Sections 331-C0013E-5H-1, shot with no core liner, and 7L-1, cored using the BHI hard rock system, which recovered rocks in this core). These two headspace samples also had low levels of butane, ethylene, and propylene, suggesting that all these gases resulted from the breakdown of larger organic molecules because of frictional heating during drilling. Although Site C0013 was one of the highest temperature sites we cored, none of the other 18 safety gas samples showed clear evidence for contamination. Figure F32 shows the observed methane to ethane ratio for samples with >400 ppm methane in the safety gas vial. In this figure, the sample with the lowest ratio is from Section 331-C0013E-5H-1, one of the samples that showed clear evidence for hydrocarbon contamination. The other samples show ratios >2000, with most >6000. A peak in methane combined with a consistently high methane-to-ethane ratio suggests biogenic hyperthermophilic methane production at this site, bounded above and below by zones of methane destruction and/or dilution.

Hydrogen was observed in 10 of the science gas samples from Holes C0013B–C0013E (Fig. F32). Its concentration increases logarithmically with depth, exceeding 200 nM at 12 mbsf and reaching micromolar concentrations at 20 mbsf. The logarithmic increase with depth suggests a deep hydrogen source and a relatively shallow H₂ sink at ~10 mbsf. The sink could be methanogenesis and/or sulfate reduction. Significant hydrogen was observed in all science gas samples from Holes C0013F–C0013H. We suspect that this hydrogen is a consequence of using aluminum core liners during drilling.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO₃) content calculated from inorganic carbon concentration at Site C0013 is generally low, with most samples having <0.1 wt% CaCO₃ (mean = 0.7 wt%). Only three samples have concentrations >1 wt% (Table T9; Fig. F33); one of these, Sample 331-C0013E-2H-CC, has 41.9 wt% and is likely anhydrite containing inclusions of calcite, dolomite, and talc, based on shipboard core descriptions (see “Lithostratigraphy”). Without these three samples, the mean CaCO₃ concentration is 0.08 wt%.

For Holes C0013B–C0013E, total organic carbon (TOC) content ranges from below detection (<0.001 wt%) to 1.76 wt%. Six samples have TOC >1 wt%. Of concern is the fact that Holes C0013B–C0013E were cored using plastic core liners, which melted at this site. Also of note, Section 331-C0013C-1H-1 has a TOC concentration of 1.2 wt%, but its total nitrogen (TN) content was too low to be detected. We were initially concerned that the low TN suggests the TOC derives from melted core liner. However, of the six samples with >1 wt% TOC, two are from Holes C0013F–C0013H, which were drilled using aluminum core liners. These two samples (at 5.8 and 9.6 mbsf) also have no detectable TN, indicating that some carbon-rich samples also have high C/N. Still, it remains likely that the melted core liners contributed a significant amount of organic carbon to some samples recovered from Holes C0013B–C0013E.

Beyond this observation, it is difficult to interpret the TOC and TN data except to state that sediments at Site C0013 are characterized by low organic carbon content. Excepting the two samples with TOC >1 wt% (at 5.8 and 9.6 mbsf), TOC in Holes C0013F–C0013H decreases with depth, from ~0.1 wt% in the uppermost few meters below seafloor to 0.01 wt% by 8 mbsf (Fig. F31). This trend suggests that hydrothermal alteration and possibly also microbial activity remineralize sedimentary organic carbon relatively quickly.

Total sulfur (TS) concentrations are generally high, ranging from 0.9 to 65.8 wt% (Table T9), reflecting the significant amount of sulfide and/or sulfate minerals observed throughout the sediments. TS decreases with depth (Fig. F31): all values >20 wt% are found in the uppermost 10 mbsf, and values >32 wt% are restricted to the uppermost 2 mbsf in Hole C0013E, where pyrite is abundant.

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