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doi:10.2204/iodp.proc.331.107.2011 GeochemistryInterstitial waterA total of 21 whole-round samples were processed at Site C0017 from four different holes (Table T6). Many samples were collected immediately above or below whole rounds dedicated for microbiology; however, there were more whole rounds collected for interstitial water analyses than for microbiology at this site. We squeezed whole rounds at the laboratory temperature of ~20°C rather than at in situ temperature. More than half of the samples are from Hole C0017D, from 63 to 141 mbsf. Major and minor cations and anionsNa concentrations (Fig. F8) are nearly constant with depth, whereas Cl (Fig. F10) increases, yielding a small decrease in Na/Cl in Hole C0017D relative to the shallower holes. Potassium is generally above seawater concentrations in the uppermost 10–20 mbsf, probably because of irreversible ion exchange with clays, but it decreases to below seawater values below ~70 mbsf and then increases again to seawater concentration in the bottommost 10 m of Hole C0017D. The decrease likely results from uptake into alteration minerals at low temperature, but the increase requires a source of potassium near the bottom of the hole. One possible source is dissolution of sylvite (KCl), which was reported in the core at 143 mbsf (see “Mineralogy and pore fluid chemistry”). A second possible source is lateral incursion of a K-rich solution at this depth, which could be either seawater or a high-temperature hydrothermal solution. Seawater incursion would increase K but would also increase Mg and lower Ca, and neither of these changes is observed; a hydrothermal solution therefore seems more likely. Enrichments in Ca at depth are consistent with sediment alteration and release of Ca to the pore water. Mg is slightly depleted in both the upper and lower 20 m of the hole. These slight depletions coincide with the presence of dolomite in the corresponding intervals, at 10–20 and 110–140 mbsf (Table T3). Distributions of B, Li, and Sr are likewise consistent with low-temperature weathering deep in the sequence (Fig. F11). Like K, Li is higher than in seawater within the uppermost 20 mbsf. Sulfate, alkalinity, ammonium, phosphate, and siliconSulfate displays a minimum concentration at 15 mbsf (Fig. F10) that roughly coincides with maxima in alkalinity, ammonium, phosphate, and Si at 11–15 mbsf (Fig. F9) and likely results from sulfate reduction coupled with oxidation of organic matter in the sediments. Sulfate increases again to slightly in excess of seawater concentration below 15 mbsf and is roughly uniform at greater depth, with some scatter. As interpreted in “Petrology,” the increase in sulfate at ~25 mbsf, along with the accompanying decreases in alkalinity, ammonium, phosphate, manganese, and silicon, almost certainly results from lateral intrusion of unaltered ocean bottom water into the sediment at ~30 mbsf. Below this zone of seawater intrusion, phosphate remains low. Alkalinity exhibits some structure below 50 mbsf, increasing significantly below ~75 mbsf, probably because of low-temperature alteration reactions. Silica generally decreases with depth, suggesting some uptake into authigenic minerals and/or a second deep zone of seawater recharge. This constituent scatters about concentrations that are lower than those found at Sites C0013 and C0014 (Fig. F9). SummaryThe uppermost 25–35 m at Site C0017 is characterized by organic matter diagenesis in the upper half and intrusion of seawater in the lower half. Below ~50 mbsf, this site is characterized by low-temperature alteration reactions and considerable scatter in the pore water data, probably resulting from interaction between chemical and hydrologic processes. The shape of the temperature profile indicates overall downwelling at Site C0017, with irregularities suggestive of nonsteady-state processes including lateral flow. The temperature gradient increases from 70 to 80 mbsf, becomes nearly flat at 85–110 mbsf, and increases again to 90°C at the bottom of the hole (see “Physical properties”), suggesting a complex three-dimensional hydrologic regime on which are superimposed the effects of temperature-dependent water-rock reactions. Headspace gas analysisMethane concentrations (Fig. F12) are quite low, mostly <1 µM. The highest values are near the bottom of the deepest Hole C0017D, where a mean value of 1.8 µM was found between 112 and 141 mbsf. The highest concentration is 3.4 µM at 112 mbsf. Methane measured in safety gas (Table T7) and science gas samples (Table T8) shows similar trends, but science gas analyses had a higher detection limit because the gas chromatograph used shows a large nitrogen peak near the methane peak. Science gas samples are thus mostly below detection, except for samples at ~137 mbsf, which show 1–2 µM methane. We conclude that methanogensis is not a robust metabolism at Site C0017. The small concentration of methane in pore water from deep horizons (112–137 mbsf) may be hydrothermal in origin or it may result from methanogenesis occurring deep at this site. Hydrogen was not observed except for small peaks at six horizons scattered down the sediment column (Table T8). Sediment carbon, nitrogen, and
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