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Inorganic geochemistry

The main geochemical objective at this site, located seaward of the megasplay, was to document the variations in interstitial water chemical composition. Such data may be used to elucidate origins, volume and nature of flow, and fluid-rock interactions that may affect the state and geotechnical properties of the formation and how they are affected by the presence of MTDs. A total of 33 interstitial water samples were squeezed from selected whole-round sections for chemical analyses. Sample depths ranged from 0 to 309 mbsf. One sample per core was collected when possible. Samples were collected at a higher spatial resolution in the uppermost 10 m in order to define the sulfate-methane transition (SMT).

Fluid recovery

Whole-round lengths ranged from 20 to 31.5 cm, with larger samples collected from cores recovered deeper within the hole where the sediments are more consolidated.

The interstitial water volumes normalized by interstitial water section length, which were recovered from whole-round sections by squeezing at a maximum pressure of 2500 pounds per square inch (17.2 MPa), are presented as a function of depth in Figure F37A. There is a considerable change in interstitial water recovery at ~130.5 mbsf. Interstitial water volumes per centimeter of core range from 1.95 to 2.15 mL/cm between 0 and 130.5 mbsf (Cores 333-C0018A-1H to 15H). The higher recovered volumes correspond to the less compacted lithologies. Below Core 333-C0018A-15H, the average interstitial water volume per section length fell to 1.3 ± 0.4 mL/cm; thus, the whole-round section length was increased to obtain a sufficient volume of interstitial water.

Because of the consolidated nature of the formation in the deeper portion of this site and the coring techniques used, some of the deeper cores were quite disturbed. Although we attempted to remove as much of the outer layers of the samples as possible, complete removal was not always feasible in fractured samples, and thus the interstitial waters can exhibit contamination by drilling fluid (Fig. F37C). Little to no drilling disturbance was noted in cores taken by HPCS, which included the SMT. The disturbed cores taken via EPCS and ESCS fell below the SMT, where in situ SO4 concentration is expected to be 0 mM. Samples that were contaminated by drilling fluid exhibit slightly elevated SO4 values. We used the sulfate concentration to identify and quantify contamination as indicated in “Inorganic geochemistry” in the “Methods” chapter (Expedition 333 Scientists, 2012). Drilling fluid contamination below the SMT reaches ~5%, thus the samples that were obtained using the EPCS and ESCS were sulfate corrected for drilling contamination following the methods of IODP Expedition 322 (Saito, Underwood, Kubo, and the Expedition 322 Scientists, 2010). Given the low levels of drilling-induced contamination in HPCS coring, these samples were not sulfate corrected. The uncorrected interstitial water data collected at Site C0018 are listed in Table T11. In addition, the composite uncorrected HPCS data and sulfate-corrected EPCS and ESCS data are listed in Table T12 and illustrated in Figures F38 and F39.

Salinity, chlorinity, and sodium

Interstitial water salinity rapidly decreases in the upper 30 m of Hole C0018A (Fig. F38). A general increase is observed from 30 to ~100 mbsf, with the trend becoming discontinuous below ~100 mbsf. The decrease in salinity in the upper ~30 m of the sediment section may reflect active sulfate reduction and the subsequent precipitation of authigenic carbonates consuming interstitial water SO4, Ca, alkalinity, and Mg within this interval (Figs. F38, F39). Chlorinity increases rapidly in the upper ~30 m of Hole C0018A and then gradually increases with depth. Dissolved sodium increases in the upper ~30 m of the sediment section of Hole C0018A then broadly increases with depth to 480 mM at 308 mbsf.

Biogeochemical processes

Sulfate and alkalinity

Sulfate concentration decreases rapidly in the upper 15 m of Hole C0018A and then remains steady at less than ~0.8 mM from 15 to ~190 mbsf. SO4 levels increase slightly from ~190 to 310 mbsf, where EPCS and ESCS coring systems were used (Fig. F37B). The rapid decrease in SO4 in the upper 15 m of Hole C0018A may reflect microbial sulfate reduction. Higher SO4 below ~190 mbsf may result from seawater intrusion due to disturbance by the coring system or the contamination with drilling fluid.

Interstitial water alkalinity increases sharply in the upper 15 mbsf of Hole C0018A, coinciding with sulfate depletion, and then remains steady at 26 mM from 15 to 29.5 mbsf. Alkalinity increases gradually from 26 mM at 29.5 mbsf to 31 mM at 84 mbsf and then gradually decreases to 22 mM from 84 to ~190 mbsf. It increases slightly at 220 mbsf and then decreases gradually to 23 mM at 308 mbsf. The elevated alkalinity in the upper 15 m of Hole C0018A reflects rapid alkalinity production in the region of intense sulfate reduction, likely occurring through methane oxidation (see “Organic geochemistry”).

Ammonium, phosphate, and bromide

Dissolved ammonium increases monotonically in Hole C0018A from 0.8 to ~220 mbsf. The dissolved ammonium then slightly decreases from 220 to 290 mbsf. The steady ammonium increase is interpreted as being the result of production by microbially mediated decomposition of organic matter. Ammonium remains elevated below the SMT due to ongoing organic matter degradation fueled by CO2 reduction and higher burial rates during remobilization of sediment resulting in MTDs. The decline with depth likely reflects decreasing metabolic rates, and thus declining ammonium production, as well as NH4 sorption on clay minerals.

Phosphate increases sharply to 128.5 µM in the upper part of Hole C0018A, peaking at 30 mbsf, and then rapidly decreases to 97 µM at 58 mbsf before starting to increase again, peaking at 114.2 µM at 84 mbsf. Below this peak, PO4 displays an overall decreasing trend with a broad interval of negative excursion between 120 and 180 mbsf, coinciding with the occurrence of MTD 6 (see “Lithology”). The initial rapid increase in phosphate reflects active organic matter decomposition, which occurs below the zone of most active sulfate reduction. The decreasing and low phosphate below the depth of maximum concentration is likely controlled by the solubility of apatite, which is a major sink for phosphate.

Bromide concentration in Hole C0018A displays a monotonic increase with depth from 0.9 mM at the surface to 1.1 mM at 308 mbsf. In general, bromide is released during organic matter diagenesis and the observed trend suggests a bromide source deeper than the base of the borehole. However, the curvature of the Br/Cl profile, though chlorinity follows a constant gradient, suggests that part of the bromide is released in situ.

Major cations (Ca, Mg, and K)

Calcium in Hole C0018A decreases from 9.4 mM at the surface to 1.6 mM at 21 mbsf, below which the profile reverses and Ca gradually increases to 6.8 mM at 308 mbsf. A positive excursion coinciding with a large ash horizon at ~50 mbsf is observed. The initial decreasing trend reflects Ca consumption during authigenic carbonate formation in the zone of the most intense sulfate reduction. The increase in Ca with depth may reflect progressive ash alteration and carbonate diagenesis downhole and/or diffusion from a deeper source.

Magnesium decreases throughout Hole C0018A from 51 mM at the surface to 29 mM at 308 mbsf. The decrease is more rapid in the upper 40 m of the hole. This zone of rapid Mg depletion approximately coincides with the SO4 reduction zone, indicating some precipitation of Mg with authigenic carbonates within this depth interval as well as uptake in clay minerals. The general decrease in Mg may indicate that uptake of magnesium in hydrous silicate minerals (mainly clays) that were formed during volcanic ash alteration.

Potassium decreases throughout Hole C0018A from 13 mM at the surface to 8.4 mM at 308 mbsf. In the upper 58 m, the values are greater than the concentration of K in seawater. This is likely a sampling artifact resulting from pressure changes during core recovery and ion exchange with NH4 on clay surfaces. The overall decline in K likely reflects the uptake of dissolved potassium in clays and, possibly, in authigenic zeolites formed during the alteration of volcanic ash and feldspars.

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

Boron in Hole C0018A increases from 490 µM at the surface to 530 µM at 15 mbsf (Fig. F38). From 15 to ~220 mbsf, B decreases gradually to 175 µM. From 225 mbsf, B slightly decreases to 170 µM at 308 mbsf. Boron concentration decreases with depth is commonly observed in marine sediment and generally attributed to boron uptake in clays at low temperature.

Lithium rapidly decreases from 42 µM at the surface to 21 µM at 21 mbsf. Below 21 mbsf, Li gradually increases to 49 µM at 308 mbsf with a positive excursion at 122 mbsf, which may correspond to the volcaniclastic material observed at ~125 mbsf (see “Lithology” results).

Dissolved silica is higher than modern values at all depths, though no Si trend is apparent.

Strontium is discontinuous, ranging from 73 to 83 µM in the upper ~30 m of Hole C0018A, and then increases from 71 µM at 40 mbsf to 85 µM at 160 mbsf with two negative excursions at 122 and 144 mbsf, which may correspond to volcanic material and a chaotic bedding interval, respectively. Between ~170 and ~190 mbsf, Sr is generally lower, ranging from 75 to 81 µM. This may correspond to a region of slightly higher density observed at this interval (see “Physical properties”). Below 223 mbsf, Sr increases with depth to 95 µM at 308 mbsf.

Barium is not detected above 10 mbsf and exhibits a rapid increase across the SMT from 5.2 µM at 11.5 mbsf to 184 µM at 21 mbsf. Ba decreases with depth from 100 µM at 30 mbsf to 21 µM at 244 mbsf and then increases slightly with depth to 28.5 µM at 308 mbsf.

Dissolved manganese rapidly decreases in Hole C0018A from 27.5 µM at the surface to 0.2 µM at 15 mbsf. Below 15 mbsf, Mn is relatively constant around >2 µM. Some increased intervals can be recognized at ~70 and ~190 mbsf. The rapid increase of dissolved Mn in the upper 15 m at this site may reflect MnO2 reduction by microbially mediated organic matter decomposition.

Dissolved iron exhibits considerable variability throughout Hole C0018A.

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

Rubidium at Hole C0018A shows a rapid decrease from 2000 nM at the surface to 1350 nM at 20 mbsf, followed by a gradual decrease to 1200 nM at ~300 mbsf (Fig. F40).

Cesium rapidly decreases from 5.4 nM at the surface to 3.2 nM at 20 mbsf then increases to 6.3 nM at ~110 mbsf. This is followed by a decrease to 4.5 nM at ~200 mbsf. Below this depth concentrations are generally higher and more variable.

Vanadium scatters in the upper 21 m of Hole C0018A. V then increases from 95 nM at 29 mbsf to 125 nM at 58 mbsf, below which it rapidly declines to 65 nM at 68 mbsf. Below 68 mbsf, vanadium increases to 110 nM at 122 mbsf, followed by a rapid decline to 54 nM at 130 mbsf. Below 130 mbsf, V remains relatively constant at ~50 nM.

Copper is relatively depleted (<100 nM) from 15 to 100 mbsf in Hole C0018A compared with the surface (averages 1000 ± 300 nM from 0 to 15 mbsf) with the exception of a positive excursion to 2830 nM at 39 mbsf, which corresponds to a zone of mixed sediment as described in “Lithology.” Below 100 mbsf, Cu exhibits a wide range.

Zinc rapidly decreases in the upper ~20 mbsf of Hole C0018A from ~1500 to 130 nM, followed by a broad increase to ~2500 nM at ~300 mbsf.

Molybdenum exhibits considerable variability throughout Hole C0018A, ranging from ~50 nM to ~350 nM. An interval of low concentration (generally <50 nM from 4 to 23 mbsf) is observed.

Lead is generally low in Hole C0018A. The average background concentration is 1.07 ± 0.74 nM.

Uranium is generally low throughout with small peaks observed at ~50, ~100, and ~175 mbsf.