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doi:10.2204/iodp.proc.333.105.2012

Inorganic geochemistry

The main objective of the inorganic geochemistry program at this site was to document the geochemical properties of subduction inputs at a site located at a basement high, near the crest of the Kashinosaki Knoll. Such data will increase our understanding of how basement topography and concomitant changes in temperature regime and stratigraphy may control fluid composition, flow, and water-rock interactions in the presubduction equivalent of the seismogenic zone. A total of 29 interstitial water samples were squeezed from selected whole-round sections for chemical and isotopic analyses. Sample depths ranged from 0 to ~525 mbsf. One sample per core was collected when possible; however, samples were collected at a higher spatial resolution in the uppermost 10 m in order to potentially help define the sulfate–methane transition. Additionally, two samples were collected from the red claystone in Core 333-C0012E-3X.

Fluid recovery

To obtain enough interstitial water for shipboard and shore-based analyses, 19–31 cm long whole-round sections were squeezed from Holes C0012C and C0012D. In the red claystone located directly above the basement in Holes C0012E and C0012G, 49–61 cm long whole-round sections were squeezed. Interstitial water volumes normalized by section length (mL/cm) 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 F45A. Interstitial water volumes per length of interstitial water section show a decrease with depth from 2.75 to 0.71 mL/cm in the upper ~180 m of Holes C0012C and C0012D. In the deeper material above the basement, volumes were significantly lower, ranging from 0.08 to 0.22 mL/cm.

The strata here are slightly to moderately lithified; however, the core quality, particularly for core obtained via the HPCS, was good. Therefore, we expect contamination by drilling fluid at this site to be minimal. The dissolved sulfate profile (Fig. F45B) shows quite a bit of structure. The observed distribution precludes any correction for potential contamination, which is consistent with the findings of Expedition 322 (Expedition 322 Scientists, 2010). The uncorrected interstitial water data collected at Site C0012 are listed in Table T11 and illustrated in Figures F46, F47, and F48.

Salinity, chlorinity, and sodium

Interstitial water salinity in Holes C0012C and C0012D increases slightly from 35.72 at ~0.6 mbsf to 36.33 at ~47 mbsf then decreases with depth to 33.56 at ~180 mbsf. The salinity profile shows good continuity with that of Hole C0012A drilled during Expedition 322. Chlorinity in Holes C0012C and C0012D decreases from 549 mM at the surface to 566 mM at 47 mbsf, which is followed by a slight decrease to 557 mM at 180 mbsf. The data from Expedition 322 indicate an increasing trend in chlorinity below this depth. This is consistent with what is observed in Holes C0012E and C0012G near the sediment/basalt interface where Cl values are high, ranging from 600 to 640 mM. The trend of increasing Cl is likely caused by hydration reactions during alteration of volcanic ash, volcanic sand, and basaltic basement, as suggested by Expedition 322 Scientists (2010).

Sodium remains relatively constant at ~470 mM from the surface to 180 mbsf in Holes C0012C and C0012D. This is consistent with the observed Na profile in Hole C0012A. Sodium in Holes C0012E and C0012G is low, ranging from 260 to 300 mM. The decrease in dissolved Na in the lower sections may be a result of carbonate formation or the formation of zeolites from the alteration of volcanic glass.

Biogeochemical processes

Sulfate and alkalinity

Sulfate in Holes C0012C and C0012D slightly decreases from 28.6 mM at the surface to 26.3 mM at 74 mbsf. The dissolved sulfate profile below 74 mbsf indicates continued depletion with depth, which is consistent with the observations of Expedition 322. The relatively constant sulfate profile from the surface to 74 mbsf exhibits SO₄ concentration similar to modern seawater. Below 74 mbsf, sulfate decreases downward, almost linearly with slight curvature, suggesting most sulfate reduction occurs below the depths cored during Expedition 333 and therefore that the sulfate reduction zone must lie much deeper than at Site C0011, where it is observed around 80 mbsf. The sulfate reduction zone at Site C0011 is, itself, relatively deep compared to other sites along the Kumano transect (Tobin et al., 2009), including IODP Site C0018, where the sulfate reduction zone is found within the upper 15–20 mbsf. During Expedition 322 it was observed that maximum sulfate depletion, which occurs at ~300 mbsf, coincided with a marked increase in methane concentration (Expedition 322 Scientists, 2010). They interpreted the sulfate profile at Site C0012 as being driven by anaerobic methane oxidation. The deep sulfate reduction zone at Site C0012 may be attributed to the bathymetric high on which Site C0012 sits, leading to slower average sedimentation rates, particularly over the last 3 m.y. (see “Paleomagnetism”), and lower average organic matter content than at Site C0011. Additionally, in Holes C0012E and C0012G and the deepest sections of Hole C0012A, sulfate concentration increases slowly in the interval below ~450 mbsf, which may indicate diffusional exchange with fluids from the basalt basement.

Alkalinity in Holes C0012C and C0012D increases with depth from 2.9 mM near the surface to 4.0 mM at 47 mbsf and then decreases to 1.8 mM at 106 mbsf. Below 106 mbsf, alkalinity increases again with depth to 2.6 mM at 138 mbsf and then decreases to 0.7 mM at 180 mbsf. In Holes C0012E and C0012G, alkalinity is <1 mM, which is consistent with Hole C0012A (Fig. F46).

Ammonium, phosphate, and bromide

Ammonium in Holes C0012C and C0012D increases with depth from 0 mM at the surface to 0.58 mM at 120 mbsf and then stays almost constant at ~0.6 mM between 120 and 170 mbsf. Ammonium increases again with depth from 0.62 mM at 170 mbsf to 0.72 mM at 180 mbsf. Ammonium throughout Holes C0012C and C0012D and into Holes C0012A, C0012E, and C0012G is quite low (<1 mM).

Phosphate in Holes C0012C and C0012D gradually decreases with depth from 4.4 µM near the surface to 0.5 µM at ~120 mbsf. Below ~170 mbsf, dissolved phosphate is not detected. Bromide increases monotonically throughout Holes C0012C and C0012D. This trend continues throughout the sediment drilled in Holes C0012E/C0012G and C0012A to the sediment/basalt interface.

Major cations (Ca, Mg, and K)

Calcium increases with depth throughout Holes C0012C and C0012D from 11 mM at ~0 mbsf to 43 mM at 180 mbsf. The rate of increase changes abruptly at ~70 mbsf. This trend is consistent with the observed data from Hole C0012A. The deeper sediment of Holes C0012E and C0012G has notably higher Ca (177–207 mM at 502–524 mbsf), which is also consistent with the Hole C0012A data. The Ca trend observed at Site C0012 is consistent with carbonate precipitation even at very low alkalinity (<2 mM). Carbonate was recovered from veins in sandstones at ~170 mbsf and the red claystone directly above the basement.

Magnesium in Holes C0012C and C0012D decreases with depth from 52 mM at the surface to 7.9 mM at 180 mbsf. The rate of the decrease becomes higher at ~70 mbsf. The Mg trend is well correlated with that found during Expedition 322 and continues to Holes C0012E and C0012G, with low Mg values (<10 mM).

Potassium in Holes C0012C and C0012D generally decreases with depth from 11.5 mM at ~0 mbsf to 2.4 mM at 180 mbsf. The rate of change in the K decrease becomes higher at ~70 mbsf. This is consistent with the trend from Hole C0012A, which continues to Holes C0012E and C0012G. Decreasing Mg and K in the upper ~200 mbsf may be interpreted as Mg and K uptake by clay formation (i.e., smectite) during volcanic ash alteration.

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

Boron in Holes C0012C and C0012D decreases with depth from 462 µM at ~0 mbsf to 380 µM at 64 mbsf and then abruptly decreases to 287 µM at 74 mbsf. Below 74 mbsf, B is discontinuous. The average B in Hole C0012D (317 ± 60 µM) is consistent with the observations of Hole C0012A. The B data in Holes C0012E and C0012G are well correlated.

Lithium in Holes C0012C and C0012D generally increases with depth from 2.3 µM near the surface to 215 µM at ~150 mbsf and then remains relatively constant from 150 to 180 mbsf. In the interval from 74 to 92 mbsf, the rate of Li increase is quite high (3 µM/m). The Li profile of Holes C0012C and C0012D is slightly offset from that of Hole C0012A, particularly below 60 mbsf and below 500 mbsf (Fig. F47).

Dissolved silica is generally higher than Si in modern seawater (160 µM) and exhibits a rapid increase from ~460 µM at the seafloor to ~840 µM at 64 mbsf. This is followed by a significant decrease to ~290 µM at ~91 mbsf, below which it is fairly scattered. This trend is consistent with the observed profile of dissolved Si in Hole C0012A, which displays a step at 200 mbsf, where Si concentration further drops to values around 100 µM near the base of the hole. The deeper Holes C0012E and C0012G fall along the trend of slightly increasing Si below 400 mbsf, which was observed in Hole C0012A. The distinct decrease in Si at 91 mbsf is consistent with a sharp decrease in porosity, which is observed between ~70 and 100 mbsf (see “Physical properties”). In conjunction with the findings of Expedition 322, we interpret the distribution of Si, as well as Mg and K, below ~100 mbsf as being controlled by the formation of montmorillonite phases during the alteration of volcanic glass (Expedition 322 Scientists, 2010).

Strontium in Holes C0012C and C0012D increases from 87 µM at ~0 mbsf to 190 µM at 180 mbsf. The rate of Sr increase downhole changes at ~80 mbsf from 0.2 to 0.75 µM/m. The Sr profile correlates relatively well with that observed in Hole C0012A. The increase in Sr with depth is likely related to the reaction of volcaniclastic material. Sr in Holes C0012E and C0012G is distinctly high (360–420 µM at 502–524 mbsf) and may be related to the alteration of basalt.

Barium in Holes C0012C and C0012D generally increases from 0.5 µM near the surface to 2.1 µM at 180 mbsf. The rate of Ba increase changes at ~90 mbsf from 3 to 16 nM/m. The trend in Ba concentration is continued to Hole C0012A, and trends at the base of Hole C0012 continue into Holes C0012E and C0012G.

Manganese in Holes C0012C and C0012D increases from 7.2 µM at ~0 mbsf to 120 µM at 160 mbsf and then rapidly decreases to 46 µM at 180 mbsf. The pattern of rapid decrease is consistent with Mn in Hole C0012A. Mn in Holes C0012E and C0012G is relatively low (10–36 µM at 502–524 mbsf) and consistent with that of Hole C0012A.

Iron in Holes C0012C and C0012D, though displaying some scatter in the data, broadly decreases with depth from a maximum of ~15 µM at 9 mbsf to <1 µM at ~100 mbsf. The trend of low Fe concentration continues into Hole C0012A. Below ~200 mbsf, Fe is generally not detected, which is also the case in Hole C0012A, and any Fe that is detected is <1 µM.

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

Rubidium decreases with depth from ~1770 nM near the surface to 180 nM at 180 mbsf in Holes C0012C and C0012D. In Holes C0012E and C0012G, Rb is slightly lower (~100–130 nM). Rb is consistent with Hole C0012A from Expedition 322 (Fig. F48).

Cesium decreases with depth from 4.8 nM at ~0 mbsf to 1.5 nM at 47 mbsf in Holes C0012C and C0012D and then abruptly increases to ~2 nM at ~70 mbsf. Then, Cs increases from 1.3 nM at 92 mbsf to 2.1 nM at 180 mbsf with little fluctuation. Cs in Holes C0012E and C0012G is relatively low (1.1–2.0 nM at 502–524 mbsf). These values correspond well to the observed Cs in Hole C0012A.

Vanadium ranges from 25 to 46 nM in the upper 30 mbsf. V slightly increases with depth from 28 nM at 35 mbsf to 30 nM at 74 mbsf. Below 92 mbsf, V remains relatively constant at ~25 nM. V in Holes C0012C and C0012D is ~15 nM higher than in Hole C0012A in the overlapping interval from 90 to 180 mbsf. In Hole C0012E, V ranges from 28 to 38 nM in the interval from 502 to 524 mbsf. In Hole C0012A, a broader range of V was observed; V concentration in C0012E and C0012G falls within this range.

Copper is on average 2300 ± 210 nM in the upper 17 m of Hole C0012C. In the interval from 28 to 180 mbsf, Cu ranges from 500 to 7600 nM. In the red claystone of Holes C0012E and C0012G, Cu is relatively low, ranging from 560 to 1600 nM.

Zinc ranges from 500 to 1500 nM in the upper 150 m of Holes C0012C and C0012D. Below 150 mbsf, Zn is generally <500 nM. In Holes C0012E and C0012G, Zn exhibits a large range with concentration from 300 to 1400 nM in the interval from 502 to 524 mbsf. Large variation is also observed within the data from Expedition 322.

Molybdenum in Holes C0012C and C0012D slightly decreases with depth from 134 nM at the surface to 93 nM at 64 mbsf and then increases to ~300 nM at 180 mbsf. This is consistent with Mo from Hole C0012A. Mo in Holes C0012E and C0012G ranges from 270 to 360 nM, which is consistent with Hole C0012A.

Lead in Holes C0012C and C0012D ranges from 2.7 to 3.6 nM in the upper 9 mbsf with a broad decrease with depth to 0.7 nM at 64 mbsf. Below 91 mbsf, Pb generally increases to 2.7 nM at 130 mbsf, followed by a decrease to 0.6 nM at 180 mbsf. In Hole C0012A, Pb is relatively constant with average Pb values of 10 ± 0.37 nM. The Pb values of the red claystone in Holes C0012E and C0012G are consistent with this value.

Uranium decreases with depth from 5.9 nM near the surface to ~2.4 nM at ~50 mbsf. U remains relatively consistent at ~2.4 nM in the interval from 50 to 130 mbsf and then decreases with depth to 0.4 nM at 180 mbsf. This trend is broadly consistent with the data from Expedition 322, as is the U within the deep sediment in Holes C0012E and C0012G.

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