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doi:10.2204/iodp.proc.333.104.2012

Inorganic geochemistry

The main geochemical objective in Holes C0011C and C0011D, located seaward of the trench, was to document the variations in interstitial water chemical composition at shallow depths that could not be recovered during Expedition 322. In combination with the data obtained during Expedition 322, data from this site 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 strata at the décollement and their evolution postsubduction. A total of 48 interstitial water samples were squeezed from selected whole-round sections for chemical analyses. Sample depths ranged from 0 to 379 mbsf, with samples from 360 to 379 mbsf overlapping Hole C0011B, which was cored during Expedition 322 (Expedition 322 Scientists, 2010). 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 define the sulfate–methane transition (SMT) zone.

Fluid recovery

In order to obtain enough interstitial water for shipboard and shore-based analyses, whole-round sections from 19 to 41 cm long were squeezed from Holes C0011C and C0011D. Longer sections were squeezed from cores recovered deeper within the holes where the sediments were more consolidated. Interstitial water volumes normalized by interstitial water section length (mL/cm) are presented as a function of depth in Figure F42A. Interstitial water recovery changes significantly at ~184.5 mbsf. Interstitial water volumes per centimeter of core range from 1.7 to 2.8 mL/cm between 0 and 184.5 mbsf (Cores 333-C0011C-1H to 333-C0011D-21H). Below Core 333-C0011D-21H (~184.5 mbsf), the average interstitial water volume per section length fell to 0.7 ± 0.3 mL/cm.

Because of the semilithified nature of the sediment in many of the deeper cores and the coring methods used, many of the cores taken by the EPCS and ESCS were moderately to highly disturbed. As much of the outer layers of these fractured samples as possible was removed prior to squeezing. Complete removal of the outer layers was not always possible, and thus, the interstitial waters may be contaminated by drilling fluid. Little to no drilling disturbance was noted in cores taken by the HPCS, which included the SMT. The disturbed cores taken via the EPCS or ESCS fell below the SMT; thus sulfate concentration is used to identify and quantify contamination as indicated in “Inorganic geochemistry” in the “Methods” chapter (Expedition 333 Scientists, 2012). The magnitude of possible drilling fluid contamination in the interval recovered by the EPCS and ESCS reaches ~13%; thus the samples that were obtained using the EPCS and ESCS were sulfate corrected for possible drilling contamination following the methods of Expedition 322 (Expedition 322 Scientists, 2010). The HPCS samples were not sulfate corrected. The uncorrected interstitial water data collected at Site C0011 are listed in Table T12. In addition, Table T13 presents a composite data set of uncorrected HPCS data and sulfate-corrected EPCS and ESCS data. The composite data are illustrated in Figures F43 and F44.

Salinity, chlorinity, and sodium

Interstitial water salinity rapidly decreases in the upper 70 m of Holes C0011C and C0011D (Fig. F43). Below this point, salinity continuously decreases. Salinity shows good correlation with the data from Expedition 322 for the overlapping depths. We interpret the rapid decrease in salinity in the upper ~70 m of the sediment section as reflecting active sulfate reduction and the subsequent precipitation of authigenic carbonates, which consumes interstitial water SO₄^2–, Ca, and Mg within this interval (Figs. F42B, F43, F44).

Chlorinity increases rapidly in the upper ~25 m of Hole C0011C, as generally observed below the seafloor. In the interval between ~25 and ~250 mbsf, chlorinity values are relatively constant. Below ~250 mbsf, chlorinity decreases with increasing depth. This is consistent with the results of Expedition 322 and may reflect diffusional exchange with a deeper stratigraphic interval where freshened interstitial water migrated along sandy turbidites. Freshening of the interstitial water probably occurred at depths considerably greater than the coring limits for this hole, where clay mineral dehydration reactions have advanced.

Dissolved sodium increases in the upper ~36 m of Site C0011. Sodium values are relatively constant from 36 to ~250 mbsf, followed by a slightly decreasing trend with depth, which was also observed during Expedition 322 in Hole C0011B.

Biogeochemical processes

Sulfate and alkalinity

Sulfate concentration decreases rapidly in the upper 70 m of Hole C0011C, followed by a less dramatic decrease from 70 to 183 mbsf, which corresponds to the end of HPCS coring (Fig. F42B). Sulfate is less continuous in the interval from ~200 to 380 mbsf, in which the EPCS and ESCS were used. The sulfate profile documents a sulfate reduction zone that is deeper than is observed at other sites in the Nankai area (e.g., Moore, Taira, Klaus, et al., 2001; Tobin et al., 2009). However, this site and Site C0012 are the only sites with known interstitial water sulfate concentration that are located in the Shikoku Basin. Site 1173 had overlying trench turbidites, the upper part of Site 1177 was not cored, and other sites were either in the trough or on the margin. Considering that the sedimentation within the uppermost 300 m at Site C0011 is dominantly hemipelagic with <0.5 wt% total organic carbon (TOC) deposited at relatively low sedimentation rates (<100 m/m.y.,) a relatively deep sulfate reduction zone is understandable. The scattering of sulfate below ~190 mbsf may reflect seawater intrusion resulting from disturbance during coring.

Interstitial water alkalinity increases sharply in the upper part of Hole C0011C and reaches a maximum at 79 mbsf. Alkalinity then decreases with depth to ~300 mbsf and increases again below this level (Fig. F43). The alkalinity trend shows a good correlation with interstitial water measured in the deeper sediments in Hole C0011B. A concurrent maximum in alkalinity and progressive change in sulfate concentration gradient at ~80 mbsf defines an upper zone of sulfate reduction at 40–150 mbsf. However, this zone does not correspond to the SMT. Methane concentration only reaches detection threshold below 260 mbsf (see “Organic geochemistry”) then progressively increases downward and reaches relatively high but scattered values in lithologic Unit II. Furthermore, sulfate is present at small concentrations to at least 183 mbsf. Barium concentration in solution rises sharply below 300 mbsf, indicating the remaining sulfate is removed from solution in this depth range, concurrently with a small increase of alkalinity. The SMT thus appears located some distance above the Unit I/II boundary (see “Lithology”). Its depth may be influenced by a minor methane source in Unit II, which has a slightly higher average TOC content than the hemipelagites above and/or by lateral migration in the tuffaceous sandstones.

Ammonium, phosphate, and bromide

Dissolved ammonium rapidly increases in the upper 95 mbsf of Holes C0011C and C0011D, which is followed by a slight decrease from 95 to 380 mbsf (Fig. F43). We interpret the initial increase as being the result of microbially mediated decomposition of organic matter. The decline with depth likely reflects decreasing metabolic rates, thus declining ammonium production as well as NH₄ sorption onto clay minerals.

Phosphate increases sharply with depth in Hole C0011C and the uppermost part of Hole C0011D, peaking at 36 mbsf and then exponentially decreases with depth to 380 mbsf (Fig. F43). The initial rapid increase in phosphate reflects active organic matter decomposition, which occurs at the top of the zone of 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 increases with depth in Hole C0011C and in the uppermost portion of Hole C0011D and then slightly decreases from ~50 to 380 mbsf (Fig. F43). There is some discontinuity between the Br trends in Hole C0011C/C0011D drilled during this expedition and Hole C0011B drilled during Expedition 322.

Major cations (Ca, Mg, and K)

Dissolved calcium in Holes C0011C and C0011D decreases with depth from 0 to ~60 mbsf, below which the profile reverses and Ca gradually increases (Fig. F44). There is good continuity with Hole C0011B. The initial decreasing trend may reflect Ca consumption during authigenic carbonate formation in the zone of most intense sulfate reduction. The increase in Ca with depth may reflect progressive ash alteration and carbonate diagenesis downhole.

Magnesium decreases throughout Holes C0011C and C0011D, which is consistent with the trend seen in Hole C0011B during Expedition 322 (Fig. F44). The decrease is more rapid in the upper 61 m of the sedimentary section. This zone of rapid Mg depletion roughly corresponds to the sulfate 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 uptake of magnesium in hydrous silicate minerals (mainly clays) formed during volcanic ash alteration.

Potassium decreases throughout Holes C0011C and C0011D (Fig. F44). The overall decline in K likely reflects the uptake of dissolved potassium during the alteration of volcanic ash and feldspars.

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

Boron in Hole C0011C rapidly decreases from 480 µM at 0.6 mbsf to 240 µM at ~80 mbsf (Fig. F43). Between 80 and ~280 mbsf, boron decreases downhole slightly with some fluctuations then increases with depth to 380 mbsf. These values are consistent with those of the upper portion of Hole C0011B.

Lithium gradually increases from 34.6 µM at 0 mbsf to 166 µM at 250 mbsf and then slightly decreases with depth. Li shows good correlation between Holes C0011C/C0011D and C0011B (Fig. F44). The overall increasing lithium trend may reflect authigenic clay formation.

Dissolved silica is generally higher than silica concentration in modern seawater and is generally stable from 0 to 250 mbsf with concentrations of ~650 µM throughout that interval. At ~250 mbsf, silica exhibits a rapid decline to ~100 µM before increasing to ~500 µM at ~380 mbsf. This trend continues in Hole C0011B. The rapid decrease in silica is consistent with the observed decrease in porosity between 240 and 250 mbsf (“Physical properties”) as well as the dissolved silica profiles from Sites 1173 and 1177, the reference sites of the Muroto and Ashizuri transects, respectively (Moore, Taira, Klaus, et al., 2001). The secondary silica maximum around 560 mbsf correlates with the tuffaceaous sandstones in lithologic Unit II that still contain unaltered volcanic glass (see “Lithology”).

Strontium gradually increases with depth in Holes C0011C and C0011D from 80 µM at the seafloor to 100 µM at 180 mbsf. From 180 to 270 mbsf, Sr rapidly increases from 100 to 130 µM (Fig. F44). Sr stays constant or slightly decreases with depth below 170 mbsf. This trend continues in Hole C0011B and may be due to alteration of volcaniclastic material.

Barium gradually increases from 0.6 µM at 0 mbsf to 13 µM at 270 mbsf in Holes C0011C and C0011D and then drastically increases to 95.7 µM in the interval from 270 to 380 mbsf (Fig. F44). The increase at depth may be attributed to a greater amount of barite dissolution or fluid migration in volcaniclastics at this interval.

Dissolved manganese in Holes C0011C and C0011D rapidly increases with depth from 4.2 µM at 0.6 mbsf to 16 µM at 20 mbsf and then decreases to 4.1 µM at 70 mbsf (Fig. F44). Below 70 mbsf, Mn rapidly increases again with depth to 30 µM at 170 mbsf and then generally increases to 50 µM at 380 mbsf with negative excursion in the interval between 270 and 310 mbsf. The rapid incline in the upper interval from 0 to 20 mbsf at this site reflects MnO₂ reduction within a depth interval where MnO₂ is a favorable and important electron acceptor for microbially mediated organic matter decomposition.

In the upper ~70 m of Site C0011, dissolved iron does not exhibit a clear trend (Fig. F44). Below 70 mbsf, Fe is generally constant at >2 µM and although there was little reported data from Expedition 322, the existing data are consistent with the data observed in Holes C0011C and C0011D.

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

Rubidium in Holes C0011C and C0011D shows a linear decrease from 3600 nM at 0 mbsf to 123 nM at 130 mbsf and then stays constant from 130 to 210 mbsf, followed by a gradual decrease to 570 nM at ~300 mbsf (Fig. F45). Below 300 mbsf, Rb increases to 1090 µM at 350 mbsf and then decreases to 650 µM at 380 mbsf.

Cesium fluctuates twice between ~90 and 350 mbsf (Fig. F45). It decreases with depth from 3.2 nM at 0 mbsf to 1.8 nM at 20 mbsf, followed by an increase to 3.1 nM at 210 mbsf, and then decreases again to 1.3 nM at ~320 mbsf, followed by an increase to 4.2 nM at ~350 mbsf.

Vanadium increases downhole with scattering in Hole C0011C and in the upper part of Hole C0011D from ~40 nM at the surface to ~60 nM at ~100 mbsf (Fig. F45). V concentration abruptly decreases to 36 nM at ~120 mbsf and then decreases to 23 nM at 380 mbsf. Copper exhibits considerable variability throughout Holes C0011C and C0011D, ranging from below detection limit to 4300 nM (Fig. F45). Zinc concentration in Holes C0011C and C0011D fluctuates between ~1000 and ~2000 nM in the upper ~120 m, below which there is no clear trend (Fig. F45).

Molybdenum concentration fluctuates throughout Site C0011 (Fig. F45). Mo decreases from ~100 nM at the surface to ~30 nM at 100 mbsf and then increases to ~200 nM at 300 mbsf, followed by a decrease to ~70 nM at 380 mbsf. Lead is generally low (0.8 ± 0.4 nM) in the upper ~80 mbsf and higher with a less cohesive trend (2.3 ± 1.8 nM) below 80 mbsf (Fig. F45). Uranium is generally low (3.5 ± 2.1 nM) throughout. U exhibits a peak of 24.7 nM at the surface (Fig. F45).

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