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doi:10.2204/iodp.proc.322.104.2010

Inorganic geochemistry

The main objective of the inorganic geochemistry program at Site C0012 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, fluid and chemical transport, and water-rock interactions in the presubduction equivalent of the seismogenic zone. A total of 42 pore fluid samples were squeezed from selected whole-round sections for chemical and isotopic analyses. Sample depths ranged from 89.4 to 529.2 m CSF. One sample per core was collected when possible. Because of poor core recovery, no pore fluids were recovered from Cores 322-C0012B-1R, 2R, 3R, 4R, 12R, 22R, 26R, 30R, 38R, and 51R.

Fluid recovery

To obtain enough interstitial water for shipboard and shore-based analyses, 19–48 cm long sections were squeezed. Interstitial water volumes recovered from whole-round sections by squeezing at a maximum pressure of 25,000 psi are presented as a function of depth in Figure F48. Interstitial water volume shows a general decrease with depth, from 28 to 3 mL. In contrast to the distribution observed in Hole C0011B, there is no correlation between water volume and lithologic units or sand content of the samples.

Even though the strata at Site C0012 are moderately lithified, core quality was significantly better than that at Site C0011; thus, contamination with drilling fluid at this site was minimal. The dissolved sulfate profile (Fig. F49) shows quite a bit of structure, which is consistent with biogeochemical processes at this site (see "Biogeochemical processes"). The observed distribution precludes any correction for potential contamination because a total depletion of sulfate in the formation fluids could not be assumed. The interstitial water data collected at Site C0012 are listed in Table T20 and illustrated in Figures F49, F50, and F51. These distributions reflect the balance between reactions in the sediment section, reactions with underlying oceanic crust, and possibly diffusive interaction with seawater-like fluid within basaltic basement.

Biogeochemical processes

The dissolved sulfate profile indicates continued depletion with depth to ~300 m CSF. There is a small amount of ammonium at this site (<1 mM), which reflects the production of this metabolite during organic matter degradation. However, the actual production and consumption pathways cannot be fully constrained because of a lack of samples shallower than 89 m CSF. Alkalinity is also probably produced in the shallower section, and the observed low values throughout the hole reflect low carbon cycling rates at this site, superimposed on the consumption of bicarbonate ions by ash alteration and carbonate formation (Fig. F49).

The sulfate profile clearly documents a sulfate reduction zone that is significantly deeper than that recovered at other sites in this margin (Fig. F52). Other sites have rapid deposition and burial of sediment by turbidites, with TOC content ranging from 0.3 to 1 wt% (see "Organic geochemistry" in the "Site C0011" chapter and Shipboard Scientific Party, 2001). Sedimentation rates ranging from 6 to 11 cm/k.y. at ODP Sites 1173, 1175, and 1176 lead to sulfate consumption in the upper 20 m CSF because of a lessening resupply by diffusion from overlying water. Site C0012 sits on a bathymetric high; here a lower sedimentation rate (see "Biostratigraphy") and the generally low organic matter content of the sediment (TOC 0.2 wt%, see "Organic geochemistry") lead to extremely slow sulfate consumption. Thus, the topography of Site C0012 provides an end-member system to study processes associated with metabolic carbon pathways. It is worth noting that sulfate depletion at ~300 m CSF coincides with a marked increase in methane concentration (see "Organic geochemistry"). It is possible that the sulfate profile at Site C0012 is driven by anaerobic oxidation of this methane. This reaction leads to the production of hydrogen sulfide (Fig. F52), as manifested by a marked increase in concentration concomitant with the methane peak. Sulfide produced by this process is removed as a variety of iron sulfide minerals, with pyrite being its most stable phase. Pyrite is commonly observed in sediments from this depth range (see "Lithology").

Another interesting aspect of the observed sulfate profile is the increase in concentration in the lower strata below ~450 m CSF, which also feed the anaerobic methane oxidation (AMO). Detailed sampling of the expanded sulfate–methane transition zone at this site and postcruise analyses of organic metabolites and their isotopic composition will enhance our understanding of the operating microbial pathways (see "Microbiology" and "Organic geochemistry").

Halogen concentration (Cl and Br)

The chlorinity profile at Site C0012 increases by 12% from ~560 mM to a maximum value of 627 mM at 509 m CSF. The probable cause is hydration reactions during alteration of volcanic sand, dispersed volcanic glass, and basement. At Site C0012, there is no indication of any of the freshening patterns observed at Site C0011 and at sites drilled seaward of the deformation front during ODP Legs 131 and 190 (Gieskes et al., 1993; Shipboard Scientific Party, 2001). At Site 1177, where heat flow is relatively low, the freshened interstitial waters are thought to have originated from greater depth arcward and reflect fluid migration updip through high-permeability horizons (Steurer and Underwood, 2003; Saffer et al., 2008; Saffer and McKiernan, 2009). If this interpretation is correct, then the lack of a freshening pattern at Site C0012 indicates that a similar system of fluid flow might advance to Site C0011 but not reach the crest of the Kashinosaki Knoll. Although noteworthy when compared to results from Site C0011, these chlorinity data are not the highest observed along the transect defined by IODP Expeditions 315, 316, and 322. Along this transect, IODP Site C0004, located to access the shallow portion of the megasplay fault system, has even higher chlorinity values (Expedition 316 Scientists, 2009).

Chemical changes due to alteration of volcaniclastics

Changes in dissolved magnesium, potassium, and silica concentrations are driven by several processes, which are dominated at Site C0012 by alteration of volcanic ash, volcanic rock fragments, and underlying basement rock (e.g., Gieskes et al., 1990; Lawrence and Gieskes, 1981). Decreasing magnesium, potassium, and silica concentrations in the upper 220 m CSF illustrates the importance of Mg and K uptake by clay formation during volcanic ash alteration. Rubidium, zinc, and uranium also show a marked decrease in concentration, suggesting uptake of these minerals during smectite formation (Fig. F51). As shown for Site C0011, first-order equilibrium thermodynamic calculations indicate that the distribution of magnesium, potassium, and silica below 250 m CSF is controlled by formation of montmorillonite phases (Fig. F53).

For calcium, the predicted increase because of ash alteration is overprinted by carbonate precipitation and by deep-seated reactions within the lower sediment section. The high levels of dissolved calcium support carbonate precipitation, even at very low alkalinity (<2 mM), as shown by values above saturation for calcite and dolomite (Fig. F53). Carbonate was recovered throughout the cored section as vein filling, carbonate cements, and discrete layers up to 16 cm thick (see "Lithology"). Zeolite formation from volcanic glass alteration is also favored by high calcium concentration in the basal fluids. The decrease in dissolved Na in the lower sections sampled is consistent with the formation of these minerals, and the low dissolved potassium content in the fluids probably hinders formation of K-clinoptilolite (Fig. F53).

The increase in strontium in interstitial waters from reaction with volcanoclastic material is well established (e.g., Gieskes et al., 1990) and is likely the cause of the observed increase in dissolved Sr with depth at Site C0012 (Fig. F50). In contrast to observations at Site C0011, lithium distribution shows a decrease with depth below a maximum value of ~208 µM at 227.5 m CSF. Field and laboratory observations document remobilization of lithium at temperatures >70°C and uptake at lower temperatures (Edmond et al., 1979; Seyfried et al., 1984; You et al., 1996). The causes for the unusual Li profiles at Site C0012 are not yet clear. Analyses of the distribution of this element among other sites drilled in this margin and isotopic data collected postcruise will help constrain the processes leading to the observed Li distributions.

Basal fluids

Profiles of most major cations and sulfate show an intriguing reversal to near-seawater values in Cores 322-C0012B-48R and 49R (between 490 and 500 m CSF). Similar reversals have been observed at the flanks of the Juan de Fuca Ridge (e.g., Elderfield et al., 1999; Wheat and Mottl, 1994) and in the equatorial Pacific (Baker et al., 1991; Oyun et al., 1995), which were cited as evidence for the presence of fluid flow in the underlying oceanic crust. To provide a preliminary evaluation of the nature of the anomalous pore fluid at Site C0012, we estimate the fraction of seawater needed to produce the reversal in the potassium profile to be ~20%. We then estimate major ion concentration in the fluids if no anomaly was present by interpolating data from above and below Cores 322-C0012B-48R and 49R and calculate the concentration that would be created by a 20% dilution of this signal with seawater. The results (shown in Table T21) show good consistency between the expected and observed values. We tentatively attributed the observed reversals to a seawater-like fluid migrating through the permeable basaltic basement. At this moment, the hydrology controlling this flow, including potential recharge and discharge zones, remains unidentified.

These results outline the intriguing possibility of two fluid regimes that alter the chemical composition of the interstitial water in the sediments seaward of the Nankai Trench. One regime is characterized by in situ dehydration of clay minerals in areas of high heat flow (Steurer and Underwood, 2003). At Site 1177, where heat flow is lower, there is evidence of local migration through high-permeability turbidite units in the incoming sediment section. Patterns of fluid freshening (negative chloride anomalies) (Saffer et al., 2008; Saffer and McKiernan, 2009) and the presence of methane and higher hydrocarbons are similar at Sites 1177 and C0012; the methane and ethane measured may indicate a very small contribution of fluid flow at this site, or alternatively, these hydrocarbons may be generated in situ by microbial processes. Identification of the hydrocarbon sources remains to be resolved by shore-based investigations, and high-quality borehole temperature data will allow better assessments of in situ reaction conditions.

Here, we postulate the presence of another flow regime driven by migration of a seawater-like flow through the upper basaltic crust, which modifies the chemical composition observed within the sediment pile. In particular, the observed increase in sulfate below 490 m CSF cannot be supplied by the methane-rich fluids that originate from greater depth arcward because those fluids are depleted in sulfate. Furthermore, the fact that we see an increase in hydrogen sulfide produced by AMO in the overlying sediments, argues for the sustained presence of sulfate in upper basaltic basement, which must be supplied by active flow within the basaltic crust. This fluid in basaltic basement is actively exchanging with the ocean and altering the interstitial fluid composition of the deep sediments by either diffusional exchange, or a via a hydrologic connection to the sandstone turbidites in lithologic Unit V.

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