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doi:10.2204/iodp.proc.311.105.2006

Interstitial water geochemistry

The main objectives at this site were to document the depth distribution of gas hydrate, to quantify the amount of gas hydrate in the sediment by comparing the pressure core degassing data with the IW chlorinity data, and to identify the microbially mediated and inorganic reactions associated within and below the gas hydrate stability zone (GHSZ). A total of 145 IW samples were processed from three holes drilled at Site U1327. In Hole U1327C, 76 whole-round samples, most 10–30 cm in length, were squeezed and analyzed. These include 65 IW samples, 5 of which were divided into subsamples based on IR images, generating 7 additional samples for a total of 72 samples for Hole U1327C. Four samples of poor quality were not squeezed. The whole-round samples were collected with a sampling frequency of four in the second core, three in the third core, two per core in the first core and from the fourth core to a depth of 231.91 mbsf, and one per core for the remaining cores. In addition, five samples were squeezed from pressure Cores 311-U1327C-15P and 24P, retrieved from 121.8 and 197.3 mbsf, respectively.

In Hole U1327D, the first two APC cores were dedicated to high-resolution microbiological and geochemical sampling, from which a total of 24 whole-round samples, 15 cm in length, were processed. These samples cover the interval from the seafloor across the SMI to 15.9 mbsf. Whole-round samples for microbiological studies and headspace (HS) samples for gas analyses were taken adjacent to each whole-round IW sample (see "Microbiology" and "Organic geochemistry"). The sampling frequency was four whole-round samples per section to the SMI depth and one per section in the first two sections below the SMI; however, the location of the SMI appears to fall in an interval that was not recovered, between Cores 311-U1327D-1H and 2H. In addition to the high-resolution sampling in the uppermost 15.9 m, six whole-round samples were collected by XCB coring between 129.0 and 221.0 mbsf, with the purpose of documenting the distribution of gas hydrate and calibrating the IR images. Sampling of these cores was guided by IR imaging, and only sections of the core with medium–large amplitude negative thermal anomalies were selected. Two of these samples (311-U1327D-11X-1, 62–72 cm, and 15X-3, 49–59 cm) that contained clearly distinct sand and clay lithologies were divided, and the sand and clay were squeezed separately. Samples were also processed from the three types of pressure cores deployed in this hole. Two samples were taken from Core 311-U1327D-4E, one from 6Y, three from 10P, and two from 17P.

Hole U1327E was drilled to conduct critical logging runs that could not be completed in Hole U1327D because of operational complications (see "Operations"). From this hole, we processed 25 samples from APC Core 311-U1327E-1H, which was intended for high-resolution sampling of the SMI interval that was missed during coring in Hole U1327D. Four whole-round samples were collected per section and two samples from pressure Core 311-U1327E-3P.

Because of the more lithified nature of the formation at this site, XCB coring was used for the collection of a large portion of IW samples. Because XCB coring yields relatively more disturbed cores, these cores are more likely to be contaminated by the drilling fluid than APC cores. Four of the whole-round samples showed extreme disturbance and were deemed unsuitable for IW extraction. Data from these samples are marked by an asterisk in Table T3. Sulfate concentration below the depth of the SMI was used to identify and quantify occasional contamination by the drilling fluid. It is worth noting that samples from the pressure cores, even when pressurized, consistently show high values of IW contamination, with sulfate values between 2.0 and 5.9 mM. The chemical data were therefore corrected for contamination by the drilling fluid. The sample collected from Core 311-U1327D-6Y, which was obtained with the FPC, shows no detectable sulfate, suggesting that the FPC may be more suitable for collecting uncontaminated IW samples than the PCS. Similarly, one of the samples taken from an HRC core (311-U1327D-4E-1, 95–110 cm) had no detectable sulfate; however, a second sample (4E-1, 0–20 cm) collected from a "disturbed" interval at the top of the same core was highly contaminated (sulfate = 13.9 mM).

The IW data collected at Site U1327 are listed in Table T3. In addition, Table T4 lists sulfate-corrected data that represent the composition of the IW corrected for drill fluid contamination. The sulfate-corrected data are illustrated in Figures F22, F23, and F24.

Salinity and chlorinity

The salinity profile shows three distinct zones:

1. A continuous decrease in salinity with depth from 34.0 at the seafloor to 23.0 at ~128 mbsf;
2. Discrete excursions to fresher values (<20) from 128 mbsf to the depth of the BSR, suggesting that gas hydrate was present in the cores and dissociated prior to processing the samples; and
3. Values that range between 23 and 20 below the BSR to TD at 298.4 mbsf (Fig. F22).

The zone of inferred gas hydrate occurrence based on salinity data correlates well with the zone of distinct thermal anomalies in IR scans, which are also indicative of gas hydrate dissociation (see "Physical properties"). Indeed, some of the lower salinity points shown in Figure F22 represent samples collected to specifically target the more pronounced IR temperature anomalies. The lowest salinities of 5.5 and 3.7 in Samples 311-U1327D-5X-2, 84–94 cm, and 15X-3, 49–59 cm, recovered at 128.7 and 222.0 mbsf, respectively (Table T3; Fig. F22), were identified by IR imaging and represent discrete zones of concentrated gas hydrate. Interestingly, both samples contain sand layers, and analyses revealed significantly lower salinity values in the sand than the adjacent clay matrix.

The chlorinity versus depth profile mimics the salinity trend with depth at this site, showing a striking overall continuous freshening trend with depth that corresponds to that previously observed at ODP Sites 889 and 890 (purple symbols in Fig. F22; Westbrook, Carson, Musgrave et al., 1994). The shallowest sample collected at this site (Sample 311-U1327C-1H-1, 25–40 cm) has a chlorinity of 548.6 mM, which is lower than the value of seawater and most likely reflects the modern bottom-water value at this site. Chlorinity decreases from 548.6 mM at 1.5 mbsf to ~390 mM (~71% of the bottom water value) at 128 mbsf, and except for the excursions to lower chlorinity values from ~128 mbsf to the depth of the BSR, it remains fairly constant at 387 ± 3 mM to TD. The discrete low chlorinity values are conspicuously clustered between 128 mbsf and the BSR and likely represent freshening caused by gas hydrate dissociation during core recovery, consistent with IR-inferred temperatures lower than in situ values (see "Physical properties") and high-resistivity zones in the logging data (see "Downhole logging"). The lowest chlorinity values of 67.8 and 70.4 mM correspond to two sand subsamples collected from whole-round Samples 311-U1327D-5X-2, 84–94 cm (128.6 mbsf), and 15X-3, 49–59 cm (222.0 mbsf), respectively. These are the same samples that have the lowest salinities.

In the gas hydrate–bearing interval between ~128 mbsf and the BSR, the chlorinity values of samples with decomposed gas hydrate range from 386 to 275 mM, except for two discrete sand samples with chlorinities of 68 and 70 mM. Assuming no disseminated gas hydrate and, therefore, taking the background chlorinity as 398 mM at this depth interval (Fig. F22; Table T4), the percent dilution by gas hydrate dissociation ranges between 3% and 30%. The dilution of the two discrete sand samples having the lowest chlorinity values of 67.8 and 70.4 mM is ~82%. The sand layers in these samples are ~3 cm thick.

The observed freshening trend in the Cl concentration versus depth profile to ~71% of the bottom water value at 128 mbsf does not reflect in situ production of water from clay dehydration (smectite to illite reaction), because temperatures of ~50°–60°C are required to initiate this reaction. At a geothermal gradient of ~61°C/km (see "Physical properties"), the temperatures between the seafloor and 128 mbsf are 3°–11°C. If instead we assume that the observed continuous Cl-dilution profile between the seafloor and ~128 mbsf is caused by dissociation of disseminated gas hydrate, the calculated pore volume occupancy by gas hydrate at ~50% porosity (see "Physical properties") ranges from zero to 15%–18% from the seafloor to 128 mbsf, respectively. Such high concentrations of gas hydrate should have been clearly detected by the IR scans and by methane concentrations in the pressure cores. Because no chlorinity and salinity anomalies are superimposed on the overall continuous freshening trend and no IR temperature anomalies were observed between the seafloor and 128 mbsf, we conclude that the Cl-dilution profile in this depth interval is primarily dominated by diffusive communication with an advective low-Cl fluid system at greater depth and not by gas hydrate dissociation. The absence of gas hydrate in the upper 128 m, which is within the GHSZ, suggests that not enough methane was available for gas hydrate formation.

The nature of the Cl concentration versus depth profile may indicate a nonsteady-state advective system, in which advection is presently not intense. Another possible interpretation of this profile is that the rather constant concentration of Cl beneath the BSR to TD reflects advection of the deep-seated low-Cl fluid. Accordingly, the change in the slope of the Cl profile at ~90 mbsf and, in particular, the diffusive profile in the uppermost 60–70 m reflect a relaxation of the advective regime, potentially induced by a change in sedimentation rate. Modeling is required to distinguish between the two interpretations of the Cl concentration versus depth profile and to constrain the change in sedimentation rate that would be required to produce the observed change in the slope of the Cl profile at 90 mbsf.

Biogeochemical processes

Similar to Site U1329, the IW concentration versus depth profiles show extensive evidence of microbially mediated reactions and abiological fluid-rock reactions. The microbially mediated bacterial sulfate reduction, carbonate reduction, and methane oxidation as well as the associated authigenic carbonate formation, are particularly intense in the uppermost 20 m of the sediment section, as reflected in the concentration versus depth profiles of sulfate, alkalinity, ammonium, and phosphate (Fig. F23).

As seen in Figure F25, there is a slight shift in the sulfate reduction slope between Holes U1327D and U1327E. The integrated, almost linear decrease in sulfate concentration with depth from 28 mM at 1.8 mbsf to zero concentration at 9.5 mbsf provides a reduction rate of ~3.6 mM/m. Note that the uppermost 1.8 m in Hole U1327D shows a constant sulfate concentration of 28 mM, indicating intense bioirrigation. There is an apparent depth offset of ~1 m between Cores 311-U1327D-1H and 311-U1327E-1H. In addition, Figure F25 clearly shows that ~3 m is missing between Cores 311-U1327D-1H and 2H. High heave during the coring operations most likely caused this gap in recovery, which was rectified later by the recovery of Core 311-U1327E-1H. The dissolved sulfate data from Core 311-U1327E-1H place the SMI depth between 9 and 10 mbsf. The depth of sulfate depletion at the SMI depth is concomitant with increasing methane concentration (see "Organic geochemistry"), consistent with anaerobic methane oxidation at the SMI. The greater than zero sulfate concentrations below the SMI depth of 9–10 mbsf (Fig. F23; Table T3) were used to correct for drill fluid contaminations (Table T3). In anoxic sediments, sulfate becomes the dominant electron acceptor during organic matter respiration, which can be described by the following net reaction (Claypool and Kaplan, 1974):

2CH₂O + SO₄^2– 2HCO₃^– + H₂S,

which produces 2 mol alkalinity for each consumed mole of sulfate. This ratio changes when methane is anaerobically oxidized by sulfate. This reaction only produces 1 mol alkalinity per mole sulfate, as shown in the following reaction:

CH₄ + SO₄^2– HCO₃^– + HS^– + H₂O.

The slope of the alkalinity corrected for carbonate (Ca, Mg) versus sulfate concentration indicates that anaerobic methane oxidation is the dominant (~80%) sulfate reduction reaction at this site.

Within the zone of sulfate reduction and alkalinity generation, Ca, Mg, and Sr profiles show rapid decreases with depth, which reflect authigenic carbonate formation. In the uppermost 33 m, this reaction preferentially consumes Ca; therefore, calcite and/or Mg-calcite formation leads to the observed increase in Mg/Ca ratios to a value of 34 at 33 mbsf, more than double the maximum value observed at Site U1329 and about six times the value of seawater (Fig. F24; Table T4). At such a high Mg/Ca ratio, high alkalinity values, and low sulfate values, dolomitization is the favored authigenic carbonate reaction. This suggestion is corroborated by the subsequent drop of the Mg/Ca ratio and the decrease in alkalinity.

The Mg profile shows a steady decrease with depth and is strongly correlated with Cl (r² = 0.98 from 28 to 300 mbsf). This suggests that the observed Mg concentrations are mainly a function of a mixing process between seawater and a deep-seated low-chlorinity fluid that also has low Mg concentrations.

Deep-seated fluid

The concentration with depth data in Table T4 provide important information on the nature of the deep-seated low-Cl fluid. As indicated in Figures F22, F23, and F26, the fluid is depleted in Mg and B; somewhat enriched in Ca, Sr, K, and Na; and enriched in Li concentrations. These characteristics are typical of subsurface fluids at temperatures >150°–200°C. At a geothermal gradient of 61°C/km (see "Physical properties"), the 150°–200°C source fluid is at >2 km depth. The silica concentration profile at this site is controlled by the in situ lithology and, therefore, does not provide an insight on the silica concentration of the deep fluid. The marked increase in silica concentration from an average of ~550 mM to an average of ~750 mM at ~70 mbsf does not coincide with the lithostratigraphic Unit I/II boundary (see "Lithostratigraphy"); it is ~20 m shallower. The sharp increase in silica concentration at ~70 mbsf suggests a heterogeneous presence of diatoms from this depth to TD.

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