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doi:10.2204/iodp.proc.311.106.2006

Interstitial water geochemistry

The main objectives of the IW analyses at this site, located within an active vent, were to document the depth distribution of gas hydrate and their relationship to fluid chemistry, lithology, vent faults, and related fractures and to determine the fluid advection rate. Surprisingly, however, at this site diffusion controls the chemical profiles below ~30 mbsf, except perhaps in the deepest 50 m (250–300 mbsf) where chemistry seems to be controlled by advection, as suggested by the Cl and salinity data. The IW chemistry in the uppermost ~30 m is controlled by in situ gas hydrate formation from a methane-rich fluid transported through a network of faults and fractures.

A total of 90 IW samples were processed from four holes cored at Site U1328. From Hole U1328B, we processed 16 whole-round samples, 10–30 cm in length, which were collected with a sampling frequency of five per core in the first core and one to two samples per core in the remaining APC and XCB cores, depending on the length and condition of the retrieved core. In addition, six samples were collected from two pressure cores at ~15 and 26 mbsf (Cores 311-U1328B-4P and 7P). Hole U1328B was terminated at 56 mbsf because of strong winds and severe ship heave. Coring continued in Hole U1328C, which was offset from Hole U1328B by 15 m, from 58 mbsf to a TD of 300 mbsf.

From Hole U1328C we collected 48 whole-round IW samples, two of which were discarded because they were very disturbed and unsuitable for IW extraction. Three of the IW samples were divided into special sections based on IR images, generating three additional samples. Also, three samples were collected from pressure Core 311-U1328C-5P deployed at 92 mbsf. A total of 53 samples were processed from Hole U1328C.

Two XCB cores were retrieved from Hole U1328D, from which we collected and processed six samples, all of which were highly disturbed by drill water and gas hydrate dissociation during recovery. The three shallowest IW samples consisted of mostly drill water (Table T3).

Hole U1327E was drilled to deploy special tools and pressure coring systems. From this hole, we processed five samples, one of which was divided to differentiate between "dry" sediment and "wet" material, which presumably has the larger signature from gas hydrate dissociation. Sample resolution was one or two samples per core, depending on the condition of the recovered sediment. Three pressure core deployments were conducted in this hole (see "Pressure coring"), and we collected three samples from pressure Core 311-U1328E-13P.

Because of the lithified nature of the formation and the presence of gas hydrate in the shallow sediments at this site, XCB coring was used for the collection of a large portion of IW samples. XCB coring yields relatively more disturbed cores, therefore these cores are more likely to be contaminated by drilling fluid than APC cores. Two whole-round samples showed extreme disturbance and were deemed unsuitable for IW extraction. They are marked by an asterisk in Tables T3 and T4. Sulfate concentration below the depth of the SMI was used to identify and quantify occasional contamination by drilling fluid. In addition, samples collected from the first three sections of Core 311-U1328D-1X were highly disturbed but, because these whole-round samples contained gas hydrate, the samples were processed for analyses. As expected, these samples show very high contamination values, with sulfate values ranging from 22 to 26 mM (Table T3). These data are excluded from Figures F25, F26, and F27.

The IW data collected at Site U1328 are listed in Table T3. In addition, in Table T4 we list sulfate-corrected data, which represent the composition of the IW corrected for drill-fluid contamination assuming zero sulfate concentration below the SMI. The sulfate-corrected data from Holes U1328B and U1328C and selected analyses from Hole U1328E are illustrated in Figures F25, F26, and F27.

In addition to IW whole-round samples and in coordination with gas analyses conducted by the organic geochemists (see "Organic geochemistry"), a total of nine gas hydrate samples (five from Hole U1328B, two from Hole U1328D, and two from Hole U1328E) were collected on the catwalk and allowed to dissociate for analyses of the lattice water. Because of the very low volumes of recovered water from the gas hydrate samples, shipboard analyses were confined to sulfate and Cl concentrations. Results of the gas hydrate–water analyses are listed in Table T5.

Salinity and chlorinity

The salinity and chlorinity profiles at this site show four distinct zones. In the uppermost ~60 m, the fluids collected from APC and PCS cores show a striking increase in both of these parameters, with maxima in the interval between 5 and 20 mbsf that reach salinity and chlorinity values of 48.9 and 855.2 mM, respectively. Values slightly above seawater were observed from 1.4 to 5 mbsf and from 20 to 60 mbsf (Fig. F25). Shallow gas hydrate is concentrated within the zone of salty, high-chlorinity fluids, which likely reflects recent and rapid formation of gas hydrate at shallow depth. The observed excess solutes result from salt exclusion from the water lattice structure during in situ gas hydrate formation and have not been removed by advective or diffusive processes.

A second zone, which extends from ~60 to ~150 mbsf, is characterized by relatively constant salinity values decreasing from 35 to 32 and a steady decrease in chlorinity from 570 to 538 mM. No gas hydrate was recovered from this zone, and no indication of gas hydrate dissociation was apparent in the IR images (see "Physical properties").

A third zone, extending from ~150 to 250 mbsf across the BSR, shows discrete excursions to fresher salinity (~21 mM) and chlorinity values (~348 mM), suggesting that gas hydrate was present in the cores and dissociated prior to processing the samples. The salinity and chlorinity anomalies are consistent with observations of distinct negative thermal excursions in the IR scans (see "Physical properties"). Indeed, some of the lower salinity and chlorinity points shown in Figure F25 represent samples collected to specifically target the more pronounced IR temperature anomalies. These samples were processed with the goal of establishing the relationship between lithology and hydrate accumulation. Our observations indicate a predominance (>90% of analyzed samples) of gas hydrate occupancy in the sandy layers. However, in the case of Sample 311-U1328C-13X-1, 0–10 cm, where the presence of gas hydrate was inferred by IR imaging and supported by salinity and chlorinity data, the sample consisted of a sizeable homogeneous clayey-silt biscuit. The fourth zone is observed from 250 mbsf to the TD of 300 mbsf. Here salinities remain constant with values ranging from 29.5 to 29.7. Chloride concentration is also relatively constant at 493 ± 3 mM, suggesting communication with a fluid at greater depth that is remarkably different in composition from the deep-seated fluid sampled at Sites U1327 and 889/890 (Westbrook, Carson, Musgrave, et al., 1994). At Site U1328, the Cl concentration of the deep fluid is ~495 mM, whereas at Sites U1327 and 889/890 it is 370 mM. This distinct difference in the deep fluid chlorinities suggests that at Site U1328 the temperature in the source region of the fluid is lower and, therefore, the fluid has undergone less freshening than at Sites U1327 and 889/890.

To form and maintain the shallow gas hydrate deposits, methane has to be supplied continuously. Furthermore, the high salinity and chlorinity values of 36–49 and 720–855 mM, respectively, observed between ~5 and 25 mbsf would dissipate rapidly if they were not continuously maintained by ongoing gas hydrate formation. However, the dissolved chlorinity profile below the subsurface brine is characteristic of a diffusion-controlled system. Assuming an average sediment diffusion coefficient of 5 x 10^–6 cm²/s, ~1.5 m.y. is required to establish the observed diffusion path length of 210 m observed in the chlorinity profile from 30 to 250 mbsf (Fig. F25). These observations indicate that in Hole U1328B the methane needed to sustain the shallow gas hydrate formation must be supplied from a laterally located source, most likely from a fault or fracture zone that intersects this site between 5 and 25 mbsf.

Below the high-salinity/chlorinity zone at 25–50 mbsf, Cl concentrations decrease from 572 to 560 mM. These values are slightly higher than the seawater value of 559 mM, suggesting diffusion from the high-chlorinity zone. Gas hydrate distribution at this site is bimodal: the first zone occurs close to the seafloor and the second zone is in a restricted depth interval above the BSR (from ~150 mbsf). Within these two zones, gas hydrate is mostly associated with sand horizons but also occurs in clayey-silt horizons, either in a disseminated mode or in thin layers parting the low-permeability clay-rich sediments, as observed in the X-ray scans of some pressure cores. Shore-based analyses should be able to show whether these two gas hydrate–bearing zones contain methane gas from the same source.

Sodium and potassium concentration profiles are overall very similar to the chlorinity profile.

Biogeochemical processes

Biological activity exponentially decays with depth, and in most cases limits its observable influence on IW chemistry to the upper part of the sediment. At Site U1328, the zone of highest biological activity coincides with the occurrence of gas hydrate and ongoing gas hydrate formation. Intense microbial activity at this site results in sulfate depletion in the upper 2 m, phosphate production, and significant accumulation of hydrogen sulfide in the interstitial water.

At this site, gas hydrate formation just below the seafloor results in the formation of highly saline fluids, caused by the exclusion of solutes from the gas hydrate and, therefore, increasing the solute concentrations in the interstitial water, including those affected by biological reactions. It is therefore difficult to delineate biogeochemical processes in the uppermost 30 m at this site (Fig. F26). Nevertheless, alkalinity values are significantly higher (up to 49.2 mM) than at Sites U1327 and U1329 than at Site U1328. This is a direct consequence of the higher organic matter accumulation rate, which in turn depends on the sedimentation rate. Compared to Sites U1329 (9.2 cm/k.y.) and U1327 (22 cm/k.y.), Site U1328 has a sedimentation rate of ~36 cm/k.y. (all rates refer to the uppermost unit; see "Biostratigraphy") and consequently much higher alkalinity values.

Further downhole, the IW concentration profiles show extensive evidence of biological fluid/rock reactions (Fig. F27; Table T4). This is most evident in the Mg/Ca ratio, which shows a distinct maximum at ~220 mbsf, coincident with a minimum in Ca and Mg concentrations and the depth of BSR. In the upper part, where biological mediated reactions cause an increase in alkalinity and the subsequent precipitation of carbonate, primarily calcium carbonate is the precipitating phase, as indicated by the large increase in the Mg/Ca ratio. It is difficult to estimate how much carbonate has precipitated because at this shallow burial depth the interstitial water may be in communication with bottom seawater. If no communication with seawater is assumed, based on the alkalinity corrected for authigenic carbonate formation, the amount of authigenic carbonate formation seems insignificant between 0 and 30 mbsf (see Fig. F28). The first two sections in Core 311-U1328D-1X contained some authigenic carbonate in the form of small, disseminated, incipient micronodules.

Between 150 and 220 mbsf, two distinct authigenic carbonate reaction zones are observed. In the first zone, at the alkalinity maximum at ~150 mbsf, Ca also shows a maximum concentration. The Mg concentration, however, begins to decrease and the Mg/Ca ratio shows a broad minimum. These relations are indicative of authigenic carbonate formation that utilizes Mg and releases Ca. Such changes are typical of dolomitization of a precursor calcium carbonate. In the second zone, at ~220 mbsf and near the depth of the seismically inferred BSR, both alkalinity and Ca concentrations show minima. Here the Mg concentration remains equal to that of the deep fluid, and the Mg/Ca ratio shows a maximum, suggesting the formation of an authigenic carbonate enriched in Ca relative to Mg. Indeed, massive carbonate was recovered in Core 311-U1328C-19X, 216–226 mbsf. Iron is commonly involved in the formation of authigenic carbonates at greater burial depths. Shore-based analyses of the IW and diagenetic carbonates will indicate the involvement of Fe at this site.

Deep-seated fluid

The salinity and Cl, Na, K, and Li concentration profiles in Table T4 provide important information on the nature of the deep-seated fluid at this site. As indicated in Figures F25, F26, and F29, the fluid is depleted in Cl, Mg, Na, and somewhat in K and enriched in Si and Ba. The Mg/Ca ratio is ~6, which is only slightly higher than the seawater ratio of 5.4, and the fluid has approximately seawater Sr, Li, and B concentrations. This is consistent with the Cl concentration data that show less dilution by hydrous mineral dehydration reactions, suggesting a lower temperature at the deep-fluid source. The other major and minor element concentrations also indicate a less-altered fluid source than at Site U1327.

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