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doi:10.2204/iodp.proc.311.104.2006

Interstitial water geochemistry

The main objectives at this site were to document the evolution of the interstitial water (IW) chemistry and the distribution of gas hydrate at the westernmost uplifted ridge of the Cascadia accretionary prism.

A total of 75 IW samples were processed from two holes cored at Site U1326. In Hole U1326C, 27 whole-round samples, 10–40 cm in length, were retrieved at the catwalk. The sampling frequency was two per section in the uppermost 10 m (Sections 311-U1326C-1H-1 through 2H-4), followed by one to three samples per core to TD of the hole. The sediment collected from Core 311-U1326C-5X, the first XCB core, was too disturbed and unsuitable for IW extraction. Two samples contained gas hydrate (Samples 311-U1326C-6X, 83–96 cm, and 7X-3, 74–101 cm). Based on lithology, infrared (IR) camera images, and visual core observations, these samples were split into five subsamples, thus three extra samples were processed. In addition, two samples from pressure Core 311-U1326C-12P were collected, generating a total of 30 IW samples from this hole.

In Hole U1326D, 37 IW samples were collected at a sampling frequency of one to two per core depending on recovery. Most whole-round IW samples were 10–50 cm in length, except for the deepest cores, from which we collected 100 cm long samples in an effort to obtain sufficient volumes of interstitial fluid. Because of the weather conditions and severe ship heave, the recovered sediment was highly disturbed and only a few clean sediment "biscuits" were retrieved from these long whole-round samples. Six of the samples were divided based on IR imaging and lithology (as described below), yielding eight additional samples for processing for a total of 45 IW samples from this hole.

In an effort to characterize the IW chemistry associated with gas hydrate–bearing sediment and to compare it with sediment with an IW composition more typical of background conditions, whole-round samples were chosen from intervals with the highest IR temperature anomalies and from zones with less anomalous thermal signals within the zone of gas hydrate occurrence. Three samples from Hole U1326C and eight samples from Hole U1326D were collected to evaluate the chlorinity of the IW associated with intervals where gas hydrate presence was inferred from IR anomalies. These samples were imaged again with a handheld IR camera, photographed, and described prior to the squeezing process, documenting the association of gas hydrate mostly with sand horizons, as previously observed at Sites U1325 and U1328. Sand and clay lithologies were split and squeezed separately.

The IW data collected at Site U1326 are listed in Table T3. Because of the lithified nature of the formation, 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 with the drilling fluid than APC cores. Sulfate concentration below the depth of the sulfate/methane interface (SMI) was used to identify and quantify contamination by the drilling fluid, and the sulfate-corrected data are listed in Table T4. The sulfate-corrected data for Holes U1326C and U1326D are illustrated in Figures F21, F22, F23, and F24.

Salinity and chlorinity

The salinity and chlorinity profiles at this site point to a deeper fluid with a chloride concentration higher than seawater (>585 mM; Fig. F21), indicative of low-temperature diagenetic reactions in the deeper parts of the site. As suggested for Site U1325, a plausible candidate for such a reaction is the alteration of volcanic ash to clay minerals and/or zeolites. These reactions consume H₂O and, therefore, increase the in situ chlorinity values.

In the zone extending from ~45 to 270 mbsf (the seismically inferred BSR is at ~230 mbsf), salinity and chlorinity data show discrete excursions to fresher values, indicating that gas hydrate was present in the cores and dissociated prior to processing the samples, consistent with observations of distinct negative thermal excursions in IR scans (see "Physical properties"). Indeed, the lower salinity and chlorinity points shown in Figure F21 represent samples collected to specifically target the more pronounced IR temperature anomalies. Similar to Sites U1325 and U1328, our observations at this site indicate that gas hydrate predominantly occupies the sandy layers. The minimum salinity and chlorinity values recorded are 4.6 and 90.1 mM (13.5% and 16% of seawater value, respectively) and were measured in a gas hydrate–bearing, 6 cm thick coarse-sand layer recovered in Sample 311-U1326C-6X-4, 83–96 cm. This sample, retrieved from 44.9 mbsf, also represents the first IR temperature anomaly in Hole U1326C. At 53.4 mbsf, Sample 311-U1326C-7X-3, 74–101 cm, contained small amounts of gas hydrate. Beneath these intense chlorinity and IR anomalies, there is little indication of gas hydrate presence until ~83.8 mbsf, below which multiple chlorinity excursions, with values as much as 39% fresher than seawater, were recorded in various sand-bearing intervals.

The degree of dilution of the IW by gas hydrate dissociation shows an approximate linear decrease with depth, from ~48 mbsf, where the lowest chlorinity and salinity values are observed, to ~220 mbsf (Fig. F21). This zone overlaps with the highest gas hydrate concentrations inferred from a zone of high resistivity in the LWD records, which lies between ~70 and 100 mbsf (see "Downhole logging"). This trend in the gas hydrate distribution suggests that a fault or a steeply dipping sand layer supplies methane to sediment at shallower depths. This is supported by the distinct molecular composition of the gas hydrate at the cemented, coarse sand at 44.9 mbsf and by the observation of a normal fault in the sediment (see "Organic geochemistry" and "Lithostratigraphy;" Fig. F15). The fluid that transports the methane to this layer also has elevated ethane, as indicated in the gas data (see "Organic geochemistry"). There are three additional interesting observations in Figure F21:

1. In the ~20 m above the seismically inferred BSR, no IR cold anomalies were observed, and the Cl concentration values are equal or close to the background value.
2. Low chlorinity values associated with IR cold anomalies are observed below the seismically inferred depth of the BSR, at the approximate depth of the base of the gas hydrate stability zone (GHSZ), which is at 275 ± 25 mbsf based on a geothermal gradient of ~60°C/km (see "Physical properties").
3. The lack of chlorinity and IR anomalies at 215–225 mbsf reveals the absence of gas hydrate, the reason for which is as yet unclear.

Biogeochemical processes

Intense microbial activity at Site U1326 results in sulfate depletion, phosphate and alkalinity production, and significant Ca and Mg depletion in the IW of the uppermost ~3 m (Figs. F22, F23). Linear interpolation of sulfate concentration versus depth places the SMI at ~2.5 mbsf; however, we noted a hydrogen sulfide smell to a depth of ~10 mbsf while cleaning the whole-round samples. Unexpectedly, Sample 311-U1326C-2H-1, 60–75 cm (4.6 mbsf), yielded a sulfate concentration of 7.6 mM. The appearance of sulfate below the SMI depth could be caused by rapid sediment redeposition (i.e., a slump) or by contamination with drill water during the drilling process. Based on only the shipboard data, it is impossible to distinguish between the two theories.

A first maximum in the Mg/Ca ratio is observed in the interval from just below the depth of the SMI at 3.7 mbsf (Fig. F23) to 9.8 mbsf. In this zone, alkalinity and ammonium concentrations also increase. Alkalinity reaches its first maximum of 23.1 mM at 2.9 mbsf and has a second peak of 19.7 mM at 9.8 mbsf (Fig. F22). This uppermost zone of carbonate precipitation is dominated by the preferential loss of Ca relative to Mg. The Ca concentration falls to ~33% of the original seawater value, but the Mg/Ca molar ratio increases to about twice that of seawater.

A second zone of intense carbonate diagenesis is indicated by large minima in the Mg/Ca ratio and alkalinity, and a minor minimum in ammonium concentration at ~70 mbsf, which corresponds to the depth of a sharp Ca maximum to almost twice seawater value (Fig. F23). These distributions are indicative of dolomitization of the calcite that precipitated in the upper zone of carbonate diagenesis. The Mg involved in the dolomitization reaction has a mixed origin. Some of the Mg is supplied by the original IW and some was expelled from clay ion exchange sites by the high ammonium concentrations, as suggested by the presence of an ammonium minimum between 40 and 100 mbsf.

A third zone of carbonate diagenesis occurs at ~115 mbsf, where Ca concentrations show a second intense minimum to ~32% seawater value. Carbonate precipitation is driven by the high alkalinity and calcium concentration almost twice that of seawater at ~70 mbsf. At such a high Ca concentration, coupled with the high alkalinity and a very low Mg/Ca ratio of 1.1, dolomite cannot form and authigenic calcite precipitates instead.

Sr concentration does not play an important role in the above described carbonate formation and recrystallization zones of the uppermost ~130 m because the various diagenetic carbonates that form or recrystallize have similar Sr concentrations at the prevailing low temperatures.

The decline in the Mg/Ca ratio and increase in Ca concentrations below ~130 mbsf are primarily caused by interaction with the deeper fluid, which is forming by alteration of volcanic ash into a Mg-rich clay and zeolites, as suggested by the K and Sr depth profiles (Figs. F21, F23).

Inorganic diagenetic processes

Two zones of high silica concentrations (lithostratigraphic Units I and III; see "Lithostratigraphy") and a zone of low silica concentrations (Unit II; see "Lithostratigraphy") are observed (Fig. F24). These zones reflect the depth distribution of diatoms at this site (see "Biostratigraphy").

Lithium shows an interesting concentration depth profile (Fig. F24). Li is depleted in the uppermost 7 m and remains low to ~150 mbsf, suggesting that some volcanic ash is interspersed in the sediment throughout this depth interval. At the depth where the ammonium concentration reaches the 8.1 mM maximum (157 mbsf), Li is expelled from the clays by ion exchange reactions. Both Mg and Li are released from clay ion exchange sites, but the Mg binding energy in the clay ion exchange sites is considerably lower than that of Li, explaining the observed stepwise behavior. Li has slightly higher concentrations relative to seawater below the BSR, and therefore is in the deep fluid.

Diagenetic deep fluid

As indicated in Figures F21, F22, and F24, the deep fluid has higher than seawater chlorinity values, is enriched in Ca and Sr, and is depleted in Mg, K, and B. These distributions are consistent with low-temperature volcanic ash alteration reactions that form Mg and K clay minerals and K-zeolites. As observed at Site U1325, the original IW is evolving with depth into a Ca-Cl brine. Extrapolating the slopes of the Mg and Ca concentration versus depth profiles suggests that Mg should reach zero concentration at 600–700 mbsf, which is ~200 m deeper than at Site U1325, and at this depth Ca concentration will be about three times that of seawater.

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