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doi:10.2204/iodp.proc.311.103.2006

Interstitial water geochemistry

The main objectives at this site were to describe the evolution of the IW chemistry and the distribution of gas hydrate at the first tectonic slope basin, which is presently being deformed and eventually will be incorporated into the Cascadia accretionary prism.

A total of 92 IW samples were processed from two holes cored at Site U1325. In Hole U1325B, 62 whole-round samples, 10–33 cm in length, were processed. The samples were collected at a frequency of two per section in the first core and two to three per core in the remaining XCB cores, except for Core 311-U1325B-9X, which had low recovery. In an effort to characterize the IW chemistry associated with gas hydrate–bearing sediment and compare it with sediment with an IW composition more typical of background conditions, whole-round samples were chosen from intervals with infrared (IR) camera temperature anomalies and from zones with a less anomalous thermal signal within the zone of gas hydrate occurrence. Seven of the samples where the presence of gas hydrate was inferred from IR anomalies were carefully described and imaged prior to the squeezing process, with the goal of establishing a relationship between lithology and gas hydrate occurrence. The maximum IR temperature anomalies were associated with sandy layers ranging from several millimeters to ~3 cm in length. Sand and clay lithologies were split and squeezed separately. In addition, before starting coring operations in Hole U1325B, a 10.5 cm³ bottom water sample was recovered from ~20 m above the seafloor using the WSTP tool. The bottom water sample was split for microbiology and geochemistry studies.

In Hole U1325C, 24 whole-round IW samples (20 samples were 10–50 cm long, one sample was 80 cm long, and three samples were 134 cm long) were processed at a sampling frequency of one to two per core depending on recovery. One sample was discarded because it came from a highly disturbed XCB core with no intact "biscuits" present. As in Hole U1325B, some of the whole-round sampling was guided by IR temperature anomalies. Emphasis was also given to obtain at least one representative "background" sample from each core. As seen in Hole U1325B, the IR low-temperature anomalies were primarily associated with sands. In five of the samples, the sand and clay lithologies were split and squeezed separately, except Sample 311-U1325C-4X-1, 112–124 cm, where the sand layer was ~9 cm thick and the clay portion of the sample was too small for squeezing. In addition, we analyzed two samples from Core 311-U1325C-10P, recovered from 256.2 mbsf.

Hole U1325D recovered one APC core with the purpose of establishing the mudline at Site U1325. Four IW samples were collected from this core.

The IW data collected at Site U1325 are listed in Table T3. Also, in Table T4 we list sulfate-corrected data, which represent the composition of the IW corrected for drill-fluid contamination and include data from the bottom water sample collected with the WSTP tool. The sulfate-corrected data for Holes U1325B and U1325C are illustrated in Figures F23, F24, and F25.

Salinity and chlorinity

Salinity and chlorinity profiles at this site are characterized by an advective system that transports water with higher than seawater salinity and chlorinity (~36 and 600 mM, respectively) from a deeper source (Fig. F23). The chlorinity of this fluid is 7.3% above the modern seawater value, a percent increase that is more than twice the value of the interglacial maximum ocean value (3.5%). Because Site U1325 is located within an isolated basin, it seems unlikely that it is connected to a deep-seated brine source. The elevated solute concentration may instead be caused by low-temperature diagenetic reactions in the deeper parts of the basin. 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 salinity and chlorinity values. This inference is supported by the Ca, Mg, Sr, K, and B concentration versus depth profiles discussed below (Figs. F25, F26).

In the zone extending from ~70 to 240 mbsf (the BSR is at ~230 mbsf), salinity and chlorinity data show discrete excursions to fresher values. The minimum salinity and chlorinity values recorded are 13 and 286 mM (221 mbsf), respectively; 42% and 51% of seawater values and 38% and 49% of the background saline fluid values, indicating 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 IR scans (see "Physical properties"). Indeed, the lower salinity and chlorinity points, shown in Figure F23, represent samples collected to specifically target the more pronounced IR temperature anomalies. Similar to Site U1328, our observations at this site indicate that gas hydrate predominantly occupies the sandy layers.

Two interesting observations are noted in Figure F23 between ~180 mbsf and the depth of the seismically inferred BSR:

These observations suggest that in the 50 m above the BSR depth the average concentration of gas hydrate is higher, and consequently, although sands contain more gas hydrate, some of the gas hydrate is also associated with the clay lithology, either in a disseminated mode or as discrete layers separating the clays.

Biogeochemical processes

Intense microbial activity at Site U1325 results in sulfate depletion, phosphate and alkalinity production, and significant Ca and Mg depletion in the IW of the uppermost 3 m (Figs. F24, F25). Hydrogen sulfide smell was notable while cleaning the first two whole-round samples (311-U1325B-1H-1, 65–80 cm, and 1H-1, 135–150 cm).

At this site, the sulfate/methane interface (SMI) was placed between 4 and 5 mbsf, based on samples recovered from Hole U1325D (Fig. F27). A comparison between data from Holes U1325B and U1325D shows an offset in the depth of the SMI of ~ 2.5 m, which suggests that the mudline was missed in Hole U1325B.

In the uppermost lithostratigraphic units at Sites U1329, U1327, and U1328, sedimentation rates are 9.2, 22, and 34.3 cm/k.y., respectively (see "Biostratigraphy"). At Site U1325, the sedimentation rate is ~38.3 cm/k.y. (see "Biostratigraphy" and "Lithostratigraphy"). Because of the high sedimentation rates, organic matter accumulation rates are likely higher than at the other sites drilled during Expedition 311. This in turn allows for higher organic matter respiration rates, resulting in the highest alkalinity (52.9 mM at 26.7 mbsf), ammonium (16 mM at 297.8 mbsf), and phosphate (121 mM at 26.7 mbsf) concentrations among the Expedition 311 sites (Fig. F24).

The first peak in the Mg/Ca ratio (Fig. F25) at ~2.2 mbsf in Hole U1325B shows that this uppermost zone of carbonate precipitation is dominated by the preferential loss of Ca relative to Mg. Ca concentration falls to ~25% of the original seawater value, and the Mg/Ca molar ratio increases to about three times the seawater value. A second zone of intense carbonate diagenesis is indicated by a large maximum in the Mg/Ca ratio at ~35 mbsf, which corresponds with the approximate depth of the alkalinity maximum and the Ca minimum (Fig. F25). Between the two Mg/Ca maxima, Ca concentration decreases by <1 mM and Mg concentration increases by ~5 mM. The increase in Mg concentration coincides with a peak in ammonium values. Despite consumption by carbonate diagenesis, the Mg concentration and Mg/Ca ratio increase, indicating that clays must be the source of the Mg supplied by ion exchange with ammonium. The decline in the Mg/Ca ratio below 35 mbsf is most likely being caused by recrystallization of carbonates where Ca is exchanged in favor of Mg. However, for each Ca ion released, 3.5 Mg ions are removed from solution, suggesting that Mg is involved in more than one reaction, most likely in the alteration of volcanic ash into a Mg-rich clay at the depth of the saline fluid formation. Similarly, Ca and Sr concentration versus depth profiles indicate production at greater depth, also consistent with alteration of volcanic material.

From the seafloor to ~200 mbsf, silica concentrations range between 400 and 800 mM, considerably above the bottom water value of 157 mM. These concentration values are controlled by diatom dissolution. Below ~200 mbsf, silica concentrations drop to 200–500 mM. This change in silica concentrations coincides with the depth where the terrigenous input into the sediment rapidly increases with a concomitant decrease in diatom abundance (see "Biostratigraphy").

Diagenetic deep fluid

The salinity and concentration profiles of Cl, Na, K, and B provide important information on the nature of the deep-seated advecting fluid at this slope basin site. As indicated in Figures F23, F24, and F26, the fluid has higher than seawater salinity and chlorinity values, is enriched in Ca and Sr, and 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. At this site, the original IW evolves with depth from seawater composition into a CaCl brine. Extrapolating the slopes of the Mg and Ca concentration versus depth profiles indicates that at ~500 mbsf Mg concentration should reach zero and Ca concentration will be above its seawater value.

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