Two holes were drilled at Site 1178. Fifty-six pore fluid samples were squeezed from selected 10- to 50-cm-long whole-round samples for chemical and isotopic analyses from Holes 1178A and 1178B. Hole 1178A sample depths ranged from 1.4 to 408.0 mbsf, and Hole 1178B sample depths ranged from 397.5 to 638 mbsf. Samples were collected from every Hole 1178A core except for Cores 190-1178A-10X, 30X, and 39X because of poor recovery. Samples were collected from every Hole 1178B core from Core 190-1178B-2R to 13R, except for Cores 8R and 11R because of low recovery. From Core 190-1178B-13R to 27R, samples were collected from every second or third core because of the low pore- water recovery and long squeezing times required. Pore fluid recovery from Cores 190-1178B-24R through 27R was <0.5 mL after long squeezing times; therefore, no deeper whole-round samples were squeezed from the bottom of this hole.
Elemental concentrations are reported in Table T12 and plotted as a function of depth in Figure F22. Because of time constraints, seven, instead of nine, major and minor dissolved anions and cations that sensitively reflect inorganic or microbially mediated water-rock reactions were determined for each sample with >10 mL recovered pore fluid from this site. The anions are Cl, Ca, Mg, and Si, and the cations are sulfate, alkalinity, and ammonium. K and Na concentrations will be determined at a shore-based laboratory. The smaller volume samples were not analyzed for Si, alkalinity, and ammonium. Salinity and pH were also determined.
The outstanding characteristics of the pore fluid concentration-depth profiles at Site 1178 are the variability in Cl concentrations superimposed on an overall continuous trend of Cl dilution with depth relative to seawater composition. Both features, the continuous dilution and sharp concentration variabilities, reflect the inhomogeneous distribution of gas hydrate in these sediments. The Ca and Mg profiles indicate that they are coupled at some depth intervals but decoupled at others. The sulfate-containing interval is much thicker than at any of the other sites drilled during Leg 190. It spans more than ~30 m. All other microbial reaction products also are produced at greater depths at this site. Alkalinity and ammonium maxima occur between 100 and 200 mbsf. Unlike at Sites 1175 and 1176, despite the rather low temperature gradient, both carbonate and silicate diagenetic reactions significantly imprint the pore fluid chemistry. Also, similar to the deep-water sites but unlike the other two shallow-water sites, the shallow increase in Cl is consistent with diffusion of lower-chlorinity interglacial water into the sediment. At this shallow site, sedimentation rates at the top of the section are considerably lower than at Sites 1175 and 1176 (see "Biostratigraphy"); therefore, the glacial signal (although partially depleted by diffusion) is clearly seen.
Cl concentrations were determined with a relative analytical uncertainty of 0.1% based on duplicate to quadruplicate titrations of all samples. Chloride concentrations increase in the top 32 mbsf to 562 mM. This is consistent with diffusion of lower chlorinity interglacial seawater into the sediment. At this site, volcanic ash is insignificant in this depth interval, and therefore ash hydration cannot be responsible for the observed increase. Below this maximum and throughout most of the rest of the section, between ~35 and ~500 mbsf, Cl exhibits a continuous trend of freshening with depth; the trend is steeper between 90 and 200 mbsf. Superimposed on this background Cl dilution profile are numerous smaller Cl minima produced by in situ dissociation of disseminated gas hydrate that occurs during core recovery operation (see "Gas Hydrates" immediately below).
Although no solid gas hydrate was recovered during Leg 190, its presence was documented indirectly. Both temperature measurements of cores on the catwalk and pore fluid Cl concentrations indicate the existence of gas hydrates at two slope sites, Sites 1176 and 1178. Gas hydrate dissociates upon recovery, as its stability depends exponentially on temperature and linearly on pressure. Recovery of solid hydrate is unlikely unless it is abundant or specialized equipment is used. As a result, mainly massive hydrates have been recovered in the drilling program.
Sites 1176 and 1178 are within the stability field of seawater-methane hydrate from the seafloor to the BSR. Because methane is the dominant gas in the sediments at these sites, any gas hydrate present should be primarily methane hydrate as it is at other nonthermogenic oceanic sites. Dissociation of methane hydrate is a highly endothermic reaction; therefore, its decomposition consumes heat and cools the sediment. At Site 1176, temperatures colder than background by 4°-5°C between ~220 and 240 mbsf were measured in two cores (190-1176A-25X and 26X). Because of no or poor core recovery, there are no data for the 240-320 mbsf interval. Pore fluid Cl concentrations suggest minor addition of water, ~1% beyond other dilution processes.
At Site 1178, gas hydrate is considerably more abundant than at Site 1176. Based on pore fluid Cl concentrations, methane hydrate (inferred from gas composition) is present between ~120 and 400 mbsf, with highest concentrations between 150 and 200 mbsf. At ~200 mbsf, in Core 190-1178A-23X, the lowest catwalk core temperature of -0.5°C was measured. Temperatures colder than background by 4°-6°C were measured in several cores, mostly between 150 and 200 mbsf.
The Cl concentration-depth profile has a steep continuous trend of freshening between 90 and 200 mbsf with two intense Cl minima. One minimum is present at 170-185 mbsf; the second, with the lowest Cl value of 524 mM compared with bottom-water value of 557 mM, was measured in Core 190-1178A-23X (T = -0.5°C), which corresponds to >6% dilution by methane hydrate dissociation. The background dilution throughout the 150-200 mbsf interval is 3%. Between 200 and 400 mbsf, Cl concentrations continue to gradually decrease with depth from 545 to a minimum of 517 mM at the BSR depth, which corresponds to >7% dilution. Superimposed on the background Cl dilution profile are numerous smaller Cl minima. This indicates that throughout the sediment section (90-400 mbsf), disseminated gas hydrate is present and is responsible for the background 3%-4% Cl dilution and that specific sediment horizons, probably the coarsest-grained ones, have higher hydrate concentrations, equivalent to 6%-7% Cl dilution. Concentration-depth profiles of other components, specifically Si and Ca, show similar fluctuations in concentration with depth, supporting the conclusion that they have a common origin of dilution by methane hydrate dissociation.
Cl concentrations sharply decrease below the BSR depth and reach a minimum of 470 mM, almost a 16% dilution, centered at ~500 mbsf. The origin of this low-Cl zone is as yet unclear. It may represent a more methane hydrate-rich paleo-BSR that has not had enough time to dissipate the dilution signal caused by dissociation of the hydrate. Consistent with this scenario, higher methane concentrations are found at this depth. Sedimentation and/or tectonics could have caused such an upward migration of the BSR.
Dissolved Si concentrations increase steeply with depth in the top 35 m and below, and although variable, they remain almost constant at 800 ± 50 µM to ~200 mbsf. In this depth interval, diatoms are abundant. Despite their abundance, Si concentrations are slightly lower than the opal-A solubility value. This may be because competing reactions consume Si released by diatom dissolution or because the opal-A dissolution rate is slower than the Si diffusion rate. This becomes important where sedimentation rates are slow to medium. At greater depth, where diatoms and sponge spicules are present but less abundant and not very well preserved, Si concentrations gradually decrease with depth. A sharp decrease in Si concentrations is observed at ~300 mbsf, where diatoms steeply disappear with depth (see "Biostratigraphy"). Because of the low volumes of pore fluid recovered below this depth and the disappearance of biogenic Si remains, the deeper part of this sediment section was not analyzed for Si.
Ca and Mg concentrations steeply decrease with depth in the top ~35 mbsf, the sulfate-containing zone. Ca decreases by ~7.5 mM from seawater concentration to 3.3 mM, whereas Mg concentrations decrease by ~15 mM from seawater concentration to 39 mM. Because the Ca/Mg molar ratio in dolomite is 1, it is clear that Mg is intensely involved in more than one diagenetic reaction at this depth interval. One reaction is authigenic dolomite precipitation; the other is either dolomitization of a calcite precursor and/or involvement in a silicate reaction. Alkalinity increases sharply in this depth interval, promoting carbonate diagenesis. Between ~35 mbsf and the depth of the BSR, Ca and Mg have an inverse relationship, suggesting either dolomitization of calcite or involvement in silicate reactions. The former is probably less important because of the low abundance of biogenic calcite in this section (2-5 wt%) (see "Organic Geochemistry"). The most common silicate reactions that increase Ca concentrations above seawater concentrations are volcanic ash dissolution and alteration. Simultaneously, a Mg-rich clay may form. At the present prevailing low temperatures, because of the moderate geothermal gradient (~4°C/100 m), the formation of an authigenic clay mineral must be extremely slow. Thus, dissolution of volcanic ash and Mg clay formation are likely decoupled reactions. The Mg diffusional profile suggests a sink at great depth, whereas the Ca profile indicates a sink at the depth of the BSR. The possibility of a linkage between the BSR depth and carbonate diagenesis is intriguing; if correct, the involved reactions are most probably linked with bacterial activity.
Alkalinity steeply increases in the sulfate-containing zone to ~20 mM and then remains approximately constant to a depth of ~200 mbsf, suggesting involvement in carbonate reactions. Below, it decreases monotonically with depth and reaches seawater concentrations at ~500 mbsf. This suggests a sink at greater depth, probably a common sink with Mg.
Ammonium produced by bacterially mediated decomposition of organic matter increases in concentration to 12 mM at ~200 mbsf. Because of the few data points below this depth, ammonium's behavior is unclear; most likely it has a sink as at all the other sites. Because K and Na data are as yet unavailable and Mg is involved in more than one reaction, the ion exchange influence is less clear.
Sulfate concentration reaches zero at ~35 mbsf and remains practically zero throughout the section. At this anaerobic condition, magnetite would become unstable and weaken the sediment magnetic properties at depth.