Twelve interstitial-water samples were collected from Hole 1121B at depths ranging from 5.95 to 132.90 mbsf (Table T6, also in ASCII format). Sampling frequency is one per 10 m, with the exception of an ~30-m-long interval between 91.90 and 120.80 mbsf, which could not be sampled because of poor core recovery (Fig. F20).
Salinities of the interstitial-water samples are almost constant (34.5) throughout the hole, except for one value (34.0) at 120.80 mbsf. Chloride (Cl-) concentrations increase from the subseafloor value of 554 mM at 5.95 mbsf to a maximum value of 565 mM at 15.40 mbsf. From 15.40 mbsf to 91.90 mbsf, chloride concentration remains relatively uniform, between 561 and 564 mM. The lowermost two samples collected in this hole show a relatively sharp decrease in chloride. Interstitial water pH values vary in a relatively narrow range between 7.55 and 7.64 (excluding two data points at 74.10 and 83.80 mbsf). These two values are probably erroneous and caused by instrumental error.
Interstitial-water alkalinity remains at a near-seawater value, with a relatively narrow range between 2.70 and 3.17 mM throughout the hole. Even the most reducing interstitial water at Site 1121 is only slightly anoxic. Alkalinity in truly reducing sediments can reach levels that are tenfold higher than the highest concentration found at Site 1121, such as those sampled at Site 1119 (see "Inorganic Geochemistry" in the "Site 1119" chapter).
Sulfate (SO42-) concentrations decrease gradually with depth from the seafloor to 91.90 mbsf. Microbial degradation of organic matter consumes sulfate, producing alkalinity that generates concave-downward and concave-upward profiles for sulfate and alkalinity, respectively. However, an alkalinity increase is not evident with depth at this site, indicating that some chemical reactions have masked the alkalinity increase caused by sulfate reduction. One possible reaction is the silicate reconstitution process, which decreases the alkalinity (Gieskes, 1974).
Phosphate (HPO42-) concentrations show a slightly decreasing trend from a subseafloor value of 1.5 µM at 5.95 mbsf to a minimum value of 0.7 µM at 74.10 mbsf. Below this depth, phosphate concentration increases downhole to a maximum value of 2.1 µM at 132.90 mbsf.
Ammonium (NH4+) concentrations are relatively low, less than ~50 µM in the upper and middle parts of the hole (<74.10 mbsf). In the lower part of the hole, a generally increasing trend can be seen, probably because of organic matter decomposition corresponding to the slight decrease of sulfate concentration.
Dissolved silica (H4SiO4) concentrations increase gradually from a subseafloor value of 588 µM at 5.95 mbsf to a local maximum value of 738 µM at 57.90 mbsf, as a result of pore-fluid migration and/or diffusion driven by the concentration difference between seawater and sediments. The values of samples below 96.10 mbsf are more variable, ranging from 599 to 757 µM. A relatively high concentration zone with respect to dissolved silica corresponds to lithostratigraphic Subunit IIB, which consists of diatom and nannofossil ooze, suggesting the dissolution of siliceous tests in the sediments (see "Lithostratigraphy"). Dissolved silica concentration at 120.80 mbsf shows a relatively small value of 599 µM. This sample is taken from lithostratigraphic Subunit IIC, which consists of thin chert layers and carbonate concretions in nannofossil ooze. The decrease in dissolved silica concentration may be attributed to chert formation. Sharp decreases in dissolved silica have been recorded in association with chert horizons at other DSDP and ODP sites (Site 315, Gieskes and Lawrence, 1981; Site 495, Harrison et al., 1982).
Calcium (Ca2+) concentration shows a near-seawater value in the shallowest sample (10.6 mM at 5.95 mbsf) and increases gradually downhole to a maximum value of 16.9 mM at 120.80 mbsf.
Magnesium (Mg2+) also has a near-seawater value in the shallowest sample (52.0 mM at 5.95 mbsf). There is a relatively rapid increase in the magnesium concentration in the upper part of the hole, to a maximum value of 54.1 mM at 34.10 mbsf. A relatively large shift in magnesium concentration can be seen in the interval between samples at 34.10 and 46.70 mbsf. This shift can be related to the boundary between lithostratigraphic Units I and II (34.0 mbsf), although the exact lithologic boundary is located above both of those samples. Between 46.70 and 83.80 mbsf, magnesium remains almost constant at ~51.1 mM. In the bottom part of the hole (>83.80 mbsf), magnesium concentrations decrease to a minimum value of 47.3 mM at 132.90 mbsf. The interval of magnesium decrease spans from lithostratigraphic Subunit IIB (nannofossil and diatom ooze) through Subunit IIC (chert breccia-bearing nannofossil ooze) to Subunit IID (nannofossil-bearing clay). According to the core descriptions (see "Lithostratigraphy"), the contact between lithostratigraphic Subunits IIC and IID probably coincides with the interval used for interstitial-water sampling (interval 181-1121B-17X-2, 140-150 cm).
An unusual increase in the magnesium concentration occurs in the upper part of the hole, shallower than 34.10 mbsf. At most DSDP and ODP sites, decreasing magnesium concentrations with depth have been reported, and magnesium transport from the surface downhole is in thought to be controlled primarily by alteration reactions involving volcanic or igneous minerals (Gieskes and Lawrence, 1981). However, the increase in magnesium at Site 1121 suggests dominant ion-exchange of clay minerals and silicate reconstitution.
Strontium (Sr2+) concentration generally tracks the profile of calcium concentration, showing a steady increase with depth. Strontium to calcium ratios (Sr2+/Ca2+) remain in a relatively narrow range between 8.7 × 10-3 and 10.3 × 10-3 mol/mol throughout the hole, although an increasing trend with depth suggests a limited effect from carbonate recrystallization.
The potassium (K+) concentration steadily decreases throughout the hole to a minimum value of 8.1 mM at 132.90 mbsf. Potassium normally decreases with increasing burial depth at deep-sea sites. Possible sinks for potassium are authigenic K-feldspar associated with the opal transformation process (Kastner and Gieskes, 1976), illite-smectite formation (Perry and Hower, 1970), or K+ uptake in the alteration of volcaniclastic sediment and basalt (Gieskes and Lawrence, 1981).
Concentrations of lithium (Li+) increase slightly with depth to 91.90 mbsf. Below this depth, concentrations rapidly increase to 48 µM by 132.90 mbsf. Sodium (Na+) concentrations do not change downcore from the subseafloor to 91.90 mbsf. Na+ values subsequently decrease to a minimum of 458 mM at 132.90 mbsf.
The profiles of interstitial-water constituents at Site 1121 show local fluctuations, suggesting dominant lithologic control rather than a simple diffusion process of diagenetic fluids. The primary chemical reactions are silica diagenesis, dissolution of carbonate, and, possibly, ion-exchange reactions of clay minerals. In particular, relatively high concentrations of dissolved silica in the interval between ~35 and 120 mbsf are related to the dissolution of biosiliceous sediments. In the lower part of the hole, relatively large shifts in concentration-depth profiles of some interstitial-water constituents (Cl-, Mg2+, HPO42-, and MH4+) occur. A local decrease in dissolved silica concentration at ~120 mbsf can be related to chert formation in lithostratigraphic Subunit IIC. Low-porosity chert layers and calcium carbonate concretions may act as diffusion barriers for interstitial-water constituents.