Twenty-five interstitial-water samples were collected at Site 1119: 13 samples from Hole 1119B at depths ranging from 2.95 to 153.10 mbsf and 12 from Hole 1119C from 156.70 mbsf to 472.30 mbsf (Table T12, also in ASCII format). Sampling frequency was one per every 10 m for the upper 100 m and one per every 30 m thereafter. Results from these two holes are considered to constitute a single depth profile in this report and the data are plotted together in Figure F21.
Salinities of the interstitial-water samples range from 31.0 to 34.0. The topmost samples show slightly higher values; below 100 mbsf, salinity remains relatively uniform (Fig. F21). Chloride (Cl-) concentrations in the upper 80 mbsf show a decreasing trend with depth, from the subsurface value of 552 mM to a minimum value of 545 mM at 77.05 mbsf. The concentrations then increase steadily to the bottom. A possible explanation for the occurrence of low chloride concentrations at 77.05 mbsf is freshwater penetration from the land through permeable sandy sediments (Gieskes, 1981). Interstitial-water pH values vary from 7.53 to 8.09. The highest pH is measured at 67.60 mbsf, which coincides with the minima of salinity and chloride. Below 77.05 mbsf, pH values decrease toward the bottom.
Interstitial-water alkalinity increases rapidly in the upper few tens of meters of the hole, reaching a maximum of 26.7 mM at 29.65 mbsf. Concentrations then decrease to 14.9 mM at 96.10 mbsf, below which the alkalinity continues to decrease steadily downhole to a minimum value of 5.4 mM at 472.30 mbsf.
Sulfate (SO42-) reduction is evidenced by the rapid decrease in sulfate concentration from 28.6 mM at 2.95 mbsf to zero at 20.15 mbsf. Sulfate concentration remains near zero from 20.15 to ~300 mbsf. Below 328.20 mbsf, the sulfate concentrations are slightly raised (<2.4 mM), which is probably caused by contamination from the drilling process. Sulfate is removed by sulfate-reducing bacteria in these rapidly deposited sediments, even though the organic carbon contents of the sediments in this site are relatively low (see "Organic Geochemistry"). The observed alkalinity increase can be attributed to the production of bicarbonate (HCO3-) ions in the sulfate reduction zone.
Phosphate (HPO42-) also increases from the uppermost sample (4.5 然 at 2.95 mbsf) to a maximum value of 206 然 at 29.65 mbsf, which is located just below the alkalinity maximum and the top of the sulfate-depleted layer. The phosphate concentrations decrease from ~50 mbsf, presumably because of coprecipitation into carbonate. Below 50 mbsf, phosphate concentrations generally remain less than 10 然.
Ammonium (NH4+) concentrations increase with depth from 0.1 mM at 2.95 mbsf to 5.1 mM at 39.05 mbsf. The rapid increase of ammonium concentration is also related to sulfate reduction. Ammonium values then remain between 4 and 5 mM from 39.05 to 96.10 mbsf. Below 96.10 mbsf, concentrations again increase gradually to a maximum of 8.2 mM at 357.00 mbsf.
Dissolved silica (H4SiO4) concentrations increase from a near-seawater value of 281 然 at 2.95 mbsf to 944 然 at 10.65 mbsf, as a result of pore-fluid migration and/or diffusion driven by the concentration difference between seawater and the sediments. From 10.65 to 96.10 mbsf, concentrations are measured within a relatively narrow range, between 682 and 710 然. The values of samples below 96.10 mbsf are more variable, ranging from 723 to 944 然. A positive correlation can be seen between the concentration of dissolved silica and the relative abundance of diatoms, especially below 100 mbsf. This is consistent with the dissolution of siliceous tests in the sediments (see "Biostratigraphy").
Calcium (Ca2+) concentrations are near-seawater values in the shallowest sample (10.8 mM at 2.95 mbsf), and decrease rapidly to under 4 mM by 20.15 mbsf. Magnesium (Mg2+) shows a very gradual decrease from near-seawater values in the shallowest sample to a minimum value of 17.2 mM at 270.70 mbsf. The decrease in calcium and magnesium at 20.15 mbsf (the depth of the sulfate minimum) suggests precipitation of diagenetic carbonate promoted by the rapid increase in alkalinity during sulfate reduction (Compton, 1988). Calcium values remain between 3.2 and 4.1 mM in the interval between 2.95 and 328.20 mbsf. Below this depth, calcium concentration increases to a maximum value of 13.26 mM at 472.30 mbsf, which is close to the seawater value. Generally, the interval of increasing calcium concentrations corresponds to lithostratigraphic Subunit IIB (see "Lithostratigraphy"), which contains significant amounts of broken shells, benthic foraminifers, and calcareous nannofossils, suggesting effects of carbonate dissolution in interstitial waters.
The potassium (K+) concentration remains fairly constant in the interval shallower than ~160 mbsf. A minimum value of 9.4 mM at 20.15 mbsf may correspond to the ammonium maximum, suggesting cation exchange has occurred between the pore water and clay minerals. Below 160 mbsf, potassium decreases gradually to a value of 7.6 mM at 414.90 mbsf. In the bottom part of the hole, a small decrease in potassium concentration can be seen between samples at 414.90 and 443.80 mbsf, which corresponds to the boundary between lithostratigraphic Units II and III (428.2 mbsf).
Concentrations of lithium (Li+) exhibit little change in the first 70 mbsf of Hole 1119B, and subsequently increase to ~80 然 by 357 mbsf. Below this depth, lithium concentrations increase rapidly to the maximum value of 184.5 然 at 472.30 mbsf. The interval of rapidly increasing lithium concentrations is coincident with lithostratigraphic Subunit IIB (see "Lithostratigraphy"). The general trend of the Li+ concentration profile is similar to that of Ca2+, suggesting possible control by recrystallization of biogenic carbonate.
Concentrations of sodium (Na+) show a minimum value of 453 mM at 29.65 mbsf. Values subsequently increase to a maximum of 498 mM at 328.20 mbsf, before decreasing further downhole.
The profiles of interstitial-water constituents at Site 1119 reflect the sulfate reduction of organic matter by microbial activity, the recrystallization of biogenic carbonate, and silica diagenesis.
A sulfate-depleted layer occurs at the shallow depth of 20 mbsf. Alkalinity, ammonium, and phosphate increase in the sulfate reduction zone. These increases are all controlled by the degradation of sedimentary organic matter in the shallow burial environment. Sulfate concentrations are near zero from 20.15 mbsf through to the bottom of the hole, suggesting enhanced diagenetic processes occurred throughout the sequence, perhaps including the dissolution of magnetic minerals.
Evidence of carbonate remineralization at Site 1119 is derived from variations in the Mg2+/Ca2+ ratio (Fig. F22). The significant increase in the Mg2+/Ca2+ ratio in the sulfate reduction zone may indicate the dissolution of high-magnesium calcite. The decreases in calcium and magnesium concentrations below the sulfate reduction zone might be attributed to the precipitation of diagenetic carbonate promoted by a rapid increase in alkalinity.