Twenty-nine interstitial-water samples were collected at this site: 11 from Hole 1125A at depths ranging from 2.95 to 190.70 mbsf, and 18 from Hole 1125B between 197.70 and 545.45 mbsf. Sampling frequency is one per 20 m from the seafloor to 462.25 mbsf. Below 462.25 mbsf, one sample per 30 m was taken. Results from these two holes are considered to constitute a single depth profile and the data are plotted together in Figure F17. Analytical results are summarized in Table T10 (also in ASCII format).
Salinities of the interstitial water decrease from 34.5 at 2.95 mbsf to 31.5 at 231.90 mbsf. Below 286.6 mbsf, salinities remain almost constant (32.0), with the exception of the lowermost sample (32.5; Fig. F17).
The chloride (Cl-) concentrations increase gradually with depth from 556 mM at 2.95 mbsf to a maximum of 583 mM at the bottom of the hole. The increasing gradient in chloride in the bottom part of the hole, below 422.65 mbsf, is steeper than that of the middle part of the hole, between 38.75 and 422.65 mbsf. The higher Cl- values below 422.65 mbsf may be attributed to the sediment reacting with the pore water (e.g., the hydration of clay minerals).
Interstitial water pH values decrease gradually from 7.51 at 2.95 mbsf to 7.09 at 443.35 mbsf. Below 443.35 mbsf, pH values increase downcore with a significantly high value of 7.79 at 519.65 mbsf. Although this high pH value seems to be erroneous, there is some possibility that this value represents the real pH value of the pore water. The pH profile is generally mirrored by that of dissolved silica and this high-pH sample shows a significantly low concentration of dissolved silica, as discussed below.
Sulfate, alkalinity, ammonium, and phosphate concentrations are controlled by organic matter decomposition processes including sulfate reduction and methanogenesis. The sulfate (SO42-) concentration decreases gradually from 26.1 mM at 2.95 mbsf to zero at 190.70 mbsf; below this depth, sulfate remains zero or near-zero downcore. Small fluctuations of sulfate in the bottom part of the hole may be attributed to contamination resulting from drilling disturbance. Methane concentration starts increasing just below the top of the zero sulfate interval, suggesting methanogenesis resulting from anaerobic organic matter decomposition by fermentation.
The alkalinity of interstitial water increases with depth to 10.51 mM at 212.70 mbsf; below this depth alkalinity decreases downcore, showing the minimum value of 0.96 mM at 519.65 mbsf. The alkalinity maximum is relatively small compared to maxima observed at Sites 1119 (26.7 mM) and 1122 (40.5 mM) on this leg, where intensive sulfate reduction also occurs. The increase in alkalinity results from the production of bicarbonate ions during bacterial degradation of organic matter.
Ammonium (NH4+) concentrations generally track the profile of alkalinity. Ammonium values increase with depth from the subsurface value of 158 然 at 2.95 mbsf to a maximum of 2578 然 at 308.90 mbsf, which is located ~180 m below the alkalinity maximum. Below 308.90 mbsf, ammonium values decrease gradually downhole to the bottom. An increase in the ammonium concentration reflects the intensive bacterial degradation of organic matter, whereas a decrease may be the result of ion-exchange reactions with clay minerals and/or the subsequent incorporation into diagenetically formed clay minerals.
The phosphate (HPO42-) concentrations decrease from a subsurface value of 6.8 然 at 2.95 mbsf to ~4 然 at 212.70 mbsf, at which depth the alkalinity maximum occurs. Below this depth, phosphate concentrations show relatively large fluctuations and these roughly correspond to the profile of methane concentrations, suggesting that leaching of the phosphate from organic matter has occurred during methanogenesis.
Dissolved silica (H4SiO4) concentrations increase gradually from a value of 447 然 at 2.95 mbsf to a maximum value of 1100 然 at 385.60 mbsf. This is a result of diffusion, driven by the concentration difference between seawater and the sediments and/or upward pore-fluid migration resulting from burial compaction. Dissolved silica concentrations show a pronounced minimum of 181 然 at 519.65 mbsf. A local decrease of dissolved silica in the deeper part of the hole is usually attributed to chert formation (see "Inorganic Geochemistry" in the "Site 1123" chapter), although no chert layers are found at this site (see "Lithostratigraphy"). However, this part of the section below 500 mbsf shows an abrupt increase in density and hardness (responsible for longer drilling times; see "Physical Properties"), which may be indicative of incipient silica concentration. The preservation of the diatom and radiolarians is poor in core-catcher samples (Samples 181-1125B-54X-CC to 58X-CC; 512.73-548.22 mbsf) (see "Biostratigraphy"), suggesting possible changes in paleoproductivity of siliceous planktonic organisms. It would be another explanation for the local decrease of dissolved silica. Low abundance of siliceous fossils mirrored by low silica concentrations in interstitial waters has been reported from Site 1123 of this leg (see "Inorganic Geochemistry" in the "Site 1123" chapter) and the Ceara Rise in the Atlantic (Mikkelsen and Barron, 1997). Yet another possible interpretation of the distinct spike of low dissolved-silica concentration is the localized enhancement of silica dissolution. As described above, the profile of dissolved silica is mirrored by that of pH in interstitial waters. High-pH conditions may result in a low saturation of opaline silica, which may enhance silica dissolution. However, the cause of high pH at this depth is not clear.
The calcium (Ca2+) concentrations decrease slightly from 10.0 mM at 2.95 mbsf to 6.62 mM at 111.75 mbsf and then increase gradually with depth to a maximum of 25.2 mM at the bottom of the hole. The carbonate precipitation caused by the increase of alkalinity resulting from the oxidation of organic matter is responsible for the decreasing trend of Ca2+ concentrations in the uppermost part of the core. On the other hand, the increasing concentrations of Ca2+ in the lower part of the core are attributed to the progressive dissolution of carbonate-rich sediment in the interstitial waters.
The magnesium (Mg2+) concentrations decrease consistently with depth from 51.7 mM at 2.95 mbsf to 9.1 mM at the bottom of the hole. Decrease of magnesium concentrations with depth have been reported at most DSDP and ODP sites, and magnesium transport from the surface downhole is thought to be controlled by alteration reactions of volcanic or igneous minerals (Gieskes, 1981). The rate of decrease in the Mg2+ concentrations diminishes below 200 mbsf.
The potassium (K+) concentration steadily decreases downhole from the subsurface volume of 11.8 mM at 2.95 mbsf to a minimum of 3.7 mM at 545.45 mbsf. Potassium normally decreases with increasing burial depth at deep-sea sites. Sodium (Na+) concentrations increase steadily from 472 mM at 2.95 mbsf to a maximum value of 510 mM at 545.45 mM mbsf. The profile of Na+ can generally be related to chloride concentration in interstitial waters.
Sulfate reduction occurs intensively in the upper part of the hole (<200 mbsf), while the methanogenesis zone is located below the sulfate reduction zone. Sulfate decreases gradually with depth, to zero at ~200 mbsf and remains near zero to the total depth. Alkalinity shows a maximum value of 10.5 mM at 222 mbsf. Organic carbon degradation processes are inferred from the profiles of phosphate and ammonium. Phosphate shows a similar profile to that of methane concentration, whereas ammonium concentrations track the profile of alkalinity, with both ammonium and alkalinity showing gradual changes downcore. Dissolved silica concentrations show almost equilibrated values with respect to opaline silica in the lower part of the hole. A pronounced minimum in dissolved silica corresponds to the poor preservation of diatoms and radiolarians in the bottom part of the core. The increased hardness of the sediments suggests a high silica concentration in this horizon.