Forty-seven interstitial-water samples were collected at this site: 28 from Hole 1123A at depths ranging from 1.40 to 154.50 mbsf, 13 from Hole 1123B between 151.80 and 483.65 mbsf, and 6 from Hole 1123C between 494.35 and 609.75 mbsf. Insufficient sample volume was obtained for measurements from the two lowermost intervals of Hole 1123C, which are micritic limestones of lithostratigraphic Unit IV (see "Lithostratigraphy"). Therefore, only chloride, sulfate, magnesium, and calcium concentrations were measured from Sample 181-1123C-31X-4, 135-150 cm, and no measurements were made on the lowermost Sample 181-1123C-33X-1, 123-135 cm. Sampling frequency is one sample per 3 m for the upper 50 mbsf and one sample per 10 m thereafter to 100 mbsf. Below 100 mbsf one sample per 30 m was taken downhole to the total depth. Results from these three holes are considered to constitute a single depth profile and the data are plotted together in Figure F32. The upper part of the profiles (<200 mbsf) are shown separately in Figure F33. Analytical results are summarized in Table T17 (also in ASCII format).
The salinity profile shows a stepwise decrease from the subsurface value of 34.5 to 33.5 at 151.80 mbsf. Taking the relatively low precision of salinity measurements (0.5) into account, this stepwise decrease with depth may be interpreted as a gradual decrease in salinity with depth. On the other hand, chloride (Cl-) concentrations increase from 555 mM at 1.40 mbsf to 562 mM at 26.50 mbsf. Below 26.50 mbsf, chloride increases slightly downhole, but also shows some fluctuations.
Interstitial water pH values increase from 7.45 at 1.40 mbsf to 7.66 at 22.80 mbsf. From 22.80 to 97.50 mbsf, pH values remain in a narrow range between 7.56 and 7.66, showing some erroneous values probably caused by instrument error (7.92 at 42.00 mbsf and 7.75 at 59.50 mbsf). The pH decreases rapidly below 97.50 mbsf to a minimum value of 7.19 at 135.50 mbsf. Below 135.50 mbsf, pH values again remain close to 7.2, although erroneous data occur at 293.60, 466.25, 483.65, and 532.85 mbsf.
Sulfate, alkalinity, phosphate, and ammonium concentrations are controlled by sulfate reduction, which occurs in the upper part of the hole. The sulfate (SO42-) concentration decreases gradually from 27.9 mM at 1.40 mbsf to 13 mM at ~200 mbsf (Fig. F33); below this depth sulfate values remain almost constant (Fig. F32).
The alkalinity of interstitial water increases with depth to 8.1 mM at 69.00 mbsf; below this depth, alkalinity remains almost constant to ~120 mbsf. Alkalinity shows a relatively small maximum value of 8.5 mM at 107.00 mbsf, compared to maxima observed at Sites 1119 (26.7 mM) and 1122 (40.5 mM), but it is about three times higher than the maximum value that occurred at Site 1121 (3.2 mM). The increase in alkalinity results from the production of bicarbonate ions during bacterial degradation of organic matter by sulfate reduction. A rather steep alkalinity decrease in the topmost part of the hole (<20.00 mbsf) presumably represents an interval of relatively intensive sulfate reduction (Fig. F33). Below 120 mbsf, alkalinity decreases downhole to 1.7 mM at 466.25 mbsf, probably caused by carbonate recrystallization, which depletes the bicarbonate ion in the interstitial water. Small fluctuations of alkalinity in the bottom part of the hole may be attributed to contamination resulting from drilling disturbance.
The intensity of sulfate reduction is governed by several factors, most importantly the abundance of organic matter and the sedimentation rates. The sedimentation rate of the upper part of the hole is not high compared to Sites 1119 and 1122 which show intensive sulfate reduction. However, a relatively high organic carbon content compared to normal deep-sea carbonate sediments may have caused moderate sulfate reduction at this site (see "Organic Geochemistry").
Ammonium (NH4+) concentrations increase with depth to a maximum of 779 µM at 151.80 mbsf, which is located ~50 m below the alkalinity maximum. Below this interval, 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 indicates the results of ion exchange reactions with clay minerals and/or the subsequent incorporation into diagenetically formed clay minerals.
The phosphate (HPO42-) concentrations decrease from a subseafloor value of 15.2 µM at 1.5 mbsf to ~4 µM at 151.80 mbsf, at which depth the ammonium maximum occurs. Below this depth, phosphate concentrations generally remain smaller than 3 µM. A rapid decreasing trend in the upper part of the hole suggests a diagenetic uptake of dissolved phosphate into sedimentary mineral phases.
Dissolved silica (H4SiO4) concentrations increase gradually from a value of 546 µM at 1.50 mbsf to a local maximum value of 1014 µM at 293.60 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 of sediments. The values of samples below 351.00 mbsf are more variable, ranging from 274 to 1062 µM. This interval corresponds to lithostratigraphic Units II and III, which consist of nannofossil chalk overlain by an unconformity located at 587 mbsf (Fig. F34; see "Lithostratigraphy"). Dissolved silica concentrations show three pronounced minima at 351.00, 466.25, and 532.85 mbsf. A local decrease of dissolved silica in the deeper part of the hole is usually attributed to chert formation (DSDP Site 315, Gieskes and Lawrence, 1981; and Site 1121, see "Inorganic Geochemistry" in the "Site 1121" chapter). However, no chert layers are found at this site (see "Lithostratigraphy"). Therefore, these local decreases of dissolved silica are not likely to be caused by silica diagenesis. The preservation of radiolarian fauna is generally good at this site (see "Biostratigraphy"); only three core-catcher samples are barren of radiolarians (Sample 181-1123B-38X-CC, 354.79 mbsf; 181-1123B-50X-CC, 468.6 mbsf; and 181-1123C-23X-CC, 536.71 mbsf). Each of these three samples corresponds to a sharp decrease in the dissolved silica concentration, suggesting possible changes in paleoproductivity of siliceous planktonic organisms including radiolarians and diatoms. Partly consolidated chalk layers of low permeability might contribute to the preservation of low dissolved silica concentrations, even accounting for the smoothing effect caused by fluid migration and diffusion. Low abundance of diatoms mirrored by low silica concentrations in interstitial waters has been reported from Paleogene sediments of the Ceara Rise in the Atlantic (Mikkelsen and Barron, 1997).
The calcium (Ca2+) concentration shows a near-seawater value in the shallowest sample (10.8 mM at 1.40 mbsf) and decreases with depth to a minimum of 7.55 mM at 78.50 mbsf, followed by a steady increase downhole to a maximum value of 31.6 mM at 609.75 mbsf. Magnesium (Mg2+) also has a near-seawater value in the shallowest sample (53.29 mM at 1.40 mbsf). There is a steadily decreasing trend in the magnesium concentration throughout the hole, to a minimum value of 22.5 mM at 580.95 mbsf. At most DSDP and Ocean Drilling Program (ODP) sites, decreases of magnesium concentrations with depth have been reported, and magnesium transport from the surface downhole is interpreted to be controlled primarily by alteration reactions involving volcanic or igneous minerals (Gieskes, 1981).
The strontium (Sr2+) concentration shows a steadily increasing trend with depth to ~400 mbsf. The content of strontium in inorganically precipitated calcite is about a factor of three lower than in biogenic calcite (Baker et al., 1982). Therefore, calcite diagenesis increases the strontium in the surrounding interstitial waters (calcite purification). The steady decrease in strontium concentration may be caused by carbonate recrystallization. Below 400 mbsf depth, strontium concentration remains in the relatively narrow range between 1481 and 1702 mM, suggesting the termination of active carbonate recrystallization. Another possible explanation for the constant dissolved strontium concentration in the interval is that celestite (SrSO4) formation masks the increase of dissolved strontium concentration. Concentrations of strontium and sulfate in the lower part of this site (>400 mbsf) are ~1600 µM and 10 mM, respectively, and these values are close to those at DSDP Leg 90 sites on the Lord Howe Rise in the Tasman Sea where celestite nodules were recovered (Baker and Bloomer, 1988).
The potassium (K+) concentration steadily decreases downhole to a minimum of 6.3 mM at 609.75 mbsf. Potassium normally decreases with increasing burial depth at deep-sea sites.
Up to ~150 mbsf, concentrations of lithium (Li+) increase slowly with depth. Below 150 mbsf, the slope of the depth-concentration profile becomes slightly steeper. The lithium concentration generally tracks the profile of the calcium concentration. Although lithium concentrations are related to the opal transformation process, including biogenic silica dissolution as described at some DSDP sites (Gieskes, 1981), the lithium profile of this site is apparently not influenced by the silica concentration.
Sodium (Na+) concentrations are related to chloride concentrations, showing a local maximum of 479 mM at 51.50 mbsf and a minimum of 468 mM at 466.25 mbsf.
Interstitial water compositions are dominantly controlled by the high carbonate content of the sediments. Sulfate reduction is moderate in the upper part of the hole, probably related to relatively high organic carbon content compared to normal deep-sea carbonate sediments. Sulfate decreases gradually with depth to 13 mM at ~200 mbsf and remains almost constant below this depth. Alkalinity shows a maximum value of 8.5 mM at 107 mbsf. Carbonate diagenetic reactions are inferred from the profiles of dissolved calcium, magnesium, and strontium. The variation of dissolved silica in the lower part of the hole may imply a possible paleoproductivity change.