Twenty-four interstitial-water samples were collected from Site 1124, two from Hole 1124B at depths ranging from 2.90 to 8.30 mbsf, and 22 from Hole 1124C at depths ranging from 21.94 to 471.75 mbsf. One interstitial-water sample was taken from approximately every 10 m to a depth of 100 mbsf. From 100 mbsf to the bottom of the hole, every third core was sampled. Analytical results are summarized in Table T15 (also in ASCII format) and plotted in Figure F26.
Salinities of the interstitial-water samples generally show an increasing trend in the lower part of the core. From 2.90 to 299.90 mbsf (299.90 is the depth of the lithologic boundary between Subunit IC and Unit II), salinity varies between 34.5 and 35.0 and then increases to a maximum of 36.0 at 443.05 mbsf (Fig. F26). The high salinity in the lower part of the hole may be caused by enrichment of major ion concentrations including calcium, strontium, lithium, sodium, and chloride as described below.
The profile of chloride (Cl-) concentration is similar to that of salinity (Fig. F26). The Cl- concentration increases from 556 mM at 2.90 mbsf to 572 mM at 80.60 mbsf and varies between 561 and 572 mM from 80.60 to 328.75 mbsf. Below 328.75 mbsf, the Cl- concentration shows high values with a maximum of 579 mM at the same depth as the salinity maximum. Part of the chloride concentration profile may result from the hydration of clay minerals.
Interstitial water pH values generally decrease with depth, ranging between 7.01 and 7.41 (Fig. F26). The highest pH in Hole 1124C (7.50) occurs at 471.75 mbsf, but this value may be erroneous.
Concentrations of sodium (Na+) increase from 475 mM at 2.90 mbsf to 492 mM at 109.10 mbsf and then decrease toward the bottom of the hole (Fig. F26).
The extent of oxidation, itself a function of the availability of organic matter, generally influences the concentrations of alkalinity, sulfate, phosphate, and ammonium. The profile of alkalinity in the interstitial water shows the typical pattern that results from organic matter oxidation and carbonate reprecipitation (Fig. F26). The alkalinity has a maximum (5.58 mM) at 21.94 mbsf, representing the peak of organic matter oxidation. The alkalinity then decreases gradually down to 193.90 mbsf, reflecting carbonate recrystallization and/or silicate reconstitution processes that use the bicarbonate ion from interstitial water (Gieskes, 1974). Below 193.90 mbsf, which is just below the lithologic boundary between Subunits IB and IC (see "Lithostratigraphy"), the variation of alkalinity is somewhat scattered, implying the occurrence of a variety of chemical reactions.
The sulfate (SO42-) concentration varies in a small range between 19.1 and 27.4 mM, which means that no active sulfate reduction occurred (Fig. F26). However, the SO42- concentration generally decreases with depth, suggesting a limited amount of organic matter degradation. The main reason for the lack of sulfate reduction at this site is the paucity (and/or unavailability) of organic matter (see "Organic Geochemistry"), and the relatively slow sedimentation rate (see "Age Models and Sedimentation Rates").
Ammonium (NH4+) concentrations increase from 59 mM at 2.90 mbsf to a maximum of 330 mM at 52.10 mbsf (Fig. F26). This NH4+ increase results from organic matter oxidation. Below this interval, the NH4+ values steadily decrease down to 193.90 mbsf, which lies near to the lithologic boundary between Subunits IB and IC. The NH4+ concentrations decrease continuously with depth, but a small enrichment in Unit II reflects an additional input into the pore water. The decrease of NH4+ concentrations is caused by ion-exchange reactions on the surfaces of clay minerals and/or the subsequent incorporation of ions into interlayers of diagenetically formed clay minerals (Gieskes, 1981).
The phosphate (HPO42-) concentrations decrease with depth from the seafloor down to 71.10 mbsf (Fig. F26). Below 71.10 mbsf, the HPO42- concentrations are relatively constant with a small variation. In the upper part of the hole, the phosphate profile is consistent with diagenetic uptake of dissolved phosphate, most likely into sedimentary mineral phases.
Calcium (Ca2+) concentrations generally increase with depth, except for at the top of the core, where Ca2+ concentrations are controlled by carbonate precipitation resulting from the buildup of alkalinity during organic matter oxidation (Fig. F26). The Ca2+ increase is attributed to the dissolution of carbonate-rich sediment in the interstitial waters. The gradual increase of Ca2+ concentrations ceases at ~300 mbsf, which is the lithologic boundary between Subunit IC and Unit II. Below this depth the Ca2+ concentrations remain relatively constant, but the lowermost part of hole shows another increasing trend.
The profile of magnesium (Mg2+) concentrations is almost opposite to that of the calcium concentration (Fig. F26). There is a gradual decrease of the Mg2+ concentration downhole to 200 mbsf, which can be explained by reactions that remove Mg2+ diagenetically from solution. Below 200 mbsf, which also marks the lithologic boundary, the Mg2+ concentrations remain at a consistent level.
Dissolved strontium (Sr2+) concentrations increase gradually from 93 然 at 2.90 mbsf to a maximum of 598 然 at 357.75 mbsf and then decrease with depth (Fig. F26). Strontium concentrations in interstitial waters are supplied by the recrystallization of biogenic carbonate. However, the decrease of Sr2+ concentrations in the lower part of the hole suggests that a different chemical reaction is occurring in addition to carbonate diagenesis. This reaction probably involves lithium and silica, as discussed below.
Dissolved silica (H4SiO4) concentrations increase slightly from 607 然 at 2.90 mbsf to 816 然 at 165.10 mbsf, which is the lithologic boundary between Subunits IB and IC (Fig. F26). The increasing silica concentration results from diagenetic dissolution of the biogenic silica in the sediments. At the interval of Subunit IC, silica concentrations become abruptly low and then increase again down to another lithologic boundary between Unit II and Unit III. Below Unit III, the dissolved silica decreases again to the bottom of the hole. The decrease of dissolved silica in the deeper part of the hole may be attributed to local chert formation (Gieskes and Lawrence, 1981; see "Lithostratigraphy"). In contrast to the bottom of the hole, the local decrease of silica in Subunit IC is not caused by chert formation. Instead, the low preservation of biogenic siliceous sediments suggests low paleoproductivity (see "Biostratigraphy"). In addition, the sudden decrease in tephra layers in Subunit IC is partly responsible for the low silica concentration (see "Lithostratigraphy").
Potassium (K+) concentrations decrease gradually from 12.0 at 2.9 mbsf to 5.5 mM at 299.90 mbsf. Below 299.90 mbsf, the potassium concentrations are greater than 5.5 mM (Fig. F26). A depth of 299.90 mbsf coincides with the lithologic boundary between Subunit IC and Unit II (see "Lithostratigraphy"). The decrease of potassium in the interstitial water is a result of its removal into clay minerals that are forming within the sediments. However, below Unit II, the enrichment of potassium may reflect another chemical reaction.
The profile of dissolved lithium (Li+) concentrations is similar to that of strontium (Fig. F26). The Li+ concentrations increase slightly from 29 然 at 2.90 mbsf to 66 然 at 137.60 mbsf, which is just above the lithologic boundary between Subunits IB and IC. Throughout Subunit IC, the gradient of increasing Li+ concentrations becomes steeper down to the depth of 299.90 mbsf, which is the lithologic boundary between Subunit IC and Unit II. However, within Unit II, the Li+ concentrations remain relatively constant; below Unit II, the lithium decreases to the bottom of the hole. Generally, the Li+ concentrations depend on release of the element into the pore water from the sediments during recrystallization of biogenic sediments. Because the Li+ concentration is related to the biogenic silica content (Gieskes, 1981), the general increase indicates release of Li+ during diatom dissolution and silica transformation. However, based on the calcium, magnesium, and strontium concentrations, the decreasing pattern of lithium in the lower part of the hole, which is similar to strontium, may be caused by chert formation (Gieskes, 1983).
The dominant chemical reactions controlling the interstitial-water element concentrations at Site 1124 include organic matter degradation, carbonate dissolution/precipitation, silica dissolution, chert formation, and reactions with clay minerals. Without active sulfate reduction, the element profiles of alkalinity, phosphate, and ammonia are typical, reflecting the organic matter oxidation and carbonate precipitation. The behavior of Ca2+, Mg2+, and Sr2+ in the bottom part of hole implies the presence of another chemical reaction. The decrease of Sr2+ corresponds to the decrease of Li+, which is related to silica utilization to form the chert observed in the lowermost part of core. However, low levels of dissolved silica in the middle of the core are attributed to the poor preservation of biogenic siliceous sediments, presumably because of low paleoproductivity. The general chemical zonations of interstitial waters at Site 1124 can be related to the lithostratigraphic units, paleontological age divisions, and the presence of hiatuses.