Three interstitial-water samples were collected from each core from the upper 50 mbsf at Site 1120. Below this depth one sample per core was taken down to 100 mbsf and one from every third core between 100 mbsf and the total depth. In total, 27 interstitial-water samples were obtained at this site, 24 from Hole 1120B at depths ranging from 1.45 to 164.90 mbsf, and three from Hole 1120D between 163.30 and 211.50 mbsf. Analytical results are summarized in Table T13 (also in ASCII format) and the depth profiles of geochemical constituents are plotted in Figure F14.
Salinities of the interstitial-water samples vary slightly between 34.0 and 35.0 (Fig. F14). The topmost sample has the lowest value (34.0). The salinity remains constant throughout the hole, except for the uppermost samples and elevated values between 69.70 and 97.70 mbsf.
Chloride (Cl-) concentrations increase gradually with depth (Fig. F14), except for the uppermost 26.65 m, where concentrations are almost constant. Below 26.65 m, there is a rapid increase in concentration from 550 to 555 mM, and then the concentration shows a steadily increasing trend with depth, reaching a maximum of 564 mM at 163.3 mbsf.
interstitial-water pH values vary from 7.17 to 7.83. Two extremely low values, of 7.02 at 45.70 mbsf and 6.90 at 102.80 mbsf, may be analytical errors considering that no significant lithologic change occurs (see "Lithostratigraphy"). The highest pH lies between 40 and 50 mbsf, below which it decreases with depth to 7.3 at the bottom of the hole.
Concentrations of sodium (Na+) vary within a small range from 463 to 475 mM, generally following the salinity trend.
The alkalinity increase from 2.99 mM at 1.45 mbsf to 5.96 mM at 211.5 mbsf is sharply different than Site 1119 (see "Site 1119" chapter, "Inorganic Geochemistry"). The principal reasons for this trend are the lack of active sulfate reduction throughout the profile and the influence of the diagenetic dissolution of carbonate sediments.
The profile of sulfate (SO42-) concentration is inversely correlated with the alkalinity profile (Fig. F14). Below 23 mbsf, the sulfate concentration decreases slightly from 27.0 mM to 24.7 mM, which may be, together with the increase in alkalinity and ammonium, indicative of sulfate reduction. However, because of the low organic carbon content, there may be another contribution to the alkalinity increase, such as bicarbonate ion (HCO3-) from the dissolution of carbonate-rich sediments (see "Organic Geochemistry").
The phosphate (HPO42-) concentrations scatter considerably. Concentrations are below 2.0 µM in the upper part of the core, down to 69.70 mbsf (Fig. F14). The concentrations decrease slightly down to the bottom, below a maximum of 1.9 µM at 75-85 mbsf. The concentration of HPO42- is possibly controlled by the oxidation of organic matter, the formation or dissolution of carbonate fluorapitite (CFA), and absorption reactions of CaCO3. The decreasing trend in the lower part of the hole may be caused by the incorporation of phosphate into the reprecipitated carbonate.
Ammonium (NH4+) concentrations are low in the upper 40 m of the hole and increase steadily with depth from 68 µM at 39.20 mbsf to 242 µM at 211.50 mbsf (Fig. F14). In general, ammonium, a by-product of organic matter degradation, increases systematically with decreasing sulfate (Gieskes, 1981).
Dissolved silica (H4SiO4) concentrations increase continuously from a value of 203 µM at 1.45 mbsf to 819 µM at 164.90 mbsf. The decrease in H4SiO4 in the bottom of Hole 1120D (Fig. F14) may be caused by a compositional change in the sediments. The silica concentration is controlled by the diffusion process of diagenetic dissolution of biogenic silica in the sediments.
Calcium (Ca2+) concentrations increase steadily from 10.8 mM at 1.45 mbsf to 14.7 mM at the bottom of the hole (Fig. F14). This increase is attributed to the dissolution of carbonate-rich sediments in the interstitial waters throughout the hole (see "Lithostratigraphy"). This dissolution effect is opposite to the magnesium profile and similar to the strontium concentration.
The profiles of magnesium (Mg2+) concentrations are the mirror image of the calcium concentration profiles (Fig. F14). There is a gradual decrease of the Mg2+ concentration downhole, which can be explained by reactions that remove Mg2+ diagenetically from solution.
Dissolved strontium (Sr2+) concentrations exhibit a similar pattern to calcium (Fig. F14). Strontium concentrations increase from 106 µM at 1.45 mbsf to 607 µM at 211.50 mbsf. The Sr2+ is supplied to pore fluids by the dissolution and recrystallization of carbonate to diagenetic low-Mg calcite. The variations of Sr2+ and Ca2+ concentrations at this site depend mainly on the lithologic characteristics (see "Lithostratigraphy").
Potassium (K+) concentrations are low (<11.2 mM), and show a slight decrease from the top to the bottom of the hole (Fig. F14). The low K+ concentration is caused by the sediment properties (carbonate rich and organic-matter poor) because the K+ concentration is partly controlled by the oxidation of organic matter (see "Organic Geochemistry"). Overall, changes in the Na+ and K+ gradients are similar and appear to be influenced primarily by depth.
The dissolved lithium (Li+) remains almost constant above 75.50 mbsf, below which the concentration starts to increase, reaching 52 µM by 211.50 mbsf (Fig. F14). The Li+ concentration depends on release of the element into the pore water from the sediments during recrystallization of biogenic carbonate. In addition, another source of Li+ may be the occurrence of early diagenesis of opal-A (Gieskes, 1983).
The biogenic sediments are the primary influence on many of the chemical gradients in the interstitial waters at Site 1120. The profiles of interstitial-water constituents at this site, controlled by simple diffusion diagenetic processes, show gradually increasing (alkalinity, H4SiO4, Cl-, Ca2+, Sr2+, Li+) and decreasing (K+, Mg2+) trends with depth and no signature of sulfate reduction. Such characteristics result from the uniform lithologic (calcareous dominated) features throughout the hole. The dominant chemical reactions are probably dissolution of carbonate, silica diagenesis, precipitation of low-Mg calcite, and possibly ion-exchange reactions in clay minerals.