Twenty-six interstitial water (IW) samples were collected at Site 1118. Samples were taken every other core between 258 and 526 mbsf and every third core until Core 180-1118A-67R, at 842 mbsf just above the sandstone/limestone boundary. No samples are available between the mudline and ~200 mbsf because this interval was drilled but not cored. All whole-round samples yielded sufficient IW to allow shipboard determination of the same full suite of constituents as analyzed at Sites 1109 and 1115. Data from this site complement those obtained at the latter two, and the resultant north-south IW chemistry transect provides a basis for evaluating sediment diagenesis in the Woodlark Rise on a more regional basis.
The IW was analyzed for salinity, pH, alkalinity, major cations (Na+, K+, Ca2+, and Mg2+) and anions (Cl- and SO42-), Li+, Sr2+, SiO2, and NH4+. Results of shipboard inorganic chemical analyses are presented in Table T9. Profiles of inorganic constituents are presented in Figures F59 and F60. Large changes in the concentrations of many of the dissolved constituents were observed below 400 mbsf.
The pH remains in a narrow range (8.1-8.6) throughout the entire sedimentary column. The samples from 427 to 635 mbsf, however, define a broad subtle maximum, whereas lower values are found below 650 mbsf. The titration alkalinity profile displays a large decrease from a maximum of 13 mM in the first sample collected to 396 mbsf (Fig. F59A). The highest alkalinity value here is nearly identical to the maximum of 13.4 mM observed at Site 1109, but substantially greater than that at Site 1115. Below 400 mbsf, alkalinity remains mostly <2 mM until 635 mbsf. A few samples near the bottom of lithostratigraphic Unit IV define a submaximum that occurs at 783 mbsf. Overall, the alkalinity profile is more similar to what was observed at Site 1109 than at Site 1115. Dissolved SO42- is essentially absent from the pore water with only a few very small excursions at depth until 635 mbsf. An unexpected increase in the concentration of SO42- was observed below 700 mbsf. A concentration of 13 mM, or nearly half the seawater concentration, is at 811 mbsf. No such increase was observed at the bottoms of Sites 1109 and 1115. Dissolved NH4+ concentrations are higher here than seen in the two other deep sedimentary profiles. The highest value of NH4+ (2822 然) coincides with the highest alkalinity value and is in the first sample collected (Fig. F59A). The sharp decrease in NH4+ downhole to a minimum of 1375 然 at 526 mbsf and subsequent increase to a broad submaximum further downhole is also reminiscent of what was observed at Site 1109 (see Fig. F70A, in the "Inorganic Geochemistry" section of the "Site 1109" chapter). Unlike the latter, however, the deep NH4+ submaximum here is characterized by substantially lower concentrations than observed at Site 1109. Additionally, the maintenance of relatively elevated concentrations of NH4+ near the bottom of the hole is surprising in light of the elevated SO42- concentrations.
Salinity fluctuates only slightly between 32 and 34 (Table T9). Lower values were observed in the upper section of the sampled sediments (258-413 mbsf). A salinity of 34 at 427 mbsf is followed by a general decrease downhole and a return to this same value in the bottom three samples. The latter samples correspond to the samples defining the deep-seated SO42- maximum. As noted at Site 1115, the salinity variations do not correlate with Na+ or Cl-; however, the range of variations of the latter two is much narrower here than observed at both Sites 1109 and 1115 (see "Inorganic Geochemistry" in the "Site 1109" chapter and "Inorganic Geochemistry" in the "Site 1115" chapter). The sharp drop in Cl- noted in the brackish water-derived lagoonal sediments immediately above the dolerite at Site 1109 is notably absent here. Dissolved Na+ and Cl- display a rough inverse correlation throughout the sedimentary column, although the Na+ minimum is present at a shallower depth than the Cl- maximum.
The K+ profile (Fig. F59B) displays a sigmoidal shape that is more similar to that observed at Site 1109 than at Site 1115. Yet, substantial differences exist in the K+ profiles of all three sites. Concentrations of K+ are slightly elevated relative to seawater near 300 mbsf, but decrease to a minimum of 4.1 mM by 543 mbsf. This minimum is lower than a comparable minimum of 6 mM near 300 mbsf at Site 1109 (see "Inorganic Geochemistry" in the "Site 1109" chapter). The return below this depth to near-seawater concentrations of K+ at 725 mbsf also resembles such a feature at Site 1109, although it is at much shallower depths at the latter.
Dissolved Li+ (Fig. F59B) exhibits a nearly steady increase in concentration downhole with concentrations near 150 然 in the volcaniclastic sand-rich sediments at the bottom of the hole. These values are greater than observed at other Leg 180 sites. Furthermore, the Li+ profile here displays none of the variations observed at Sites 1109 and 1115.
Dissolved Ca2+ exhibits the narrowest range of concentrations (2.6-26 mM) of the three northern sites. This constituent is most depleted relative to seawater at 258 mbsf, below which it increases to a well-defined maximum near 500 mbsf. Although Ca2+ concentrations decrease to about a 10% enrichment over seawater by 725 mbsf, significant increases are observed in the underlying volcaniclastic sand-rich sediments of lithostratigraphic Unit V (see "Lithostratigraphic Unit V"). The Mg2+ profile remains in a narrow concentration range of 35-38 mM from 258 to 604 mbsf. Decreases in concentration that are below this depth contrast sharply with those observed at Sites 1109 and 1115. The minimum of 24 mM in the deepest IW sample is ~45% of the seawater value. This is a much greater concentration than observed at the other northern sites where Mg2+ concentrations were as low as 11 and 2 mM, respectively.
The Sr2+ profile at Site 1118 fluctuates in the range 53-83 然 between 257 and 544 mbsf (Table T9; Fig. F59C), below which a smooth and substantial increase in concentration is found to the bottom of the hole. The maximum Sr2+ concentration of 631 然 is in lithostratigraphic Unit V within volcaniclastic sands and coincides with the most elevated Li+ concentrations. Excluding the upper 250 mbsf of the hole, which was not sampled, the Sr2+ profile seems most similar to that observed at Site 1109, although the maximum concentration is only about one third of that at the latter site. Additionally, the Sr2+ maximum is not as sharply defined as the increase noted in the aragonitic shell-rich sediments of lithostratigraphic Unit VII at Site 1109 and the subsequent decrease toward the underlying dolerite. The latter feature is absent here (see "Lithostratigraphic Unit VII").
The dissolved SiO2 profile (Fig. F60) displays more complexity than those of other pore-water constituents at this site, as it did at Sites 1109 and 1115 (see "Inorganic Geochemistry" in the "Site 1109" chapter and "Inorganic Geochemistry" in the "Site 1115" chapter). Because the upper 250 mbsf of the profile are missing here, it is impossible to know at what depth in the sediments dissolved SiO2 exhibits its primary transitions from low concentrations to the range of 400-500 然, which is usually observed in the upper several hundred meters. Additionally, the return to concentrations of SiO2 <200 然, observed at depth at Sites 1109 and 1115, does not occur here. Rather, the highest SiO2 concentrations (822-871 然) are present in the samples near the boundary between lithostratigraphic Units IV and V (see Fig. F60). Comparing the profiles from the three northern margin sites, it appears that the profile here is most similar to that of Site 1109. A major difference is a substantial downhole shift of the SiO2 concentration maximum that was observed in the deeper of the two high-porosity intervals at Site 1109 (see Fig. F71 in the "Site 1109" chapter).
Although many of the data needed to interpret the changes in the chemistry of interstitial fluids at Site 1118 were not processed as fully as those from other sites because of time constraints, certain similarities in some of the IW constituent profiles from Sites 1109, 1115, and 1118 suggest that the same diagenetic reactions mediate the pore-water composition at Site 1118. Furthermore, the IW chemistry appears most similar between Sites 1109 and 1115. Among the similarities can be included the behavior of pH, alkalinity, SO42-, and NH4+ in the upper section of these sites. Although the upper 200 m of Site 1118 was not cored, it appears that the chemistry of the biologically influenced constituents follows similar trends in the upper 500 m at Site 1109 and the upper 700 m at Site 1118. This is not entirely surprising because the sediments of the thick onlap sequence at Site 1118 represent an expanded version of part of the section cored at Site 1109. It is also thought that within these sediments volcanic matter alteration reactions and clay-mineral diagenesis occur. The variations in the Ca2+, K+, and, to a lesser extent, Mg2+ profiles are consistent with this inference. Differences in the profiles of these same constituents are greater deepest in the sediments at each site. This probably arises as a result of the substantially different lithologies encountered deep in the sediments at each site.
Thus, concentrations of IW constituents in the upper portions of all holes primarily reflect the oxidation of organic matter mediated by microbial activity and the concomitant early diagenesis of biogenic carbonates. Specific reactions believed to occur include the dissolution of aragonite and its recrystallization into low-magnesian calcite. The dominant lithologies and mineralogies at the three sites support this inference (see "Lithostratigraphy," "Lithostratigraphy" in the "Site 1109" chapter; and "Lithostratigraphy" in the "Site 1115" chapter).
Deeper downhole, volcanic alteration and authigenesis become more important. These processes are common to each site and include alteration of volcanic ash layers as well as volcaniclastic sands disseminated in carbonates, precipitation of authigenic clay minerals (e.g., chlorite and smectite), as well as conversions of pre-existing detrital clays (e.g., illite and/or illite/smectite interlayering).
Some of the marked differences between the three sites result primarily from variations in the thickness of selected lithologies and/or the presence of different lithologies at one site relative to the other. Paramount among these are the existence of a lagoonal/brackish water to freshwater transitional sediment sequence overlying dolerite at the bottom of Site 1109, a connectivity between the forearc sediment sequence and the synrift sediments at Site 1115, and the absence of the transitional lagoonal/brackish water to freshwater sequence at Site 1118. The occurrence of a marked limestone/coarse sandstone neritic sediment sequence at Site 1118 likely imposes important constraints on changes in the pore-water chemistry at this site.
Profiles of dissolved K+, Li+, Ca2+, Sr2+, and SiO2 (Figs.F59A, F59B, F60) demonstrate the importance of the presence of abundant volcaniclastic sands and of a high porosity on the pore-water chemistry. Because rates and stratigraphic occurrences of silica diagenesis in sediments are mediated strongly by temperature (Torres et al., 1995, and references therein), it is interesting to speculate that the more elevated temperature gradient at Site 1118 compared to Sites 1109 and 1115 (see "Temperature Data") should have contributed to the existence of lower concentrations of dissolved SiO2. The presence of abundant and only slightly altered to relatively unaltered volcanic matter deep in the coarse-grained sediments of lithostratigraphic Unit V imparts, however, an important additional constraint. This is manifested by substantially more elevated dissolved SiO2 than might have been expected simply based on depth and temperature. The highest dissolved Li+ concentration in the pore water from northern sites is also observed here and is consistent with a higher temperature alteration of volcanic matter. Additionally, the elevated Sr2+ concentrations, in the absence of a biogenic carbonate source (e.g., aragonitic gastropod shells observed at Sites 1109 and 1115), also support the inference of a volcanic source.
The chemical composition of the IW is influenced by a series of sedimentary diagenesis reactions. The alteration of volcanic matter (whether as ash layers or dispersed throughout the sediments), carbonate recrystallization reactions mediated by the microbially driven oxidation of organic matter, and silicification reactions all contribute to the observed profiles of pore-water constituents.