The interstitial water (IW) sampling program at Site 1108 was initially designed to obtain a high-resolution profile of pore-water constituents throughout the sedimentary column. However, poor recovery, the occurrence of heavily indurated sediments, and subsequent drilling difficulties deeper downhole resulted in the collection of only 13 whole-round samples. The 5-cm whole rounds that were collected rapidly became insufficient to produce enough IW to conduct the routine shipboard analyses, therefore, as much as 15-cm whole rounds were collected from cores recovered in the deeper portions of the hole. Three whole-round core samples failed to produce any pore water, even upon extended squeezing (12-24 hr) at the maximum pressure available on the Carver press.
The IW was analyzed for salinity, pH, alkalinity, major cations (Na+, K+, Ca2+, Mg2+) and anions (Cl-, SO42-), SiO2, Sr2+, and Li+. Because generally less than 10 mL of IW was recovered from the whole-round core sections, the alkalinity was only determined in four samples to allow time for other more critical analyses. Results of shipboard inorganic chemical analyses are presented in Table T7. For reasons described in "Interstitial Water Sampling and Geochemistry" (in "Inorganic Geochemistry" in the "Explanatory Notes" chapter), pH measurements presented here should be considered only semiquantitative.
The titration alkalinity at Site 1108 is near 3.2 meq/L in the shallowest two core samples (Cores 180-1108B-1R and 10R) and then drops to ~1 meq/L in the two other samples analyzed for this constituent (Table T7).
Salinity decreases between 0.7 and 83 mbsf and then increases downhole with a maximum of 39 observed at ~392 mbsf. The slightly lower salinity (33) observed at 83 mbsf can be attributed primarily to a depletion of SO42- because no decrease in Na+ or Cl- is observed in this section. Sulfate displays a rapid depletion downhole, with 75% depletion observed by 83 mbsf, the depth where headspace analyses reveal that CH4 concentrations become elevated (see "Organic Geochemistry"). Although SO42- returns to near 18 mM at 164 mbsf, complete depletion of this constituent is observed below this depth. The IW analysis at 164.5 mbsf also corresponds to the core in which the CaCO3 maximum was observed (see "Organic Geochemistry") and is within the fault zone identified between 160 and 220 mbsf ("Structural Geology"); this anomalous SO42- concentration is puzzling and may be related to a fluid input along the fault zone.
Concomitant with the increase in salinity observed downhole, Na+ and Cl- concentrations increase, reaching maxima of ~500 and 678 mM, respectively, at 392 mbsf. (Fig. F35). Dissolved K+ concentrations, however, decrease downhole from near seawater values (12 mM) immediately below the mudline to a minimum of 2.7 mM at 304 mbsf, then fluctuate slightly below this depth.
The dissolved Li+ profile at Site 1108 shows substantial variations over the range of 20-136 然 (Table T7; Fig. F35). Immediately below the mudline, near-seawater concentrations decrease to a minimum of 20 然 at 83 mbsf. Elevated values (43-63 然) are observed in sediments near the fault zone identified between Cores 180-1108B-18R and 19R, but Li+ concentrations subsequently decrease to near-seawater values by 202 mbsf. Below this depth, Li+ increases steadily downhole and reaches a maximum of 135 然 at 392 mbsf. Within sediments near the fault zone, the Li+ concentration is relatively low (43 然) in the interval coinciding with the CaCO3 maximum, but nearly 50% more concentrated (62 然) immediately below in the interval containing approximately half as much CaCO3 (see "Organic Geochemistry").
Calcium (Fig. F35) increases from a near-seawater concentration in the first IW sample to a local maximum of 46.4 mM at 267 mbsf, with a slight reversal noted in Section 180-1108B-33R-1 (304 mbsf). Magnesium generally displays a trend opposite that of Ca2+, decreasing from a near-seawater value at 0.7 mbsf to a minimum of 18.4 mM at 267 mbsf. A slight reversal of the concentration trend for these constituents is observed below this depth, but the initial trend returns in the deeper portion of the hole. The Sr2+ profile exhibits an order of magnitude increase between the mudline and 202 mbsf, with the largest increase to 759 然 between 165 and 202 mbsf. Below this depth, Sr2+ concentrations decrease to 225 然 in the deepest sample analyzed for this constituent (344 mbsf).
Dissolved SiO2 displays a substantial increase over seawater below the mudline, reaching 374 然 at 83 mbsf, then decreases downhole, remaining in a narrow range of 140-180 然 to the bottom of the hole.
Although insufficient inorganic geochemical data were collected to fully characterize sedimentary diagenesis caused by a paucity of available fluid, the SO42- profile, when combined with the CH4 profile obtained in the upper section of Hole 1108B by the organic geochemistry program, indicates that these constituents reflect bacterially mediated oxidation of organic matter (e.g., Claypool and Kaplan, 1974; see "Microbiology"). Extensive depletion of SO42- below 83 mbsf coincides with the rise in CH4 concentrations observed in headspace analyses (see "Organic Geochemistry"). The 18-mM SO42- concentration observed at 164 mbsf, however, is difficult to explain in terms of an input of seawater because other major dissolved constituents do not display more seawater like signatures. However, an input of previously altered seawater extant within the fault could readily explain the anomalous concentrations observed here. Furthermore, all IW constituent profiles, except salinity, display an offset at 169 mbsf relative to the 164 mbsf sample, attesting to nonsteady state or normal diffusive profiles within this active zone.
The characteristics of the IW at Site 1108 suggest sedimentary diagenesis under a mildly elevated temperature regime (see "In Situ Temperature Measurements"). The principal processes at Site 1108 responsible for the observed pore-water profiles include the alteration of volcanic matter and the formation of clay minerals. Evidence in support of this hypothesis includes an increasing Ca/Mg value (Fig. F36) with increasing depth below seafloor attributable to a release of Ca2+ from the alteration of plagioclase and a concomitant uptake of Mg2+ from pore water during the formation of chlorite/smectite (McDuff and Gieskes, 1976; McDuff, 1981; Martin et al., 1995), and increasing dissolved Li+ concentrations in the deeper portions of Hole 1108B. Although the slight reversal in the trend of decreasing Mg2+ and increasing Ca2+ concentrations observed at 303 mbsf could possibly arise from contamination with seawater during drilling; a lack of a return to more seawater-like concentrations of other pore-water constituents in this interval suggests this is not the case. Rather, the presence of calcite cements in various portions of the cores suggests that precipitation of this mineral may have influenced the concentration of dissolved Ca2+. A continued influence of volcanic matter on the alkaline earth profiles downhole is suggested by the return to the original trend observed above 303 mbsf. A generally linear decrease in the concentration of K+ downhole is consistent with its uptake from pore water during the formation of clay minerals resulting from the alteration of volcanic matter (Gieskes, 1981). The large increase in dissolved Sr2+ at Site 1108 can be attributed to either (1) release to pore water during the recrystallization of biogenic carbonate, especially aragonite (Baker, 1986), and/or (2) release during the alteration of volcanic ash/glass (Gieskes, 1981). Aragonite is not particularly abundant in the sediments of Site 1108, but the dissolved Sr2+ maximum at 202 mbsf is consistent with the presence of shell fragments within Core 180-1108B-22R (see "Lithostratigraphy"). Nonetheless, alteration of volcanic minerals dispersed throughout the sediments also likely contributes some of the dissolved Sr2+ in the pore water from Site 1108. The downhole decrease in Sr2+ observed in IW from the lower portion of the sedimentary column is consistent with a lack of ash layers that could act as source material for this constituent, although plagioclase remains an important mineral constituent throughout the sedimentary column (see "Lithostratigraphy").
Variations in SiO2 concentrations often reflect reactions under local lithologic control to a much greater extent than those of other IW constituents such as Mg2+, Ca2+, and Sr2+, which often display diffusion-controlled profiles and reflect diagenesis of the sedimentary column as a whole (McDuff, 1981). Although high dissolved SiO2 concentrations often arise when volcanic glass is altered in sediments, IW from Site 1108 generally contains <200 然 SiO2, except in the upper 100 mbsf. Dissolved SiO2 concentrations in marine sediments can reflect diagenesis of biogenic silica with conversion of Opal-A to Opal-CT, followed by recrystallization to quartz, as well as reactions involving the alteration of volcanic ash/glass (Gieskes, 1981). Additionally, the local lithology can exert a strong influence on dissolved SiO2 profiles. Often SiO2 profiles reveal an initial increase immediately below the mudline to ~400-600 然 that reflects dissolution of biogenic silica (Opal-A). Subsequently, recrystallization to Opal-CT and/or the alteration of volcanic glass result in elevated concentrations that can exceed 1 mM, before recrystallization to quartz or uptake by clay mineral formation removes most of the dissolved SiO2 from the pore water. No large increase in dissolved SiO2 is observed deeper downhole at Site 1108. The highest concentrations are observed immediately below the mudline in the sediments of lithostratigraphic Units I and II (see "Lithostratigraphic Unit I" and "Lithostratigraphic Unit II"); they subsequently drop to <200 然 in lithostratigraphic Units III and IV. This observation may be consistent with removal of a significant portion of the sedimentary column at Site 1108 (see "Lithostratigraphy," "Biostratigraphy," and "In Situ Temperature Measurements"), in which dissolution and recrystallization of Opal-CT and the alteration of volcanic glass/ash layers would dominate SiO2 profiles. The abundance of clay minerals in the deep sediments at Site 1108 is also consistent with low dissolved SiO2 concentrations (Gieskes, 1981). Although uptake of dissolved SiO2 also occurs during precipitation of quartz (e.g., De Carlo, 1992, and references therein), it is not possible to distinguish between this process and uptake by formation of clay minerals at Site 1108 without further knowledge of the origin of the quartz found in the deeper portions of the sedimentary column.
Lithium is another element that can reflect both overall sedimentary diagenesis and localized reactions. Dissolved Li+ generally increases with depth in deep-sea sediments (Gieskes, 1981). It is released during the diagenesis of biogenic silica and the alteration of fresh volcanic matter and can be removed from solution during the formation of clay minerals. At Site 1108 we observe fluctuations that are interpreted to derive from a combination of reactions involving clay minerals and local lithologic control, including the alteration of volcanic layers. Gieskes et al. (1983) have attributed coincident dissolved Li+ and SiO2 fluctuations to the diagenesis of opaline silica and an alteration of volcanic materials. However, no such correlation exists in the sediments at Site 1108. Rather, the highest dissolved Li+ concentrations, with the exception of the local submaximum within sediments of the fault zone, occur in the deepest sections of Hole 1108B. It is thus likely that this trend reflects deep-seated water-rock reactions under an increasing thermal regime. Hydrothermal alteration of basalt, for example, is known to release Li+ to the aqueous phase (Edmond et al., 1979). Downhole temperature measurements (see "In Situ Temperature Measurements") are consistent with this observation.