Inorganic chemical analyses were conducted on 23 interstitial water samples from Hole 1148A squeezed from whole-round samples at a frequency of one per core in the first five cores and one every second core thereafter. Analytical methods are detailed in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. The concentrations of dissolved interstitial constituents are presented in Table T16, and the profiles with depth are shown in Fig. F22. Interstitial water profiles in these sediments are dominated by sediment/water exchanges driven by sulfate reduction in the upper 110 mcd, and volcanic ash alteration, clay mineral diagenesis, and calcite recrystallization at depth. The most striking changes in some of the profiles occur between 458 and 492 mcd below a slumped interval (Unit VI; see "Lithostratigraphy"). Within this interval, salinity and chlorinity values are variable, and ammonium and silica values increase. In conjunction with evidence from hydrocarbon gas and sedimentological data (see "Organic Geochemistry" and "Lithostratigraphy"), the slump may act as a barrier to diffusion of gas and possibly to some elements. The microenvironments within fractures may also lead to variable interstitial water concentrations in this lower interval.
Chloride (Cl-) concentrations in interstitial waters at Site 1148 increase from 548 mM near the surface to 559 mM at 39 mcd, are relatively constant between 39 and 458 mcd, and only decrease slightly toward the base of this interval (Fig. F22A; Table T16). Below 458 mcd, Cl- decreases to 548 mM and then increases to a maximum of 577 mM near the base of the hole. The interstitial water salinity profile is similar (Fig. F22B; Table T16). Salinity decreases from 35 to 34 between 10 and 100 mcd, is relatively constant between 100 and 300 mcd, decreases to 33 at 372 mcd, and stays constant to 458 mcd. Below 458 mcd, salinity decreases to 31 and then increases to a maximum of 36 at 653 mcd. The large decrease in Cl- and salinity at 458 mcd occurs after a sampling gap of nine cores in Hole 1148A. Above this interval is an ~20-m section of slumped sediments (lithologic Unit VI), the base of which is a nannofossil chalk (see "Lithostratigraphy"). Below this chalk layer the density increases, and P-wave velocity decreases substantially (see "Physical Properties"), suggesting that these slump deposits have sealed the underlying sediments. Descriptions of the sediments in the low-recovery interval primarily reflect drilling disturbance but also include features such as elongated burrows, extensive shearing, and microfaulting (see "Lithostratigraphy"). The chalk layer appears to act as a barrier to vertical migration of hydrocarbons (see "Organic Geochemistry"). This evidence for impedance of the diffusion of gases and the presence of an interval of higher density in the slump and chalk layer overlying a less consolidated, heavily stressed sedimentary sequence suggest that the highly variable Cl- and salinity values below 470 mcd are trapped and cannot diffuse upward. Furthermore, the extensive fracturing and alteration of fractures, as well as mineralogic alteration observed in fractures (see "Lithostratigraphy"), suggests that the more variable Cl- and salinity values at depth are the expression of microenvironments within a heavily altered sedimentary sequence.
Sulfate (SO42-) concentrations decrease from 21.6 mM at 10 mcd to 12.6 mM at 110 mcd (Fig. F22C; Table T16). Below 120 mcd, SO42- increases slowly to a maximum of 16.2 mM at ~285 mcd and then decreases to 4.9 mM at 518 mcd. Sulfate values never reach zero, indicating that sulfate reduction is incomplete and methanogenesis is not an important process in these sediments. As a result, the upper sediments at Site 1148 have low methane concentrations. High methane values at depth are related to thermogenic production of hydrocarbons (see "Organic Geochemistry"). Decreasing SO42- values with depth below the zone of sulfate reduction at Site 1148 suggest a continuous sink for sulfate, either its incorporation into "iron sulfides," which were observed throughout the sediments at this site (see "Lithostratigraphy"), or into another mineral phase.
Ammonium (NH4+) increases from 0.87 mM at 10 mcd to a local maximum of 1.19 mM at 110 mcd, decreases to a minimum of 0.61 mM at 256 mcd, and then increases downhole (Fig. F22D; Table T16). Dissolved phosphate (HPO42-) concentrations are highest at the top of the hole, 63 mM, and then decrease below the detection limit of the technique by 110 mcd (Fig. F22E; Table T16). Alkalinity increases slightly between the first and second cores at Site 1148 to a maximum of 11.8 mM by the third core, then decreases downhole (Fig. F22F; Table T16). All three of these elements increase in the upper 110 mcd at Site 1148 in response to sulfate reduction and, to a lesser extent, methanogenesis. Changes in ammonium, as well as chloride and salinity, below 470 mcd can also be related to the dehydration reaction of clay minerals. In this interval, X-ray diffraction data show that below 470 mcd, smectite, illite, and kaolinite are absent and mixed layer clays increase (see "Lithostratigraphy").
Magnesium (Mg2+)
concentrations progressively decrease downhole from near-seawater values at the
top (51.3 mM) to a minimum of ~22.4 mM at the bottom of the hole (Fig. F22G;
Table T16).
Dissolved calcium concentrations (Ca2+) decrease significantly from
8.8 mM in the second core to a minimum of 6.8 mM around 79 mcd, then increase
gradually to reach 30.2 mM at the bottom of the hole. The Ca2+
minimum coincides with the base of the upper alkalinity peak and the first
pore-water sulfate minimum, which is consistent with carbonate precipitation at
the top of the sulfate reduction zone (Fig. F22H;
Table T16). As
a result of sulfate reduction, the slope of 
Ca/Mg
is positive above ~110 mcd and negative below (Fig. F23),
with two distinct slopes. The Ca/Mg
slope in the interval 110-256 mcd is -7 and decreases to -0.6 at the bottom of
the hole. The slope of -0.6 may indicate the uptake of Mg2+ and
release of Ca2+ during in situ alteration of volcanic material. This
change in slope at 256 mcd corresponds to a change in the lithology (transition
between lithologic Subunits IIA and IIB; see "Lithostratigraphy").
Dissolved potassium (K+) concentrations decline sharply downhole from 11.7 mM near the surface to 5.5 mM at 198 mcd and then decrease more slowly to 3 mM at the base of the hole (Fig. F22; Table T16). This decrease most likely reflects uptake of K+ during the alteration of volcanic material and/or diagenesis of the detrital minerals.
Dissolved silica (H4SiO4) concentrations are high (>600 mM) in the upper part of the hole, then decrease abruptly to 227 mM at 79 mcd and more slowly thereafter to reach a minimum of 167 mM at 227 mcd (Fig. F22J; Table T16). Below this level, H4SiO4 concentrations increase slowly to a maximum of 406 mM at 401 mcd, increase abruptly to a second maximum of 658 mM at 518 mcd, and then decrease abruptly to 104 mM at 629 mcd. The two high-concentration intervals (0-79 mcd and 518-592 mcd) are associated with intervals of higher biogenic silica content (see "Biostratigraphy"). This suggests, as at all sites of this leg, that higher dissolved silica reflects a greater presence of soluble amorphous silica in the form of microfossils.
The lithium (Li+) profile is relatively flat in the upper 256 mcd and subsequently increases continuously to a maximum of 2020 mM at 518 mcd (Fig. F22K; Table T16). Below this depth, Li+ concentrations decrease slightly to the bottom of the hole. The depth of the abrupt increase of Li+ (256 mcd) corresponds also to changes of the lithology (transition between lithologic Subunit IIA and IIB; see "Lithostratigraphy"), most likely a result of the alteration of volcanic material in the sediment.
Strontium concentrations (Sr2+) increase slowly from seawater values at the top of the hole (87 mM) to 136 mM at 79 mcd, and then increase continuously with depth to a maximum of 1008 mM at 372 mcd (Fig. F22L; Table T16). Below this level, the profile of Sr2+ is flat to the bottom of the hole. This profile is consistent with relatively constant %CaCO3 at this site and, therefore, with relatively constant calcite recrystallization with depth. The plateau, which is seen in many interstitial water Sr2+ profiles, suggests that interstitial water strontium/calcium ratios have reached steady state with respect to the ratios in biogenic and authigenic calcite, so that calcite recrystallization no longer enriches the interstitial waters in Sr2+.
