INORGANIC GEOCHEMISTRY

Forty-five whole-round samples were collected from Hole 1172A (36) and Hole 1172D (9) for interstitial water (IW) analysis at the following frequency: three per core in the upper 60 m, one per core from 60 to 100 mbsf, and one every third core to total depth. Thirty-five of the IW samples were used for shipboard analyses; the balance of samples from the upper 60 mbsf of the hole were archived for shore-based analyses, with one exception. The whole-round sample collected from Core 189-1172D-19R showed visual signs of disturbance and evidence of contamination with drilling fluid (surface seawater) and was thus excluded from the data set. Results of IW analyses are reported in Table T19 and Figure F28.

Chloride (Cl^-) and sodium (Na⁺) concentrations increase immediately below the seafloor and remain above mean seawater concentrations in the upper 500 mbsf (Fig. F28A). The increase in Cl^- at the top of the hole centered at ~30 mbsf is a 1.6% increase from 557 mM in the uppermost sample to 566 mM. Maximum Cl^- values in the upper 500 mbsf reach 574 mM (2.8% increase) at 530 mbsf. Not surprisingly, with such minor variations in Na⁺ and Cl^-, salinity changes very little in the upper 530 mbsf. Below ~500 mbsf, Cl^-, Na⁺, and salinity are highly variable. Chloride values reach a minimum of 476 mM (14.8% decrease from the standard seawater value), yet concentrations higher than seawater are also observed in this interval (567 mM at 691 mbsf). The low-Cl^- fluids at Site 1172 are in what appears to be an overall background increase in the Cl^- profile with depth (Fig. F28A). Thus, the actual freshening may be greater by ~2%, or a total of 17%, because of the elevated pore-fluid Cl^- (574 mM) immediately above the horizon of freshening.

The presence of low-Cl^- waters on the East Tasman Plateau and at sites on the STR and western Tasmania margin demonstrates that the observed pore-water freshening is clearly a regional phenomenon. Although a determination of the origin of the low-Cl^- waters requires shore-based analyses, Site 1172 profiles provide new insight into the possible mechanisms. At Sites 1168, 1170, and 1171, pore-water freshening coincided with the onset of methanogenesis, suggesting a possible link either to methane hydrate dissociation or as an organic matter degradation by-product. However, at Site 1172, pore-water freshening is present without appreciable methane (see "Volatile Hydrocarbons" in "Organic Geochemistry"). Together with the previous observations that the low-Cl^- fluids are found below the calculated base of the gas hydrate stability zone, the lack of methane suggests that neither gas hydrate dissociation nor a methanogenic by-product universally explains the regional extent of low-Cl^- pore fluids. The other possible hypotheses to describe low-Cl^- pore fluids include (1) dehydration reactions of hydrous minerals, such as clays and biogenic opal; (2) clay-membrane ion filtration; and perhaps (3) connate fluids. Postcruise isotopic analysis of the IW will be required to determine the origin of these regionally extensive low-Cl^- pore waters.

Strontium (Sr²⁺) increases with depth in the upper ~200 mbsf, reaching a maximum of 914 µM, at 181 mbsf (Fig. F28A). Below ~200 mbsf, Sr²⁺ decreases downhole reaching a fairly constant value of ~270 µM at ~500 mbsf. As observed at the previous sites, the increase in Sr²⁺ concentration and Sr²⁺/Ca²⁺ ratios (Fig. F28A) coincides with Neogene-Oligocene pelagic carbonates, indicating that these sediments are actively undergoing recrystallization (e.g., Baker et al., 1982).

Unlike the setting at the other deep penetration sites, complete sulfate reduction is not achieved by the base of the cored interval (Fig. F28B). Sulfate decreases gradually from seawater values to 4 mM, with a steeper gradient across the boundary between lithostratigraphic Units III and IV (~500 mbsf) (see "Lithostratigraphy"). Alkalinity increases downsection in the upper 300 mbsf, decreases across Unit II, then increases again reaching a maximum of 11 mM at ~500 mbsf. Below 500 mbsf, alkalinity declines to a minimum of 1.2 mM at 691 mbsf. The pH varies inversely with alkalinity (r = 0.86) reaching a minimum of 7.0 and a maximum of 8.6 at the alkalinity maximum and minimum, respectively. Ammonium steadily increases downcore to 500 mbsf (984 µM), below which the profile is highly variable, ranging from 542 to 1277 µM. The variability in NH₄⁺ may correspond to fluctuations in pH and alkalinity (i.e., when alkalinity is high, NH₄⁺ is high, and vice versa).

The major changes in SO₄^2-, NH₄⁺, and alkalinity are largely a function of microbially mediated organic matter degradation. Despite appreciable TOC concentrations (0.5-1 wt%) and the presence of very labile marine organic matter (see "Sedimentary Geochemistry" in "Organic Geochemistry"), sulfate reduction is not complete and methane does not increase significantly within the cored sequence.

Dissolved silica concentrations range from 45 to 1149 µM in the upper 500 mbsf, with several distinct negative shifts (~58 and 320 mbsf) in a generally increasing profile (Fig. F28B). Below 500 mbsf, H₄SiO₄⁰ drops precipitously, reaching a minimum of 45 µM.

The distinct shifts in pore-fluid H₄SiO₄⁰ correspond to shifts in biogenic silica abundance in the sediments (see "Biostratigraphy" and "Lithostratigraphy"). In particular, the negative H₄SiO₄⁰ excursions are in intervals that are barren or contain only trace amounts of diatoms and radiolarians. The rapid decrease below 500 mbsf is likely a response to the general absence of biogenic silica. The transformation of opal-A to opal-CT may also play a role in the H₄SiO₄⁰ decrease (e.g., Baker, 1986).

Similar to previous sites, Mg²⁺ and K⁺ concentrations decrease from the topmost sample to minimum values of 19.5 and 3.3 mM, respectively, at the base of the cored interval (Fig. F28C). The magnitude of the decrease is similar for both elements?% for Mg²⁺ and 68% for K⁺. Lithium concentrations steadily increase to 584 µM at ~500 mbsf. Below ~500 mbsf, Li⁺ concentrations decrease and are slightly variable, ranging from 415 to 263 µM. In the upper 500 mbsf, the Ca²⁺ and Li⁺ profiles are very similar, with Ca²⁺ increasing to a maximum of ~28 mM (~2.7 times normal seawater). Below 500 mbsf, Ca²⁺ decreases and is more variable than Li⁺ over the same interval, ranging from 20.8 to 25.2 mM.

Gradients in Ca²⁺, Mg²⁺, K⁺, and Li⁺ profiles are largely the result of silicate mineral alteration. As at the previous sites, the Mg²⁺ and K⁺ decreases are correlated (r = 0.92). However, in contrast to the previous sites, Mg²⁺ and Ca²⁺ are strongly inversely correlated (r = 0.987) in the upper 500 mbsf (Fig. F29). This relationship has been noted at numerous DSDP and ODP sites and has been interpreted to be the result of alterations involving sedimentary volcanic material or basaltic material at depth (i.e., Layer II) (e.g., Lawrence et al., 1975; Lawrence and Gieskes, 1981). The strong positive correspondence between Ca²⁺ and Li⁺ (r = 0.915) (Fig. F29) also suggests that the production of Li⁺ is controlled by volcanic material alteration. Volcanic material from the nearby Cascade Seamount was observed in smear-slide sediment analysis, in particular in lithostratigraphic Units II and III (see "Lithostratigraphy"). Below Unit III, there is little indication of ash. The coincident maximums of Li⁺ and Ca²⁺ at the base of Unit III (~500 mbsf), the decrease of each below, and the lack of correspondence between Mg²⁺ and Ca²⁺ below Unit III indicate that the reactions are being driven by sedimentary volcanic material rather than from basaltic rocks at depth.

Many of the same reactions noted at Sites 1168-1171 control the IW chemical profiles at Site 1172. In contrast, however, Site 1172 shows clear evidence of volcanic material alteration, which results in large increases in Ca²⁺ and Li⁺ and contributes to the consumption of Mg²⁺ and K⁺. The presence of fresher pore waters on the East Tasman Plateau has demonstrated that the low-Cl^- fluids are a regional feature in the older sediments. In addition, the pore-water freshening observed at Site 1172 is not coincident with methanogenesis, which likely eliminates gas hydrates as a possible source of the low-Cl^- fluids at this site.