INORGANIC GEOCHEMISTRY

Shipboard interstitial water (IW) analyses were performed on 39 of 58 whole-round samples taken from Hole 1171A (12 of 23 samples), Hole 1171C (5 of 13 samples), and Hole 1171D (22 samples). Samples were taken at the frequency of three per core in the upper ~50 mbsf, one per core from 50-100 mbsf, and one every third core to total depth in Hole 1171A. In Hole 1171C, samples were collected at two per core between 50 and 80 mbsf. Below, one whole-round core every third core to total depth was taken, continuing in Hole 1171D at one whole-round core every third core down to the bottom of the hole. Table T20 and Figures F34, F35, F36, F37, and F38 present the results of interstitial water geochemistry. The balance of the samples was archived for shore-based investigations.

Based on the conservative parameters salinity and chloride (Cl^-), the downcore record at Site 1171 can be separated into three units. The uppermost ~270 mbsf shows little change in salinity (34 to 35) (Table T20) and Cl^- (554-564 mM; Fig. F34). The Cl^- concentrations are close to seawater concentrations although they increase by ~1% to a maximum of 564 mM at ~50 mbsf. In the second unit from ~270 to ~600 mbsf, salinity and Cl^- decrease to 30-31 and 544-551 mM, respectively. Below ~600 mbsf in the lowermost unit, both parameters decrease to lower values (salinity 27-28; Cl^- below ~520 mM). Chlorinity values exhibit a 13% decrease from seawater values. Sodium (Na⁺) generally follows the same pattern as salinity and Cl^-, with concentrations ranging between ~465 and ~485 mM in the uppermost ~350 mbsf and increasing to 475-490 mM from ~350 to ~600 mbsf. Below ~600 mbsf, Na⁺ concentrations become highly variable (~440 to ~490 mbsf) and are mostly below seawater values.

The ~1% increase in Cl^- within the upper ~40-60 mbsf was also observed at Sites 1168, 1169, and 1170. The slight chlorinity change may be attributed to a salinity increase during the last glacial maximum as proposed by McDuff (1985) and Schrag et al. (1996). Farther downcore, a Cl^- decrease provides evidence of deep pore-water freshening at this site. Note that in contrast to the Na⁺ profile, the Na⁺/Cl^- downcore record exhibits gradually increasing values from 0.84 at the surface to 0.98 at depth.

The advection of meteoric waters as a possible source of low-Cl^- interstitial waters (e.g., Austin, Christie-Blick, Malone, et al., 1998) can be ruled out because of the isolated location of Site 1171 on the STR. The Cenozoic sequence is mostly absent between the STR and the Tasmanian shelf, so there is no readily conceivable aquifer link to Tasmania. We suspect that as at Site 1170, the presence of low-Cl^- fluids implies internal processes such as gas hydrate dissociation, dehydration reactions of hydrous minerals (e.g., clay minerals and biogenic opal), and/or clay-membrane ion filtration (e.g., Kastner et al., 1991; Hesse and Harrison, 1981; Paull, Matsumoto, Wallace, et al. 1996). Similar to Sites 1168 and 1170, pore-water freshening is restricted to older strata and apparently coincides with the onset of methanogenesis (Fig. F34). Whether the ongoing maturation of organic matter affects the freshening of pore water is not yet answered.

The sulfate-concentration profile can be differentiated into two units. From 0 to ~ 270 mbsf (a zone with ~90% CaCO₃ and low organic matter content), sulfate concentrations gradually decrease from near-seawater values (26-27 mM) at the top of the hole to ~18-19 mM at ~270 mbsf (~30% decrease) (Fig. F35). The decrease in interstitial water SO₄^2- concentrations at Site 1171 exhibits the same amount of depletion through comparable depths as observed at Site 1169. Below ~270 mbsf, SO₄^2- sharply decreases to values close to zero, coincident with a distinct increase in methane (the onset of methanogenesis unfortunately was not documented as a result of technical problems but likely occurred at ~320 mbsf; see "Volatile Hydrocarbons" in "Organic Geochemistry").

The pH and alkalinity reveal a three-step downcore change. The pH decreases from ~7.6 at the top to <7.2 at ~70-80 mbsf. Subsequently, pH increases and stays nearly constant at 7.4-7.5 between 80 and 270 mbsf (Fig. F35). From ~270 to ~520 mbsf, pH increases from ~7.5 to ~7.7, below which the pH increases to ~8.3 and remains high to the bottom of the hole. This pH increase corresponds to an increase in methane content.

Titration alkalinity values from surface-sediment pore waters are ~3.1 mM and show a slight increase to ~4.0-5.5 mM within the upper ~270 mbsf of calcareous ooze (Fig. F35). From 270 to ~500 mbsf, alkalinity varies considerably from 4 to 7 mM. Below ~500 mbsf, the values decrease sharply to minimum values of 1.5-2.6 mM. Ammonium (NH₄⁺) steadily increases from 10 µM at the surface to a maximum of ~3.0 mM at ~930 mbsf, thus reaching considerably higher values than at previous sites (Fig. F35).

The downcore change in SO₄^2-, alkalinity, and NH₄⁺ is most likely the result of organic matter remineralization. The nearly complete exhaustion of SO₄^2- below the carbonate-rich sediment sequence (lithostratigraphic Units I and II) should be immediately followed by the onset of methanogenesis (e.g., Claypool and Kaplan, 1974). Bacterial methane production, which normally proceeds by the mechanism of CO₂ reduction in marine environments, at the same time tends to raise the pH and also favors carbonate precipitation (Claypool and Kaplan, 1974) by the following generalized formula: Me²⁺ + 2HCO₃^- + 8H⁺ CH₄ + MeCO₃ + 3H₂O.

The calcareous lithostratigraphic Units I to III at Site 1171, which cover the uppermost ~ 270 mbsf, have relatively constant Ca²⁺ concentration. Apart from the topmost sample where Ca²⁺ concentrations are close to seawater values (Fig. F36), Ca²⁺ concentrations increase to an average of 11.6 mM with a maximum of 12.3 mM at ~90 mbsf. In the underlying siliciclastic unit (lithostratigraphic Units IV and V), Ca²⁺ concentrations exhibit a wider variability from 10.3 to 11.8 mM but still are close to seawater values. Below ~700 mbsf within lithostratigraphic Unit IV, Ca²⁺ concentrations significantly decrease to minimum values of 5.6 mM at the bottom of the hole.

Strontium (Sr²⁺) concentrations gradually increase with depth from 117 µM at the seafloor to ~420 µM at ~80 mbsf and, subsequently, decline to 178 µM at the base of the hole (Fig. F36). The distribution of Li⁺ is inverse to that of Sr²⁺. Concentrations vary between ~17 and 50 µM with minimum values in the uppermost sediments and gradually and steadily increase downcore. Maximum values of ~600 µM are observed at ~930 mbsf. Normalizing to Cl^- to account for the downcore pore-fluid freshening does not affect the records of either Ca²⁺, Sr²⁺, or Li⁺.

The slight increase in Ca²⁺ and Sr²⁺ concentrations and Sr²⁺/Ca²⁺ ratios within the calcareous ooze are most likely the result of dissolution and reprecipitation of calcite. A release of strontium into interstitial fluids is likely caused by the recrystallization and/or dissolution of biogenic carbonate to diagenetic low-Mg calcite (e.g., Manheim and Sayles, 1974; Baker et al., 1982).

The significantly increasing Li⁺ concentrations below the carbonate sequence suggest that silicate phases may act as a major source for Li⁺, which at the same time removes Mg²⁺ and K⁺. Volcanic matter alteration could be an alternative mechanism to describe increased Li⁺ pore-water concentrations.

The downcore magnesium (Mg²⁺) and potassium (K⁺) profiles exhibit steadily decreasing concentrations from the uppermost sample to the bottom of Hole 1171D (Fig. F37). The uppermost Mg²⁺ and K⁺ concentrations of 51.3 and 10.0 mM, respectively, are close to normal seawater concentrations, decreasing by ~86% to 7.8 and 1.7 mM at depth, respectively. Note, however, that below ~270 mbsf (lithostratigraphic Unit III) Mg²⁺ and K⁺ concentrations decrease more rapidly. Normalizing these values to Cl^- (Fig. F37) indicates that the elemental downcore gradients are much greater than can be attributed to pore-water salinity changes. The change in concentration of both elements is highly correlated (r = 0.98), as in the previous sites, and suggests that both elements are being removed from pore waters by similar processes. Most likely, basement alteration and/or ion-exchange reactions associated with clay minerals (Gieskes, 1983; De Carlo, 1992) take place below the drilled sequence and drive the Mg²⁺ and K⁺ distribution within the noncarbonate lithostratigraphic Units IV-VI. For example, the conversion of kaolinite to montmorillonite will remove varying amounts of Mg²⁺ and K⁺ from pore fluids (Loughnan, 1969). Additional processes within the calcareous oozes may act as sinks for Mg²⁺ and K⁺, although these processes have yet to be identified.

Dissolved silica concentrations (H₄SiO₄⁰) range from ~100 to 1100 µM. Within the upper ~300 mbsf (lithostratigraphic Units I-III and part of Unit IV), H₄SiO₄⁰ exhibits a near-continuous increasing downward profile (Fig. F38) with the highest concentrations (>900 µM) in the lower part of lithostratigraphic Subunit IB and within Units II-IV (~170 to 320 mbsf). Below, silica concentrations decrease in a two-step manner: to ~500-700 µM (mainly restricted to lithostratigraphic Subunits VA and VB) and to ~100-200 µM (below 500 mbsf within lithostratigraphic Subunit VC and Unit VI).

The dissolved silica concentrations within the calcareous oozes (lithostratigraphic Units I-III) at Site 1171 are comparable to the concentrations found in the carbonate sequences of Sites 1169 and 1170 while being consistently higher than those at Site 1168. The lowest H₄SiO₄⁰ values of ~100 to 200 µM within the siliciclastic deposits below ~500 mbsf compare to H₄SiO₄⁰ concentrations below the diffusional boundary at Site 1170. In contrast to Site 1170, we observe a gradation in H₄SiO₄⁰ between the calcareous oozes and the underlying siliciclastic deposits.

The distinct H₄SiO₄⁰ variations are likely a response to the abundance of biogenic silica and the formation of opal-CT. Most silica dissolution is apparently in sediments rich in biogenic silica, and H₄SiO₄⁰ concentrations are a reflection of the opal content of the sediments, as seen in the previous Leg 189 sites. A significant removal of H₄SiO₄⁰ from pore waters, in contrast, may be induced by the recrystallization of opal-A to opal-CT (e.g., Baker, 1986). The highest abundance of siliceous fauna estimated from smear slides are present within lithostratigraphic Unit IV at Site 1171 (see "Lithostratigraphy"), whereas above and below, the abundance of biogenic silica is significantly lower. These trends are compatible with observed dissolved silica concentrations in the pore waters. Furthermore, increased radiolarian abundance (see "Biostratigraphy") matches peaks in the dissolved silica record. X-ray diffraction analysis shows the existence of opal-CT between ~480 and 520 mbsf (Fig. F38; see "Lithostratigraphy"), coincident with a significant dissolved silica drop. Gieskes (1981), however, suggested that a significant change in H₄SiO₄⁰ does not occur until the transition of opal-CT to quartz.

Site 1170 provided valuable insight into the location of sources and sinks for many of the pore-water constituents because of the presence of a distinct and probably low-permeable lithologic change, which acts as a diffusional boundary (see "Inorganic Geochemistry" in the "Site 1170" chapter). Although the geochemistry is characterized by a similar abrupt lithologic change from upper Oligocene to Pleistocene carbonate oozes to mainly Eocene siliciclastic deposits, a diffusional boundary is not present at Site 1171. Nevertheless, mechanisms for the distribution of various pore-water constituents become apparent by comparing sites.

At Site 1171, concentrations and the progressive removal of Mg²⁺ and K⁺ downcore compare with Site 1168 and 1170, suggesting that Mg²⁺ and K⁺ are most likely being consumed by silicate reactions (e.g., basement alteration and/or ion-exchange reactions associated with clay minerals) either within the siliciclastic sediments or below the cored section.

The increasing Sr²⁺ and Ca²⁺ concentrations within the pore waters of the pelagic calcareous oozes (observed at Sites 1168-1171) and the continuous loss of Sr²⁺ within the underlying siliciclastic sections point to dissolution and/or reprecipitation of calcite. The significant loss of Ca²⁺ at depth, which is similar to observations of Site 1168, is most likely attributed to silicate reactions. Pore-water Li⁺ concentrations from the calcareous oozes of Site 1171 resemble those of Sites 1168 and 1170, suggesting that an effective Li⁺ sink is present within the nannofossil ooze and chalks.

In general, sulfate reduction is complete within or below the calcareous oozes and is followed at depth by the onset of methanogenesis. Coincident with increasing methane concentrations, pore waters become distinctly fresher as deduced from decreasing Cl^- values, whereas pH increases. Whether pore-water freshening is caused by organic matter maturation, dehydration reactions of hydrous minerals, clay-membrane ion filtration, and/or gas hydrate dissociation is not yet answered.

H₄SiO₄⁰ downcore variations reflect the abundance of biogenic opal and opal-CT within the sediments. Most silica dissolution apparently is present in sediments enriched in biogenic opal, leading to higher pore-water H₄SiO₄ concentrations. In contrast, the diagenetic conversion of opal-A to opal-CT may extract H₄SiO₄ from the interstitial fluids.