INORGANIC GEOCHEMISTRY

At Site 1208, 20 interstitial water samples were collected; 11 samples between 0 and 90.2 mbsf (one sample per core) and 9 between 118.7 and 363.4 mbsf (approximately one sample every three cores). Details of analytical methods are provided in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Filtered (0.45 µm) samples were analyzed for pH, salinity, chlorinity (Cl^-), alkalinity, sulfate (SO₄^2-), phosphate (HPO₄^2-), ammonium (NH₄⁺), silica (Si(OH)₄), boron (H₃BO₃), iron (Fe²⁺), manganese (Mn²⁺), and major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Li⁺, Sr²⁺, and Ba²⁺). A compilation of data is provided in Table T8. Cited values for average seawater composition are from Millero and Sohn (1992) and Broecker and Peng (1982); values for North Pacific Deep Water composition are from Rea et al. (1993).

The interstitial water samples span the range of sediments in lithologic Units I (0.0-328.15 mbsf) and II (328.15-392.3 mbsf). The major lithologic trends that affect pore water chemistry include a transition from nannofossil clay and ooze to nannofossil chalk (Subunit IA/IB boundary) and the subsequent appearance of clayey sediments (Subunit IC [311.65-328.15 mbsf]). Throughout Units I and II there are intermittent occurrences of pyrite and volcanic ash. In Unit II, nannofossil ooze is the dominant lithology and chert is common.

The pH decreases downcore from a value of 7.51 in the shallowest sample to 7.19 at 147.2 mbsf (Table T8). Subsequently, the pH abruptly increases to 7.47 at 175.7 mbsf. Salinity was nearly constant throughout Hole 1208A, with an average value of 34.7 ± 0.2 g/kg (1 ).

Chloride (Cl^-) concentrations show little variation downcore (560 ± 4 mM). Sodium (Na⁺) concentrations, calculated by charge balance (see Broecker and Peng, 1982), exhibit little variability, averaging 479 ± 5 mM (1 ). This is only slightly higher than the average seawater concentration of 470 mM.

The distinct downcore alkalinity curve (Fig. F30) reflects a change in the redox conditions affecting the generation of HCO₃^-, likely a result of organic matter degradation. Alkalinity increases from a surface value of 2.89 mM to a maximum of 8.17 mM at 61.7 mbsf. At this depth, a reversal occurs, and alkalinity decreases to 4.79 mM at the bottom of the hole (363.4 mbsf). The subsurface maximum implies an enhanced reducing environment extending downcore from ~60 mbsf. This is supported by the sulfate (SO₄^2-) and NH₄⁺ downcore trends (Fig. F30). At Site 1208, the uppermost interstitial waters have SO₄^2- concentrations of 27.7 mM—only slightly lower than average seawater (28 mM). Below 4.7 mbsf, pore water SO₄^2- decreases gradually, reaching a minimum concentration of 19.5 mM at 118.7 mbsf. Here, the pore water SO₄^2- content slowly increases to a subsurface maximum of 22.8 mM at 295.9 mbsf, before declining to 21.7 mM at 363.4 mbsf.

Figure F31 compares ammonium and phosphate (NH₄⁺ and HPO₄^2-) concentrations with alkalinity data. The correlation among the data (R² = 0.91 and 0.89, respectively) implies that the enhanced carbonate ion production (alkalinity) is linked to the decomposition of organic matter. SO₄^2- reduction is also reflected in the sediments by the presence of pyrite (FeS₂), observed intermittently throughout the core (see "Lithostratigraphy").

In core from Hole 1208A, the NH₄⁺ and HPO₄^2- concentrations range from 125 to 556 µM, and 0 to 22 µM, respectively (Fig. F30). The NH₄⁺ concentrations are lowest at the sediment/water interface (125 µM) and steadily increase to 556 µM at 71.2 mbsf. Below this interval, NH₄⁺ decreases steadily to the bottom of the hole. Based on the relationship between alkalinity and NH₄⁺ (Fig. F31), it follows that NH₄⁺ is being generated by decomposition of organic matter. Because both NH₄⁺ and HPO₄^2- are produced largely in pore waters by the decomposition of organic matter (Gieskes, 1983), the observed trends in both species should be similar. However, the HPO₄^2- profile exhibits a trend opposite of NH₄⁺ in the upper section of the hole. The HPO₄^2- profile shows a maximum concentration of 22 µM at 4.7 mbsf, declining steadily downcore (Fig. F30). Only below ~100 mbsf are the NH₄⁺ and HPO₄^2- trends coincident. Consequently, the HPO₄^2- and alkalinity relationship is significant only for HPO₄^2- data below 147.2 mbsf. In the upper section of the hole, HPO₄^2- maybe being consumed during the adsorption or precipitation reactions (e.g., a calcium phosphate phase).

Large downcore fluctuations in pore water iron (Fe²⁺) concentrations (83 ± 56.7 µM [1 ]) occur in Hole 1208A (Fig. F32). Variations in the Fe²⁺ profile are attributed to the close proximity of iron sources, such as volcanic ash, to areas of sulfate reduction and pyrite (FeS₂) formation. Below 209.8 mbsf, ash content declines and Fe²⁺ concentrations decrease to a minimum of 19 µM at 295.5 mbsf. The decreasing Fe²⁺ may also reflect diffusion between sites of high and low Fe²⁺ concentrations.

The manganese (Mn²⁺) concentration profile in these pore waters is characterized by a subsurface maximum (28 µM) at 23.7 mbsf and subsequent decrease to 5 µM at 90.2 mbsf (Fig. F32). The elevated Mn²⁺ concentrations in the shallowest samples suggest that the degradation of organic matter is sufficient to deplete oxygen in the upper part of the sediment column, above the depth at which the sulfate and alkalinity profiles imply that SO₄^2- reduction is occurring (~50 mbsf). The sharp decrease to low and uniform concentrations suggests that a manganese-rich phase may be forming within this interval. The requisite reducing conditions in the sediment column are consistent with the alkalinity and SO₄^2- data discussed previously and may also be implied by low red/blue (680/420 nm) ratios in color reflectance (see "Lithostratigraphy").

The Mn²⁺ concentration curve (5-8 µM) between 90.2 and 209.8 mbsf is likely the result of upward diffusion of Mn²⁺ between high concentrations associated with Mn²⁺ minerals and lower concentrations in overlying sediments. This is supported by a dramatic decrease from 32 µM (295.9 mbsf) to 3 µM (334.5 mbsf). The excursion to a maximum Mn²⁺ concentration is coincident with the upper boundary of Subunit IC (311.7-328.2 mbsf), at which an unconformity, with inferred Mn-rich phases, is present. For this reason, the Mn²⁺ profile is interpreted to reflect both upward and downward diffusion of Mn²⁺ cations from this Mn-rich horizon.

Potassium (K⁺) concentrations decrease gradually downcore from a maximum of 11.8 mM at 14.2 mbsf to values close to that of average seawater (10.2 mM) in the deepest sample at 363.4 mbsf (Fig. F33). Elevated pore water concentrations of K⁺ (relative to seawater) correspond to the distribution of volcanic ash in the upper section of the hole (see "Lithostratigraphy"), suggesting that the major source for K⁺ in the pore water is glass-rich silicic volcanic material. K⁺ is liberated from the solid phase via leaching and weathering reactions that produce clays (i.e., smectite). The downcore decrease in K⁺ to concentrations below that of average seawater likely reflects a diminishing source of volcanic ash, diffusion toward greater depths, and possible exchange with basement.

The dissolved silica (Si(OH)₄) concentrations of the interstitial waters from Hole 1208A (above ~300 mbsf) (871 ± 95 µM [1 ]) are greater than in the overlying water (North Pacific Deep Water value = 160 µM). At ~300 mbsf, the pore water Si(OH)₄ concentrations (Fig. F32) start decreasing to a minimum of 204 µM at 363.4 mbsf. This dramatic decrease coincides with the appearance of chert (Subunit IC/Unit II boundary). It is likely that this significant removal of Si(OH)₄ from pore waters may be induced by the recrystallization of opal-A to opal-CT or quartz (e.g., Baker, 1986; Gieskes, 1981).

In Hole 1208A, there are several processes influencing the downcore calcium (Ca²⁺) concentrations. The pore water Ca²⁺ content of the uppermost sample is 10.25 mM, which is approximately the Ca²⁺ concentration of average seawater (10.30 mM). Ca²⁺ concentrations decrease between 0 and 80.7 mbsf, then increase to 12.5 mM by the base of the hole (Fig. F33). The interval of low Ca concentrations centered around 75 mbsf suggests precipitation of a Ca-rich diagenetic phase. However, no obvious Ca-rich diagenetic phases were apparent from core descriptions nor the few XRD analyses performed. The correlation between Ca²⁺ and HPO₄^2- may indicate precipitation of a calcium phosphate mineral. The Ca²⁺ increase below ~100 mbsf is a result of diffusion from the base or below the cored interval. Dissolution of carbonate is a reasonable possibility for the increase in Ca²⁺ at depth. However, note that the maximum Ca²⁺ content is ~200 m lower than the maximum Sr concentration, which is a sensitive indicator of carbonate alteration (Baker et al., 1982).

Strontium concentrations show a gradual downcore increase from seawater-like values (87 µM; 0 mbsf) to a maximum of 305 µM at 209.8 mbsf. Below this depth, the profile reverses and the Sr²⁺ interstitial water concentration decreases to 226 µM. The Sr²⁺ and Sr/Ca ratio data are interpreted to reflect the dissolution and/or recrystallization of carbonates at ~200 mbsf, release of Sr²⁺ cations into solution, and subsequent diffusion. The decreasing Sr/Ca ratios below ~150 mbsf may also be indicative of dissolution of carbonate but only if Ca²⁺ is not being sourced from a different reaction(s) below.

The downcore magnesium (Mg²⁺) profile ranges between a low of 46.1 mM (175.7 mbsf) and seawater-like concentrations of 53.2 mM in the uppermost pore water sample. The linear decrease in Mg²⁺ between the surface and 175.7 mbsf implies that Mg²⁺ is being consumed and a diffusion gradient exists between high and low concentrations. However, this trend does not extend below ~175 mbsf. The change in the behavior of Mg²⁺ is evident in Figure F33E, where K⁺ and Mg²⁺ are compared. The linearity of these data for pore water samples from the upper section (between 0.0 and 209.8 mbsf; R² = 0.90) implies that Mg²⁺ is being consumed and K⁺ is being generated via volcanic weathering reactions and subsequent diffusion (see above). In addition, formation of Mg-rich phases (e.g., saponitic green laminae) could be responsible for the decrease in pore water Mg²⁺ (see "Diagenesis" in "Lithostratigraphy" in the "Site 1210" chapter). Below 209.8 mbsf, Mg²⁺ concentrations are essentially uniform (29.5 ± 0.4 mM [1 ]), yet K⁺ concentrations continue to decline. This change in the slope of the profile implies that Mg²⁺ cations are being mobilized into the pore waters below 209.8 mbsf. However, the process responsible for such a reaction cannot be determined with available data.

The interstitial barium (Ba²⁺) concentrations are consistently higher than that of average seawater (0.10 µM), averaging 0.72 ± 0.49 µM (1 ) (Table T8). These elevated Ba²⁺ concentrations relative to seawater are likely a consequence of the presence of skeletal debris and ash (Ba²⁺ sources), as well as local dissolution of biogenic BaSO₄ in reducing environments. Boron (H₃BO₃) concentrations do not change appreciably, with the exception of a large excursion in the H₃BO₃ profile at 52.2 mbsf (449 ± 14 µM [1 ], excluding excursion datum). The significance of this change is not understood, although there are deviations at this depth in the Ba²⁺, Sr²⁺, Ca²⁺, Mg²⁺, and Cl^- data. Similarly, alkalinity, Ca²⁺, Si(OH)₄, HPO₄^2-, and NH₄⁺ profiles show reversals in trends or are significantly altered at approximately this depth. Based on the available data, it cannot be determined whether these fluctuations are related, much less whether a specific process is simultaneously affecting the interstitial water chemistry of these species.