INORGANIC GEOCHEMISTRY

Eleven interstitial water samples were collected from Hole 1211A: nine samples between 0 and 100 mbsf (one sample per core) and two between 120 and 160 mbsf. Details of analytical methods can be found in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Filtered (0.45 µm) samples were analyzed for pH, salinity, chlorinity, 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 T11. Cited values for average seawater composition are from Millero and Sohn (1992) and Broecker and Peng (1982).

The pH of pore waters in Hole 1211A ranges from 7.28 to 7.54, with an average value of 7.42 ± 0.08 (Table T11). All values are lower than the average seawater value of 8.1. Much of the variability in the pH profile is contained within the Neogene sediments, which are characterized by a higher proportion of noncarbonate sediment (biogenic silica, ash, and detrital silicates) relative to underlying sediments (see "Lithologic Unit I" in "Lithostratigraphy"). The buffering capacity of the carbonate-dominated Cretaceous-Paleogene sediment keeps pH values constant below ~90 mbsf. Salinity ranges from 34.5 to 35.5 g/kg, with the highest value occurring at 37.25 mbsf.

The chloride (Cl^-) concentrations increase from a minimum value of 551 mM in the shallowest sample (1.45 mbsf) to a maximum of 563 mM at 37.25 mbsf (Fig. F22). Concentrations remain relatively uniform below this depth, averaging 560 ± 1 mM. Sodium (Na⁺) concentrations, calculated by charge balance using the methods described in Broecker and Peng (1982), average 476 ± 3 mM. Broad downcore variations in pore water Na⁺ and Cl^- profiles of pelagic sediments have been linked to variations in mean ocean salinity associated with changes in ice volume (McDuff, 1985; Schrag et al., 1996).

As at Sites 1209 and 1210, the pore water profile at Site 1211 is characterized by generally low and invariant concentrations of SO₄^2-, alkalinity, NH₄⁺, and HPO₄^2- (Fig. F23A, F23B, F23C, F23D). Alkalinity increases slightly from 2.6 mM in the shallowest sample at 1.45 mbsf to 3.2 mM at 8.75 mbsf. A broad maximum in alkalinity occurs between 8.75 and 65.75 mbsf; the average concentration through this interval is 3.2 ± 0.1 mM. Below this maximum, alkalinity decreases steadily to 2.7 at 157.75 mbsf. The alkalinity maximum corresponds to a broad, albeit minor, excursion to higher concentrations in the NH₄⁺ profile between 1.45 and 56.25 mbsf (Fig. F23C). The NH₄⁺ concentrations are the lowest so far observed at any Leg 198 site, with values ranging between 4 and 15 µM. The pore water HPO₄^2- concentrations are extremely low, with the highest values (1.5-1.8 µM) occurring in the upper 9 m of the profile. The extraordinarily low and uniform concentrations of SO₄^2-, alkalinity, NH₄⁺, and HPO₄^2- reflect the very low organic matter content of the sedimentary section recovered at Site 1211 (see "Carbonate" in "Organic Geochemistry"). The Mn²⁺ and Fe²⁺ pore water profiles exhibit concentration peaks at 8.75 and 18.25 mbsf, respectively (Fig. F23E, F23F). These peak concentration occurrences are consistent with the series of reactions that occur because of a loss of oxygen in sediment pore water, wherein reduction of MnO₂ is followed by the reduction of Fe(III) to Fe(II) (at deeper depths).

Throughout the pore water profile at Site 1211, K⁺, Ca²⁺, and Mg²⁺ differ little from concentrations in average seawater (Fig. F24A, F24B, F24C). Downcore trends, scarcely distinguishable, are consistent with those resulting from exchange with basaltic basement at depth (Gieskes, 1981), wherein K⁺ and Mg²⁺ decrease and Ca²⁺ increases slightly with depth. The convex-upward nature of the upper parts of the Sr²⁺ and Sr/Ca pore water profiles (Fig. F24D) suggests that carbonate dissolution is occurring in the upper ~40-60 m of the sediment column (e.g., Baker et al., 1982). Below this depth, there is little change in either the Sr²⁺ or Sr/Ca profiles, indicating little additional Sr²⁺ input. This lack of variability likely reflects the buffering capacity of the carbonate-dominated Paleogene-Cretaceous sediment, which has not been buried to sufficient depth for the onset of pressure solution. The Li⁺ concentrations decrease sharply from 26 µM in the shallowest sample (1.45 mbsf) to a minimum of 18 µM at 27.75 mbsf (Fig. F24E). Below this depth, concentrations gradually increase to 25 µM at the base of the profile (157.75 mbsf).

The morphology of the dissolved silica profile at Site 1211 is generally similar to those observed at Sites 1207-1210, although concentrations in Unit I are lower than in the correlative sections of the other sites (Fig. F25). This is in agreement with lower abundance of both biogenic silica and volcanic ash relative to Sites 1207-1210. Pore water silica concentrations are highest in the Neogene sediments, with an average concentration of 554 ± 39 µM prevailing through the upper ~40 m of the profile. Concentrations decrease gradually to a low of 136 µM at the base of the profile (157.75 mbsf). Elevated concentrations in the upper part of the profile are interpreted to reflect the leaching and weathering of volcanic ash and biogenic silica in the Pliocene-Pleistocene sediments. Lower concentrations occur in the Paleogene-Cretaceous carbonate-dominated sediments, in which biogenic silica is minor and ash is absent. Chert occurs as layers and nodules in the Cretaceous section.

Given that the significance of variations in the Ba²⁺ and H₃BO₃ concentrations in pore waters of pelagic sediments is poorly understood, pore water profiles for these parameters at Site 1211 are described largely for purposes of documentation (Table T11). The average boron concentration (445 ± 16 µM) is higher than that of average seawater (416 µM); concentrations tend to increase downcore. The Ba²⁺ concentrations average 0.5 ± 0.4 µM and show little variability with depth. Concentrations are extremely low but, on average, are higher than those of average seawater, indicating that Ba²⁺ is being added to the system. Possible sources of Ba²⁺ include skeletal debris and minor volcanic ash in the Neogene section, which may be undergoing leaching and/or dissolution.