INORGANIC GEOCHEMISTRY

Thirteen interstitial water samples were collected at Site 1212. In Hole 1212A, nine samples were taken between 0 and 90 mbsf (one sample/core). Hole 1212B was sampled only when depths exceeded those reached in Hole 1212A. The sampling resolution was decreased (one sample per three cores) in Hole 1212B such that four samples were collected between 100 and 200 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).

pH, Salinity, Chloride, and Sodium

The pH of pore waters at Site 1212 ranges between 7.37 and 7.63, with an average value of 7.45 ± 0.07 (Table T11). All values are lower than the average seawater value of 8.1. As at Sites 1209-1211, variability in the pH profile occurs within the upper lithologic unit, which is characterized by a lesser proportion of carbonate sediment (72 wt% carbonate through Subunit IA) relative to underlying sediments (82 wt% carbonate in Subunit IB) (see "Carbonate" in "Organic Geochemistry"). Below ~40 mbsf, the pH becomes relatively uniform as the carbonate content of the sediments increases, reflecting the buffering capacity of carbonate-dominated sediments. Salinity ranges from 34.0 to 35.5 g/kg.

The chloride (Cl^-) pore water profile fluctuates around a mean of 559 ± 2 mM, with an increase in pore water Cl^- anion content between ~30 and 90 mbsf (Fig. F21). Sodium (Na⁺) concentrations, calculated using the methods described in Broecker and Peng (1982), average 474 ± 3 mM (Fig. F22). Such 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., 1995).

Alkalinity, Sulfate, Ammonium, Phosphate, Iron, and Manganese

The concentrations of SO₄^2-, alkalinity, NH₄⁺, and HPO₄^2- in the interstitial waters at Site 1212 (Fig. F22) are relatively low and invariant. Downcore, SO₄^2- concentrations decrease steadily from 28 mM (average seawater concentration) in the uppermost sample to 22 mM at 88.35 mbsf. Below this depth, SO₄^2- remains uniform at 22 mM down to the bottom of the profile (186.60 mbsf). Alkalinity decreases from 2.8 mM in the surface pore waters (10.85 mbsf) to 2.2 mM at 159.10 mbsf. Here, the trend reverses, and the alkalinity increases to 2.8 mM at 186.60 mbsf.

Similar to Sites 1209-1211, the NH₄⁺ concentrations are low (between 5 and 130 µM), implying that the organic matter content of the sediment at Site 1212 is low (see "Carbonate" in "Organic Geochemistry"). This interpretation is supported by the limited downcore decrease in SO₄^2- and the low pore water HPO₄^2- concentrations, the highest values of which are <2 µM (1.5-1.8 µM) (Fig. F22). However, Site 1212 has the greatest degree of sulfate reduction on the Southern High. A corresponding increase in pyrite was observed through lithologic Unit I in cores from Site 1212 relative to other Southern High sites (see "Lithologic Unit I" in "Lithostratigraphy").

The Mn²⁺ profile exhibits a downcore decrease from 16 µM at 2.95 mbsf to 1 µM at 39.35 mbsf. Below this depth, there is little variation in the Mn²⁺ profile and concentrations remain low (1-4 µM). As at Sites 1209 and 1210, elevated concentrations of Mn²⁺ in the upper part of the sediment section are likely related to the occurrence of a condensed interval containing Mn-rich phases. Consequently, the minor excursion at ~60 mbsf is interpreted to reflect the dissolution of Mn minerals and diffusion of Mn²⁺ away from Mn-rich sediments.

After an increase to 4 µM in the uppermost sediments, the downcore Fe²⁺ concentrations decrease significantly through the upper ~50 mbsf of the section. Concentrations decrease from 42 µM at 2.95 mbsf to 4 µM at 53.35 mbsf; the lower part of the profile is uniform with average concentrations of 3 ± 1 µM (Fig. F23). The inflection at ~50 mbsf coincides with the middle Miocene to lower middle Eocene unconformity in Core 198-1212A-7H (see "Lithostratigraphy"). The elevated concentrations of Fe²⁺ through the sediment section corresponding to lithologic Unit I (0.00-53.60 mbsf) are likely related to the close proximity of volcanic ash (Subunit IA) and other Fe²⁺ sources to sites of pyrite formation.

Potassium, Calcium, Magnesium, Strontium, and Lithium

The concentrations of K⁺, Ca²⁺, and Mg²⁺ in the upper pore waters at Site 1212 differ little from concentrations in average seawater (12, 11, and 51 mM, respectively) (Fig. F24). In lithologic Unit II (~60 mbsf) of the section, downcore trends are consistent with those resulting from exchange with basaltic basement at depth (Gieskes, 1981), wherein K⁺ and Mg²⁺ concentrations decrease downcore and Ca²⁺ increases. However, in lithologic Unit I, it is likely that processes other than exchange with basement are affecting the distribution of Mg²⁺ cations. In the pore waters contained within Unit I (see "Lithologic Unit I" in "Lithostratigraphy"), Mg²⁺ concentrations decrease at a rate that is inconsistent with what might be expected for a diffusional gradient associated with basement exchange. Consequently, it is likely that a Mg-rich phase is precipitating within lithologic Unit I. Below ~50 mbsf, where the lithology is dominated by carbonate-rich phases, Mg²⁺ concentrations become uniform (46.7 ± 0.3 mM). Assuming that volcanic weathering reactions with basement are removing Mg²⁺ from interstitial waters at depth, the trend observed below ~50 mbsf implies that Mg²⁺ cations are being mobilized into the pore waters. Although there is a 5-mM increase in Ca²⁺ through this interval, the Sr profile and Sr/Ca ratios imply that carbonate dissolution is not responsible for the increase in Mg²⁺.