INORGANIC GEOCHEMISTRY

A total of 14 interstitial water samples were collected from Hole 1209A: 11 samples between 0 and 100 mbsf (one sample per core) and 3 between 137 and 250 mbsf. The lower sampling resolution toward the bottom of the hole reflects an effort to avoid disturbing intervals of critical stratigraphic importance. 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 (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 T11. Cited values for average seawater composition are from Millero and Sohn (1992) and Broecker and Peng (1982).

The pore water samples span lithologic Units I (0.0-84.2 mbsf) and II (84.2-235.0 mbsf) and extend into the uppermost part of Unit III (235.0-297.0 mbsf). The compositional differences among these lithologic units play a role in controlling downcore trends in pore water geochemistry. Lithologic Unit I, primarily nannofossil ooze with varying amounts of clay, contains disseminated volcanic glass and discrete ash layers. Pyrite in laminae and blebs is common in the upper ~37 m. Lithologic Unit II, composed of nannofossil ooze and clayey nannofossil ooze with clay-rich horizons, is marked by a higher overall carbonate content and an absence of volcanic ash and pyrite. The sediment of lithologic Unit III consists of clay-poor nannofossil ooze, and chert in discrete layers.

The pH of pore waters in Hole 1209A range from 7.12 to 7.54, with an average value of 7.36 ± 0.12 (Table T11). All values are lower than that of average seawater (pH = 8.1). Much of the variability in the pH profile is contained within the Neogene sediments of lithologic Unit I, which is characterized by a high proportion of noncarbonate sediment (biogenic silica, ash, detrital silicates, and hemipelagic mud) relative to underlying sediments (see "Lithostratigraphy"). The relatively narrow range of values below ~100 mbsf reflects the buffering capacity of the carbonate-dominated Paleogene-Cretaceous sediment. Salinity remains at 35 g/kg throughout the profile.

As at other Leg 198 sites, the chloride (Cl^-) profile exhibits only minor fluctuations. Concentrations increase from 551 mM in the shallowest sample (5.95 mbsf) to 569 mM near the bottom of the profile at 164.40 mbsf (Table T11). Sodium (Na⁺) concentrations, calculated by charge balance, show a slight increase downcore from ~470 mM at the top to ~480 mM near the bottom of the profile (Table T11). 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).

Relative to Sites 1207 and 1208, pore water SO₄^2-, alkalinity, NH₄⁺, and HPO₄^2- concentrations are low and uniform (Fig. F29A, F29B, F29C, F29D). The SO₄^2- concentrations decrease steadily from the average seawater value of 28 mM in the shallowest sample (5.95 mbsf) to 23 mM at the base of the profile. Given that sulfate reduction tends to increase alkalinity at the rate of 2 moles of HCO₃^- for every mole of SO₄^2- reduced, a downcore increase in alkalinity on the order of 10 mM would be expected at Site 1209. Alkalinity values, however, are low throughout the profile (2.94 ± 0.32 mM) and show a slight decrease downcore from values slightly higher to values similar to that of the overlying seawater (Fig. F29A). The decreasing trend suggests that ~7-10 mM of alkalinity have been consumed. Carbonate precipitation is one process that might be responsible for the observed alkalinity decrease. Ammonium (NH₄⁺) and HPO₄^2- concentrations are extremely low throughout Hole 1209A, with values ranging from 49 to 64 µM and 1 to 3 µM, respectively. These low and uniform values reflect the very low organic matter content of the sedimentary section (see "Organic Geochemistry").

The Fe²⁺ and Mn²⁺ concentrations decrease sharply through the upper ~40 m of the sediment column (Fig. F29E, F29F). The Fe²⁺ concentrations decrease from 47 to 17 µM over this interval; the lower part of the Fe²⁺ profile fluctuates between values of 16 and 28 µM. These variations are attributed largely to the proximity of iron sources (i.e., volcanic ash) to sites of pyrite formation in the Neogene section (see "Lithostratigraphy"). Variations in pore water Fe²⁺ are generally more subdued than those observed at Sites 1207 and 1208 and reflect lower rates of sulfate reduction at Site 1209, as indicated by the SO₄^2- profile (Fig. F29B).

The Mn²⁺ profile at Site 1209 is similar to the profiles at Sites 1207 and 1208 (Fig. F29F). Manganese concentrations decrease from 8 µM in the shallowest sample at 5.95 mbsf to 1 µM at ~40 mbsf. Elevated concentrations in the upper part of the sediment column suggest that degradation of the little organic matter present is sufficient to drive manganese reduction. An excursion to higher manganese concentrations in the lower part of the profile (~80-30 mbsf) encompasses a number of condensed intervals and disconformities within the lower Neogene and upper Paleogene section, which contain inferred Mn-rich phases (see "Lithostratigraphy"). As such, this increase in Mn²⁺ concentrations is interpreted to reflect the dissolution of Mn minerals and diffusion of Mn²⁺ away from Mn-rich horizons.

Potassium (K⁺) concentrations show a gradual decrease downcore in Hole 1209A from a maximum of 11.8 mM in the shallowest sample (5.95 mbsf) to values approaching that of average seawater (10.2 mM) in the deepest sample at 257.15 mbsf (Fig. F30A). Elevated concentrations correspond to the distribution of volcanic ash in the upper ~80 m 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⁺ is interpreted to reflect the absence of ash in the Paleogene and Cretaceous sediments, diffusion toward greater depths, and possible exchange with basaltic basement.

Concentrations of Ca²⁺ and Mg²⁺ are similar to that of average seawater (10.3 and 53.2 mM, respectively) at the top of the pore water profile and show an antithetic relationship downcore (Fig. F30B, F30C). Ca²⁺ concentrations increase downcore from 10.8 mM at the top to 15.7 mM at the base of the profile, whereas Mg²⁺ concentrations decrease from 53.1 to 44.8 mM. The most likely influences on Ca²⁺ and Mg²⁺ concentrations in pore waters at Site 1209 include calcium carbonate dissolution, weathering reactions involving volcanic ash in the Neogene section, and alteration processes involving the volcanic basement. Whereas carbonate dissolution releases both cations to pore waters, the latter two processes release Ca²⁺ and remove Mg²⁺ from the system.

The Li⁺ profile (Fig. F30D) shows a gradual increase from a concentration similar to that of overlying seawater throughout the upper ~100 m of the profile (19.4 ± 1.6 µM) to 69.0 µM at the base of the profile (247.15 mbsf). The lower concentrations through the upper ~100 m of the profile may reflect uptake by clay minerals formed through the weathering of volcanic material and, possibly, zeolites (Gieskes, 1981) in lithologic Unit I. The similarity of the Li⁺ profile to the Ca²⁺ profile (increasing concentration with increasing depth) and its antithetic relationship with the Mg²⁺ and K⁺ profiles suggests that increasing concentrations of Li⁺ with depth may reflect alteration reactions involving volcanic basement (~850 mbsf). Geochemical studies of seafloor vent systems have demonstrated that significant quantities of Li⁺ may be released as a consequence of exchange between seawater and basalt, even at low temperatures (Millero and Sohn, 1992).

The convex-upward nature of the upper parts of the Sr²⁺ and Sr/Ca pore water profiles (Fig. F30E) suggests that the highest rates of carbonate alteration are occurring within the upper ~50-80 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 profile, indicating that little additional Sr²⁺ is being added to the system. The lack of variability in the lower part of the profile likely reflects buffering capacity of the carbonate-dominated Paleogene-Cretaceous sediment, which has not been buried to sufficient depth for the onset of pressure solution.

Although maximum concentrations are lower, the morphology of the dissolved silica (Si(OH)₄) profile at Site 1209 is similar to those observed at Sites 1207 and 1208 (Fig. F31; also see Fig. F42 in the "Site 1207" chapter and Fig. F32 in the "Site 1208" chapter). Pore water silica concentrations are highest in the upper ~100 m of the profile in Neogene sediments, with average concentrations of 620 ± 55 µM. Concentrations decrease sharply to 287 µM at 137.65 mbsf. The lowest Si(OH)₄ concentration, 197 µM, is encountered at the bottom of the profile. High concentrations in the upper part of the profile are interpreted to reflect the leaching and weathering of volcanic ash in lithologic Unit I. Lower pore water Si(OH)₄ concentrations correspond to the disappearance of ash at ~100 mbsf and the appearance of chert in Cretaceous sediments encountered in Holes 1209B and 1209C. The removal of Si(OH)₄ from pore waters may be induced by the recrystallization of opal-A to opal-CT or quartz (Baker, 1986; Gieskes, 1981).

The significance of variations in the Ba²⁺ and dissolved boron (generated mostly as boric acid, H₃BO₃, at the measured pH levels) concentrations in pore waters of pelagic sediments are not well understood. Pore water profiles at Site 1209 are described largely for purposes of documentation. The data are provided in Table T11. The average dissolved boron concentration (444 ± 13 µM) is somewhat higher than that of average seawater (416 µM). H₃BO₃ concentrations show an increase below ~100 mbsf to 469 µM. The Ba²⁺ concentrations in interstitial waters of Hole 1209A average 0.5 ± 0.4 µM and show little variability with depth. Concentrations in the upper half of the profile are extremely low, but on average, higher than those of average seawater, indicating that Ba²⁺ is being added to the system. Possible sources of Ba²⁺ include skeletal debris and volcanic ash, which may be undergoing leaching and/or dissolution.