INORGANIC GEOCHEMISTRY

A total of 15 interstitial water samples were collected in Hole 1210A: 11 samples between 0 and 100 mbsf (one sample per core) and 5 between 144.8 and 230.3 mbsf. Sample resolution was decreased downcore to preserve critical stratigraphic intervals (e.g., Eocene/Oligocene boundary). A total of 19 samples were collected in Hole 1210B: 18 between 0 and 100 mbsf and 1 at 284.8 mbsf. The samples from 0 to 100 mbsf for Hole 1210B were collected for postcruise analyses. With the exception of chloride titrations, the shipboard analytical suite was not produced for these samples. The deep interstitial water sample in Hole 1210B was collected after a significant change in sediment lithology was observed at ~260 mbsf; this lithological shift was not encountered in Hole 1210A, which extends only to ~240 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 T13. Cited values for average seawater composition are from Millero and Sohn (1992) and Broecker and Peng (1982).

At Site 1210, there is little variation in the pH of the interstitial waters (7.37 ± 0.07) (Table T13). As at Site 1209, variability of the pH profile (0-100 mbsf) may be a consequence of variations in sediment composition. The relatively narrow range of values below ~100 mbsf reflects the buffering capacity of the carbonate-dominated sediment, whereas the variation in the upper section of the profile may be a consequence of a higher proportion of noncarbonate material (e.g., biogenic silica, ash, and detrital silicates) (see "Lithostratigraphy"). Salinity remains at 35 g/kg throughout the profile.

The chloride (Cl^-) profile exhibits slight fluctuations (Fig. F24). Concentrations increase from 553 mM in the shallowest sample (4.45 mbsf) to 569 mM at the bottom of the profile at 284.8 mbsf (Table T13). Sodium (Na⁺) concentrations, calculated by charge balance using the methods described by Broecker and Peng (1982), show only a slight increase downcore from 469 mM (4.45 mbsf) to 483 mM near the bottom of the profile (Table T13).

As in Hole 1209A, the effects of organic matter reactions are subdued (Fig. F25). The SO₄^2- concentrations decrease steadily from 29 mM in the shallowest sample (4.45 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 HCO_3- per mole of SO₄^2- reduced, a downcore increase in alkalinity of ~10 mM would be expected. Alkalinity values, however, are low throughout the profile (2.92 ± 0.34 mM) and decrease downcore from ~3.30 to 2.40 mM (Fig. F25). The difference between the observed and expected alkalinity implies that 7-10 mM of HCO_3- have been removed from the system, possibly by precipitation of carbonate minerals. Phosphate (HPO₄^2-) and ammonium (NH₄⁺) concentrations range from 0 to 3 µM and 61 to 118 µM, respectively. Although the NH₄⁺ pore water content is approximately twice that in Hole 1209A, the downcore concentrations are significantly less than at Sites 1207 and 1208, implying that there is little organic matter within the sedimentary section at Site 1210 (see "Carbonate" in "Organic Geochemistry").

The Fe²⁺ and Mn²⁺ concentrations decrease significantly through the upper ~40 m of their respective profiles (Fig. F25). The Fe²⁺ concentrations decrease from 53 to 12 µM over this interval; the lower part of the Fe²⁺ profile is uniform with average concentrations of 9 ± 3 µM, with the exception of a single excursion to 27 µM at 203.35 mbsf. This variation occurs in proximity to the K/T boundary, where clays with pyrite blebs and oxides are likely related to the deviation in pore water Fe²⁺ concentrations (see "Lithostratigraphy").

The shape of the Mn²⁺ profile at Site 1210 is similar to those at Sites 1207, 1208, and 1209 (Fig. F25), implying that similar processes are controlling Mn²⁺ concentrations in the pore waters. Manganese concentrations decrease from 18 µM (4.45 mbsf) to 1 µM at ~40 mbsf. Two large positive excursions occur in the lower section of the profile at Site 1210; 8 µM (95.85 mbsf) and 10 µM (230.80 mbsf). As in Hole 1209A, these deviations coincide with a number of condensed intervals containing inferred Mn-rich phases (see "Lithostratigraphy"). Consequently, the increase in Mn²⁺ concentrations is interpreted to reflect the dissolution of Mn-minerals and diffusion of Mn²⁺ away from two separate Mn-rich horizons.

Potassium (K⁺) concentrations gradually decrease downcore from a maximum of 12.6 mM in the shallowest sample (4.45 mbsf) to values slightly less than that of average seawater (10.2 mM) in the deepest sample at 284.80 mbsf (Fig. F26). Elevated concentrations may correspond to the occurrence of volcanic ash in the upper ~80 m of the hole (see "Lithologic Unit I" in "Lithostratigraphy"), suggesting that K⁺ is liberated from glass-rich, silicic volcanic material via leaching and weathering reactions that produce clays (i.e., smectite). The downcore decrease in K⁺ is interpreted to reflect the absence of ash relative to the overlying sediments, diffusion of K⁺ toward greater depths, and possible exchange with basaltic basement.

As at Site 1209, concentrations of Ca²⁺ and Mg²⁺ in the upper pore waters at Site 1210 are close to average seawater (10.3 and 53.2 mM, respectively) and show an inverse relationship downcore (Fig. F26): Ca²⁺ concentrations increase downcore from 10.9 to 14.1 mM, whereas Mg²⁺ concentrations decrease from 55.3 to 44.6 mM. The most likely influences on the downcore distribution of Ca²⁺ and Mg²⁺ cations are calcium carbonate dissolution (releases Ca²⁺) and weathering reactions involving volcanic ash and/or basement (release Ca²⁺ and consume Mg²⁺). However, Ca²⁺ continues to increase below the depth of maximum Sr²⁺ concentration, which suggests that carbonate dissolution may not be the source of excess. The distribution of Mg²⁺ may also be influenced by the formation of Mg phases (i.e., saponite), observed intermittently throughout lithologic Unit I as green laminae (see "Lithologic Unit I" in "Lithostratigraphy"). The relative contribution of Ca²⁺ and Mg²⁺ to interstitial waters by these processes cannot be determined from the available data.

Initially, the concentration of Li⁺ in Holes 1210A and 1210B (Fig. F26) decreases from 23 µM to a minimum of 16 µM at 49.85 mbsf. Below this depth, Li⁺ cation concentrations increase linearly to 33 µM at 284.4 mbsf. The lower concentrations through the upper ~50 m of the profile may reflect Li⁺ adsorption to clay minerals forming through the weathering of volcanic material and, possibly, zeolites (Gieskes, 1981). The similarity of the Li⁺ profile to the Ca²⁺ profile (increasing concentration with depth), and the inverse relationship between these and the Mg²⁺ and K⁺ profiles, implies that weathering reactions involving volcanic basement may be adding Li⁺ to pore waters. 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 similar curvature of the Sr²⁺ and Sr/Ca pore water profiles (Fig. F26) suggests that the highest rate of carbonate dissolution and/or recrystallization is occurring within the upper ~100 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 indicates the sediment has not been buried to a depth sufficient for the onset of pressure solution.

The downcore dissolved silica (Si(OH)₄) profile is similar to those observed at Sites 1207, 1208, and 1209 (Fig. F27). Pore water silica concentrations average 619 ± 60 µM in the upper ~100 m of the profile, decrease sharply to 229 µM at 144.80 mbsf, and remain low and uniform (226 ± 25 µM) down to 284.8 mbsf. The elevated concentrations in the upper part of the profile are interpreted to reflect the leaching and weathering of volcanic ash in Neogene sediments. Lower pore water Si(OH)₄ concentrations likely correspond to a decrease in ash content and the appearance of chert in calcareous sediments. 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).

At Site 1210, dissolved boron (H₃BO₃) concentrations are relatively uniform (459 ± 58 µM), with one significant negative excursion at 30.85 mbsf (282 µM). Above 100 mbsf, the Ba²⁺ concentrations average 0.4 ± 0.2 µM and show only slight variability. Although Ba²⁺ concentrations in the upper half of the profile are extremely low, they are, on average, higher than those of average seawater, indicating that Ba²⁺ is being added to the pore waters. Possible sources of Ba²⁺ include leaching and/or dissolution of skeletal debris and volcanic ash. The significance of variations in the H₃BO₃ and Ba²⁺ concentrations in pore waters of pelagic sediments is not well understood. Pore water of these constituents profiles at Site 1210 are described largely for purposes of documentation. The data are provided in Table T13.