A total of 17 interstitial water samples were collected at Site 1207: 16 from Hole 1207A at depths ranging from 2.95 to 203.80 mbsf and 1 from Hole 1207B at 208.5 mbsf. This report includes data from Hole 1207A only; the sample recovered from Hole 1207B was too severely disturbed to provide reliable geochemical information. 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 (SO42-), phosphate (HPO42-), 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 T17. Cited values for average seawater composition are from Millero and Sohn (1992) and Broecker and Peng (1982); values for North Pacific Deep Water (NPDW) composition are from Rea et al. (1993).
The pore water samples span the range of sediments in lithologic Unit I (0-162.50 mbsf) and extend into lithologic Unit II (162.5-335.3 mbsf). The major lithologic trends that affect pore water geochemistry include a transition from nannofossil ooze with diatoms and siliceous clayey nannofossil ooze to nannofossil clay just above the boundary between lithologic Units I and II (see "Lithologic Unit I" in "Lithostratigraphy"). Volcanic ash, typically concentrated in discrete layers, is common throughout lithologic Unit I. Abundant zeolites (i.e., phillipsite) and manganese micronodules occur within the lowermost portion of Unit I. Chert is common below this depth.
In Hole 1207A, the pH of pore waters ranged from 7.17 to 7.43, with the minimum value at 39.25 mbsf and a mean value of 7.33 ± 0.06 (1 
). All values are lower than the average seawater value of 8.1. The relatively narrow range of values reflects the buffering capacity of the carbonate-dominated sediment. Salinity was uniform throughout the profile, with an average value of 35.0 ± 0.25 g/kg.
In Hole 1207A, Cl- concentrations range from a low of 547 mM at 10.75 mbsf to a high of 566 mM at 58.25 mbsf (Fig. F40B). The Cl- concentration in the shallowest sample (2.75 mbsf) is slightly higher than the Pacific Ocean Deep Water (PODW) value of 554 mM. Below the minimum value at 10.75 mbsf, Cl- concentrations exhibit a gradual increase to a maximum value of 566 mM at 58.25 mbsf. Below this depth, concentrations fluctuate around a mean value of 561 mM. The broad downcore fluctuations in Cl- may reflect the ongoing adjustment of the pore water Cl- concentrations to variations in mean ocean salinity, which has generally increased over the past few million years in response to increasing continental ice volume (McDuff, 1985).
Sodium (Na+) concentrations, calculated by charge balance (see Broecker and Peng, 1982), range from a low of 469 mM at 10.75 mbsf to a high of 485 mM at 77.25 mbsf (Fig. F40A), with average concentrations (477 ± 4 mM) slightly higher than that of average seawater (470 mM). The broad trend of increasing concentrations to ~58 mbsf below the minimum value at 10.75 mbsf is similar to that observed in the Cl- data and may reflect the salinity changes discussed above. The cause of significant variation in Na+ below this depth is unclear but may be related to the alteration of volcanic ash in lithologic Unit I and zeolites in Subunit IC (see "Lithostratigraphy").
Although alkalinity variations in the interstitial waters of Hole 1207A are minor, a distinct downcore trend is evident. Alkalinity increases steadily from 3.09 mM at 2.75 mbsf to 4.32 mM at 96.25 mbsf. Below 96.25 mbsf, alkalinity values remain relatively uniform, averaging 4.18 ± 0.12 mM (Fig. F41A). The shape of the downcore trend indicates that the highest rate of alkalinity production, generated mostly as bicarbonate ion (HCO3-) at the measured pH, is occurring within the upper ~90 m of the profile. This interpretation is supported by the sulfate (SO42-) profile, which shows a decrease from near the average seawater value of 28 mM to 25.5 mM over the same depth interval (Fig. F41B). SO42- reduction is reflected in the sediments by the presence of pyrite in lithologic Unit I (see "Lithologic Unit I" in "Lithostratigraphy"). The uniformity of the alkalinity and SO42- values below ~90 mbsf suggests the absence of significant alkalinity production via SO42- reduction in the lower part of the pore water profile.
Ammonium (NH4+) and phosphate (HPO42-) in pore waters in the upper parts of sequences dominated by biogenic sediments are produced largely through the decomposition of organic matter (Gieskes, 1983). NH4+ and HPO42- concentrations are low throughout Hole 1207A, with values of 79-220 µM and 0.5-5.0 µM, respectively (Fig. F41E, F41F). Although more variable, the first ~100 m of the NH4+ profile is similar to that of alkalinity. This similarity suggests that NH4+ is being produced through the anaerobic reduction of organic matter. NH4+ concentrations generally decrease below this depth, suggesting that no NH4+ is being produced in the lower part of the profile. This interpretation is consistent with the uniformity of the alkalinity and SO42- profiles below ~90 mbsf. Fluctuations in the NH4+ profile between ~50 and 100 mbsf may reflect the irregular addition of NH4+ to the pore water system as a result of the leaching and weathering of the abundant silicic volcanic material in lithologic Unit I (see "Lithologic Unit I" in "Lithostratigraphy"). HPO42- concentrations are highest in the shallowest interstitial water sample (5.0 µM) and decrease sharply to a value of 1.3 µM at 29.75 mbsf. Below this depth, HPO42- concentrations show a gradual downcore decrease to a minimum value of 0.5 µM at the bottom of the profile (Fig. F41E). This broad trend is interrupted by a brief excursion to a concentration of 2.9 µM at 39.25 mbsf. The very low levels of dissolved HPO42- below the uppermost pore water sample likely reflect the effective adsorption of any phosphate produced via organic matter degradation onto the surfaces of the carbonate sediment (Walter and Burton, 1986). As indicated above, the alkalinity and SO42- profiles suggest that rates of organic matter degradation are low in the sediments of lithologic Units I and II.
Iron (Fe2+) profiles show significant variation downcore, with concentrations ranging from 0.1 to 35.4 µM (Fig. F41D). The overall trend is toward decreasing concentration with increasing depth. Variations in the Fe2+ profile are attributed to the proximity of iron sources, such as volcanic ash to areas of sulfate reduction and pyrite precipitation in lithologic Unit I (see "Lithologic Unit I" in "Lithostratigraphy"). The lowest observed concentration of dissolved Fe2+, 0.1 µM at 178.25 mbsf, may reflect the formation of the manganese micronodules that occur in lithologic Subunit IC. The overall trend of decreasing Fe2+ concentrations is related to the absence of volcanic debris, the major Fe2+ source, below ~162 mbsf and reflects diffusion between sites of high and low Fe2+ concentrations.
Pore water manganese (Mn2+) concentrations decrease from 10.0 µM in the shallowest sample to a minimum of 1 µM at 48.75 mbsf. Concentrations remain uniformly low to 134.25 mbsf, increase to 7.9 µM between 153.25 and 178.25 mbsf, and decrease to 1.2 µM at the base of the profile (Fig. F41C). Elevated Mn2+ concentrations in the two 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 indicate that SO42- reduction occurs. The sharp decrease to low and uniform concentrations between 48.75 and 134.25 mbsf suggests that a manganese carbonate phase may be forming within this interval. The requisite reducing conditions in the sediment column are consistent with the occurrence of pyrite and may be reflected in the low red/blue (680/420 nm) ratios in color reflectance through this interval (see "Lithostratigraphy"). The excursion to higher Mn2+ concentrations in the lower part of the pore water profile centers on lithologic Subunit IC, which encompasses a major unconformity between the middle Miocene and the Campanian. The unconformity surface is characterized by the presence of a 5-cm-thick manganese oxide crust; the interval contains abundant manganese oxide micronodules (see "Subunit IC" in "Lithostratigraphy"). The sediment below the unconformity is described as yellowish brown to pale yellowish brown with relatively high red/blue (680/420 nm) ratios in color reflectance (see "Lithologic Unit II" in "Lithostratigraphy"). These features suggest that the sediment column remained relatively oxic during deposition. By contrast, the sediments above the unconformity appear to have been less oxidizing, and perhaps mildly reducing, at shallow depths during deposition. Accumulation of Miocene sediments atop the unconformity led to the development of redox conditions within the Campanian deposits, which ultimately became reducing enough to support the dissolution of manganese oxides associated with the unconformity surface. This interpretation is consistent with the SO42- profile through this interval, which suggest that redox conditions in sediments below the unconformity are now reducing. The progressive dissolution of manganese oxides led to the development of a diffusion gradient between high concentrations associated with the unconformity surface and low concentrations in the pore waters of overlying sediments.
Potassium (K+) concentrations decrease gradually downcore in Hole 1207A from a maximum of 12.2 mM in the shallowest sample (2.75 mbsf) to values approaching that of average seawater (10.2 mM) in the deepest sample at 203.80 mbsf (Fig. F42A). Elevated concentrations correspond to the distribution of volcanic ash and pumice in the upper ~160 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+ likely reflects a diminishing source of volcanic ash and pumice, diffusion toward greater depths, and possible exchange with basaltic basement.
The dissolved silica, Si(OH)4, concentrations throughout the upper ~150 m of Hole 1207A (765 ± 126 µM) are distinctly higher than in the overlying water (NPDW value = 160 µM). Below ~150 mbsf, values decrease to a minimum of 216 µM at 203.8 mbsf (Fig. F42B). High Si(OH)4 concentrations in the upper part of the hole are interpreted to reflect the leaching and weathering of volcanic ash and pumice in lithologic Unit I. Si(OH)4 concentrations likely approaching saturation levels in the pore waters of lithologic Unit I have allowed for excellent preservation (based on analysis of smear slides) of the abundant siliceous microfossils that occur throughout this interval. Lower pore water Si(OH)4 concentrations correspond to the appearance of chert in the sediment column. A significant removal of Si(OH)4 from pore waters may be induced by the recrystallization of opal-A to opal-CT (e.g., Baker, 1986) or quartz (e.g., Gieskes, 1981).
Lithium (Li+) concentrations decrease abruptly from a value approaching that of modern seawater (27 µM) in the shallowest sample (2.75 mbsf), to relatively uniform values at greater depths that fluctuate around a mean of 17.7 ± 1.2 µM (Fig. F42C). The observed depletion may reflect uptake by clay minerals forming through the weathering of volcanic material and possibly zeolites (Gieskes, 1981), which occur in the lower 3 m of lithologic Unit I (see "Lithologic Unit I" in "Lithostratigraphy").
Concentrations of calcium (Ca2+) and magnesium (Mg2+) are similar to that of average seawater at the top of the pore water profile and show an antithetical relationship downcore (Fig. F43A, F43B). Ca2+ concentrations increase slightly downcore from 10.4 mM in the upper part of the profile to 11-12 mM, whereas Mg2+ concentrations decrease from 53.8 to 51.5 mM. The most likely influences on the relative abundances of Ca2+ and Mg2+ in pore waters at Site 1207 include calcium carbonate dissolution, weathering reactions involving volcanic ash in lithologic Unit I, and weathering reactions involving the volcanic basement. Each of these processes serves to create a source of Ca2+ and a sink for Mg2+. The alteration of volcanic ash typically releases Ca2+ to solution while sequestering Mg2+in alteration products such as magnesian smectites or sepiolite (McDuff and Gieskes, 1976; Lawrence and Gieskes, 1981). No shipboard data are available to determine the influence of such weathering processes on the pore water profile at Site 1207.
Strontium (Sr2+) concentrations show a gradual increase from values similar to that of average seawater at 86 µM in the shallowest sample at 2.75 mbsf to 258 µM at 153.25 mbsf (Fig. F43C). Below this depth, Sr2+ concentrations decrease to 233 µM at the bottom of the profile (203.80 mbsf). The dominant influence on the Sr2+ profile is likely carbonate dissolution and/or recrystallization (Baker et al., 1982). This interpretation is supported by smear slide observations (see "Lithostratigraphy") and by increasing Sr/Ca ratios with increasing depth (Fig. F43C). If weathering of relatively calcium-enriched volcanic ash was a major contributor to Sr2+ levels in the pore fluids, Sr/Ca ratios would be expected to decrease. Generally, low Sr2+ concentrations suggest that little alteration of carbonate is occurring relative to most other pelagic sites (Baker et al., 1982).
Boron, generated mostly as borate, H3BO3, at the measured pH levels, shows little downcore variation (Fig. F43D), with average concentrations (438 ± 25 µM) slightly higher than that of average seawater (416 µM). These concentrations and the lack of variability in the profile suggest that boron may not be involved in the sediment-pore water interactions discussed above. Average Ba2+ concentrations in interstitial waters of Hole 1207A are extremely low (0.5 ± 0.4 µM) but consistently higher than that of average seawater (0.1 µM). Ba2+ concentrations vary significantly with depth, trending toward higher values near the base of the profile (Fig. F43E). Possible sources of Ba2+ in the sediments of Site 1207 include skeletal debris and volcanic ash, which may be undergoing leaching and/or dissolution (see above), and BaSO4, which may be undergoing dissolution as a consequence of sulfate reduction.
