GEOCHEMISTRY

We collected interstitial waters from six samples at Site 1215: five from Hole 1215A at depths ranging from 5.65 to 64.15 mbsf and one from Hole 1215B at 43.95 mbsf (Table T11; Fig. F14). The samples from both holes were taken to constitute a single depth profile. However, major ion concentrations of the interstitial water taken from the Hole 1215B indicate contamination with seawater, most likely caused by the cherty (nodular) nature of the sediment rendering the core more permeable to seawater. Chemical gradients in the interstitial waters at this site primarily reflect the limited amount of organic matter diagenesis, the generally nonbiogenic character of the sediments, and possibly a small diffusive influence of reactions in the underlying basalt.

Chlorinity, as measured by titration, increases with depth from values of ~556 mM at 5.65 mbsf to values of ~565 mM at 64.15 mbsf (Fig. F14). The values at the top of the section are slightly lower than the standard seawater value of 559 mM, consistent with the lower than average chlorinity of modern Pacific bottom waters (~542 mM). Sodium concentrations determined by charge balance were on average 2.5% higher than those measured by ion chromatograph. Sodium concentrations as determined by charge balance generally increase downcore from values of ~495 mM at 5.65 mbsf to values of ~500 mM at 64.15 mbsf. Salinity, as measured by a handheld refractometer, was lowest in the shallowest sample from 5.65 mbsf from Hole 1215A (34.5); all other interstitial waters were measured as 35.0.

Alkalinity increases with depth in the top 25 m of the section to values of ~2.8 mM and, thereafter, decreased with depth, reaching ~2.4 mM at 64.15 mbsf. The pH generally increases with depth, from values of ~7.1 at 5.65 mbsf to values of ~7.5 at 64.15 mbsf. Dissolved silica concentrations increase with depth, from values of ~260 µM at 5.65 mbsf to values of ~420 µM in Hole 1215A. Silica concentration values then decrease to ~350 µM at 64.15 mbsf in Hole 1215A. Silica concentrations of all interstitial water samples were below saturation levels, which is consistent with the absence of radiolarians from all cored sediments. Interstitial water silica contents ranged from ~325 to 425 µM in the interval where chert nodules were present (between ~23 mbsf to basement).

Sulfate concentrations are >27 mM throughout the section, indicating that the amount of labile organic matter available for oxidation is extremely low. Ammonium is a by-product of organic matter degradation and is present in extremely low levels, which is consistent with the high sulfate values. Phosphate levels are equal to or less than five times the detection limit (0.2 µM) in all samples (Table T14; not shown in Fig. F14).

Dissolved manganese concentrations range from 0.12 to 1.63 µM throughout the interstitial water profile at Site 1215. Strontium concentrations are similar to seawater (i.e., 85.4-89.3 µM) throughout the pore water profile. The small magnitude of the middepth strontium maximum (to ~90 µM over the upper 25 m of the section) likely reflects the low carbonate content of the upper sediments.

Lithium pore water values increase steadily from values of ~32 µM at 5.65 mbsf to values of ~57 µM at 64.15 mbsf. These are the highest lithium pore water values encountered in Leg 199 profiles.

Calcium concentrations increase slightly with depth (from 10.0 to 10.8 mM), whereas magnesium concentrations generally range from 49.9 to 52.9 mM. These profiles may reflect a small influence of alteration of basement and subsequent diffusion, with magnesium replacing calcium in altered basement rocks. The low levels of magnesium in the shallowest sample, from 5.65 mbsf in Hole 1215A, may reflect authigenic mineral precipitation. Potassium concentrations show a similar profile to that of magnesium, presumably reflecting the uptake of potassium during basement alteration. Dissolved barium concentrations are low (from 0.19 to 0.49 µM) and show no systematic variation with depth. Levels of dissolved boron decrease slightly, from values of ~677 µM at 5.65 mbsf to values of ~610 µM at 24.65 mbsf. Below 24.65 mbsf, dissolved boron values increase to 644 µM at 64.15 mbsf.

In summary, the pore water profiles from this site primarily reflect the limited amount of organic matter diagenesis, the dissolution of biogenic silica, and, possibly, a small influence of alteration of underlying basalt and subsequent diffusion. High levels of sulfate and concomitant low levels of ammonium suggest a relatively oxic environment, consistent with the presence of metalliferous oxides. Silica levels in the interstitial waters are higher than seawater values, indicating that biogenic silica was possibly a more important component of the original sedimentary deposits than is obvious by visual inspection of the cores. Calcium, magnesium, and potassium profiles may be influenced by the alteration of basalt and subsequent diffusion to the sediment-water interface.

We collected bulk sediment samples adjacent to the interval sampled for physical properties, resulting in a sampling resolution of approximately one per section from 0.51 to 67.46 mbsf in Hole 1215A (Table T12; Fig. F15), except for the critical interval (P/E boundary). We measured silicon, titanium, iron, manganese, calcium, magnesium, phosphorus, strontium, and barium concentrations in the sediment by inductively coupled plasma-atomic emission spectrophotometer (ICP-AES). Bulk sediment geochemistry primarily reflects the changing lithology of the sediments with depth from red clay to nannofossil ooze and clay to metalliferous oxide ooze.

Silicon decreases from ~25 wt% at 0.51 mbsf to ~4 wt% at the transition from red clay to nannofossil ooze and clay (~28.54 mbsf). Subsequently, silicon remains at or below ~5 wt%. Aluminum and titanium follow similar trends to silicon. Between 5 and 30 mbsf, the Al/Ti ratio is high relative to the Post-Archean Average Shale value of 16.7 (Taylor and McLennan, 1985) (see Fig. F19 in the "Leg 199 Summary" chapter). Aluminum decreases from ~9 wt% at 0.51 mbsf to <1 wt% at 28.54 mbsf and remains below 1 wt% until the deepest sample (67.46 mbsf), which has a value of 1.81 wt%. Similarly, titanium decreases from ~0.5 wt% to <0.05 wt% at 28.54 mbsf and generally remains below 0.05 wt% until 67.46 mbsf.

Iron and manganese show similar patterns to each other, with a gradual overall increase downcore to their maxima at 22.44 mbsf of ~21 and ~5 wt%, respectively, in lithologic Unit I. Iron decreases to <5 wt% and manganese to <1 wt% between 28.54 and 66.44 mbsf, corresponding to the transition to lithologic Unit II. Maximum concentrations of ~26 and ~9 wt% for iron and manganese, respectively, are present at 67.46 mbsf in lithologic Unit III.

Calcium is <2 wt% in the red clay (from 0 to ~23 mbsf) but increases to 25-35 wt% in the nannofossil ooze and clay between 28.54 and 66.44 mbsf. Calcium decreases to <5 wt% in the metalliferous oxide ooze near basement. Strontium follows a similar pattern to calcium, with highest values of ~1000-~1200 ppm between 28.54 and 66.44 mbsf. Magnesium varies between 1 and 3 wt% in the clay (0.51-26.94 mbsf) and decreases to between 0.5 and 1.5 wt% in the nannofossil ooze and clay (~30-65 mbsf).

Phosphorus and barium follow similar trends except for the interval from 0 to 10 mbsf, where phosphorus increases from 0.04 to 0.47 wt% and barium decreases from 969.40 to 378.11 ppm with depth. Phosphorus and barium peak values are 1.6 wt% and 2000 ppm, respectively, in the hydrothermal sample at 67.46 mbsf.

Ash layers at 7.39 and 8.28 mbsf contain slightly elevated concentrations of silicon and aluminum and slightly lowered concentrations of most other elements. These layers are also characterized by elevated Al/Ti ratios (see Fig. F19 in the "Leg 199 Summary" chapter). Flow-in contamination at 28.24 mbsf is characterized by a low in calcium and strontium concentration and spikes in all other elements measured. The metalliferous oxide ooze at 67.46 mbsf contains elevated levels of all elements measured except calcium and strontium. Iron, manganese, phosphorus, and barium reach their highest concentrations at this level.

CaCO₃ (in weight percent) and organic carbon (C_org in weight percent) were determined for approximately two samples per core for Hole 1215A. CaCO₃ is <1 wt% from 0.51 to 23.93 mbsf and subsequently increases to ~85 wt% from 26.94 to 66.44 mbsf (Table T13). CaCO₃ values calculated from Ca contents (in weight percent) yielded similar trends to CaCO₃ measured via coulometer, although absolute values by calculation are lower when CaCO₃ is <1 wt% (Table T13) (see also "Geochemistry" in the "Explanatory Notes" chapter). C_org is uniformly low (0-0.19 wt%) for all samples measured (Table T13).

In summary, the bulk geochemistry of the sediments characterizes the lithology, with high silicon, aluminum, titanium, iron, manganese, and magnesium in the red-clay unit; high calcium and strontium in the nannofossil ooze and clay unit; and high iron, manganese, magnesium, phosphorus, and barium in the hydrothermal unit.