GEOCHEMISTRY

We collected interstitial waters from five samples in Hole 1216A at depths ranging from 4.45 to 47.65 mbsf (Table T8; Fig. F11). However, major ion concentrations of the interstitial water sample taken from 47.65 mbsf indicate contamination with seawater, most likely caused by the incomplete core recovery and the resulting contact with seawater (this sample is not plotted in Fig. F11). Similar to results from Site 1215, 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 signal of chemical reactions in the underlying basalt.

Chlorinity, as measured by titration, increases with depth from values around standard seawater (559 mM) at 4.45 mbsf to values of 564 mM at 32.65 mbsf (Fig. F11). Sodium concentrations determined by charge balance were, on average, 2% lower than those measured by ion chromatograph. Sodium concentrations as determined by charge balance generally remain steady at values similar to that of average seawater. Salinity, as measured by a handheld refractometer, did not vary downcore; all interstitial waters were measured as 35.0.

Alkalinity and pH did not vary significantly downcore in noncontaminated samples at Site 1216. Dissolved silica concentrations increase with depth, from values of around 210 µM at 4.45 mbsf to values of ~610 µM at 32.65 mbsf. Dissolved silica concentrations increase more rapidly with depth than at Site 1215, consistent with the presence of radiolarians in deeper cores (from ~40 mbsf).

Interstitial water sulfate concentrations are high (>27 mM) throughout the section, indicating that the amount of labile organic matter available for oxidation is extremely low. Ammonium is a byproduct of organic matter degradation and is present in extremely low levels (<10 µM), consistent with the high sulfate values.

Dissolved manganese concentrations are low throughout the interstitial water profile at Site 1216. Manganese concentrations slightly increase downcore, from levels of ~1 µM at 4.45 mbsf to levels of ~3 µM at 32.65 mbsf. Strontium concentrations are similar to seawater throughout the pore water profile. The absence of a middepth strontium maximum likely reflects the low carbonate content of the sediments. Lithium pore water values are generally low (between 30 and 40 µM) and show a slight increase downcore.

Calcium concentrations in the pore waters from Site 1216 are similar to that of seawater (~10 mM), with a slight increase at 32.65 mbsf to values of ~11 mM. Magnesium concentrations also increase with depth, reaching levels similar to that of seawater at 32.65 mbsf. Magnesium concentrations in the pore waters from the upper three cores from Site 1216 (4.45-23.15 mbsf) are lower than that of seawater, perhaps indicating magnesium uptake in the precipitation of authigenic minerals. Potassium concentrations decrease downcore, consistent with chemical alteration of basement rocks, suggesting that the hydrothermal signal in the pore water magnesium and calcium profiles is masked by the effects of additional processes. Dissolved barium concentrations are low (<0.45 µM) and show no systematic variation with depth. Levels of dissolved boron decrease with depth, from values of ~650 µM at 4.45 mbsf to values of ~530 µM at 32.65 mbsf.

In summary, the pore water profiles from this site primarily reflect the dissolution of biogenic silica, authigenic mineral precipitation, possible 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 occurrences of metalliferous oxides within the sediment. Silica and alkalinity levels in the interstitial waters are higher than seawater values, indicating that biogenic silica was a more important component of the original sedimentary deposits than is obvious by visual inspection of the cores.

We collected bulk-sediment samples adjacent to the interval sampled for physical properties (see "Physical Properties"), resulting in a sampling resolution of approximately one per section from 0.72 to 52.79 mbsf in Hole 1216A (Table T9; Fig. F12). We also collected samples from 1.18 to 8.68 mbsf in Hole 1215B (Table T9). We measured silicon, aluminum, titanium, iron, manganese, calcium, magnesium, phosphorus, strontium, and barium concentrations in the sediment by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Bulk-sediment geochemistry primarily reflects the predominant red-clay lithology of the sediments.

Silicon varies between ~17 and ~25 wt% (Fig. F12). Silicon decreases from ~25 wt% at 0.72 mbsf to ~17 wt% at 29.22 mbsf and is ~22 wt% at 52.79 mbsf. The exception to this trend is a spike of silicon of ~29 wt% at 39.29 mbsf that corresponds to a relative minimum in all other elements measured, presumably because of dilution by the large amount of silicon. Possibly, the sample at 39.29 mbsf may have included a small fragment or nodule of chert.

Aluminum and titanium in Hole 1216A follow similar trends to one another. Between 0.72 and 39.29 mbsf, aluminum decreases from 8.5 to 1.4 wt% and titanium decreases from 0.51 to 0.07 wt%. After reaching minimum values at 39.29 mbsf, aluminum remains steady at ~2.8 wt%, and titanium remains steady at ~0.12 wt%. The Al/Ti ratio is ~25 and 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).

Iron contents are relatively constant (~5 wt%) to ~21 mbsf, then increase to a peak of 17.89 wt% at 29.22 mbsf. Below this depth, iron remains at ~12 wt% in the rest of the hole, except for a low of 7.14 wt% at 39.29 mbsf. Manganese increases from 0.91 wt% at 0.72 mbsf to a peak of 3.76 wt% at 29.22 mbsf and for the rest of the hole varies between 1.78 and 3.6 wt%.

Calcium is <2 wt% in the entire hole, consistent with the lack of carbonate. Likewise, strontium is low, at ~200 ppm, throughout. Magnesium decreases from 2.2 wt% at 0.72 mbsf to 1.5 wt% at ~10 mbsf. Subsequently, Mg shows a modest increase downcore to values of almost 3 wt% at 52.79 mbsf, with the exception of a low at 39.29 mbsf.

Phosphorus is uniformly low at <0.5 wt% in Hole 1216A. Barium increases downcore from ~1090 ppm at 0.72 mbsf to a peak value of 4200 ppm at 52.79 mbsf.

Calcium carbonate (CaCO₃; in weight percent) and organic carbon (C_org) (in weight percent) were determined for approximately two samples per core from Hole 1216 (Table T10). CaCO₃ measured by coulometer is low (less than or equal to ~1 wt%) for all of the sediments from Site 1216. CaCO₃ values calculated from Ca contents (in weight percent) yielded similar trends to CaCO₃ measured via coulometer, although absolute values are negative, indicating a problem with the calibration at very low carbonate values (Table T10) (see "Geochemistry" in the "Explanatory Notes" chapter). C_org is uniformly low (0-0.09 wt%) for all samples measured (Table T10).

In summary, the bulk geochemistry of the sediments from Site 1216 is very similar to that in the red-clay unit of Site 1215.