GEOCHEMISTRY

We collected interstitial waters from seven samples in Hole 1217A at depths ranging from 2.95 to 84.65 mbsf (Table T10; Fig. F18). Chemical gradients in the interstitial waters from Site 1217 reflect the dissolution of biogenic opal, the limited amount of organic matter diagenesis, and possibly the precipitation of authigenic minerals and the diffusive influence of reactions in the underlying basalt.

Chlorinity, as measured by titration, increases with depth from values of ~555 mM at 2.95 mbsf to values of ~565 mM at 84.65 mbsf. Chlorinity of the interstitial water samples from the upper two cores are lower than standard seawater (559 mM). Sodium concentrations as determined by charge balance (on average 1% lower than those measured by ion chromatograph) are similar to that of average seawater (480 mM) and show no consistent trend downhole. Salinity, as measured by a handheld refractometer, does not vary downhole; all interstitial waters were measured as 35.0, with the exception of a measurement of 36.0 at 55.15 mbsf.

Alkalinity values for the entire hole are higher than International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater (2.325 mM) and increase slightly downhole from 2.43 mM at 2.95 mbsf to 3.40 mM at 84.65 mbsf. The pH ranges from 7.20 at 2.95 mbsf to a peak of 7.37 at 29.45 mbsf. Dissolved silica concentrations increase with depth from ~280 µM at 2.95 mbsf to values of ~789 µM at 84.65 mbsf. Dissolved silica concentrations are higher than at Sites 1215 and 1216, which is consistent with the presence of radiolarian ooze (from ~50 to 90 mbsf) at Site 1217.

Interstitial water sulfate concentrations remain at or above seawater concentrations (~28 mM) throughout the hole, indicating that the amount of labile organic matter available for oxidation is extremely low. Consistent with high sulfate values, ammonium is present at extremely low levels (<10 µM).

Dissolved manganese concentrations are low throughout the interstitial water profile at Site 1217 (<2.5 µM). Lithium and strontium concentrations are approximately or only slightly higher than seawater values (20 and 90 µM, respectively) throughout the pore water profiles.

Calcium concentrations in the pore waters from Site 1217 are similar to IAPSO concentrations (~10 mM). Magnesium concentrations are lower than IAPSO values (from 2.95 to 55.15 mbsf). These low magnesium concentrations in the pore waters may indicate magnesium uptake in the precipitation of yet unidentified authigenic minerals. Potassium concentrations increase from 11.9 to 12.6 mM for 0-19.45 mbsf. Below 19.45 mbsf, there is an overall decrease in potassium concentrations from 11.8 to 10.5 mM, which is consistent with higher levels of clay in the upper sediments (0-50 mbsf). The lack of increasing calcium and decreasing magnesium concentrations with depth likely indicates that the diffusive influence of reactions in the underlying basalt is small in these pore waters. Dissolved barium concentrations are low (~<0.50 µM). Boron concentrations range from 461 to 659 µM, higher than seawater value (416 µM).

In summary, the pore water profiles from Site 1217 are influenced by the dissolution of biogenic silica, authigenic mineral precipitation, possibly alteration of underlying basalt, and subsequent diffusion. High levels of sulfate and concomitant low levels of ammonium indicate an 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 (and higher than values from Sites 1215 and 1216), indicating the relative importance of biogenic silica as a component of these sediments.

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 0.79 to 128.83 mbsf in Hole 1217A (Table T11; Fig. F19). We also collected samples from 31.74 to 72.73 mbsf in Hole 1217B to fill in gaps in the Hole 1217A sediment column (Table T11; Fig. F19). We show all data plotted against depth in mcd for Site 1217 (Fig. F19). Depth in mbsf and mcd for each hole is provided (Table T11). We measured silicon (Si), aluminum (Al), titanium (Ti), iron (Fe), manganese (Mn), calcium (Ca), magnesium (Mg), phosphorus (P), strontium (Sr), and barium (Ba) concentrations in the sediment by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Bulk-sediment geochemistry primarily reflects the changing lithology of the sediments from red clay to nannofossil ooze in Unit I to radiolarian ooze and clay in Subunit IIA to nannofossil chalk in Unit III.

Silicon generally ranges between 20 and 30 wt%, which is consistent with the dominance of red clay and siliceous sediment throughout most of the depth range (Fig. F19). The exceptions are a decrease in silicon to ~11 wt% in the nannofossil ooze section between 27 and 29 mcd and a decrease to ~4 wt% in a sample from 129 mcd in the nannofossil chalk.

Aluminum and titanium in Site 1217 sediments follow similar trends of aluminum and titanium in sediments from Sites 1215 and 1216. Aluminum decreases overall downhole from ~9 wt% at 0.79 mcd to <1 wt% at 129.09 mcd (Fig. F19). The nannofossil ooze is marked by lower aluminum values (<3 wt%). Titanium content decreases from 0.91 wt% at 0.79 mcd to 0.01 wt% at 129.09 mcd. Titanium values in the shallowest sediments (0-5 mcd) are higher than observed in Sites 1215 and 1216 sediments. Al/Ti ratios in these shallow sediments are significantly lower (as low as 10) than at Sites 1215 and 1216.

Iron and manganese contents show similar trends to each other (Fig. F19). In the red clay Unit I (~0-50 mcd), iron varies between ~5 and ~9 wt% and manganese varies between 0.5 and ~1.5 wt%. Both iron and manganese decrease in the radiolarian ooze and the nannofossil chalk (Units II and III).

Consistent with the lack of carbonate in most sections, calcium is generally <2 wt% at Site 1217, with the exception of nannofossil ooze, where calcium is as high as 23 wt% (from ~27 to 29 mcd), and nannofossil chalk, where calcium is measured at ~42 wt% (129 mcd) (Fig. F19). This 42 wt% value for Ca cannot be accurate because 100 wt% CaCO₃ is 40 wt% Ca. Possible reasons for this discrepancy are given in "Geochemistry" in the "Explanatory Notes" chapter. A more accurate value for CaCO₃ (wt%) determined by coulometer is 83.55% (see Table T12). Likewise, strontium is low (at ~200 ppm throughout) with the exception of peaks of ~1000 ppm in the nannofossil ooze and chalk (~27-29 mcd). Magnesium varies between 1.2 and 3 wt%.

Similar to Sites 1215 and 1216, phosphorus is low (generally <0.5 wt%) in Site 1217 sediments (Fig. F19). Barium contents in Site 1217 sediments are highest between 25 and 50 mcd (>2500 ppm) and are generally higher than at Site 1215 or 1216.

Calcium carbonate (CaCO₃) (in weight percent) and organic carbon (C_org) (in weight percent) were determined for approximately two samples per section from 0.79 to 128.83 mbsf in Hole 1217A and for one sample per core from 31.74 to 72.73 mbsf in Hole 1217B to fill in gaps in the Hole 1217A sediment column (Table T12). CaCO₃ measured by coulometer is low (less than or equal to ~1 wt%) for all of the sediments from Site 1217 except for samples between 26 and 31 mbsf and a further sample at 128.83 mbsf in Hole 1217A. CaCO₃ values calculated from Ca contents yielded similar trends to CaCO₃ measured via coulometer, although absolute values are lower for low carbonate values (<1 wt%) and higher for values >1 wt% (Table T12). Calculated values were negative for some samples, indicating a problem with the calibration at very low carbonate values (see "Geochemistry" in the "Explanatory Notes" chapter). C_org is uniformly low (0-0.40 wt%) for all samples measured (Table T12).

In summary, the bulk geochemistry of the sediments from Site 1217 reflects the varying lithology of the sediments between red clay, nannofossil ooze/chalk, and the increasing proportion of siliceous biogenic sediments relative to earlier sites.