GEOCHEMISTRY

We collected interstitial waters from 14 samples from Hole 1218A at depths ranging from 5.95 to 273.65 mbsf (Table T16; Fig. F21). Chemical gradients in the interstitial waters from Site 1218 reflect the dissolution of biogenic opal, the limited amount of organic matter diagenesis, and possibly the diffusive influence of reactions in the underlying basalt.

Chlorinity, as measured by titration, increases with depth from values of ~555 mM at 5.95 mbsf to values of ~560 mM at 22.15 mbsf and remains nearly constant until 200.35 mbsf. Chlorinity generally decreases between 200.35 and 273.65 mbsf to a minimum value of ~548 mM. Sodium concentrations, as determined by charge balance (on average 1% lower than those measured by ion chromatograph), increase from ~476 to ~487 mM between 5.95 and 22.15 mbsf and then generally decrease to values of ~468 mM at 273.65 mbsf. Salinity, as measured by a handheld refractometer, does not vary much downhole; most interstitial waters were measured as 35.0.

Alkalinity variations of the pore waters follow those of chlorinity. Alkalinity increases downhole from 2.8 mM at 5.95 mbsf to 3.9 mM at 164.65 mbsf. Alkalinity decreases downhole between 164.65 and 273.65 mbsf to values of ~2.26 mM. pH generally decreases downhole from 7.27 at 5.95 mbsf to 7.15 at 88.65 mbsf, remains relatively low until 229.15 mbsf, and then increases to 7.32 at 273.65 mbsf. Dissolved silica concentrations increase with depth, from values of ~548 µM at 5.95 mbsf to values of ~1116 µM at 229.15 mbsf, followed by an abrupt decrease to 626 µM at 273.65 mbsf. The increase in interstitial water silica concentration with depth probably reflects the dissolution of biogenic silica and subsequent diffusion.

Interstitial water sulfate concentrations remain at or above seawater concentration (28 mM) throughout most of the hole, which indicates that the amount of labile organic matter available for oxidation is low. The lowest sulfate values (26.3 mM), which are lower than at any of the previous sites (Sites 1215-1217), are reached at ~136.15 mbsf. Ammonium is present at extremely low levels (<13 µM) at Site 1218 interstitial pore waters, which is consistent with high sulfate values.

Dissolved manganese concentrations found at Site 1218 decrease from ~30 to ~1.5 µM between 5.95 and 41.15 mbsf. Manganese concentrations remain low (<10 µM) from 41.15 mbsf to basement. Lithium concentrations in the interstitial waters decrease from seawater values (~27 µM) to ~24 µM between 5.95 and 88.65 mbsf and remain constant downhole until increasing to a maximum of ~33 µM near basement (273.65 mbsf). Strontium concentrations in the interstitial waters at Site 1218 increase from ~88 to 119 µM between 5.95 and 229.15 mbsf and subsequently decrease to ~92 µM at 273.65 mbsf.

Calcium concentrations in the pore waters at Site 1218 are similar to seawater concentrations (~10 mM) from the top of the hole to ~165 mbsf. Between ~200 and 260 mbsf, Ca values increase to an average of 15 mM, then decrease to 13 mM at 273 mbsf. Magnesium concentrations decrease with depth and are lower than seawater values from 5.95 to 273.65 mbsf. The trends of slightly decreasing magnesium and slightly increasing calcium with depth are the first observed during Leg 199 and may reflect alteration of basalt and subsequent diffusion. Nevertheless, pore water gradients in magnesium and calcium are modest. Potassium concentrations decrease in a near-linear fashion downhole from values of ~12 mM at 5.95 mbsf to values of ~10 mM at 273.65 mbsf. Dissolved barium concentrations are low (~<1 µM) throughout the sediment column from Site 1218. Boron concentrations range from 466 to 531 µM.

In summary, the pore water profiles at Site 1218 are influenced by the dissolution of biogenic silica, the lack of organic matter diagenesis, and possibly the alteration of underlying basalt and subsequent diffusion.

At Site 1218, we collected bulk-sediment samples adjacent to the intervals sampled for physical properties (see "Physical Properties"), resulting in a sampling resolution of approximately one per section from 0.74 to 273.75 mbsf at Hole 1218A (Table T17; Fig. F22). 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 downhole from red clay (Unit I) to nannofossil ooze and chalk (Unit II) to radiolarian ooze (Unit III) to nannofossil chalk with dolomite (Unit IV) (Fig. F22).

Silicon varies between 1 and 30 wt% at Site 1218, which primarily reflects sediment lithology. Silicon concentrations are ~20 wt% in Unit I, 1-3 wt% in Unit II, and ~30 wt% in Unit III. Silicon concentrations decrease through Unit IV, reaching ~4 wt% near basement.

Aluminum and titanium concentrations are highest in the clay and siliceous sediments (Units I and III) and lowest in the carbonate sediments (Units II and IV). Aluminum concentrations decrease downhole from ~7 to ~0.5 wt% between 0.74 and 213.23 mbsf and then increase to almost 2 wt% at 218.03 mbsf. Farther downhole, aluminum concentrations decrease to 0.61 wt% at 274.23 mbsf. Titanium content decreases from ~0.3 to 0.2 wt% within Unit I, is very low (<0.10 wt%) throughout the carbonate sediments (Units II and IV), and is ~1.1 wt% in Unit III.

Al/Ti ratios vary between 12.5 and 34.0 in the silica-rich units (I and III). Si/Ti and Ba/Ti ratios are higher in Unit III than in Unit I, possibly reflecting the increased biogenic component of the silicon of Unit III relative to Unit I.

Iron and manganese contents show similar trends to aluminum and titanium, with maximum values in Units I and III and values near 1 wt% in Unit III (Fig. F22). Iron varies between 3 and 6 wt% in Units I and III and between 0.18 and 1.15 wt% in Units II and IV. Manganese concentrations are ~0.2 wt% in Unit II and reach ~1.5 wt% within Units I and III.

Calcium and strontium concentrations are highest in the carbonate-rich lithologies. Calcium concentrations vary between 0.38 wt% at 0.74 mbsf (Unit I) and 43 wt% at 162.44 mbsf (Unit II). However, this value of 43 wt% is too high to be real, as a result of causes discussed in "Geochemistry" in the "Explanatory Notes" chapter. Similarly, strontium concentrations are lowest (~250 ppm) in the siliceous-rich lithologies (Units I and III) and highest (~2000 ppm) in the carbonate-rich lithologies (Units II and IV). Magnesium varies between 1.4 and 2.2 wt% in the red-clay and radiolarite units and between 0.2 and 0.5 wt% in the carbonate-rich sediments. Magnesium concentrations increase to ~5 wt% near the base of the hole (~273 mbsf), which is consistent with the presence of dolomite in Unit IV.

Phosphorus concentrations are low (generally <0.5 wt% in siliceous and clay sediments and <0.2 wt% in carbonate sediments). Barium concentrations in Site 1218 sediments are higher than at previous sites. Barium concentrations are highest in the red-clay and radiolarite sediments, varying between 4,500 and 10,000 ppm with isolated peak values in Unit I reaching 15,000 ppm. Barium concentrations are lower (between 400 and 2000 ppm) throughout the nannofossil ooze and chalk and nannofossil chalk with dolomite (Units II and IV). The Ba/Ti ratio of Unit II is intermediate between that of Units I and III (see Fig. F19 in the "Leg 199 Summary" chapter).

CaCO₃ (in weight percent) was determined by coulometric methods for approximately three samples per core from 0.74 to 274.23 mbsf in Hole 1218A (Table T18; Fig. F23). CaCO₃ is low (1 wt%) for the clay interval (Unit 1), high (~60-90 wt%) in the nannofossil ooze and chalk (Unit II), varies widely (<1-60 wt%) in the radiolarite (Unit III), and high (up to 83 wt%) in the nannofossil chalk and dolomite (Unit IV). CaCO₃ values calculated from Ca ICP-AES data (in weight percent) yielded similar trends to CaCO₃ measured via coulometer. However, CaCO₃ (wt%) values calculated from ICP-AES data are overestimated at high values because of analytical problems (see"Geochemistry" in the "Explanatory Notes" chapter). C_org (in weight percent) determined for one sample per core is uniformly low (0-0.30 wt%) for the samples measured.

In summary, the bulk geochemistry of the sediments from Site 1218 reflects the varying lithology of the sediments between red clay, nannofossil ooze and chalk, radiolarite, and nannofossil chalk with dolomite.