GEOCHEMISTRY

We collected interstitial waters from eight samples in Hole 1221A at intervals of approximately one sample every core for the first six cores and every third core thereafter, at depths ranging from 4.45 to 108.95 mbsf (Table T10; Fig. F13).

Chlorinity, as measured by titration, increases with depth from 551 mM at 4.45 mbsf to 564 mM at 108.95 mbsf (Fig. F13). The lower than average seawater value of interstitial water at shallow sediment depth (4.45 mbsf) at this site is consistent with the chlorinity of modern Pacific bottom waters (~542 mM). Sodium concentrations determined by charge balance increase with depth from 485 mM at 4.45 mbsf to 501 mM at 108.95 mbsf (Table T10). Salinity, as measured by a handheld refractometer, increases with depth from 34.5 at 4.45 mbsf to 35.5 at 108.95 mbsf.

Alkalinity generally increases with depth from 2.53 mM at 4.45 mbsf to a maximum of 3.20 mM at 80.41 mbsf and subsequently decreases to 2.86 mM at 108.95 mbsf.

The pH varies between 7.21 and 7.40. Like all other Leg 199 sites, average sulfate concentrations are high (29.1 mM) and ammonium concentrations are low (12 µM), indicating little oxidation of organic matter.

Dissolved silica concentrations increase with depth, from 388 µM at 4.45 mbsf to ~900 µM at 108.95 mbsf. These high interstitial water silica values are consistent with dissolution of biogenic silica throughout the sediment.

Site 1220 calcium, magnesium, potassium, and lithium profiles are similar to those seen at all other Leg 199 sites except Site 1219 and show little evidence for exchange with basalt and subsequent diffusion. Calcium and potassium concentrations are relatively constant at ~10 mM (about seawater concentration), magnesium concentrations are relatively constant at 48 mM, and lithium pore water values are slightly higher than that of seawater (27 µM).

Strontium concentrations (81 µM) are slightly lower than seawater value (87 µM) at the top of the hole and do not show much increase with depth, consistent with the lack of carbonate at this site. Dissolved manganese averages ~2.6 µM and boron averages ~444 µM, slightly above seawater value (416 µM). Dissolved barium concentrations are below the detection limit.

In summary, the pore water profiles from Site 1220 primarily reflect minor organic matter degradation and the dissolution of biogenic silica. Most of the dissolved chemical constituents show a lack of gradient with depth and values similar to or slightly higher than seawater. Overall, the profiles from Site 1221 are very similar to the profiles from Site 1215, except lithium, which is elevated at the base of the hole at Site 1215.

We collected bulk-sediment samples in every other section, adjacent to the interval sampled for physical properties (see "Physical Properties") at Site 1221, resulting in a sampling resolution of approximately three samples per core. The resulting depth profile is based primarily on data from sediments in Hole 1221A (Table T11; Fig. F14). We also analyzed continuous scrapings (31 samples of 2.5-5 cm in width) from the P/E boundary section (in Section 199-1221C-11X-3, between 153.50 and 154.91 mbsf), discussed separately below.

Bulk-sediment geochemistry does not show much variation. It is difficult to resolve Unit I (clay) from Unit II (diatom-radiolarian-nannofossil ooze), although the radiolarian ooze (Subunit IIIA) and nannofossil chalk (Unit IV) are distinctive (Fig. F14).

Silicon is <20 wt% in Units I-II, increases to ~30 wt% in Unit III, and decreases to 2 wt% in Unit IV (Fig. F14). Aluminum and titanium concentrations show little variation and are generally <2 and <0.10 wt%, respectively. Iron, manganese, and magnesium contents show similar trends to each other (Fig. F14). Iron, manganese, and magnesium concentrations are relatively constant throughout Unit II at levels of 1.5, 0.3, and 1.3 wt%, respectively.

Levels of calcium and strontium are highest in the more carbonate-rich lithologies, ~10-30 wt% for calcium and ~700-1200 ppm for Sr (Units II and IV). Calcium and strontium are <1 wt% and <100 ppm, respectively, in the radiolarian ooze (Subunit IIIA). Phosphorus concentrations are generally <0.5 wt%, and barium concentrations are generally <3000 ppm.

Calcium carbonate (CaCO₃; in weight percent) was determined by coulometric methods for two to three samples per core from 5.23 to 103.23 mbsf for Hole 1221A and for one sample each from Holes 1221B (2.23 mbsf) and 1221C (104.73 mbsf) (Table T12; Fig. F15). CaCO₃ is variable in Units I and II (20-60 wt%) and is low in Subunit IIIA (<1 wt%). CaCO₃ values calculated from Ca inductively coupled plasma-atomic emission spectroscopy data (in weight percent) and salt-fraction data (in weight percent) yielded similar trends to CaCO₃ measured via coulometer, except at high CaCO₃ values, when the calculated values overestimate carbonate, and at values <1 wt%, when calculated values underestimate carbonate (see "Geochemistry" in the "Explanatory Notes" chapter). Organic carbon (C_ORG; in weight percent) determined for one sample per core is uniformly low (<0.2 wt%) for the samples measured.

In summary, the bulk geochemistry of the sediments from Site 1221 reflect the shifts in lithology between sediments dominated by silica and carbonate.

Changes of both shape and absolute concentration in bulk geochemistry seen across the P/E boundary of Site 1221 (Section 199-1221C-11X-3) are similar to those seen at Site 1220 (Table T11; Fig. F16). Silicon, aluminum, and magnesium have elevated concentrations (~20, 5, and 0.3 wt%, respectively) above their Unit IV background concentrations. Over the same interval, calcium is below its Unit IV background concentration, as low as 0.1 wt%. Strontium generally decreases downsection and has a distinct minima centered at interval 199-1221C-11X-3, 72.5-77.5 cm. Strontium levels in the carbonate lithology at the base of the section generally increase downsection (between 154.15 and 154.91 mbsf), possibly reflecting a decrease in the extent of carbonate recrystallization. Iron concentrations peak at ~4 wt% at interval 199-1221C-11X-3, 85-87.5 cm, and titanium concentrations peak at 0.25 wt% at interval 199-1221C-11X-3, 75-77.5 cm, in the section. Manganese concentrations are low overall but show a single peak (11 wt%) at interval 199-1221C-11X-3, 70-72.5 cm, corresponding to the darkest sediments in the section. Phosphorus and barium concentrations show similar trends to each other. Unlike Site 1220 where the maxima were coincident, the barium maximum is at interval 199-1221C-11X-3, 62.5-67.5 cm, and the phosphorus maximum is at interval 199-1221C-11X-3, 85-87.5 cm.

Calcium is extremely high (35-58 wt%) between 10 and 50 cm in this section. Calcium values higher than 35.9% are out of the range of the standards (see "Geochemistry" in the "Explanatory Notes" chapter for more discussion).