GEOCHEMISTRY

We collected interstitial waters from 11 samples from Site 1220 at intervals of approximately one sample every core for the first six cores and every third core thereafter (Table T13; Fig. F19). Eight samples from Hole 1220A, at depths ranging from 4.45 to 110.45 mbsf, and three samples from Hole 1220B, at depths ranging from 125.89 to 173.55 mbsf, together comprise the interstitial water profile for Site 1220. Chemical gradients in the interstitial waters at Site 1220 primarily reflect relatively limited organic matter diagenesis, dissolution of biogenic silica, little carbonate, and possibly diffusive influence of reactions in the underlying basalt.

Chlorinity, as measured by titration, increases with depth from 554 mM at 4.45 mbsf to a mid-depth maximum of 566 mM at 80.45 mbsf (Fig. F19). This pattern is very similar to the chlorinity pattern at Site 1219. 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 were on average 2.4% lower than those measured by ion chromatograph and are consistent overall with the chlorinity values (Table T13). Sodium concentrations determined by charge balance are low (477mM, as compared to the average seawater value of 480 mM) at 4.45 mbsf and increase to a middepth maximum (487 mM) at 80.45 mbsf. Salinity, as measured by a handheld refractometer, varies slightly from 35.0 in the shallower part of the hole (4.45-51.95 mbsf) to 35.5 in the deeper part of the hole (80.45-173.55 mbsf).

Alkalinity generally decreases with depth from 4.45 to 173.55 mbsf. The pH varies between 7.14 at the seafloor (4.45 mbsf) to 7.39 at the bottom of the profile (173.55 mbsf). Like all other Leg 199 sites, sulfate concentrations are high (28.6 ± 1 mM) throughout the profile, indicating little oxidation of labile organic matter. Ammonium, a byproduct of organic matter degradation, is present in extremely low levels (5 µM).

Dissolved silica concentrations increase with depth, from 556 µM at 4.45 mbsf to ~900 µM at 154.45 mbsf. These high interstitial water silica values are consistent with dissolution of biogenic silica throughout the sediment.

Calcium and magnesium concentrations are similar to those seen at all other Leg 199 sites except Site 1219. At Site 1220, calcium, magnesium, and potassium profiles show little evidence for exchange with basalt and subsequent diffusion. Calcium concentrations increase slightly downhole, from 10.7 mM (4.45 mbsf) to 13.1 mM (173.55 mbsf). Magnesium concentrations increase from 51.5 mM (4.45 mbsf) to 59.0 mM (125.89 mbsf) and then decrease to 55.0 mM in the deepest sample (173.55 mbsf). Potassium concentrations show only a small decrease with depth, from 12.4 mM (4.45 mbsf) to 10.6 mM (154.45 mbsf). Lithium pore water values are similar to that of seawater (27 µM) at 4.45 mbsf and show a small decrease with depth, consistent with alteration with basalt and subsequent diffusion.

Strontium concentrations (82 µM) are lower than seawater value (87 µM) at the top of the hole but show a small increase to seawater value at 23 mbsf. Dissolved manganese was 27 µM in the shallowest sample (4.45 mbsf) but was below detection limit for all other samples. Barium concentrations are low (<0.6 µM) throughout the sediment column; the deepest sample at 173.55 mbsf in Hole 1220B was below detection limit (0.04 µM). Manganese concentrations are below detection limit. Boron concentrations range from 423 to 505 µM.

In summary, the pore water profiles from Site 1220 primarily reflect minor organic matter degradation, the dissolution of biogenic silica, and minor alteration of underlying basalt. Overall, the profiles from Site 1220 are very similar to the profiles of all other Leg 199 sites except Site 1219.

We collected bulk-sediment samples adjacent to the interval sampled for physical properties in every other section (see "Physical Properties") at Site 1220, resulting in a sampling resolution of approximately three samples per core. The profile is composed of sediments from 2.24 to 112.73 mbsf for Hole 1220A and from 114.23 to 193.14 mbsf for Hole 1220B (depths in both mcd and mbsf are provided in Table T14 and Fig. F20). We also analyzed continuous scrapings (26 samples of 2-7 cm in width between 198.90 and 199.79 mbsf) from the P/E boundary core section (199-1220B-20X-2), resulting in the first ever shipboard analysis of bulk-sediment geochemistry across this paleoceanographically important interval.

Bulk-sediment geochemistry primarily reflects the changing lithology of the sediments downhole from clay (Unit I) to radiolarian ooze (Unit II) to nannofossil ooze (Unit III) to radiolarian ooze (Unit IV) and, finally, to nannofossil chalk (Unit V) (Fig. F20). Trends in the P/E boundary section are discussed separately.

Silicon in Site 1220 sediments is <25 wt% in lithologic Units I-III (0-70 mcd) and increases to ~30 wt% in Unit VI (70-190 mcd). Silicon concentrations fall to <5 wt% in Unit V (Fig. F20). Aluminum and titanium concentrations are linearly related (R = 0.96). Aluminum concentrations are high in the clay sediments (~6 wt%; Unit I) and low in the carbonate and siliceous sediments (generally <3 wt%; Units II-V). Titanium concentrations show the same pattern and are generally 0.30 wt% in Unit I and <0.05 wt% in Units II-V.

Iron, manganese, and magnesium contents show similar trends to each other (Fig. F20). All are generally highest in the clay sediments (Unit I), decrease to a mid-depth minimum in Unit III at ~75 mcd, and then increase in Unit IV. Iron decreases from 4.5 to <1 wt% between 2.24 and 76.39 mcd then increases to a peak of 4.2 wt% at 100.06 mcd. Iron content then decreases to 0.30 wt% at 208.84 mcd and remains low throughout Unit V. Manganese concentrations are slightly elevated (up to 1.34 wt%) in Unit I (between 0 and 30 mcd) and subsequently decrease to <0.60 wt% throughout Unit V. Magnesium concentrations decrease from 2.10 wt% at the top of Unit I to 0.30 wt% at the bottom of Unit III and increase slightly to ~1.3 wt% in Units IV and V.

Calcium and strontium concentrations are highest in the more carbonate-rich lithologies (Units II, III, and V). Calcium concentrations are low in Units I and IV (<1 wt%) and high in Units III and V (as high as 40 wt%, although these values are out of the range of the standards) (see "Geochemistry" in the "Explanatory Notes" chapter). The variability in calcium (7-35 wt%) throughout Unit III reflects layers of alternating nannofossil ooze and radiolarian ooze in the sediment (see "Unit III" in "Lithostratigraphy"). Strontium concentrations are lowest (as low as 100 ppm) in Units I and IV and highest (as high as 2000 ppm) in the carbonate-rich lithologies (Units III and V).

Phosphorus and barium concentrations in Site 1220 sediments follow each other (R = 0.8). Phosphorous concentrations are low (generally <0.5 wt%). Barium concentrations in Site 1220 sediments are variable (between 800 and 13,000 ppm).

CaCO₃ (in weight percent) was determined by coulometric methods for three samples per core from 2.24 to 211.85 mcd for Site 1220 (Table T15; Fig. F21). CaCO₃ is low (1 wt%) in clay-rich Unit 1, variable (~20-100 wt%) in Units II and III, low (<1 wt%) in siliceous Unit IV, and high (up to 87 wt%) in Unit V. CaCO₃ values calculated from inductively coupled plasma-atomic emission spectroscopy determined Ca data (in weight percent) and salt fraction data (in weight percent) yielded similar trends to calcium carbonate measured via the coulometer, although absolute values are generally lower for CaCO₃ <1 wt% and higher for CaCO₃ >1 wt% (see "Geochemistry" in the "Explanatory Notes" chapter).

In summary, the bulk geochemistry of the sediments from Site 1220 reflect the shifts in lithology between sediments dominated by silica and carbonate.

Significant changes in bulk geochemistry are seen across the P/E boundary (Table T14; Fig. F22). Continuous scrapings at intervals of 2-7 cm from Section 199-1220B-20X-2 (between 198.90 and 199.79 mbsf) were analyzed. The highest concentrations of Fe, Mn, Ca, Mg, Ba, and P for Site 1220 are present in this core section. Silicon, aluminum, and magnesium show similar trends of elevated concentrations (~20, 4, and 3.7 wt%, respectively) above their Unit V background concentrations between 47 and 77 cm in this section. Over the same interval, calcium shows the opposite trend (~6 wt% below its Unit V background concentration). Strontium generally decreases downsection to the dark brown lithology, has a distinct minima centered at 65-67 cm, and an isolated peak at 69-73 cm in the section. Strontium concentrations in the carbonate lithology at the base of the section generally increase downsection, away from the color change, possibly reflecting a decrease in the extent of carbonate recrystallization. Manganese concentrations are low overall but show a single peak (9 wt%) at 60-63 cm, corresponding to the darkest sediments in the section. Iron concentrations increase to ~4 wt% at 47-54 cm and peak at 7 wt% at 75-77 cm in the section. Likewise, titanium concentrations increase to ~0.31 wt% at 52-54 cm and peak at 0.42 wt% at 65-67 cm in the section. Phosphorus and barium concentrations show similar trends, increasing between 45-47 cm and 58-60 cm, decreasing to a minima at 65-67 cm, and increasing to a maximum at 69-73 cm.