GEOCHEMISTRY

We collected interstitial waters from 11 samples in Hole 1219A at depths ranging from 2.95 to 200.45 mbsf, one sample every core for the first six cores and every third core thereafter (Table T14; Fig. F25). Samples were not obtained from the lowermost 40 m of Hole 1219A because the core designated for an interstitial water sample (Core 199-1219A-25X) recovered only chert and subsequent cores were short and possibly contaminated with seawater. Chemical gradients in the interstitial waters at Site 1219 primarily reflect the changing bottom-water salinity of Pacific deep waters through time, relatively limited organic matter diagenesis, dissolution of biogenic silica, and diffusive influence of reactions in the underlying basalt.

Chlorinity (as measured by titration) increases with depth, from values of ~552 mM at 2.95 mbsf to values of ~565 mM at 38.95 mbsf (Fig. F25). The lower than average seawater value of interstitial water at shallow sediment depth (2.95 mbsf) at this site is consistent with the chlorinity of modern Pacific Bottom Waters (~542 mM). Chlorinity increases to a middepth maximum of 565 mM at 38.95 mbsf. Sodium concentrations determined by charge balance were on average 2.6% higher than those measured by ion chromatograph (Table T14). Consistent with the pattern of the chlorinity values, sodium concentrations as determined by charge balance, increase from 481 mM at 2.95 mbsf to a maximum value of 494 mM at 38.95 mbsf and then decrease to 484 mM at 200.45 mbsf. Salinity, as measured by a handheld refractometer, slightly increases downhole from 35.0 at 2.95 mbsf to 35.5 at 200.45 mbsf.

Alkalinity, pH, and sulfate generally follow the same trend of a decrease to a mid-depth minimum and subsequently an increase to the deepest sample. Alkalinity decreases with depth in the uppermost 135 m, to 0.89 mM, and thereafter slightly increases with depth to ~1.06 mM at 200.45 mbsf. The pH also decreases with depth from 7.16 at 2.95 mbsf to 6.82 at 135.45 mbsf and then increases with depth to 7.24 at 200.45 mbsf. Sulfate concentrations are higher than 25 mM throughout the section, and ammonium is present in extremely low levels (<10 µM). Both indicate that the amount of labile organic matter available for oxidation is extremely low. Minima in sulfate and ammonium coincide at 105.45 mbsf.

Dissolved silica concentrations increase with depth, from ~500 µM at 2.95 mbsf to ~1000 µM at 200.45 mbsf. These high interstitial water silica values are consistent with dissolution of biogenic silica throughout the sediment.

The magnitude of the calcium concentration increase and magnesium concentration decrease with depth (2.95-200.45 mbsf) at Site 1219 is significantly greater than that seen at any other Leg 199 site (Sites 1215-1222). This pattern is consistent with alteration of basement rocks, with magnesium replacing calcium during the formation of chlorite. In addition, potassium concentrations show a profile similar to that of magnesium, presumably reflecting the uptake of potassium during basement alteration. These interstitial water profiles are consistent with the recovery of highly altered basalt at Site 1219, unlike at previous sites in the Leg 199 transect.

Strontium concentrations are similar to seawater (87 µM) at 2.95 mbsf, then slightly increase with depth to ~126 µM at 200.45 mbsf. This pattern likely reflects dissolution of the carbonate sediments that occurs between 50 and 150 mbsf, which is consistent with micropaleontological observations (see "Calcareous Nannofossils," "Planktonic Foraminifers," and "Benthic Foraminifers," all in "Biostratigraphy"). Dissolved manganese concentrations are low (<1 µM) from the seafloor to 21.45 mbsf and then rapidly increase downhole (to 76.3 µM at 105.45 mbsf) before showing a subsequent decrease to 61.4 µM at 200.45 mbsf. This Mn profile is the exact opposite of that observed at Site 1218 where Mn values were high at shallow depths and decreased with depth. Lithium pore water values are slightly higher than that of seawater (33 µM) at 2.95 mbsf but decrease with depth to values of 25-26 µM between 38.95 and 200.95 mbsf, which is consistent with lithium uptake during low-temperature basalt alteration. Barium concentrations are low (1 µM), and boron concentrations range from 0.23 to 1.10 µM.

In summary, the pore water profiles from Site 1219 primarily reflect the record of changing deepwater chlorinity, little organic matter degradation, the dissolution of biogenic silica, the alteration of underlying basalt, and subsequent diffusion. The alkalinity, pH, sulfate, and ammonium profiles in the interstitial waters reflect the small amount of organic matter degradation occurring in these sediments. The silica interstitial water profile indicates dissolution of biogenic silica. Calcium, magnesium, potassium, and lithium profiles appear to be controlled by the alteration of basalt and subsequent diffusion to the sediment/water interface.

We collected bulk-sediment samples adjacent to the interval sampled for physical properties in every other section (see "Physical Properties") resulting in a sampling resolution of approximately three per core from 2.23 to 244.38 mbsf in Hole 1219A (Table T15; Fig. F26). 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 radiolarian ooze and clay (Unit I) to nannofossil ooze (Unit II) to radiolarian ooze (Units III) to calcareous chalk (Unit IV) (Fig. F26).

Silicon at Site 1219 averages ~25 wt% between 2.23 and 27.23 mbsf (Unit I), decreasing to an average of 10 wt% in Unit II (30-150 mbsf). Throughout Unit III, silicon increases slightly from 26 wt% at 152.59 mbsf to 38 wt% at 223.35 mbsf. Silicon concentrations reach ~2 wt% near basement (Fig. F26).

Aluminum and titanium concentrations are highest in the clay and siliceous sediments (lithologic Units I and III) and lowest in the carbonate sediments (lithologic Units II and IV). Aluminum concentrations decrease downhole from ~6 to ~0.63 wt% between 2.23 and 150.35 mbsf and then increase to ~3 wt% at 152.59 mbsf. Farther downhole, aluminum concentrations decrease to an average of 0.6 wt% between 184.81 mbsf and basement. Titanium content decreases from ~0.3 to 0.1 wt% within Unit I, is very low (<0.07 wt%) throughout the carbonate sediments (Units II and IV), and is ~0.08 wt% at the top of Unit III, decreasing to 0.03 wt% at the base of Unit III decreasing toward basement.

Al/Ti ratios vary slightly downhole with the greatest variation (8.5-25) throughout Unit III (see Fig. F19 in the "Leg 199 Summary" chapter). The average Al/Ti ratio is high relative to the Post-Archean Average Shale value of 16.7 (Taylor and McLennan, 1985). The Si/Ti ratio is higher in Unit III than in Unit I, possibly reflecting the increased biogenic component of the silicon in the radiolarian ooze relative to the clay (see Fig. F19 in the "Leg 199 Summary" chapter).

Iron and manganese contents show similar trends to aluminum and titanium, with maximum values in Units I and III and values at or near 0 wt% in Unit II (Fig. F26). Iron varies on average between 1 and 3 wt% in Units I and III, with a maximum of 6.3 wt% at 152.59 mbsf at the top of Unit III, and between 0.2 and 2 wt% in Units II and IV. Manganese concentrations are ~0.2 wt% in Unit II and average ~0.5 wt% within Units I and III, with a maximum of 1.4 wt% at 152.59 mbsf (top of Unit III).

Calcium and strontium concentrations are highest in the carbonate-rich lithologies (Units II and IV). Calcium concentrations vary between 0.58 wt% at 2.23 mbsf (Unit I) and 37.4 wt% at 39.73 mbsf (Unit II). The variability in calcium (16-37 wt%) throughout Unit II reflects layers of clay-rich sediment within the nannofossil radiolarian ooze and can be seen as dark to light color cycles in the sediment (see "Unit II" in "Lithostratigraphy"). Strontium concentrations are lowest (~250 ppm) in the siliceous-rich lithologies (Units I and III) and highest (~1600 ppm in Unit II and 600 ppm in Unit IV) in the carbonate-rich lithologies (Units II and IV). Magnesium varies between 0.3 and 2 wt% in the siliceous units (I and III) and between 0.1 and 0.6 wt% in the carbonate-rich sediments.

Phosphorus concentrations are low (generally <0.5 wt% in siliceous and clay sediments and <0.2 wt% in carbonate sediments). Barium concentrations in Site 1219 sediments are highest in the siliceous sediments, varying between 4,500 and 10,000 ppm. Barium concentrations are lower (between 700 and 3000 ppm) throughout the nannofossil oozes and chalks (Units II and IV).

Calcium carbonate (CaCO₃) (in weight percent) was determined by coulometric methods for approximately three samples per core from 2.23 to 244.38 mbsf in Hole 1219A (Table T16; Fig. F27). CaCO₃ is low (1 wt%) in clay-rich Unit 1, high (~60-100 wt%) in Unit II (where carbonate defines the lithology), low to moderate (<1-40 wt%) in siliceous Unit III, and high (up to 96 wt%) in Unit IV. CaCO₃ values calculated from Ca ICP-AES data and salt fraction data (in weight percent) yielded similar trends to CaCO₃ measured via coulometer, although absolute values differ by up to ~10% (see "Geochemistry" in the "Explanatory Notes" chapter). Organic carbon (C_org) (in weight percent) determined for one sample per core is uniformly low (<1 wt%) for the samples measured.

In summary, the bulk geochemistry of the sediments from Site 1219 reflects the shifts in lithology between sediments dominated by silica and carbonate.