A total of 34 headspace samples from Site 1263 (all from Hole 1263A) were analyzed (Table T11). The concentration of CH4 (C1) in most of the samples was at an atmospheric background level (range = 1.6–2.0 µL/L [ppmv]) and did not exceed 3 ppmv in any sample. No hydrocarbon gases higher than C1 were detected.
Interstitial waters from 30 samples were collected at Site 1263: 26 from Hole 1263A (10.2–378.2 mcd) and 4 from Hole 1263B (281.3–313.4 mcd). The samples from the two holes were taken to constitute a single depth profile using the composite depth scale. Slight differences in lithology may cause minor breaks in the concentration-depth gradients of some chemical parameters. Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Explanatory Notes" chapter. Results of the chemical analyses are presented in Table T12.
The pH of pore waters at Site 1263 ranges from 7.29 to 7.70, although this maximum value appears to be anomalous. Excluding the 7.70 maximum, the average pH of Site 1263 interstitial waters is 7.44 ± 0.07 (Table T12). All values are lower than the average seawater value of 8.1, and there is no distinct depth trend. Salinity typically ranges from 34.0 to 35.5 g/kg (mean = 34.8 ± 0.4 g/kg).
Alkalinity is relatively constant with depth and, ignoring a suspiciously high value of 5.93 mM at 55.0 mcd and an anomalously low value of 2.09 mM at 97.5 mcd, the average value is 3.05 ± 0.21 mM (Fig. F28A). Apart from the anomalously low value, all interstitial water samples have a higher alkalinity than average seawater (2.33 mM; International Association of Physical Sciences of the Ocean [IAPSO] certified value).
Chloride concentrations are generally constant with depth (average = 559 ± 18 mM) with the exception of a low value of 476 mM, which occurs at a depth of 77.4 mcd (Fig. F28B). The sodium profile does not exhibit any significant downhole trend, with concentrations between 463 and 486 mM (Fig. F28C).
Site 1263 downhole trends in potassium, calcium, and magnesium are consistent with those resulting from exchange with basaltic basement at depth (Gieskes, 1981), with potassium and magnesium decreasing and calcium increasing slightly with depth (Fig. F28D, F28E, F28F). Calcium values increase from 12.3 mM (10.2 mcd) to 20.5 mM (378.2 mcd) (Fig. F28E). The shallowest Site 1263 interstitial water calcium value of 12.3 mM is higher than that from Site 1262 (8.49 mM at 4.45 mcd) and the mean seawater value of 10.6 mM (IAPSO certified value). The calcium concentration of the deepest (231 mcd) sample from Site 1262 of 17.6 mM is similar to that from the equivalent depth (231.8 mcd) at Site 1263 of 16.9 mM.
The magnesium pore water profile (Fig. F28F) from Site 1263 is characterized by a general decrease with depth, from 54.9 mM in the shallowest sample (10.2 mcd) to 46.9 mM at the base of the section (378.2 mcd). Pore water potassium concentrations decrease slightly with depth from 11.0 mM (10.2 mcd) to 9.30 mM (378.2 mcd) (Fig. F28D).
Strontium concentrations increase downhole from 125 µM in the shallowest sample (10.2 mcd) to peak at values >250 µM for ~70 m (279.5–347.1 mcd) (Fig. F28G). Below 347.1 mcd, strontium values appear to decrease slightly; but with only three samples below this depth, no real assessment of trends can be made. The strontium pore water profile indicates a source of strontium to the interstitial waters between 279.5 and 347.1 mcd and diffusion of this strontium into the sediments above. This input of strontium is most likely from the dissolution and recrystallization of carbonates (e.g., Baker et al., 1982).
Lithium concentrations increase gradually from 28.6 µM in the shallowest sample (10.2 mcd) to 38.4 µM toward the base of the section (378.2 mcd) (Fig. F28H). A very similar lithium pore water profile was observed at Site 1262. The increase with depth suggests a source of lithium from the sediment to the pore waters.
Pore water boron concentrations increase from 462 to 528 µM over the depth interval from 10.2 to 108.4 mcd (Fig. F28I) then decrease downhole to a value of 458 µM at the bottom of the section (378.2 mcd). Laboratory experiments under controlled temperatures and pressures have shown that boron is leached from terrigenous sediments into fluids (e.g., James et al., 2003), and a study of Leg 186 interstitial water samples concluded that the removal of boron from clays and volcanic ash was responsible for boron enrichment in the pore waters (Deyhle and Kopf, 2002). Therefore, the pore water boron peak at 108.4 mcd could indicate either increased concentrations of terrigenous sediment or the enhanced dissolution of terrigenous components in this interval. Barium values fluctuate between ~0.3 and 3.29 µM downhole, with zones of consistently low barium concentrations occurring from 153.7 to 207.0 mcd and below 347.1 mcd (Fig. F28J).
The sulfate pore water profile at Site 1263 is characterized by relative stability at values between 25 and 26 mM from 55.0 to 292.6 mcd. The uppermost samples (10.2–32.5 mcd) exhibit more variability and lower values, whereas samples from below 292.6 mcd display greater variability and generally higher values (Fig. F28K). Below 313.4 mcd XCB drilling was used, and it is possible that the increased sulfate values from this depth and below were caused by increased seawater contamination associated with the XCB technique, although none of the other chemical profiles exhibit such a shift toward seawater values. Overall, the sulfate concentrations at Site 1263 (average = 25.73 ± 1.18 mM) are higher than those recorded at Site 1262 (average = 22.54 ± 0.65 mM), which suggests the sediments at Site 1263 contain even less organic matter than those recovered at Site 1262 (see "Carbonate and Organic Carbon" in "Sediment Geochemistry").
The Site 1263 manganese pore water profile exhibits a large, broad peak extending from 108.4 to 347.1 mcd, climaxing at 231 mcd with a concentration of 7.82 µM (Fig. F28L). The mean concentration above and below this interval is 0.60 ± 0.47 µM. The maximum concentration of manganese in seawater is 3.6 nM (Burton, 1996). Pore water concentrations of dissolved Fe are typically low and invariant throughout the upper 300 m of the section (Fig. F28M). Below 347.1 mcd, iron concentrations increase dramatically to 3.83 µM at 378.2 mcd. The broad peak in pore water manganese concentrations and the subsequent increase in iron below the manganese peak suggest elevated reduction associated with enhanced anaerobic microbial activity (e.g., Malone et al., 2002). Under conditions where reducible iron oxides and manganese oxides are limited, continuing anaerobic microbial activity would shift to reduction of sulfate, and lower sulfate concentrations and increased alkalinity would be expected (e.g., Gieskes, 1981). However, both sulfate and alkalinity pore water profiles are relatively constant through the section, suggesting the large manganese peak and increase in iron may not be associated with anaerobic microbial activity. Instead, it is possible that the manganese and iron pore water enrichments at Site 1263 are the result of inorganic sedimentary diagenesis, which requires further investigation.
Dissolved silicon in pore fluids from Site 1263 increases slightly from 262 µM at a depth of 10.2 mcd to 454 µM at 140.0 mcd. Below 140.0 mcd, the silicon concentrations increase more rapidly downhole and peak at 1498 µM at the bottom of the hole (378.2 mcd). The maximum concentration of dissolved silicon measured in the pore waters of Site 1263 sediments is almost three times the maximum observed at Site 1262 and during the study of Leg 74 interstitial waters (Gieskes et al., 1984). Ash layers have been observed in the sediments from Site 1263 below ~185 mcd and substantial chert deposits were recovered below ~225 mcd, but it seems that these minor siliceous lithologies do not occur in sufficient volume to account for the high silicon concentrations in the pore waters.
Zinc concentrations from Site 1263 pore waters varied between 4.32 and 0 µM (below detection limits) in the upper section (10.2–207.0 mcd). Below 207.0 mcd, the zinc concentrations of the interstitial waters were consistently low and often below the detection limits (F28O).
Although the calcium, potassium, and magnesium interstitial water profiles at Site 1263 suggest that a simple diffusion profile between seawater and basement basalt is responsible for the chemistry of the pore waters, other elements including strontium, lithium, manganese, iron, and silicon indicate that diagenetic processes occurring in the sediments also have a strong impact on the interstitial water chemistry. Further geochemical study of the sediments (shipboard by sediment dissolution inductively coupled plasma–atomic emission spectroscopy) is required to understand the nature of the diagenesis of Site 1263 sediments.
Carbonate determinations by coulometry were made for a total of 163 samples from Site 1263 (Table T13). The values for carbonate are generally high (mean = 85.5 wt%) but range from 1.33 to 98.96 wt% (Table T13; Fig. F29). High-resolution (every 2 to 10 cm) samples were analyzed for carbonate content across the P/E boundary section from Hole 1263C (Fig. F29B) and show a drastic drop from ~85 to 1.33 wt% within <10 cm (mcd) of sediment at the initiation of the P/E event. Above 335.64 mcd, the carbonate values recover to ~40 wt% over a few centimeters before slowly returning to >80 wt% by 334.6 mcd.
Elemental analysis of total carbon indicates low (0.86–0.00 wt%; mean = 0.09 wt%) concentrations of organic carbon in Site 1263 sediments (Table T13), although no samples from the carbonate-barren P/E boundary sediments were analyzed.
Extraction of organic matter was attempted on several sample residues after squeezing interstitial water. Analyzable amounts of extracts were obtained from 40 g of carbonate-rich Samples 208-1263A-17H-5, 145–150 cm (175.2 mcd), and 36X-2, 140–150 cm (357.0 mcd).
The aliphatic hydrocarbon fractions of the samples are dominated by C12–C22 iso- and anteiso-alkanes. It is notable that C17–C19 n-alkanes of algal origin are not major components in Sample 208-1263A-17H-5, 145–150 cm. Long chain alkanes, namely C29, C31, and C33, were detected in Sample 208-1263A-36X-2, 140–150 cm, indicating a contribution of organic matter from higher terrestrial plants.