A total of 23 headspace samples from Site 1262 (17 from Hole 1262A, 3 from 1262B, and 3 from 1262C) 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 ppmv [µL/L]). Ethane (C2), ethylene (C2=), propane (C3), and propene (C3=) were detected in three samples from the top of Hole 1262B and near the bottom of Hole 1262A, showing low C1/C2 ratios; however, the concentration of these gases never exceeded 10 ppmv.
Interstitial water from 20 samples was collected at Site 1262: 14 from Hole 1262A (13.5–179.0 mcd), 4 from Hole 1262B (4.5–231.0 mcd), and 2 from Hole 1262C (201.4–210.2 mcd). The samples from the three holes were taken to constitute a single depth profile using the composite depth scale. However, slight differences in lithology may cause minor breaks in 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 1262 ranges from 7.4 to 7.6 (average = 7.50 ± 0.06) (Table T12). All pH values are lower than the average seawater value of 8.1, and the general trend is a decrease with depth. Salinity typically ranges from 34.0 to 35.5, although an anomalous value of 37.0 was recorded at 128.5 mcd.
Alkalinity decreases slightly from 2.6 mM in the shallowest sample at 4.5 mcd to 2.3 mM at 231.0 mcd (Fig. F29A). The maximum alkalinity of 2.9 mM occurs at 54.4 mcd; the average alkalinity through this interval is 2.50 ± 0.15 mM. Below ~150 mcd, alkalinity values vary more than those in the upper 150 m.
The chloride concentrations generally increase with depth from a minimum value of 565 mM (13.5 mcd) to 574 mM (210.2 mcd) (Fig. F29B) (mean = 564 ± 18 mM). Several samples in this interval contain lower chloride values (527 mM at 43.5 mcd and 550 mM at 201.4 mcd). The highest value (577 mM) occurs at 170.9 mcd.
Sodium concentrations are more variable than those of chloride, with values ranging from 457 to 474 mM (Fig. F29C). The sodium profile does not exhibit any significant downcore trend or correlation to changes in chloride concentration.
Site 1262 downcore 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. F29D, F29E, F29F). Pore water potassium concentrations decrease slightly from 10.8 mM (4.5 mcd) to 9.9 mM (231.0 mcd) (Fig. F29D). Calcium values increase from 8.5 mM (4.5 mcd) to 17.6 mM (231.0 mcd) (Fig. F29E). The magnesium profile (Fig. F29F) is characterized by a general decrease with depth, from 53.0 mM in the shallowest sample (4.5 mcd) to 40.4 mM at the base of the section (231. 0 mcd).
Strontium concentrations increase from 110 µM (4.5 mcd) to 194 µM (54.4 mcd). Below this depth, strontium values decrease to 146 µM (231.0 mcd), indicating removal of strontium at depth (Fig. F29G). The slight convex-upward nature of the upper part of the strontium pore water profile suggests that some carbonate dissolution has occurred in the upper ~75 mcd of the sediment column (e.g., Baker et al., 1982). In general, lithium concentrations increase gradually from 29 µM in the shallowest sample (4.5 mcd) to 40 µM toward the base of the section (210.2 mcd) (Fig. F29H). The increase with depth suggests a source of lithium from the sediment or underlying basement rock into the pore waters.
Boron values exhibit a slight increase with depth, from 468.6 µM (4.5 mcd) to 530.9 µM (231.0 mcd) (Fig. F29I). Superimposed on this trend is a peak to 757.1 µM (65.7 mcd). Values decrease back to 499.2 µM at 103.4 mcd. Pore water barium concentrations increase from 0.6 µM to 2.5 µM over the depth interval from 4.5 to 13.5 mcd (Fig. F29J) then decrease downcore to a value of 0.04 µM at the bottom of the section (231.0 mcd).
The pore water profile at Site 1262 is characterized by a general downhole decrease in sulfate from 4.5 to ~140 mcd (Fig. F29K). The relatively high concentrations of sulfate (mean = 22.54 ± 0.65 mM) reflect the very low organic matter content of the sedimentary section recovered at Site 1262 (see "Carbonate and Organic Carbon" in "Sediment Geochemistry"). The manganese pore water profile exhibits a broad maximum extending from 23.2 to 54.4 mcd (Fig. F29L) and reaching values up to 2.9 µM. The mean concentration above and below this interval is 0.26 ± 0.21 µM. The peak concentrations may be consistent with slightly enhanced reduction of manganese oxides; however, there is no subsequent increase in reduced iron below this manganese peak that would indicate enhanced microbial reduction. The base of the dissolved manganese peak also corresponds to the upper portion of a lithologic boundary between carbonate ooze above ~45 mcd and clay below (see "Lithostratigraphy").
Pore water concentrations of dissolved iron are typically low and invariant throughout the interval analyzed (Fig. F29M). The mean iron concentration is 0.15 ± 0.12 µM. Given the relatively high pore water sulfate and lack of elevated iron concentrations throughout the interval, anaerobic microbial activity was probably not sufficiently intense to cause reduction of Fe(III) to Fe(II).
Pore water silicon concentrations (Fig. F29N) decrease from the shallowest value of 385 µM (4.5 mcd) to 239 µM (43.5 mcd) then increase to 369 µM (54.4 mcd). The remainder of the profile is variable with a slight overall silicon increase to 412 µM (231.0 mcd). Zinc concentrations from Site 1262 pore waters were low and relatively constant, ranging from 0.9 to 1.4 µM (Fig. F29O). One sample (33.8 mcd) yielded a concentration of 2.9 µM, but this is most likely an anomalous value.
The Site 1262 pore water profiles described above are dominated by the diffusional gradient between seawater and basalt of the underlying basement. This relationship is demonstrated in the potassium, calcium, and magnesium profiles. Little evidence of enhanced microbial influence exists in these profiles, as is reflected in the sulfate, manganese, and iron profiles.
Carbonate determinations by coulometry were made for a total of 134 samples from Site 1262, 11 from Hole 1262A, 21 from Hole 1262B, and 2 from Hole 1262C (Table T13). Samples were selected to provide a measure of the carbonate content in different lithostratigraphic units (Fig. F30A) and to assess the influence of carbonate content on L*. High-resolution samples (every 2–5 cm) were also taken to assess the change in carbonate content across the P/E boundary (Fig. F30B). The carbonate values in Unit I are ~90 wt%, except for one anomalous value of 46 wt%. Carbonate contents in Subunit IIA are very low (mean = 4.3 wt%). Subunit IIB contains higher carbonate contents (mean = 68.1 wt%), whereas Subunit IIC values increase downsection from 8 to 93 wt%. Subunit IIIA is characterized by high carbonate contents (mean = 92.4 wt%). Values of Subunit IIIB sediments are lower and more variable, ranging from 20 to 97 wt%. High-resolution carbonate determinations across the P/E boundary section of Hole 1262A (Fig. F30B) reveal a gradual decline in carbonate content below the clay interval from ~85 wt% above 140.31 mcd to ~77 wt% at 140.16 mcd. Between 140.13 and 140.11 mcd, the carbonate content drops from >70 to <1 wt%. Carbonate contents of <1 wt% persist from 140.11 mcd uphole until 139.96 mcd (15 cm). From 139.93 to 139.51 mcd, carbonate contents rise to >90 wt%.
Elemental analysis of carbon indicates generally low concentrations of organic matter from Site 1262 sediments (Table T13). Several of the samples analyzed contained organic carbon concentrations slightly greater than zero, with values ranging from 0.0 to 0.3 wt%. Eleven samples yielded conspicuously high organic carbon values, most likely attributable to analytical artifacts and error (i.e., subtracting inorganic carbon values obtained by coulometry from total carbon values obtained by combustion to calculate organic carbon). None of the analyzed samples contained measurable nitrogen.
Extraction of organic matter was attempted on several sample residues after squeezing interstitial water. Analyzable amounts of extracts were obtained from carbonate-rich interval 208-1262B-1H-3, 145–150 cm (4.5 mcd), and clay-rich intervals 208-1262A-6H-5, 145–150 cm (65.6 mcd), and 8H-2, 145–150 cm (83.4 mcd). Carbonate-rich sediments from intervals 208-1262A-12H-5, 140–150 cm (128.5 mcd), and 13H-4, 140–150 cm (138.0 mcd), did not yield hydrocarbons detectable with the gas chromatography–mass spectrometry selective detector (GC-MSD).
Total ion chromatography indicates that the aliphatic hydrocarbon fraction of interval 208-1262B-1H-3, 145–150 cm, is dominated by n-alkanes of algal origin. In contrast, the aliphatic fraction of clay-rich interval 208-1262A-8H-2, 145–150 cm, contains branched isoprenoids, specifically anteiso-alkanes against n-alkanes (Fig. F31), suggesting a cyanobacterial contribution to the sedimentary organic matter. No long-chain alkanes indicative of terrestrial input were detected. Compounds identified with the GC-MSD are listed in Table T14.