A total of 30 headspace samples from Site 1265 (all from Hole 1265A) 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) and did not exceed 2.4 ppmv in any sample. No hydrocarbon gases higher than C1 were detected.
Interstitial waters from 26 samples were collected at Site 1265: 19 from Hole 1265A (9.1–351.9 mcd) and 7 from Hole 1265B (165.2–273.3 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 1265 ranges from 7.18 to 7.42 (average = 7.30 ± 0.06) (Table T12). All values are lower than the average seawater value of 8.1, and no distinct depth trend is recognizable. Salinity typically ranges from 34.0 to 35.5 (mean = 35.1 ± 0.4).
Alkalinity is relatively constant with depth (excluding two suspiciously high values of 3.48 mM at 29.7 and 165.2 mcd, plus three anomalously low values of 2.22, 2.17, and 2.25 mM at 9.1, 326.6, and 351.9 mcd, respectively), and the average value is 2.60 ± 0.05 mM (Fig. F31A). Apart from the three anomalously low values, all interstitial water samples have a higher alkalinity than average seawater (2.33 mM; International Association of Physical Sciences of the Ocean certified value).
The chloride concentrations at Site 1265 increase from 556 mM at 9.1 mcd and peak at values of ~568 mM between 19.6 and 73.9 mcd (Fig. F31B). Below this chloride peak, concentrations drop to 560 mM at 116.2 mcd and then generally increase downhole to a maximum of 574 mM at the bottom of the section (351.9 mcd). The sodium profile does not exhibit a significant downhole trend, with concentrations varying between 423 and 479 mM (Fig. F31C).
Site 1265 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 with depth (Fig. F31D, F31E, F31F). Calcium values increase from 11.0 (9.1 mcd) to 21.5 mM (351.9 mcd) (Fig. F31E). The magnitude of the downhole increase in calcium concentrations (10.5 mM) is larger than those observed at Sites 1262, 1263, and 1264.
The magnesium pore water profile (Fig. F31F) at Site 1265 is characterized by a general decrease with depth from 50.0 mM in the shallowest sample (9.1 mcd) to 42.3 mM at the base of the section (351.9 mcd). Pore water potassium concentrations decrease with depth from 10.7 (9.1 mcd) to 7.0 mM (351.9 mcd) (Fig. F31D).
Strontium concentrations increase downhole from 136 µM in the shallowest sample (9.1 mcd) and peak at values between 277 and 319 µM below 148.7 mcd (Fig. F31G). The strontium pore water profile indicates a source of strontium to the interstitial waters below 148.7 mcd and diffusion of this strontium into the sediments above. This strontium source is most likely the result of the dissolution and recrystallization of carbonates (e.g., Baker et al., 1982).
Lithium concentrations increase gradually from 28.3 µM in the shallowest sample analyzed (29.7 mcd) to 40.0 µM at the base of the section (351.9 mcd) (Fig. F31H). Similar lithium pore water profiles were observed at Sites 1262 and 1263 and also below ~150 mcd at Site 1264. The increase with depth suggests a source of lithium from the sediment to the pore waters.
Pore water boron concentrations generally increase from 457 to 501 µM over the depth interval from 9.1 to 72.6 mcd then decrease downhole to a value of 416 µM at the bottom of the section (351.9 mcd) (Fig. F31I). Laboratory experiments under controlled temperatures and pressures have shown that boron is leached from terrigenous sediments into fluids (e.g., James et al., 2003). Likewise, a study of Ocean Drilling Program Leg 186 interstitial water samples (Deyhle and Kopf, 2002) concluded that the removal of boron from clays and volcanic ash was responsible for boron enrichment in the pore waters. Therefore, the pore water boron peak centered at ~72.6 mcd could indicate either increased concentrations of terrigenous sediment or the enhanced dissolution of terrigenous components in this interval.
Pore water barium values fluctuate between 0.19 and 2.97 µM downhole, with a zone of consistently low barium concentrations occurring from 83.5 to 168.8 mcd (Fig. F31J).
The sulfate pore water profile at Site 1265 displays a slight decrease downhole from 26.8 mM in the shallowest sample (9.1 mcd) to 22.8 mM at the base of the section (351.9 mcd) (Fig. F31K). The sulfate concentrations at Site 1265 (average = 24.7 ± 1.1 mM) are high enough to indicate a lack of sulfate reduction associated with the microbial decomposition of organic matter (e.g., Gieskes, 1981).
The Site 1265 manganese pore water profile (Fig. F31L) increases sharply from 0.22 µM at the top of the section (9.1 mcd) to 2.11 µM at 29.7 mcd. Manganese pore water concentrations continue to rise with depth to peak at a value of 3.38 µM at 72.6 mcd before dropping to values between 1.15 and 1.67 µM in the section spanning from 94.5 to 148.7 mcd. After this interval of low values, pore water manganese increases downhole to peak at values between 3.60 and 4.66 µM from 196.1 to 273.3 mcd before dropping to 1.35 µM at the base of the section (351.9 mcd). Below ~182 mcd, dark grains and nodules were apparent in the sediments. A few large (~600 µm in diameter) nodules incorporating planktonic foraminifer tests were picked from washed core catchers 208-1265A-18H and 27H for geochemical analysis (Fig. F32). Similar nodules were analyzed by XRD and found to be composed of calcite and a manganese oxide called lithiophorite (see "Lithostratigraphy"). The manganese oxide nodules are associated with the peak in pore water manganese between 196.1 and 273.3 mcd, but it is unclear whether the nodules contributed to the enhanced pore water concentrations or the higher values of dissolved manganese in this interval resulted in oxide precipitation. Analysis of the manganese oxide nodule samples is necessary before the chemical composition of the nodules can be used to aid the interpretation of the pore water profiles at Site 1265.
Pore water concentrations of dissolved iron are typically low and often below the detection limit throughout the section (Table T12; Fig. F31M), but distinct peaks with iron concentrations between 0.41 and 0.94 µM are present at 40.7, 72.6, 116.2, 206.2, and 351.9 mcd.
Dissolved silicon in pore fluids from Site 1265 decreases slightly from 369 µM at a depth of 9.1 mcd to values of ~250 µM between 29.7 and 105.0 mcd. Below this, silicon concentrations increase to values of ~330 µM between 116.2 and 165.2 mcd before increasing significantly downhole to a peak of 1428 µM at 326.6 mcd. The maximum concentration of dissolved silicon measured in the pore waters of Site 1265 sediments is very similar to that observed at Site 1263 (1498 µM) and is almost three times the maximum observed at Site 1262 and during the study of Leg 74 interstitial waters (Gieskes et al., 1984). Chert deposits were recovered below ~285 mcd (see "Lithostratigraphy"), and it is possible that chert formation processes contributed to the high dissolved silicon concentrations (Hesse, 1990).
Zinc concentrations are consistently low and vary between 1.2 and 0 µM (below detection limits) (Fig. F31O).
Although the calcium, potassium, and magnesium interstitial water profiles at Site 1265 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, and silicon indicate that diagenetic processes occurring in the sediments also have a strong impact on the interstitial water chemistry. The pore water profiles of strontium, lithium, boron, and silicon from Site 1265 show remarkable similarities with the same profiles from Site 1263, suggesting similar process are controlling the pore water chemistry at both sites.
Carbonate determinations by coulometry were made for a total of 182 samples from Site 1265 (Table T13). The values for carbonate are generally high (mean = 89.1 wt%) and range from 30.0 to 98.4 wt% (Table T13; Fig. F33). Closely spaced (every 2 to 10 cm) samples from Hole 1265A were analyzed for carbonate content across the P/E boundary section (Fig. F33B). Carbonate content drops from ~90 to 30 wt% below 316 mcd within <10 cm of sediment. The lowest carbonate values are significantly higher than the 1.33 wt% carbonate values observed in the P/E sediments at Site 1263. Above 315.8 mcd, the carbonate values increase uphole to ~70 wt% over ~30 cm before climbing back to >80 wt%.
Extraction of organic matter was attempted on several sample residues after squeezing interstitial water. Analyzable amounts of extracts were obtained from 15 to 40 g of carbonate-rich lithologies in Samples 208-1265A-5H-5, 140–150 cm (52.1 mcd), 9H-5, 140–150 cm (94.5 mcd), 14H-5, 140–150 cm (148.7 mcd), and 208-1265B-20H-5, 140–150 cm (206.2 mcd). All samples were extracted in an ultrasonic bath for 12 hr or longer in an attempt to collect heavier compounds.
The aliphatic hydrocarbon fraction of samples was dominated by unidentified branched isoprenoids and mono-unsaturated isoprenoids over C12–C22 n-alkanes (Fig. F34). Regular isoalkanes and anteisoalkanes were not observed. Most of the peaks other than n-alkanes remained unidentified; they are divided into two homologous groups based on their mass spectral patterns. Molecules belonging to Group A are saturated hydrocarbons that show mass spectral patterns dominated by m/z 14n + 1, where n represents the carbon number (Fig. F34). Each compound of Group A generally shows a similar mass spectral distribution as n-alkanes, which exhibit an exponential decrease of mass fragment abundances for m/z 71 and larger. However, Group A exhibits considerable depression at m/z 91, 141, 183, and 225 and higher abundances at m/z 155 and 169 relative to the n-alkane with the same carbon number (Fig. F35A, F35B). The m/z 113 from each molecule of Group A characteristically associates with m/z 111, showing twin spectra. Each molecule belonging to Group B is interpreted to be unsaturated. The mass spectral pattern shows prominent abundances at m/z 111 and 153 and a depressed abundance at m/z 97, suggesting similar structure with molecules of Group A in configuration (Fig. F35C, F35D).
Samples 208-1265A-14H-5, 140–150 cm, and 20H-5, 140–150 cm, showed almost identical total ion chromatograms and mass fragment ratios at m/z 85 of aliphatic hydrocarbons with a predominance of homologous Groups A and B, suggesting that the majority of aliphatic hydrocarbons from these samples have a unique source (i.e., molecules belonging to Groups A and B are most likely from a single contaminant). The samples analyzed here are too lean in extractable organic matter to be accurately characterized under the mask of contamination.
Any long-chain alkanes indicative of terrestrial higher plants, namely n-C27–33 with strong odd carbon number preference in hexane eluates, or longer straight-chain alkenones (C37 or larger) in dichloromethane eluates were not detected in any samples.