The concentration of CH4 (C1) in 28 headspace samples analyzed from Site 1266 was at an atmospheric background level (range = 1.6–2.0 µL/L); no hydrocarbon gases higher than C1 were detected.
Interstitial water from 27 samples was collected from Hole 1266A, extending from 10.5 to 342.1 mcd. 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 T11.
The pH of pore waters from Site 1266 ranges from 7.1 to 7.5 (average = 7.3 ± 0.1) (Table T11). All values are lower than the average seawater value of 8.1 and generally decrease with depth. Salinity values range from 35.0 to 35.5 g/kg.
Alkalinity decreases slightly from 3.3 mM in the shallowest sample at 10.5 mcd to 1.9 mM at 330.2 mcd (Fig. F27A). One sample had an anomalously high value of 4.9 mM at 151.5 mcd.
The pore water chloride concentration increases slightly with depth from a minimum value of 561 mM (10.5 mcd) to 569 mM (330.2 mcd) (Fig. F27B). Three samples contain lower chloride values of 503 mM (107.8 mcd), 513 mM (129.7 mcd), and 510 mM (173.4 mcd). Excluding these values, the mean pore water chloride value is 565 mM (standard deviation = 6 mM).
Sodium concentrations average 480 mM (standard deviation = 10.2 mM) over the entire pore water profile, with no distinct trend aside from an initial increase from 452 to 497 mM from 10.54 to 21.47 mcd (Fig. F27C).
Site 1266 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 downhole (Fig. F27D, F27E, F27F). Pore water potassium concentrations decrease slightly from 9.9 mM (10.54 mcd) to 6.8 mM (342.1 mcd) (Fig. F27D). Calcium values increase from 10.4 mM (10.54 mcd) to 26.9 mM (319.2 mcd) (Fig. F27E). The magnesium profile (Fig. F27F) is characterized by a general decrease with depth from 53.6 mM in the shallowest sample (10.54 mcd) to 40.1 mM at the base of the section (342.1 mcd). Superimposed upon this trend is an increase from 42.4 mM (151.5 mcd) to 49.0 mM (238.9 mcd).
Strontium concentrations increase from 165 µM (10.54 mcd) to 452 µM (319.2 mcd). Below this depth, strontium values remain high (>420 µM) to the base of the analyzed record (342.1 mcd) (Fig. F27G). These high pore water concentrations may be related to carbonate diagenesis, where the dissolution of biogenic calcite and subsequent reprecipitation of inorganic diagenetic calcite supplies dissolved strontium to the interstitial waters (e.g., Baker et al., 1982).
The lithium concentration profile exhibits a slight decrease from 24.6 to 23.7 µM over the interval from 10.54 to 30.89 mcd then increases gradually to 47.0 µM at the base of the section (342.1 mcd) (Fig. F27H). This trend suggests a minor shallow subsurface sink for dissolved lithium and a deeper source of lithium from the sediment into the pore waters.
Boron values increase with depth from 435 µM (10.54 mcd) to 618 µM (97.94 mcd), remain relatively high (>485 µM) to 246.9 mcd, then decrease to 426 µM at the base of the profile (342.1 mcd) (Fig. F27I). Pore water barium concentrations range between background values of <0.5 and 2.6 µM (Fig. F27J). Elevated barium concentrations do not correlate with any decrease in pore water sulfate concentrations that might suggest enhanced barite solubility.
The pore water profile from Site 1266 is characterized by a general decrease in sulfate from 26.2 mM (10.5 mcd) to 20.9 mM (330.2 mcd) (Fig. F27K). One sample had a relatively low sulfate concentration of 19.1 mM (265.4 mcd). The relatively high concentrations of sulfate (mean = 23.8 ± 1.5 mM) reflect the very low organic matter content of the sedimentary section recovered at Site 1266 (see "Carbonate and Organic Carbon").
Pore water manganese values decrease from 3.82 µM at the top of the profile (10.5 mcd) to values <1.0 µM in the interval from 30.9 to 65.2 mcd. Below this depth, values increase to a local maximum value of 4.32 µM (87.0 mcd) and then decrease again to 0.47 µM (151.5 mcd). Manganese values remain low (<1.0 µM) to 205.3 mcd then increase to >5.0 µM (271.8 mcd) (Fig. F27L).
Pore water concentrations of dissolved iron are below detection limit throughout much of the interval analyzed (Fig. F27M). However, concentrations rise from 0.26 to 8.34 µM at 54.2 mcd. Dissolved iron concentrations then decrease to 0 at 87.0 mcd. Maximum iron concentrations correspond to a decrease in sediment chromaticity (a*), suggesting a source of reduced iron from the sediments to pore waters in the interval from ~8 to 55 mcd (see Fig. F13; "Lithostratigraphy").
Pore water silicon concentrations (Fig. F27N) average 269 µM over the interval from 10.5 to 205.3 mcd then increase to 1446 µM at the base of the profile (342.1 mcd). This increase coincides with the increase in pore water manganese concentrations at ~239 mcd.
The Site 1266 pore water profiles of potassium, calcium, and magnesium reflect the diffusional gradient between seawater and basalt. In contrast, the profiles of strontium, lithium, boron, silicon, and part of the manganese and iron records are dominated by sedimentary contributions of dissolved ions to the pore waters. Little evidence of microbial influence exists in these profiles as reflected in the sulfate, manganese, and iron profiles.
Carbonate determinations by coulometry were made for a total of 257 samples from Site 1266 (Table T12). Low-resolution samples were selected to provide a measure of the carbonate content within different units, and two sets of high-resolution samples were taken to assess the magnitude of carbonate decrease within the P/E boundary interval (305.0–307.6 mcd) and clay horizon (292.0–293.0 mcd). The carbonate values in lithostratigraphic Unit I average 94.7 wt%. In Unit II, carbonate values decrease to 69.6 wt% (97.9 mcd) then increase to 93.9 wt% (129.0 mcd). From 129.0 to 274.9 mcd, carbonate values average 90.0 wt% (Table T12; Fig. F28).
The interval spanning 281.2 to 306.8 mcd contains two notable clay-rich horizons (intervals of low carbonate): the P/E boundary interval (305.0 to 307.5 mcd) and another clay horizon spanning 292 to 293 mcd (Fig. F28). Description of these detailed records will proceed from the base of the interval upward. A composite record of the P/E clay interval (Fig. F28B) constructed from all three holes (Sections 208-1266A-31X-3, 208-1266B-6H-7, and 208-1266C-17H-3) indicates that carbonate contents drop from 92.6 wt% below the base (Sample 208-1266A-31X-3, 70 cm; 306.8 mcd) to 0.5 wt% (Sample 208-1266C-17H-3, 110 cm; 302.8 mcd) at the base of the P/E clay interval. Carbonate values remain below 1.0 wt% for 6 cm (306.69 mcd) and then gradually increase upsection to ~90 wt% at 306.18 mcd.
High-resolution sampling of an additional clay-rich interval (Fig. F28) indicates that carbonate values decrease from 91.9 wt% (Sample 208-1266B-5H-5, 100 cm; 292.78 mcd) to 47.7 wt% (Sample 5H-5, 88 cm; 292.66 mcd). Above this minimum, carbonate values gradually increase to 94.1 wt% (Sample 208-1266B-5H-5, 75 cm; 292.53 mcd).
Four samples were selected for organic matter extraction: squeeze cake Samples 208-1266A-2H-5, 140–150 cm (21.5 mcd), and 13H-5, 140–150 cm (140.6 mcd), and core catcher Samples 208-1266C-11H-CC, 0–1 cm (185.2 mcd), and 12H-CC, 0–1 cm (196.7 mcd).
Mass chromatograms (m/z 85) for all samples analyzed were dominated by n-C14 through n-C18 alkanes (Fig. F29), with minor occurrences of branched isoprenoids in each aliphatic hydrocarbon fraction (except for pristane and phytane). This feature is generally observed in the sediments of the Miocene interval and above from the previous sites of Leg 208. Samples 208-1266C-11H-CC, 0–1 cm, and 12H-CC, 0–1 cm (Fig. F29), indicate that this character of aliphatic hydrocarbons extends down to at least the upper lower Oligocene but no deeper than the middle Eocene as shown by an analysis of Sample 208-1263A-17H-5, 140–150 cm. The Site 1266 results support findings from other Leg 208 sites that a turnover of the major primary producer occurred between the middle Eocene and late early Oligocene.
Pristane and phytane were also abundant in the aliphatic hydrocarbon fraction of all samples (Figs. F29, F30). This character is particularly distinct in Sample 208-1266A-13H-5, 140–150 cm (Fig. F30), in which pristane and phytane showed comparable abundances of n-C17 and n-C18, respectively.
The aliphatic hydrocarbon fraction from Sample 208-1266C-2H-5, 140–150 cm, contained the long-chain alkanes n-C27 through n-C33, with a strong predominance of the odd carbon number indicating a contribution of terrestrial higher plants (Fig. F30). Leaf wax of terrestrial higher plants is believed to be the sole source of long-chain n-alkanes with a strong odd carbon preference in thermally immature sediments (e.g., Tissot and Welte, 1984). The largest abundance among the n-C27–33 in Sample 208-1266C-2H-5, 140–150 cm, is characteristically marked by n-C31 instead of n-C29, suggesting a specific origin among a variety of terrestrial plants. A similar distribution of long-chain n-alkanes was observed in Sample 208-1262B-1H-3, 140–150 cm.