Concentrations of volatile hydrocarbon gases were measured from every core using the standard Ocean Drilling Program (ODP) headspace sampling technique and gas chromatographic analysis. Methane only occurred in very minor concentrations (1.7-6.6 ppmv) (Table T8).
The low gas content at Site 1198 is likely a function of appreciable pore water SO42- concentrations limiting methanogenesis and the lack of mature organic matter that could provide a thermogenic component to the gas fraction.
Pore waters were extracted from 40 samples at Site 1198 (Fig. F17; Table T9). Twenty-three samples were taken approximately every 10 m in Hole 1195A, from 2.90 to 204.40 mbsf through lithologic Unit I (see "Lithostratigraphy and Sedimentology"). In Hole 1198B, one sample was taken from a depth of 196.60 mbsf at the base of lithologic Unit I. From 197 to 350 mbsf, poor recovery of the unconsolidated sediments of lithologic Subunits IIA and IIB precluded pore water sampling. Sampling resumed in Hole 1198B at 350 mbsf, with samples taken at ~10 m intervals to a depth of 504.98 mbsf. Below 460 mbsf, it was not possible to extract sufficient water to complete the entire suite of shipboard chemical analyses. Results from Holes 1198A and 1198B are discussed together (Fig. F17; Table T9).
Chloride concentration increases from 560 to 565 mM over the upper 30 mbsf, and then remains between 564 and 561 mM to a depth of 115 mbsf (Fig. F17A). In the interval from ~100 to ~160 mbsf, chloride concentration increases to 569 mM; the concentration then falls to 563 mM at 180 mbsf. In the lower 20 m of lithologic Unit I, the concentration varies between 563 and 568 mM. Below the interval of no recovery, chloride remains ~567 mM from 350 to 390 mbsf. From 390 mbsf to the bottom of the cored interval, the concentration varies considerably, from 558 to 570 mM (Fig. F17A).
Alkalinity values rise steadily from typical bottom-water values of ~2.5 mM in the shallowest sample to ~4.5 mM at 100 mbsf (Fig. F17B). Values fall back to ~2.5 mM over the next 100 mbsf. This pattern observed in pore water alkalinity in the upper 200 mbsf is a pattern repeated in several other pore water constituents. In lithologic Subunit IIC and Unit III, alkalinity varies within narrow limits, from 2 to 2.5 mM.
Sulfate concentrations fall from 28.6 to ~22 mM over the interval from 2.9 to ~80 mbsf, remain constant to 115 mbsf, and then rise again to 28.6 mM at 200 mbsf (Fig. F17C). At the top of lithologic Subunit IIC at 350 mbsf, the sulfate concentration is 29.9 mM. From 350 to 505 mbsf, sulfate decreases linearly from 29.9 to 16.42 mM.
Ammonium concentration rises from 145 to ~850 µM in the upper 60 m of Hole 1198A (Fig. F17D). The concentration remains relatively constant to a depth of 115 mbsf then decreases to 246 µM at the base of lithologic Unit I. At the top of lithologic Subunit IIC at 350 mbsf, the ammonium concentration is 103 µM. Thereafter, the concentration rises to 210 µM at 488 mbsf then falls again to 152 µM in the lowermost sample at 505 mbsf.
Pore water magnesium concentration decreases from a near-seawater value of 56.45 to ~42 mM from the top to bottom of lithologic Subunit IA (Fig. F17E). Through lithologic Subunits IB and IIA, the concentration increases to 48 mM. Below the interval in which pore waters could not be sampled, the magnesium concentration is initially 52 mM and remains between 52 and 50 mM to a depth of 450 mbsf. From 480 to 505 mbsf, the magnesium concentration decreases rapidly to 29.5 mM at the base of Hole 1198B.
Calcium concentration remains almost constant at ~10 mM in the upper 60 mbsf then rises linearly to 19.1 mM at the base of lithologic Unit I (Fig. F17F). At the top of lithologic Unit III, the calcium concentration is 17.1 mM. Below this point, the calcium concentration rises rapidly with depth, reaching 135.3 mM at 505 mbsf, just above the basaltic basement.
Strontium concentration rises rapidly in the upper 60 mbsf, reaching a concentration of 616 µM at 58 mbsf (Fig. F17G). Concentration remains between 624 and 588 µM from 58 to 144 mbsf and then decreases downhole to a value of 246 µM at 204.4 mbsf, the base of lithologic Unit I. The increase in strontium with depth at this site is more rapid than for other Leg 194 sites as a result of the significant aragonite component in the sediments. The constant concentration between 60 and 140 mbsf suggests a solubility control, possibly as saturation with celestite is reached. In lithologic Unit III, the strontium concentration is 276 µM at the top and increases to ~700 µM between 440 and 460 mbsf.
Potassium concentration changes little in the upper 200 mbsf (Fig. F17H), varying between 11 and 9.6 mM. Nevertheless, a clear curved pattern of initially rising then falling concentration is seen. From 200 to 350 mbsf is found the interval of no recovery. At 350 mbsf, the concentration is 10.4 mM, and it decreases steadily through the remainder of the cored interval to a value of 3.58 mM near the base of Hole 1198B.
For several pore water constituents, nearly symmetrical, arcuate pore water profiles are found in the upper 200 mbsf. Concentrations increase (decrease) from near seawater values in the interval from 0 to 100 mbsf then decrease (increase) in the interval from 100 to 200 mbsf, returning to concentrations close to those found in seawater. The changes in concentration seen in the lower half of this sediment package are quite unusual. Sulfate concentration, for example, initially decreases as a result of bacterial sulfate reduction. A cessation of this process in the lower part of lithologic Unit IB is an unlikely cause for the increase in sulfate concentration because there is no decrease in the organic carbon content within this unit. More likely is that sulfate reduction occurs throughout this sediment package, but sulfate is supplied by diffusion from both above and below. The near-seawater concentration of sulfate (and other constituents) at the base of these sediments strongly suggests active circulation of seawater through the sediment package between 200 and 350 mbsf. Neither the mechanism nor direction of fluid flow can be determined from the available data.
The high concentration of calcium in the lower portion of the hole is clear evidence for a diagenetic flux of calcium out of basaltic basement (Fig. F17F). During this mineralogical reaction, calcium production must be balanced by the uptake of another element. The observed magnesium concentrations cannot alone provide a balance for calcium. The only other significant cation in solution is sodium. Sodium concentration, however, is usually calculated by charge balance, which automatically leads to rapidly decreasing sodium concentration as calcium rises because there is no offsetting rise in the concentration of any anion. Sodium is also measured during ion-chromatographic determination of calcium, magnesium, and potassium, although the values are not usually reported. Figure F18 shows a comparison of sodium concentration as determined by both charge balance and ion chromatography. The results match well except for the lower 50 m of Hole 1198B, where results from the charge-balance calculation are significantly below the ion chromatography measurements. Both pore water profiles, however, show a large decrease in sodium concentration in the lower portion of the hole. Thus, the large increase in calcium concentration is dominantly offset by a decrease in sodium, suggesting formation of a sodium-rich mineral in the basaltic basement. The most likely candidate is natrolite (Na2Al2Si3O10 · 2H2O), a zeolite that is a common alteration product in the vugs and pore spaces of the basalt.
A total of 109 samples were analyzed for carbonate mineralogy from Site 1198 (Fig. F19; Table T10). At the sediment surface, lithologic Unit I contains ~30% aragonite and 60% calcite. The latter is a near-equal mixture of high- and low-magnesium calcite. The aragonite and high-magnesium calcite content decrease over the upper 120 mbsf and are not present below this depth. From 110 to 200 mbsf, minor amounts of dolomite are found. The transition from lithologic Units II to III is marked by an increase in dolomite content to ~5-8%, with one sample at 230 mbsf containing 20% dolomite. Below 300 mbsf, dolomite content decreases to between 0% and 3%. Dolomite content increases again in lithologic Unit IV, accompanied by a decrease in overall calcium carbonate content. Dolomite varies between 5% and 10% between 400 and 480 mbsf and then is absent to the bottom of the cored interval.
Calcium carbonate (CaCO3) content at Site 1198 ranges from ~47 to 99 wt% and generally covaries inversely with total organic carbon (TOC) content, which ranges from 0.0 to 0.38 wt% (Fig. F20; Tables T11, T12). Note that TOC values from Rock-Eval pyrolysis and carbon-nitrogen-sulfur analyses provide similar downsection profiles.
Hydrogen index (HI) values at Site 1198 range from 28 to 328 mg HC/g TOC (Fig. F20; Table T12), but the low TOC values of some intervals limit the reliability of these results. We performed duplicate and triplicate analyses on low organic carbon samples, and the results were within 10% of the mean value. Oxygen index (OI) values vary from 0 to 57,000 mg CO2/g TOC (Table T12). In general, the high OI values are attributed to the thermal degradation of carbonate minerals during pyrolysis and are not considered in this interpretation. Tmax values range from 316° to 433°C (Table T12), although the most reliable values lie between 370° and 420°C and have a mean value of 392°C.
Total sulfur content in Site 1198 sediments ranges from 0 to ~0.78 wt% (Fig. F20; Table T11), and its distribution is similar to that of TOC. C/N and C/S ratios (Fig. F20; Table T11) are compatible with interpretations of a dominantly marine depositional environment.
The inverse covariation between CaCO3 and TOC content mostly reflects fluctuations in the ratio of biogenic carbonate and terrigenous sedimentation through time. Calcium carbonate content displays an overall decrease from ~90 wt% near the seafloor to ~77 wt% at ~200 mbsf. Through the same interval, TOC content displays an overall increase from values <0.1 wt% to values >0.2 wt%, although TOC values decrease from ~190 to 200 mbsf, where effectively no organic carbon was measured; total S content shows the same generalized profile as TOC. Superimposed on these trends are higher-frequency, inversely covariant TOC and CaCO3 content excursions. These excursions are particularly well developed in lithologic Subunit IB and coincide with alternations of white to yellow skeletal grainstones and packstones and olive-gray packstones (see "Lithostratigraphy and Sedimentology"). Here, relatively elevated total S content also shows high variability, although the sample frequency is lower than for calcium carbonate and TOC, so an accurate definition of TOC-S-CaCO3 relationships cannot clearly be determined. The variations are suggestive, however, of changes in redox conditions at the seafloor, perhaps tied to episodes of increased terrigenous sedimentation.
At ~200 mbsf seismic Megasequence C/D boundary (see "Seismic Stratigraphy") is marked by a hardground (see "Lithostratigraphy and Sedimentology") that coincides with an abrupt change to higher CaCO3 (~97 wt%) and lower TOC (~0 wt%) and S (0%) contents. This abrupt change exists at the boundary between lithologic Units I and II (see "Lithostratigraphy and Sedimentology") and may record a downhole shift to a more stable oxic seafloor and/or water column. TOC and CaCO3 content remain at similarly low (average = <0.01 wt%) and high (average = ~96 wt%) values, respectively, until ~350 mbsf. This horizon of lower CaCO3 and higher TOC corresponds to seismic Megasequence B/C boundary (see "Seismic Stratigraphy") and marks the beginning of a gradual transition to lower carbonate and higher organic carbon contents through the remainder of lithologic Subunit IIC.
Lithologic Subunit IIIA, compared to those values observed in the overlying Unit II, is distinguished by an abrupt drop in carbonate content and an increase in TOC and total S content. The highest total S content (0.78%) measured at Site 1198 coincides with the lowest CaCO3 value at ~406 mbsf, and the highest TOC values exist just below this horizon, at ~409 and 418 mbsf. Interestingly, HI values within this interval of Subunit IIA (~405-410 mbsf) are low, suggesting input of more oxidized or terrigenous organic matter associated with low calcium carbonate and relatively high TOC and S deposition.
Lithologic Subunit IIIA is further characterized by inversely covariant alternations in carbonate and TOC content superimposed on the general trends observable in the data. These alternations are similar to those observed in lithologic Subunit IB. In lithologic Subunit IIIB, "cyclic" alternations are best developed in the TOC profile where they coincide with alternations of white to yellow skeletal grainstones and packstones and olive-gray packstones (see "Lithostratigraphy and Sedimentology").
Lithologic Subunit IV is characterized by high CaCO3 and low TOC. Below this subunit, all measured geochemical parameters decrease to zero in the basaltic basement rock.