Concentrations of volatile hydrocarbon gases were measured from every core using the standard ODP headspace sampling technique and gas chromatographic analysis. Methane occurred in only very minor concentrations (1.6-3.1 ppmv) (Table T9). The low gas content at Site 1197 is likely a function of appreciable pore water SO42- concentrations, limiting methanogenesis and the lack of mature organic matter to provide a thermogenic component to the gas fraction.
A total of 18 pore water samples were taken from Site 1197 sediments (Fig. F25; Table T10). Six samples were recovered from lithologic Unit I in Hole 1197A, covering the interval from 4.4 to 50 mbsf. In Hole 1197B, low recovery and the disturbed nature of the recovered sediment eliminated the possibility of taking pore water samples until a depth of 420.91 mbsf. This low-recovery interval corresponds approximately to seismic Megasequence C. Below 420.91 mbsf, samples were taken approximately every 10 m to a depth of 548.91 mbsf. Below this depth, sediments were highly indurated with low porosity. It was therefore not possible to extract pore water from the samples, despite repeated attempts and squeezing at pressures of 40,000 lb for up to 5 hr.
In sediments from the upper 50 mbsf of the hole, concentrations of dissolved constituents vary little from seawater values (Fig. F25), particularly for chloride, sulfate, ammonium, strontium, and potassium. Alkalinity decreases slightly from 2.5 to 1.7 mM in the upper 12 mbsf, then remains approximately constant to 50 mbsf (Fig. F25B). Magnesium concentration decreases from 55 to 50 mM in the upper 21.5 mbsf and then remains constant to 50 mbsf (Fig. F25E). Calcium alone shows a significant change, increasing steadily over the upper 50 mbsf from 11 to 20 mM (Fig. F25F).
The concentrations of various ions from the sample at 50 mbsf and at the top of the next sampled interval (420.91 mbsf) are similar. Calcium, for example, has essentially the same concentration, 20 mM, immediately above and below the sampling gap (Fig. F25F). Within the lower interval, chloride shows significant variation, with one value as low as 541 mM, the cause of which is uncertain (Fig. F25A). Alkalinity is 2.5 mM at 420 mbsf and increases slightly to 3.5 mM at 514 mbsf. Sulfate values decrease from 21.59 to 10.66 mM, whereas ammonium values increase from 477 to 856 µM in the lower sampled interval (Fig. F25C, F25D). Magnesium decreases from 39.36 mM at the top of this interval to 29.89 mM at the base. Calcium changes little, remaining near 20 mM. Strontium increases from 594 to 1124 µM (Fig. F25G). Finally, potassium values decrease from 8.2 to 4.6 mM in the lower interval (Fig. F25H).
Because of the paucity of samples, few inferences may be drawn from the pore fluid chemistry at this site. Nevertheless, the relative lack of change in chemistry of the pore waters immediately above and below the sediments of Megasequence C may be significant. The data show that the fluids within this sequence have a composition similar to seawater, as at Site 1198 on the other side of the carbonate platform. It is thus possible that fluid flow occurs within these sediments, as was clearly expressed in the pore water chemistry at Site 1198.
A total of 133 sediment samples were analyzed for carbonate mineralogy from Site 1197 (Fig. F26; Table T11). The sediments of lithologic Unit I have typical mineralogy for shallow-water hemipelagic sediments deposited above the aragonite compensation depth. The sediments are comprised of ~20 wt% aragonite at the surface, with abundant high-magnesium calcite (HMC). Over the upper 60 mbsf, aragonite and HMC content decrease and a minor dolomite component appears. Below the unconformity marking the boundary between lithologic Units I and II, there is a sharp disappearance of aragonite and a sharp increase in dolomite concentration to ~70 wt%. High dolomite content is maintained to a depth of 280 mbsf and then decreases to ~5-20 wt% at 380 mbsf, the base of lithologic Subunit IVA. Both dolomite and total calcium carbonate are quite variable in lithologic Subunit IVB. Dolomite disappears completely below 600 m and only calcite is present in lithologic Unit V.
Calcium carbonate (CaCO3) content at Site 1197 ranges from ~4 to 104 wt%. Carbonate measurements are calibrated for calcite; thus, the presence of dolomite can generate values >100 wt%. CaCO3 content covaries inversely with total organic carbon (TOC) content, which ranges from 0.0 to 0.54 wt% (Fig. F27; Tables T12, T13). Note that TOC values from Rock-Eval pyrolysis and carbon-nitrogen-sulfur (CNS) analyses provide similar downsection profiles but differ in absolute values. However, the Rock-Eval profile indicates the lack of TOC in the uppermost 100 mbsf, whereas the CNS profile displays values up to ~0.1%.
Hydrogen index (HI) values at Site 1197 range from 0 to 171 mg HC/g TOC (Fig. F27; Table T13), but the low TOC values of some intervals limit the reliability of some of these results. We performed duplicate and triplicate analyses on these samples, and the results were within 10 wt% of the mean value. Oxygen index (OI) values vary from 0 to 51,100 mg CO2/g TOC (Table T13). 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 305° to 418°C (Table T13), although the most reliable values lie between 405° and 415°C.
Total sulfur content in Site 1197 sediments ranges from 0.0 to 1.56 wt% (Fig. F27; Table T13), and its distribution is somewhat similar to that of TOC. C/N ratios and HI values show an inconsistent relationship (Fig. F27; Table T13). C/N values from Site 1197 indicate the presence of marine organic matter, whereas HI values indicate the presence of terrestrial or oxidized organic matter in the sediments. It must be noted that the low TOC content, and particularly the low total N content of these sediments, must be considered when assessing these data sets.
Variations in the generally high CaCO3 content and the inversely covariant relation between CaCO3 and TOC content at Site 1197 mostly reflect variations in carbonate mineralogy as these values were not recalculated to account for the presence of dolomite and the ratio of biogenic carbonate and terrigenous sedimentation through time. Calcium carbonate content displays an overall increase from ~90 wt% near the seafloor to >100 wt% CaCO3 at ~100 mbsf. Between ~100 and 300 mbsf, in an interval characterized by high dolomite, TOC content is effectively zero.
Below ~260 mbsf, TOC and carbonate content display high frequency variability. The amplitude of these variations increases downsection through lithologic Unit IV. The maximum amplitude of variability is observed at ~495 mbsf in lithologic Subunit IVB. Here, the highest TOC value (0.54 wt%), lowest above-basement CaCO3 value (~59 wt%), and second lowest total sulfur value (0.83 wt%) at Site 1197 were measured. These variations correspond to lower-frequency alternations that are well defined by natural gamma ray data (see "Core Physical Properties"). The highest total S value (~0.83 wt%) measured at Site 1197 was also obtained within this interval (at ~544 mbsf). The variations in CaCO3 content become less pronounced below ~550 mbsf, whereas TOC variations continue to display amplitudes similar to those observed above.
Interestingly, HI values suggest that lithologic Subunit IVB contains either terrestrial or oxidized organic matter. The presence of highly oxidized organic matter within an interval characterized by high total S content is unlikely; therefore, organic matter content may have been dominated by terrestrial input. This conclusion is supported by the presence of clay in Subunit IVB, suggesting that enhanced terrigenous input to the seafloor diluted CaCO3 content during this time.
At ~600 mbsf, more consistent CaCO3 values ranging from ~92 to 98 wt% and TOC values of zero roughly correspond to lithologic Unit V. Beneath this unit, CaCO3 values decline through lithologic Unit VI and reach a low value of ~4 wt% in the basement.