Concentrations of volatile hydrocarbon gases were measured from every core using the standard ODP headspace sampling technique and gas chromatographic analysis. Methane only occurred in very minor concentrations (1-3 ppmv); the highest value obtained was ~7 ppmv at ~485.6 mbsf (Table T9).
The low gas content at Site 1193, as at Site 1192, is likely a function of low total organic carbon (TOC) content of the sediments that prevents sulfate reduction from going to completion, low pore water SO42- concentrations that limit bacterial methanogenesis, and the immaturity of organic matter present in the sediments relative to hydrocarbon expulsion and petroleum generation.
Thick sequences of cemented shallow-water carbonates at Site 1193 limited pore water sampling to finer-grained, more clastic-rich sediments above, below, or within these carbonates. Some of the sediments were extremely well compacted or cemented, making it impossible to extract sufficient water for all shipboard chemical analyses from every sample. However, it was still possible to obtain a good picture of the pore water chemistry.
Chloride concentrations are ~565 mM in the upper 40 mbsf at Site 1193 (Table T10; Fig. F43A) within the hemipelagic sediments of lithologic Units I and II (see "Lithostratigraphy and Sedimentology"). Within the lower part of the carbonate platform sediments of lithologic Unit III, the chloride concentrations increase slightly to 567 mM. Just below the thickest section of shallow-water carbonates at ~240 mbsf, a rapid decrease in chloride is observed with values as low as 552 mM. The observed decrease is about 2%-3% of the total ion concentration and, therefore, is difficult to detect in other dissolved constituents but is probably expressed as small decreases observed in potassium and magnesium over the same depth interval (Fig. F43C, F43D). The origin of the rapid decline in chloride is not certain. Below 250 mbsf, chloride concentration generally increases downhole, with a value of 566 mM at the base of the hole.
Alkalinity is near typical bottom-water values (2.1 mM) at the top of the hole. Values vary only slightly to 300 mbsf then decrease to <1 mM at the base of the cored interval (Fig. F43B). In all likelihood, alkalinity is removed from the pore waters by calcium carbonate precipitation.
Potassium concentration changes little from near the sediment surface to ~200 mbsf, with values between 11 and 12 mM (Fig. F43C). Below 200 mbsf, the concentration decreases to below 4 mM at the bottom of the cored interval.
Dissolved magnesium decreases slightly downhole in the upper 40 mbsf from 56 to 53 mM and decreases further to 48 mM just below lithologic Unit III (Fig. F43D). Thereafter, the concentration remains nearly constant to the base of the hole.
Calcium concentrations increase almost linearly from the seafloor to the base of the sediment column. Just above basement, the concentration is 56.25 mM, approximately three times the seawater concentration (Fig. F43E). The linear profile suggests a simple diffusion gradient between the basement and seawater, with low-temperature alteration of the mafic basement rocks supplying calcium to the pore waters.
Strontium concentration remains low, below 200 µM, from the shallowest sample at 4.4 mbsf through the base of lithologic Unit III at 220 mbsf (Fig. F43F). Below this interval, strontium increases sharply to over 500 µM and then increases gradually to over 600 µM at 375 mbsf. This increase is likely to be caused by the steady release of strontium during calcite recrystallization. As at Site 1192, high strontium and sulfate concentrations result in celestite precipitation, as confirmed by X-ray diffraction.
Sulfate and ammonium profiles are essentially mirror images of one another (Fig. F43G, F43H). The concentrations remain near seawater values until a depth of 220 mbsf below the carbonate platform sediments of lithologic Unit III. Below this interval, sulfate reduction begins. Sulfate values decrease to 22 mM at 300 mbsf then decrease more slowly to below 20 mM at the base of the hole. Ammonium values increase in antipathetic fashion over the same interval. Ammonium concentration is 419 µM at 300 mbsf and further increases to 559 µM at 485 mbsf. The single ammonium value above 800 µM could be an artifact.
Iron and manganese were measured on most samples from Site 1193, but all values were within the measurement error. Small clots of black iron sulfide staining occur over large parts of the more hemipelagic sediments. The Fe sulfides attest to release of iron during sediment diagenesis, with the iron rapidly removed in the zone of sulfate reduction.
Overall, the pore water geochemistry from Site 1193 indicates that fluids within lithologic Units I-III are little evolved from seawater. Although pore water samples could not be taken from within the shallow-water facies of the platform, the highly permeable lithologies suggest that similar values found above and below this interval can be extrapolated to provide an estimate of the values within it. Even the strontium concentrations, which might be expected to increase because of ongoing carbonate recrystallization, show no increase until below the platform. The best explanation for the lack of change in fluid chemistry is that fluids are moving through the sediments.
The percentages of carbonate minerals were determined on 147 samples (Table T9; Fig. F44). Lithostratigraphic Unit I is entirely calcitic in mineralogy, whereas the percentages of dolomite and calcite vary greatly through the rest of the sedimentary section (Table T11). Lithostratigraphic Subunit IIA contains the highest concentration of dolomite that exists in the alternating 20- to 30-m-thick layers that are either 80-90 wt% dolomite or 80-90 wt% calcite (Table T11). These variations are observed in both Holes 1193A and 1193B (Fig. F44). Dolomite content decreases in lithologic Subunit IIB and in lithologic Unit III, averaging ~30 wt%, although several ~5-m-thick layers seem to contain up to 50 wt% dolomite. The dolomite content decreases further in lithologic Unit IV to 10-20 wt% (Table T11). In lithologic Unit V, total carbonate decreases greatly and the sediments are primarily calcite with only a few layers containing up to 10 wt% dolomite.
Calcium carbonate (CaCO3) content at Site 1193 ranges from ~5 to >100 wt% (Fig. F45; Table T12). Measurements are calibrated for calcite; thus, the presence of dolomite can result in calcium carbonate values >100 wt%. The TOC content of all measured samples at Site 1193 is <0.5 wt%, with the exception of an interval at 366 mbsf where 0.96 wt% TOC was measured. Note that percent TOC values from Rock-Eval pyrolysis and carbon-nitrogen-sulfur (CNS) analyses provide similar profiles with differences in absolute values (Fig. F45; Tables T12, T13) and that TOC values covary inversely with percent CaCO3 (Fig. F45).
Hydrogen index (HI) values range from 0 to 233 mg HC/g TOC at Site 1193 (Fig. F45; Table T13). Oxygen index (OI) values vary from 0 to 925 mg CO2/g TOC (Table T13). The low percent TOC limits the reliability of some HI and OI values. To ensure they are reproducible, duplicate and triplicate analyses were performed on these samples. Tmax values range from 311° to 435°C (Table T12), although the most reliable Tmax values cluster between 400° and 420°C below ~200 mbsf.
Total sulfur (S) content in Site 1193 sediments ranges from 0 to >1.05 wt% (Fig. F45; Table T12) and displays a similar distribution to percent TOC.
Variations in the generally high calcium carbonate content (average = ~82 wt%) (Fig. F45) of sediments at Site 1193 mostly reflect fluctuations in the ratio of biogenic carbonate to terrigenous sediment input through time. Carbonate contents exhibit a decrease from ~89 wt% at ~2.25 mbsf to ~72 wt% at 11.85 mbsf followed by an increase to 83 wt% at 33.85 mbsf. These sediments contain well-preserved foraminifers with lesser amounts of calcareous nannofossils (see "Biostratigraphy and Paleoenvironments") and correspond to lithologic Unit II (see "Lithostratigraphy and Sedimentology"). The slightly elevated TOC content with mostly low HI values in this interval indicates the presence of terrigenous or reworked refractory organic matter and/or oxic seafloor conditions. The abrupt change in calcium carbonate content at the lithologic Unit II/III boundary coincides with a major exposure surface. In the underlying carbonate platform from ~35 to 167 mbsf (lithologic Subunit IIIA [see "Lithostratigraphy and Sedimentology"]), calcium carbonate content is effectively 100 wt%. A slight decrease in CaCO3 concentration at ~61 mbsf appears to coincide with the presence of two exposure surfaces between ~60 and 67 mbsf (see "Lithostratigraphy and Sedimentology"). The carbonate platform interval contains effectively no organic matter, as is common in these types of sediments.
Within the interval from ~167 to 245 mbsf, at least three horizons are well defined by decreased CaCO3 content, increased percent TOC values, low HI values, and relatively elevated S concentrations. The uppermost horizon (~167-172 mbsf) correlates to the top of lithologic Subunit IIIB and is described as mudstone with large shallow-water benthic foraminifers that appear to have undergone transport (see "Biostratigraphy and Paleoenvironments"). Two additional horizons occur at ~215 and 245 mbsf within lithologic Unit IV. Unit IV is a mudstone with fine sand-sized bryozoan and benthic foraminifer fragments and few associated worn planktonic foraminifers, suggestive of transport (see "Biostratigraphy and Paleoenvironments"). The combination of geochemical, paleontological, and lithological characteristics is compatible with an interpretation of the horizons as basinward facies shifts. In this conceptual sequence stratigraphic model, increased terrigenous input may have caused carbonate dilution, whereas increased sulfur content may be attributable to pyrite formation due to iron associated with the terrigenous influx. Therefore, these horizons may represent relative sea-level lowstands.
Between ~245 and 365 mbsf, CaCO3 contents are generally >80 wt%, TOC values are ~0.2 wt%, and HI values are relatively high, suggesting that organic matter preservation occurred on the most distal openmarine, deepest, or dysoxic seafloor recorded at the site. Therefore, the overall downsection trend from Subunit IIIB to Unit V may represent an overall landward facies shift or deepening.
Other conspicuous horizons exist at ~365 (CaCO3 = ~54 wt%) and ~385 mbsf (CaCO3 = ~72 wt%), below which a zone of bioclastic material (see "Biostratigraphy and Paleoenvironments") displays slightly elevated CaCO3 content and low TOC values reminiscent of lithologic Subunit IIIB. The highest TOC content measured at this site (0.96 wt%) occurs at ~365 mbsf. This organic matter is most likely terrigenous in origin.
The widest range in carbonate values (~5-92 wt%) at Site 1193 is observed from ~420 mbsf to the base of the drilled section, including two horizons with the lowest CaCO3 contents observed (5 and 9 wt%) (Fig. F45). TOC (up to ~0.5%) and total S concentrations are relatively elevated through this interval. Generally low carbonate and high TOC contents above basement suggests that this basal unit was deposited in a setting most affected by terrigenous inputs.