The shipboard organic geochemistry program at Site 1172 included studies of volatile hydrocarbons, total organic and inorganic carbon, total nitrogen, total sulfur, and hydrogen and oxygen indexes. Rock-Eval pyrolysis and gas chromatography were performed (see "Organic Geochemistry" in the "Explanatory Notes" chapter) on headspace residues sampled at one per core. The CNS analysis and carbon coulometry (see "Organic Geochemistry" in the "Explanatory Notes" chapter) were performed on samples taken from Sections 1, 3, and 5 in each core.
Carbonate (CaCO3) content for the strata sampled at Site 1172 ranges from 0 to >95 wt% (Fig. F25; Table T16). In general, the carbonate distribution exhibits a two-tiered profile as observed at Sites 1170 and 1171. Here at Site 1172, sediments from ~355 to 766 mbsf commonly contain <5 wt% CaCO3 (with broader zones of elevated values up to 20% from ~360 to 410 mbsf and >5 wt% from ~500 to 545 mbsf), whereas carbonate content increases to mostly >85% above 355 mbsf. Within the upper high-carbonate content strata, carbonate content values are consistently >90 wt% from ~175 to 335 mbsf. From the seafloor to ~175 mbsf, carbonate content displays an overall declining trend with values mostly >80 wt%. A decrease to ~30 wt% is observable at ~22 mbsf.
The TOC content for most intervals at Site 1172 is <1 wt% (Fig. F25; Tables T16, T17). Total nitrogen content ranges from 0 to 0.07 wt% (Fig. F25; Table T16); nitrogen content covaries with the TOC content (Fig. F25). From ~370 to 766 mbsf, total sulfur content ranges from 0.2 to ~1.4 wt% (Fig. F26; Table T16). Zones of elevated total sulfur content >1 wt% exist at ~455 mbsf and below ~635 mbsf. No appreciable sulfur content was noted above ~370 mbsf. We calculated the C/S ratios for strata below ~370 mbsf at the site assuming that all of the sulfur exists as pyrite. The C/S method of estimating paleosalinities is discussed in "Organic Geochemistry" in the "Site 1168" chapter, "Organic Geochemistry" in the "Site 1170" chapter, and "Organic Geochemistry" in the "Site 1171" chapter; the implications of these ratios at Site 1172 are discussed below.
Organic matter type was assessed using Rock-Eval pyrolysis and CNS analyses; Rock-Eval pyrolysis data were obtained only for samples from ~400 mbsf to total depth. A discussion of this methodology is included in "Organic Geochemistry" in the "Site 1168" chapter, and "Organic Geochemistry" in the "Site 1170" chapter. The hydrogen index (HI) values from Rock-Eval pyrolysis range from 68 to 775 mg of hydrocarbon per gram of TOC at Site 1172 (Fig. F25; Table T17). The highest HI values are between ~560 and 620 mbsf. The oxygen index values vary between 31 to 778 mg of CO2 per gram of TOC. The Tmax values obtained from Rock-Eval pyrolysis range from 402° to 598°C, although the most reliable Tmax values cluster between 400° and 420°C (Table T17). The Tmax values provide an estimate of organic matter thermal maturity with the "oil window" generally considered to range between Tmax values of 435° and 465°C (see "Organic Geochemistry" in the "Site 1168" chapter, "Organic Geochemistry" in the "Site 1170" chapter, and "Organic Geochemistry" in the "Site 1171" chapter).
The high carbonate content of Oligocene through Holocene sediments at Site 1172 primarily reflects dominance of calcareous nannofossils; foraminifers are secondary in importance (see "Biostratigraphy"). This observation is similar to those made for Sites 1168, 1169, 1170, and 1171. The overall upward increase in carbonate content through the sequence is again a direct consequence of a change from shallow marine to pelagic open-ocean conditions. Here, the transition from carbonate-poor to carbonate-rich sediments appears to be relatively abrupt, although sampling at higher resolution demonstrates that the change in carbonate content is gradational from ~343 to 356 mbsf. This observation suggests that more sampling across this boundary at Sites 1168, 1170, and 1171 may provide important information with regard to the nature of this carbonate-rich/carbonate-poor boundary. Specifically, the presence of a condensed section can be inferred if the carbonate content increases gradationally across the boundary at all sites, whereas the presence of an unconformity can be considered where an abrupt change in carbonate content is observed. In either case, a relatively rapid change in depositional environments is recorded at all Leg 189 sites penetrating the Eocene/Oligocene boundary interval.
In the upper ~360 mbsf at Site 1172, the CNS results indicate TOC content values commonly up to 0.5 wt%. However, Rock-Eval pyrolysis verification of the organic carbon content was not performed on these samples. At previous sites, Rock-Eval pyrolysis results indicate very little organic carbon content in the Oligocene and younger carbonate-rich sediment. For this reason, and because CNS results are determined by difference, the organic carbon content determined by CNS analysis is considered questionable. Therefore, two modes of carbonate and total organic carbon preservation are inferred to exist at Site 1172, as at the other Leg 189 sites.
The TOC content values determined by Rock-Eval pyrolysis and CNS analysis mostly provide similar TOC content profiles below ~360 mbsf at Site 1172, although the absolute values differ for each method. Replicate Rock-Eval pyrolysis analyses were not performed, so some of the highest and lowest TOC content and HI values may be erroneous. In general, average organic carbon content displays a downward increase from the uppermost middle Eocene to the Upper Cretaceous (~370 mbsf to total depth). Organic matter type (as determined by Rock-Eval pyrolysis) is best characterized as dominantly marine through this trend, but shows an overall downward decrease in marine character. However, the CNS analyses suggest a mixed marine-oxidized marine or marine-terrestrial signature. The total sulfur content also increases downward, and C/S ratios become increasingly marine downward in the core. Various issues regarding the organic carbon content and type will be resolved through postcruise analyses.
The overall increase in the marine character of organic matter at Site 1172 is similar to observations from Site 1171. Interestingly, organic matter type is bundled into four intervals at Site 1172—Upper Cretaceous to upper Paleocene, upper Paleocene to lower Eocene, lower Eocene to middle Eocene, and middle Eocene to the Eocene/Oligocene boundary. The upper three intervals appear to correspond to intervals in organic matter type identified at Site 1171. This similarity between geochemical parameters is significant because it suggests regional-scale changes in Paleogene Pacific Ocean seafloor and water-column conditions along the STR.
Relatively elevated TOC contents exist in the middle Eocene and the upper Paleocene/lower Eocene; both of these intervals contain very labile marine organic matter. The upper Paleocene/lower Eocene interval contains C/S values indicative of fluctuations between marine and brackish depositional settings, whereas at Site 1171, Paleocene through lowermost Eocene sediments show alternations between horizons containing marine and terrestrial organic matter. These alternations in organic matter type at Site 1171, and in C/S ratios at Site 1172, suggest rapidly changing environmental conditions across the Paleocene/Eocene boundary on the STR.
The middle Eocene interval of slightly elevated organic carbon content is more marine in character than surrounding sediment and exists at the base of a decreasing trend in the Th/U values (see "Downhole Measurements"). Here, the Th/U ratios attain values indicative of dysoxic to anoxic seafloor conditions. Elevated TOC content was also observed in the middle Eocene sediment at Sites 1170 and 1171. At Site 1170, the middle Eocene organic matter is best characterized as terrestrial, whereas at Site 1171 the organic matter is considered marine. These observations suggest that a middle Eocene episode of elevated organic carbon burial was widespread in shallow and deep marine environments around the STR.
The Tmax values obtained from Rock-Eval pyrolysis of sub-Oligocene samples at Site 1172 display three distinct populations. First, a group of Tmax values fall between ~400° and 425°C, which is interpreted to represent mostly immature organic matter relative to thermal maturation and hydrocarbon generation. A second population exists at nearly 600°C, which we attribute to the presence of bitumen in the sediments. The third population lies between the first two and likely represents a "smearing" of the output signal during Rock-Eval pyrolysis of samples containing a mix of immature organic matter and bitumen.
Concentrations of volatile hydrocarbon gases were measured from every core using the standard ODP headspace-sampling technique and gas chromatographic analysis. Profiles of methane content and various methane and heavier volatile hydrocarbon ratios are presented in Figure F27 (also see Table T18). Methane only occurred in minor concentrations (~2-185 ppmv) at the base of the hole. Here, the ratio of methane vs. ethane (C1/C2) shows maximum values at ~610 mbsf and from ~740 to 760 mbsf.
The low headspace-gas content of sediment at Site 1172 is unique compared to the other sites drilled on Leg 189. At Sites 1168, 1169, 1170, and 1171, low gas content was mostly confined to the upper carbonate-rich lithologies and was considered to be a function of very little organic matter as a source of gas and pore-water profiles with sulfate reduction limiting methanogenesis. At Site 1172 the gas is likely best characterized as biogenic in origin. Here, low gas content continues downhole into the organic carbon-bearing siliciclastic sediments, where pore-water profiles show the presence of appreciable dissolved sulfate; hence, sulfate reduction processes are likely inhibiting methanogenesis (see "Inorganic Geochemistry"). This observation is unusual because the total organic carbon content (0.5%-1%) of these low-gas sediments appears to be sufficient to have already driven sulfate concentrations to zero.