The objectives (other than hydrocarbon monitoring for safety reasons) of the shipboard organic geochemistry studies included assessments of the amount, type, and maturity of the organic matter preserved in cores recovered from the Nankai Trough. Another point of interest was to determine the genetic characterization of light hydrocarbons that can be generated by biogenic or thermogenic (thermal cracking) maturation of organic matter and the relationship of gas production to accretionary processes. The strong correlation of temperature to the formation and production of light hydrocarbons represents a sensitive indicator of the thermal history of the accretionary wedge. In addition, several samples were collected in an attempt to measure bulk hydrogen, a first for an ODP drilling project. This work involved developing a sample collection and handling protocol that would minimize contamination and ensure reproducibility of the measurements (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter).
At Hole 1173A, 74 sediment samples were collected at ~10-m intervals from 4.5 to 724 mbsf. All sediments were analyzed for methane concentration and light hydrocarbon composition during headspace analyses (Fig. F27; Table T16). In addition, nine gas samples were collected with gas-tight syringes (vacutainer) from gas pockets trapped in the core liner and were analyzed for molecular composition. TOC, inorganic carbon (carbonate), nitrogen, and carbon/nitrogen (C/N) ratios were determined on all sediments collected for headspace analyses (Fig. F28; Table T17).
TOC contents in the analyzed sediment samples are low, ranging from 0.14 to 0.85 wt% and averaging 0.3 wt% (Fig. F28). A decrease in the amount of organic matter with increasing depth is reflected in the low concentrations (average = 0.2 wt%) in the deepest 220 m of the hole. Variations in the TOC content can also be correlated with changes in the lithostratigraphy. For example, a sharp decrease in the amount of TOC was observed in the transition from upper Shikoku Basin (hemipelagic mudstone with abundant volcanic ash layers) to lower Shikoku Basin facies (hemipelagic mudstone with rare siliceous and calcareous mudstones). The decrease continues to the bottom of the hole with TOC values as low as 0.14 wt%. Inorganic carbon (carbonate) concentrations are also low (average = ~5 wt%), except from 100 to 300 mbsf, where values increase slightly up to ~10-15 wt%, and in some thin sediment layers below 500 mbsf, where values increase sharply up to 70 wt% (Fig. F29). High concentrations of carbonate nodules were seen in these sediments, which explains these high carbonate values (Fig. F29).
Nitrogen contents of the sediments are also low (0.06-0.15 wt%) and decrease with increasing depth, similar to organic matter. The C/N ratios are consistent with a marine origin; however, as was previously noted at Site 808 (Leg 131), the upper Shikoku Basin sediments may also contain a significant terrigenous component from the flux of terrestrial organic matter to these sediments. Postcruise measurements of the carbon isotopic compositions of kerogens and dissolved organic matter extracted from pore-water fluids in whole rounds will allow for a more careful assessment of the contributions of various sources (marine vs. terrestrial) to these sediments. At lower depths, the C/N ratios remain constant with a slight increase in the lower 150 mbsf, possibly reflecting thermogenic or catagenic hydrocarbon generation that has been observed in previous legs (see the Leg 131 Initial Reports volume). Unlike at Site 808, however, the generation of light hydrocarbons is extremely low (1-3 ppm) and is attributed to the higher temperature gradient in Hole 1173A. For example, temperatures at depths below 300 mbsf are already >50°C (see "In Situ Temperature and Pressure Measurements"), which is in the range of thermogenic hydrocarbon generation (Tissot and Welte, 1984). Below 400 mbsf, temperatures are in excess of 80°C and are approaching levels at which catagenic stages of hydrocarbon production occur. However, the very low TOC content, coupled with low methane and light hydrocarbon concentrations, is consistent with the low hydrocarbon production and/or preservation of organic-rich material in these sediments.
Headspace gas concentrations are moderately low (~700 ppm) in the first core (5.85 mbsf) within the sulfate reduction zone (see "Inorganic Geochemistry"). A significant increase in methane concentrations is observed just below this zone (up to 16,914 ppm) through the first 100 mbsf. Methane then decreases to lower concentrations (~1000-9000 ppm) down to 300 mbsf. As has been observed in past legs, an increase in methane in sediments below the sulfate reduction zone is indicative of bacterial origin (Claypool and Kvenvolden, 1983). The presence of small concentrations of methane in the sulfate reduction zone, however, may be due to migration from below, with methane partly consumed by sulfate reducing bacteria. Shore-based measurements of the stable carbon isotopes of methane and kerogens will allow for a more complete assessment of the origin of hydrocarbons from these zones. Just below 300 mbsf, small concentrations of ethane and propane (1-3 ppm) first appeared and show an increase below 500 mbsf (up to 25 ppm) to the bottom of the hole. At depths >350 mbsf, methane is very low (10-73 ppm), increasing slightly to 1632 ppm below 600 mbsf. The low concentrations of methane between 350 and 550 mbsf may suggest that the production of light hydrocarbons is due to thermogenic processes. There is no evidence of migration of hydrocarbons below the facies boundary at ~343 mbsf nor does there appear to be migration of hydrocarbons from the deeper section (>550 mbsf). This is in contrast to observations at Site 808 that indicated an enrichment of lighter hydrocarbons, methane, ethane, and propane in sediment that was attributed to their higher mobility.
An increase in the sulfate concentration from ~400 mbsf to total depth (TD) may have inhibited the production of biogenic methane in these sediments; however, in the deeper part of the section (550 mbsf to TD), a slight increase in both methane and light hydrocarbons is observed, consistent with the production of hydrocarbons from either thermogenic or catagenic processes. The increase in both carbon and nitrogen concentration at ~480 mbsf is coincident with an increase in the number of microbes detected (see "Microbiology"). Shore-based measurements will further address the sources and production mechanisms for hydrocarbons.
For the first time, bulk hydrogen was measured in sediments collected at an ODP coring site. In this study, we measured hydrogen concentrations in sediments at in situ temperatures over time (days) in cores from Hole 1173A. Sediment samples were taken throughout the core to establish an appropriate sampling protocol.
Preliminary results suggest that hydrogen is detectable and correlates with other biogeochemical data. Although the observation of some correlations with hydrogen concentrations and other geochemical measurements is encouraging, these results are preliminary and thus should be considered as a first attempt at understanding the role hydrogen plays in the various geochemical environments encountered in Hole 1173A.
Organic geochemical analysis at Site 1173 leads to the following conclusions: