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doi:10.2204/iodp.proc.322.103.2010

Organic geochemistry

The organic geochemistry program during Expedition 322 aimed to characterize the composition of sediments entering the subduction system with respect to their role as a habitat for the deep subseafloor biosphere. The main objectives were to characterize (1) potential energy sources of the deep biosphere (i.e., the amount and quality of organic matter within the sediment); (2) the availability of hydrogen (H₂), which represents an alternative energy source that results from the degradation of organic matter, mineral–water interactions, or radiolysis of water; (3) the presence of methane that could be exploited as an energy source in the process of anaerobic methane oxidation and which may lead to the formation of authigenic carbonate; and (4) sources of methane and other hydrocarbon gases that formed in the course of biogenic and thermogenic alteration of organic matter during sediment burial.

To achieve these objectives, we (1) measured the quantity and composition of hydrocarbon gases (C₁–C₄) and hydrogen by headspace technique, (2) determined the potential of the sediments to produce and consume hydrogen in incubation experiments, (3) characterized the composition of the particulate sedimentary organic matter by elemental analysis and Rock-Eval pyrolysis, and (4) measured the concentration of total inorganic carbon in the sediments.

Hydrocarbon gases

At Site C0011, hydrocarbon gas concentrations increase with depth (Table T19). Methane (C₁) was present in all samples of Hole C0011B. Ethane (C₂) was detected in all cores taken from depths >425 m CSF, except for Core 322-C0011B-30R. Propane (C₃) was first observed at 569 m CSF and was present in almost all deeper cores. Isobutane (C₄) occurred sporadically at depths >675 m CSF. The occurrence of ethane below 422 m CSF results in low C₁/C₂ ratios ~280 ± 80 that generally decrease with depth but show a reversal to higher C₁/C₂ values at the bottom of the hole (Fig. F61).

The C₁/C₂ ratio is generally used to obtain information on the origin of the hydrocarbons. When high amounts of C₁ are present (>10,000 ppm), very high C₁/C₂ ratios indicate methane formation by biological processes. In contrast, major amounts of higher molecular weight hydrocarbons in shallow depths suggest thermogenic hydrocarbon sources and gas migration. However, minor amounts of C₂–C₄ can also be generated in situ during early, low-temperature diagenesis of organic matter, and it is becoming increasingly recognized that C₂–C₄ gases can also be produced biogenically along with C₁, although not in high concentrations (Vogel et al., 1982; Wiesenburg et al., 1985; Oremland et al., 1988; Hinrichs et al., 2006). The very low C₁/C₂ ratios (~280 ± 80) at Site C0011 are unusual for sediments with organic carbon contents of <0.5 wt%. These data are within the normal range in the context of safety considerations (Fulthorpe and Blum, 1992; Shipboard Scientific Party, 1995) when plotted versus an estimated temperature, assuming a geothermal gradient of 0.056°C/m and a bottom water temperature of 3°C (Fig. F62). The potential sources of the higher hydrocarbon gases (i.e., in situ production and/or migration from deeper, hotter sources) remain to be explored by additional shore-based investigations.

Hydrogen gas

At Site C0011, we aimed to determine H₂ concentrations directly using the extraction method. This method is based on the assumption that H₂, which is initially present in a wet sediment sample, exsolves from the liquid phase when it is slurried with an NaCl solution and can be captured in the defined headspace of a closed vial. Using mass balance considerations, the initial concentrations of dissolved H₂ were calculated from the analyzed concentrations of H₂ in the headspace gas after accounting for the contribution of H₂ from the reagent blank.

H₂ concentrations in the headspace gas cover a large range (from 0.27 to 62.9 ppmv) (Table T20). The lowest concentrations are only slightly higher than the reagent blank of 0.214 ppmv, but in the majority of samples, H₂ concentrations are at least three times higher. The corresponding dissolved H₂ concentrations range from 0.049 to 17.4 µM and average 1 ± 3 µM. The dissolved H₂ concentrations do not show a clear trend with depth (Fig. F63).

Because H₂ concentrations were unexpectedly high, we investigated the potential contamination of samples with H₂ that might be generated and introduced into the sample in the course of RCB drilling. Starting with Core 322-C0011B-23R, samples were taken from the free fluids that had accumulated between the cut core and the core liner whenever there was enough fluid inside the core liner. Samples were retrieved when the core was cut into sections and headspace vials were filled completely with drilling fluid and crimp capped before a headspace of pure N₂ was introduced into the headspace vial. As opposed to sediment samples, the drilling fluid was not slurried with an NaCl solution. In a total of 13 samples, the undiluted drilling fluids yielded 1.28 to 544 ppmv H₂ in the headspace gas (Table T21). The corresponding dissolved H₂ concentrations range from 0.030 µM to 1.86 µM (Table T21) and do not show a clear trend with depth (Fig. F63).

The high levels of H₂ in the drilling fluid demonstrate the high potential for contaminating sediment with H₂ in the course of RCB drilling and coring. There is, however, no clear relationship between H₂ concentrations in the drilling fluid and sediment samples (Fig. F63). Within the data set, H₂ concentrations in the interstitial water of sediments are both higher and lower than H₂ concentrations in the drilling fluid. Because cores in general were severely disturbed and intensively exposed to drilling fluid in the course of RCB drilling and coring, the extraction method does not yield reliable information on in situ interstitial water H₂ concentrations at Site C0011.

In order to better constrain interstitial water H₂ concentrations, we subsampled whole-round cores that had been taken for shore-based analyses (biogeochemistry and deep biosphere studies) and investigated H₂ concentrations using the incubation method. This method allows the determination of dissolved H₂ concentrations based on two fundamental assumptions: (a) gaseous H₂ is in equilibrium with dissolved H₂, and (b) the incubation of samples in the laboratory leads to the establishment of a steady state between production and consumption of H₂ that is representative of in situ equilibrium. The suitability of this method for deep subseafloor sediments is questionable because microorganisms in deep marine subsurface sediments metabolize at very low rates (D'Hondt et al., 2002; Parkes et al., 2005). Therefore, it is not clear whether steady state can be reached within an acceptable time frame in the laboratory or if such a steady state would be representative of in situ conditions.

At Site C0011, nine samples were investigated using the incubation method (Table T22). For each sample, incubation experiments comprised three replicates. The variability between the replicates is large, with Section 322-C0011B-45R-4 representing an extreme case (Fig. F64), and might be related to the lithification of the sediment, which hindered homogenization of the samples before they were split into replicates. In spite of the large variability between replicates, the following trends were observed. In general, an initial increase in H₂ concentrations is followed by a decrease, suggesting active consumption of H₂ in the sediments. To avoid contact with atmospheric oxygen, three samples (i.e., from Sections 322-C0011B-15R-3, 25R-2, and 32R-5) were stored prior to sample processing in the N₂ atmosphere of an anaerobic chamber that contained an admixture of 2% H₂. Though the headspace of the incubation vials was thoroughly flushed with pure N₂ and H₂ concentrations were below detection limit when the incubation experiment started, the elevated initial levels of H₂ in these three samples might result from some contamination of the sample with H₂ during storage. In samples that were incubated for >200 h, H₂ concentrations approach constant levels that suggest a steady state between H₂ production and consumption. In all but the deepest sample (322-C0011B-58R-6, 123–135 cm; 682.61–862.73 m CSF) final concentrations of dissolved H₂ ranged from 1 to 10 nM. H₂ concentrations in replicates of the deepest sample were >70 nM but <120 nM. Thus, H₂ concentrations determined using the incubation method are a factor of 100 to 1000 lower than H₂ concentrations that resulted from the extraction method.

The large difference between H₂ concentrations obtained using the two different methods could have resulted from an underestimation of H₂ concentrations by the incubation method (because of low metabolic activity in the recovered samples) and/or from an overestimation of H₂ concentrations by the extraction method (because of H₂ contamination in the course of drilling). Shipboard investigations did not allow clear differentiation between these two effects. Though both methods have severe limitations, they still allow us to delineate the lower and upper limits of in situ H₂ concentrations in Hole C0011B to ~1 nM and 17 µM, respectively.

Carbon, nitrogen, and sulfur contents of the solid phase

Carbon, nitrogen, and sulfur contents are low throughout the cored interval of Site C0011 (Table T23). In general, inorganic carbon contents (0.4 ± 0.9 wt%) are similar to organic carbon contents (0.3 ± 0.1 wt%). Inorganic carbon contents show more scatter in lithologic Unit III than in other lithologic units, but organic carbon contents are more uniform in Unit III (Fig. F65). Inorganic carbon contents correspond to a mean calcium carbonate content of 2.92 wt%, but local increases in Unit III reach up to 61.7 wt% (Table T23). Total sulfur and total nitrogen contents average 0.4 ± 1 wt% and 0.06 ± 0.02 wt%, respectively. Nitrogen content decreases with depth in Unit III but shows no clear trends in the other lithologic units. Total organic carbon (TOC)/total nitrogen (TN) ratios of ~6 ± 3 indicate a marine origin of the sedimentary organic matter (Fig. F66).

Characterization of the type and maturity of organic matter by Rock-Eval pyrolysis

At Site C0011, the type and maturity of the organic matter was characterized in a selection of 76 samples using shipboard Rock-Eval pyrolysis. Fourteen samples yielded poorly resolved S₂ peaks with peak maxima (T_max) ~605°C and were excluded from the data set presented in this report. Thermal cracking of nonvolatile organic matter (S₂) generally released <0.2 mg hydrocarbon (HC)/g sediment, and pyrolysis of the kerogen (S₃) produced <1.4 mg CO₂/g sediment (Table T24). S₂ and S₃ values show little variation throughout all four cored lithologic units (Fig. F67). The hydrogen index (HI) and oxygen index (OI) average 30 ± 20 mg HC/g TOC and 300 ± 100 mg CO₂/g TOC (Table T24; Fig. F67), respectively, thus suggesting a kerogen of Type III evolution. T_max values average 420° ± 10°C and indicate sedimentary organic matter at a thermally immature stage (Table T24). T_max did not vary with depth (Fig. F67).

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