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doi:10.2204/iodp.proc.322.104.2010

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., 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 AMO 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) 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 total inorganic carbon concentration in the sediment.

Hydrocarbon gases

At Site C0012, no dissolved hydrocarbon gases were detected in the upper 189 m of sediment (Table T22). Below this depth, low methane (C₁) levels were observed in all cores. Methane concentration increases with depth to a maximum of 244 µM at 417 m CSF (Fig. F54). In the depth range of the higher methane values (359.5–453.2 m CSF), ethane (C₂) was detected, reaching a concentration maximum of 3.9 µM at 417 m CSF. These concentrations are considerably lower than those at Site C0011. Propane (C₃) and butane (C₄) were not detected in any core. The occurrence of ethane below 359.5 m CSF results in low C₁/C₂ ratios (<100) that generally decrease with depth. The concentration maxima of methane and ethane occur in the same depth interval and are located at the interface of lithologic Units IV and V. These maxima correspond to a zone with locally elevated TOC contents in the solid phase of the sediment (Fig. F55).

The very low C₁/C₂ ratios at Site C0012 are unusual for sediment with low organic carbon contents, and data plot within the "abnormal" range in the context of safety considerations (Pimmel and Claypool, 2001; Fulthorpe and Blum, 1992; Shipboard Scientific Party, 1995) when plotted versus an estimated temperature (Fig. F56), assuming a geothermal gradient of 0.056°C/m and a bottom water temperature of 3°C. Operations continued despite the low C₁/C₂ ratio because the observed concentrations were much smaller than those at Site C0011. Potential sources of the hydrocarbon gases (i.e., in situ production and/or migration from deeper, hotter sources) remain to be determined by additional shore-based investigations.

Hydrogen gas

Because the analysis of H₂ using the extraction method at Site C0011 demonstrated the high potential for H₂ contamination with RCB drilling and coring, we only used the incubation method to investigate H₂ concentrations at Site C0012. For this approach, we subsampled nine whole-round cores that had been taken for shore-based analyses (biogeochemistry and deep biosphere studies), incubated the subsamples at estimated in situ temperatures, and monitored for the evolution of H₂ concentration in the headspace of the incubation vials until the expedition ended (i.e., over a period of 170–300 h) (see the "Methods" chapter). During this period, dissolved H₂ concentrations were uniformly low in all samples (Table T23; Fig. F57), with a maximum value of 3.26 nM in the deepest core (Section 322-C0012A-45R-3).

Carbon, nitrogen, and sulfur contents of the solid phase

In general, nitrogen and sulfur contents are low in the majority of cores retrieved from Site C0012 (Table T24). Inorganic carbon contents (0.4 ± 0.9 wt%) are of similar magnitude to organic carbon contents (0.3 ± 0.1 wt%) but show sporadic excursions toward higher values in all lithologic units (Fig. F55). Inorganic carbon contents correspond to a mean calcium carbonate content of 3.26 wt%, but elevated carbonate contents reach 63.6 wt% (Table T24). Total sulfur contents average 0.2 ± 0.5 wt% but show distinct excursions toward higher values at the Unit IV/V boundary and reach a maximum of 4.3 wt% in Unit V (Fig. F55). Except for one elevated value, nitrogen contents are nearly uniform in Units I–IV, where they averaged 0.05 ± 0.01 wt%, but they decrease with depth to 0.02 wt% in Unit V (Fig. F55). TOC/TN averaged ~7 ± 4, which indicates a predominantly marine origin of the sedimentary organic matter (Fig. F23), but ratios >25 indicate the presence of terrigenous organic matter in Unit V.

The trend toward higher TOC and sulfur contents in the solid phase of sediments in the upper part of Unit V occurs ~10 m deeper than the maxima of dissolved methane, ethane (Fig. F54), and sulfide (Fig. F23) at the lithologic Unit IV/V boundary. This observation gives rise to the question of whether or not the presence of terrigenous matter in Unit V supports the metabolic activity of deeply buried microorganisms that could produce methane and ethane in situ, whereas other microorganisms consume these dissolved gases by AMO where sulfate is available. The presence and metabolic activity of microorganisms and the origin of the hydrocarbon gases remain to be explored by shore-based investigations.

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

At Site C0012, the type and maturity of the organic matter was characterized in seven samples using shipboard Rock-Eval pyrolysis. The amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter (S₂) is <0.32 mg hydrocarbon (HC)/g sediment and the amount of CO₂ produced during pyrolysis of the kerogen (S₃) is <1.2 mg CO₂/g sediment (Table T25). S₂ and S₃ values show little variation throughout lithologic Units II, III, and IV (Fig. F58). The hydrogen index and oxygen index average 50 ± 10 mg HC/g TOC and 400 ± 200 mg CO₂/g TOC (Table T25; Fig. F58), respectively, and indicate that the sedimentary organic matter is a kerogen of Type III evolution. T_max values average 420° ± 10°C, which shows that organic matter in the samples is at a thermally immature stage (Table T25). T_max does not vary with depth (Fig. F58).

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