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doi:10.2204/iodp.proc.311.104.2006

Organic geochemistry

The shipboard organic geochemistry program for Site U1326 included analysis of the composition of volatile hydrocarbons (C₁–C₅) and nonhydrocarbon gases (i.e., O₂ and N₂) from headspace (HS) gas samples, void gas samples, gas samples recovered during PCS degassing experiments, and dissociated gas hydrate. Sediment from the IW squeezecakes was analyzed for inorganic carbon (IC) content (also expressed as weight percent CaCO₃), total carbon (TC), and total nitrogen (TN). Total organic carbon (TOC) was calculated as the difference between the TC and IC. A total of 70 samples for HS and solid-state analysis were collected at Site U1326. Most of the HS samples were collected on the cut end of core sections facing the IW samples so that the gas and IW data could be integrated. The near-surface sediments from Core 311-U1326C-1H were sampled at high depth resolution to define the SMI. Six pairs of HS samples were collected from inside and outside each IR anomaly interval and were imaged with the IR camera to confirm that the samples contained sediment from the cold section of the core.

We collected 44 void gas samples at Site U1326 from depths where gas cracks in the sediment were first observed (6.1 mbsf) to a TD of 269.1 mbsf. A gas sample was also collected from a PCS degassing experiment conducted with Core 311-U1326C-12P (see "Pressure coring").

The primary objectives of the organic geochemistry sampling program at this uplift site were to

• Determine the origin (microbial vs. thermogenic) of the gases recovered by HS gas, void gas, and PCS degassing techniques;
• Investigate the relationship between the gas composition and the distribution of gas hydrate in the system; and
• Describe the carbon and nitrogen contents of the sediments.

Hydrocarbons

Headspace gas and void gas compositions

Hydrocarbon HS gas measurements from Holes U1326C and U1326D are listed in Table T5 and plotted relative to depth in Figure F25. Results are reported in parts per million by volume (ppmv) of methane, ethane, ethylene, and propane in the air headspace of a 25.41 ± 0.18 mL serum vial and as the millimolar concentration of dissolved methane in the IW (see "Organic geochemistry" in the "Methods" chapter). Methane content increased rapidly from 37 ppmv in the near-surface sample (0.8 mbsf) to ~17,000 ppmv at 6.2 mbsf. There was a general trend toward lower HS methane content with depth below ~10 mbsf. In the deeper sections of Holes U1326C and U1326D, the methane HS concentration generally varied between ~2000 and 5000 ppmv. Values above that range were usually associated with samples targeting IR anomalies. Low concentrations of ethane (<2.0 ppmv) were occasionally observed, whereas ethylene and propane were detected in two samples. A few air samples collected from the catwalk area during Site U1326 operations had an average concentration of 2.45 ± 0.39 ppmv (n = 3) methane, which is slightly higher than the current atmospheric methane concentration (~1.7 ppmv). The data reported in Table T5 are uncorrected for the atmospheric contribution.

The composition of gas from voids in the core liner is shown in Table T6 and displayed relative to depth (excluding methane) in Figure F26. With the exception of several samples that contained a large percentage of air, the void gas was almost entirely methane. Carbon dioxide concentrations ranged between 0.0% and ~0.4%, and hydrogen sulfide was absent in all samples.

Although the C₂₊ hydrocarbon void gas concentrations were <125 ppmv for all samples, their relative abundance and distribution were valuable for describing the gas hydrate system at Site U1326. Ethane was the most abundant C₂₊ hydrocarbon in the void gas. Low concentration (<25 ppmv) in the uppermost 35.3 m was underlain by an interval extending to 72 mbsf with 44–82 ppmv ethane. In this same interval, two gas hydrate samples were collected, significant pore water freshening (see "Interstitial water geochemistry") was observed, and IR temperature anomalies (see "Physical properties") were imaged. Ethane and methane (the only void gases present in that interval) support Structure I gas hydrate formation (Sloan, 1998). The shallow sediments, therefore, most likely contained Structure I gas hydrate.

With greater depth, ethane concentration returned to the near-surface concentration before increasing again. Increasing ethane concentration was accompanied by increasing propane concentration. Maximum concentrations of ethane (122 ppmv at 244 mbsf) and propane (56 ppmv at 217 mbsf) occurred near the depth of the seismically inferred BSR (~234 ± 2.5 mbsf). Isobutane concentration was elevated within the same interval, with a maximum of ~10 ppmv at 228 mbsf. Propane and i-C₄ are known Structure II gas hydrate formers (Sloan, 1998). Therefore, enrichment of propane and i-C₄ in the void gas is an indication of decomposed Structure II gas hydrate.

There was a marked decrease in the C₁/C₂ ratio in the 44–72 mbsf interval where the near-surface gas hydrate was collected. Within that interval, C₁/C₂ ratios from void, gas hydrate, and PCS gases were virtually identical (Tables T6, T7).

The C₁/C₂ ratio returned to near-surface values below the shallow gas hydrate zone and then decreased gradually with depth to ~180 mbsf. The variation of the i-C₄/n-C₄ ratio in the deeper cores was, however, more distinct and informative than the C₁/C₂ ratio. Whereas i-C₄ is sequestered by Structure II gas hydrate, n-C₄ is not. Consequently, elevated i-C₄/n-C₄ ratio indicates decomposition of Structure II gas hydrate. The i-C₄/n-C₄ ratio from 132 mbsf to the base of Hole U1326D was elevated. The maximum value of i-C₄/n-C₄ (~19) occurred at 198.6 mbsf and is a strong indication that Structure II gas hydrate was present at that depth (Fig. F27).

Evidence of different molecular ratios associated with gas hydrate at different depths at the same location suggests that the local gas hydrate system may be supported by different gas-bearing source fluids or that the hydrocarbon composition of the fluids was altered during fluid migration from depth. An association between a steeply dipping sand layer at ~45 mbsf and concentrated gas hydrate in the shallow horizon indicates fluid migration along structural fractures (see "Interstitial water geochemistry").

Gas composition expressed as the C₁/C₂ ratio of HS and void gas is plotted relative to sediment temperature in Figure F28. Sediment temperature is based on the calculated geothermal gradient of 60°C/km (see "Physical properties"). The monitoring of the C₁/C₂ ratio in void and HS gas samples and its relationship to temperature was developed as a safety guideline by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) Pollution Prevention and Safety Panel during the Ocean Drilling Program. C₁/C₂ ratios are described as either "normal" or "anomalous" depending upon where they plot relative to the slightly diagonal line in Figure F28. All values measured at Site U1326 were within the acceptable "normal" limits for safe drilling (Pimmel and Claypool, 2001).

Biogeochemical processes

The IW and HS gases from Cores 311-U1326C-1H and 2H were sampled at high depth resolution (approximately two samples per 1.5 m section) to define the SMI. The depth of sulfate depletion was 2.9 mbsf, but a deeper "peak" was measured at 4.6 mbsf (Fig. F29). It was suggested that this second peak may be related to a slump feature or drilling fluid contamination (see "Interstitial water geochemistry"). The increasing concentration of methane to 6.2 mbsf is not necessarily consistent with the former interpretation because consumption of methane would be expected at the "second" SMI. The shallow and thin SMI is located between ~2.3 and 3 mbsf.

Sediment carbon and nitrogen composition

The sediment IC, carbonate (CaCO₃), TC, TOC, and TN concentrations and C/N ratio from Site U1326 are listed in Table T8 and plotted relative to depth in Figure F30. High-resolution depth profiles were obtained for the uppermost 13 m. No data were obtained below 198 mbsf because the ship was under way in rough seas by the time the samples from that interval were freeze dried. The solid-state carbon and nitrogen vertical profiles displayed transitions that agree remarkably well with the defined lithostratigraphic units (see "Lithostratigraphy"). The carbonate content is relatively high (average = 0.73 wt%) in the depth interval corresponding to lithostratigraphic Unit I (0–24.1 mbsf), low in Unit II (24.1–146.3 mbsf; average = 0.19 wt%), and intermediate in Unit III (146.3–271.4 mbsf; average = 0.41 wt%). The highest value was measured at 2.15 mbsf, which is the depth roughly corresponding with the SMI. TOC displays a similar distribution with average contents of 0.51, 0.34, and 0.45 wt% in lithostratigraphic Units I, II, and III, respectively. C/N ratios in lithostratigraphic Unit I are ~11.1, which indicates the highest terrestrial contribution of organic matter among all sites investigated during Expedition 311. This conclusion is interesting considering Site U1326 is the greatest distance from land. C/N ratios in lithostratigraphic Units II (average = ~8) and III (average = ~7.7) are similar, though a low C/N value of 2.1 at 131 mbsf skewed the average value for lithostratigraphic Unit II downward.

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