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doi:10.2204/iodp.proc.311.106.2006

Organic geochemistry

The organic geochemistry shipboard program for Site U1328 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 IW squeeze cakes was analyzed for inorganic carbon (IC; also expressed as weight percent CaCO₃), total carbon (TC), and total nitrogen (TN). Total organic carbon (TOC) was calculated as the difference between TC and IC. We collected 71 samples for HS and solid-state analysis at Site U1328. Most of the HS samples were collected from the cut end of core sections facing the IW samples so that the gas and IW data could be integrated. Gas hydrate dissociation during recovery of shallow cores (0–40 mbsf) affected the structural integrity of the sediments and allowed large volumes of drill fluid to contaminate the sediment IW. Consequently, we did not attempt to construct high-resolution vertical profiles of sulfate and methane in the near-surface sediments.

A total of 13 gas samples from gas hydrate pieces collected from the uppermost 40 m (seven from Hole U1328C, three from Hole U1328D, and three from Hole U1328E) were analyzed for gas composition. The lattice water from the dissociated gas hydrate was also collected for analysis (see "Interstitial water geochemistry"). We collected 49 void gas samples from depths where gas cracks in the cores were first observed (3.3 mbsf) to 299.5 mbsf. Finally, we analyzed the gas composition of five gas samples from PCS degassing experiments on cores from Holes U1328B, U1328C, and U1328E (see "Pressure coring").

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

• Characterize the composition of gas hydrate at the site;
• Determine the origin (microbial vs. thermogenic) of the gases recovered by HS, void, and gas hydrate gas and PCS degassing techniques;
• Investigate the relationship between the gas composition and the distribution of gas hydrate; and
• Compare the carbon and nitrogen contents to gas-related features in the sediments.

Hydrocarbons

Headspace and void gas composition

Hydrocarbon HS gas data from Holes U1328B, U1328C, and U1328D are listed in Table T6. 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 C₁ in the interstitial water (see "Organic geochemistry" in the "Methods" chapter). Methane and ethane HS results from Site U1328 are plotted in Figure F30 as the hydrocarbon gas component relative to depth. Methane content increased rapidly from 2.3 ppmv in the near-surface sample (0.5 mbsf) to a maximum concentration of ~65,500 ppmv at 5.3 mbsf. With the exception of a 3 m interval between 69 and 72 mbsf, where elevated HS concentrations of ~11,000 and 35,000 ppmv were observed, HS concentrations below 7.9 mbsf were fairly consistent and relatively low, ranging between ~1500 and 7000 ppmv. The apparent decrease in HS methane concentrations between 5.3 and 14.7 mbsf and the fairly consistent, relatively low concentrations below may be a sampling artifact that does not necessarily represent in situ trends. Postcruise stable carbon isotope analysis of the HS, void, and thermally desorbed gases will allow us to obtain a better understanding of which factors (degassing vs. preferential desorption from sediments) determine the composition and distribution of gas recovered by each method. A few air samples collected from the catwalk area during Site U1328 operations had a concentration of 1.81 ± 0.08 ppmv (n = 3) methane, which is similar to the current atmospheric methane concentration (~1.7 ppmv). The data reported in Table T6 are uncorrected for the atmospheric contribution.

Ethane was present in the HS samples at all depths below 1.5 mbsf (Fig. F30; Table T6). Elevated C₂ was observed from near the surface (2–30 mbsf) where gas hydrate was recovered, and at or below the depth of the seismically inferred BSR (219 mbsf). Propane was limited to a 27.8 m thick interval between 192.2 and 220 mbsf and above the BSR. It is noteworthy that ethane concentrations in this same interval were not elevated, especially when considering that chloride and IR temperature anomalies indicated that the interval contained gas hydrate (see "Physical properties" and "Interstitial water geochemistry"). Dissociation of gas hydrate with ethane is expected to enrich the surrounding gas with ethane because gas hydrate (especially Structure I gas hydrate) preferentially fractionates ethane relative to the source free-gas composition (Sloan, 1998). The same effect is observed with propane in Structure II gas hydrate (Sloan, 1998). Enrichment of propane, but not ethane, suggests an increased abundance of Structure II gas hydrate relative to Structure I gas hydrate in the interval above the BSR.

Gas composition expressed by the C₁/C₂ ratios of HS and void gas samples is plotted relative to sediment temperature in Figure F31. The sediment temperature was estimated assuming the calculated geothermal gradient of 54°C/km (see "Physical properties"). The monitoring of C₁/C₂ ratios in HS and void gas samples and its relationship to temperature was developed as a safety guideline by the Joint Oceanographic Institutions for Deep Earth Sampling Pollution Prevention and Safety Panel during ODP. C₁/C₂ ratios are described as either "normal" or "anomalous" depending upon where they plot relative to the slightly diagonal line in Figure F31. Some values from the HS and void gas samples were within the "anomalous" region (Pimmel and Claypool, 2001). The anomalous excursions, however, occurred in intervals where gas hydrate was either recovered or inferred. Because gas hydrate sequesters ethane and other higher hydrocarbons (Sloan, 1998), the C₁/C₂ values at this site were artificially deflated relative to the source signature. With this effect under consideration, it was determined that conditions were safe for drilling.

The composition of gas from voids in the core liner is shown in Table T7. With the exception of the near-surface samples, the void gas was ~90% methane. The surface samples contained slightly more air contamination and elevated H₂S in the shallow sediments. The calculated H₂S concentrations (as much as 4.5%) are based on calibration data from a previous cruise (H₂S standards above the detection limit of the gas chromatograph were not available during instrument calibration nor during finalization of this report) and will be verified postcruise. Carbon dioxide accounted for ~0.1%–1.5% of the total gas content. C₂–C₅ hydrocarbons constituted <0.2% of the total gas content in all samples; however, their relative abundances and distribution are valuable as tracers for fluid migration and interpreting the dynamics of the gas hydrate system.

The distribution of ethane and propane in the void gas samples was similar to the pattern observed in the HS data (Figs. F30, F32). Ethane concentrations were highest (~1600 ppmv at 38.2 mbsf in Hole U1328E) in the surface sediments where massive gas hydrate was recovered. Ethane was elevated from the BSR (219 mbsf) to the base of Hole U1328C.

Propane was present in trace amounts (0–5.3 ppmv) to 213 mbsf and then increased rapidly (maximum concentration = ~200 ppmv) in the 199–209 mbsf interval above the seismically inferred BSR. The highest i-butane concentration (170 ppmv) was observed in this same interval. Notably, ethane concentrations in this region of elevated propane and i-C₄ were not substantially elevated. The pattern of enriched Structure II gas hydrate–forming gases (C₃ and i-C₄) suggests that the gas hydrate inferred from the IR and chloride anomalies (see "Physical properties" and "Interstitial water geochemistry") was enriched with Structure II gas hydrate relative to the near-surface (0–40 mbsf) gas hydrate.

The molecular ratios of C₁/C₂ and i-C₄/n-C₄ are consistent with the interpretation that the deep and shallow gas hydrate accumulations contain hydrocarbons from different sources. The C₁/C₂ ratios in the shallow interval (0–40 mbsf) were relatively low (~700–2900) and the i-C₄/n-C₄ ratios were slightly elevated (~6) (Fig. F33). These results indicate a slight contribution from Structure II gas hydrate. In contrast, the i-C₄/n-C₄ and C₁/C₂ ratios above the seismically inferred BSR (219 mbsf) were both elevated (Fig. F33). These data indicate the gas hydrate within that interval was enriched in propane and i-C₄, but not ethane.

Using a source gas composition similar to that present below the BSR (Table T8), the composition of Structure I and II gas hydrate at the in situ equilibrium conditions was calculated with the CSMHYD program (Sloan, 1998). These calculations did not include H₂S or CO₂. Both Structure I and II gas hydrate would have a C₁/C₂ ratio of ~2500. However, the Structure I gas hydrate would contain no propane or i-C₄, whereas the Structure II gas hydrate would have approximately twice as much propane as ethane (C₃/C₂ = 2.25) and approximately equal values of ethane and i-C₄ (i-C₄/C₂ = 1.25). Within the region of elevated propane and i-C₄ (199–209 mbsf), C₃/C₂ ratios ranged from 0.7 to 2.0 and i-C₄/C₂ ratios ranged from 0.2 to 1.4. By applying a two-source mixing model, where the end-members are Structure I and II gas hydrate with the molar percentages provided in Table T8 and assuming that all void gas was a product of gas hydrate decomposition, we calculated that ~50%–60% of the gas in the interval above the projected BSR may have come from Structure II gas hydrate.

Gas hydrate gas composition

The data from dissociated gas hydrate (HYD) gas from Site U1328 are listed in Table T9. The HYD gas samples were collected from the uppermost 40 m and therefore only reflect the composition of the shallow gas hydrate accumulations. Methane was the dominant gas recovered from the HYD gas samples. On average, C₁ accounted for ~99.4% of the total recovered gas, whereas C₂, CO₂, and H₂S comprised ~0.14%, 0.33%, and 0.12%, respectively. Although C₂ and CO₂ were present in most of the HYD gas samples, H₂S was limited to four HYD gas samples collected between the depths of 6.1 and 8.5 mbsf. The concentrations (~350 ppm to 1.8% H₂S) were similar to values reported from the southern summit of Hydrate Ridge (Milkov et al., 2005). The source of the H₂S is presumed to be from sulfate reduced during the anaerobic oxidation of methane. The concentration of C₂–C₅ hydrocarbons was <0.001% of the total gas hydrate gas content and is consistent with a primary Structure I gas hydrate composition for the shallow gas hydrate.

Results from the analysis of gas samples collected during the PCS degassing experiments (see "Pressure coring") are shown in Table T10 and the C₁/C₂ ratios are plotted relative to depth with the HS, void, and gas hydrate gas data in Figure F34. The C₁/C₂ ratios of PCS gas samples are in agreement with the void gas sample trend at depths where direct comparisons were possible. Cores containing gas hydrate, the target for many PCS deployments, typically did not contain void gas that could be sampled. Gas hydrate dissociation degrades the mechanical stability of the sediments and allows air to flush the core liner. Therefore, opportunities to compare void gas from gas hydrate–bearing cores to gas from PCS cores are limited. The available direct comparisons of C₁/C₂ gas hydrate, PCS, and void gas generally agree within the shallow (0–40 mbsf) gas hydrate–bearing interval. The PCS samples from 92 mbsf (Cores 311-U1328C-5P and 311-U1328E-10P) did not fit the trend of the void gas C₁/C₂ data, but no void gas samples were collected within 10 m of the PCS samples. Low C₁/C₂ values in the HS data suggest the gas composition in that interval may be offset from the downhole void gas trend. The PCS gas sample from 230 mbsf (Core 311-U1328E-13P) agreed with the void gas data.

Biogeochemical processes

The depth of sulfate depletion was ~1.5 mbsf (see "Interstitial water geochemistry"), similar to the depth where dissolved methane concentrations begin to increase (Table T6). However, unlike the previous sites where the sulfate and C₁ profiles in the near-surface sediments showed clear zonation of the biogeochemical regions, high-resolution profiles from Site U1328 were not possible because of poor core recovery. Nevertheless, the shallow depth of sulfate depletion and occurrence of methane indicates a high flux of methane supporting the near-surface gas hydrate accumulations.

Sediment carbon and nitrogen composition

Sediment IC, carbonate (CaCO₃), TC, TOC, and TN concentrations and C/N ratios from Site U1328 are listed in Table T11 and plotted relative to depth in Figure F35. Elevated IC was observed at two depths (9.7 wt% at 18.4 mbsf and 1.5 wt% at 43 mbsf) in the shallow Hole U1328B (52.8 mbsf). These elevated values suggest production of authigenic carbonate. In the deeper Hole U1328C, we observed a region of elevated carbonate between 190 and 210 mbsf. Within that region, the highest value of carbonate (1.1 wt%) occurred at 190.2 mbsf, and values declined gradually with depth to 0.62 wt% at 208 mbsf. There is no clear evidence for elevated carbonate from 216 to 226 mbsf, the interval where carbonate concretions and carbonate cements (see "Lithostratigraphy") were observed. TOC ranged from 0.18 to 1.24 wt% (average = 0.5 wt%) and TN ranged from 0.02 to 0.14 wt% (average = 0.06 wt%) (Table T11; Fig. F35). TOC and TN contents were slightly lower in the uppermost 40 m relative to the deeper sediment at this site. C/N ratios ranged between 6.0 and 11.4 and averaged 8.3 for all samples. The values show no apparent trend with depth and indicate a primary marine source with limited terrestrial influence.

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