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doi:10.2204/iodp.proc.311.105.2006

Organic geochemistry

The onboard organic geochemistry program for Site U1327 included analysis of the composition of volatile hydrocarbons (C₁–C₅) and nonhydrocarbon gases (i.e., O₂ and N₂) from HS gas samples, void gas samples, and gas samples recovered during PCS degassing experiments. We analyzed sediment from the IW squeeze cakes 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 the TC and IC. We collected a total of 145 samples for HS and solid-state analysis at Site U1327. Most of the HS samples were collected from the ends of core sections facing the IW samples so that the gas and IW data could be integrated. The sampling plan (see "Interstitial water geochemistry") was designed to maximize vertical resolution in shallow sediments near the SMI. We collected 78 HS and sediment samples from Hole U1327C, which was sampled to 298.5 mbsf. In Hole U1327D, 37 of 43 HS samples were obtained at high vertical resolution from near the seafloor (0.4 mbsf) to 15.9 mbsf. It was later determined that the SMI interval was not recovered in this hole. We collected six additional samples from Hole U1327D to a maximum depth of 220 mbsf for safety monitoring purposes. The SMI interval missed in Hole U1327D was later recovered with a single APC core deployment in Hole U1327E. From this core, we collected 24 samples between 3.4 and 12 mbsf. In Hole U1327C, we collected a total of 46 void gas samples from depths where gas cracks in the sediment were first observed (25.6 mbsf) to 299.3 mbsf. Finally, we collected six gas samples from five PCS degassing experiments (see "Pressure coring").

The primary objectives at this site were to:

• Determine the origin (microbial versus thermogenic) of the gases recovered by HS gas, void gas, and PCS degassing techniques;
• Investigate the relationship between the gas composition and the presence of gas hydrate;
• Evaluate the fate of methane near the SMI; and
• Compare the carbon and nitrogen contents to gas related features in the sediments.

Hydrocarbons

Headspace gas composition

Hydrocarbon HS gas data from Holes U1327C, U1327D, and U1327E are listed in Table T5. 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 containing the sediment volume listed in Table T5 and as the millimolar concentration of dissolved methane in the IW (see "Organic geochemistry" in the "Methods" chapter). Methane and ethane HS results from Hole U1327C are plotted in Figure F27 as the hydrocarbon gas component relative to depth. The results from high-resolution Holes U1327D and U1327E will be discussed below within the context of the sulfate data. Methane content increased rapidly from just above background levels (3–7 ppmv from 1.5 to 7.6 mbsf) to ~18,400 ppmv from 9.5 to 28.1 mbsf. A few air samples collected from the catwalk area during Site U1327 operations had an average concentration of 1.87 ± 0.03 ppmv (n = 4) methane, which is similar to the current atmospheric methane concentration (~1.7 ppmv). Because the HS vials contained atmospheric air prior to sampling, this contribution should be considered in future isotopic analysis of the HS methane for low-concentration samples collected near the seafloor. The data reported in Table T5 are uncorrected for the atmospheric contribution. Methane values below the region of increasing gas concentrations (>28.1 mbsf) decreased slightly to ~3000 ppmv at 49.1 mbsf and generally remained within the range of ~2000–6000 ppmv to 298.5 mbsf. Quantitative sampling of methane by the HS method is limited by the low solubility of methane in solution at atmospheric pressure (~1.2 mM). Methane exsolution from IW caused by depressurization occurs during core recovery, an effect that may be exacerbated in deeper samples with low porosity or sediments with different lithologies (i.e., sand versus clay). Low-porosity samples are compacted, which makes sampling difficult, and have a greater fraction of their sediment surface exposed to air during collection. Consequently, the apparent decrease in HS methane concentrations with depth may be a sampling artifact that does not necessarily represent in situ conditions. For example, the calculated solubility for methane at 153 mbsf is ~92 mM, whereas the calculated methane concentration was 14.6 mM. A PCS sample from 155 mbsf contained gas hydrate (see "Pressure coring"); therefore, it is likely that the IW at 153 mbsf was saturated. Considering that the HS sample from 153 mbsf was suspected to contain some gas hydrate (concentrations above and below that depth were 2.7 and 2.0 mM, respectively), it is clear that the HS technique underestimates in situ concentrations. The reported concentrations are, therefore, minimal estimates.

Trace amounts of HS ethane (0.8–3.0 ppmv) were present between 8.8 and 134.5 mbsf in both Holes U1327C and U1327D. With greater depth, the concentration of ethane increased gradually from ~5 to 15 ppmv at the projected depth of the BSR. One sample contained anomalously high concentrations of ethane (26.4 ppmv) and methane (~21,512 ppmv) within this interval. Given that this interval also contained evidence of gas hydrate (see "Interstitial water geochemistry"), this elevated value may be the result of a small piece of gas hydrate having been captured in the sample vial. Below the projected BSR depth, the HS ethane concentrations decreased to values within the approximate range of 6–11 ppmv. Ethylene and propane were absent in HS samples from Site U1327.

Void gas composition

The composition of gas from voids in the core liner is shown in Table T6. Void gas was, on average, >90% methane. Void gas reflects the composition of the combined gas from all phases (dissolved, free, and hydrate bound) but not the amount.

Although the concentrations of C₂–C₅ hydrocarbons were low relative to methane, their occurrence and distribution are useful for inferring the presence and type of gas hydrate. Structure I gas hydrate selectively fractionates ethane relative to the source composition, which has a greater lattice volume. Structure II gas hydrate also accommodates propane and i-butane (Sloan, 1998). Localized enrichment of void gases with these compounds may be used as a proxy for dissociated gas hydrate and a specific indicator for the type of hydrate (Structure I or Structure II) that was present.

Ethane was present in all void gas samples (Table T6). The concentrations of ethane increased gradually from 42 ppmv in the shallowest void gas sample (25.5 mbsf) to 157.7 ppmv at 113.5 mbsf (Fig. F28). With greater depth, ethane increased rapidly to ~1150 ppmv at 200.5 mbsf and then decreased to 314 ppmv at the base of the hole. The interval of elevated ethane concentration corresponds to the interval where gas hydrate was inferred from LWD logs (see "Downhole logging"), low-temperature IR anomalies (see "Physical properties"), and low-chloride anomalies (120–223 mbsf; see "Interstitial water geochemistry"). This interval extends to the approximate depth of free gas (~240 mbsf), as inferred from a velocity decrease in the Hole U1327D VSP (see "Downhole logging"). We can, therefore, infer that the ethane enrichment in the void gas resulted from ethane-enriched dissociated gas hydrate.

Propane, i-butane, n-butane, i-pentane, and n-pentane were also present in the void gas, primarily in the same interval where ethane was enriched (Fig. F28). Peak concentrations of propane (17.5 ppmv at 181 mbsf) and i-butane (16.9 ppmv at 214 mbsf) roughly correspond to the ethane peak described above. Normal butane was present from 155 mbsf to the box at the hole in the concentration range of 0.2–2.8 ppmv. Enrichment of propane and i-butane in this inferred gas hydrate–bearing interval suggests Structure II gas hydrate was present. Hydrogen sulfide was absent from all void gas samples.

Gas ratios

C₁/C₂ ratios from the HS gas samples were consistently lower than those of the void gases from equivalent depths (Fig. F29). Although the gases recovered by the two sampling techniques clearly represent different fractions of the total gas pool, it is unclear if the offset reflects in situ differences or a sampling effect. Above the BSR, where free gas generally does not occur, the presumed origin of void and HS gas is either gas hydrate or gas that has come out of solution (exsolution). Void gas is collected directly from the core liner (see "Organic geochemistry" in the "Methods" chapter), which is "sealed" by sediment at the ends of the core barrel and frequently pressurized, whereas HS gas has been depressurized and exposed to atmosphere for a period of time (as long as 20 min). Given the lower solubility and supersaturated condition of methane in recovered cores, methane will preferentially exsolve relative to ethane. The initial gas coming out of solution (captured in the core liner) should contain a greater fraction of methane than that exsolving later (captured by the HS technique). The observation of methane enrichment in the void gas and ethane enrichment in the HS gas may be explained by the difference in sampling time described above and is a factor that should be considered when comparing HS and void gas data. Further research is required to conclusively understand the relationship of the offset between the C₁/C₂ ratios of the gas components.

The C₁/C₂ ratios for the void and HS gas show downhole transitions related to the inferred distribution of gas hydrate at this site (see "Interstitial water geochemistry" and "Downhole logging"). C₁/C₂ of the HS gas decreased gradually from a maximum value of ~14,500 at 13.6 mbsf (the shallowest occurrence of HS ethane) to a minimum of 179 at 224.1 mbsf (the projected BSR). Void gas C₁/C₂ ratios decreased gradually from 25.5 (the shallowest occurrence of void gas) to 108.1 mbsf and then rapidly to the depth of the projected BSR. The transition from a gradual to a rapid decrease in C₁/C₂ ratios of the void gas was accompanied by the occurrence of C₃–C₅ hydrocarbons (Table T5), gases with a primary thermogenic origin. The source of ethane driving the decrease in C₁/C₂ ratios is low-temperature diagenetic ethane production, migration of thermogenic ethane from a deep thermal source, or a combination of both. (Claypool and Kaplan, 1974). The chemical composition of the IW suggests fluids from depths where the temperature exceeds 150°–200°C have migrated into the system (see "Interstitial water geochemistry"). Given that temperatures >90°C are sufficient for thermal generation of hydrocarbons (Claypool and Kaplan, 1974), the fluids and thermogenic hydrocarbons may share a similar origin.

Elevated i-C₄/n-C₄ ratios in the gas hydrate–bearing interval indicated Structure II gas hydrate was present. Normal butane is too large to occupy the Structure II gas hydrate lattice, whereas i-C₄ is not (Sloan, 1998). Consequently, n-C₄ migrates through the region of gas hydrate stability, whereas i-C₄ is sequestered. Subsequent dissociation of the gas hydrate during core recovery results in an elevated quantity of i-C₄ relative to n-C₄ in the void gas. Below the depth where free gas occurs (~240 mbsf; see "Downhole logging") to the base of the hole, the i-C₄/n-C₄ ratio averaged 0.94 (Fig. F30). This value represents a signature unaltered by gas hydrate sequestration and is similar to values reported from cuttings from the Mallik 5L-38 well (Lorenson et al., 2005). Above this depth the ratio increased to a maximum value of ~10 at 214 mbsf; a depth within 10 m of where a low-temperature IR anomaly was observed (see "Physical properties") and a discrete IW sample collected from a 3 cm sand layer had a salinity of 3.7 (see "Interstitial water geochemistry"). The i-C₄/n-C₄ ratios remained elevated to 128.5 mbsf, the shallowest depth where the salinity data showed excursions from background values (see "Interstitial water geochemistry"). Additional research is required to address the possibility that the observed ratios occurred as a result of low-temperature diagenetic reactions, but the agreement between the gas data and other proxies for the presence of gas hydrate described above suggests the observed ratios are related to the occurrence of gas hydrate. Gas composition expressed by the C₁/C₂ ratios of HS and void gas is plotted relative to sediment temperature in Figure F31. Sediment temperature is based on the calculated geothermal gradient of 59.0°C/km (see "In situ temperature profile"). Monitoring of C₁/C₂ in void and HS 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 samples measured at Site U1327 were within the "anomalous" region (Pimmel and Claypool, 2001). However, all the void gas samples, which are better indicators of the in situ gas composition for reasons described above, were within the acceptable limits for safe drilling.

Results from the analysis of gas samples collected during PCS degassing experiments (see "Pressure coring") are shown in Table T7 and are plotted relative to depth with the HS and void gas data in Figure F29. The C₁/C₂ ratios of PCS gas samples were similar to those of void gas samples from similar depths. Because PCS gas samples are purported to reflect the in situ gas composition (including gas from dissociated gas hydrate), agreement between the two data types suggests that void gas samples also represent the in situ gas C₁/C₂. Furthermore, concentrations of methane in the void gas and PCS gas were similar (~90% of the total gas). The i-C₄/n-C₄ ratios of PCS and void gas samples displayed similar downhole trends, but the magnitude of the values differed (Fig. F29).

Biogeochemical processes

The sulfate concentration profile established the base of the sulfate reducing zone at 9.5 mbsf (see "Interstitial water geochemistry"). IW sulfate data are plotted with dissolved methane concentrations in Figure F32 to illustrate the relationship between reduction of sulfate and anaerobic oxidation of methane. All data from Hole U1327D and select data from Hole U1327E are included in Figure F32. An interval of ~1.5 m, which happened to contain the SMI, was not recovered between Cores 311-U1327D-1H and 2H. Core 311-U1327E-1H was collected to capture the missed SMI interval. By design, we drilled to 3.2 mbsf before shooting this core so the SMI would fall approximately in the center of the core. Consequently, no surface sediment data were obtained from Hole U1327E. In Figure F32, the surface data are from Core 311-U1327D-1H. The methane concentration profile was extended with data from Core 311-U1327E-2H to show the decreasing trend of methane concentration below 14.4 mbsf.

The SMI was centered at 9–10 mbsf where the dissolved methane and sulfate concentrations were 1.2 and 2.0 mM, respectively.

Sediment carbon and nitrogen composition

The sediment IC, carbonate (CaCO₃), TC, TOC, and TN concentrations and C/N ratios from Site U1327 are listed in Table T8 and plotted relative to depth (0–300 and 0–13 mbsf) in Figure F33. The IC and carbonate contents were relatively low with 82 of 83 samples containing <1 wt% carbonate and an average IC of 0.34 wt%. The variability reflects primary changes in biogenic and authigenic carbonate and does not show a clear relationship with depth from 0 to 300 mbsf. The near-surface Cores 311-U1327D-1H and 311-U1327E-1H were selected to obtain high-resolution profiles in the uppermost 13 m. The IC and carbonate contents were low throughout the uppermost 13 m (average IC = ~0.35 wt%) and showed no relationship to the depth of the SMI (9 mbsf). TOC ranged from 0.02 to 1.42 wt% (average = 0.62 wt%), and TN ranged from 0.00 to 0.19 wt% (average = 0.08%) (Table T8; Fig. F33). Within the high-resolution cores, the TOC and TN were highest in the sample nearest the sediment/water interface (0.89 wt% TOC and 0.07 wt% TN at 0.3 mbsf) and decreased gradually with depth to uniform values of ~0.4 wt% TOC and ~0.03 wt% TN. C/N ratios in the near-surface sediments had an average value of ~10, reflecting primary marine input with some terrestrial influence. Low values in the range of 3.7 to ~6 at ~200 mbsf are lower than what one would expect from pure marine organic matter or microbial biomass (C/N = ~7). These values most likely reflect absorption of inorganic nitrogen to clay minerals because of high concentrations of ammonium in the pore water (~6–8 mM at 100–200 mbsf) (see "Interstitial water geochemistry").

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