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doi:10.2204/iodp.proc.311.107.2006

Organic geochemistry

The shipboard organic geochemistry program for Site U1329 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. 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 the TC and IC. A total of 55 HS gas samples (Holes U1329C and U1329E) and 51 samples for solid-state analysis (Holes U1329C and U1329E) were collected from the cut end 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 resolution in shallow sediments near the SMI. We collected 39 HS gas and sediment samples from Hole U1329C, which was sampled to a depth of 186.7 mbsf. The additional 16 HS gas samples were obtained from Hole U1329E, which was sampled at high resolution from the surface to a depth of 30 mbsf to capture the SMI. A total of 29 void gas samples were collected from Hole U1329C from depths where gas expansion features in the sediment were first observed (26.6 mbsf) to a TD of 186.2 mbsf. Two gas samples were collected during degassing experiments of PCS cores recovered at 56 and 190 mbsf (see "Pressure coring"). Hole U1329D, which yielded one whole-round sample for IW analysis, was extensively washed with drilling fluid and was therefore deemed unsuitable for HS analysis.

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

• Determine the origin (microbial versus thermogenic) of gases recovered by HS gas, void gas, and PCS degassing techniques;
• Investigate the fate of methane near the SMI; and
• Quantify the carbon and nitrogen contents in the sediments.

Hydrocarbons

Hydrocarbon measurements from Holes U1329C and U1329E HS gas samples are listed in Table T4. 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). The data are normalized to 3 mL sediment volumes. In Hole U1329E, methane and ethane results are plotted in Figure F24 relative to depth. Methane content increased rapidly from just above background levels (2–37 ppmv) at 1.5–6.5 mbsf to ~16,413 ppmv at 23.5 mbsf. Below the elevated surface values and above the seismically inferred BSR (126 mbsf), HS methane ranged from ~5000 to 8000 ppmv. Below the BSR, the HS methane values decreased to a minimum value of ~1600 ppmv at 180.4 mbsf. An air sample collected from the catwalk area during Site U1329 operations had a concentration of 2.1 ppmv methane, which is slightly elevated relative to atmospheric methane concentrations (~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. The data reported in Table T4 are uncorrected for the atmospheric contribution.

Quantitative sampling of methane by the HS method is limited by the low solubility of methane in solution. Methane exsolution from IW caused by depressurization occurs during core recovery; an effect that may be exacerbated in deeper samples with low porosity. 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. The concentrations reported are, therefore, minimal estimates.

Trace amounts of HS ethane were present below 12.6 mbsf (1.2 ppmv) in Hole U1329C and below 16.0 mbsf in U1329E. In Hole U1329C, HS ethane increased slightly to a mid-depth maximum of 3.6 ppmv at 108.1 mbsf. Trace levels of ethylene (0.8 ppmv) were also observed in this interval. Below the projected depth of the BSR (126 mbsf), the concentration of HS ethane, in general, decreased and was scattered between 0 and 4.1 ppmv until the base of the hole (186.7 mbsf), where the highest HS ethane concentration at Site U1329 was observed. Propane was only observed at the base of Hole U1329C at a concentration of 19.9 ppmv.

The composition of gas from voids in the core liner is shown in Table T5 and concentrations of methane and ethane plotted relative to depth in Figure F25. The void gas was, on average, >90% methane with minimal air contamination (indicated by the concentrations of nitrogen and oxygen). Void gas samples reflect the composition of gas in the subsurface, but not the amount. The subsurface gas content is probably proportional to the general abundance and internal pressure of core voids, but it is difficult to quantify. Trace levels of n-butane (0.3–1.5 ppmv) and i-pentane (0.2–0.3 ppmv) were observed from 59.7 to 125.2 mbsf. With increasing depth to 180.8 mbsf and below the boundary that roughly coincides with the lithostratigraphic Unit II/III boundary (see "Lithostratigraphy"), propane and i-butane were first observed and the concentrations of n-butane and i-pentane increased. The sample collected at the base of the hole (Section 311-U1329C-22X-5; 186.2 mbsf) contained the highest concentrations of C₂–C₅ hydrocarbons. The complementary IW sample from this depth had the lowest salinity and dissolved chloride concentration of all samples from this site (see "Interstitial water geochemistry").

Gas composition expressed as the C₁/C₂ ratios of HS gas is plotted relative to depth with the void gas and PCS data in Figure F26. The C₁/C₂ ratios from the HS gas samples are consistently lower than those from the void gas samples at equivalent depths. A possible explanation for the difference is that the in situ dissolved gases are preferentially enriched with ethane, but it is also possible that the difference may be caused by preferential exsolution of C₁ relative to C₂ during the formation of the gas cracks. The observation of methane enrichment in the void gas and ethane enrichment in the HS gas, which is produced from extraction of dissolved gases from solution that may have already lost some of its methane, is consistent with preferential exsolution of methane. PCS gas samples had a C₁/C₂ composition similar to the void gas data. Further research is required to conclusively understand the relationship of the offset between the C₁/C₂ ratios of the gas components. Hydrogen sulfide was absent in all void gas samples.

The routine 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. The C₁/C₂ ratios of HS and void gas samples are plotted relative to sediment temperature in Figure F27. The sediment temperature was estimated assuming the calculated geothermal gradient of 76.4°C/km (see "Physical properties"). C₁/C₂ ratios are described as either "normal" or "anomalous" depending upon where they plot relative to the slightly diagonal line. All values measured at Site U1329 were within the acceptable "normal" limits for safe drilling (Pimmel and Claypool, 2001).

The HS and void gas C₁/C₂ ratios show downhole transitions related to the lithology and, possibly, the BSR. The HS gas C₁/C₂ ratios are high in the surface sediments (15.6 mbsf, the shallowest depth of ethane occurrence) and decreased gradually throughout and slightly beyond lithostratigraphic Units I and II (see "Lithostratigraphy"). The high HS gas C₁/C₂ ratios in the surface sediments (from ~6,000 to 12,500) may be explained by production of methane in the shallow sediments (dilution of C₂) or as a sampling artifact. High-porosity sediments have a lower surface to volume ratio and, therefore, may retain more dissolved methane before they are sampled. Lower C₁/C₂ ratios (from ~1900 to 3300) above the estimated depth of the BSR (125 mbsf) reflect an increased contribution from ethane (Fig. F24), which was either produced in situ by low-temperature diagenetic reactions or by migration from a thermogenic source. The increase in the C₁/C₂ ratios below the BSR is the result of a decrease in C₂ (Figs. F25, F26). With increasing depth in lithostratigraphic Unit III, the C₁/C₂ values decreased to a minimum value of 422 at the base of Hole U1329C, which indicates a contribution from a deep thermogenic hydrocarbon source.

The void and PCS gas data followed a pattern similar to the downhole pattern of the HS gas data. The highest C₁/C₂ values (from ~15,900 to 18,100) were observed near the depth where void gas first appeared (~18,100 at 29.7 mbsf) and slightly below the BSR (~20,275 at 139.2 mbsf). With increasing depth within lithostratigraphic Unit II (see "Lithostratigraphy"), the C₁/C₂ ratios decreased to the estimated depth of the BSR. This decrease may be attributed to the dissociation of gas hydrate (because gas hydrate preferentially concentrates C₂ relative to methane [Sloan, 1998]), low-temperature diagenetic production of ethane, or an increased thermogenic gas contribution (Pimmel and Claypool, 2001). PCS gas C₁/C₂ data from 55.8 and 73.5 mbsf compared favorably with the void gas data from similar depths, although at 73.5 mbsf the PCS C₁/C₂ ratio is slightly higher.

The lowest void gas C₁/C₂ ratio from Site U1329 was measured at the base of Hole U1329C (~3900 at 186.2 mbsf). Gas collected during the degassing of PCS Core 311-U1329C-23P extended the observation of decreasing C₁/C₂ ratios with depth in lithostratigraphic Unit III to 188.5 mbsf (Table T6). The C₁/C₂ ratio of the PCS gas was 475, which was substantially lower than the void gas sample collected 2.3 m above the top of the PCS core depth. Also, the concentrations of C₂–C₅ hydrocarbon gases in the PCS sample were an order of magnitude greater than those of the void gas sample, and CO₂ was considerably higher (8,270 versus 22,247 ppmv). Salinity and chloride concentrations (22 and 380 mM, respectively) from 201.7 mbsf in Hole U1329D were also lower than values from the bottom portion of Hole U1329C (28 and 480 mM, respectively; 183.6 mbsf) (see "Interstitial water geochemistry"). The freshening of the IW was interpreted to be the result of migration of fluids from a deeper, fresh fluid source (see "Interstitial water geochemistry"). The similar pattern of rapid change in the void gas composition suggests a similar deep source for the hydrocarbon gas and CO₂. Determining the source of the dissolved hydrocarbons and associated fluids is a subject for further investigation.

Biogeochemical processes

The sulfate concentration profiles established the base of the sulfate reducing zone at 9.4 mbsf (see "Interstitial water geochemistry"). The IW sulfate data are plotted with the dissolved methane concentrations in Figure F28 to illustrate the relationship between the reduction of sulfate and the anaerobic oxidation of methane (AOM). The concentrations of sulfate and methane at the SMI were both 0.2 mM. At sediment depths above (7.9 mbsf) and below (11.0 mbsf) the SMI, the concentration of methane or sulfate were near or below the limit of analytical detection and are presumed to be insufficient for supporting the AOM. Delineating the processes occurring within this biogeochemically active region can only be achieved with high-resolution sampling and postcruise stable carbon isotope analysis of the various carbon phases.

Sediment carbon and nitrogen composition

The sediment IC, CaCO₃, TC, TOC, and TN concentrations and C/N ratios from Hole U1329C are listed in Table T7 and plotted relative to depth in Figure F29. IC was relatively low with 48 of 51 samples containing <1 wt% IC and an average IC of 0.48 wt%. The variability reflects primary changes in biogenic and authigenic carbonate. The samples that contained elevated carbonate were concentrated near the depth of the AOM-generated alkalinity peak at 17.8 mbsf (see "Interstitial water geochemistry") and from 69.9 to 74.4 mbsf. TOC ranged from 0.1 to 1.7 wt% (average = 0.8 wt%) and TN ranged from 0.03 to 0.17 wt% (average = 0.08 wt%) (Table T7; Fig. F28). Lithostratigraphic Unit I (see "Lithostratigraphy") had the lowest TOC (average = 0.52 wt%) and TN (average = 0.06 wt%) values, although it is notable that a distinct peak in TOC (1.14 wt%) and TN (0.12 wt%) concentrations occurred near the SMI, a region where methane is converted to microbial biomass. Maximum TOC and TN concentrations occurred at 119.4 to 123.9 mbsf, which is near the projected depth of the BSR. Beyond the high concentrations observed near the projected BSR, TOC and TN contents displayed no apparent trend with depth in lithostratigraphic Units II and III (TOC = 0.93 wt% and TN = 0.10 wt%). Based on the C/N ratios, which averaged ~10.0 and showed no apparent trend with depth, organic matter in the sediment has a predominately marine origin with some terrestrial influence.

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