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doi:10.2204/iodp.proc.311.103.2006

Organic geochemistry

The shipboard organic geochemistry program for Site U1325 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, and a gas sample recovered during the PCS degassing experiment. Sediment from IW squeeze cakes was analyzed for content of 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. A total of 78 samples for HS and solid-state analyses were collected at Site U1325. 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. Core 311-U1325B-1H was sampled twice per 1.5 m section to characterize the SMI. It was later determined that this core may have missed the mudline, so an additional hole was drilled to definitively establish the SMI depth (Core 311-U1325D-1H). Three sets of HS samples were collected from inside and outside each IR anomaly interval and imaged with the IR camera to confirm that the samples contained sediment from the cold section of the core.

A total of 45 void gas samples were collected from Site U1325 from depths where gas cracks in the sediment were first observed (9.1 mbsf) to 297.84 mbsf. A gas sample was collected from a PCS degassing experiment conducted with Core 311-U1325C-10P (see "Pressure coring").

The primary objectives of the organic geochemistry sampling program at this slope basin 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 distribution of gas hydrate in the system; and
• Compare the carbon and nitrogen contents to gas related features in the sediments.

Hydrocarbons

Headspace gas and void gas compositions

Hydrocarbon HS gas measurements from Holes U1325B, U1325C, and U1325D 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 and as the millimolar concentration of dissolved methane in the IW (see "Organic geochemistry" in the "Methods" chapter). Methane content increased rapidly from 10 ppmv in the near-surface sample (0.8 mbsf) to ~14,000 ppmv at 7.5 mbsf. In the deeper sections of Holes U1325B and U1325C, the methane HS concentrations generally varied between ~2000 and 6000 ppmv and showed no relationship with depth (Fig. F28). The two HS samples outside this range (~11,000 and 26,000 ppmv) were collected from cold spots identified with the IR camera. Trace amounts of ethane (<1 ppmv) were present in four samples from sediment depths >248 mbsf. There was no evidence of ethylene or propane in any HS samples collected from Site U1325. Air samples collected from the catwalk area during Site U1325 operations had an average concentration of 1.84 ± 0.29 ppmv (n = 2) methane, which is similar to 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 in Figure F29. With the exception of several samples that contained a large percentage of air, the void gas was composed almost entirely of methane with a small percentage of carbon dioxide (~0.1%–1.2%). Trace quantities of C₂₊ hydrocarbons (<13 ppmv) were present above the seismically inferred BSR (230 mbsf). With greater depth, the concentrations of ethane, propane, and i-butane increased but did not exceed 100 ppmv. Trace concentrations of C₂₊ hydrocarbons suggest limited contribution of thermogenic gases. Source identification will be verified by postcruise stable carbon isotope analysis of methane.

The molecular ratios of methane to ethane (C₁/C₂) were the highest observed during Expedition 311. Values from where gas voids were first observed (9.1 mbsf) to the seismically inferred BSR ranged from ~61,000 to 205,000 and increased slightly with depth (Fig. F30). Below the BSR, the values decrease slightly to a minimum of ~10,000 at the base of Hole U1325C. This "low" value, however, is among the highest values measured at the other sites investigated during Expedition 311.

The hydrocarbon molecular ratios (e.g., C₁/C₂) from Site U1325 did not provide clear evidence for gas hydrate in the interval above the seismically inferred BSR, whereas IR temperature anomalies (see "Physical properties") and low chloride anomalies (see "Interstitial water geochemistry") clearly did. It appears that the concentrations of C₂₊ hydrocarbons in this system were too low to have had a measurable effect on the molecular signature of the gas hydrate. We suspect the gas hydrate identified by the IR and IW data was essentially pure methane hydrate with a molecular composition indistinguishable from the background void gas signature. Postcruise stable carbon isotope analysis of the methane and ethane should help resolve this issue.

Gas composition expressed by the C₁/C₂ ratio of HS and void gas is plotted relative to sediment temperature in Figure F31. The sediment temperature was estimated assuming the calculated geothermal gradient of 60°C/km (see "Physical properties"). The 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 (JOIDES) 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. All values measured at Site U1325 were within the acceptable "normal" limits for safe drilling (Pimmel and Claypool, 2001).

Pressure coring system gas composition

Splits from gas recovered during the Core 311-U1325-10P PCS degassing experiment contained as much as 1.5% ethane. The void gas from above (Core 311-U1325C-9X) and below (Core 11X) this core contained ethane at concentrations of 54 and 67 ppmv. Furthermore, the check samples analyzed in the PCS van did not contain elevated ethane. We concluded that the samples were contaminated during storage and did not reflect the composition of the PCS gas.

Biogeochemical processes

The IW and HS from Core 311-U1325B-1H were sampled at high depth resolution to define the biogeochemical zones associated with the SMI. The depth of sulfate depletion for Core 311-U1325B-1H was 2.2 mbsf, but it was later determined that this core missed the mudline. Core 311-U1325D-1H, which was collected near the end of Site U1325 operations specifically to establish the mudline, placed the actual depth of sulfate depletion (by extrapolation) to between 4 and 5 mbsf (see "Interstitial water geochemistry"). The dissolved methane concentration data from Core 311-U1325D-1H were complementary to the sulfate data and established the depth of the SMI at ~1.5 mbsf (Fig. F32). The concentrations of sulfate (1.1 mM) and methane (0.5 mM) at the SMI should support the net anaerobic oxidation of methane (AOM) (Hoehler et al., 1994). Microbiological and geochemical studies of AOM should utilize the high-resolution samples from Core 311-U1325B-1H and assume an SMI depth of 4–5 mbsf (as determined from Core 311-U1325D-1H) for sulfate and methane flux calculations.

Sediment carbon and nitrogen composition

The sediment IC, carbonate (CaCO₃), TC, TOC, and TN concentrations and C/N ratios from Site U1325 are listed in Table T7 and plotted relative to depth in Figure F33. With the exception of the interval from 127.9 to 143.7 mbsf, where IC ranged between 1.0 and 1.4 wt%, IC was relatively low throughout the core and showed no clear trend with depth. The TOC was highest in the surface (0–6 mbsf) sediments (several values exceeded 1.5 wt%) and tended to decrease with depth. TN showed no apparent trend with depth. The C/N ratios averaged ~8 in the uppermost 100 to 150 m at Site U1325, which is consistent with a primary origin of marine organic matter. Below 150 mbsf, the C/N ratios averaged 5.5. This value is more enriched with nitrogen than expected for marine organic matter (~6–7). Ion exchange of ammonium for magnesium on clays (see "Interstitial water geochemistry") may explain the low C/N ratios.

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