RESULTS

The results of the shipboard gas chromatographic analyses of gas samples are presented in the individual site chapters of the Leg 204 Initial Reports volume (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Figure F2 summarizes the chemical composition of gases in terms of the C₁/C₂ ratio. Four general patterns of C₁/C₂ composition are illustrated by the different sites cored during Leg 204. At Sites 1245 and 1247 the C₁/C₂ values are high (10⁵–10⁴) in the uppermost 100–120 m beneath the seafloor then decrease rapidly to low values (10²) at depths corresponding to the occurrence of seismic Horizon A (150–175 mbsf), a high-amplitude feature found in cores to be an ash/turbidite layer. Sites 1244, 1246, and 1251 are on the landward (east) side of Hydrate Ridge and are not underlain by Horizon A. These sites show a significant offset in C₁/C₂ values at the base of gas hydrate stability, from high to intermediate (10³) values. Site 1252 is in a similar landward position with respect to the ridge but shows a gradual decrease in C₁/C₂. Sites 1248, 1249, and 1250 at or near the summit of Hydrate Ridge show a pattern inverse to that typically found in ODP cores in that C₁/C₂ values are low to intermediate near the seafloor and increase to high values at 50–100 mbsf. With further increase in depth at Sites 1248 and 1252 the C₁/C₂ values decrease again toward the BSR and Horizon A.

Table T2 lists ¹³C values of CH₄ and CO₂ in void gas samples, and Figure F3 illustrates the variation in ¹³C of CH₄ as a function of depth of burial, which shows patterns similar to those observed for C₁/C₂ ratios. That is, Sites 1245 and 1247 show the heaviest (least negative) ¹³C₁ values at the depth of seismic Horizon A. Sites 1244, 1246, 1251, and 1252 show more or less monotonic increases in ¹³C₁ with increasing depth of burial. Sites 1248, 1249, and 1250 show heavy ¹³C₁ values at or near the seafloor and become lighter with increasing depth to ~80 mbsf, and then become heavy again. The ¹³C values of CO₂ are not plotted but are generally subparallel with the ¹³C₁ values.

Table T3 gives ¹³C values of C₂–C₅ hydrocarbon components in void gas samples that contained elevated amounts of higher hydrocarbons. Most of the measured ¹³C values for C₃–C₅ hydrocarbons are isotopically heavy (from –27 to –23) compared with the ¹³C₁ values. However, the ¹³C values of ethane appear to represent different populations, as shown in the plot of C₁/C₂ vs. ¹³C₂ (Fig. F4). Ethane components in gas samples with relatively low C₁/C₂ ratios have isotopically heavy values (from –32 to –28), whereas ethane in samples with high C₁/C₂ values is light (from –54 to –44).

Table T4 gives the D values of CH₄ in selected void gas samples. The D values of methane range from –208 to –187, are generally parallel to ¹³C values, and are in the same range observed for D when measured in other DSDP/ODP cores.

The ¹³C values of CH₄ in gas samples from the PCS cores are listed in Table T5 and plotted vs. depth in Figure F5. The methane given off during the PCS degassing experiments had ¹³C values that were consistent with those of void gas samples at the nearby depths. Gas collected in multiple aliquots showed random variations of ±1.

The isotopic compositions of gases liberated from gas hydrate samples collected during Leg 204 are presented in Table T6. More complete discussion of the chemical and isotopic compositions of gas hydrate–bound gases and their relation to void gases is given in Milkov et al. (2005).

Table T7 presents the ¹³C values of dissolved inorganic carbon (DIC), present in the subsurface mainly as bicarbonate ion (HCO₃^–). Figure F6 is a plot of the ¹³C of DIC in pore water samples from 0 to 50 mbsf at Sites 1244, 1245, 1246, 1247, 1251, and 1252. All of these sites show a similar pattern, with seawater values (assumed ¹³C of ~0) at the seafloor decreasing to minimum values from –23 to –18, except for Sites 1245 and 1246, which have minimum values of –25 and –30, respectively. The depth of the minimum in the ¹³C-DIC curve corresponds exactly with the depth at which dissolved sulfate is depleted, as discussed below.

Values for ¹³C-DIC in pore water samples from Sites 1248, 1249, and 1250 at and near the summit are not plotted, but are all ¹³C-enriched (from +13 to +17), except for three samples from interval 0–1.4 mbsf at Site 1248 that have negative ¹³C values (from –24 to –1).

Leg 204 pore water analyses are presented in the individual site chapters of the Leg 204 Initial Reports volume (Tréhu, Bohrmann, Rack, Torres, et al., 2003). These results provide the framework for the following discussion of diagenetic processes occurring in the shallow sediments at Hydrate Ridge. The main results discussed are concentration-depth profiles of dissolved sulfate, alkalinity (bicarbonate), calcium, magnesium, ammonium, and phosphate.