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doi:10.2204/iodp.proc.311.210.2009

Results

The carbonate mineralogy is generally very complex, with mixtures of calcium carbonates (calcite and aragonite), dolomite, and occasionally rhodochrosite and siderite. There are no obvious carbonate mineralogical variations with depth, especially in relation to the location of the base of the gas hydrate stability zone (BGHSZ) (Blanc-Valleron et al.). However, it is important to notice that siderite and rhodochrosite occurrences are limited to sedimentary layers lower than 125 meters below seafloor (mbsf).

The analytical results for authigenic carbonates are reported in Table T1 with both carbonate mineralogy and oxygen and carbon isotopic compositions.

Oxygen and carbon isotopic compositions of carbonates from all sites

Oxygen and carbon isotope compositions display large ranges of variations (Fig. F2). The widest ranges are measured in calcite/aragonite: –1.27‰ < δ18O < 6.58‰ and –51.61‰ < δ13C < 23.38‰. There is one sample (311-U1329C-22X-CC, 72–74 cm) that falls far off this range with a δ18O value of –8.68‰; its microsparitic facies does not correspond to that of an authigenic carbonate (Blanc-Valleron et al.), and it was thus excluded from the plots. Dolomite also exhibits large variations: 3.31‰ < δ18O < 7.73‰ and –38.35‰ < δ13C < 33.46‰. Carbonate mixtures appear very similar to dolomite: 2.10‰ < δ18O < 6.79‰ and –41.49‰ < δ13C < 37.55‰.

The δ18O values of dolomite, as well as of carbonate mixtures, are generally shifted by a few permil compared to the calcite/aragonite values, as it is expected that dolomite, siderite, and rhodochrosite are enriched in 18O relative to calcite precipitated in similar conditions. There is also a significant difference in the δ13C values of the carbonate phases that are ~10‰ lower in calcite/aragonite compared to dolomite and most of the carbonate mixtures.

These values are very similar to those measured on authigenic carbonates sampled during previous cruises on the Cascadia margin (Ocean Drilling Program [ODP] Legs 146 and 204) and Hydrate Ridge (SO109 and SO110 cruises of GEOMAR) (Kastner et al., 1995a, 1995b; Kopf et al., 1995; Sample and Kopf, 1995; Greinert et al., 2001).

Oxygen and carbon isotopic distributions with depth

Site U1325

Site U1325, drilled to 2195 mbsf, is located within the first slope basin in the southwestern part of the transect. The BGHSZ depth was estimated at 240.5 mbsf. Based on pore water chemistry, the presence of gas hydrates has been inferred in the sandy layers from 70 to 240 mbsf (see the "Site U1325" chapter; Torres et al., 2008).

Two samples were taken above and one sample was taken below the BGHSZ (Table T1). These three samples exhibit very similar δ18O values, but their δ13C values decrease abruptly from –3.94‰ at 221.98 mbsf to –30.23‰ at 258.29 mbsf, below the BGHSZ.

Site U1326

Site U1326, drilled to 1828 mbsf, is located on top of the first uplifted ridge of accreted sediments at the far western downslope end of the transect. The BGHSZ depth was estimated at 260 mbsf. Pore water analysis indicated that gas hydrates were disseminated in the sandy layers below 40 mbsf (see the "Site U1326" chapter; Torres et al., 2008).

There are important variations of δ18O and δ13C values with depth at this site (Fig. F3A, F3B). The diagenetic carbonates are characterized by high δ18O values and low δ13C values to 118.75 mbsf, and they exhibit low δ18O values and slightly positive δ13C values from 155.9 to 263.75 mbsf. The sample located at 235.9 mbsf has the lowest δ18O value (–1.27‰) measured at this site.

Site U1327

Site U1327, drilled to 1304 mbsf, is located on the midcontinental slope off Vancouver Island, several hundred meters from Site 889 (ODP Leg 146). The BGHSZ depth was estimated at 230 mbsf. Gas hydrates were found above the BGHSZ to 128 mbsf (see the "Site U1327" chapter; Torres et al., 2008).

The δ18O and δ13C values are randomly distributed with depth, being alternatively low and high between 126.4 and 178.43 mbsf (Fig. F4A, F4B). The two samples of authigenic carbonates at 228 mbsf (i.e., very close to the BGHSZ) have similar relatively low δ18O values (0.45‰ and 0.53‰) and high δ13C values (23.33‰ and 23.38‰).

Site U1328

Site U1328, drilled to 1268 mbsf, is located 3.7 km southeast of Site U1327 on the midcontinental slope off Vancouver Island. The BGHSZ depth was estimated at 219 mbsf (see the "Site U1328" chapter). Gas hydrates were present in two main layers: at the subsurface (0–35 mbsf) and just above the BGHSZ (215–222 mbsf).

δ18O values show a rapid decrease by ~3‰ from the uppermost levels down to 36.46 mbsf followed by an increase by ~2.5‰ just at the BGHSZ depth; below the BGHSZ, δ18O values fluctuate between 4.66‰ and 7.42‰ in authigenic dolomite (Fig. F5A). There is a strong difference in δ13C values of the authigenic carbonates that are very low above the BGHSZ (as low as –51.61‰) and increase abruptly by 53‰ below the BGHSZ to reach values of 11.3‰ (Fig. F5B).

Site U1329

Site U1329 was the easternmost and shallowest site (946 mbsf) of the transect. The BGHSZ depth is much shallower than at the other sites and was estimated at 124 mbsf (see the "Site U1329" chapter). At this site, only minor amounts of gas hydrates are supposed to occur above the BGHSZ, but there is no significant chloride anomaly in pore water.

δ18O values are more dispersed in calcite and carbonate mixtures than in dolomite (Fig. F6A, F6B). δ18O values of dolomite show an obvious depth variation from high values (7.07‰ at 31.13 mbsf to 6.93‰ at 104.35 mbsf) above the BGHSZ to progressively decreasing values below the BGHSZ with the lowermost value of 4.30‰ at 176.3 mbsf. There is also a significant change in δ13C values of dolomite with depth, with negative values (–22.55‰ to –6.39‰) above the BGHSZ and highly positive values (21.18‰ to 33.46‰) below the BGHSZ. It is at Site U1329 that the highest δ13C values have been measured in the authigenic dolomite and in a siderite-rich carbonate mixture (δ13C = 37.55‰) at 150.5 mbsf. Such high δ13C values have been reported in very rare authigenic carbonate occurrences as in Hydrate Ridge (Greinert et al., 2001).