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doi:10.2204/iodp.proc.311.204.2009

Results

XRD analyses show that SED samples from Sites U1325–U1329 are dominated by detrital minerals, mainly quartz, plagioclases and potassium feldspars, micas, clay minerals, amphiboles, and sometimes pyroxenes (e.g., diopside). Some pyrite and carbonate minerals are also present as minor components. Magnetic minerals (magnetite and greigite) (Esteban and Enkin, 2007) are also present but were not identified in the raw samples analyzed by XRD, even if they sometimes seemed relatively abundant during clay fraction extraction.

Because of the inhomogeneous nature of the sediment (Expedition 311 Scientists, 2005) the mineralogical composition varies considerably vertically and laterally. We did not attempt to quantify the mineral phases present in the bulk samples because

• Phyllites, amphiboles, and feldspars s.l. have variable compositions;

• Many minerals are present near or below the detection limit; and

• Superposition of peaks is frequent.

In this report we focus only on the carbonate phases, whereas Bartier et al. focus on the clay mineralogy (<2 μm fraction) of selected samples.

SED samples

Carbonate (MCM) analyses indicate that SED samples are carbonate poor (mean values ≈ 2–3 wt%) (Table T1). Calcite, mainly low-Mg calcite (LMC) with a d₁₀₄ peak between 3.035 and 3.030 Å (Table T2), is the dominant carbonate phase with some dolomite (d₁₀₄ peak usually located between 2.885 and 2.900 Å); other carbonates were generally not detected. In these low-carbonate samples, the relative abundance of detrital minerals makes quantification of calcite and dolomite percentages uncertain, as albite interferes with high-Mg calcite (HMC) and microcline interferes with nonstoichiometric dolomite; therefore, only the total carbonate percentage is given.

Site U1326 is located on top of the first uplifted ridge of accreted sediments at the far western downslope end of the transect (Fig. F1). Sediments from this site show the lowest carbonate contents of the transect (Fig. F3; Table T2). Slightly higher values in lithostratigraphic Unit I may correspond to calcareous microfossil remains, as observed in Sample 311-U1326C-3H-5, 80–82 cm (5.5 wt% carbonate with nearly stoichiometric calcite and abundant foraminifers on sieved fractions). However, carbonate contents in Samples 311-U1326C-4H-3, 75–77 cm (26.65 meters below seafloor [mbsf]), 311-U1326D-2X-4, 75–77 cm (93.65 mbsf), and 15X-1, 73–75 cm (214.33 mbsf), may correspond to some authigenic carbonate cement scattered in the sediment, as suggested by the composition of calcite (5–7.7 mol% MgCO₃) (Table T2). Sample 311-U1326D-2X-1, 72–74 cm (MCM values ≈ 10 wt%), originally a SED sample, was added to the CARB series because irregular carbonate grains (~6 μm in diameter and 6.5 mol% MgCO₃ in calcite) and almost no calcareous microfossil remains were observed in sieved fractions and smear slides. In addition, the mousselike texture observed in the section may be associated with dissociation of gas hydrate.

Site U1325 is located within the first slope basin in the southwestern part of the transect cored during Expedition 311 (Fig. F1). In lithostratigraphic Units I and III (see the "Site U1325" chapter), biogenic components (very abundant foraminifers sometimes observed in the sieved fractions) are more abundant, as reflected by the slightly higher amount of carbonate observed in the sediments (Fig. F4; Table T2), although they drop to very low values in lithostratigraphic Units II and IV (almost no foraminifers observed in the sieved fractions). In sediments, calcite is generally nearly stoichiometric, except in Samples 311-U1325B-10X-1, 75–77 cm (73.55 mbsf), 311-U1325C-7X-6, 75–77 cm (233.42 mbsf), and 8X-4, 75–77 cm (241.40 mbsf), with 7.7, 5.0, and 4.4 mol% MgCO₃ in calcite, respectively (Table T2). This is interpreted as corresponding to biogenic components.

Site U1327 is located near the middle of the coring transect on a broad uplifted ridge of accreted sediments about mid-slope up the margin, near Ocean Drilling Program (ODP) Leg 146 Site 889 (375 m southeast from Hole 889C) (Fig. F1). In lithostratigraphic Unit I, interstitial water geochemistry results predict authigenic carbonate formation within the sulfate reduction and alkalinity generation zone (see the "Site U1327" chapter); however, no authigenic carbonate cement was found. Slightly higher carbonate contents were found in samples from 0 to 32.5 mbsf (0–2 mol% MgCO₃ in calcite). Carbonate contents of sediments throughout the lithostratigraphic units are generally low, with slightly higher values in Samples 311-U1327C-3H-1, 75–77 cm (16.35 mbsf), 4H-3, 75–77 cm (28.85 mbsf), 9H-6, 75–77 cm (73.35 mbsf), and 13X-4, 75–77 cm (107.75 mbsf), which are associated with foraminifer-rich intervals and substoichiometric calcite. However, as at Site U1326, carbonate contents in Samples 311-U1327C-12X-6, 75–77 cm (99.87 mbsf), 18X-3, 75–77 cm (144.60 mbsf), and 31X-5, 75–77 cm (263.81 mbsf), might correspond to some authigenic carbonate particles scattered in the sediment, as suggested by the composition of calcite (~6 mol% MgCO₃) (Table T2; Fig. F5). Sample 311-U1327C-19X-1, 75–77 cm (MCM value ≈ 20 wt%), was added to the original CARB series, as the carbonate value is not explained by calcareous organisms and corresponds mainly to HMC (11.8 mol% MgCO₃).

Site U1329 is the easternmost and shallowest site of the transect and is interpreted to be located near the eastern limit of gas hydrate occurrence on the northern Cascadia margin (Fig. F1). Moreover, it is the only site with Miocene sediments (lithostratigraphic Unit III) occurring in discordance below Pleistocene sediments. The sediments at Site U1329 have the highest carbonate values (up to 18.6 wt% in Sample 311-U1329C-3H-2, 75–77 cm) (Table T2; Fig. F4) of the transect and mainly correspond to foraminifer- or calcareous nannoplankton–rich levels, as shown by examination of some smear slides and sieved fractions and to substoichiometric calcite. Miocene sediments of lithostratigraphic Unit III contain almost no carbonates (0–3 wt%; average value ≈ 1 wt%) compared to the upper part of the series. At Site U1328, the average value of carbonates is 3.2 wt% (~3.7 wt% if Miocene sediments are excluded), whereas Sites U1326–U1327 contain lower carbonate contents (~2 wt%); along the transect, carbonate contents decrease with distance to the shoreline. As at Sites U1326 and U1327, some authigenic carbonate may be scattered in the sediment (calcite, siderite, and/or dolomite), but their identification is difficult except in Sample 311-U1329C-21X-1, 75–77 cm (170.05 mbsf), the carbonate paragenesis of which (5.4 mol% MgCO₃ calcite + siderite) is similar to nearby CARB Sample 311-U1329C-21X-2, 118–120 cm (mainly siderite + 7.4 mol% MgCO₃ calcite).

Site U1328 is an active cold seep (Bullseye vent site) associated with faults located 3.7 km southeast of Site U1327, on the mid-continental slope off Vancouver Island (British Columbia) (Fig. F1). It is characterized by a massive accumulation of gas hydrate in the upper ~40 m of the series. Carbonate contents of the sediments (average = 2.6 wt%) (Tables T1, T2; Fig. F7) are slightly higher than at nearby Site U1327 (average = 2.2 wt%). This may be due to the presence of dispersed authigenic carbonate crystals throughout the cores. For example, the presence of HMC in the upper part of the cores (0–38 mbsf) is inferred from the intensity of the peak around 3.0 Å (corresponding to 10–11 mol% MgCO₃ in calcite) (Table T2). However, LMC (0–3.5 mol% MgCO₃) is the main carbonate present, and most of the carbonate-rich sediments correspond to foraminifer-rich levels (Table T2).

CARB samples

XRD diffractograms of the carbonate-rich samples show that the d₁₀₄ reflection peak for some calcites and dolomites is asymmetrical and formed by the superposition of two peaks. This indicates that those carbonates are heterogeneous, with different populations contributing to the composite diffractogram peaks.

From the distribution of d₁₀₄ values (Fig. F2; Table T2) we distinguish the following:

• Stoichiometric to low-Mg calcite (3.035–3.025 Å) corresponding to 0–3.4 mol% MgCO₃ (associated with foraminifers and nannoplankton remains) or medium-Mg calcite (MMC) (3.025–3.01 Å) corresponding to 3.4–8.4 mol% MgCO₃;

• Very high magnesium calcite (3.010–2.984 Å) corresponding to 8.4–17.1 mol% MgCO₃;

• Nonstoichiometric dolomite (LMD) with d₁₀₄ varying between 2.944 and 2.905 Å and variable chemical composition (Ca, Mg, some Fe, but no Mn in EDS spectra recorded); and

• More stoichiometric dolomite (HMD) with d₁₀₄ varying between 2.905 and 2.887 Å.

Site U1326

At Site U1326, salinity and chlorinity data (see the "Site U1326" chapter) indicate that gas hydrate is present in the zone extending from ~45 to 270 mbsf, predominantly occurring in the sandy layers. Seven carbonate-rich samples were analyzed from Site U1326 (Table T3; Fig. F3); calcite, dolomite, and sometimes rhodochrosite and siderite were identified.

In lithostratigraphic Unit I, carbonates correspond mainly to unlithified carbonate cement, as well as partly lithified carbonate, observed between 0.97 and 2.15 mbsf in intervals 311-U1326B-1H-1, 95–102 cm; 1H-1, 149–150 cm; and 1H-2, 62–80 cm (see the "Site U1326" chapter). The concretion sampled at 6.32 mbsf (Sample 311-U1326C-2H-2, 92–93 cm) is composed of a mixture of LMD (d₁₀₄ = 2.914 Å) and magnesian calcite (~7 mol% MgCO₃).

In lithostratigraphic Unit II, carbonates correspond mainly to unlithified carbonate cement, observed between 40 and 119 mbsf with some lithified carbonates (e.g., Sample 311-U1326C-9X-2, 92–93 cm). Dolomite (d₁₀₄ = 2.900–2.905 Å) is the main carbonate present.

In lithostratigraphic Unit III, carbonate contents are lower than above (16–33 wt%) and correspond to carbonate cements composed of magnesian calcite (8.7 and 7.7 mol% MgCO₃ in Samples 311-U1326D-9X-1, 0–1 cm, and 17X-3, 0–100 cm, respectively) or to a mixture of rhodochrosite (d₁₀₄ ≈ 2.841 Å instead of 2.850 Å), very high magnesium calcite (13.8 mol% MgCO₃), siderite (d₁₀₄ ≈ 2.795 Å), and dolomite in Sample 311-U1326D-20X-3, 0–1 cm, which is near the base of the gas hydrate stability zone (BGHSZ) (264 mbsf).

Site U1325

At Site U1325, salinity and chlorinity data (see the "Site U1325" chapter) indicate that gas hydrate is present from ~70 to 240 mbsf, predominantly occupying the sandy layers and some of it also being associated with the clay lithology in the 50 m above the bottom-simulating reflector (BSR) (~180–230 mbsf). No carbonate cement or concretion was observed in the upper part of these slope basin sediments (lithostratigraphic Units I and II), only one in Unit III (~127 mbsf), and two in Unit IV (~222 and 258 mbsf) (Table T3; Fig. F4). These carbonates contain variable proportions of calcite and dolomite.

In lithostratigraphic Unit III, a layer of cemented carbonate occurs in interval 311-U1325B-16X-6, 27–33 cm. XRD analysis of Sample 311-U1325B-16X-6, 28–30 cm, confirms a combination of LMD (d₁₀₄ = 2.902 Å) and HMC (10.7 mol% MgCO₃).

In lithostratigraphic Unit IV, Sample 311-U1325C-6X-4, 47–48 cm, is mainly composed of a mixture of LMD (d₁₀₄ = 2.924 Å) and HMD (d₁₀₄ = 2.895 Å), whereas HMC (10.4 mol% MgCO₃) dominates in Sample 311-U1325C-11X-1, 119–122 cm.

Site U1327

At Site U1327, salinity and chlorinity data (see the "Site U1327" chapter) indicate that gas hydrate is present in the zone extending from ~128 mbsf to the BSR (~223 mbsf). Fifteen of the diagenetic carbonate levels sampled were analyzed and showed calcite, dolomite, and sometimes siderite (Table T3; Fig. F4). Most of them occur in the gas hydrate zone (see the "Site U1327" chapter). The first authigenic carbonate occurrence corresponds to a light-colored unlithified cement zone (interval 311-U1327C-10H-1, 95–103 cm) (see Fig. F15 in the "Site U1327" chapter) and is mainly composed of 6.7 mol% MgCO₃ HMC and some dolomite at 96–98 cm.

In lithostratigraphic Unit II, few unlithified carbonate cements and numerous lithified carbonates are present. Magnesium calcite (6–16 mol% MgCO₃) and dolomite (HMD, d₁₀₄ = 2.893–2.903 Å; LMD, d₁₀₄ = 2.92–2.93 Å) are the dominant carbonate phases. Siderite is present either as a major component of the carbonate fraction (72% in Sample 311-U1327C-16X-2, 120–150 cm; d₁₀₄ ≈ 2.794 Å) or as a minor phase (7% in Sample 311-U1327C-20X-5, 6–7 cm; d₁₀₄ ≈ 2.786 Å).

In lithostratigraphic Unit III, few unlithified carbonate cements, visible as faint light spots, occur; XRD shipboard analyses shows a combination of HMC with LMD in most of the cores or a combination of rhodochrosite with HMC and calcium-rich dolomite in Samples 311-U1327C-29X-4, 59–60 cm, and 30X-3, 80–81 cm (~243 mbsf). Lithified carbonate rocks (Table T3) correspond to dolomite (d₁₀₄ = 2.897 Å in Sample 311-U1327C-21X-7, 0–1 cm) or ~5.5 mol% MgCO₃ calcite (Samples 311-U1327E-16X-CC, 10–11 cm, and 18–19 cm). Previous shipboard analyses of lithified carbonate rocks from interval 311-U1327C-24P-1, 0–69 cm, show different carbonate mineralogies (substoichiometric dolomite to LMD and/or HMC).

Site U1329

Site U1329 is at the eastern limit of gas hydrate occurrence on the northern Cascadia margin. Seismic indicators of gas hydrate are present but more subdued than at all the other sites. Only minor cold anomalies were detected in the cores from this site, consistent with evidence of only minor amounts of gas hydrate based on logging-while-drilling resistivity measurements and chlorinity analyses (see the "Site U1329" chapter). Thirty-three carbonate samples were analyzed from this site (Table T3; Fig. F6) showing calcite, dolomite, and sometimes siderite.

In lithostratigraphic Unit I, authigenic carbonates correspond to unlithified carbonate cements, as well as lithified and partly lithified carbonates. All are mainly composed of HMD (d₁₀₄ = 2.897–2.905 Å) with some LMD (d₁₀₄ = 2.922–2.906 Å) and sometimes substoichiometric calcite. Previous interstitial water geochemistry results (Mg, Ca, and Mg/Ca values) suggesting dolomitization reactions at ~30 mbsf may explain these dolomite occurrences around 31 mbsf.

In lithostratigraphic Unit II, authigenic carbonates, more or less lithified, are dispersed throughout the series. Calcite (mainly HMC with 13–14 mol% MgCO₃) is the dominant phase except in two samples (311-U1329C-5H-6, 61–63 cm, and 311-U1329E-8Y-1, 35–37 cm), where HMD (d₁₀₄ = 2.905 and 2.894 Å, respectively) is dominant.

In lithostratigraphic Unit III, unlithified carbonate cements and lithified and partly lithified carbonates are abundant throughout the cores. Pure dolomites (HMD; d₁₀₄ = 2.894–2.897 Å) or calcites (~12.5 mol% MgCO₃ [160–165 mbsf], 5.4–6 mol% MgCO₃ [172.91 mbsf], and 2.3 mol% MgCO₃ [187.75 mbsf]) occur, as well as mixtures of calcite and/or dolomite and/or siderite. Sample 311-U1329C-22X-CC, 72–74 cm (187.75 mbsf), with pure LMC (2.3 mol% MgCO₃) exhibits a microsparitic facies quite different from the clotted micrite facies usually observed in the other sampled carbonate facies and may be related to the conglomerate deposit found just below in pressure Core 311-U1329C-23P and interpreted as a major event, possibly a debris flow (see the "Site U1329" chapter). HMD (d₁₀₄ = 2.894–2.905 Å), HMC (10.4–13.4 mol% MgCO₃), and MMC (5.4–8.1 mol% MgCO₃) are the dominant carbonate phases present in lithostratigraphic Unit III, whereas siderite (4%–93% of the carbonate fraction; d₁₀₄ ≈ 2.805–2.809 Å) is less abundant.

Site U1328

Site U1328 (Bullseye vent site) is located within an area of active cold vents, where previous dives conducted in 2000 and 2001 with the remotely operated vehicle ROPOS found 10–15 cm thick carbonate sheets covering an area of >10 m² of the seafloor. Gas hydrate distribution at Site U1328 is bimodal, as gas hydrates occur mainly close to the seafloor (0–35 mbsf) and just above the BSR (215–222 mbsf) (see the "Site U1328" chapter). Thirty-one authigenic carbonate samples were analyzed from this site (Table T3; Fig. F7) with calcite and/or dolomite and/or aragonite. The authigenic carbonates were found in lithostratigraphic Units I (1–37 mbsf) and III (220–277 mbsf) and Sample 311-U1328C-5P-1, 0–2 cm (92 mbsf?), probably originating from Unit I (fallen block?).

In lithostratigraphic Unit I, to 36.46 mbsf, HMC (8.7–17.1 mol% MgCO₃) is the dominant carbonate phase. The HMC is sometimes pure (Sample 311-U1328E-2X-1, 52–54 cm) but is most often mixed with dolomite (d₁₀₄ = 2.894–2.939 Å) and/or aragonite.

In lithostratigraphic Unit III HMC (12.4–16.1 mol% MgCO₃) is the dominant carbonate phase near the BGHSZ, whereas near-stoichiometric dolomite (d₁₀₄ = 2.887–2.895 Å) appears between 253 and 276 mbsf.

Site U1328 differs from the other sites because of the presence of aragonite (8%–46% of the carbonate fraction) between 6 and 16 mbsf. This site is also characterized by less abundant dolomite and the absence of siderite and rhodochrosite.

Main occurrences of authigenic carbonate minerals based on XRD analyses of CARB samples

Aragonite is only present in the upper 16 mbsf of the active vent site (U1328), and is associated with calcite and dolomite (Fig. F8A, F8B). The occurrence of aragonite fits well with the predicted gas hydrate content (highest in the upper 15 mbsf; see the "Site U1328" chapter). In the samples studied, aragonite is associated with abundant HMC (13.1–16.8 mol% MgCO₃) and with some LMD (d₁₀₄ = 2.911–2.939 Å). The link between aragonite and gas hydrate or methane venting was often observed in recently studied sediments (e.g., Hydrate Ridge on the Cascadia continental margin) (Teichert et al., 2005; Teichert and Bohrmann, 2006).

Calcite is most abundant at the active vent Site U1328 (average content = ~68% in the carbonate fraction and 47%–53% in the transect sites), where it corresponds mainly to HMC (8.7–17.1 mol% MgCO₃) (Fig. F8B–F8D). Calcite at transect sites shows more variable compositions with MMC (5.4–8.1 mol% MgCO₃) and/or HMC (8.7–14.1 mol% MgCO₃). LMC is rare and is mainly associated with calcareous biological or detrital remains, based on thin section or smear slide examinations.

Dolomite is well represented in all trasect sites, whereas it is less abundant at Site U1328 (mean average value in carbonate fraction is ~28%, instead of 40%–53%). Dolomite shows variable compositions (d₁₀₄ = 2.987–2.944 Å), sometimes with two phases associated in the same sample. LMD, mainly represented at Site U1328, often shows rounded dolomite grains with a knobbly surface (Fig. F8E), whereas HMD often shows euhedral crystals.

From our data, siderite (Fig. F9A–F9C) is mainly observed at Sites U1329 (associated with calcite) and U1327 (associated with calcite and dolomite). This phase was identified from XRD diagrams (d₁₀₄ ≈ 2.795–2.809 Å instead of 2.796 Å), but EDS analyses of the more carbonate rich part of Sample 311-U1329C-21X-3, 129–131 cm (d₁₀₄ ≈ 2.809 Å), reflect a high content of Mg, Fe, and Ca corresponding to an intermediate phase between magnesite, siderite, and calcite. Complementary analyses such as transmission electron microscopy coupled with microanalyses of individual particles are needed to better characterize what we are recognizing as siderite.

Rhodochrosite is briefly observed from our data at Sites U1326 and U1327 near or below the BGHSZ. This phase was identified from XRD diagrams (d₁₀₄ ≈ 2.850 Å). As it is only a minor component (<15 wt%), SEM observation of this phase is difficult. A very fine grained facies was observed in Sample 311-U1326D-20X-3, 0–1 cm (d₁₀₄ ≈ 2.841 Å instead of 2.850 Å), showing areas <1 μm in size composed of bladelike particles or euhedral nanocrystals (Fig. F9D, F9E, F9G, F9H). From EDS analyses (Fig. F9F) rhodochrosite may not exist, as these carbonate-rich areas are enriched in Ca, Mg, and Fe but show little Mn. As for the presence of siderite, further studies are needed.

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