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doi:10.2204/iodp.proc.333.101.2012

Principal results

Site C0018

The primary goals of drilling Site C0018 were to establish a well-dated Quaternary mass-movement event stratigraphy and to sample the distal part of an exceptionally thick MTD for analyzing its rheological behavior to constrain sliding dynamics and tsunamigenic potential (Fig. F3). Identification of MTD intervals in cores relied on a combination of observations:

• Indications for sediment remobilization and internal deformation during mass transport: Evidence for sediment remobilization includes convolute strata, chaotic and brecciated facies, mud pebbles, absence of bioturbation, and mixed sediments of different grain sizes. Deformation styles can be highly variable, and parts that only experienced moderate plastic deformation concentrated to restricted zones may contain intervals of coherently bedded sediments. Dipping beds, small folds, and faults are common occurrences in these parts. However, assessment of bedding dips and fault orientation is needed to distinguish deformation attributable to slope instability from postdepositional tectonic deformation, which could result from strain in the underlying accretionary wedge. Paleomagnetism (i.e., for HPCS coring: dispersed paleomagnetic declination and inclination) can provide additional arguments to recognize zones of sediment perturbation, notably in the absence of bedding. Contrasts of physical properties and physical composition may also help define the boundaries between intact and remobilized sediments.

• Upper boundary/contact: The upper contact surface of MTDs may be identified from the presence of a boundary between undisturbed subhorizontal and bioturbated sediment above and disturbed sediments below, without evidence for shearing. A turbidite, originating from erosion resulting from viscous drag of the frontal part of the moving mass, as has been shown experimentally, could be a marker associated with the larger events (Lee et al., 2004; Solheim et al., 2005).

• Lower boundary/contact of MTDs: The lower contact may appear as an erosional boundary between flowed and/or sheared sediments above and flat-lying sediments below. However, deformation could also occur in the underlying sediments as a consequence of shear during flow and/or postdepositional loading (e.g., Schnellmann et al., 2005; Frey-Martínez et al., 2006; Alves and Lourenço, 2010). The lower contact may also appear as subhorizontal shear zone. A shear zone in the context of the Nankai accretionary wedge can be of tectonic origin, and, when related to gravity sliding, is not necessarily tied to a single event. Multiple shear zones can also result from nonuniform strain distribution within the MTD. However, structural relationships and stratigraphy may lead to propose a shear zone as forming the base of an MTD.

• Biostratigraphy and chronology: Age constraints, as inferred from biostratigraphy, magnetostratigraphy, tephrostratigraphy, and (postcruise) isotope stratigraphy, can resolve conformable MTD-overlying and MTD-underlying strata and/or age gaps, as well as the possible admixture of older nannofossil and/or plankton assemblages within MTDs. The age of the hemipelagic/pelagic sediments deposited immediately above the MTD can define the minimum age of the underlying landslide, considering the slide as one single, discrete event. The base of a MTD may, however, have a different significance depending on where observations are performed. In the distal downslope part of the MTD, the base may represent a depositional surface, on which the remobilized material ceased downslope movement. In this case, the sediment immediately below the MTD can, in principle, have a similar or only slightly older age as strata immediately above. In more proximal parts, however, basal erosion is likely a frequent process that can result in a hiatus. In the source region, the base of the slide is presumably a zone of concentrated shear that may not necessarily be described as a stratigraphic contact.

Overview

Slope sediments cored in Hole C0018A are divided into two lithologic subunits. Lithologic Subunit IA is primarily composed of hemipelagic mud (i.e., silty clay) with interbedded volcanic ash layers and is affected by MTDs (Fig. F4). Lithologic Subunit IB is a sandy turbidite sequence. Two thick ash layers were tentatively correlated on the basis of characteristic microscopic features observed in smear slides with onland tephra deposits (Azuki and Pink) that originated from Kyushu volcanoes (Hayashida et al., 1996). The Pink ash layer, dated on land as 1.05 Ma, lies at 183.8–190.65 mbsf, close the base of Subunit IA and the base of the thick MTD. The Azuki ash layer, dated on land as 0.85 Ma, is found at 125.65–126.45 mbsf, <1 m above the top of the thick MTD. These correlative ages were used to constrain magnetostratigraphy and are compatible with preliminary micropaleontological data.

Six intervals with evidence for MTDs are observed within Subunit IA and numbered from top to bottom for convenience (Fig. F4). The upper boundary/contact is well defined for MTDs 1, 2, and 6 and is marked by a turbidite for two of them (MTDs 2 and 6) (Fig. F6 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). MTD 1 extends over 2.9 m of chaotic and convolute bedding in Core 333-C0018A-1H. MTD 2 comprises in its lower part several intervals of coherent bedding limited by probable shear zones. The lowermost one defines the base of the MTD 2 interval (Fig. F7 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). MTD 3 comprises an interval with visual evidence for remobilization near its top, and examination of X-ray computed tomography (CT) scan and structural data lead to considering this interval as part of a thicker MTD zone. MTD 4 is a relatively thin interval (50 cm) associated with a fluidized ash layer (Fig. F8 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). MTD 5 extends over cores that were also disturbed by the coring process. A zone of remobilization is identified based on visual evidence and CT scan in Core 333-C0018A-9H, although this core was damaged during extraction from the core barrel. Evidence for a shear zone with a sharp lower boundary on X-ray CT scan images defines the base of MTD 5; however, it is yet unclear whether this MTD interval corresponds to a single event deposit. MTD 6 is a 61 m interval between 127.55 and 188.57 mbsf and corresponds to the main MTD body identified in the seismic data by Strasser et al. (2011). As already noted, a turbidite deposit is found immediately above its upper boundary. Chaotic and convolute bedding (Fig. F9 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]) and mixing of ash with hemipelagite deposits are observed in the cores, but other intervals remain coherently bedded. Several shear zones are identified from CT scans in the lower part of MTD 6, but none could be positively identified as the basal surface. The base of the thick MTD was thus defined at the top of the Pink ash layer.

Important findings are a correlation of MTD occurrence with a change in sedimentation from turbidite to hemipelagite dominated and the presence of a thick ash layer at or near the base of the thick MTD. Whereas the presence of a thick ash layer at, or near, the lithologic boundary is likely coincidental, the peculiar stratigraphic context of the thick MTD suggests that sediment properties, and their variations, have a major influence on MTD occurrence and size.

Lithology

Hole C0018A was entirely drilled in a slope basin stratigraphic succession. Slope deposits overlying the accretionary wedge were defined as lithologic Unit I in previous expedition reports. In Hole C0018A, a marked difference was observed in cores between a dominantly hemipelagic section bearing ash layers, volcaniclastic sands, occasional siliclastic turbidites and intercalated MTDs forming Subunit IA (above 190.65 mbsf), and a sequence of sandy turbidites forming Subunit IB and interpreted as a sand-rich slope basin (Fig. F10 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). These subunits correlate with seismic Units 1a and 1b of Kimura et al. (2011) and Strasser et al. (2011). Within Subunit IA, the interval between 24.04 and 57.51 mbsf contains more abundant silty and sandy turbidites than intervals above and below. Three facies (IAi, IAii, and IAiii) are thus defined within Subunit IA. Turbidites in Facies IAii are dominantly of volcaniclastic origin. Conversely, samples with relatively higher plagioclase content, lower quartz content, and higher clay content in Facies IAii reflect an altered ash component (Fig. F5 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). Sequences of thin (~5 cm), silty to very fine sand turbidites are also found above ash layers in the lower part of Facies IAiii. Sands in Subunit IB have a mixed composition with quartz, plagioclase, and abundant lithic fragments of both metamorphic and volcanic origin, and thus are not considered to be volcaniclastic sands. An increase in quartz content and in the variability of mineral composition from Subunits IA to IB is observed in the X-ray diffraction (XRD) data (Fig. F5 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). Below 190.65 mbsf there is also a marked decrease in the relative abundance of nannofossils, diatoms, and spicules. Calcite content from bulk powder XRD, calcium content from X-ray fluorescence (XRF) (Fig. F11 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]), and carbonate content from coulometric analysis are consistently higher in Subunit IA than in Subunit IB and tend to increase uphole within Subunit IA.

Two remarkably coarse-grained ash layers occur within Unit I and are attributed to cataclysmic eruptions in Kyushu, known from widespread tephra deposits on land. A normally coarse (sand-sized) ash 80 cm thick is found at 125.65–126.45 mbsf and interpreted as the Azuki event, dated on land as 0.85 Ma (Hayashida et al., 1996) (Fig. F8 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). A dark sand-sized to fine ash layer, which was fluidized upon coring, is found at 183.8–190.65 mbsf. Our provisional correlation is with the Pink event, dated on land as 1.05 Ma (Hayashida et al., 1996) (Fig. F8 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). These preliminary correlations are based on visual observations of the shape of the glass shards and dominant associated minerals in smear slides. The Subunit IA/IB boundary lies below this ash layer.

Structural geology

Structural features of Site C0018 mainly record gravity-driven deformation and mass-movement processes. The main structural features from visual core description at this site are subhorizontal and southeast-dipping beds and northward- and southward-dipping normal faults, outside MTD intervals in the upper part of the borehole (0–127.26 mbsf); scattered bedding dips, fault zones, and flow structures within the thick MTD (127.55–188.57 mbsf); and subhorizontal beds and fissility below the thick MTD (188.62–313.655 mbsf) (Fig. F12 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d] and Fig. F5). A remarkable overturned fold was also observed within MTD 2 at 41.2 mbsf (Fig. F16 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). CT scans may provide additional evidence for shearing. Lines with very high CT values (3000 or more) were identified as early diagenetic pyrite, which may constitute a marker of ductile deformation. Furthermore, small faults and thin shear bands (<3 mm) associated with MTDs appear generally denser on CT scan images (Fig. F18 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]), although still in the usual range for high-porosity sediment (<1500). Several intervals with pyrite mineralizations organized as lineations and bearing low-angle planar discontinuities with associated increased CT values are tentatively interpreted as shear zones (e.g., Fig. F8 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]).

Biostratigraphy

Preliminary biostratigraphic determination for Site C0018 was based on examination of calcareous nannofossils. All core catcher samples and additional samples from sections were examined. Several nannofossil events were recognized with mostly good preservation. Although some reworked specimens were identified in the examined intervals, most specimens can be considered in situ. The result shows no significant time gap in Hole C0018A. The lowest core catcher at 313.61 mbsf indicates an age younger than 1.67 Ma. Ages derived from nannofossil events suggest a slower sedimentation rate (~6 cm/k.y.) in the interval of 0–25.525 mbsf corresponding to Facies IAi and a higher apparent sedimentation rate (~23 cm/k.y.) in the interval of 25.53–313.61 mbsf. This interval includes most MTDs within Subunit IA as well as Subunit IB, which is composed of sandy turbidites.

Paleomagnetism

Shipboard paleomagnetic studies for Site C0018 were performed with remanent magnetization and magnetic susceptibility measurements of discrete samples. Recovered sediments showed considerable variation in magnetic properties and demagnetization behavior. The natural remanent magnetization (NRM) intensities span more than two orders of magnitude, and variations in magnetic susceptibility are consistent with the variations in NRM intensity. Remagnetization imparted by the coring process is commonly encountered and is characterized by NRM inclinations that are strongly biased toward vertical (mostly toward +90°) in a majority of cores. Alternating-field (AF) demagnetization to 30 mT effectively removed this drilling-induced remagnetization, as observed by a significant decrease in intensity and a shift of inclination toward shallower or negative values for intervals with normal or reversed polarity, respectively. Magnetostratigraphy is determined based on the inclination data demagnetized at 30 mT. The most diagnostic feature in the paleomagnetic polarity is a change from normal to reversed polarity within Core 333-C0018A-14H, which corresponds to the Brunhes/Matuyama Chron boundary (0.78 Ma) (Fig. F4). According to tephra chronological data, the base of a normal chron below the thick MTD horizon is correlated to the base of Jaramillo Subchron (1.07 Ma). Cores recovered with HPCS generally show a clustered distribution in declination within each core, but the declinations in the MTD intervals are scattered. This supports irregular rotation of sediments in MTDs.

Physical properties

From the surface to ~200 m core depth below seafloor (CSF), bulk density generally increases and porosity decreases downhole, accompanied by increasing penetration and shear strength, thermal conductivity, and resistivity (Fig. F6). Porosity and resistivity are generally correlated and, arguably, MTD intervals display an increased compaction gradient compared with the average porosity-depth trend, and slight reversals (porosity increasing with depth) are observed near the base of MTDs 2, 3, 5, and 6. For the latter case (MTD 6), this could be in part related to the lithologic change between Subunits IA and IB. Shear strength displays more scatter within MTD intervals. Within the thick MTD, all physical property values (porosity, thermal conductivity, strength, and resistivity) show greater scatter than above or below. A drop in shear strength below the thick MTD at ~190 m CSF may at least in part be an artifact related to the change of coring system from HPCS to EPCS, and then ESCS. Below 200 m CSF, physical properties show no distinct trend.

Inorganic geochemistry

The main geochemical objective at this site was to document the variations in chemical composition of the interstitial water. Whole-round lengths ranged from 20 to 31.5 cm, and interstitial water volumes per centimeter of core ranged from 1.95 to 2.15 mL/cm between 0 and 130.5 mbsf (Fig. F37 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). Because of the consolidated nature of the formation in the deeper portion of this site and the coring techniques used, some of the deeper cores were quite disturbed. Sulfate concentration, calcium concentration, and alkalinity point to a sharp methane-sulfate reaction zone at ~15 mbsf. Below this level, we could use the sulfate concentration to identify and quantify contamination. Chlorinity increases rapidly in the upper ~30 m of Hole C0018A and then gradually increases with depth (Fig. F38 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). The gradual increase in chlorinity in Hole C0018A may reflect ash alteration, which consumes water. The distribution of major and minor cations documents extensive alteration of volcanogenic sediments as well (Fig. F39 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). The thick MTD interval is associated with lower phosphate concentration. This feature is not explained.

Organic geochemistry

Methane is the predominant hydrocarbon component in most cores and ranges between 0 and 19,339 ppmv (Fig. F41 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). Ethane is either below detection or present in low concentrations (i.e., <2 ppmv). No heavier hydrocarbon gases were detected. All C1/C2 ratios are >4000, suggesting that methane is biogenic and organic matter is immature. Calcium carbonate (CaCO₃) content averages 12.5 wt% and varies in a wide range (0.2~25.4 wt%) (Fig. F42 in the “Site C0018” chapter [Expedition 333 Scientists, 2012d]). Total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents are low in the majority of cores, averaging 0.58 ± 0.16, 0.07 ± 0.02, and 0.21 ± 0.19 wt%, respectively. Parameters including CaCO₃, TOC, TN, and TS contents all show larger variations in the upper ~87 mbsf and become less variable in deeper sediments. The atomic ratios of TOC to TN (TOC/TN_at) fall in the range of ~3.5–15, suggesting organic matter is mostly derived from marine sources but also contains terrestrial material in some horizons.

Core-seismic integration

At Site C0018, lithologic Subunits IA and IB correlate with seismic Units 1a and 1b of Kimura et al. (2011) and Strasser et al. (2011). The base of the thick MTD (MTD 6) appears very reflective in profiles from 3-D seismic data and of negative polarity. This could be related to a step increase of porosity observed at the base of MTD 6 (Fig. F6). It is unclear whether the change of physical properties observed at this level is the consequence of deformation within the MTD or of a change of lithology at the transition between lithologic Subunits IA and IB, or both. Seismic onlap surfaces above MTD 6 define the base of several suspected MTDs in seismic data. These correlate with intervals in cores with evidence for remobilization (MTDs 2, 4, and 5) (Fig. F7). The layered sequence below MTD 6 corresponds to lithologic Subunit IB, which is composed of turbidites, but these reflectors cannot be interpreted as individual events in the turbidite sequence.

Downhole temperature measurements

During HPCS operations, downhole temperature was measured with the advanced piston corer temperature tool (APCT-3) at ~30 m intervals. Starting from 35.15 m drilling depth below seafloor (DSF), six data points were obtained to 190.65 mbsf, but the deepest measurements at 161.65 and 190.65 m DSF may be unreliable. Other data follow a nearly linear increase in temperature with depth (0.0581°C/m) (Fig. F8). Heat flow, which was calculated by taking into account thermal conductivity measured on cores, is 62 mW/m².

Site C0011

The main reasons for returning to Site C0011 were to perform temperature measurements for heat flow determination and expand the age-depth models into the Pliocene and Quaternary. This was necessary because the upper stratigraphic intervals of the Shikoku Basin were not adequately sampled during Expedition 322. The additional coring provided complete profiles of organic and interstitial water geochemistry, and sampled across prominent discontinuities in physical properties identified on LWD data. Site C0011 is located on the northern flank of the Kashinosaki Knoll (Fig. F9, F10).

Lithology

A total of 380 m of strata was drilled in Holes C0011C and C0011D (Fig. F3 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Two lithologic units were identified with very good recovery rates. Unit I corresponds to Shikoku Basin hemipelagic/pyroclastic facies and Unit II corresponds to a volcanic turbidite facies that was originally designated middle Shikoku Basin facies (Underwood et al., 2010). In addition, two lithologic subunits were interpreted within Unit I, Subunit IA (youngest), and Subunit IB. Cored lithologies include silty clay, clayey silt, clay, and mudstone interbedded with coarse to fine volcanic ash (Fig. F3 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). The Subunit IA/IB boundary occurs at 251.52 mbsf and is defined by the appearance (below) of more-indurated dark gray mud and mudstone with abundant bioturbation and by the disappearance of volcanic ash layers with unaltered glass. A more dramatic lithologic change occurs below 347.82 mbsf where we found an abrupt shift into coarser-grained tuffaceous sandstone and heterolithic gravel and sand. This change marks the Unit I/II boundary.

Subunit IA comprises a 251.5 m thick succession of greenish gray silty clay with minor amounts of grayish silty clay and <50 cm intercalations of volcanic ash, often bearing unaltered glass shards. Bioturbation is particularly observed in the upper part of the unit. The genera Zoophycos and Chondrites burrows are abundant. Overall, ash layers are more abundant in the upper half of the subunit to ~100 mbsf (Fig. F3 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). In addition, XRD data from the dominant silty clay lithology indicate an average content of 57 wt% clay minerals, 18 wt% quartz and feldspar, and 7 wt% calcite for the subunit. Variations in the relative percentage of calcite are recorded at specific intervals with a maximum in calcite abundance of 26 wt% at 3.16 mbsf (Fig. F13 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Although uncommon, siliceous fossils (diatoms, radiolarians, and sponge spicules) are present in Subunit IA.

The dominant lithology of Subunit IB is a greenish brown to dark gray weakly lithified mudstone with minor contribution of altered volcanic ash beds (Fig. F3 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Clay minerals and altered volcanic glass are the most abundant particles on smear slides, and an increase in the severity of glass alteration as depth increases is observed below the transition from Subunit IA to IB (Fig. F8 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Clay mineral content from XRD data averages 67 wt% for the mudstone, ranging from 74 to 46 wt% (Fig. F13 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Quartz content ranges from 12 to 19 wt% and averages 17 wt%, and feldspar ranges from 9 to 18 wt% and averages 12 wt%. Calcite is present in the subunit, varying from 0 to 31 wt% (Fig. F13 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]) and reaching a maximum near the transition from Subunit IA to IB. Siliceous microfossils are absent from smear slides (with one exception) in Subunit IB.

The upper part of Unit II is dominated by coarser-grained tuffaceous sandstone and heterolithic gravel, gravelly sandstone, and sandstone with sharp and well-defined upper and lower boundaries (Fig. F11 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]), similar to recoveries during Expedition 322 (Expedition 322 Scientists, 2010). These beds are separated by indurated mudstone very similar to that of Subunit IB. The relative clay mineral abundance drops in association with a higher percentage of sandy material. From XRD data, the content of quartz is consistent at ~16 wt%, whereas feldspar values vary from 11 to 46 wt%. Biotite, orthopyroxene, and hornblende are common in the tuffaceous sandstones (Fig. F3 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]) (see Site C0011 smear slides in “Core descriptions”). Siliceous microfossils and fresh glass shards are found again in Unit II.

Four volcanic ash layers comprise provisional stratigraphic markers at Site C0011. Preliminary correlations are based on visual observations of the shape of the glass shards and dominant associated minerals in smear slides. The first ash layer, which correlates with the Azuki volcanic ash layer on land (0.85 Ma; Hayashida et al., 1996), occurs at 21.18 mbsf. The second ash layer, which the Pink ash correlates with (1.05 Ma; Hayashida et al., 1996), occurs at 31.30 mbsf. The correlative products of both events are preserved on land as thin discrete accumulations of ash. In addition, a bed correlative to the onland Ohta ash layer (4.0 Ma; Satoguchi et al., 2005) is present at 157.26 mbsf and a provisional match to the onland Habutaki ash layer (2.8–2.9 Ma; Nagahashi and Satoguchi, 2007) is located at 80.56 mbsf.

XRF analyses were performed on 57 samples from Holes C0011C and C0011D to estimate the bulk chemical composition of the sediments and to characterize compositional trends with depth and/or lithologic characteristics (Fig. F11). As shown by the underlying units (Underwood et al., 2010), major element contents in the hemipelagic mud and mudstone of Unit I span a relatively small range of values, and the compositions resemble those of the upper continental crust as defined by Taylor and McLennan (1985). A few samples of volcanic ash and volcaniclastic layers were analyzed. They all have higher silica and lower aluminum content than the background sediment and slightly lower iron content but variable alkaline content. Several samples have a high-potassium rhyolite composition. Other samples, and notably the tuffaceous sandstones in Unit II, have relatively high sodium and calcium but low potassium content and correspond to more typical calco-alkaline rhyolite and dacite compositions. This variability in bulk chemistry suggests that volcanic materials originate from different sources (presumably Kyushu vs. Izu-Bonin).

The deposition mode for Subunit IA was dominated by hemipelagic settling. A small MTD is observed in the uppermost few meters of Holes C0011C and C0011D, between 0.85 and 2.77 mbsf (Core 333-C0011C-1H). However, several volcanic ash layers display turbidite characteristics (basal lamination and upward fining), suggesting these were remobilized by subaqueous gravity flow. Deposition of Subunit IB was dominated by hemipelagic settling with minor contributions of coarse (sand-sized) ash and fine (silt- and clay-sized) ash that becomes more abundant toward the base of the subunit. During the deposition of Unit II, the paleoenvironment was dominated by transport of sand from a volcanic source. Miocene sandy turbidites have been previously identified in the Nankai Trough and Shikoku Basin. The Miocene siliciclastic turbidites at Ocean Drilling Program (ODP) Site 1177 are generally older and were derived from a relatively large land mass, most likely southern Japan. That middle–late Miocene (15–7 Ma) dispersal system spread terrigenous sediment over a broad area of the Shikoku Basin (Moore, Taira, Klaus, et al., 2001; Fergusson, 2003). Conversely, the younger tuffaceous sandstones at Site C0011, on the northeast side of the basin, have been linked to a provenance in the Izu-Bonin volcanic arc (Underwood et al., 2010).

Structural geology

Holes C0011C and C0011D provide sparse and subtle structures. Structural features encountered in Hole C0011B are subhorizontal to moderately dipping beds, high- to moderately dipping small faults, planar shear zones, and low-angle healed faults (found in lithologic Subunit IB and Unit II) (Fig. F15 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Sediment-fill veins (vein structures) also develop in the shallow part. In an interval from 100 mbsf to the base of logging Unit 1A (250 mbsf), beds dip consistently to the northwest of up to 30°, which exceeds the dip of strata above and below as well as the local slope. This tilting may tentatively be attributed to slope instability, affecting the same stratigraphic interval as observed at Site C0012. Most faults and shear zones are observed over the 30–190 mbsf interval, which may be in part due to better observations on HPCS rather than ESCS cores. Two sets of conjugate normal faults, striking north-northeast–south-southwest and northwest–southeast can be distinguished (Fig. F18 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). The strikes of these conjugate sets are, respectively, parallel to the N25° maximum horizontal stress direction inferred from borehole breakouts at Site C0011 (Saito et al., 2010), and orthogonal to Nankai subduction convergence (300°–310°N; Henry et al. 2001; Loveless and Meade, 2010). A north-northeast–south-southwest compression would be consistent with the local direction of compression inferred from kinematic modeling of the Zenisu-Izu fault system (20°N; Mazzotti et al., 2001). Furthermore, focal mechanisms of the 2004 earthquakes off Kii Peninsula, which occurred within the oceanic plate north and northeast of Site C0011 indicate dominantly north–south compression (Ito et al., 2005). It is therefore likely that the state of stress at Site C0011 is influenced by intraplate compressive deformation within the Philippine Sea plate.

Biostratigraphy and paleomagnetism

Shipboard paleomagnetic studies for Holes C0011C and C0011D were performed with remanent magnetization and magnetic susceptibility measurements of discrete samples. Recovered sediments showed considerable variations in magnetic properties and demagnetization behavior in Holes C0011C and C0011D. Because AF demagnetization of 30 mT can effectively remove drilling-induced remagnetization, the remanent magnetization of 30 mT inclinations were used for magnetostratigraphy. Shipboard paleomagnetic interpretations agree well with the preliminary identification of dated tephra events from smear slide visual description (Fig. F12). Nannofossil biostratigraphy was done postcruise, and although nannofossils were not well preserved in some intervals, datums have excellent consistency with paleomagnetic interpretations, except for the deepest one, which corresponds to the paracme end of Reticulofenestra pseudoumbilica (>7 µm) (Fig. F12). This event is considered to have a low level reliability (see “Biostratigraphy” in the “Methods” chapter [Expedition 333 Scientists, 2012a]). On the other end the shipboard paleomagnetic interpretation has good continuity with Expedition 322 data, and yields an age of 7.6 Ma for the transition between lithologic Units I and II (Fig. F13), and the age of the Subunit IA/IB boundary is constrained to 5.32 Ma from magnetostratigraphy. Based on our data, the geologic age throughout the cores is from 0 to ~7.8 Ma. An average sedimentation rate in the cores is calculated at ~4.6 cm/k.y. The sedimentation rate changes downhole from ~2.7 to ~7.5 cm/k.y. at ~80 mbsf (or ~3.0 Ma). There is also an interval of slower sedimentation in lithologic Subunit IB.

Physical properties

Bulk density and resistivity values generally increase from the surface to 50 m CSF, reflecting normal consolidation of sediments (Fig. F36 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). However, from ~50 to 80 m CSF density and resistivity decrease and then remain anomalously constant from 80 to 240 m CSF. Below 240 m CSF, density and resistivity increase abruptly and then continue along a normal consolidation trend. This behavior is similar to that observed at ODP Leg 190 Sites 1173 and 1177, where the base of the zone showing retarded compaction was ascribed to dissolution of opal-CT cement and precipitation of quartz (Spinelli et al., 2007). This interpretation is further supported by the profile of dissolved silica at Site C0011, which shows a sharp decline beginning at ~240 m CSF.

Sediment strength generally increases with depth, though vane shear and penetrometer measurements could not be made below 160 m CSF because of high sediment strength (Fig. F33 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Resistivity and acoustic velocity measurements conducted on discrete cubes from below 215 m CSF show normal trends of increasing resistivity and velocity with depth, including elevated resistivity and velocity in volcaniclastic sands at the top of Unit II (Fig. F37 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Anisotropies in the vertical plane show that with depth the z-direction becomes slower and more resistive, which is consistent with uniaxial consolidation of transversely isotropic, bedded sediments.

Two transitions were shown by LWD data at Site C0011 (Saito et al., 2010), one at 212 m LSF (downward decrease in gamma ray with a small associated decrease in resistivity) and one at 251.5 m LSF (downward increase in resistivity and gamma ray). The transition at 251.5 m LSF is obviously associated with the strong porosity and resistivity gradient observed between Subunits IA and IB. At this date, the upper transition is not correlated with any remarkable observations on cores except, perhaps, a 1 m thick silt layer at 200.4–201.4 m CSF.

During HPCS operations, downhole temperature was measured with the APCT-3 at ~30 m intervals. We completed one measurement in Hole C0011C at 22.5 mbsf and eight measurements in Hole C0011D from 49 to 184 mbsf. Data show a nearly linear increase in temperature with depth (0.0913°C/m) (Fig. F8), corresponding to a heat flow value of 89.5 mW/m². Temperature extrapolated at basement (~1050 m, from seismic profile), taking into account heat conductivity variations in the cored intervals, is ~80°C.

Inorganic geochemistry

Inorganic geochemical objectives at this site were to document the variations in interstitial water chemical composition in shallower depth, which were recovered during Expedition 322. Whole-round lengths ranged from 19 to 41 cm and interstitial water volumes per centimeter of core range from 1.7 to 2.8 mL/cm between 0 and 378.6 mbsf (Fig. F42 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Because of the more consolidated nature of the formation in the deeper portion of this site and the coring techniques used, some of the deeper cores were quite disturbed. We were able to use sulfate concentration to identify and quantify contamination in the deeper cores. Results are generally consistent between Hole C0011B (Expedition 322) and Hole C0011C/C0011D (Expedition 333) data, except for bromide, for which there is an offset, still unexplained.

Chlorinity increases rapidly with depth in the upper ~25 m of Holes C0011C and C0011D, stays close to seawater value from 25 to 250 mbsf in Hole C0011D, and then gradually decreases below 130 mbsf in Hole C0011D (Fig. F43 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). The chlorinity decrease, which continues to the deeper interstitial waters of Hole C0011B taken during Expedition 322 (Underwood et al., 2010), may reflect the updip migration of interstitial water freshened by the smectite-illite reaction at greater depths below the trench and prism toe.

Another remarkable feature is a sharp drop in silica concentration (also coincidental with a lithium concentration maximum) at the level of the transition from lithologic Subunit IA to IB (Fig. F44 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Remarkably, this drop correlates with a decrease in porosity from ~65% to ~55% observed over <10 m in moisture and density data as well as with a concurrent resistivity increase in LWD (Saito et al., 2010) and core data. The high (~800 µM) silica concentration in the fluid in Subunit IA supports the hypothesis that the retarded compaction in lithologic Subunit IA is due to a opal-CT cement, as proposed for Sites 1173 and 1177 (Spinelli et al., 2007). At the level of the tuffaceous sandstones (350–370 mbsf), a secondary silica concentration maximum is observed, which may be related to ash alteration. This interval is also characterized by higher barium concentration and slightly decreased lithium and strontium relative to the trend defined from concentrations above and below.

Organic geochemistry

Methane was the only hydrocarbon gas detected at Site C0011 and it occurs only in low concentrations (2 ppmv) in the uppermost 250 m of the cored sequence (Fig. F46 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Thereafter, methane concentration gradually increased to reach highest values (~900 ppmv) at 380 mbsf.

TOC, TN, and TS contents are generally low in all lithologic units, averaging 0.31 ± 0.10, 0.06 ± 0.01 and 0.14 ± 0.97 wt%, respectively (Fig. F47 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). At the base of Subunit IA and top of Subunit IB, several samples display higher TS values, including a pyrite-rich specimen at ~12 wt%. TOC/TN_at was 5.67 ± 1.60, indicating a marine origin for the sedimentary organic matter. The calcium carbon content varied between 0.2 and 24.5 wt% and shows highest concentration in the upper part of Subunit IA and over the transition from Subunit IA to IB. Rock-Eval derived T_max values range from 399° to 414°C and indicate a thermally immature state of the organic matter.

Site C0012

The main objectives of returning to Site C0012 were to perform temperature measurements for heat flow determination, to expand the age-depth models into the Pliocene and Quaternary, and to core the basement to at least 100 m below the sediment/basement interface. Knowledge of thermal state, interstitial water geochemistry, hydrologic properties, and basement alteration are needed to characterize the state of the subduction inputs and model their evolution with downdip increases in temperature and pressure.

Lithology

In Holes C0012C and C0012D, 180 m of lithologic Unit I (hemipelagic/pyroclastic facies) and the upper part of lithologic Unit II (volcanic turbidite facies) were drilled during Expedition 333 (Fig. F3 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). The remaining holes (C0012E, C0012F, and C0012G) aimed at drilling red calcareous claystone and basalt at the contact between sediments in the Shikoku Basin and the igneous oceanic crust. The cores brought new information on lithologic Unit I not acquired during Expedition 322 because of severe coring disturbance and poor recovery.

Three lithologic subunits were interpreted in Unit I: Subunit IA (youngest), Subunit IB, and Subunit IC. The subunits are distinguished based on the presence, frequency of occurrence, and thickness of volcanic ash layers. The lithologies in Holes C0012C and C0012D include dark greenish gray clay and silty clay and silt interbedded with volcanic ash and minor occurrences of thin sand. A major change in the frequency of the occurrence of ash layers is recorded at ~71.5 mbsf, thereby defining the Subunit IA/IB boundary. Ash alteration was observed from 91.2 mbsf to the lower part of Subunit IB. At Site C0011, a comparable alteration front occurs at the top of Subunit IB. Within Subunit IB, ash layers are scarce to ~123.3 mbsf, below which another interval of dark greenish gray clay/silty clay with abundant ash layers extends to 149.77 mbsf. This depth for the base of Unit I matches closely with the designation of 150.9 m CSF that was made during Expedition 322 (Underwood et al., 2010).

Below 149.77 mbsf, Unit II comprises turbidite sands and sandstones with sharp and well-defined upper and lower boundaries. Commonly, beds have normal grading, but some are intercalated with intervals of massive beds with or without clay clasts. At the base of normal graded beds, pebble and sand clasts are composed of coarse ash and lapilli tuff. The lower part of Unit II comprises several layers of carbonate-cemented sandstones with calcite and barite veins. These beds are separated by mudstone very similar to that of Subunit IC. The base of Unit II was not cored during Expedition 333 but was fixed at 219.8 m CSF in Hole C0012A (Underwood et al., 2010).

Nannofossils are the dominant group of microfossils and are found in both Units I and II. Sponge spicules, diatoms, and radiolarians occur as a rare or trace component in most of Subunit IA, being relatively more abundant at the top of the subunit. Below ~85 mbsf they were not identified. Bulk powder XRD data indicate an average content of 63 wt% clay minerals, with the lowest content (~45 wt%) characterizing the uppermost 15 m. There is on average about 19 wt% quartz and 18 wt% feldspar; both show uniform values throughout Units I and II (Fig. F6 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). Calcite content is highest in the upper 15 m, where it reaches ~25 wt% (Fig. F6 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). Below 15 mbsf calcite content of Subunit IA is very low, dropping the average for the subunit to 4 wt% except for Subunit IB where it varies from 0 to 20 wt% and averages 6 wt%. Major element concentrations of Unit I and II from XRF analyses span a relatively small range of values (Fig. F14) and resemble those of the upper continental crust as defined by Taylor and McLennan (1985).

The cored portion of Units I and II has an estimated Holocene–late Miocene age range (~0–8.3 Ma). Three ash beds were correlated to known tephra dated on land based on visual observations of the shape of the glass shards and dominant associated minerals in smear slides. The Azuki volcanic ash bed, dated on land as 0.85 Ma (Hayashida et al., 1996) was identified at 5.7 mbsf. The Pink ash bed, dated on land as 1.05 Ma (Hayashida et al., 1996), correlates with a characteristic ash layer at 7.7 mbsf. A third major volcaniclastic event, the Ohta ash bed, dated on land as 4.0 Ma (Satoguchi et al., 2005), correlates with an ash bed at 44.95 mbsf (Fig. F3 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]).

The unusually shallow depths of the presumed Azuki and Pink ash beds implies very slow sedimentation rates over the last ~1 Ma. This interpretation is, however, consistent with the depth of the Brunhes-Matuyama reversal and of the Jaramillo Chron from natural magnetic remanence data. This interval of condensed sedimentation lies on an angular unconformity with a hiatus of ~2 m.y. at ~14 mbsf, interpreted as the top of a slump. Other, yet unresolved, hiatuses may account for the very slow apparent sedimentation rate above this unconformity.

The deposition of Subunits IA and IC was dominated by hemipelagic settling and frequent volcanic eruptions, whereas the time interval of Subunit IB experienced fewer volcanic eruptions. During deposition of the upper part of Unit II, the paleoenvironment was dominated by deposition from a sandy system with a volcanic provenance (Izu-Bonin arc). In comparison to Holes C0011C and C0011D, the Miocene sandy turbidites are finer grained on top of the bathymetric high, and the thick tuffaceous sandstone layers characteristic of the volcanic turbidite facies were not observed. On the other hand, pebbly and mixed layers evocative of MTDs were found.

Hole C0012E recovered two cores of greenish yellow mudstone intercalated with thin sandstone layers from 500 mbsf, corresponding to the base of lithologic Unit V (volcaniclastic-rich facies) defined during Expedition 322 (Underwood et al., 2010), and one core from 519 mbsf that recovered 6.8 m of reddish brown calcareous claystone with lighter green layers, overlying altered pillow basalts. The interface between the red calcareous claystone and the basaltic basement was also recovered in Holes C0012G and C0012F. The red calcareous claystone corresponds to Expedition 322 lithologic Unit VI (pelagic claystone) (Underwood et al., 2010) and holds veins of calcite with traces of barite as well as several layers with accumulations of manganese oxide forming millimeter- to centimeter-sized lumps.

Hole C0012G cored pillows and massive phyric basalts from 525.69 mbsf to the base of the hole at 630.5 mbsf. Two units are defined in the basalt: Unit I is composed of phyric or highly phyric pillow basalt and Unit II is composed of sheet flows with pillow basalt interlayers. As observed during Expedition 322, the basalt in Unit I is highly altered and some voids remaining between basalt pillows are filled with analcime. Observations of thin sections showed that all olivine and glass (except in one sample) as well as a large fraction of the plagioclases have been replaced by secondary phases—dominantly saponite and zeolites—that are also present as vesicle fillings. In Unit II, the massive flows are more crystalline and generally less altered. Basalt experienced localized alteration under iron oxidizing conditions with accumulation of iron hydroxides in veins and alteration halos. Celadonite and saponite are present in the rock mass and, locally, pyrite. This suggests two stages of alteration, under iron oxidizing and iron reducing conditions.

Structural geology

Structures observed at Site C0012 mainly consist of bedding planes, faults, shear zones, and chaotic structures (Fig. F15). Bedding planes show a large range of dipping angles from 3° to 70° but are organized in zones of low and high bedding dips. Bedding planes with low dips are characteristically observed in Zone I (0–14 mbsf) (Fig. F15B) and Zone III (85–145 mbsf). However, high dip angle beds are found in Zone II (14–85 mbsf), where they consistently strike northeast–southwest (Fig. F15C) and dip southeast, and in Zone IV (145–180 mbsf), where the distribution of strikes and dips appears scattered. Sediment above this unconformity (between 10 and 14 mbsf) comprises disturbed intervals suggesting it has been reworked. Faults are mostly normal faults with high dip angles, striking northwest–southeast, and dipping northeast or southwest, suggesting northeast–southwest extension (Fig. F15D). Shear zones generally have high dip angles with large displacements. Chaotic structures are composed of disrupted beds, folds, and injections of sand or mud and observed at the bottom of Zones II and IV. Considering the location of Site C0012 on a topographic high southwest of a steep slope evocative of a slide scar, one interpretation proposed is that Zone II corresponds to a slump that was later covered by flat-lying sediments, whereas Zone IV may have been affected by multiple (and older) sliding events.

Paleomagnetic and biostratigraphic data suggest that the unconformity between Zone I and Zone II correlates with an age hiatus of ~2.0 m.y. Similar observations within the upper part of Unit III during Expedition 322 suggested that a slumping event associated with the remobilization of the uppermost sedimentary layers also occurred about 9.5 m.y. ago (Underwood et al., 2010) (Fig. F13).

Biostratigraphy and paleomagnetism

Paleomagnetic shipboard studies for Site C0012 were performed with remanent magnetization and magnetic susceptibility measurements of discrete samples (Fig. F31 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). Magnetostratigraphic correlations indicate the age of the sediments in Holes C0012C and C0012D range from present day at the seafloor to >8 Ma at the bottom of the hole (~180 mbsf). Following AF demagnetization to 30 mT, inclination values were variable. Bedding correction performed on the specimens clustered the data near the expected ~52° but did not affect the chron and subchron boundaries. The Brunhes/Matuyama boundary was detected in Core 333-C0012C-2H, and provisional correlation of tephra chronology identified the Pink tephra layer in Core 333-C0012C-3H within the lower portion of the Jaramillo Subchron and the Ohta tephra layer in Core 333-C0012C-6H within the Gilbert Chron. A thickness of only 9 m between these two chrons indicates that the lower section of the Matuyama Chron is probably missing and this interpretation is also consistent with correlation of the magnetic susceptibility between Sites C0011 and C0012 (Fig. F31 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). This indicates a hiatus or condensed sedimentation interval spanning roughly 2 m.y. (~1 to ~3 Ma) occurring between 10 and 14 mbsf. In this interval the core presents both evidence of disturbance from drilling (flow-in below Pink ash layer) and of in situ disturbance, which precludes detailed magneto-stratigraphic interpretation. The nannofossil assemblage found in Section C0012C-2H-CC corresponds to the 0.9–1.1 Ma time interval (see Table T4 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). It is thus proposed the hiatus occurs between Cores C0012C-2H and C0012C-3H at 14 mbsf and coincides with an angular unconformity (see “Structural geology”). Below this level, shipboard paleomagnetic interpretations and nannofossil datums from both expeditions are generally consistent, except in the 70 to 90 mbsf interval that corresponds to the base of the slump (Fig. F12). Paleomagnetic data suggest a small hiatus (<1 Ma) may be present at ~85 mbsf across a chaotic zone, representing the probable base of the slump. Below this level, the sedimentation appears continuous. The sedimentation rate displays some variations and, notably, is faster before 7.4 Ma in lithologic Subunit IB and Unit II. The transition from Subunit IA to IB is constrained at 4.42 Ma from magnetostratigraphy, and thus appears slightly younger at Site C0012 than at Site C0011 (Fig. F13). However, a change in physical properties was observed below this level and therefore would better correlate in age with a comparable change observed at Site C0011 than the lithologic boundary. The change of the frequency of ash layer occurrence at the Subunit IB/IC boundary is constrained to 7.13 Ma from magnetostratigraphy. The age models at the depth of the transition from Unit I to Unit II are very consistent and provide an age of 7.8 Ma, slightly older than at Site C0011, but it is unclear whether this variation is significant.

Physical properties

From the surface to ~10 m CSF, porosity decreases downhole, as expected for progressive burial. Below 10 m CSF, porosity slightly increases and then remains relatively constant until ~70 m CSF, which coincides with the lithologic Subunit IA/IB boundary (Fig. F34 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). Within this anomalous interval, electrical resistivity also remains constant while shear strength increases, and there is an anomalous shift in the concentration of dissolved silica in the interstitial water. Between 70 and 100 m CSF, porosity sharply decreases and electrical resistivity and shear strength increase. Porosity increases and then decreases between 100 and 170 m CSF, followed by a steady compaction trend, with some scatter in sand-rich units, to the base of the borehole. Sandstones are also indicated by spikes of high magnetic susceptibility and low natural gamma ray.

Site C0012 porosity values from below 240 m CSF are generally lower than those from similar depths at Site C0011 (Fig. F16). A possible explanation for the lower porosity at Site C0012 is removal of overlying material by erosion or slope failure. This interpretation would be consistent with the observed time gap of ~2 m.y. found between 10 and 14 m CSF and the structural evidence of an angular unconformity. Calcareous claystones from below 500 m CSF range in porosity from 0.28 to 0.46 and show P-wave velocities ~2000 m/s (Fig. F17). Within the basalt, measured porosity is extremely variable and ranges from 0.09 to 0.37. Measured P-wave velocities vary between 3000 and 5000 m/s.

Electrical resistivity is greater in the z-direction within the claystones but lower within the basement. Higher resistivity in the z-direction in the sediments is consistent with a transversely isotropic medium in which the bedding planes are approximately horizontal (Fig. F17). Within the basement, electrical conductivity may be enhanced by subvertical veins or fractures filled with more conductive material, resulting in a lower resistivity in the z-direction.

Thermal gradient values are evaluated from the APCT-3 measurements made at 10 depths in Holes C0012C and C0012D together, and the mean thermal gradient value determined is 0.135 K/m (Fig. F8). The estimated heat flow value at this site is 141 mW/m², amounting to ~50% higher than the 89.5 mW/m² determined for the adjacent Hole C0011C that was drilled during this expedition. Based on the determined heat flow value of 141 mW/m², as well as the thermal conductivity values from core measurements (see also, Expedition 322 Scientists, 2010), temperature at the top of basement (at 526 mbsf) is estimated to be ~65°C, which is significantly lower than the estimated value of 80°C at ~1050 m CSF at Site C0011. We suspect that hydrothermal fluid convection in the basement is transferring heat between the two sites, thus resulting in a higher heat flow at Site C0012, which is located on the topographic high. However, the effect of sedimentation on heat flow, and also of sediment removal by gravity sliding at Site C0012, should also be considered in models.

Inorganic geochemistry

The main objective of the inorganic geochemistry program at this site was to document the geochemical properties of subduction inputs at a site located above a basement high, near the crest of the Kashinosaki Knoll. A total of 28 interstitial water samples were squeezed from selected whole-round sections for chemical and isotopic analyses. To obtain enough interstitial water for shipboard and shore-based analyses, 19–31 cm long sections were squeezed in Holes C0012C and C0012D. In the red claystone located directly above the basement in Hole C0012E, sections 59.5–61 cm in length were squeezed. Interstitial water volumes per length of interstitial water section decrease with depth, from 2.75 to 0.71 mL/cm of core in the upper ~180 mbsf of Holes C0012C and C0012D (Fig. F45 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]). In the deeper material above the basement, volumes were lower, ranging from 0.22 to 0.08 mL/cm.

The sulfate profile for Site C0012 documents a much deeper sulfate reduction zone than observed at the other sites drilled during this expedition. At Site C0011, the zone occurs at ~80 mbsf, and at Site C0018 it occurs at ~15 mbsf. During Expedition 322 it was observed that minimum sulfate concentration, which occurs at ~300 m CSF at Site C0012, coincides with a marked increase in methane concentration (Saito et al., 2010). The Expedition 322 scientists interpreted the sulfate profile at Site C0012 as being driven locally by anaerobic methane oxidation. The deeper anomaly was attributed to slower sedimentation rates (because of the bathymetric high Site C0012 sits on), to a lower average organic matter content at Site C0012, or to differences in fluid migration. Additionally, in both Holes C0012E and C0012A sulfate increases in concentration in the interval below ~450 m CSF, which may indicate diffusional exchange with fluid in basaltic basement that sustain a higher sulfate concentration.

In general, analytical results are consistent between Expeditions 333 and 322 (Figs. F46, F47 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]) and interpretations proposed after Expedition 322 are not put in question by the newly acquired data. However, shipboard Br concentrations during Expedition 333 tend to be systematically lower, whereas Li and Sr concentrations tend to be higher. Overall, the combined data set reflects in situ alteration of volcanic ash in the sediment and basalt alteration in the upper igneous crust as well as exchange by diffusion. More specifically, the decrease in Mg and K, increase in Li and Sr, and variations in silica (Fig. F47 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]) in the upper 200 m are probably controlled by volcanic ash alteration and equilibration of the interstitial fluid with clay minerals.

Silica concentration drops below the Subunit IA/IB boundary at ~70 m, where a decrease in porosity and an increase in resistivity are also observed. Alteration of volcanic glass shards also becomes more pronounced. This again suggests a relationship between interstitial water composition, the presence or dissolution of opal cement, and ash diagenesis. However, the silica concentration remains relatively high and variable in the 200–500 µM range from 90 to 200 m and then, according to Expedition 322 data, decreases again. These local variations do not fit existing models for opal dissolution and quartz precipitation (Spinelli et al., 2007), which suggests that additional sources of silica are present (e.g., volcanic ash alteration) and that dissolved silica concentration in this interval is not limited by quartz precipitation.

The trend of increasing chlorinity (Fig. F46 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]), Ca, and Sr (Fig. F47 in the “Site C0012” chapter [Expedition 333 Scientists, 2012c]) concentrations and decreasing Na in the lower part of the borehole is interpreted as a consequence of diffusion between the lowermost sediment and basement fluids.

Organic geochemistry

At Site C0012, methane and ethane were either below detection or present at only low concentrations. No heavier hydrocarbon gases (i.e., C3 and C4) were found. The only two samples that contained both methane and ethane were found at depths of ~501 and ~520 mbsf and had C1/C2 ratios <100, indicating a possible thermogenic origin of these hydrocarbon gases (Fig. F18). Calcium carbonate (CaCO₃) concentration ranged between 0.2 and 45.1 wt% and averaged 6.1 wt% (Fig. F19). Sediment below 500 mbsf contained higher amounts of CaCO₃ compared to the upper 180 mbsf of the cored sequence. TOC, TN, and TS concentrations were low, ranging between 0.03 and 0.46 wt%, 0.03 and 0.07 wt%, and 0 and 0.49 wt%, respectively. The TOC/TN_at fell in the range of 1.3–9.3, suggesting a marine origin of the organic matter. The variations of these four elemental parameters shared a similar pattern, including scattered values but displaying a generally increasing trend with depth in the upper 52 mbsf relatively uniform values between 52 and 180 mbsf, as well as low values between ~500 and 525 mbsf.

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