Drilling sites and studied cores
The sediment cores used in this study were collected from three different drilling sites around the Nankai Trough (Sites C0002, C0021, and C0022) by D/V Chikyu during various Integrated Ocean Drilling Program (IODP) expeditions (Fig. 1).
The longest core was from Site C0002 in the Kumano Forearc Basin. The sediments at this site were lithologically categorized into five units (Moore et al. 2013; Expedition 348 Scientists and Scientific Participants 2014): in descending order, Unit I (shallower than 126 mbsf) consists of hemipelagic mud with fine sand, silt, and volcanic ash deposited since 1 Ma; Unit II (126–826 mbsf), which has a similar lithology to Unit I but consists of more consolidated sediments, was deposited between 1.67 and 1 Ma; Unit III (826–1025.5 mbsf) is composed of slope sediments deposited during a period of slow sedimentation from 3.65 to 1.67 Ma; Unit IV (1025.5–1740.5 mbsf) is composed of accretionary prism sediments older than 5.6 Ma (Ashi et al. 2008); and Unit V (1740.5–2220 mbsf) consists of accretionary prism sediments older than those of Unit IV. Cuttings and logging data suggest that Unit V is somewhat sandy with many conductive fractures. A bottom-simulating reflector (Moore et al. 2009; Strasser et al. 2014) and onboard infrared-camera observations of low-temperature anomalies at 369, 388, and 392 mbsf in the recovered sediment cores (Moore et al. 2013) confirm the presence of methane hydrate at 200 to 400 mbsf in Unit II. Reduced chlorinity in interstitial water (IW) from 200 to 400 mbsf also supports the presence of the methane hydrate in this depth interval (Fig. 2, Moore et al. 2013).
Site C0022 is located on the toe of the forearc slope of the Nankai Trough (Fig. 1). On the basis of their overall lithology, the cored sediments down to 415.9 mbsf were classified into a single unit, although two subunits were identified in logging data during drilling (Moore et al. 2013): Subunit II-a (0–383.5 mbsf) is composed dominantly of silty clay with variable amounts of calcareous nannofossils, foraminifers, siliceous biogenic debris, and volcanic ash; Subunit II-b (383.5–415.9 mbsf) consists of a series of mud-encrusted gravels interbedded with thin sand, clayey silt, and silty clay layers in its upper part and with mainly silty clay layers in its lower part. Several shear zones and minor faults indicate the presence of a splay fault in the interval from 80 to 140 mbsf. Below this fracture zone, chlorinity of the IW was higher than that of ambient seawater (Fig. 3c), presumably because of upwelling of deep-seated water along the fault (Moore et al. 2013).
Site C0021, located on the basin slope seaward of the splay fault, is composed of dominantly silty clay sediments that were relocated by a submarine landslide (Moore et al. 2013). The chlorinity of the IW, which was the same as that of seawater, was uniform from the seafloor to 200 mbsf (Fig. 3g), probably as a result of homogenization of the collapsed sediments.
Most core samples used for our As analyses were collected by IODP Expedition 338 (October 2012 to January 2013) from Hole C0021B (drilled interval 0-204 mbsf), Hole C0022B (drilled interval 0–204 mbsf), Holes C0002K, L (200–505 mbsf), and holes C0002J and H (902–940 and 1100.5–1120 mbsf, respectively) (Moore et al. 2013). Both stored and newly recovered cores from Site C0002 were analyzed to document As behavior from the seafloor to as deep as possible in the ocean bottom sediments. Sediment samples from 0 to 204 mbsf and 475–1057 mbsf in Holes C0002D and B, respectively, were collected by Expedition 315 (November–December 2007) (Ashi et al. 2008), and those from 2110 to 2220 mbsf in Holes C0002P were collected by Expedition 348 (September 2013 to January 2014) (Expedition 348 Scientists and Scientific Participants 2014).
Analytical methods
In order to examine As in coexisting water and solids, we analyzed As in both IW and the squeezed cake (SC, residual sediment after extraction of IW). IW was extracted from the sediment cores on board with a Manheim-type titanium squeezer (Manheim 1966), filtered through a polytetrafluoroethylene (PTFE) membrane filter (0.45 μm mesh), preserved with HCl in a plastic vial (Nalgene), and stored at room temperature. The SC was stored onboard at − 20 °C during the cruises. Then, in the shore-based laboratory, the SC was freeze-dried and powdered manually for the analyses. In our analyses, we used stored IW from shallow sediments above 200 mbsf in Site C0002 that had previously been used for onboard analysis of alkalinity. Among the samples taken during Expedition 348, we analyzed only the SC, because these core samples rarely contained an extractable amount of IW.
We determined total As concentrations in the bulk sediment (i.e., SC) and dissolved in the IW, As species extracted from the sediments by a sequential chemical extraction method, and As species in the sediments separated by high-performance liquid chromatography (HPLC) as described below.
Total As concentration
One gram of powdered sediment was fused with NaCO3 at 900 °C until the silicates were completely melted, and then the residue was dissolved in 0.05 M HNO3. IW was diluted with 0.05 M HNO3 to make a 1/300 solution. Total As concentrations in the sample solutions were quantified by inductively coupled plasma-mass spectrometry (ICP-MS; SPQ9700, Hitachi), with H2 gas used to prevent the formation of ArCl+. Commercially distributed standard As solution (FUJIFILM Wako Pure Chemical Corp.) was used for the calibration. Accuracy of the results was checked by using rock standard samples (JSd-1, As 2.42 ppm; JSd-2, 38.6 ppm; and JSl-1, 14.9 ppm) from AIST (National Institute of Advanced Industrial Science and Technology), which were prepared in a similar manner to the studied samples. The analytical error was ± 8% at maximum.
Sequential chemical extraction of As
The BCR sequential chemical extraction method (Rauret et al. 1999) was used to separate chemical phases of As in sediment. This method cannot accurately specify the As host phases, but As behaviors associated with changes in the environmental redox condition can be roughly evaluated. As phases were extracted in five steps. In step 1, 1 g of sediment powder was put in a PTFE centrifuge tube, which was shaken with 20 ml 0.22 M acetic acid (pH adjusted to 5 with HNO3) for 16 h at room temperature. Then, the mixture was centrifuged at 10,000 rpm for 10 min to extract acid-soluble phases, namely, components weakly adsorbed onto detrital materials or contained in carbonates into the supernatant fluid. In step 2, the residual sediment was washed with ultra-pure water; then, 20 ml 0.5 M hydroxylammonium chloride (NH3OHCl, pH adjusted to 2 with HNO3) was added and the mixture was shaken for 16 h at room temperature to extract As fixed in reducible phases, mainly, Fe-oxyhydroxides and Mn-oxides. In step 3, the residue from the previous step was heated with 20 ml 0.1 M sodium pyrophosphate solution for 2 h and then shaken with 5 M ammonium acetate (CH3COONH4, pH 2) for 16 h. This reagent cannot completely decompose sulfide minerals, so the most probable phase dissolved at this step is organic matter. Then, the mixture of residue and solution was centrifuged to separate. In step 4, the residue from step 3 was shaken with a mixture of concentrated nitric and perchloric acid for 16 h, then gently heated for 24 h, and centrifuged. Most insoluble phases, silicates, and sulfides were decomposed at this step. In step 5, the residue from the previous step was decomposed by alkaline fusion to decompose minerals resistant to the reagents used in steps 1–4. The As extracted at each step was quantified by ICP-MS as described in the previous section.
As species in sediments
Water-soluble As compounds in SC from Sites C0002 and C0022 were extracted by the method of Ellwood and Maher (2003). The sediment sample was mixed with a solution of 0.1 M hydroxylammonium chloride and 0.5 M phosphoric acid in a PTFE centrifuge tube and shaken at 25 °C for 1 h. The hydroxylammonium chloride solution is reductant and can decompose amorphous and weakly crystalized Fe-oxyhydroxides (e.g., ferrihydrite and goethite) and Mn-oxides, and the phosphoric acid helps to preserve AsIII and AsV species, especially in solutions containing high amounts of Fe and Mn (Ellwood and Maher 2003). Frozen sediment was used for this analysis, because dried samples may become oxidized, changing the chemical form of the As. After the reaction, the solid phase was separated by centrifugation (1880×g for 10 min) and removed by filtration through a PTFE membrane filter (0.45 μm mesh). The obtained solution was analyzed by HPLC–ICP-MS at the National Institute for Environmental Studies (8800 ICP-QQQ, Agilent Technologies, Inc., USA) to determine the concentrations of each separated As phase. Eight As compounds (commercially distributed by FUJIFILM Wako Pure Chemical Corp.), As(III), As(V), methylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB), trimethylarsine oxide (TMO), tetramethylarsonium salt (TMAO), and arsenocholine (AsC), were separated in a GL-Science InertSil AS column (4.6 i.d. × 250 mm long), using an eluent composed of 10 mM butane-1-sulfonic acid sodium salt, 4 mM malonic acid, 4 mM tetramethylammonium hydroxide, and 0.05% methanol. The flow rate was 1.0 ml/min at 40 °C.