Masuda et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0269-y(2019) 6:28

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Vertical profiles of arsenic and arsenic
species transformations in deep-sea
sediment, Nankai Trough, offshore Japan
Harue Masuda1* , Haruka Yoshinishi1, Shigeshi Fuchida1,2, Tomohiro Toki3 and Emilie Even1

Abstract
Concentrations of arsenic (As) and its chemical forms were determined on deep-sea sediments drilled at three sitesof Nankai Trough, off the Kii Peninsula, Japan. Those sediments were analyzed to document the behavior of As in
relation to methane hydrate formation and the deep biosphere.
The analytical results showed the total As concentration of interstitial water (IW) and squeezed cake (SC) ranged
from 0.9 to 380 ppb and from 3 to 14 ppm (average, 6.4 ppm), respectively. The sediments from Site C0002, of
which sediment column was the longest down to 2200 m below the seafloor (mbsf) among the studied threedrilling sites, were analyzed for the host phase transformation of As. The total concentration of As of IW and SC
from 200 to 500 mbsf, where methane hydrate zone was included, was higher than those from the uppermost 200
m. Concentration of As was ultimately controlled by pH. Also, organoarsenicals, such as methylarsonic acid (MMA)
and arsenobetaine (AsB), were detected in the sediment column, implying that these organoarsenicals appeared inrelation to the in situ microbial activities. These observations suggest that As becomes mobilized directly or
indirectly as a result of microbial activity in deep-sea sediments.
Keywords: Nankai Trough, Deep biosphere, Organoarsenical, Convergent margin

Introduction
Distribution and behavior of arsenic (As), one of themost common toxic elements, in the hydrosphere andsubareal and under-water shallow sediments have been
well documented as a part of geologic cycle of arsenic in
the Earth’s surface and crust (e.g., Masuda 2018, andreferences therein). The As is generally present in clean,
open-ocean water at an average concentration of 1.5 μg/L
(e.g., Andreae 1979). Marine organisms, particularly
photosynthetic organisms, take up As from seawater andaccumulate it in their bodies, where metabolic reactions
result in As species transformations. Biogeochemical processes affecting As in ocean water have been well documented (e.g., Neff 2002, and references therein); in marineorganisms, As is present as both inorganic species such asarsenate (AsV) and arsenite (AsIII) (e.g., Sanders 1979;
Raab et al. 2005; Duncan et al. 2015) and organic species,* Correspondence: harue@sci.osaka-cu.ac.jp
1
Department of Biology and Geosciences, Graduate School of Science, Osaka
City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Full list of author information is available at the end of the article

such as arsenosugars in marine algae (e.g., Morita andShibata 1990; Edmonds et al. 1997; Raab et al. 2005) andarsenobetaine in marine animals (e.g., Edmonds et al.
1977; Duncan et al. 2015).
In general, the dominant As phases in sediments are
the inorganic species: that is, arsenite in reducing
environments and arsenate in oxidizing environments(Neff 2002). Unpolluted nearshore marine and estuarine
sediments usually contain from 5 to 15 μg/g As, but
deep-sea sediments occasionally contain much higher
concentrations, probably owing to precipitation fromsubmarine hydrothermal solutions. Neff (2002) documented As phase changes in marine sediments due tomicrobial decomposition and synthesis of organoarsenicals and reported that sulfides such as realgar (As4S)
and pyrite (FeS2), in which As was included as an impurity element, and oxides such as arsenolite (As2O3) are
the main final inorganic products fixed in the sediments.
In coastal areas, As in sediments and interstitial water
has been studied mainly in relation to biotoxicity. In
deep-sea environments, although As has been documented© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

(2019) 6:28

Page 2 of 14

in seafloor surface sediments (e.g., Francesconi and Edmonds 1996) and in hydrothermal sediments (e.g., Handleyand Lloyd, 2013), vertical profiles of As in deep-sea sediments have not been systematically studied. In this study,
we first traced concentrations and chemical forms of As in
deep-sea sediments from the seafloor surface, through amethane hydrate layer, and down to 2200 mbsf. Then, we
examined the sources of As and effect of microbial activity
on its behavior in deep-sea sediments.

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 upwellingof 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 ofhomogenization of the collapsed sediments.
Most core samples used for our As analyses were collected by IODP Expedition 338 (October 2012 toJanuary 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 andH (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 fromthe 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–December2007) (Ashi et al. 2008), and those from 2110 to2220 mbsf in Holes C0002P were collected by Expedition 348 (September 2013 to January 2014) (Expedition
348 Scientists and Scientific Participants 2014).

Masuda et al. Progress in Earth and Planetary Science

Samples and analytical methods
Drilling sites and studied cores

The sediment cores used in this study were collectedfrom 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 ofmore consolidated sediments, was deposited between
1.67 and 1 Ma; Unit III (826–1025.5 mbsf ) is composedof 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 inthis 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 overalllithology, the cored sediments down to 415.9 mbsf wereclassified into a single unit, although two subunits wereidentified 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, andvolcanic ash; Subunit II-b (383.5–415.9 mbsf) consists of aseries of mud-encrusted gravels interbedded with thinsand, clayey silt, and silty clay layers in its upper part andAnalytical 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 aplastic vial (Nalgene), and stored at room temperature.
The SC was stored onboard at − 20 °C during thecruises. 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 beenused for onboard analysis of alkalinity. Among the samples taken during Expedition 348, we analyzed only theSC, 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.

(2019) 6:28

Masuda et al. Progress in Earth and Planetary Science

35°
NPage 3 of 14

km
0

a

50

Kii
Peninsula
34°

Kumano Basin
C0002

C0022

C0021
33°

Philippine
Sea
Plate

gh

rou
T
i
a
k

Nan
135° E136°

Site C0002

Site C0009

Depth (kmbsl)

2

Sites

C000

Site C0022
C0021/8/1Site0 Site C0018

1/3/4

Kumano Basin

4
6

137°

138°

b
Site C0006
Site C0007

Trenchturbidites

Shikoku Basin
sediments

Site C0011

Site C0012

Accretionary prism
Old accretionary sediments

8

Oceanic basement

lt

y fau

aspla

Meg

nt

Décolleme

10

plate

h

cting P

Subdu

Sea
ilippinekm

Fig. 1 Location of study sites (Sites C0002, C0021, and C0022, Kumano Basin, Nankai Trough) (a) and geological profile of the area (b) (Strasser et al. 2014)

Total As concentration

One gram of powdered sediment was fused with NaCO3
at 900 °C until the silicates were completely melted, andthen 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 theresults was checked by using rock standard samples

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

Page 4 of 14

e

a

b

c

d

Fig. 2 Arsenic concentrations of sediments (squeezed cakes) (a) and interstitial water (b), ratio of arsenic concentrations of IW and sediment (c),and chlorinity (d) of Sites C0002, Kumano Basin, Nankai Trough, in relation to the depth, lithology and its related components (e) Lithology wasreferred to Strasser et al. (2014)(JSd-1, As 2.42 ppm; JSd-2, 38.6 ppm; and JSl-1, 14.9ppm) from AIST (National Institute of Advanced
Industrial Science and Technology), which wereprepared 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 withchanges in the environmental redox condition can beroughly 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 thesupernatant fluid. In step 2, the residual sediment waswashed 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 heatedwith 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 probablephase dissolved at this step is organic matter. Then, themixture of residue and solution was centrifuged to separate. In step 4, the residue from step 3 was shaken with amixture of concentrated nitric and perchloric acid for 16h, 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 theprevious 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 witha solution of 0.1 M hydroxylammonium chloride and
0.5 M phosphoric acid in a PTFE centrifuge tube andshaken at 25 °C for 1 h. The hydroxylammonium chloride solution is reductant and can decompose amorphousand weakly crystalized Fe-oxyhydroxides (e.g., ferrihydrite and goethite) and Mn-oxides, and the phosphoricacid helps to preserve AsIII and AsV species, especially insolutions containing high amounts of Fe and Mn (Ellwood and Maher 2003). Frozen sediment was used forthis analysis, because dried samples may become

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

Page 5 of 14

a

b

c

d

e

f

g

h

Fig. 3 Arsenic concentrations of sediment (squeezed cake) and interstitial water and chlorinity of Sites C0021 and C0022, Kumano Basin, Nankai
Trough, in relation to the depth, lithology and its related components (a) (b) (c) (d): Arsenic concentrations of sediment and interstitial water,
chlorinity and lithology of Site C0022. (e) (f) (g) (h): Arsenic concentrations of sediment and interstitial water, chlorinity and lithology of Site
C0021. Lithology was referred to Strasser et al. (2014)

oxidized, changing the chemical form of the As. Afterthe 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.

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

Results
Bulk As concentration

The bulk As concentrations in SC from Hole C0002
ranged from 3 to 14 ppm (average 6.4 ppm) (Table 1,
Fig. 2a), which is within the range of reported concentration in seafloor surface sediments surrounding theJapanese islands; for example, an average concentrationof As of 5.5 ppm (range, 0–14 ppm, except one sample
with 23.3 ppm) has been reported for 95 surface sediment samples from Kumano-nada (the sea area aroundthe study area) (AIST 2018).
The highest As concentration was measured in SC
from 116 mbsf, at the bottom of Unit I (13.5 ppm), but
the As concentration range in SC from other parts ofUnit I was low (3–5 ppm). In the upper half of Unit II,
between 110 and 500 mbsf (referred to as Unit II-1 hereafter), the As concentration was > 5 ppm, and a maximum concentration of 13.2 ppm was observed at
320 mbsf. Arsenic concentrations of SC from below
600 mbsf in Hole C0002 were mostly < 5 ppm, similar to
those of SC from Unit I. The bulk As concentration in
sediments from below 2000 mbsf was also mostly < 6ppm, while it increased in the deeper part and the maximum concentration was up to 11 ppm (Table 1). Although only bulk As concentrations were obtained fromthe deepest sediments, we inferred that As behavior did
not largely change below ~ 500 mbsf in Hole C0002.
The total dissolved As concentration of IW from Hole
C0002 ranged from 0.9 to 379 ppb (average 50 ppb). The
total As concentration was low in the Unit I sediments
and in the sediments from below 500 mbsf (Units II-2
and III), whereas it was high in the Unit II-1 sediments(Fig. 2b). The relationship between the SC concentrationand the IW concentration varied with depth (Fig. 2c):for example, As peaks in IW were observed at 200 and
400 mbsf, whereas sediment peaks were observed at 116
and < 350 mbsf. The low As concentrations in SC from
Unit I (excepting three samples from the bottom of theunit) and from below 500 mbsf did not change withdepth. Owing to the insoluble host phases of the As andthe physico-chemical conditions of the ambient IW, As
in these depth intervals would not be easily mobilized.
At Site C0022, the drilled hole penetrated the fracturezone of a splay fault at around 100 mbsf (Fig. 3). The
chlorinity of the IW increased with depth from 550 mM,
that of ambient seawater, to 600 mM in the fracturezone, reflecting the upwelling of deep-seated water, inwhich dissolved salts had been concentrated via hydration reactions, along the fault (Strasser et al. 2014). The
total As concentration in SC and IW ranged from 3 to
11 ppm and 4 to 94 ppb, respectively. The total concentration in SC did not change with depth, whereas that in
IW increased below 100 mbsf and reached a peak at
110–130 mbsf, below the fracture zone.

Page 6 of 14

Total As concentrations of SC and IW from Hole
C0021 were from 4 to 7 ppm and from 7 to 36 ppb, respectively, except the deepest sample (22.7 ppm and 94ppb), and did not change substantially with depth (Fig. 3).
This site is located on the forearc slope, and the coredsediments consist of landslide deposits. The chlorinity isstable with depth and the same as that of seawater, probably because the sediment and IW were homogenized
during relocation by the landslide. Thus, except in the
deepest samples (from 180 mbsf ), the As concentrationsin the sediment (4–9 ppm) and IW (5–20 ppb) can be
regarded as averaged shallow sediment values.
Host phases of As in the sediments

Total As recovery by sequential extraction is reported
as a percentage relative to the bulk As concentration(Table 2). In the sequential chemical extraction of As
phases from sediments of Hole C0002, the total recovery of As ranged from 65.4 to 146.7% (average and 1σ:
104 ± 20%, Table 2 and Fig. 4). Among the totally extracted As, recovery in steps 1, 2, and 3, which extracted the mobile As phases in association withchanging redox and pH conditions of coexisting
solution, ranged from 10 to 70%; As recovery in steps 2
and 3 averaged 21% and 19%, respectively. Further, As
extracted in step 3, of which main host phase is organic
matter, seemed to increase as the total As concentration of the sample increased (r = 0.72); this result
implies that biogenic As was responsible for the highaccumulation rate of As in this sediment column.
The high concentrations of As extracted from Unit II
in step 2, which extracted As associated with
Fe-oxyhydroxides/Mn oxides, imply that oxidation of
Fe-minerals occurred in this unit.
As species in sediments separated by HPLC

The mixture of hydroxyl-ammonium chloride solution
and phosphoric acid leaches inorganic and organic
phases adsorbed onto Fe-hydroxides and other particles.
The leached phases may correspond partly to As extracted by steps 1, 2, and 3 of the sequential extraction
procedure. Only sediments from Unit II of C0002 (recovery 30–100%) and from C0022 (recovery 20 to 80%)
were analyzed by this procedure. These recoveries are
generally high compared with the summed As concentrations sequentially extracted by steps 1, 2, and 3(Table 3), probably because the analytical error of theleaching procedure is slightly higher than that for the
total As concentration measurement.
Even considering the large analytical error, it is notablethat the most abundant leached As phase was arsenate(AsV) (Fig. 5); this result indicates that the As was mostly
adsorbed onto or fixed in Fe-oxyhydroxides in the(2019) 6:28

Masuda et al. Progress in Earth and Planetary Science

Page 7 of 14

Table 1 Total arsenic concentration of squeezed cake andinterstitial water of sediments from the Sites C0002, C0021 andC0022 at Nankai TroughTable 1 Total arsenic concentration of squeezed cake andinterstitial water of sediments from the Sites C0002, C0021 andC0022 at Nankai Trough (Continued)

Sample name

Sample nameDepth(mbsf)

As concentration
Squeezed cake
(ppm)

Interstitial water(ppb)

C0002D-1H-2

2

3.5

5.1

C0002D-1H-5

4

4.8

C0002D-2H-3

9C0002D-2H-7

13C0002D-3H-4

19

C0002D-4H-4

29

C0002D-5H-4

38

C0002D-6H-5

49

C0002D-7H-4

57

C0002D-9H-5

78

C0002D-10H-4

86

C0002D-11H-5

98

3.8

C0002D-12H-5

106

3.1

C0002D-13H-5

116

13.5

95.4

C0002D-14H-3

122

6.5

C0002D-14H-5

125

7.3

C0002D-15X-5

133

6.0

C0002D-16H-6

157

C0002D-17X-4

160

C0002D-18H-1

200

C0002D-18H-2
C0002K-1H-3

3.0

Depth
(mbsf)

As concentration
Squeezed cake
(ppm)

Interstitial water(ppb)

C0002L-12X-7

388

8.8

220.4

C0002P 2R-1

2173

2.5

3.1

C0002P 2R-2

2175

2.5

0.9

C0002P 2R-3

2176

3.6

3.0

C0002P 2R-4

2177

8.7

2.0

C0002P 3R-1

2182

6.6

4.0

C0002P 4R-1

2192

4.9

6.4

C0002P 4R-4

2196

4.3

11.1

C0002P 4R-6

2199

5.4

18.8

C0002P 4R-7

2203

2.5

15.5

C0002P 4R-8

2212

6.4

10.0

C0002P 4R-9

2213

11.6

C0002P 4R-10

2214

8.0

C0002P 4R-11

2217

8.9

101.5

C0021B-1H-4

3.7

5.7

6.6

60.1

C0021B-2H-3

81.6

4.8

15.2

27.8

C0021B-3H-4

91.7

7.9

14.0

125.1

C0021B-4H-6

104.9

7.6

13.0

7.5

196.8

C0021B-5H-2

109.9

4.6

25.2

7.4

378.7

C0021B-6H-3

119.5

4.4

10.8

201

7.3

57.5

C0021B-7H-4

129.9

5.7

13.5

202

4.3

21.2

C0021B-8H-2

138.0

5.0

13.9

C0002K-3T-5

210

7.8

41.0

C0021B-9H-2

147.5

6.9

14.0

C0002K-4T-2

216

9.1

77.9

C0021B-10H-8

162.1

5.4

36.1

C0002K-5T-6

224

5.7

16.3

C0021B-12H-2

170.0

5.9

15.3

C0002K-6T-3

231

9.6

31.9

C0021B-13T-3

177.4

7.1

27.8

C0002K-7X-5

242

5.3

66.3

C0021B-14T-2

186.4

22.7

93.7

C0002K-8X-2

250

6.1

21.4

C0022B-1H-2

1.2

8.0

4.0

C0002K-9X-3

260

6.6

18.6

C0022B-2H-5

23.4

6.0

6.5

C0002K-10X-2

268

5.1

41.6

C0022B-3H-2

30.3

4.4

3.0

C0002K-11X-2

277

8.5

84.8

C0022B-4H-2

39.5

3.5

7.4

C0002L-1X-4

281

6.1

27.4

C0022B-5H-2

49.3

5.1

3.7

C0002L-3X-2

297

3.0

25.3

C0022B-6H-2

58.6

8.9

5.6

C0002L-4X-2

307

3.9

10.3

C0002L-13X-1

392

4.3

124.8

C0002L-5X-3

317

9.8

93.6

C0002L-14X-3

403

8.4

58.4

C0002L-6X-4

327

13.2

115.9

C0002L-15X-4

412

5.2

25.8

C0002L-7X-2

335

11.6

36.4

C0002L-16X-3

422

7.4

82.3

C0002L-8X-4

346

10.5

177.8

C0002L-17X-4

432

8.5

70.0

4.6

3.3

C0002L-9X-5

356

9.1

105.7

C0002L-18X-3

440

7.5

47.8

C0002L-10X-3

365

11.5

145.5

C0002L-19X-3

451

6.6

140.6

C0002L-11X-4

375

9.7

157.2

C0002L-20X-2

459

5.1

55.3

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Table 1 Total arsenic concentration of squeezed cake andinterstitial water of sediments from the Sites C0002, C0021 andC0022 at Nankai Trough (Continued)

Table 1 Total arsenic concentration of squeezed cake andinterstitial water of sediments from the Sites C0002, C0021 andC0022 at Nankai Trough (Continued)

Sample name

Sample nameDepth(mbsf)

As concentration
Squeezed cake
(ppm)

Interstitial water
(ppb)

Depth
(mbsf)

As concentration
Squeezed cake
(ppm)

Interstitial water(ppb)

C0002L-21X-4

471

6.6

96.9

C0002L-24X-5

501

7.6

54.2

C0022B-13T-1

104.5

4.4

12.8

C0022B-14X-4

114.1

6.5

23.9

C0002B-4R-2

505

3.9

C0002B-8R-2

535

46.2

C0022B-15X-3

118.5

5.3

22.0

34.4

C0022B-16X-4

127.5

5.9

85.8

C0002B-9R-3
C0002B-10R-2

545

35.9

C0022B-17X-2

134.8

2.9

41.7

554

23.1

C0022B-18X-4

145.6

4.7

35.6

C0002B-11R-3

564

39.2

C0022B-19X-2

153.4

5.5

94.1

C0002B-13R-2

582

17.8

C0022B-20X-4

193.6

6.8

10.3

C0002B-16R-3

612

21.1

C0022B-21X-6

204.3

5.3

8.5

C0002B-19R-3

641

20.7

C0022B-22X-1

210.5

4.8

22.0

C0002B-21R-3

659

12.1

C0022B-23X-4

221.2

10.1

30.3

C0002B-23R-3

678

23.9

C0022B-24X-3

268.7

8.6

18.8

C0002B-24R-2

686

C0002B-27R-2

714

C0002B-29R-2
C0002B-30R-2

3.9

52.3

C0022B-26X-3

288.3

5.2

13.3

17.3

C0022B-27X-4

294.3

6.1

10.9

734

17.8

C0022B-28X-3

297.4

5.4

6.4

743

25.4

C0022B-29X-2

301.3

5.2

3.8

C0002B-32R-5

764

21.7

C0022B-30X-3

307.3

4.8

20.5

C0002B-33R-2

771

14.8

C0022B-31X-2

315.9

5.7

23.4

C0002B-37R-2

807

4.0

20.4

C0022B-33X-7

341.7

7.8

17.8

C0002B-38R-3

818

4.4

33.3

C0022B-34X-3

345.4

7.1

24.7

C0002B-40R-2

836

25.3

C0022B-35X-5

357.8

6.9

7.2

C0002B-41R-3

847

30.5

C0022B-36X-3

364.2

11.3

13.1

C0002B-43R-5

868

5.5

C0022B-37X-2

373.1

6.3

14.2

C0002B-46R-4

895

17.6

C0022B-38X-3

384.3

6.6

26.6

C0002B-48R-5

915

8.8

C0022B-39X-2

391.0

5.7

8.7

C0002B-49R-3

922

7.7

C0022B-40X-3

403.2

7.5

7.9

C0002B-51R-5

944

3.5

C0022B-41X-2

411.4

5.4

5.4

C0002B-55R-2

978

5.1

C0002B-56R-2

987

C0002B-59R-2

1011

C0002B-61R-3

1021

5.3

C0002B-62R-2

1024

21.3

3.7

5.5
6.5

C0002B-64R-2

1043

3.9

25.5

C0002J-3R-3

913.6

5.5

3.3

C0022B-7H-7

74.0

6.3

5.6

C0022B-8H-1

76.5

5.5

5.7

C0022B-9T-2

84.8

4.4

14.6

C0022B-10T-2

90.3

5.7

22.1

C0022B-11T-1

94.5

4.8

21.2

C0022B-12T-1

99.5

4.6

8.8

sediments. In addition, chromatogram peaks indicatingsmall amounts of MMA and AsB and two unidentified As
compounds were detected. Chromatogram peaks for theidentifiable compounds were confirmed by co-injection ofstandards of As-compounds. The presence of above organoarsenicals indicates that As in the sediment column hada biological source.

Discussion
We examined the factors controlling As behavior indeep-sea sediments mainly on the basis of the analyticalresults for the cored sediments from Hole C0002. Vertical profiles of As and other chemical components in IW
from the sediment column at Site C0002 are shown in
Fig. 6 (Moore et al. 2013). As described above, chlorinity

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

Page 9 of 14

Table 2 Arsenic concentrations of sequentially extracted fractions of sediments from Site C0002 at Nankai TroughSample nameDepth(mbsf)

Total
As(ppm)As concentration (ppm)
Step 1

Step 2

Step 3

Step 4

Step 5

Total

Recovery (%)

C0002D 1H-2

2

3.5

0.18

1.27

0.36

1.67

0.29

3.76

107.3

C0002D 5H-4

38

4.6

0.10

0.15

1.03

3.80

0.76

5.84

127.7

C0002D 7H-4

57

3.3

0.13

0.57

0.42

2.34

0.54

3.99

121.9

C0002D 11H-5

98

3.8

0.12

0.76

0.54

2.31

0.72

4.46

116.2

C0002D 12H-5

106

3.1

0.11

0.49

0.97

1.86

0.67

4.10

132.7

C0002D 13H-5

116

13.5

0.15

1.24

6.12

2.64

0.66

10.81

80.2

C0002D 15X-5

133

6.0

0.08

1.23

0.57

1.88

0.82

4.58

76.7

C0002D 17X-4

160

7.5

0.44

3.69

1.30

2.19

0.31

7.92

105.8

C0002D 18H-1

200

7.4

0.21

2.24

1.14

4.18

0.33

8.11

110.2

C0002D 18H-2

201

7.3

0.06

1.41

0.73

4.73

0.77

7.71

104.9

C0002K 7X-5

242

5.3

0.30

1.19

1.44

2.33

0.49

5.75

107.6

C0002K 9X-3

260

6.6

0.44

0.80

1.86

3.05

0.75

6.91

105.4

C0002L 3X-2

297

3.0

0.25

0.49

0.53

1.04

0.64

2.95

100.0

C0002L 6X-4

327

13.2

0.75

2.74

2.76

2.86

0.43

9.53

72.3

C0002L 8X-4

346

10.5

0.45

1.78

0.77

3.17

0.70

6.86

65.4

C0002L 9X-5

356

9.1

0.76

1.66

0.85

2.15

0.70

6.13

67.7

C0002L 11X-4

375

9.7

0.62

1.76

3.52

3.65

1.06

10.62

109.2

C0002L 12X-7

388

8.8

0.76

2.01

1.21

4.01

1.22

9.22

104.8

C0002L 13X-1

392

4.3

0.35

1.34

0.46

2.69

0.58

5.42

125.2

C0002L 16X-3

422

7.4

0.28

1.57

0.81

2.79

0.61

6.07

81.6

C0002L 17X-4

432

8.5

0.41

1.15

2.33

3.38

0.87

8.14

96.1

C0002L 19X-3

451

6.6

0.21

1.46

1.40

2.70

0.27

6.05

91.8

C0002L 24X-5

501

7.6

1.06

2.56

2.10

1.63

0.63

7.97

104.9

C0002B 16R-3

612

3.9

0.44

0.61

0.89

2.71

0.24

4.90

125.5

C0002B 27R-2

714

3.7

0.03

0.96

0.64

1.74

0.65

4.03

108.6

C0002L 37R-2

807

4.0

0.10

1.95

0.81

1.67

0.22

4.74

117.5

C0002B 38R-3

818

4.4

0.11

0.96

1.07

2.36

0.51

5.01

114.9

C0002B 59R-2

1011

6.5

0.07

0.35

0.69

7.35

1.03

9.48

146.7

decreases linearly in Units I, III, and IV, whereas in Unit
II, freshwater derived from the dissociation of methane
hydrate diluted the IW, lowering the chlorinity of that
unit. Here, we first discuss differences in the vertical variation of chemical components likely controlled by in situ
microbial activity; later, we discuss sources of the IW.
In Unit I, reduction of SO42− occurred just beneaththe seafloor, leading to a low SO42− concentration there.
The Fe concentration is also low in Unit I, probably
owing to the precipitation of FeS2 (pyrite), because framboidal pyrite was observed throughout the sediment column (Strasser et al. 2014). Br−, PO43−, and NH4+
increase with depth in Unit I; these results can be plausibly attributed to the decomposition of organic matter.
Peak concentrations of these three components were observed in the uppermost part of Unit II; below that, theirconcentrations decreased with depth down to 400 mbsf.

The SO42− concentrations of Unit II are higher thanthose of the Unit I, in which SO42− reduction occurs justbeneath the ocean bottom surface (Fig. 6). The similar
decrease (3–6 mbsf) and increase (> 400 mbsf) of SO42−
concentrations were observed in the deep-sea sediments
at IODP Sites 1173 and 1174 at Nankai Trough, about
100 km SW from this study site. In those sediment columns, high SO42− was supplied from diffusive flux of sulfate from the oceanic basement into the overlaying
sediments (e.g., Shipboard Scientific Party 2001a, b; Heueret al. 2017). It would not be the case for the sediment column studied here, since the upward migration of deep
fluid was limited (Toki et al. 2017). However, the SO42−
would be derived diffusive flux of seawater and/or
deep-sourced fluid (BG, described in the next paragraph).
We examined variations of IW chemistry in relation to
the end-member composition of the IW (Fig. 7). Toki et al.

Depth (mbsf)

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

Page 10 of 14

Table 3 Arsenic concentrations of soluble fractions extracted byhydroxylammonium chloride and phosphoric acid solution

1.517
38.33
57.35
97.86
106.3
115.5
133.2
159.8
200.4
200.6
242
260.1
297.2
327.2
346.4
356.4
375.4
388.1
391.8
422
431.8
450.5
500.7
611.6
714.3
807.1
818
1011

Sample name

I

C0002K-1H-3

II

IV
0

2

4
6
8 10
As conc. (ppm)12

Fig. 4 Sequentially extracted arsenic concentrations of the sedimentsof Site C0002, Kumano Basin, Nankai Trough(2017) inferred that IW at Site C0002 is a mixture of three
end members: SW (seawater); MH, freshwater derived fromthe dissociation of methane hydrate; and BG, defined as a
deep-sourced fluid probably formed by ultrafiltration during
burial diagenesis of IW in the old accretionary prism(Fig. 7a). SW, MH, and BG are characterized by chlorinity
values of 550, 140, and 460 mM, respectively, and δ18O
values of 0‰, + 1‰, and − 2.5‰, respectively. These three
end members are typically found at the seafloor surface,around the methane hydrate concentration peak at about
400 mbsf, and below 1000 mbsf, respectively. Toki et al.
(2017) suggested that the IW initially formed by the mixingof SW and BG via slow advection and was later modified by
the addition of MH to the mixture. Most IW from Units I,
III, and IV represents a mixture of SW and BG, whereas IW
from Unit II is a mixture of MH with pre-existing SW–BG
mixture. Concentrations of dissolved components associatedwith microbial activity and redox conditions are not just a
simple function of the mixing ratios of the three end members. Figure 7 shows relationships among chemical components plausibly vary associated with As. Some components
show contrasting distributions in four separate depth
intervals: for example, Br− is enriched but the Cl−
concentration is unchanged from that of SW in Unit I,
both Cl− and Br− are depleted in MH, and Cl− is depleted with no change in the Br− concentration in BG

Depth(mbsf)

Concentration
As(V)
(ppm)

As(III)As(III)
(ppm)

MAA
(ppm)

AsB
(ppm)

202

2.64

0.13

0.19

0.01

C0002K-5 T-6

224

5.23

0.13

0.17

0.00

C0002K-7X-5

242

3.20

0.57

0.00

0.00

C0002K-10X-2

268

3.22

0.74

0.01

0.02

C0002L-3X-2

297

2.87

0.10

0.02

0.01

C0002L-5X-3

317

5.29

0.77

0.03

0.01

C0002L-7X-2

335

3.37

0.14

0.09

0.00

C0002L-9X-5

356

5.42

0.50

0.12

0.01

C0002L-11X-4

375

7.07

1.22

0.11

0.01

C0002L-13X-1

392

6.81

0.21

0.17

0.00

C0002L-15X-4

412

3.73

0.14

0.08

0.01

C0002L-17X-4

432

5.25

0.69

0.00

0.01

C0002L-19X-3

451

6.49

0.27

0.06

0.01

C0002L-21X-4

471

6.11

0.05

0.41

0.01

C0022B-1H-2

1

2.56

0.32

0.00

0.01

C0022B-2H-5

23

1.95

0.29

0.00

0.00

C0022B-4H-2

39

0.66

0.07

0.01

0.01

C0022B-8H-1

77

2.01

0.21

0.01

0.01

C0022B-10 T-2

90

4.56

0.11

0.09

0.00

C0022B-13X-1

105

4.87

0.06

0.24

0.00

C0022B-15X-3

118

6.26

0.09

0.09

0.00

C0022B-19X-2

153

6.02

0.19

0.04

0.00

C0022B-20X-4

194

3.01

0.41

0.07

0.00

C0022B-21X-6

204

3.08

0.09

0.18

0.01

C0022B-22X-1

210

5.91

0.21

0.00

0.01

C0022B-24X-3

269

1.01

0.02

0.05

0.00

C0022B-29X-2

301

3.00

0.14

0.04

0.00

(in Units III and IV) (Fig. 7b). Br−, PO43−, and NH4+
show weakly positive correlations in Unit I, whereas
those components show strongly positive correlationsin Unit II-1. IW from Units III and IV is characterized
by low PO43− without changes in Br− and NH4+
concentrations (Fig. 7c, d). In Unit II-1, the Fe concentration seems to increase with the PO43− concentration(Fig. 7e). These features suggest that the IW geochemistry reflects the degree of biological degradation, andthat microbial activity was most active in Unit II-1.
The Br− concentration (Fig. 6b), but not the Cl− concentration, of IW increased with depth in Unit I, probably because Br was released from sediments by thedecomposition of Br-containing organic matter derived
from algae and marine animals, some of which(2019) 6:28

Masuda et al. Progress in Earth and Planetary Science

Page 11 of 14

0

a

b

Depth (mbsf)

100

c

d

C0022
C0002

200

300

400

500
0

2

4 6 8
As(V) ppm

10 0

0.5 1 1.5
As(III)ppm

02 0 100 200 300 400
MAA ppb

5

10 15
AsB ppb

20

Fig. 5 Arsenic speciation of sediments separated by HPLC-ICP-MS after extraction by mixing solution of hydroxylammonium chloride andphosphoric acid in relation to the depth a Arsenate. b arsenite. c methylarsonic acid. d arsenobetaine

experiment conducted with sediment from 4.7 mbsf. The
freshwater derived from the dissociation of methane hydrate is not only low in Cl− but also in Br− (Fig. 7a). The
BG end member is slightly depleted in Cl− but not in Br−
compared with SW. IW from Unit II was a mixture of
MH, SW, and BG. The Br− concentration increased withincreasing PO43− in Units I and II-1. High Br− and PO43−
concentrations are probably attributable to the decomposition of algae, and the relationship between them showed
different trends between Units I and II-1. Moreover, theconcentrate Br in various, mostly toxic, compounds suchas bromoperoxidases (Hewson and Hager 1980), bromoform, and dobromoacetic acid (DBA) (e.g., Paul et al.
2006). Further, alkyl bromide compounds (Ma et al. 2010)
have been extracted from algae, and 2,4-dibromophenol is
produced by animal activity in coastal sediments (King1988). Dehalogenation bacteria were recovered from thesediment column of Hole C0002 down to 150 mbsf, andFutagami et al. (2013) observed dehalogenation (bromide
release) of 2,4,6-tribromophenol in an incubation
0

I
200

II-1

Depth (mbsf)

Peak of methane hydrate

400

600

II-2

800

III
IV

1000

a

b

c

d

e

f

g

1200
0

10 20 30
Alkalinity mM

0

0.5
1
Br- (mM)

0

100 200 300
3PO (mM)
4

0

5
10
+
NH (mM)4

0 5 10 15 20 25
2SO (mM)

0

20

40

60

0

2000 4000 6000
As(IW) (nM)

4Fig. 6 Relationships of interstitial water chemistry to the subsurface depth of sediment column at C0002. Data from a to f are from Strasser et al.
(2014), and g from this study

Masuda et al. Progress in Earth and Planetary Science(2019) 6:28

bPage 12 of 14

c

d

a

e

f

g

h

Fig. 7 Relationships of total As and δ18 O and dissolved chemical components in IW sensitive to changing redox condition a: chlorinity and δ18O;
b: chrolinity and Br- concentration; c: Br- and PO43- concentrations; d: NH4+ and PO43- e: PO43- and Fe; f: PO43- and As; g: pH and As; Fe and As. SW,
MH, and BG are the seawater, dissolved water from methane hydrate, and diagenetically altered interstitial water, respectively (a referred Toki et al.
(2017) was modified). Data except As concentrations were from Moore et al. (2013) and Toki et al. (2017)

relationship between NH4+ and PO43− in IW showed different trends even more clearly between Units I and II-1.
These differences between Units I and II-1 suggest thatthe sources of Br−, PO43−, and NH4+ differed between
these Units (Fig. 7c, d). IW was depleted PO43− in Unit III
and IV, and the relationship to Br− and NH4+ can be explained by the mixing of BG and MH.
The high bulk As concentrations in both sediment and
IW in Unit II-1 (Fig. 2), along with the high Br−, PO43−,
and NH4+ concentrations of IW, imply that these components had similar sources, i.e., marine biology especiallysuch as algae. In Unit II-1, the As concentration increased
with increasing pH (Fig. 7g) but not in relation to either
PO43− or Fe (Fig. 7f, h). From the combined results of sequential extraction and leaching analyses, we can inferthat a considerable amount of AsV was associated with
Fe-oxyhydroxides and that the As concentration of IW
was largely controlled by the pH. The adsorption–desorption affinity of As onto Fe-oxyhydroxides is mainly controlled by the solution pH; adsorption of arsenate (AsV)
occurs at pH 5–6, whereas desorption becomes dominant
at higher pH (e.g., Pearce and Moore 1982; Bowell et al.
1994; Dixit and Hering 2003).
MMA, one of the simplest organoarsenicals, is derived
from the decomposition of arsenosugars contained inphytoplankton and algae (Edmonds et al. 1997; Morita
and Shibata 1990). AsB (arsenobetaine), the simplest
protein containing As, is derived from marine animals(e.g., Francesconi and Edmonds 1996). Decomposition
rates of organic As in sediments are high: Takeuchi et al.
(2005) observed that in surface sediments from a coastal

area, which were plausibly more oxic than the deep-sea
sediments studied here, organic As was largely decomposed within approximately 60 years. The ages of thesediments from 200 to 500 mbsf in Hole C0002K and
C0002L, corresponding to Unit II-1 in this study, wereconstrained from 1.04 to 1.34 Ma by biostratigraphy andmagnetostratigraphy conducted during Expedition 315(Strasser et al. 2014; Expedition 315 Scientists 2009).
MMA can form in situ by microbial methylation (Reimer
and Thompson 1988) in conjunction with methane formation. Sulfate-reducing and nitrogen-fixing bacteria weredetected with methanogen, which mostly uses CO2 andH2 for producing CH4, from the sediments down to193.29 mbsf at IODP Site 1173, located in the Nankai
Trough and about 100 km SW of our study site (Newberry
et al. 2004). In the same sediment column down to
194 mbsf, epsilon-proteobacterial clones, of which species
used a diverse spectrum of electron accepters, such as O2
in low concentrations, NO3−, S°, and S2O32−, were also detected, and it was notable that these clones could also respire with Fe(III) and arsenate (Kormas et al. 2003). The
similar microbial assemblage would be expected in the
Unit II-1 of our studied column, of which IW contained
SO42−. AsB, an organic As compound that is synthesizedand degraded in biotransformation cycles, is ubiquitous inmarine, terrestrial, and deep-sea hydrothermal systems(Hoffmann et al. 2018). In light of the results reported by
Hoffmann et al. (2018), we can infer that the AsB in the
deep-sea sediments studied here was also a product of insitu microbial activity. Thus, the detection of organic As
compounds suggests that considerable amounts ofMasuda et al. Progress in Earth and Planetary Science(2019) 6:28

mobile As in the studied sediment column may havebeen derived in relation to microbial activity via bothdegradation and synthesis of organic As compounds inthe sediment column.

Conclusions
Arsenic is actively released into IW at depths of methane
hydrate layer of deep-sea sediments of the Nankai Trough,offshore Japan. The release of As accompanies the degradation of organic matter and following change of redox
condition to control the adsorption/desorption affinity of
As onto the Fe-oxyhydroxides. The sources of the As inthe sediment column are not clear at present. However, aconsiderable amount of mobile As must be derived frommarine organisms. It is notable that As mobilization in
deep-sea sediments is associated with microbial activity in
deep-sea sediments where methane hydrate appears.
Acknowledgements
The authors sampled the studied sediments and IW in collaboration with thecrews and onboard scientists of Expeditions 338 and 348. The stored samples
taken during Expedition 315 were provided by the Kochi Core Center, which is
directed by Dr. Lalan Gupta. T. Shimonaka and K. Okazaki supported for
conducting shore-based laboratory analyses. The comments from Dr. T. Kakegawa
and two anonymous reviewers were helpful to improve the manuscript. We
thank to all of them. This study was financially supported by post-cruise IODP
studies to HM, SF and EE, and KAKENHI (No. 17K18810).
Funding
Samples were collected on Drilling Vessel “Chikyu” under IODP program during
Expeditions 338 and 348.
Post cruise research fund from JAMSTEC for HM, SF and EE and KAKENHI(No. 17K18810) were used for As analyses.
Availability of data and materials
All data newly obtained from this study is in the manuscript and data sharing is
not applicable.
Authors’ contributions
HM carried out the sample collection and pretreatments on Exp. 338, helpedin the analysis of water-soluble As, wrote the manuscript, and drew the figures.
HY helped in the analyses of As of interstitial water and sediments. SF helpedin the sample collection and pretreatments on Exp. 348, analysis of watersoluble As with HY, and discussion. TT helped in the sample collection andpretreatments on Exp. 338, and discussion. EE contributed to the sample
collection and pretreatments on Exp. 348. All authors read and approved thefinal manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Department of Biology and Geosciences, Graduate School of Science, Osaka
City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
2
Present Address: Marine Environment Section, Center for Regional
Environmental Research, National Institute for Environmental Studies, 16-2
Onogawa, Tsukuba, Ibaraki 305-8506, Japan. 3Department of Chemistry,
Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1
Senbaru, Nishihara, Okinawa 903-0123, Japan.

Page 13 of 14

Received: 30 April 2018 Accepted: 27 January 2019

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