EXPRESS LETTER Kikai Caldera south of Kyushu Island, Japan

Geochemical Journal, Vol. 49, pp. e1 to e7, 2015
doi:10.2343/geochemj.2.0374
EXPRESS LETTER
Sulfides in oxic seawater over the submarine hydrothermal area of
Kikai Caldera south of Kyushu Island, Japan
N ORIKO NAKAYAMA,* K OTARO SHIRAI , Y UJI SANO, TOSHITAKA GAMO and HAJIME OBATA
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
(Received December 19, 2014; Accepted May 29, 2015; Online published June 26, 2015)
The concentration of total metal sulfide throughout a water column over a submarine hydrothermal vent in Kikai
Caldera south of Kyushu Island, Japan, at ~350 pmol/kg, was higher than that reported in previous studies, at <50 pmol/
kg below the halocline. Seawater filtered at 0.2 µ m pore size and unfiltered seawater exhibited almost identical metal
sulfide concentrations throughout the water column, indicating that most metal sulfide existed in dissolved and particulate
forms with diameters <0.2 µ m. By using a mass balance calculation with the observed sulfide species of free and metal
sulfides and carbonyl sulfide, we showed that ~70% of the metal sulfide supplied from hydrothermal vents were contained
in the water column beyond the halocline without undergoing oxidative loss even after mixing into overlying oxic seawater.
Our findings clearly indicate that sulfide and trace metals emitted from hydrothermal vents form a stable metal-sulfide
complex with diameters <0.2 µm. These results also strongly support the recently proposed theory such that metal-sulfide
complexation/nanoparticles play an important role in the long-distance transportation of trace metals in the ocean.
Keywords: dissolved sulfide, long distance transportation of trace metal, hydrothermal source, oxic seawater
Koschinsky, 2011). Only a few researchers reported on
metal sulfide and free sulfide (H2Sgas + HS– + S2–) as a
ligand of trace metals in oxic seawater for the Atlantic
Ocean (Andreae and Ferek, 1992; Radford-Knoery and
Cutter, 1994; Cutter et al., 1999); no new data have been
reported since the 1990s.
In the present study, we present for the first time depth
concentration profiles of free and metal sulfides in oxic
seawater from the surface to the bottom over the submarine hydrothermal area of Kikai Caldera, and we estimate
the metal sulfides derived from the hydrothermal vent
throughout the water column by combining observed
sulfide species data with a simple mass balance model.
INTRODUCTION
Trace metals such as Fe, Cu, and Zn are known as
essential micronutrients for biological production; thus,
their availability impacts the biogeochemical cycles of
oceans (e.g., Bruland et al., 1991). Although oceanic hydrothermal vents are important sources of bioactive trace
metals (Wu et al., 2011; Nishioka et al., 2013; Saito et
al., 2013), the current understanding of the stability and
chemical speciation of oceanic trace metals remains incomplete and inadequate for describing the
biogeochemical cycles and processes of these trace metals (Benner, 2002). A large portion of metal sulfides in
hydrothermal fluid was previously believed to precipitate onto the seafloor in the direct vicinity of the hydrothermal vent or to be rapidly oxidized and removed mostly
within the first few meters above the vent (German et al.,
1990; Feely et al., 1994). Several recent studies have suggested the formation of metal sulfide nanoparticles in
hydrothermal plumes and have pointed out the potential
importance of complex nanoparticle-formation in the
transportation of trace metals over longer distances than
that previously expected (Yücel et al., 2011; Sander and
OBSERVATION AND METHODS
Located approximately 90 km from the southern part
of Kyushu Island, Japan, Kikai Caldera is one of the largest submarine calderas in the country. The area is relatively shallow with a maximum depth of approximately
590 m. Active hydrothermal activity has been reported
within the caldera (Onodera et al., 2009). The current
observation was conducted as a part of the KS-14-10 research cruise aboard the R/V Shinsei-Maru on June 25 to
29, 2014. Observations including seawater sampling and
geophysical surveys were conducted at two locations.
Station 1, at 30°42.61′ N, 130°22.36′ E, is in the south-
*Corresponding author (e-mail: [email protected])
Copyright © 2015 by The Geochemical Society of Japan.
e1
Fig. 1. Geographical setting of sampling site and sampling locations (stations 1 and 2) at the Kikai Caldera submarine volcano
(source: Hydrographic and Oceanographic Department, Japan Coast Guard, available at http://www1.kaiho.mlit.go.jp/
GIJUTSUKOKUSAI/kaiikiDB/kaiyo30-2.htm (accessed June 20, 2014)).
ern part of the caldera at a maximum depth of approximately 468 m. Station 2, at 30°47.38′ N, 130°25.00′ E, is
in the northeastern part of the caldera near southern
Takeshima Island at a maximum depth of approximately
521 m (Fig. 1).
Seawater samples were collected from appropriate
depths in the water column by using 24
polytetrafluoroethylene (PTFE)-coated Ocean Test Equipment (OTE) bottles, each having a 12 L capacity and an
external stainless steel spring closure. These bottles were
mounted on a Sea-Bird carousel system (Sea-Bird Electronics (SBE), Bellevue, Washington), equipped with an
SBE 9 plus conductivity, temperature, and depth (CTD)
package, an SBE 43 dissolved oxygen sensor, and a
Seapoint fluorometer (Seapoint Sensors, Exeter, New
Hampshire) for chlorophyll-a measurement. Dissolved
oxygen and salinity detected by the sensors were validated by authentic Winkler titration and Autosal salinometer methods, respectively, for discrete seawater samples.
Measurement of pH was conducted by using a PHM93
Reference pH Meter (Radiometer, Copenhagen, Denmark)
within a day after sampling. Two artificial seawater buffer
solutions were used for calibration: Tris (2-Amino-2hydroxymethyl-1,3-propanediol, lot WEK8350; Wako
pure chemical industries, 287-77321) and AMP (2Aminopyridine, lot WEK8351; Wako pure chemical industries, 284-77321).
Seawater samples for the measurements of free sulfide
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N. Nakayama et al.
(free-S, = H2S gas + HS– + S2–) and total sulfide (total-S =
free-S + total metal-S) were carefully transferred to 200
mL glass vials directly from the OTE bottle by gravity
using Tygon tubing. Special care was taken for the gas
exchange with air by overflowing at least one volume and
preventing the formation of space or air bubbles during
the seawater sample transportation. For the dissolved total sulfide (dissolved total-S = free-S + dissolved metalS), seawater samples were filtered with a 0.2 µm-pore
size filter. To reduce the possibility of contamination, a
capsule filter (AcroPakTM 200 Capsules with Supor membrane, Pall Corp., San Diego, California) was used for
the filtration by attaching it to the OTE bottle by Tygon
tubing when the seawater samples were transferred. Each
glass vial was then sealed with a Teflon-lined septum with
an aluminum crimp cap and was stored in an on-board
refrigerator at 4°C until gas extraction and analysis. All
seawater samples were analyzed within 24 h after the sampling. Storage tests were conducted several times on board
during the cruise to confirm that there was neither a significant loss nor production of sulfide species in the
seawater within 24 h after sampling.
The quantification of sulfides in the seawater samples
was conducted by extracting the sulfides using a dissolved
gas stripping/trapping system coupled with a gas chromatography system with a flame photo detector (GC-2010
with FPD, Shimadzu, Kyoto, Japan), which is a modified
version of the analytical method developed by Radford-
Knoery and Cutter (1993). In brief, 200 mL of seawater
samples was transferred into a gas stripping vessel and
was purged with He (>99.9999%) carrier gas streams for
the free-S analysis. For the total-S and dissolved total-S
analysis, 8 mL of 1 M phosphoric acid was added to generate H2S gas in the stripping vessel prior to the stripping. The stripped H 2S gas, which was cryogenically
trapped using liquid nitrogen, was swept into the GC-FPD
for chemical separation and quantification. The
preconcentration system was constructed with Sulfinert®treated tubing to prevent any sulfur compound adsorption by the inner wall of the system during the analysis.
A wide-bore capillary column of 30 m × 0.53 mm inner
diameter × 40 µ m (HP-PLOT/Q + PT, Agilent Technologies, Santa Clara, California) was used at a constant temperature of 105°C in the gas chromatography-flame photometric detector (GC-FPD) instead of the packed column used by Radford-Knoery and Cutter (1993). Carbonyl sulfide (OCS), the hydrolysis of which is one of the
main sources of H2S in seawater (Elliott et al., 1987),
was also measured simultaneously with H2S gas for freeS analysis. The detection limits were approximately 0.5
pmol/kg and 0.1 pmol/kg for H2S and OCS, respectively,
and the precision was estimated to be better than ±5%
based on the deviation of multiple measurements of the
same seawater samples. The calibration curves were obtained by using standard gas for 4.81 ppm (v/v) of H2S
and 4.59 ppm (v/v) of OCS for nitrogen dilution (Taiyo
Nippon Sanso, Tokyo). Sulfinert ® -treated gas sample
loops equipped with six-way valves were used to inject
the aliquots of standard gases with He gas streams, and
the standard gases were measured by using the same procedure as that used for the seawater samples. Although
the six-way valves are not treated with Sulfinert®, we confirmed through laboratory testing that these valves negligibly affected our H 2S quantification. The total and dissolved metal-S concentrations were calculated by subtracting free-S from total-S and dissolved total-S, respectively.
RESULTS AND DISCUSSION
Figure 2 shows the hydrographic and chemical data
of the two stations in Kikai Caldera. The water column
was generally divided into four layers for both stations.
The surface mixed layer extended from the surface to a
depth of 50 m followed by the halocline layer from 50 m
to 100 m. At station 1, at the southern part of the caldera,
the halocline layer had higher salinity due to an enhanced
inflow of saline (≥34.5) Kuroshio surface water with
higher temperature (≥25°C). The lower salinity in the
surface layer of station 2 may be attributed to local waters from the islands.
Below the 100 m depth, both stations had similar
Fig. 2. Vertical profiles of salinity (‰), potential temperature
(°C), pH, dissolved oxygen (µmol/kg), and intensity of chlorophyll fluorescence (arbitrary unit) for station 1 (solid lines and
filled circles) and station 2 (dotted lines and open circles) in
Kikai Caldera during the KS-14-10 research cruise on June
25–29, 2014.
hydrographic and chemical structures such as an inflow
of cool and saline Kuroshio subsurface water at 100–330
m followed by a stable uniform layer below 330 m. The
observed depth profile suggests that the water column
from the 330-m depth to the bottom was within the rim
of the Kikai Caldera Basin and was thus vertically stratified. No anomalies of bottom temperature or pH due to
hydrothermal activity were observed at either station. It
should also be noted that the dissolved oxygen concentration remained high below 330 m at 165 µmol/kg.
Vertical profiles of free-S, total metal-S, dissolved
metal-S, and OCS are shown in Fig. 3. In the surface
water, the free-S concentration was ~30 pmol/kg. This
value increased with depth and reached its maximum of
54.5–59.5 pmol/kg around the bottom, which is inside
the caldera rim. The observed surface concentrations are
quite similar to those reported from surface surveys at
the western North Atlantic Ocean (Andreae et al., 1991;
Radford-Knoery and Cutter, 1994). Below the halocline,
however, the free-S concentrations were significantly
Metal sulfides in oxic seawater
e3
Fig. 3. Vertical profiles of observed sulfides species for stations 1 (diamonds) and 2 (circles): (a) free sulfide (H2S + HS – +
S 2–), (b) metal sulfide complexes (closed: unfiltered, open: after filtering with a 0.2-µ m pore size filter, (c) carbonyl sulfide
(OCS). Error bars represent the current analytical precision.
higher than those reported for the open ocean with values
of 19 pmol/L in the top 50 m and below 5 pmol/kg at
other depths (Radford-Knoery and Cutter, 1994; Cutter
et al., 1999).
The concentrations of total and dissolved metal-S remained constant within the analytical error throughout
the water column at 332–354 pmol/kg. The observed uniform vertical profiles clearly indicate that most of the
metal sulfide existed in a dissolved form throughout the
water column, whether completely dissolved or in
particulate form with diameters <0.2 µm, with high stability even in the oxic conditions of the study area.
The average concentration of OCS was ~40 pmol/kg
in the upper 40 m, which is slightly higher than that below the 40 m at ~30 pmol/kg. OCS in seawater has two
production processes: that through the photolysis of dissolved organic sulfur compounds in surface waters
(Andreae and Ferek, 1992) and that through bacterial respiration/regeneration of organic matter in deeper waters
in a process known as dark production (Radford-Knoery
and Cutter, 1994; Ulshöfer et al., 1996; Flöck and
Andreae, 1996). Hydrothermal fluids contain little OCS
and are thus not the source of OCS (Chin and Davis,
1993).
In order to assess the contribution of hydrolysis of
OCS and other potential sources for the H2S, we first examined the ratio between the free-S and OCS concentrations, since OCS has been considered as the primary
source of H2S in oceans. Table 1 shows the depth variation of the ratios at both stations. The ratios were lowest
at the surface, at 0.7–0.9, and increased with depth to
reach the maximum at 1.6–2.0 below 330 m. In previous
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N. Nakayama et al.
studies conducted at the continental shelf, open ocean,
and high-latitude regions in the North Atlantic Ocean, the
ratio ranged from 0.1 to 0.7 throughout the water columns and was approximately 0.3 at the surface (RadfordKnoery and Cutter, 1994; Cutter et al., 1999).
The 0.7–0.9 ratio observed in the surface layer during
this study is roughly within the range of that previously
reported but is higher than the ~0.3 average value observed at the surface of the North Atlantic Ocean. Moreover, the ratios of 1.6–2.0 observed above the bottom in
this study significantly exceed the values reported in the
North Atlantic Ocean. It appears probable that the direct
emissions of biogenic H2S at the surface, where high chlorophyll-a concentrations were observed at approximately
the top 100 m, would have contributed to the observed
high free-S in the surface mixed layer. However, such a
direct biogenic emission is quite unlikely to have occurred
below the halocline. The higher concentrations of free-S
below the halocline and the subsequent higher free-S to
OCS ratio are likely due to H2S released from hydrothermal vents.
We qualitatively assessed the contribution of hydrothermal emission to the total-S in the water column at
Kikai Caldera area by using a simple mass balance model
formulated by Cutter et al. (1999). Without a fresh input
of sulfide from hydrothermal activity into the ocean, the
total-S concentration is regulated by the balance between
its production through the hydrolysis of OCS and its
oxidative loss (Millero et al., 1987) by the reactions with
O2 and IO3– (Zhang and Whitfield, 1986; Radford-Knoery
and Cutter, 1994). The major products formed from the
oxidation of H2S by dissolved O2 or IO3– in seawater are
Table 1. Concentrations of free sulfide and carbonyl sulfide (OCS) and their ratios at different depths at
stations 1 and 2
Depth (m)
Free sulfide (pmol/kg)
COS (pmol/kg)
Ratio (Free-S/OCS)
20
40
60
100
200
250
300
350
410
458
31
28
29
37
36
40
49
49
59
60
40
39
35
31
29
30
29
28
30
35
0.8
0.7
0.8
1.2
1.2
1.3
1.7
1.8
2.0
1.7
20
40
60
100
200
300
320
370
470
510
29
31
30
33
38
43
47
47
54
55
34
40
36
31
31
31
31
29
34
34
0.9
0.8
0.8
1.0
1.2
1.4
1.5
1.6
1.6
1.6
Station 1 (30°42.61′ N, 130°22.36′ E)
Station 2 (30°47.38′ N, 130°25.00′ E)
SO32–, S2O 32–, and SO42–; the lifetime of H2S in typical
seawater conditions has been estimated to be a few hours
to a few days (Millero et al., 1987; Zhang and Millero,
1993, and references therein). Although other loss processes of H 2S occur such as air-sea exchange (Andreae et
al., 1991) they are considered to be negligible compared
with the oxidative loss process (Cutter et al., 1999). The
mass balance model in the steady state (ss) can be thus
simplified:
[Total sulfidess ] =
khyd ∗ [OCS]
[ ]
koxid ∗ [O 2 ] + kIO − ∗ F ∗ IO 3−
3
0.5
,
(1)
where k hyd is the rate constant of OCS hydrolysis calculated on the basis of the result reported by Radford-Knoery
and Cutter (1994) in addition to the observed salinity, pH,
and temperature. koxid is the oxidation rate constant calculated by using the equation proposed by Millero et al.
(1987); kIO3– is the rate constant for the oxidation of iodate-sulfide; and F is the fraction of free-S to the total-S
(Cutter et al., 1999). We adopted 630 mol–0.5/h for kIO3–
as reported by Zhang and Whitfield (1986). Because iodate was not determined in the present study, we estimated its content by applying the salinity-iodate relationship observed in the East China Sea (Wong and Zhang,
2003; Wong et al., 2004). The fraction of free-S to the
total-S (F), 0.08 to 0.09, was obtained from the current
field observations.
The model calculation showed that the observed total-S values were 3 times higher than those predicted by
the model (total sulfidess) in the upper 100 m and were
10–14 times higher below the depth of 330 m, which is
inside the caldera rim (Fig. 4). It should be also noted
that the observed total-S was still two to three times higher
than that predicted by the model even if we exclude the
effect of the oxidation loss by IO3–. In the open oceanic
region, the exclusion of the IO 3– oxidation process is
known to lead to overestimations of the total-S concentration (Cutter et al., 1999).
The difference between our observation and the model
prediction, even without considering the IO3– oxidation
process, clearly indicates the existence of an additional
source of total-S in the present study area. The most plausible explanation is the direct emission of free-S and
sulfide-forming metals such as copper, iron, and zinc from
hydrothermal vents to the ocean in Kikai Caldera. Metal
sulfide can also be formed through bacterial H2S production within macroscopic particles in an oxic water environment and in anoxic ocean sediment (Cutter and
Krahforst, 1988; Radford-Knoery and Cutter, 1994). In
this studied area, however, low primary production was
estimated on the basis of previous studies using in situ
chlorophyll-a sensor data (Yamaguchi et al., 2012, 2013).
In the East China Sea, low organic sedimentation was
reported below the continental shelf slope (Oguri et al.,
Metal sulfides in oxic seawater
e5
Fig. 4. Vertical profiles of observed total sulfide for stations 1
(black diamonds) and 2 (black circles) and model-predicted
total sulfide concentrations in the steady-state condition by
assuming its production through carbonyl sulfide (OCS) hydrolysis and different oxidation processes (by IO3– and O2) for
stations 1 (solid line) and 2 (dashed line). Red lines represent
the model-predicted total sulfide with full production and loss
chemistry. Blue lines represent the total sulfide predicted by
the same assumption as above, excluding IO 3– oxidation. Gray
lines represent estimated hydrothermal content calculated by
subtracting the modeled total sulfide (red lines) from the observed total sulfide (black lines).
2003), which is similar to that in the Kikai Caldera area.
These biogenic processes may have contributed to the
observed high metal sulfide concentrations although likely
to a lesser extent than the hydrothermal process. It is also
possible that a high concentration of metal sulfide was
formed at the shallow continental shelf in the East China
Sea, as was observed in the coastal Atlantic Ocean (Cutter and Krahforst, 1988), and was then transported to the
Kikai Caldera area. In this case, the inflowing cool and
saline Kuroshio subsurface water at 100–330 m, as observed in this area, would contain a high concentration of
metal sulfides. However, insufficient data prevents discussion of this process at the present. Thus far, as we consider the higher free-S to OCS ratio at the bottom, it is
likely that the H2S was released from the seafloor bottom. We thus conclude that the observed high metal sulfide
is mainly of hydrothermal origin, although further study
is required to fully understand this process.
In the aforementioned model analysis, the contribution of hydrothermally derived sulfide to the observed
e6
N. Nakayama et al.
total-S over the water column was estimated to be 60–
90%. The observed rather uniform vertical profiles of the
total-S also indicate that approximately 70% of the totalS released from the hydrothermal vents is transported to
the surface layer with little oxidative loss from the water
column, suggesting the existence of stable and oxidationresistant metal sulfide. The present results clearly show
that the dissolved metal-S able to pass through a 0.2-µmpore filter contributed most of the sulfide observed in
Kikai Caldera, and its concentrations are significantly
higher than those observed in open oceans. The observed
uniform concentration of metal sulfides throughout the
water column also suggests that the metal sulfide of hydrothermal vent origin escaped from rapid precipitation
and removal near hydrothermal vents and was dispersed
upward to the surface in the oxic environment. This process resulted in the long transportation of trace metals from
their source to remote oxic environments. Currently, we
are unable to distinguish between the completely dissolved form and particulates <0.2 µm in the observed dissolved metal sulfide. More detailed size-fractionated
chemical speciation is crucial for exploring the formation mechanism and stability of the dissolved metal
sulfide.
CONCLUSION
Vertical distributions of sulfide species in oxic
seawater over the submarine hydrothermal field were
obtained for the first time at Kikai Caldera south of
Kyushu Island, Japan. Free-S was found to gradually increase with depth at maximums of 31.1 pmol/kg in the
surface layer and 59.5 pmol/kg below ~330 m within the
Caldera rim, whereas total metal-S was virtually constant
at ~345 pmol/kg throughout the water column. Interestingly, total and dissolved metal-S exhibited uniform and
quite similar concentrations within the water column,
showing that metal sulfide exists in the dissolved phase
with complete dissolution or in particulate form with diameters <0.2 µm with high stability even in an oxic environment. A mass balance calculation suggested that 70%
of the total-S originating from hydrothermal vents reached
the upper layer without oxidation. The present results
clearly show that dissolved metal-S originating from hydrothermal vents contributes to a large portion of the observed high dissolved total-S concentration even in the
surface water, indicating that metal-sulfide complex is
quite stable in preventing its rapid removal from the water column by metal precipitation or oxidation in an oxic
environment. The present study suggests that metalsulfide complexation and nanoparticles play an important role in transporting trace metals far from their sources
in oxic environments and in biogeochemical cycles of
trace metals in the ocean.
Acknowledgments—We thank Dr. Nobuo Tsurushima for lending us the FPD equipment and the scientists who participated
in the KS-14-10 (leg 1) research cruise for their kind assistance with the sample collection. We also thank the captain and
crew of R/V Shinsei Maru for their support. NN also thanks
Dr. Hiroshi Furutani for valuable discussion at various stages
of this study. We also thank two anonymous reviewers for their
valuable comments which improved the manuscript. This study
was partly supported by a Grant-in-Aid for Scientific Research
(A) (No. 2353001) from MEXT and a Grant-in-aid for Young
Scientists (B) (No. 24710002) from the Japan Society for the
Promotion of Science.
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