Journal of Nuclear and Radiochemical Sciences, Vol. 20, pp. 1-11, 2020

ArticlesPlutonium Isotopes Research in the Marine Environment: A synthesis

Wang Jinlonga,*, Du Jinzhoua and Zheng JianbaState Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200241,
P.R.ChinabResearch Center of Radiation Protection, National Institute of Radiological Sciences, Chiba 263-8555, Japan
Received July 22, 2019; Accepted December 18, 2019; Published online January 8, 2020
Plutonium (Pu) isotopes are one of most important artificial radionuclides. Recent advances in analyticalmethodology of Pu have enabled Pu to play a vital role in tracing biogenic elements and pollutants transport,
sediment deposition/resuspension and other marine process. The various ratios of Pu isotopes (238Pu, 239Pu, 240Pu
and 241Pu), which are dependent on the Pu sources, are useful for identifying the contributions from different
sources, and these ratios can be utilized to indicate the marine processes. The aim of this manuscript is to provide
an overview of the applications of Pu for studying various processes in the marine environment, such as thedetermination of recent sedimentation rate, to assess sediment transport and deposition/resuspension, water masstransport, etc, based on the knowledge and advances in the analytical methodology and the geochemical behaviorof Pu. Furthermore, this work is expected to provide new insights for broadening the application of Pu in themarine environment and to help better understand land-sea interactions and global climate change.

160

Plutonium (Pu), mainly derived from thermonuclear bomb
testing, nuclear accidents, nuclear reprocessing facilities andnuclear power plants since the 1950s, entered the ocean by
global fallout and by direct release and was then transported
by ocean current, exchanged, transformed and eventually buried in the ocean. Pu in the marine environment consists
mainly of four isotopes, namely,238Pu (T1/2 = 87.7 a), 239Pu (T1/2 =24100 a), 240Pu (T1/2 = 6561 a) and 241Pu (T1/2 = 14.3 a), in whicha very small fraction of 239Pu originates from uranium minerala nd most 2 39 P u come s f rom a nt h rop oge n ic a ct iv it y.
Additionally, Pu has two isotopes with extremely low concentrations namely, 242Pu (T1/2 = 376000 a) and 244Pu (T1/2 = 8.7
×107 a).1 The distribution of Pu concentration in the marine
environment is influenced by ocean current distribution andbiogeochemical cycles, and therefore, Pu isotopes are typicallyutilized to trace water mass exchange, particle scavenging andbiogeochemical cycles.2-4 Compared to 90Sr (Kd: 10-102 L kg-1)
and 137Cs (Kd: 102-103 L kg-1), Pu has a much stronger particle
affinity in marine environments (Kd: 104 -107 L kg-1) and thus
can serve as a better tracer for indicating transport, scavenging
and particle deposition.5-8
To use Pu as a tracer for environmental process, the geochemical behavior of Pu should be understood and the analytical method for Pu need to be improved. With the developmentof analytical methods for Pu, the detection limit of Pu in seawater and sediments has continuously decreased,9-11 whichallows increasing numbers of researchers (Figure 1) to focus
on the sources, geochemical behaviors, distribution and environmental implications.12-18 Pu from different sources or incidents has unique atom (or activity) ratios,19 e.g., 240Pu/239Pu,
241
Pu/239Pu and 238Pu/239+240Pu, and these ratios can be used toquantitatively evaluate the source of Pu and to study different
marine processes along with 239+240Pu activity concentration.
Therefore, this study aims to synthesize the application of Pu
to marine processes based on a summary of its sources, geo*Corresponding author. Tel.:+86-21-54836467; Fax: +86-2154836458. E-mail address: jlwang@sklec.ecnu.edu.cn

120

Article numbers

1. Introduction

80

40

0
1990-1995

1996-2000

2001-2005

2006-20102011-20152016-2018

Figure 1. Publication numbers of Pu in the marine environments
du r ing 1990 and 2018 (Statistics on Web of Science using“Plutonium” and “sea” as keywords).

chemical behaviors, distribution and analytical methods.
2. Sources of Pu isotopes
2.1. Thermonuclear bomb testing. Nuclear testing is the
predominant source of Pu isotopes in the marine environment.
There were 2055 instances of nuclear testing from 1945 to2016 (https://www.armscontrol.org/factsheets/nucleartesttally), including 18 tests in 1951 and 178 tests in 1962, which
released a total 170 PBq of Pu. The resulting Pu was released
into the troposphere and stratosphere and was subsequently
deposited on the earth’s surface, thus affect the local andglobal Pu distribution. 20 In the Northern Hemisphere, Pu
entered the troposphere via aerosol and was scavenged via
rainfall or natural settling after approximately 71 days. 21
Therefore, two time-markers are used in most marine environment: one marker corresponding to the introduction of Pu tothe environment in 1952, and the second marker correspond-© 2020 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 01/08/2020

2

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Wang
240

239

ing to the peak fallout levels in 1963. The Pu/ Pu atom
ratios vary depending on the plutonium sources. The fractionation of atmospheric nuclear weapons testing in Novaya
Zemlya (Russia) to stratosphere was 78 Mt, which released
amount of Pu with 240Pu/239Pu atom ratios of 0.03.13 Nuclear
weapons testing in the Pacific Proving Grounds (PPG) produced a total yield of 26 Mt with 240Pu/239Pu atom ratios of
0.33-0.36.22 Nuclear weapons testing in these two regions contributed 70% of the global fallout Pu 20 and significantly
affected Pu distribution in local regions.23,24 The Pu with different sources and 240Pu/239Pu atom ratios entered the stratosphere and mixed completely, which led to global fallout with240
Pu/239Pu atom ratio of 0.18.19 Additionally, at Mururoa andFangataufa Atolls in French Polynesia, France conducted as eries of nuclear weapons tests, and released Pu into themarine environment, with 240Pu/ 239Pu atom ratios of 0.020.05.25 It is estimated that the total amount of 238,239,240Pu in
French Polynesia lagoons is approximately 30 TBq.26
2.2. Nuclear accidents. Accidental releases are importantsources of Pu in the marine environment. The Chernobyl
accident in April 1986 released 6 PBq of Pu into the environment and the Pu from this event was mainly deposited in the
Scandinavian countries and in the Baltic regions. 20,27,28
However, Holm 29 estimated that the Chernobyl accident contributed less than 10% of the total 239+240Pu in the Baltic Sea.
The Fukushima accident released a mass of artificial radionuclides (340-800 PBq)30 but produced much less Pu; Zheng etal31 using the Pu isotopic ratios, which was estimated that thisPu release using the Pu isotopic ratios to be less than 0.01% ofthat the amount released from the Chernobyl accident. Bu etal32 reported that 239+240Pu concentrations in the surface sediments near the Fukushima nuclear power plant were 0.01-5.81
Bq kg-1 before the accident and 0.48-3.53 Bq kg-1 after the accident, and reported 240Pu /239Pu and 241Pu/239Pu atom ratios of0.170-0.280 and 0.0012-0.0016 before the accident, respectively, and 0.188-0.255 and 0.0014-0.0016 after the accident,
respectively. Zheng et al31 reported that the Fukushima accident released Pu with 240Pu/239Pu and 241Pu/239Pu atom ratios of
0.30-0.33 and 0.103-0.135, respectively. Both the 239+240Pu concentration and 240Pu /239Pu and 241Pu/239Pu atom ratio after theaccident clearly did not increased compared to the levels
before the accident and, thus, the c ontribution from the
Fukushima accident to the Pu in the marine environment was
negligible.
The SNAP-9A satellites accident released 0.63 PBq of 238Pu
into the stratosphere on 21 April 1964, and most of this Pu wasdeposited in the Southern Hemisphere.33 Roos et al34 observedthat the 238Pu/239+240Pu activity ratios (0.21) in the Antarctic
Peninsula area were much higher than those from global fallout (0.019). Lee et al35 reported that the 238Pu/239+240Pu activity
ratios in the southern Indian Ocean also showed higher valuesof 0.08-0.20. In Palomares, Spain (1966), and Thule, Greenland(1968), two aircraft aircrafts crash accidents released 5.5×10 -5
and 0.01 PBq of 239+240Pu, respectively, with 240Pu/239Pu atom
ratios of 0.05-0.06, which significantly affected the local Pu
distribution. For example, the 240Pu/239Pu atom ratios in thesoils of Palomares, Spain were clearly below 0.18 and close to
0.05.36,37 However, most of this Pu was deposited on the land
and the contribution to the marine environment was verysmall.3
2.3. Nuclear reprocessing facilities. Effluents from thelater part of the nuclear fuel cycle in nuclear reprocessing
facilities contain small fraction of Pu; however, due to thelarge eff luent f low quantities, large amounts of Pu haveentered the marine environment. The nuclear reprocessing
facilities in Sellafield (UK) and La Hague (France) have discharged 22 PBq and 0.14 PBq of Pu into the ocean, respec-

240

239

tively, with Pu/ Pu atom ratios of 0.06-0.25 and 0.34,
respectively.6,38 It was reported that 0.5-1.4 TBq 239+240Pu wastransported into the Irish Sea through the English Channel,
and 99% of this amount was contributed by Sellafield (UK)
activities, causing 240Pu/239Pu atom ratios to be lower in the
Irish Sea.6,38-40 In addition, the nuclear reprocessing facilities
in Mayak, Krasnoyarsk and Tomsk, Russia, Hanford andSavannah River, USA, Marcoule, France discharged small
amounts of Pu into the Kara Sea, eastern Pacific Ocean, thewestern Atlantic Ocean and the Mediterranean Sea, respectively, with lower 240Pu/239Pu atom ratios of <0.13.41-44 For
example, the Savannah River production plants discharged 140
GBq of Pu into the atmosphere and 23 GBq of Pu into local
small rivers and ponds from 1954 and 1989.45 Due to theeffects of the Marcoule reprocessing plant, the sedimentary
inventory of 239+240Pu in the Rhone River and Lions Gulf wasreported to be 172 GBq from 1945 and 2000.46
The incidents discussed above comprise the main sources of
Pu in the marine environment, in which 26 PBq Pu was stored
in the ocean as estimated by Aarkrog 47. Additionally, the
USSR Navy nuclear submarine ‘‘Kosmomolets” sank in the
Barents Sea, and the dumping of radioactive wastes from thenuclear industry Sea into Northeast Atlantic and the Arctic
Oceans are potential sources of oceanic Pu.47 However, thesepotential sources might affect the distribution of Pu in marine
environments in the future, but the contribution to Pu is currently negligible.
3. Analytical method for Pu isotopes
Since the 1960s, researchers have begun to use alpha spectrometry to measure Pu and there was a report regarding theanalysis of Pu using mass spectrometry in 1971; the detection
limit improved from 10 -13 g in 1980s to 10 -18 g in 2015.10,48,49
The analytical procedure for Pu usually consists of sample
pretreatment, chemical separation and purification, source
preparation and instrumental measurement. A total of 2-25 gof solid samples (soil, sediment and biological samples) are
first ashed at 400–600°C to decompose organic matter and are
spiked with a tracer (generally 242Pu). The residue is thendigested using alkali f usion or acid digestion consisting of acidle a ch i ng ( p a r t ia l d ige s t ion) a nd t ot a l d i s s olut ion . 9
Concentrated HNO3, HCl, HNO3+HCl, or HNO3+H 2O2 areused to leach Pu.50-53 Some samples containing Pu oxide canbe completely dissolved by HF+HCl+HClO4,54,55 but total dissolution also creates more matrix components/ interfering elements and is further detrimental to subsequent chemical
separation and purification. Alkali fusion can completely
decompose the samples and completely melt the Pu but could
also melt the container, thus producing more interfering elements. Pu is concentrated by coprecipitation using NdF 3,
LaF 3, CaF 2 , CeF 3, Ca(OH) 2 , Mg(OH) 2 , Fe(OH) 2 , Fe(OH) 3,
MnO2, CaHPO 4, Ca 3(PO 4)2 or CaC2O 4, etc. The Pu in water
sample is concentrated by using coprecipitation after filtrationand the volume of the water samples for determining Pu levels
varied varies between 20 and 500 L.9,11
The Pu in acid leaching solution usually existed as Pu 3+,
Pu4+, PuO2+ and PuO22+, in which the partition coefficient ofPu(NO3)62- in resin is highest in nitric acid systems, and thus
the Pu in the sample should remain in IV valence before it isloaded into the column for separation and purification. In thecase of acidic condition, Pu can be adjusted to Pu 3+ using I−,
Fe2+, Ti3+, NH 2OH or N2H4 and then be oxidized to Pu4+ using
H2O2, NO3− or NaNO2.9,56 The method for separating and purifying Pu involves solvent extraction, ion exchange chromatography or extraction chromatography. Solvent extraction
introduces major organic pollutants and thus is seldom used toanalyze Pu in environmental samples; this procedure is mainly
utilized for the separation of high-level liquid waste.57 Ion

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Plutonium Isotopes Research in the Marine Environment: A synthesis

3

TABLE 1: Comparison between different methods for detecting Pu isotopes (modified after Qiao et al.9 and Wang et al.61)

α-spectrometry

Measurable isotope
238

Pu
Pu
241
Pu
241
Pu, 238Pu, 239,240Pu
239,240

LSC
ICP-MS

239

Pu

240

Pu

241

Pu

AMS

RIMS

TIMS

239

Pu
240
Pu
241
Pu
239
Pu
240
Pu
241
Pu
239
Pu
240
Pu
241
Pu

Interfering species
241

Am, 210Po, 228Th
Pa, 243Am
210
Po, 228Th
All beta emitters
238 1
U H, 208Pb31P, 206Pb33S, 204Hg35Cl,
202
Hg37Cl, 199Hg40Ar, 204Pb35Cl
238 2
U H, 209Bi31P, 207Pb33S, 205Tl35Cl,
204
Pb36Ar, 200Hg40Ar
204
Hg37Cl, 204Pb37Cl, 206Pb35Cl
similar m/z as 239
similar m/z as 240
similar m/z as 241
Matrix elements
231

Detection
Limit (mBq)
0.01-0.1
0.01-0.1
0.1-1.0
5-50

Operation
time
Day-weeks

Fair/highhours

Fair/high10 -4-10 -2

minutes

Fair/lowminuteshighminuteshighminuteshighexchange chromatography and extraction chromatography are
the most commonly used method for analyzing Pu.58 Due tothe advantage of simple operating procedures and short operating times, extraction chromatography has attracted increasing attention.59 Commonly used resins, e.g., AG 1×4, AG 1×8,
Bio-Rad AG 1×8, Dowex 1×4, Dowex 1×8, UTEVA+TRU and
Chelating+TRU are used singly or are combined, to separate
and purify Pu.50,60,61 TEVA resin with good selectivity for Pu
and a high decontamination factor for U and chemical yields,
serves as one of most commonly used resins for Pu analysis.9,11
Since the environmental samples contain large quantities of U,
decontaminating the U is very important due to the formationof polyatomic ions (e.g., 238U1H+ and 238U1H 2+) when measurements are conducted by using inductively coupled plasma
mass spectrometry (ICP-MS) (Table 1).
There are seve ral types of detection techniques for thedetermination of Pu: α-spectrometry, liqu id scintillation
counting (LSC), ICP-MS, thermal ionization mass spectrometry (TIMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS) (Table 1).
α-spectrometry, as a traditional counting method, was themost commonly used method in previous years, but thismethod has a higher detection limit and longer operating times
and cannot determine 240Pu/239Pu atom ratios. LSC also has avery high detection limit and cannot determine 240Pu/239Pu
atom ratios. AMS has the advantages of high sensitivity andshort operating time but is more costly. TIMS and RIMS havehigh precision and sensitivity with short operating times but
requires complex chemical procedures for the separation andpurification of Pu. ICP-MS, with the advantages of high sensitivity, reasonable costs and relatively short operating times,
presently serves as the most commonly used method for
detecting 239Pu and 240Pu. Reports regarding Pu measurement
also show that ICP-MS has been widely used to determine Pu
in recent years (Figure 2). As discussed above, the major
problem in Pu analysis in marine samples using MS is the separation of U at high concentrations due to the high salinity ofseawater. Initially, researchers use an anion exchange resin to
separate U, but the decontamination factor was not high andthe chemical yield of the whole procedure was relatively low(e.g., Zheng and Yamada62). Subsequently, researchers began

0.36-5
10 -4-10 -3
10 -4-10 -3
0.01-0.1
0.01-0.1
5-50
10 -4-10 -3
10 -4-10 -3-







160

120

Cost

10 -4-10 -2

Matrix elements

Article numbers

Method



80

40

0
-1995

1990

-2000

1996

-2005

2001

-2010

2006

-2015

2011

-2018

2016

Figure 2. Publication numbers of Pu using different instruments
du r ing 1990 and 2018 (Statistics on Web of Science using“Plutonium” and related instrument as keywords).

using TEVA resin which has a higher selectivity for Pu topurify Pu samples (e.g., Ketterer and Szechenyi56). Recently,
researchers have used UTEVA to remove U and have combined TEVA for purifying Pu with SF-ICP-MS detection,61,63
which showed a very low detection limit (i.e., 10 -4 mBq for
239
Pu, 10 -4 mBq for 240Pu, and 0.36 mBq for 241Pu).
4. Distribution of Pu in the marine environment
4.1. Distributions of Pu in seawater. Since Pillai et al64
first reported the activity of 238Pu and 239,240Pu in seawater,
there are increasing number of studies focused on the distributions and geochemical behaviors of Pu and the implications formarine processes in the marine environment. Pu distribution
in the ocean are affected by global fallout which has a latitude
effect (Figure 3). The area with the maximum amount of
239,240
Pu from global fallout is distributed between 40 and
50 °N (~80 Bq m-2) and decreases toward north and south. The
maximum levels of 239,240Pu in the Southern Hemisphere, 1/4 ofthose in the North Hemisphere, are distributed between 40 and4

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Wang

1000

Global fallout

239,240

Pu (Bq m-2)

Burial in sediments

100

10

1

0.1

90

80

70

60
50
40
30
Northern Hemisphere

20

10

0

10

20

30

40

50

60

70

80

90

Southern Hemisphere

Figure 3. The atmospheric fallout and marine sedimentary deposition of 239,240Pu between different latitudes: The atmosphericfallout 239,240Pu values are estimated by using the activity ratio between 239,240Pu and 90Sr to multiply 90Sr amount in Table 8 in
UNSCEAR.20 The sedimentary deposition is summarized from the literature.5,15,18,75-90

TABLE 2: Oceanic inventory (PBq) of Pu isotopes originating from globally dispersed debris, and local deposition from atmospheric nuclear tests (Till to January, 2000, Modified after Aarkrog47)
Ocean
Arctic Ocean
Atlantic Ocean
North Atlantic
South Atlantic
Indian Ocean
Pacific Ocean
North PacificSouth Pacific
Total

238

Pu
0.00180.044
0.035
0.0088
0.018
0.31
0.15
0.062
0.35

239

Pu
0.050
1.4
1.1
0.30
0.56
4.5
2.7
1.8
6.5

240

Pu
0.040
0.89
0.70
0.20
0.36
4.0
2.3
1.7
5.4

50°S and decrease toward the north and south, and the minimum levels are observed in the Antarctic.20 Based on the estimation by Aarkrog47 (Table 2), most 239,240Pu is stored in the
Pacific Ocean (8.45 PBq), accounting for 75% of worldwide
total 239,240Pu in the ocean, for which Pu in the North Pacific
Ocean accounts for 59% of the total 239,240 Pu in the whole
Pacific Ocean. Pu in the Atlantic accounts for 17% of total Pu
in the ocean, and that in the North Atlantic Ocean accounts for
79% of the total Pu in the Atlantic Ocean. Pu in the Indianand Arctic Oceans only accounts for 6% and 0.5% of Pu,
respectively, in the global ocean.
In the 2000s, Aoyama and Hirose65 found that the maximum
239,240
Pu activity was observed in the Irish Sea (>100 mBq m-3)
and relatively high values were observed in the North Atlantic,
B a r e n t s S e a , No r t h S e a , E n g l i s h C h a n n el a n d t h eMediterranean Sea (10-50 mBq m-3); lower 239,240Pu activity
was distributed in the Pacific Ocean, South Atlantic Ocean
and the Indian Ocean (1-10 mBq m-3); the lowest 239,240Pu activit y was found in the So uthern Ocean (<0.5 mBq m -3).65-71
239,240
Pu in the surface seawater of the Pacific ranged from 0.3
to 2.7 mBq m-3, and higher values were observed in the Japan
Sea, subarctic Pacif ic Ocean and the Easter n Pacif ic
Ocean.72,73 239,240 Pu in the surface water of the Japan Sea
decreased in winter due to downward scavenging.72 239,240Pu
levels in the surface water decreased with time after 1970 due
to the influence of water mass transport, biological activity
and decrease in global fallout.74
239,240
Pu activity generally showed maximum values in thesubsurface layer (400-900 m) and then gradually decreased
with increasing water depth (Figure 4). However, a maximum

241

Pu
0.078
2.0
1.6
0.41
0.80
11
6.2
4.8
14

242

Pu
0.000010
0.000300.000230.000060
0.00020.00300.00150.00150.0040Total Pu in the Ocean
0.17
4.3
3.4
0.91
1.7
20
11
8.4
26

value for 239,240Pu wasn’t observed in the Norwegian Sea andGreenland Sea, which could be related to the remineralizationof particulate matter caused by biological activity and watermass transport.91,92 The shape of the 239,240Pu activity peak isrelated to the location of the sea area, for example, the peak for
the Japan Sea and the Mediterranean Sea are broad, and thepeak observed for the northwestern Pacific Ocean is sharp.93-95
It was estimated that the residence time of 239,240Pu in the subsurface layer is 400-600 years and thus the 239,240Pu stored in
the subsurface layer could serve as an important Pu source inadjacent seas.2
4.2. Distribution of Pu in marine sediment. As shown in
Figure 3, the sedimentary inventory of 239,240Pu is generally
greater than the global fallout. From 20-50 °N, the sedimentary inventory of 239,240Pu in the Washington shelf area, Japan
Sea, Sagami Bay, northwestern Pacific Ocean, East China Sea,
Southern Okinawa Trough and northern South China Sea are
63-417, 59-65, 36-474, 28-30, 2-807, 172-675 and 366 Bq m-2,
respectively, which are higher than those of global fallout atthe corresponding latitudes.5,18,76,79-85,87-89 In the Arctic region,
Okhotsk Sea and in equatorial waters, the 239,240Pu inventory issmall, ranging from 0.16 to 48, from 0.6 to 45 and from 1.8 to
9.0 Bq m-2, respectively, which are close to those of global fallout at the corresponding latitudes.15,75,78 Report of the sedimentary inventory of 239,240Pu in the Southern Hemisphereindicate lower levels. Pittauer et al90 reported that the 239,240Pu
inventory in the Lombok basin of the India Ocean (10°S) was
219 Bq m -2 , which was sig nificantly higher than the global
fallout value at this latitude. The distribution of the 239,240Pu

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Plutonium Isotopes Research in the Marine Environment: A synthesis
239,240

Pu (mBq m-3)

a. 0

20

40

60

239,24080

100 b. 0

Pu (mBq m-3)

20

40

60

0

0

-2000

Water depth (m)

5

-1000

-4000
-2000
-6000

Northwestern Pacific
Bonin Trench AN1 (1984)
Japan Trench JT (1984)
Sagami Bay CE-13 (1992)
Central North Pacific

-8000

-3000

Station 235 (1973)
Northeastern Pacific
Aleutian Trench St.1 (1981)-10000

1984
1993
1994
1995
1996
1997
N2 (1994)-4000

Figure 4. 239,240Pu depth profiles in ocean seawater: (Modified after Lindahl et al.3 and Hong et al.95): a. North
Pacific; b. central Northwest Pacific Ocean.

inventory in these regions is related to its geochemical behavior and to different sources (riverine input, atmospheric deposition and advective input from neighboring sea). For example,
in the East China Sea, South China Sea, and the marginal seasof the northwestern Pacific, the higher inventory of 239,240Pu
relative to global fallout levels is caused by oceanic input fromthe PPG that is transported via the Kuroshio Current andNorth Equatorial Current and riverine inputs affected the estuarine regions (inputs from the Changjiang and Pearl River).18,89
Meanwhile, Pu sediment distribution are also be influenced byocean currents and by sediment focusing/erosion and transport, which are discussed in detail below.
5. Geochemical behaviors of Pu in marine environment
Pu distributions in the water column are also related to its
geochemical behaviors and are affected by the particulate matter density, physico-chemical properties, hydrolysis, inorganicand organic matter complexation, and by high-molecularweight polymer, etc.92,96 As discussed above, Pu is present asfour different oxidation states in the aqueous environment.
The low valence state of Pu is stable when the pH is low andthe high valence Pu state is stable when the pH is high. The
Pu (III, IV) forms exhibit higher reactivity than the Pu (V, VI)
form, which are very easily hydrolyzed in aqueous environments and do not readily undergo complexation with organic
ligands (e.g., humic acid), leading to extremely low concentrations of the free state Pu (IV) form.74 The particle concentration in the open ocean are ver y low, which cause theparticulate Pu concent ration to also be lower. The Kd value for
Pu in the open ocean is lower (i.e., 10 4 L kg-1); for example,
particulate 239,240Pu represents only 10% of the total 239,240Pu in
the surface water of the Mediterranean Sea and the North
Pacific and represents much smaller portion (<2%) in the deep
water of the Pacific Ocean.97-99 The dissolved Pu mainly exists
in the Pu (V, VI) forms and is dominated by the Pu (V) form,e.g., the 239,240Pu (V) form accounts for more than 70% of thetotal dissolved phase in the Irish Sea and Mediterranean
Sea.100,101 In coastal regions Pu is dominated by the Pu (III,IV) form, with K d values of 10 6 -10 7 L kg-1. 2 Due to thesehigher Kd values and relatively higher particle concentration,
the dissolved phase of Pu in these regions (e.g., 46%-92%) areless than those in open ocean (close to 100%).102,103 The Pu(III, IV) forms are easily adsorbed onto colloids and particles
and are scavenged from the water column whereas the Pu (V,VI) forms are preferentially adsorbed onto carbonate particles(e.g., coral).23,104
Some studies have reported that Pu is remobilized by physical mixing, biological activity, or changes in redox conditions.104-106 Sediments in a lagoon in the Marshall Island thatwas polluted by the PPG contain large amounts of Pu andrelease Pu at a rate of 0.2 TBq a-1 due to the influence of sediment resuspension. 107 239,240Pu activity increases slightly inthe bottom layer of the water column, which should be related
to the release from the sediments due to the resuspension process.15 However, experimental results showed that 239,240Pu in
sediment from the Irish Sea was only desorbed by less than
1%.108 The desorption of 239,240Pu is expected to increase whenbioturbation occurs, which suggests that biological activity
plays a n i mp or t a nt role i n t he mobi l it y of P u . 10 9,110
Experimental results have also shown that the Kd values for Pu
increased with decrease in organic matter content.4 Baskaran
et al111 observed that 239,240Pu in sediment in the Florida coastal
region that contained high organic matter content (loss on
ignition >50%) exhibited pre-depositional mobility phenomena, which caused the activity peak corresponding to 1963 to
be broader. This result occurs because the degradation oforganic matter enhanced the sediment phase 239,240Pu entry
into the porewater resulting in the remobilization of 239,240Pu.
However, Pu mobility is much lower relative to 137Cs mobility
in most aqueous environments (except for environments withrich organic matters or bioturbation) and can be negligible.18,112
6. Implication of Pu isotopes for marine process
As it has been discussed above, Pu is highly particle-reactive in coastal regions (Kd >106 L kg-1) and can be used to trace
particle/sediment dynamics and biogeochemical processes
whereas it is weakly particle-reactive in the open ocean (Kd=104 L kg-1) and is useful for tracing water mass exchange.
6.1. Implication of Pu isotopes for sediment dynamics.
1) Dating recent sediments:Some Pu isotopes have long halflives (e.g., 239Pu and 240Pu) and are highly particle-reactive incoastal regions and thus can be used to trace sediment dynamics (Figure 5) 239,240Pu shares similar sources with 137Cs andcan be used to date recent sediments by using two common
time markers “1952” and “1963” corresponding to the introduction of Pu to the environment and to peak fallout levels. In
addition, 240Pu/239Pu atom ratios can also be used to date recent

6

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Wang

Figure 5. Application of Pu isotopes in the marine process tracing research: a. Dating in the ECS sediment (core OR499-16, modified after Su
and Huh81; b. The mass balance of 239+240Pu in the ECS sediment (modified after Wang et al.18).

sed i ments, for exa mple, i n t he ma rg i nal sea s of t he
Northwestern Pacific, the atom ratios were high in the 1950s
and can be used as a time marker. Recently, increasing numbers of studies have used Pu for sediment dating (Figure 5).85-89
Wang et al18 compared the dating results using 210Pb, 137Cs and239,240
Pu in sediments in the East China Sea and found that thedating results between 210Pb and 239,240Pu agreed well in bothshallow and deep waters. However, an activity peak for 137Cs
was not observed, which was probably caused by its very weakparticle affinity and the relatively strong mobility of 137Cs inmarine environment.113-115 137Cs has a shorter half-life (30.2 a,
75% of peak fallout of 137Cs has decayed away in 2019) and relatively higher mobility, whereas Pu isotopes (239Pu and 240Pu)
have longer half-lives and stronger particle affinities, and thus,
Pu would play a more important role in dating marine sediments in the future compared to 137Cs. Pu can also be used todate corals. Corals have a strong affinity for dissolved Pu in
ocean waters, and Pu could be packed into coral skeleton andaccumulate with coral growth; therefore, the signal of Pu in
the neighboring waters could be reconstructed by coral.23,116
Lindahl et al23 found that the 240Pu/239Pu atom ratios in corals
near Guam corresponded to the period of nuclear testing in the
PPG. Froehlich et al117 also found a similar phenomenon incorals from the Marshall Islands, suggesting that 240Pu/239Pu
atom ratios could be used to date corals. The historical recordof Sr/Ca ratio in coral could reflect climate changes and seawater temperature change,118,119 and Pu isotopes could provide
the chronology for Sr/Ca ratios in coral that is useful in the
study of global climate change.
2) Indicating the source of artificial radionuclides associatedwith adsorbed particles: The atom ratios (Rs) vary dependingon their differing sources, and the contribution from n source
terms can be estimated by following equation:19,120
Rs = χ1R1 + χ2R 2 +…+ χnR n(1)

χ1 + χ2 +…+ χn = 1(2)where R i is the contribution from source term I, and χi is thecontribution from source term i. It should be noted that thisequation is only suitable for stable isotopes or for long-lived
radionuclides (e.g., 239Pu and 237 Np). Assuming that Pu isotopes (239Pu and 240Pu) have two sources, equation (1) and (2)can be combined to form˖
χ1 (%) = 100 × (R m - R 2)/(R1 - R 2)

(3)

where R m is the measured 240Pu/239Pu atom ratio. In the East
China Sea, the 240Pu/239Pu atom ratios were 0.18-0.31,18 whichlies in the Changjiang input/global fallout range (0.18) and in
the PPG input range (0.33-0.36). Using equation (3), it is estim a t e d t h a t t h e PP G c o nt r i b u t io n d o m i n a t e t h e P usource(~60%), and the mas balance of Pu in the East China
Sea sediments is then obtained (Figure 5b). Zheng and
Yamada15 found that the particulate flux of 239,240Pu near the
Okinawa Trough increased with increasing water depth, suggesting that the PPG derived Pu could enter the East China
Sea by boundary scavenging. Near the Changjiang Estuary,
the 240Pu/239Pu atom ratios were 0.18, which indicate that thePu source in this region is mainly dominated by riverine
inputs.18,87 Combined with other isotopes-indicated results, it
can be concluded that these sediments are from the Changjiang
inputs.81,114,121
3)Implications for sediment transport, mixing and erosion:
The spatial distribution of the Pu inventory is useful for indicating sediment transport. The sedimentary inventories of239,240
Pu and 137Cs decrease from the coast to offshore, suggesting that these radionuclides can be transported offshore associated with other particles.18,114,122 Ketterer et al123 reported thatthe transport of drainage particles from the Danube Delta to
the Black Sea was indicated by using the spatial distribution of
240
Pu/239Pu atom ratios. Tims et al124 also reported sedimenttransport in the Herbert River by using 239Pu and 137Cs as
tracers. Buffoni et al125 evaluated the mixing depth and sedimentation rate by observing the vertical profile of 239,240Pu in

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Plutonium Isotopes Research in the Marine Environment: A synthesis

7

60°N

40°N

20°N

EQ

40°S
90°E

Ocean Data View

20°S

120°E

150°E

180°E

150°W

120°W

Figure 6. The synthesis of 240Pu/239Pu atom ratios in the areas surrounding the Pacific: Soil (black color),32,139,140,142-145 sediment (red color),
18,22,83-90,130,140,146-148
seawater (blue color)149-157 and a coral (yellow color).23,117,129 NEC: North Equatorial Current; SEC: South Equatorial Current;
KC: Kuroshio Current; OC: Oyashio Current; MC: Mindanao Current; NGCU: New Guinea Coastal Undercurrent; EAC: East Australian Current;
ITF: Indonesian Throughflow.

sediments. Huh and Su122 observed the vertical profiles ofPb, 137Cs and 239,240Pu and estimated that the average sediment mixing rate in the coastal regions and outer shel were 20
and 0.03-0.3 cm 2 a-1, respectively. Additionally, the temporal
distribution of 239,240Pu inventories could indicate sedimenterosion. Wang et al18 observed that the 239,240Pu inventory of
three sediment cores collected in same region in 1997, 2006
and 2013 decreased with increasing time, and they inferredthat this phenomenon was related to sediment erosion or channel improvement. Livingston and Bowen126 found that in theshallow waters sediment along the northeast coast of the USA,239,240
Pu/137Cs activity ratios decreased with increasing, depth
but the 238Pu/239,240Pu activity ratios did n’t change, which indicate that these radionuclides migrated downward due to bioturbation. Santschi et al127 observed that 137Cs and 239,240Pu in
the sediments of the Palos Verdes Shelf showed positive correlation with organic matter content and used the distribution ofradionuclides to indicate the source and transport of organiccarbon. Due to the distinct 240Pu/239Pu atom ratios arisingfrom different sources, Wang et al18 proposed that 240Pu/239Pu
atom ratios are useful for t racing the transport of particle-reactive species, including radionuclides (210Pb, 231Pa and 230Th, etc)
and organic/inorganic matter (Fe, P, etc) from the open ocean
to the coastal region. Therefore, it is necessary to strengthen
this research to provide technical support for understanding
the scientific difficulties of oceanography.
210

6.2. Implication of Pu isotopes for water mass transport.
1) The Pu signature in water masses: Zheng and Yamada141
studied the sedimentary profile of Pu isotopes in Sagami Bay
and found that the 239,240Pu inventory (mean: ~191 Bq m-2)80
was significantly higher than that from global fallout (42Bq m-2)128 and the 240Pu/239Pu atom ratio was higher than thatfrom global fallout (0.18) and was close to that from PPG(0.33-0.36). These authors inferred that these Pu isotope ratios
were mostly due to long-range transport from the PPG. Since
then, there has been increasing research on the distribution of

240

Pu/239Pu atom ratios in the Pacific, which corresponds towater mass exchange and further confirms the presence oflong-range Pu transport in the Pacific (Figure 6). Wang et al18
synthesized the transport of PPG-derived Pu in marginal seas(e.g., South China Sea, East China Sea, East Sea and east
coasts of Japan); it was reported that the PPG-derived Pu could
be transported to the Bering Sea, but the 240Pu/239Pu atom ratio
was 0.15-0.18 and was inf luenced by input from the Arctic
Ocean that were transported via the Oyashio Current.129 The
240
Pu/239Pu atom ratio of the bottom waters on the west coast of
North America near Washington state were determined to be
0.08 due to the inf luences from the Pu releases from the
Hanford site.129 Pittauer et al90 examined the sedimentary profiles of 241Am/239Pu activity ratios and 240Pu/239Pu atom ratios
in the Eastern Lombok Basin and found that the PPG-derived
artificial radionuclides could be transported to the India
Ocean. The 240Pu/239Pu atom ratios in the sediments of thesoutheast coast of Australia showed lower values (0.11-0.16)
caused by nuclear testing by the UK.130 However, report on
the 240Pu/239Pu atom ratios in the South Pacific are still lacking. Though a number of studies have reported the long-range
transport of Pu in the Western Pacific and its marginal seas,
their conclusions were only indirect speculation based on the240
Pu/239Pu atom ratios. Therefore, more investigations areneeded to determine the compositions of the different species
Pu isotopes (especially colloidal Pu) to understand the scavenging mechanism for Pu entering shallow waters.
At some stations in the Northwest Atlantic, the 240Pu/239Pu
atom ratios in the deep waters were less than 0.18 due to the
influence of nuclear testing at the Nevada Test Site.131,132 In the
Irish Sea, which was affected by the discharges from
Sellafield, the 240Pu/239Pu atom ratios of sediments were 0.06
during the 1960s and were greater than 0.20 after the 1980s.6
This Sellafield derived Pu was transported northward along
the west coast of Northern Europe via the North Atlantic
Warm Current and the Norwegian Warm Current. This
Atlantic derived Pu could enter the Arctic Ocean and affect the8

J. Nucl. Radiochem. Sci., Vol. 20, 2020

distribution of Pu in the Arctic Ocean with other sources. For
example, some Pu released from nuclear testing site andnuclear reprocessing facilities could be transported to the Kara
Sea via the Ob and Yenisei rivers.133-136 The transport ofChernobyl-derived Pu in the Baltic and Black seas wasobser ved from the distribution of the 240 Pu/ 239 Pu atom
ratios.137,138 In conclusion, the 240Pu/239Pu atom ratios are very
useful for tracing water mass transport, but more investigations are needed to confirm the distributions of Pu isotopes in
the South Pacific, South Atlantic and the Indian Ocean to better understand the transport of Pu as well as marine process.
2) Indicating the vertical mixing of seawater: At one stationin the eastern North Pacific (31.38 N, 150.03 W), the 239,240Pu
activity in the subsurface in 1973 was 4 times higher than thatin 2000, but 239,240Pu activity was unchanged below 1,000 m.74
A similar variation trend was also found in the mid-latitude
areas of northwestern Pacific (Figure 4a). The decrease in the239,240
Pu maximum in the subsurface layer was not caused by
downward settling of particles because the 239,240Pu values
below 1,000 m showed no increase. Instead, advective transport should be the main reason for such a phenomenon. The
subsurface maxima for 239,240Pu were 70-80, 40-60 and 20-30
mBq m-3 in the St. 235 near the PPG, Bonin Trench and Japan
Trench, respectively (Figure 4a).3,140 This decreasing trend
from east to west could be related to the westward transport of
the North Equatorial Current associated with Pu. The water
depth of the subsurface maximum of 239,240Pu in the central
Northwestern Pacific varied from 300 m in 1984 to 900 m in1997 (Figure 4b). This variation is because in this region, bioactivity is strong and p article concentrations are relatively
high, leading to settling down of the associated particles.95
Therefore, the 239,240Pu activity in the water column increases
with increasing water depth above the subsurface maximum
layer (500-800 m), whereas 137Cs activity exponentiallydecrease with increasing water depth in the upper 1,000 m.
Hirose et al74 studied the vertical profiles of 239,240Pu/137Cs
activity ratios in the subsurface layer in the upwelling region
and found that the upwelling depths in the eutrophic Subarctic
Pacific were significantly higher than those in the subtropicalNorth Pacific. Hirose et al (2011) found that the 239,240Pu/137Cs
activity ratios exponentially increased from the surface to the
1,500 m layer and estimated that the upwelling depths were
between those in the subarctic Pacific and subtropical North
Pacific. Although there have been many studies of 239,240Pu
profiles, Pu behavior in marine environments is complex, andmore investigations are needed to understand the temporalvariation in the subsurface Pu ma xima (peak position andshape) and to better understand marine processes using Pu as a
tracer.
7. Conclusions and perspectives
The main areas of progress related to the study of Pu isotopes are the following. 1) The method for determining Pu in
the sediment has been well developed and a 1-2 g sample issufficient for conducting measurement. ICP-MS, with economical cost and efficiency advantages, can determine the isotope ratios and is widely applied for measuring Pu; however,
AMS has high sensitivity for Pu measurement but is notwidely utilized due to its higher costs. 2) Research on the Pu
isotopes distribution in seawater and sediment has mainly
been focused on the North Pacific, North Atlantic and Arctic
Ocean, but research in the Southern Hemisphere is lacking,especially for the Pu isotope compositions. 3) studies on Pu
species in seawater are relatively limited and have mainly
focused locally on the Irish Sea and Mediterranean Sea withhigher 239,240 Pu, whereas studies in the Pacific and Indian
Oceans are lacking. 4) In the coastal regions, Pu isotopes havebeen used to date sediments and to trace sediment transport,

Wang
focusing and erosion; in the open ocean, Pu is used to indicate
water mass transport and vertical mixing. However, studies
on Pu isotope compositions in the water column are still lacking, which not only limits the study of Pu geochemical behavior but also restricts the study of the implication of Pu formarine processes.
More studies on Pu isotopes in marine environment worthwhile in the future: 1) It is important to optimize the analyticalmethod for measuring Pu in seawater. The minimum volume
for measuring Pu in seawater is currently at least 20 L, whichcauses difficulty when collecting samples from thousands toten thousands meter water layers. Thus, further developmentof the analytical method of Pu is required to reduce sample
volume of less than 5 L. 2) The study of Pu geochemical
behavior in field observations or laboratory experiments isvery easily inf luenced by external condition. Therefore,
stricter experimental conditions are needed for the study of theporewater chemistry of Pu and the influence of colloids andorganic matter on Pu behavior. Based on this, the mechanismof Pu transport and setting needs further clarification.
3) Using the unique isotope ratios of Pu with different sources
as tracers, e.g., 240Pu/239Pu atom ratios, the long-range transport of oceanic biogenic elements from the open ocean t ocoastal regions and the upwelling transport are worthy of further study. 4) The mobility of Pu in sediment requires further
study to assess its suitability for dating marine sediment. 5) In
addition to 240Pu/239Pu, combinations of other artificial radionuclides, e.g., 237 Np/239Pu and 241Am/239Pu, to study radionuclides source and marine processes are recommended.
Additionally, some particle-reactive natural radionuclides, e.g.,
210
Pb/239Pu and 232Th/239Pu, can also be combined and used to
trace marine processes.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (41706089), the State Key Laboratory of
Estuarine and Coastal Research (SKLEC-KF201703) and the
JSPS KAKENHI (JP17k00537).
References(1) Ofan A, Ahmad I, Greene J P, et al, New Astronomy
Reviews. 50(7), 640-643 (2006).
(2) Sholkovitz E R. Earth-Science Reviews. 19(2), 95-161(1983).
(3) Lindahl P, Lee S, Worsfold P, et al. Marine Environmental
Research. 69(2), 73-84 (2010).
(4) Santschi P H, Xu C, Zhang S, et al. Applied
Geochemistry. 85, 121-127 (2017).
(5) Baskaran M, Asbill S, Santschi P, et al. Earth andPlanetary Science Letters. 140(1-4) 243-257 (1996).
(6) Kershaw P J, Woodhead D S, Lovett M B, et al. Applied
Radiation and Isotopes. 46(11), 1121-1134 (1995).
(7) Periáñez R. Journal of Marine Systems, 22(1), 37-51(1999).
(8) IAEA. International Atomic Energy Agency: Vienna,
Austria (2004).
(9) Qiao J, Hou X, Miró M, et al. Analytica Chimica Acta.
652(1), 66-84 (2009).
(10) Wang Z, Yang G, Zheng J, et al. Analytical Chemistry.
87(11) , 5511-5515 (2015).
(11) Cao L, Bu W, Zheng J, et al. Talanta. 151, 30-41 (2016).
(12) Scott M R, Rotter R J, Salter P F. Earth and Planetary
Science Letters. 75(4), 321-326 (1985).
(13) Smith J N, Ellis K M, Naes K, et al. S Deep Sea
Research Part II: Topical Studies in Oceanography.
42(6), 1471-1493 (1995).
(14) Dai M, Kelley J M, Buesseler K O. Environmental

Plutonium Isotopes Research in the Marine Environment: A synthesis

Science & Technology. 36(17), 3690-3699 (2002).
(15) Zheng J, Yamada M. Environmental Science &Technology. 40(13) , 4103 (2006).
(16) Garcia-Orellana J, Pates J M, Masqué P, et al. Science of
the Total Environment. 407(2), 887-898 (2009).
(17) Hirose K, Povinec P. P. Scientific Reports. 5, 15707(2015).
(18) Wang J, Baskaran M, Hou X, et al. Earth and Planetary
Science Letters. 466, 32-42 (2017).
(19) Kelley J M, Bond L A, Beasley T M. Science of the
Total Environment. 237, 483-500 (1999).
(20) United Nations Scientific Committee on the Effects ofAtomic Radiation (U NSCEAR). United Nations
Publications (2000).
(21) Holloway R W, Hayes D W. Environmental Science &Technology, 16(2), 127-129 (1982).
(22) Buesseler K O. Journal of Environmental Radioactivity.
36(1), 69-83 (1997).
(23) Lindahl P, Asami R, Iryu Y, et al. Geochimica et
Cosmochimica Acta. 75(5), 1346-1356 (2011).
(24) Łokas E, Anczkiewicz R, Kierepko R, et al.
Chemosphere, 178, 231-238 (2017).
(25) Chiappini R, Pointurier F, Millies-Lacroix J C, et al.
Science of the Total Environment. 237, 269-276 (1999).
(26) Livingston H D, Povinec P P. Ocean & Coastal
Management. 43(8), 689-712 (2000).
(27) Boulyga S F, Erdmann N, Funk H, et al. Radiation
Measurements. 28(1-6), 349-352 (1997).
(28) Strumińska D I, Skwarzec B. Journal of Radioanalyticaland Nuclear Chemistry. 268(1), 59-63 (2006).
(29) Holm E. PApplied Radiation and Isotopes. 46(11), 12251229 (1995).
(30) Steinhauser G, Brandl A, Johnson T E. Science of the
Total Environment. 470 800-817 (2014).
(31) Zheng J, Tagami K, Watanabe Y, et al. Scientific
Reports. 2(3), 304, (2012).
(32) Bu W, Fukuda M, Zheng J, et al. Environmental Science& Technology. 48(16), 9070-9078 (2014).
(33) Krey P W. Science. 158(3802), 769-771 (1967).
(34) Roos P, Holm E, Persson R, et al. Journal ofEnvironmental Radioactivity. 24(3), 235-251(1994).
(35) Lee S, Povinec P P, Gastaud J, et al. Journal of
Oceanography. 65(3), 397-406 (2009).
(36) Lind O C, Salbu B, Janssens K, et al. Journal ofEnvironmental Radioactivity. 81(1), 21-32 (2005).
(37) Chamizo E, Garcia-Leon M, Synal H, et al. Nuclear
Instruments and Methods in Physics Research Section
B: Beam Interactions with Materials and Atoms. 249(1),768-771 (2006).
(38) Gray J, Jones S R, Smith A D. Journal of RadiologicalProtection. 15(2), 99-131 (1995).
(39) Cook G T, Mackenzie A B, Mcdonald P, et al. Journal ofEnvironmental Radioactivity. 35(3), 227-241 (1997).
(40) Oughton D H, Skipperud L, Salbu B, et al. Monaco:
Symposium on Marine Pollution. IAEATECDOC-1094, 123-128 (1999).
(41) Oughton D H, Fifield L K, Day J P, et al. Environmental
Science & Technology. 34(10), 1938-1945 (2000).
(42) Miralles J, Radakovitch O, Cochran J K, et al. Scienceof the Total Environment. 320(1), 63-72 (2004).
(43) Dai M, Buesseler K O, Pike S M. Journal ofContaminant Hydrology. 76(3), 167-189 (2005).
(44) Armstrong C R, Nuessle P R, Brant H A, et al.
Environmental Science & Technology. 49(3), 1286-1293(2015).
(45) Carlton W H, Murphy Jr C E, Evans A G. Health
Physics. 71(3), 290-299 (1996).
(46) Eyrolle F, Charmasson S, Louvat D. Journal ofEnvironmental Radioactivity, 74(1), 127-138 (2004).

J. Nucl. Radiochem. Sci., Vol. 20, 20209(47) Aarkrog A. Deep Sea Research Part II: Topical Studies
in Oceanography. 50(17), 2597-2606 (2003).
(48) Diamond H, Fields P R, Stevens C S, et al. PhysicalReview. 119(6), 2000, (1960).
(49) Wong K. Analytica Chimica Acta. 56(3), 355-364(1971).
(50) Kim G, Burnett W C, Horwitz E P. Analytical
Chemistry. 72(20), 4882-4887 (2000).
(51) Zheng J, Yamada M, Wang Z, et al. Analytical andBioanalytical Chemistry. 379(3) 532-539 (2004).
(52) Varga Z, Surányi G, Vajda N, et al. Microchemical
Journal. 85(1), 39-45 (2007).
(53) Cizdziel J V, Ketterer M E, Farmer D, et al. Analyticaland Bioanalytical Chemistry. 390(2), 521-530 (2008).
(54) Mellado J, Llaurado M, Rauret G. Analytica Chimica
Acta. 443(1), 81-90 (2001).
(55) Okubo A, Zheng J, Yamada M, et al. Journal ofRadioanalytical and Nuclear Chemistry. 275(2), 291-297(2007).
(56) Ketterer M E, Szechenyi S C. Spectrochimica Acta Part
B: Atomic Spectroscopy. 63(7), 719-737 (2008).
(57) Murali M S, Bhattacharayya A, Raut D R, et al. Journalof Radioanalytical and Nuclear Chemistry. 294(1), 149153 (2012).
(58) Kim C, Kim C, Martin P, et al. Journal of Analytical
Atomic Spectrometry. 22(7), 827-841(2007).
(59) Maxwell S L, Culligan B K, Jones V D, et al. Health
Physics. 101(2), 180-186 (2011).
(60) Jakopič R, Tavčar P, Benedik L. Applied Radiation and
Isotopes. 65(5), 504-511(2007).
(61) Wang Z, Zheng J, Ni Y, et al. Analytical Chemistry,
89(4), 2221-2226 (2017).
(62) Zheng J, Yamada M. Analytical Sciences, 23(5), 611615 (2007).
(63) Men W, Zheng J, Wang H, et al. Scientific Reports, 8(1),1892 (2018).
(64) Pillai K C, Smith R C, Folsom T R. Nature. 203(4945),
568-571 (1964).
(65) Aoyama M, Hirose K. The Scientific World Journal. 4,200-215 (2004).
(66) Hirose K, Sugimura Y, Aoyama M. International Journalof Radiation Applications and Instrumentation. Part A.
Applied Radiation and Isotopes. 43(1-2), 349-359(1992).
(67) Hirose K, Aoyama M, Miyao T, et al. Journal ofRadioanalytical and Nuclear Chemistry. 248(3), 771-776(2001).
(68) Hirose K, Aoyama M, Kim C S., et al. Radioactivity inthe Environment. 8, 67-82 (2006).
(69) Hirose K, Aoyama M, Fukasawa M, et al. Science of the
Total Environment. 381(1-3), 243-255 (2007).
(70) Livingston H D, Povinec P P. Health Physics. 82(5),656-668 (2002).
(71) Povinec P P, Hirose K, Honda T, et al. Journal ofEnvironmental Radioactivity. 76(1), 113-137 (2004).
(72) Miyao T, Hirose K, Aoyama M, et al. Journal ofEnvironmental Radioactivity. 40(3), 239-250 (1998).
(73) Hirose K, Aoyama M. Deep Sea Research Part II:
Topical Studies in Oceanography. 50(17), 2675-2700(2003).
(74) Hirose K, Aoyama M, Povinec P P. Journal ofEnvironmental Radioactivity. 100(3), 258-262 (2009).
(75) Cochran J K. Geochimica et Cosmochimica Acta. 49(5),1195-1210 (1985).
(76) Carpenter R, Beasley T M, Zahnle D, et al. Geochimica
et Cosmochimica Acta. 51(7), 1897-1921 (1987).
(77) Nagaya Y, Nakamura K. Journal of Oceanography.
48(1), 23-35 (1992).
(78) Huh C, Pisias N G, Kelley J M, et al. Deep Sea Research

10

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Part II: Topical Studies in Oceanography. 44(8), 17251743 (1997).
(79) Lee M H, Lee C W, Moon D S, et al. Journal ofEnvironmental Radioactivity. 41(2), 99-110 (1998).
(80) Yamada M, Nagaya Y. Journal of Radioanalytical andNuclear Chemistry. 246(2), 369-378(2000).
(81) Su C, Huh C. Marine Geology. 183(1), 163-178 (2002).
(82) Moon D, Hong G, Kim Y I, et al. Deep Sea ResearchPart II: Topical Studies in Oceanography. 50(17), 26492673(2003).
(83) Lee S, Huh C, Su C, et al. Deep Sea Research Part I:
Oceanographic Research Papers. 51(11), 1769-1780(2004).
(84) Lee S, Povinec P P, Wyse E, et al. Marine Geology.
216(4), 249-263 (2005).
(85) Wang Z L, Yamada M. Earth and Planetary Science
Letters. 233(3), 441-453 (2005).
(86) Dong W, Zheng J, Guo Q, et al. Journal ofEnvironmental Radioactivity. 101(8), 622-629 (2010).
(87) Liu Z, Zheng J, Pan S, et al. Environmental Science &Technology. 45(5), 1805-1811 (2011).
(88) Pan S M, Tims S G, Liu X Y, et al. Journal ofEnvironmental Radioactivity. 102(10), 930-936 (2011).
(89) Wu J, Zheng J, Dai M, et al. Environmental Science &Technology. 48(6), 3136-3144 (2014).
(90) Pittauer D, Tims S G, Froehlich M B, et al. Scientific
Reports. 7, 44679 (2017).
(91) Herrmann J, Nies H, Goroncy I. Radiation Protection
Dosimetry. 75(1-4), 237-245 (1998).
(92) Livingston H D, Povinec P P, Ito T, et al. Radioactivity
in the Environment. 1, 267-292 (2001).
(93) Fowler S W, Noshkin V E, Jacques La R, et al.
Limnology and Oceanography. 45(2), 446-458 (2000).
(94) Ito T, Otosaka S, Kawamura H. Journal of Nuclear
Science and Technology. 44(6), 912-922 (2007).
(95) Hong G, Hamilton T F, Baskaran M, et al. Springer, 367394 (2012).
(96) Novikov A P, Kalmykov S N, Utsunomiya S, et al.
Science. 314(5799), 638-641 (2006).
(97) Vintró L L, Mitchell P I, Condren O M, et al. Science of
the Total Environment. 237, 77-91 (1999).
(98) Hirose, K., Aoyama, M. Analytical and Bioanalytical
Chemistry. 372(3), 418-420 (2002).
(99) Hirose K, Aoyama M, Povinec P P. Deep Sea ResearchPart II: Topical Studies in Oceanography. 50(17), 26392647 (2003).
(100) Nelson D M, Lovett M B. O Nature. 276(5688), 599-601(1978).
(101) Mitchell P I, Downes A B, Condren O M, et al. Applied
Radiation and Isotopes. 46(11), 1175-1190 (1995).
(102) Sholkovitz E R, Mann D R. Estuarine, Coastal and Shelf
Science, 25(4), 413-434 (1987).
(103) Dai M H, Buesseler K O, Kelley J M, et al. Journal ofEnvironmental Radioactivity. 53(1), 9-25 (2001).
(104) Almazán-Torres M G, Ordóñez-Regil E, Ruiz-Fernández
A C. Journal of Environmental Radioactivity. 164, 395399 (2016).
(105) Xu C, Athon M, Ho Y F, et al. Environmental Science &Technology. 48(6), 3186-3195 (2014).
(106) Xu C, Zhang S, Kaplan D I, et al. Environmental
Science & Technology.) 49(19), 11458 (2015).
(107) Robison W L, Noshkin V E. Science of the Total
Environment. 237, 311-327 (1999).
(108) Mcdonald P, I Batlle J V, Bousher A, et al. TScience of
the Total Environment. 267(1), 109-123 (2001).
(109) Gouzy A, Boust D, Connan O, et al. Radioprotection.
40, S613-S619 (2005).
(110) Francis A J, Dodge C J, Gillow J B. Environmental
Science & Technology. 42(7), 2355-2360 (2008).

Wang(111) Baskaran M, Bianchi T S, Filley T R. Journal ofEnvironmental Radioactivity. 174, 10-16 (2017).
(112) Crusius J, Anderson R F. Journal of Paleolimnology.
13(2), 119-141 (1995).
(113) Foster I D, Mighall T M, Proffitt H, et al. Journal of
Paleolimnology. 35(4), 881-895 (2006).
(114) Wang J, Du J, Baskaran M, et al. Journal of GeophysicalResearch: Oceans. 121(1), 224-239 (2016).
(115) Wang J, Baskaran M, Niedermiller J. Geochimica et
Cosmochimica Acta. 218, 323-342 (2017).
(116) Kaplan D I, Powell B A, Duff M C, et al. Environmental
Science & Technology. 41(21), 7417-7423 (2007).
(117) Froehlich M B, Chan W Y, Tims S G, et al. Journal ofEnvironmental Radioactivity. 165, 197-205 (2016).
(118) Goodkin N F, Hughen K A, Cohen A L, et al.
Paleoceanography. 20(4) (2005).
(119) Yu K F, Zhao J X, Wei G J, et al. Global and Planetary
Change, 47(2-4), 301-316 (2005).
(120) Krey P W, Hardy E P, Pachucki C, et al. Transuranium
Nuclides in the Environment. 671-678 (1975).
(121) Du J, Wu Y, Huang D, et al. Journal of Marine Systems.
82(4), 286-294 (2010).
(122) Huh C, Su C. Marine Geology. 160(1), 183-196 (1999).
(123) Ketterer M E, Gulin S B, Maclellan G D, et al.
Spectrochimica Acta Part B: Atomic Spectroscopy 3,
197-201 (2010).
(124) Tims S G, Everett S E, Fifield L K, et al. Nuclear
Instruments and Methods in Physics Research Section
B: Beam Interactions with Materials and Atoms. 268(7),1150-1154 (2010).
(125) Buffoni G, Delfanti R, Papucci C. Marine Geology.
109(1-2), 159-170 (1992).
(126) Livingston H D, Bowen V T. Earth and Planetary
Science Letters. 43(1), 29-45 (1979).
(127) Santschi P H, Guo L, Asbill S, et al. Marine Chemistry.
73(2), 125-152 (2001).
(128) United Nations Scientific Committee on the Effects ofAtomic Radiation (UNSCEAR). United Nations
Scientific Committee on the Effects of Atomic
Radiation. A/37/45, New York (1982).
(129) Hain K, Faestermann T, Fimiani L, et al. Environmental
Science & Technology. 51(4), 2031-2037 (2017).
(130) Smith B S, Child D P, Fierro D, et al. Journal ofEnvironmental Radioactivity. 151, 579-586 (2016).
(131) Noshkin V E, Gatrousis C. Earth and Planetary Science
Letters, 22, 111-117 (1974).
(132) Buesseler K O, Sholkovitz E R. Geochimica et
Cosmochimica Acta. 51(10), 2623-2637 (1987).
(133) Cooper L W, Kelley J M, Bond L A, et al. Marine
Chemistry. 69(3), 253-276 (2000).
(134) M a s q u é P, C o c h r a n J K , H e b b e l n D , e t a l .
Environmental Science & Technology. 37(21), 48484854 (2003).
(135) Lindahl P, Roos P, Holm E, et al. Journal ofEnvironmental Radioactivity. 82(3), 285-301(2005).
(136) Lind O C, Oughton D H, Salbu B, et al. Earth andPlanetary Science Letters. 251(1), 33-43 (2006).
(137) Buesseler K O, Livingston H D. Radionuclides in the
Oceans, Inputs and Inventories, 199-217 (1996).
(138) Strumińska-Parulska D I, Skwarzec B. Oceanologia,
52(3), 499-512 (2010).
(139) Muramatsu Y, Yoshida S, Tanaka A. Journal ofRadioanalytical and Nuclear Chemistry. 255(3), 477-480(2003).
(140) Zheng J, Yamada M, Wu F, et al. Journal ofEnvironmental Radioactivity, 100(1), 71-75 (2009).
(141) Zheng J, Yamada M. Environmental Science &Technology. 38(13), 3498-3504 (2004).
(142) Dong W, Tims S G, Fifield L K, et al. Journal ofPlutonium Isotopes Research in the Marine Environment: A synthesis

Environmental Radioactivity. 101(1), 29-32 (2010).
(143) Bu W, Ni Y, Guo Q, et al. Scientific Reports. 5. (2015).
(144) Froehlich M B, Akber A, McNeil S D, et al. Journal ofEnvironmental Radioactivity, 205,17-23 (2019).
(145) Zhang W, Hou X. Chemosphere, 230, 587-595 (2019).
(146) Zheng J, Yamada MJournal of Radioanalytical andNuclear Chemistry. 267(1), 73-83 (2005).
(147) Xu Y, Pan S, Gao J, et al. Chemosphere. 207, 130-138(2018).
(148) Zhuang Q, Li G, Wang F, et al. Marine Pollution
Bulletin, 138, 604-617 (2019).
(149) Kim C K, Kim C S, Chang B U, et al. Science of the
Total Environment, 318(1-3), 197-209 (2004).
(150) Yamada M, Zheng J, Wang Z L. Science of the Total
Environment. 366(1), 242-252 (2006).

J. Nucl. Radiochem. Sci., Vol. 20, 202011(151) Ya m a d a M , Z h e n g J , Wa n g Z L . J o u r n a l o f
Environmental Radioactivity. 98(3), 274-284(2007).
(152)Yamada M, Zheng J. Science of the Total Environment.
408(23), 5951-5957 (2010).
(153) Yamada M, Zheng J. Science of the Total Environment.
430(430), 20-27 (2012).
(154) Hao Y, Xu Y, Pan S, et al. Marine pollution bulletin. 130,240-248 (2018).
(155) Wu J. Marine Pollution Bulletin, 135, 808-818 (2018).
(156) Wu J, Dai M, Xu Y, et al. Science of T he Total
Environment. 610, 200-211 (2018).
(157) Wu J, Dai M, Xu Y, et al. Chemical Geology, 511, 256264 (2019).
(158) Hirose K, Kim C S, Yim S A, et al. Progress in
Oceanography. 89(1), 101-107 (2011).