Patric Lindahl
email: patriclindahl@yahoo.com

Sang-Han Lee

Paul Worsfold

Miranda Keith-Roach

Plutonium isotopes as tracers for ocean processes: A review

Keywords: plutonium, isotopic ratio, inventory, source term, marine environment, marine tracer, water, sediment, biota

Since the first nuclear weapons tests in the 1940s, pulsed inputs of plutonium isotopes have served as excellent tracers for understanding sources, pathways, dynamics and the fate of pollutants and particles in the marine environment. Due to the well-defined spatial and temporal inputs of Pu, the long halflives of 240 Pu and 239 Pu and its unique chemical properties, Pu is a potential tracer for various physical and biogeochemical ocean processes, including circulation, sedimentation and biological productivity, and hence a means of assessing the impacts of global climate change. Due to the source dependency of the Pu isotopic signature, plutonium isotopes are beginning to be exploited as tools for the evaluation and improvement of regional and global ocean models that will enhance understanding of past and future changes in the oceans. This paper addresses the major sources of Pu and the physical and biogeochemical behaviour in the marine environment. Finally, the use of Pu isotopes as tracers for various oceanic processes (e.g. water mass transport, particle export, and sedimentation) is considered.

Introduction

The World Ocean plays a critical role in regulating the earth's climate and reliable tracers are required to predict water mass movement and elucidate ocean processes. Anthropogenic radionuclides play important roles as tracers for ocean processes [START_REF] Dahlgaard | Transfer of European coastal pollution to the arctic: radioactive tracers[END_REF][START_REF] Livingston | A millennium perspective on the contribution of global fallout radionuclides to ocean science[END_REF] and can be used to reconstruct past environmental conditions and therefore enhance our knowledge about past and future climate change. In addition, the distribution of anthropogenic radionuclides can be modelled to derive rates of ocean transport on timescales not attainable by direct measurement. Since the input histories of anthropogenic radionuclides to the environment are well known, it enables us to trace small-scale (near shore) and large-scale oceanic processes (circulation, sedimentation and biological productivity) and predict future climate change processes and their impact on the marine environment.

Plutonium has been released into the marine environment since the first nuclear weapons test in 1945. Plutonium isotopes are mainly produced by neutron irradiation of uranium, and the isotopic composition of Pu is characteristic of the source, as it depends on the neutron intensity and the duration of irradiation. Plutonium concentrations in the marine environment are continuously changing due to marine processes such as ocean currents, sedimentation, remobilisation and biological uptake. The well-defined spatial and temporal source terms and high particle affinity of Pu provide a powerful tool for studying a variety of processes in the marine environment such as water mass transport (advection and convection), particle fluxes and scavenging, and validating various global circulation and biogeochemical ocean models. The most frequently studied Pu isotopes in the marine environment are 238 Pu (T ½ = 87.74 a), 239 Pu (T ½ = 24 100 a), 240 Pu (T ½ = 6 561 a), 241 Pu (T ½ = 14.33 a) and 242 Pu (T ½ = 373 000 a) with 239 Pu and 240 Pu as the most abundant in the marine environment.

Recent advances in analytical procedures and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) have improved the determination of low concentrations of Pu and provide precise isotopic ratios with reasonable costs [START_REF] Kim | Determination of Pu isotope concentrations and isotope ratio by inductively coupled plasma mass spectrometry: a review of analytical methodology[END_REF][START_REF] Ketterer | Determination of plutonium and other transuranic elements by inductively coupled plasma mass spectrometry: A historical perspective and new frontiers in the environmental sciences[END_REF][START_REF] Qiao | Determination of plutonium isotopes in waters and environmental solids: A review[END_REF]. Previously, Pu analysis in marine samples was mainly performed by conventional alpha spectrometry, with the disadvantage of long measurement times and no resolution between 239 Pu and 240 Pu, due to similar alpha energies.

Comprehensive studies on the distribution and behaviour of Pu in the marine environment have been summarised in several review papers (e.g. [START_REF] Sholkovitz | The geochemistry of plutonium in fresh and marine water environments[END_REF]Baxter et al., 1995;[START_REF] Livingston | Anthropogenic marine radioactivity[END_REF][START_REF] Livingston | The behaviour of plutonium in the Pacific Ocean[END_REF][START_REF] Livingston | A millennium perspective on the contribution of global fallout radionuclides to ocean science[END_REF][START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF][START_REF] Vintró | Transuranium nuclides in the world's oceans[END_REF].

The specific aim of this paper is to provide an overview of the use of Pu isotopes as tracers for various processes in the marine environment. First, the major Pu sources to the marine environment and their isotopic composition are summarised. This is followed by an overview of the geochemical and biogeochemical processes affecting the behaviour and distribution of Pu isotopes in the marine environment. Finally, the utility of Pu isotopes as tracers for various marine processes is considered, with an emphasis on the use of specific Pu isotopic composition (e.g. 240 Pu/ 239 Pu ratio).

Sources of Pu in the oceans

The majority of Pu in the environment is derived from man-made sources with only trace amounts from natural sources [START_REF] Taylor | Environmental plutonium -creation of the universe to twenty-first century mankind[END_REF]. The main sources of Pu isotopes in the World Ocean are fallout from atmospheric nuclear weapons testing and liquid discharges from nuclear reprocessing plants ( [START_REF] Vintró | Transuranium nuclides in the world's oceans[END_REF]. Other sources that have impacted on Pu levels in the marine environment at the local and/or regional scale are the Chernobyl accident, aircraft accidents involving nuclear weapons, burn-up of nuclear powered satellites and, to a lesser extent, dumping of radioactive wastes and the loss of nuclear submarines. The composition of Pu isotopes from different sources in the marine environment varies significantly due to the characteristic isotopic composition corresponding to the means of production (see Table 1). Reactor-grade Pu typically contains more than 35 % 239 Pu with a 240 Pu/ 239 Pu atom ratio of 0.2 -1.0 after fuel burn-up [START_REF] Yamana | Isotopic ratio of Pu released from fuel cycle facilities -importance of radiochemically pure 236 Pu as a tracer[END_REF]. The 240 Pu/ 239 Pu atom ratio in weapon-grade Pu is much lower (0.02 -0.06) because burn-up is kept low to minimise the production of higher Pu isotopes [START_REF] Mark | Explosive properties of reactor-grade plutonium[END_REF]. <Table 1 here>

Atmospheric nuclear weapon tests

Between 1945 and 1980, 543 atmospheric nuclear weapons tests (440 Mt total yield) were carried out worldwide with the most intense periods in 1952-1958and in 1961-1962(UNSCEAR, 2000a)). Figure 1 shows the total annual yields (Mt per year) from atmospheric nuclear weapons tests together with the activity of 90 Sr deposited in the northern and southern hemispheres. The 90 Sr global fallout pattern is similar to that of Pu isotopes, with an average 239+240 Pu/ 90 Sr activity ratio in global fallout of about 0.025 (decay corrected to 2000) based on the normalised production rate for 90 Sr and Pu isotopes in nuclear explosions (UNSCEAR, 2000a;[START_REF] Aarkrog | Input of anthropogenic radionuclides into the World Ocean[END_REF].

The low-yield fission weapons tests up to 1951 produced mainly tropospheric fallout that was deposited around the latitude band of the test sites (close-in fallout). From 1952 until the early 1960s, high-yield thermonuclear devices were tested and the fallout was spread worldwide (global fallout). Due to the distribution of the weapons tests locations the majority (ca. 76 %) of the fallout debris was deposited in the northern hemisphere (Fig. 1) with the highest deposition density between the 40 and 50 ºN latitudes (UNSCEAR, 2000a). <Figure 1 here> It has been estimated that 12 PBq of 239+240 Pu was deposited in the World Ocean from atmospheric nuclear weapons tests [START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF] and the resulting Pu isotope inventory is shown in Table 2. The Pu isotopic composition in the debris from nuclear detonations varies considerably and depends on the explosive yield, test site location and weapon type [START_REF] Perkins | Worldwide fallout[END_REF]. The Pu isotopic composition in atmospheric fallout debris has changed over time due to changes in weapon design and yield, as demonstrated by 240 Pu/ 239 Pu time series analysis of samples from around the world [START_REF] Koide | The 240 Pu/ 239 Pu ratio, a potential geochronometer[END_REF][START_REF] Leifer | The Environmental Measurements Laboratory's Stratospheric Radionuclide (RANDAB) and Trace Gas (TRACDAB) Databases[END_REF][START_REF] Warneke | A new ground-level fallout record of uranium and plutonium isotopes for northern temperate latitudes[END_REF][START_REF] Olivier | Plutonium from global fallout recorded in an ice core from the Belukha Glacier, Siberian Altai[END_REF]. The overall integrated 240 Pu/ 239 Pu atom ratio in global fallout debris is characterised by a ratio of 0.178 ± 0.023 (2 SD; 95 % confidence level) [START_REF] Kelley | Global distribution of Pu isotopes and 237 Np[END_REF]. <Table 2 here> More than 70 % of the global input of Pu from atmospheric nuclear weapons testing was from Novaya Zemlya (Russia) and the Marshall Islands (North Pacific). The Novaya Zemlya testing grounds were used mainly for large-scale atmospheric tests, with most of the debris injected into the stratosphere, and hence contributed mainly to the global fallout with only minor local marine contamination [START_REF] Osvath | Kara Sea radioactivity assessment[END_REF][START_REF] Oughton | Accelerator mass spectrometry measurement of 240 Pu/ 239 Pu isotope ratios in Novaya Zemlya and Kara Sea sediments[END_REF]. However, a series of surface and underwater tests contributed significantly to elevated Pu levels in Chernaya Bay, off the southern coast of Novaya Zemlya. Approximately 11 TBq of 239+240 Pu from the tests remains in Chernaya Bay sediments and still acts as a source to the Barents Sea, with an average 240 Pu/ 239 Pu atom ratio of about 0.03, equivalent to low-yield nuclear detonations [START_REF] Smith | Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya[END_REF]Smith et al., 2000b). Most of the Marshall Islands tests were conducted near the surface, resulting in significant local and regional inputs of Pu to the marine environment [START_REF] Buesseler | The isotopic signature of fallout plutonium in the North Pacific[END_REF][START_REF] Robison | Radionuclide characterization and associated dose from long-lived radionuclides in close-in fallout delivered to the marine environment at Bikini and Enewetak Atolls[END_REF]. Debris from the first high-yield (Ivy Mike test, 15 Mt) thermonuclear detonation in 1952 at the Enewetak Atoll had a 240 Pu/ 239 Pu atom ratio of 0.363 ± 0.004 [START_REF] Diamond | Heavy isotope abundances in Mike thermonuclear device[END_REF]. It has been estimated that debris from this detonation contributed about 5.1 PBq of 239+240 Pu to the local marine environment [START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF]. Close-in fallout from the highest yield (Bravo test, 15 Mt) detonation performed at the Bikini Island in 1954 had a 240 Pu/ 239 Pu atom ratio of 0.33 ± 0.06 [START_REF] Muramatsu | Measurement of 240 Pu/ 239 Pu isotopic ratios in soils from the Marshall Islands using ICP-MS[END_REF]. The main Pu contamination from these sites is buried in lagoon sediments (ca. 90 TBq 239+240 Pu in 2000) which still act as a Pu source in the Pacific Ocean with an estimated annual 239+240 Pu export of about 0.2 TBq a -1 to the open ocean [START_REF] Noshkin | Plutonium and americium behavior in coral atoll environments[END_REF][START_REF] Hamilton | 137 Cs ( 90 Sr) and Pu isotopes in the Pacific Ocean: Sources and trends[END_REF][START_REF] Donaldson | University of Washington's radioecological studies in the Marshall Islands, 1946-1977[END_REF][START_REF] Robison | Radionuclide characterization and associated dose from long-lived radionuclides in close-in fallout delivered to the marine environment at Bikini and Enewetak Atolls[END_REF][START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF].

Other locations with significant input to the global or local marine environment include the Johnston Atoll and the Christmas Island (North Pacific), the Lop Nor site (China), the Mururoa and the Fangataufa Atolls (South Pacific), and the Semipalatinsk site (Kazakhstan). Contributions from other tests were very small or negligible [START_REF] Livingston | Anthropogenic marine radioactivity[END_REF]. Overall, about 85 % of the Pu injected into the atmosphere has been deposited globally and 15 % locally or regionally (UNSCEAR, 2000a).

Discharges from the nuclear industry

When spent nuclear fuel is treated in nuclear reprocessing facilities low-level liquid radioactive wastes containing variable quantities of Pu are released into the marine environment. The nuclear reprocessing facilities in Sellafield (UK) and La Hague (France) in the North East Atlantic region are the most prominent sources. In the Arctic Ocean, a potential Pu source is run off from contaminated sediment in the Ob and Yenisey river system connected to the Mayak, Krasnoyarsk and Tomsk nuclear facilities and the Seimpalatinsk test site [START_REF] Lind | Transport of low 240 Pu/ 239 Pu atom ratio plutonium-species in the Ob and Yenisey Rivers to the Kara Sea[END_REF]. Recent Pu analyses of sediment cores from the Ob and Yenisey estuaries showed 240 Pu/ 239 Pu atom ratios of about 0.11 and 0.18, respectively, indicating contributions from both global fallout and the nuclear industry in the outflow to the Kara Sea [START_REF] Skipperud | Association of plutonium with sediments from the Ob and Yenisey Rivers and Estuaries[END_REF].

Plutonium releases from other installations that may have influenced local Pu levels include Hanford plutonium production facility into the North East Pacific [START_REF] Beasley | The inventories of plutonium-239, -240, americium-241, cesium-137, and cobalt-60 in Columbia River sediments from Hanford to the Columbia River Estuary[END_REF], Oak Ridge and Savannah River production plants into the West Atlantic [START_REF] Carlton | Plutonium in the Savannah River Site environment[END_REF], Marcoule reprocessing plant in the Northwestern Mediterranean Sea [START_REF] Miralles | Multitracer study of anthropogenic contamination records in the Camargue, Southern France[END_REF][START_REF] Lansard | Spatial and temporal variations of plutonium isotopes ( 238 Pu and 239,240 Pu) in sediments off the Rhone River mouth (NW Mediterranean)[END_REF], Rokkasho and Tokaimura nuclear reprocessing plants on the west coast of Japan [START_REF] Yamada | Determination of 240 Pu/ 239 Pu atom ratio in coastal surface seawaters from the western North Pacific Ocean and Japan Sea[END_REF], and Trombay and Tarapur nuclear reprocessing plants on the west coast of India. Other nuclear facilities that discharge liquid radioactive waste into the oceans, directly or indirectly, have made a less significant contribution to Pu contamination of the marine environment. <Figure 2 here> Low-level liquid radioactive discharges from the Sellafield nuclear reprocessing plant on the coast of the Irish Sea in Cumbria, UK have been a major Pu source to the marine environment since 1952 [START_REF] Kershaw | Plutonium: the legacy of Sellafield[END_REF]. The total 239+240 Pu in liquid discharges (1952 -2007) from the plant is about 0.6 PBq, with a peak in the early to mid-1970s (Fig. 2). The Pu isotopic composition of Sellafield effluents has varied over the years due to the different plant operations and the fuel reprocessed (Kershaw et al., 1995b). The complex was designed initially for the production of low burn-up weapons-grade Pu but focused later on reprocessing high burn-up spent fuel from commercial power reactors. Studies of dated sediment cores collected in the vicinity of the Sellafield plant showed 240 Pu/ 239 Pu atom ratios of 0.06 in the 1960s, corresponding to weapons-grade Pu, increasing to > 0.20 in the 1980s, corresponding mainly to commercial spent fuel reprocessing (Fig. 2) (Kershaw et al., 1995a;[START_REF] Morris | Evidence for the remobilization of Sellafield waste radionuclides in an intertidal salt marsh, West Cumbria, U.K. Estuarine[END_REF]. A similar pattern was observed for the 238 Pu/ 239+240 Pu activity ratio, due to greater neutron capture by 238 U with longer burn-up times. Plutonium discharges from the Sellafield site decreased in the early 1980s but the Pu deposited in the Irish Sea sediment continues to act as a source for transport to the North Atlantic and the Arctic Ocean [START_REF] Hunt | Remobilisation of artificial radionuclides from the sediment of the Irish Sea[END_REF][START_REF] Cook | Remobilization of Sellafieldderived radionuclides and transport from the North-east Irish Sea[END_REF]Kershaw et al., 1999a;Kershaw et al., 1999b). The estimated 239+240 Pu inventory in the seabed of the Irish Sea is in the range 440 -560 TBq, with annual transport through the North Channel of 0.5 -1.3 TBq a -1 [START_REF] Cook | Remobilization of Sellafieldderived radionuclides and transport from the North-east Irish Sea[END_REF][START_REF] Jones | Distribution of 137 Cs and inventories of 238 Pu, 239/240 Pu, 241 Am and 137 Cs in Irish Sea intertidal sediments[END_REF]Kershaw et al., 1999a;[START_REF] Leonard | Dissolved plutonium and americium in surface waters of the Irish Sea, 1973-1996[END_REF]. Recent model predictions support the view that the major Pu source in the Irish Sea is the contaminated sediments resulting from historical Sellafield discharges [START_REF] Aldridge | Transport of plutonium ( 239/240 Pu) and caesium ( 137 Cs) in the Irish Sea: comparison between observations and results from sediment and contaminant transport modelling[END_REF].

The Pu input from the La Hague site on the north coast of France has been small compared with the Sellafield site (~0.4 %), with a total Pu release of < 0.005 PBq [START_REF] Kershaw | The transfer of reprocessing wastes from north-west Europe to the Arctic[END_REF]Kershaw et al., 1995b;[START_REF] Nyffeler | Radionuclides in the Atlantic Ocean: A survey[END_REF][START_REF] Boust | Distribution and inventories of some artificial and naturally occurring radionuclides in medium to coarse-grained sediments of the channel[END_REF].

Accidental releases

The Chernobyl accident and incidents involving aircrafts and satellites have been the main accidental releases of Pu isotopes to the marine environment. The latter generally have local or regional impact but the SNAP-9A satellite incident had a wider footprint.

The Chernobyl accident in April 1986 has been the most serious accident involving a nuclear power reactor, with significant radionuclide fallout in the Northern Hemisphere. The explosions and subsequent fire released about 2.5 PBq 239+240 Pu into the air, mostly deposited in the vicinity of the reactor (UNSCEAR, 2000b). The 240 Pu/ 239 Pu atom ratio from the Chernobyl fallout is about 0.38 (Table 1), which is considerably higher than the worldwideintegrated fallout ratio of ca. 0.18. The Pu input to the marine environment was reasonably small and mostly local, i.e. to the Baltic Sea and the Black Sea [START_REF] Holm | Plutonium in the Baltic Sea[END_REF][START_REF] Buesseler | Natural and man-made radionuclides in the Black Sea[END_REF]. About 9 % (1.5 TBq) of the total 239+240 Pu inventory in the Baltic Sea can be attributed to the Chernobyl accident [START_REF] Holm | Plutonium in the Baltic Sea[END_REF].

In addition, Pu has been released into the marine environment from two accidents involving aircraft carrying nuclear weapons; in Palomares, Spain (1966) and in Thule, Greenland (1968) [START_REF] Mitchell | Perturbation in the 240 Pu/ 239 Pu global fallout ratio in local sediments following the nuclear accidents at Thule (Greenland) and Palomares (Spain)[END_REF]. In Thule, the Pu was dispersed locally after the chemical explosive components detonated on impact. Although an extensive clean-up operation was carried out on the ice around Thule immediately after the accident, the residual 239+240 Pu contamination of the marine environment was in the range 5 -15 TBq [START_REF] Eriksson | On weapons plutonium in the Arctic environment[END_REF]. Most of the Pu released from the Palomares accident was deposited on land and only a small fraction has been transferred to the marine environment, with a maximum estimated input of 1.4 TBq [START_REF] Papucci | Time evolution and levels of man-made radioactivity in the Mediterranean Sea[END_REF][START_REF] Sanchez-Cabeza | Concentrations of plutonium and americium in plankton from the western Mediterranean Sea[END_REF]. The characteristic 240 Pu/ 239 Pu atom ratio from both these sources was around 0.06 (Table 1), which corresponds well with 240 Pu/ 239 Pu atom ratio in weapons-grade Pu.

In 1964, about 0.63 PBq of 238 Pu was injected in the stratosphere from the burn-up of nuclear-powered SNAP-9A navigational satellite [START_REF] Krey | Atmospheric burnup of a plutonium-238 generator[END_REF]. Consequently, about 78 % of the 238 Pu debris was deposited in the Southern Hemisphere, with the highest deposition between latitudes 20 °S and 50 °S [START_REF] Perkins | Worldwide fallout[END_REF]. The accident has contributed to the total inventory of 238 Pu in the World Ocean with an estimated input of 238 Pu of about 0.32 PBq decay corrected to the year 2000 [START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF]. As a result of the SNAP-9A incident the average 238 Pu/ 239+240 Pu activity ratio in global fallout increased from 0.019 ± 0.003 to 0.029 ± 0.005 in the Northern hemisphere and 0.14 ± 0.04 in the Southern hemisphere (values are decay corrected to 2000) [START_REF] Perkins | Worldwide fallout[END_REF].

Other potential Pu inputs to the marine environment

Nuclear-powered submarines lost at sea may be potential Pu sources to the marine environment. For example, the USSR Navy nuclear submarine -Kosmomolets‖ sank to a depth of 1700 m in the Barents Sea in 1989 with an estimated 239 Pu inventory of 21 TBq [START_REF] Livingston | Anthropogenic marine radioactivity[END_REF]. However, no Pu contamination from the submarine has yet been reported [START_REF] Nies | Anthropogenic radioactivity in the Arctic Ocean -review of the results from the joint German project[END_REF]. Other potential accidental Pu sources include nuclear powered generators from the interplanetary station -Mars' 96‖ (0.17 PBq of 238 Pu) and Apollo 13 (1.6 PBq of 238 Pu and 1.2 TBq of 239 Pu), both of which landed intact in the deep South Pacific Ocean [START_REF] Dobry | Transuranic elements in space nuclear power systems[END_REF]IAEA, 2001).

The dumping of radioactive wastes from the nuclear industry has been carried out at deep ocean sites, mainly in the North East Atlantic and the Arctic Oceans (IAEA, 1999), with only minor local Pu contamination [START_REF] Livingston | Anthropogenic marine radioactivity[END_REF]. An estimated inventory of 8.9 TBq 239+240 Pu in marine reactors was dumped in the Kara Sea (IAEA, 1998a) and enhanced levels of 239+240 Pu in close proximity to the objects indicates some leakage, but no evidence of any transfer from this site has been reported [START_REF] Salbu | Radioactive contamination from dumped nuclear waste in the Kara Sea --results from the joint Russian-Norwegian expeditions in 1992-1994[END_REF][START_REF] Nies | Anthropogenic radioactivity in the Arctic Ocean -review of the results from the joint German project[END_REF].

Radioactive dumping was also carried out in the western North Pacific Ocean and its marginal seas, but with considerably lower Pu inventories, and there is no evidence of elevated Pu from these sources [START_REF] Livingston | Anthropogenic marine radioactivity[END_REF]. However, the lack of studies around these dumping sites makes it difficult to predict future Pu releases, especially in the Arctic Ocean [START_REF] Nies | Anthropogenic radioactivity in the Arctic Ocean -review of the results from the joint German project[END_REF][START_REF] Osvath | Kara Sea radioactivity assessment[END_REF][START_REF] Preller | Modeling the transport of radioactive contaminants in the arctic[END_REF][START_REF] Skipperud | Plutonium in the Arctic Marine Environment -A Short Review[END_REF].

Nevertheless, these dumping sites may be potential Pu sources to the marine environment in the future.

Behaviour of Pu in the marine environment

The geochemical behaviour of Pu in the marine environment is influenced by: (i) the characteristics of the Pu source, (ii) its physico-chemical properties and (iii) biogeochemical and physical processes in the ocean.

Plutonium from a specific source may be associated with particles, colloids, pseudocolloids or low molecular mass species [START_REF] Salbu | Actinides associated with particles[END_REF] depending on the source and the environmental conditions. Global Pu fallout is mainly associated with sub-micron iron oxide particles, while Pu fallout from surface tests in the Marshall Islands was mainly attached to calcium hydroxide particles [START_REF] Buesseler | The isotopic signature of fallout plutonium in the North Pacific[END_REF]. In effluents from nuclear reprocessing plants most Pu is associated with particulate material or colloids [START_REF] Leonard | Physico-chemical characterisation of radionuclides discharged from a nuclear establishment[END_REF].

<Figure 3 here>

The behaviour of Pu in seawater is influenced by a variety of geochemical processes, e.g. solubilisation, hydrolysis, complexation, sorption and colloid formation (Fig. 3).

Microbial processes may also have a significant effect on the long-term behaviour and mobility of Pu [START_REF] Francis | Microbial transformations of plutonium and implications for its mobility[END_REF][START_REF] Francis | Reductive dissolution of Pu(IV) by Clostridium sp. under anaerobic conditions[END_REF]. The oxidation state of Pu significantly affects its geochemical behaviour. In marine waters four different oxidation states exist, Pu(III), Pu(IV), Pu(V) and Pu(VI), with Pu(V) and Pu(IV) being the most common [START_REF] Choppin | Actinide speciation in aquatic systems[END_REF]. The Pu(IV) form is highly particle reactive and easily scavenged by suspended matter and colloids, while Pu(V) is relatively soluble and can be transported in the dissolved phase over long distances. Field studies in the Irish Sea and the Mediterranean Sea have shown that Pu(V) is the prevailing species in the dissolved phase and that particulate Pu is almost entirely in the reduced Pu(IV) form [START_REF] Mitchell | Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the western Mediterranean[END_REF]. In anoxic seawater Pu(III) is the dominant oxidation state [START_REF] Sanchez | Distribution of plutonium and its oxidation states in Framvaren and Hellvik fjords, Norway[END_REF]. In general, Pu oxidation states in seawater are dependent on pH, E H and the suspended particle concentration [START_REF] Choppin | Actinide speciation in aquatic systems[END_REF].

Interaction with complexing agents and hydrolysis reactions are generally strongest for Pu(IV) and weakest for Pu(V), which leads to the preferential removal of reduced Pu species from solution by adsorption onto colloidal and suspended material [START_REF] Nelson | Oxidation state of plutonium in the Irish Sea[END_REF][START_REF] Orlandini | Colloidal behavior of actinides in an oligotrophic lake[END_REF][START_REF] Mitchell | Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the western Mediterranean[END_REF]. In contrast, oxidised Pu(V) has a high affinity for adsorption onto carbonate particles, e.g. corals, as surface complexes (Keeneykennicutt and Morse, 1985;[START_REF] Meece | The coprecipitation of Pu and other radionuclides with CaCO 3[END_REF]Gouzy et al., 2005b). Field studies of coastal seawater have shown that colloidal Pu (>1 kDa) is only a minor fraction of the total Pu in seawater [START_REF] Dai | Sizefractionated plutonium isotopes in a coastal environment[END_REF][START_REF] Mitchell | Studies of the speciation, colloidal association and remobilisation of plutonium in the marine environment[END_REF].

In shallow waters with higher particle concentrations, preferential removal of Pu from the water column to the sediment takes place while in surface open ocean waters this process is dependent on biological productivity. Thus, in biologically poor (oligotrophic) regions the main component of the deposited Pu inventory from stratospheric fallout remains in the water column while in coastal waters almost all is removed from the water column to the seabed [START_REF] Vintró | Transuranium nuclides in the world's oceans[END_REF]. Field studies in the oligotrophic western Mediterranean Sea indicate that Pu distribution in the upper water column is controlled by the degree of primary production [START_REF] Fowler | Temporal variations in plutonium and americium inventories and their relation to vertical transport in the Northwestern Mediterranean Sea[END_REF]. However, close-in Pu fallout from the surface tests at the Marshall Islands had different physico-chemical characteristics, due to interaction with calcium-rich particles from the surrounding media, which resulted in a faster scavenging rate from surface waters to deep ocean sediment [START_REF] Buesseler | The isotopic signature of fallout plutonium in the North Pacific[END_REF].

Sediment resuspension and dissolution influence the dispersion of Pu from contaminated sediment [START_REF] Lansard | Potential plutonium remobilisation linked to marine sediment resuspension: first estimates based on flume experiments[END_REF]. Plutonium is remobilised from contaminated sediment mainly in the oxidised forms Pu(V,VI) as observed in the Irish Sea [START_REF] Leonard | Dissolved plutonium and americium in surface waters of the Irish Sea, 1973-1996[END_REF][START_REF] Mitchell | Studies of the speciation, colloidal association and remobilisation of plutonium in the marine environment[END_REF]. Various environmental factors may influence the remobilisation/uptake process such as pH, temperature, light, bioirrigation, bioturbation and natural and anthropogenic physical mixing [START_REF] Mccubbin | The role of thermal and photochemical reactions upon the remobilisation of Pu from an Irish Sea sediment[END_REF]Gouzy et al., 2005a;[START_REF] Kaplan | Influence of pH on plutonium desorption/solubilization from sediment[END_REF]. Laboratory studies suggested that only 0.02 -1 % of the total Pu in the sediment is available for remobilisation [START_REF] Mccubbin | The role of thermal and photochemical reactions upon the remobilisation of Pu from an Irish Sea sediment[END_REF][START_REF] Mcdonald | The availability of plutonium and americium in Irish Sea sediments for re-dissolution[END_REF]. However, recent sequential extraction studies of Irish Sea sediment suggested that Pu is more bioavailable, with a release of at least 10 % of the total Pu (Gouzy et al., 2005a). The study indicated that Pu is loosely bound to the sediment and is oxidised readily on contact with oxygen-rich waters.

The uptake of Pu into marine biota is dependent on the species and the surrounding environment [START_REF] Iaea | Sediment distribution coefficients and concentration factors for biota in the marine environment[END_REF]. Sediment-dwelling species play a significant role in the remobilisation process, as they may redistribute deposited Pu in the sediments by mixing and agitation [START_REF] Ryan | Transuranic biokinetic parameters for marine invertebrates--a review[END_REF].

In summary, the scavenging rate of Pu in the marine environment depends on (i) the physical and chemical forms of Pu; (ii) the amount of particulate matter; (iii) the settling rate of particles; (iv) mixing processes (e.g. diffusion, resuspension and bioturbation); and (v) biogeochemical processes (e.g. mineralogical transformations, redox reactions and uptake by biota).

Use of Pu isotopes for studying marine processes

Because of the unique sources (spatial and temporal) of Pu in the World Ocean and its nonconservative behaviour, the inventory is not uniformly distributed and Pu concentrations show significant variability around the world. The 239+240 Pu activity concentration in surface waters varies from a few mBq m -3 in the open ocean up to > 100 mBq m -3 in semi-enclosed waters close to a Pu source (e.g. Irish Sea) (IAEA, 2005).

The main advantage of using Pu isotopes as tracers for marine processes is their welldefined source terms. The 240 Pu/ 239 Pu atom ratio from integrated global fallout (~0.18) is assumed to be uniformly distributed throughout the marine environment. For the calculation of the contribution of different sources with different Pu isotopic composition in marine samples, simple mixing models can be used [START_REF] Kelley | Global distribution of Pu isotopes and 237 Np[END_REF][START_REF] Ketterer | Resolving Chernobyl vs. global fallout contributions in soils from Poland using plutonium atom ratios measured by inductively coupled plasma mass spectrometry[END_REF]. In most marine regions Pu contamination consists mainly of contributions from global fallout and a local/regional source except in some cases such as the Baltic Sea [START_REF] Holm | Plutonium in the Baltic Sea[END_REF] and the Arctic Ocean [START_REF] Cooper | Sources of the transuranic elements plutonium and neptunium in arctic marine sediments[END_REF]. In general, the measured isotopic ratio, R S , in a marine sample consists of contribution from n source terms and can be expressed as:

R S = x 1 R 1 + x 2 R 2 +…+ x n R n (1) x 1 + x 2 +…+ x n = 1 (2)
where x i is the contribution from source term i and R i is the isotopic ratio of source term i. In the case of two source terms the contribution (x 1 ) of source term (in %) can be calculated by combining Eqs. ( 1) and (2) as follows:

x 1 = 100 × (R m -R 2 ) / (R 1 -R 2 ) (3)
where R m is the measured ratio. With the application of Eq. ( 3), a good estimation of the contribution of a particular source term can be helpful in understanding oceanographic processes, e.g. water mass exchange, in a specific marine region,. This has been applied in a number of marine studies such as estimation of the influence of Chernobyl Pu fallout to the Baltic Sea [START_REF] Lindahl | Studies of Np and Pu in the marine environment of Swedish-Danish waters and the North Atlantic Ocean[END_REF], the influence of the Pu-contaminated Rhone river input to the Mediterranean Sea [START_REF] Miralles | Multitracer study of anthropogenic contamination records in the Camargue, Southern France[END_REF], the contribution of Pu debris from the Nevada Test Site in deep North West Atlantic waters [START_REF] Buesseler | The geochemistry of fallout plutonium in the North Atlantic: II. 240 Pu/ 239 Pu ratios and their significance[END_REF], determination of the influence of underwater nuclear tests in the Bering Sea on the marine environment [START_REF] Dasher | An assessment of the reported leakage of anthropogenic radionuclides from the underground nuclear test sites at Amchitka Island, Alaska, USA to the surface environment[END_REF], determination of the source of radioactivity in the Arctic Ocean [START_REF] Huh | 240 Pu/ 239 Pu ratios in Sun Moon lake sediments: implications for sources of nuclear fallout in Taiwan[END_REF][START_REF] Cooper | Distributions of nuclear fuel-reprocessing tracers in the Arctic Ocean: indications of Russian river infuence[END_REF][START_REF] Cooper | Sources of the transuranic elements plutonium and neptunium in arctic marine sediments[END_REF], and estimation of the contribution of close-in fallout from the Marshall Islands to the Northwest Pacific Ocean and its marginal seas, and to the Indian Ocean [START_REF] Zheng | Sediment core record of global fallout and Bikini close-in fallout Pu in Sagami Bay, western Northwest Pacific margin[END_REF][START_REF] Lee | Distribution and inventories of 90 Sr, 137 Cs, 241 Am and Pu isotopes in sediments of the Northwest Pacific Ocean[END_REF][START_REF] Wang | Plutonium activities and 240 Pu/ 239 Pu atom ratios in sediment cores from the east China sea and Okinawa Trough: sources and inventories[END_REF]Zheng and Yamada, 2005b;Yamada et al., 2006;[START_REF] Oikawa | Plutonium concentration and 240 Pu/ 239 Pu atomic ratio in liver of squid collected in the coastal sea areas of Japan[END_REF][START_REF] Yamada | 240 Pu/ 239 Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging[END_REF][START_REF] Yamada | Determination of 240 Pu/ 239 Pu atom ratio in coastal surface seawaters from the western North Pacific Ocean and Japan Sea[END_REF].

Seawater studies

Plutonium isotopes are effective as indicators for physical and biogeochemical processes in the water column. Plutonium in seawater is usually transported via advection, diffusion and particle scavenging. Many studies carried out in the open ocean water column have shown that Pu concentration depth profiles often show a pronounced sub-surface maximum (León [START_REF] Vintró | Transuranium nuclides in the world's oceans[END_REF]. As an example, Figure 4 is showing 239+240 Pu depth profiles from different sites in the North Pacific Ocean. It has been suggested that this sub-surface maximum is related to vertical fluxes of Pu-bearing particles and subsequent remineralisation by biological activity as well as physical circulation processes [START_REF] Fowler | Temporal variations in plutonium and americium inventories and their relation to vertical transport in the Northwestern Mediterranean Sea[END_REF][START_REF] Livingston | The behaviour of plutonium in the Pacific Ocean[END_REF].

Enhanced 239+240 Pu concentrations in deep bottom waters in the Pacific Ocean have been observed and this may be explained by rapid transport of Pu isotopes attached to large particles through the water column to deep ocean sediments and resuspension by near-bottom currents [START_REF] Livingston | Large particle transport of plutonium and other fallout radionuclides to the deep ocean[END_REF][START_REF] Yamada | 238 U, Th isotopes, 210 Pb and 239+240 Pu in settling particles on the continental margin of the East China Sea: fluxes and particle transport processes[END_REF]. In shallow and coastal waters, the concentration of Pu is low as it is removed rapidly from the water column by particulate material.

<Figure 4 here> Zheng and Yamada (2006b) has decreased by about one third from the amount estimated in 1988. This is due to the vertical transport of Pu and the consequent decrease of its concentration at intermediate depths (IAEA, 2005). Sediment trap studies have indicated that particles sinking through deep waters (1 000 -2 000 m) could account for 26 % to 72 % of the derived annual 239+240 Pu loss in the Mediterranean Sea [START_REF] Fowler | Temporal variations in plutonium and americium inventories and their relation to vertical transport in the Northwestern Mediterranean Sea[END_REF]. The advection of deep water masses to the south-western part of the Mediterranean Sea from the Levantine Basin in the eastern Mediterranean has also been identified from the depth distributions of 239+240 Pu and 238 Pu together with 137 Cs [START_REF] Noureddine | Radionuclide tracing of water masses and processes in the water column and sediment in the Algerian Basin[END_REF].

Concentrations of 239+240 Pu and 137 Cs in surface seawater, coupled with oceanographic parameters, i.e. salinity and nutrient concentrations, have been utilised to demonstrate the exchange of surface water masses between different sea areas in the South Pacific Ocean [START_REF] Hirose | Plutonium and 137 Cs in surface water of the South Pacific Ocean[END_REF]. By comparing the 239+240 Pu/ 137 Cs activity ratios in surface water with the global fallout ratio they found good correlations between 239+240 Pu concentrations and particle scavenging rates, which provided additional evidence to support the suggested exchange processes. A similar approach was applied to vertical profiles of the 239+240 Pu/ 137 Cs activity ratios in shallow waters of the North Pacific [START_REF] Hirose | 239,240 Pu/ 137 Cs ratios in the water column of the North Pacific: a proxy of biogeochemical processes[END_REF]. Depth profiles of 239+240 Pu/ 137 Cs were expressed by a simple exponential function reflecting export and regeneration processes.

In addition, speciation studies have shown that the ratio between oxidised and reduced forms of Pu may be a valuable tracer for lateral water movement processes (Baxter et al., 1995). The 238 Pu/ 239+240 Pu activity ratio and Pu oxidation state ratio in the water column of two Norwegian Fjords were used as a tracer for water exchange between the fjords and the North Sea [START_REF] Sanchez | Distribution of plutonium and its oxidation states in Framvaren and Hellvik fjords, Norway[END_REF].

Sediment studies

Marine sediments are major repositories of organic matter and pollutants that are embedded in the sediment layers. Understanding the dispersion and fate of these contaminants is of great importance due to their influence on global biogeochemical cycles and marine processes [START_REF] Hedges | Sedimentary organic matter preservation: an assessment and speculative synthesis[END_REF].

There have been a large number of studies of Pu in marine sediments, including the dynamics of mixing processes and accumulation rates. Sediment studies usually include Pu isotopes in combination with other radionuclides or stable isotopes (e.g. [START_REF] Krishnaswami | Atmosphericallyderived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: evidence from 7 Be, 210 Pb, and 239,240 Pu[END_REF][START_REF] Santschi | Pu in coastal marine environments[END_REF][START_REF] Benninger | Sedimentary processes in the inner New York Bight: Evidence from excess 210 Pb and 239,240 Pu[END_REF][START_REF] Cochran | Particle mixing rates in sediments of the eastern equatorial Pacific: evidence from 210 Pb, 239,240 Pu and 137 Cs distributions at MANOP sites[END_REF][START_REF] Smith | Plutonium and 210 Pb distributions in northeast Atlantic sediments: subsurface anomalies caused by non-local mixing[END_REF][START_REF] Carpenter | Cycling of fallout (Pu, 241 Am, 137 Cs) and natural (U, Th, 210 Pb) radionuclides in Washington continental slope sediments[END_REF][START_REF] Lapicque | Interpretation of 239,240Pu in Atlantic sediments with a non-steady state input model[END_REF][START_REF] Buffoni | Accumulation rates and mixing processes in nearsurface North Atlantic sediments: Evidence from C-14 and Pu-239,240 downcore profiles[END_REF][START_REF] Huh | Natural radionuclides and plutonium in sediments from the western Arctic Ocean: sedimentation rates and pathways of radionuclides[END_REF][START_REF] Huh | Sedimentation dynamics in the East China Sea elucidated from 210 Pb, 137 Cs and 239,240 Pu[END_REF]Smith et al., 2000b;[START_REF] Gascó | Distributions of Pu, Am and Cs in margin sediments from the western Mediterranean (Spanish coast)[END_REF][START_REF] Su | 210 Pb, 137 Cs and 239,240 Pu in East China Sea sediments: sources, pathways and budgets of sediments and radionuclides[END_REF][START_REF] Moon | Accumulation of anthropogenic and natural radionuclides in bottom sediments of the Northwest Pacific Ocean[END_REF][START_REF] Garcia-Orellana | Distribution of artificial radionuclides in deep sediments of the Mediterranean Sea[END_REF]. Knowledge of particle mixing rates in sediments is essential for deconvoluting different depth-dependent sediment processes that could influence the distribution of chemical pollutants. The various Pu studies in marine sediments have contributed to developing sediment mixing models with constant, continuous, or pulsed Pu inputs [START_REF] Smith | Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya[END_REF]Smith et al., 2000b;[START_REF] Moon | Accumulation of anthropogenic and natural radionuclides in bottom sediments of the Northwest Pacific Ocean[END_REF].

The deposition of Pu in shelf or slope environments is controlled by the extent of scavenging in the upper water layers. Hence, the depth distribution of Pu is often a measure of historical Pu-flux in the water column. In areas with negligible mixing compared to sediment accumulation, global fallout 239+240 Pu (with distinct peak at around 1964), in combination with the well-established 210 Pb dating methodology, in sediment cores can be used to estimate the past flux of particulate material. For example, high particle flux events were observed in the continental margin of the East China Sea by determining 239+240 Pu and natural radionuclides in settling particles and surface sediment [START_REF] Yamada | Large particle flux of 239+240 Pu on the continental margin of the East China Sea[END_REF][START_REF] Choppin | Actinide speciation in aquatic systems[END_REF]. These high flux events were attributed to episodic lateral transport of resuspended particles that flow down the continental slope with the nepheloid layer.

Studies of Pu and natural radionuclides association with particulate matter in the ocean have considerably advanced understanding of the ocean carbon cycle and ocean particle mixing [START_REF] Bacon | Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea. Deep Sea Research Part A[END_REF]. The sinking of Pu-bearing particles (Pu-flux) in the upper euphotic zone is closely coupled with the carbon flux [START_REF] Baxter | Observations on plutonium in the oceans[END_REF] and scavenging of trace metals [START_REF] Hirose | Plutonium and 137 Cs in the western North Pacific: estimation of residence time of plutonium in surface waters[END_REF]. The techniques developed for the study of particle associated Pu in the ocean were used in the Joint Global Ocean Flux Study (JGOFS) [START_REF] Livingston | A millennium perspective on the contribution of global fallout radionuclides to ocean science[END_REF]. Plutonium isotopes have also been used at a seasonal polynia off the northeast coast of Greenland to evaluate the carbon cycle and to improve the estimation of the particulate organic carbon flux. In this study, the 239+240 Pu depth distribution in sediment was used as an indicator of particle mixing, with accumulation rates estimated from 210 Pb [START_REF] Roberts | 210 Pb and 239,240 Pu in the Northeast Water Polynya, Greenland: particle dynamics and sediment mixing rates[END_REF].

Distribution and transport of particulate matter in riverine inputs to coastal waters can be estimated by using Pu isotopes in sediments contaminated with various identified sources such as global fallout and nuclear facilities linked to the river [START_REF] Olsen | Plutonium, lead-210, and carbon isotopes in the Savannah estuary: riverborne versus marine sources[END_REF][START_REF] Thomas | Input of artificial radionuclides to the Gulf of Lions and tracing the Rhône influence in marine surface sediments[END_REF][START_REF] Eyrolle | Plutonium isotopes in the lower reaches of the River Rhône over the period 1945-2000: fluxes towards the Mediterranean Sea and sedimentary inventories[END_REF][START_REF] Miralles | Multitracer study of anthropogenic contamination records in the Camargue, Southern France[END_REF][START_REF] Duffa | 238Pu and 239+240Pu inventory and distribution through the lower Rhone valley terrestrial environment (Southern France)[END_REF][START_REF] Lansard | Spatial and temporal variations of plutonium isotopes ( 238 Pu and 239,240 Pu) in sediments off the Rhone River mouth (NW Mediterranean)[END_REF]. For example, [START_REF] Oktay | The 129 Iodine bomb pulse recorded in Mississippi River Delta sediments: results from isotopes of I, Pu, Cs, Pb, and C[END_REF] found a five-fold decrease in riverine particle discharges into the Gulf of Mexico from the Mississippi River prior to the early 1960, by applying Pu isotopes in combination with excess 210 Pb in determination of an 129 I chronology record.

Another approach for dating of deep-sea sediments may be the use of 241 Pu and from ingrowth of241 Pu [START_REF] Lee | Distribution and inventories of 90 Sr, 137 Cs, 241 Am and Pu isotopes in sediments of the Northwest Pacific Ocean[END_REF].

Retrospective studies

In research on global climate change, retrospective studies on past ocean conditions are based mainly on isotopic analyses of natural archives in the marine environment, including sediments, corals and other marine biota. These natural archives (proxies) provide information regarding historical chemical and physical conditions in the surrounding seawater. Isotopic analyses of annual bands from corals has provided sensitive time series of past ocean chemistry, ocean circulation and climate [START_REF] Druffel | Geochemistry of corals: proxies of past ocean chemistry, ocean circulation, and climate[END_REF]. As the Pu isotopic composition differs from each source, retrospective studies of Pu may provide valuable information on past ocean circulation and aid understanding of the biogeochemical cycles of oceanic carbon, particle-reactive nutrients and pollutants.

Dated coral cores have been used to retrieve historical records of Pu isotopes from global and close-in fallout in the surrounding seawater and thereby provide information about source terms, water mass movement, and the geochemical behaviour of Pu in the marine environment [START_REF] Noshkin | Transuranics and other radionuclides in Bikini Lagoon: concentration data retrieved from aged coral sections[END_REF][START_REF] Benninger | Fallout plutonium and natural radionuclides in annual bands of the coral Montastrea annularis, St. Croix, U.S. Virgin Islands[END_REF][START_REF] Purdy | Anomalous levels of 90 Sr and 239,240 Pu in Florida corals: evidence of coastal processes[END_REF][START_REF] Buesseler | The isotopic signature of fallout plutonium in the North Pacific[END_REF]. Plutonium concentrations in accumulating corals are in equilibrium with surrounding water and the determination of Pu in corals provides a better annual resolution than sediment due to better accumulation rates with distinct seasonal features and negligible mixing between the growth layers.

In onwards by the Kuroshio Current up to the Northwest Pacific Ocean and its marginal seas [START_REF] Kim | Plutonium isotopes in seas around the Korean Peninsula[END_REF][START_REF] Zheng | Sediment core record of global fallout and Bikini close-in fallout Pu in Sagami Bay, western Northwest Pacific margin[END_REF][START_REF] Lee | Distribution and inventories of 90 Sr, 137 Cs, 241 Am and Pu isotopes in sediments of the Northwest Pacific Ocean[END_REF][START_REF] Wang | Plutonium activities and 240 Pu/ 239 Pu atom ratios in sediment cores from the east China sea and Okinawa Trough: sources and inventories[END_REF]Zheng and Yamada, 2005b;Zheng and Yamada, 2006a;[START_REF] Yamada | 240 Pu/ 239 Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging[END_REF][START_REF] Yamada | Determination of 240 Pu/ 239 Pu atom ratio in coastal surface seawaters from the western North Pacific Ocean and Japan Sea[END_REF]. This hypothesis was recently proposed from elevated 240 Pu/ 239 Pu atom ratios in squid liver samples collected from the western and eastern coastal areas of Japan [START_REF] Kishimoto | Concentration of plutonium in squids collected from Japanese inshore measured by HR-ICP-MS, 1981-2000[END_REF][START_REF] Oikawa | Plutonium concentration and 240 Pu/ 239 Pu atomic ratio in liver of squid collected in the coastal sea areas of Japan[END_REF]. The high 240 Pu/ 239 Pu atom ratios (> 0.20) observed in these studies indicate a contribution from the Marshall Islands source since the tropospheric Pu fallout from the Marshall Islands tests can be assumed to be negligible in these waters [START_REF] Huh | 240 Pu/ 239 Pu ratios in Sun Moon lake sediments: implications for sources of nuclear fallout in Taiwan[END_REF]Zheng and Yamada, 2005a). Figure 5 shows a time series of 240 Pu/ 239 Pu atom ratios in sediment and coral from three sites in the Pacific Ocean. There are indications that the flow of the Kuroshio Current has been disrupted due to global climate change [START_REF] Sato | Response of North Pacific Ocean circulation in a Kuroshio-resolving ocean model to an Arctic Oscillation (AO)like change in northern hemisphere atmospheric circulation due to greenhouse-gas forcing[END_REF]. Therefore, by tracking the distinct Pu signal from the Marshall Islands in this area, past current flows can be retrospectively studied and the potential coupling with recent climate change can be elucidated.

<Figure 5 here>

The surface current transit time from the Marshall Islands to the marginal areas of the Northwest Pacific was estimated to be six years by analysing 239+240 Pu and the 240 Pu/ 239 Pu atom ratio in sediments from the Southern Okinawa Trough [START_REF] Lee | Sedimentation in the Southern Okinawa Trough: enhanced particle scavenging and teleconnection between the Equatorial Pacific and western Pacific margins[END_REF]. From this estimation, the mean velocity of NEC was calculated to be ~0.022 m s -1 , which corresponds well with the annual mean velocity, which is in the range 0.02 -0.06 m s -1 .

Another potential proxy for historical Pu records is the use of dated mollusc shells that contain calcium carbonate. For example, [START_REF] Baskaran | Reconstructing seawater column 90 Sr based upon 210 Pb/ 226 Ra disequilibrium dating of mollusk shells[END_REF] used archived bivalve shells for reconstructing historical global fallout of 90 Sr levels in shallow coastal waters.

Plutonium isotopes and 129 I in archived marine algae have been utilised as historical indicators for the transport of seawater contaminated with radioactive waste from nuclear fuel reprocessing plants to the North Atlantic and Arctic Ocean [START_REF] Cooper | Iodine-129 and plutonium isotopes in Arctic kelp as historical indicators of transport of nuclear fuel-reprocessing wastes from mid-to-high latitudes in the Atlantic Ocean[END_REF].

Other applications of Pu isotopes in the marine environment

Sea ice transport is an important factor for the long range dispersal of contaminants in the Arctic Ocean and into the North Atlantic because melting releases contaminants from the ice [START_REF] Pfirman | The potential transport of pollutants by Arctic sea ice[END_REF]. Plutonium isotopes ( 239 Pu and 240 Pu), together with other natural and anthropogenic radionuclides, have been used to track sea ice transport through the Arctic Ocean and to estimate transit times from the source to the removal area [START_REF] Landa | Transport of 137 Cs and 239,240 Pu with ice-drafted debris in the Arctic Ocean[END_REF][START_REF] Masqué | The role of sea ice in the fate of contaminants in the Arctic Ocean: plutonium atom ratios in the Fram Strait[END_REF][START_REF] Masqué | Radionuclides in Arctic sea ice: tracers of sources, fates and ice transit time scales[END_REF].

Atmospheric inputs of mineral aerosol into the ocean affect biogeochemical processes in the water column. A method for estimating the mineral aerosol flux to the Pacific Ocean using 239+240 Pu as a tracer was developed by utilising 239+240 Pu concentrations in surface waters and airborne dust together with Pu residence time and mixed layer depth [START_REF] Nakanishi | Estimation of mineral aerosol fluxes to the Pacific by using environmental plutonium as a tracer[END_REF].

There is an increasing demand for validated regional and global marine ecosystem models, as well as ocean circulation models for the prediction of future global environmental changes, and the unique chemical properties of Pu, together with its well-defined source terms, will provide important information for the evaluation and improvement of these models.

Conclusions

The overall input of Pu to the World Ocean has declined significantly in the last four decades, mainly due to the cessation of atmospheric weapons tests. However, due to the long half-life and unique chemical properties of Pu, it will persist in the marine environment for a long time.

Due to the well-defined source terms, Pu isotopes can be utilised in marine research for the study of oceanic processes such as water mass movement, particle export and sediment mixing and accumulation rates. Plutonium isotope data can be used for calibrating and validating numerical models for pollutant dispersion in the marine environment and ocean current circulation (Smith et al., 2000a;[START_REF] Nakano | Modelling the distribution of plutonium in the Pacific Ocean[END_REF][START_REF] Tsumune | Numerical simulation of 137 Cs and 239,240 Pu concentrations by an ocean general circulation model[END_REF]. In view of the increasing concerns relating to global climate change and its impact on the marine environment, the historical Pu signal in natural archives (e.g. sediments and biota) will enhance our understanding of important biogeochemical and physical processes in the ocean.

Reconstructed Pu signatures in natural archive marine samples will also provide valuable information on the impact of past climate change on ocean mixing and support the validation and calibration of ocean circulation models. , 1998b;[START_REF] Chiappini | 240 Pu/ 239 Pu isotopic ratios and 239+240 Pu total measurements in surface and deep waters around Mururoa and Fangataufa atolls compared with Rangiroa atoll (French Polynesia)[END_REF][START_REF] Mulsow | Radionuclide concentrations in underground waters of Mururoa and Fangataufa Atolls[END_REF][START_REF] Danesi | Residual radioactivity in the terrestrial environment of the Mururoa and Fangataufa Atolls nuclear weapon test sites[END_REF][START_REF] Hrnecek | Determination of plutonium in environmental samples by AMS and alpha spectrometry[END_REF]) Nevada Test Site 0.032 ± 0.006 [START_REF] Hicks | Nevada test site fallout atom ratios[END_REF]) Semipalatinsk 0.038 ± 0.016 0.037 ± 0.002 [START_REF] Beasley | Isotopic Pu, U, and Np signatures in soils from Semipalatinsk-21, Kazakh Republic and the Southern Urals, Russia[END_REF][START_REF] Yamamoto | Distributions of Pu isotopes and 137 Cs in soil from Semipalatinsk Nuclear Test Site detonations throughout southern districts[END_REF]) Chernaya Bay, underwater nuclear tests 0.0320 ± 0.0024 0.023 ± 0.009 [START_REF] Smith | Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya[END_REF]Smith et al., 2000b) Releases from the nuclear industry Sellafield (up to 1996) 0.06 -0.25 0.03 -0.4 [START_REF] Kershaw | A sediment history of Sellafield discharges[END_REF]Kershaw et al., 1995a;[START_REF] Morris | Evidence for the remobilization of Sellafield waste radionuclides in an intertidal salt marsh, West Cumbria, U.K. Estuarine[END_REF] La Hague effluent, 1996 0.34 ± 0.06 [START_REF] Oughton | Determination of 240 Pu/ 239 Pu isotope ratios in Kara Sea and Novaya Zemlya sediments using Accelerator Mass Spectrometry[END_REF]) Russian river discharges from nuclear reprocessing plants at Krasnoyarsk, Mayak, Tomsk 0.05 -0.30 [START_REF] Oughton | Plutonium from Mayak: measurement of isotope ratios and activities using Accelerator Mass Spectrometry[END_REF][START_REF] Skipperud | Plutonium isotope ratios in the Yenisey and Ob estuaries[END_REF][START_REF] Børretzen | Pu and U atom ratios and concentration factors in Reservoir 11 and Asanov Swamp, Mayak PA: an application of Accelerator Mass Spectrometry[END_REF][START_REF] Lind | Transport of low 240 Pu/ 239 Pu atom ratio plutonium-species in the Ob and Yenisey Rivers to the Kara Sea[END_REF][START_REF] Gauthier-Lafaye | Radioisotope contaminations from releases of the Tomsk-Seversk nuclear facility (Siberia, Russia)[END_REF][START_REF] Skipperud | Association of plutonium with sediments from the Ob and Yenisey Rivers and Estuaries[END_REF] Accidental releases Chernobyl accident, 1986 0.38 ± 0.07 0.42 ± 0.04 [START_REF] Boulyga | Determination of isotopic composition of plutonium in hot particles of the Chernobyl area[END_REF][START_REF] Wendt | Recent developments in and applications of resonance ionization mass spectrometry[END_REF][START_REF] Muramatsu | Concentrations of 239 Pu and 240 Pu and their isotopic ratios determined by ICP-MS in soils collected from the Chernobyl 30-km zone[END_REF][START_REF] Boulyga | Isotopic analysis of uranium and plutonium using ICP-MS and estimation of burn-up of spent uranium in contaminated environmental samples[END_REF]) Thule accident, 1968 0.055 ± 0.008 0.0161 ± 0.0010 [START_REF] Lind | Characterization of uranium and plutonium containing particles originating from the nuclear weapons accident in Thule, Greenland[END_REF][START_REF] Eriksson | U, Pu, and Am nuclear signatures of the Thule hydrogen bomb debris[END_REF]) Palomares accident, 1966 0.063 ± 0.006 0.015 ± 0.004 [START_REF] Chamizo | Determination of the 240 Pu/ 239 Pu atomic ratio in soils from Palomares (Spain) by low-energy accelerator mass spectrometry[END_REF][START_REF] Pöllänen | Multi-technique characterization of a nuclear bomb particle from the Palomares accident[END_REF][START_REF] Jiménez-Ramos | On the presence of enriched amounts of 235 U in hot particles from the terrestrial area affected by the Palomares accident (Spain)[END_REF]) SNAP- 9A, 1964 N Hemisphere S Hemisphere --0.010 ± 0.005 0.12 ± 0.07 [START_REF] Perkins | Worldwide fallout[END_REF] Dumping sites Kara Sea ~0.44 ~0.18 (IAEA, 1998a) Sunken submarine reactors 0.005 -0.13 0.12 -0.45 [START_REF] Oughton | Accelerator mass spectrometry measurement of 240 Pu/ 239 Pu isotope ratios in Novaya Zemlya and Kara Sea sediments[END_REF] * Decay corrected to the year 2000. , 1945[START_REF] Dobry | Transuranic elements in space nuclear power systems[END_REF][START_REF] Aarkrog | Input of anthropogenic radionuclides into the World Ocean[END_REF][START_REF] Hamilton | Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century[END_REF]. 238 , 1952[START_REF] Bnfl | Discharges and Monitoring in the United Kingdom[END_REF][START_REF] Gray | Discharges to the environment from the Sellafield site, 1951-1992[END_REF][START_REF] Bnfl | Discharges and Monitoring of the Environment in the UK[END_REF][START_REF] Bnfl | Discharges and Monitoring in the United Kingdom[END_REF] and 240 Pu/ 239 Pu atom ratios from dated sediment cores collected in the vicinity of the Sellafield plant (Kershaw et al., 1995a;[START_REF] Morris | Evidence for the remobilization of Sellafield waste radionuclides in an intertidal salt marsh, West Cumbria, U.K. Estuarine[END_REF]. [START_REF] Bowen | Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions, largely from GEOSECS stations[END_REF][START_REF] Nelson | Plutonium oxidation state distributions in the Pacific Ocean during 1980-1981[END_REF][START_REF] Nozaki | The distribution of radionuclides and some trace metals in the water columns of the Japan and Bonin trenches[END_REF][START_REF] Yamada | 240 Pu/ 239 Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging[END_REF].

Figure 5. Time series of 240 Pu/ 239 Pu atom ratio in three sites in the Pacific Ocean [START_REF] Buesseler | The isotopic signature of fallout plutonium in the North Pacific[END_REF][START_REF] Lee | Sedimentation in the Southern Okinawa Trough: enhanced particle scavenging and teleconnection between the Equatorial Pacific and western Pacific margins[END_REF][START_REF] Zheng | Sediment core record of global fallout and Bikini close-in fallout Pu in Sagami Bay, western Northwest Pacific margin[END_REF]. The sedimentation rate for the Sagami Bay site was estimated from the global fallout 239+240 Pu peak.

ACCEPTED MANUSCRIPT

  observed an increase in Pu flux with increased depth in the continental margins of Northwest Pacific by determining Pu concentrations in settling particles from sediment traps. They also found, from240 Pu/ 239 Pu atom ratio analyses, that dissolved Pu is transported from the open ocean to ocean margins where Pu is removed into the margin sediments by scavenging. Scavenging has also been found to have a profound effect on the net annual outflow of239+240 Pu from the Mediterranean Sea to the Atlantic, which

  the Pacific Ocean, the North Equatorial Current (NEC) and the Kuroshio Current are important currents that supply the Northwest Pacific with warm surface water from the Equatorial Pacific. The Marshall Islands, located in the path of NEC, continuously releases about 0.2 TBq of 239+240 Pu annually from contaminated sediment into the open ocean (see Section 2.1). By determining 240 Pu/ 239 Pu atom ratios in sediment and seawater it has been suggested that Pu from the Marshall Islands is transported by the westward-flowing NEC and

Figure captions Figure 1 .

 captions1 Figure captions Figure 1. Total annual yield energies (Mt per year) and annual fallout debris deposition (from 90 Sr deposition, PBq per year) in the northern and southern hemispheres produced from

Figure 2 .

 2 Figure 2. 239+240 Pu discharges from the Sellafield reprocessing plant, 1952, -2007 (Gray et al., (Gray et al.,

Figure 3 .

 3 Figure 3. Biogochemical cycle of Pu in the marine environment. Adapted from Choppin and

Figure 4 .

 4 Figure 4. 239+240 Pu depth profiles in seawater from various stations in the North Pacific Ocean

Table 1 .

 1 Principal sources in the marine environment and their specific Pu isotopic composition expressed as240 Pu/ 239 Pu atom and238 Pu/ 239+240 Pu activity ratios. Uncertainties are expressed as ± 2 SD (95 % confidence level).

	Main sources	240 Pu/ 239 Pu atom ratio	238 Pu/ 239+240 Pu activity ratio * References
	Atmospheric weapons testing, 1945-1980
	Integrated global fallout	0.178 ± 0.023	0.019 ± 0.006	(Perkins and Thomas, 1980;
				Kelley et al., 1999)
	Specific test sites
	Marshall Islands	0.29 ± 0.04		(Muramatsu et al., 2001)
	Johnston Atoll	~0.06		(Wolf et al., 1997)
	French Polynesia	0.02 -0.05	0.1 -0.4	(IAEA

Table 2 .

 2 Estimated Pu isotope inventory (PBq) in the world's oceans originating from atmospheric nuclear weapons tests

  Pu a 239 Pu 240 Pu 241 Pu a 242 Pu

	Arctic Ocean	0.002	0.05	0.04	0.17	0.00001
	Atlantic Ocean	0.05	1.38	0.89	4.24	0.0003
	North Atlantic	0.04	1.1	0.7	3.4	0.00023
	South Atlantic	0.01	0.3	0.2	0.9	0.00006
	Indian Ocean	0.02	0.56	0.36	1.73	0.0002
	Pacific Ocean b	0.35	4.47	3.98	23.89	0.003
	North Pacific	0.17	2.7	2.3	13.5	0.0015
	South Pacific	0.07	1.8	1.7	10.4	0.0015
	Southern Ocean	0.005	0.07	0.05	0.24	0.00006
	All Oceans	0.4	6.5	5.4	30	0.004

a Decay corrected to 1 January 2000. b The Mike shot assumed evenly distributed between North and South Pacific.

Am concentrations in the sediment layers with the assumption that all 241 Am is produced

Acknowledgements

This work was financially supported under the European Commission 7 th Framework, Marie Curie International Outgoing Fellowship Research Project PIOF-GA-2008-219625 -PLUTOTRACE‖.