Introduction

The ocean plays a central role in the regulation of Earth’s climate. It absorbs more than 90% of excess heat associated with greenhouse-gas forcing and takes up a substantial fraction of anthropogenic CO₂ emissions, buffering atmospheric warming while redistributing heat, carbon, freshwater and nutrients within and between basins1,2. Detecting how ocean circulation is changing is therefore a core challenge in climate science. However, direct observations of ocean transport and mixing remain difficult because the ocean is a slow-moving body with a vast surface and sparsely sampled over climate-relevant timescales3,4,5.

Isotopic and chemical tracers provide an alternative approach: substances with known sources and predictable decay can be used to infer rates of ocean ventilation, mixing and water-mass exchange. Among the most useful are anthropogenic radionuclides, including tritium (3H), carbon-14 (14C), cesium-137 (137Cs) and plutonium isotopes6,7,8 (Supplementary Table 1). These radionuclides were injected into the environment primarily as global fallout from atmospheric nuclear weapons testing in the 1950s-1960s, with additional inputs from nuclear accidents (for example, Chornobyl 1986 and Fukushima 2011) and controlled releases from nuclear reprocessing facilities (notably Sellafield in the UK and La Hague in France)9,10,11. Global fallout was the dominant oceanic source and contaminated the world ocean at the basin scale12,13 (Supplementary Fig. 1).

After deposition, radionuclides are transported by ocean currents and mixing, while also interacting with marine particles, biota and sediments, and their distributions depend on aqueous chemistry. We shall focus in this paper on 3H and 137Cs, which behave largely conservatively in seawater, while plutonium (Pu) is strongly particle-reactive and undergoes scavenging and regeneration in the water column14,15. Because their delivery histories are well documented, radionuclides act as time markers that can quantify water-mass movement and residence times on decadal, centennial and millennial timescales, and have been widely used for dating seawater and sediments, and for process studies in the ocean interior16,17.

As the global ocean is too vast to be investigated by small-scale institutional projects alone, large-scale international programs have been essential for establishing global radionuclide datasets, which contributed significantly to the advancement of ocean science. These included GEOSECS12, WOCE18, WOMARS19, SHOTS20 and related programs. GEOTRACES3 is the largest worldwide project carried out at present on global marine biogeochemistry and isotope oceanography21. Marine radionuclide data have been compiled in repositories, including the IAEA’s Marine Radioactivity Information System (MARIS; https://maris.iaea.org)22 and the University of Tsukuba ERAN database (https://eran-database.jp/list/00085.html)23.

The Pacific Ocean received a large fraction of global fallout and has been repeatedly sampled for more than five decades6,7,12,14,20,21,24,25,26,27,28,29,30,31,32,33,34,35,36,37. Therefore, it provides an ideal region to assess whether and how circulation fingerprints in radionuclide distributions have evolved under climate forcing. In this Review, we focus on 3H, 137Cs, 239Pu and 240Pu, synthesizing how observations, combined with ocean circulation and scavenging models, constrain water-mass transport, vertical mixing, inter-basin exchange and the strength of the global meridional overturning circulation (GMOC).

The global meridional overturning circulation

The GMOC, often described as the global conveyor belt, is a large-scale ocean circulation system that connects the Arctic, Atlantic, Indian, and Pacific Oceans through the Southern Ocean38,39. It represents the global-scale system of currents that connects surface and deep waters across ocean basins, redistributing heat, carbon, freshwater and nutrients. It is sustained by density differences (driven by air–sea heat and freshwater fluxes) and interior mixing, operating on timescales from decades to millennia. Warming heats the surface ocean, which enhances vertical stratification. and consequently, can modify surface circulation. contributing to a gradual weakening of the GMOC since the 1960s1,2,40,41,42,43,44. Because the GMOC is difficult to investigate directly, as it operates slowly and varies on multidecadal timescales, independent tracers are required to constrain changes in its strength and pathways5,40,41,45,46,47. Although typical flow speeds are small ( ~ 1 cm s−1), the integrated transport is enormous (roughly 30 times the total global river discharge) and strongly influences regional climate, sea level and marine ecosystems by redistributing heat and carbon39,42,48,49. For example, northward heat transport by the Atlantic circulation (including the Gulf Stream system) helps keep NW Europe several degrees warmer than it would otherwise be43.

Anthropogenic radionuclides are well suited for this purpose: their input histories provide time markers, and their distributions respond sensitively to changes in transport, mixing, and ventilation50,51 (Box 1).

Why are radionuclides sensitive indicators of ocean circulation change?

In the Pacific, major surface currents include the subtropical gyres in both hemispheres and the western boundary currents (notably the Kuroshio and its extension) that rapidly transport surface waters eastward (Supplementary Fig. 2). Radionuclide concentrations vary laterally and vertically because labeled waters are continually advected, mixed, and transformed. Since the end of the global fallout peak in the 1960s, ocean radionuclide inventories have generally declined by radioactive decay and export, but deviations from simple decay curves reflect changes in water exchange and mixing52.

To quantify these effects, the apparent environmental half-life (Tap) is used to distinguish physical removal from radioactive decay (Supplementary Fig. 3). A Tap shorter than the radioactive decay half-life indicates rapid export via advection and mixing (in the absence of additional input), while a longer Tap indicates slower ventilation and/or accumulation. Temporal changes in Tap at fixed locations, especially when based on multidecadal time series, can indicate changing ventilation and transport. The impacts of climate can be inferred from changes in radionuclide concentrations and inventories at each site. Therefore, long-term time series of radionuclide concentrations provide valuable indicators of changes in water exchange rates and climate-driven circulation53,54.

Importantly, different radionuclides respond to different processes: water-soluble tracers such as 3H and 137Cs tend to decrease rapidly with depth, while particle-reactive plutonium isotopes exhibit subsurface/intermediate maxima driven by scavenging and regeneration15,55,56. Changes in stratification, ventilation, and particle cycling therefore leave distinct, interpretable fingerprints in their vertical profiles.

Anthropogenic radionuclides in Pacific Ocean waters

Tritium (³H)

Atmospheric ³H rapidly exchanges with water vapor and is incorporated into seawater as tritiated water (HTO), where it behaves conservatively as part of the water mass. Vertical stratification arises from density contrasts between warm surface waters ( ~ 25 °C in the tropics) and cold deep waters ( ~ 1 °C). The intervening thermocline (approximately 70–1000 m) is characterized by strong temperature gradients that suppress vertical mixing of heat and dissolved substances. Therefore, strengthening stratification under warming affects both the penetration of surface tracers into the interior and the rate at which deep waters ventilate. Stratification in the upper 200 m has increased by ~5.3% since 1960 across the major ocean basins41. The apparent environmental half-life of ³H in surface waters (0–25 m) was 12.7–13.7 years (comparable with its radioactive decay half-life of 12.33 years) in the northern subtropical and tropical bands (60°N–0°), and 11.8–19.0 years in the southern bands (0°–40°S), indicating latitudinal differences in ventilation and transport34.

Repeated full-depth ³H sections measured along ~150°W in 1991, 2006 and 2015 provide a basis for assessing multidecadal changes in tracer penetration, inventories, and implied ventilation57,58,59 (Fig. 1). These sections show persistent surface maxima at mid-latitudes and in the Southern Hemisphere tropics, separated by a relative minimum associated with the North Equatorial Current region. Penetration depths increased between the 1970s and early 2000s from ~1000 to ~1200 m in the Northern Hemisphere and from ~900 to ~1000 m in the Southern Hemisphere, reaching ~1500 and ~1300 m, respectively, by 2015. Enhanced stratification contributed to this deepening by warming the mixed layer and reducing the upwelling of dense, ³H-poor deep waters.

Fig. 1: Comparison of vertical ³H distributions along the 150°W meridian in 1991, 2006 and 2015 in the Pacific Ocean.
Fig. 1: Comparison of vertical ³H distributions along the 150°W meridian in 1991, 2006 and 2015 in the Pacific Ocean.The alternative text for this image may have been generated using AI.
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Temporal changes in ³H inventories are shown for subsections between 50°N–20°N and 7°N–20°S. Two surface maxima were observed near 35°N and 7°S, separated by a minimum in the North Equatorial Current region. One tritium unit (TU) is defined as one tritium atom per 1018 hydrogen atoms in water, equivalent to 0.119 Bq L-1. 3H data from ref. 18 were used in calculations. Color mapping was performed using the DIVA gridding function of Ocean Data View (Schlitzer, R., 2024. Ocean Data View; https://odv.awi.de).

Tritium inventories declined by 4.4% yr-1 and 3.7% yr−1 from 1991 to 2006, and by 3.7% yr−1 and 1.4% yr−1 from 2006 to 2015 in the northern and southern sections, respectively. The pronounced slowdown in the decline of 3H inventory occurred in the tropical Pacific, consistent with reduced ventilation and/or altered circulation pathways in a more stratified upper ocean60.

Although recent releases of ALPS (Advanced Liquid Processing System) treated water from the Fukushima Daiichi Nuclear Power Plant (F1NPP) have attracted attention, the discharge rate is very small compared to the residual global fallout inventory and remains largely confined to coastal regions61. As fallout-derived ³H continues to decay, contributions from regional nuclear facilities may become more detectable. Surface ³H concentrations in the northern and equatorial Pacific are projected to approach natural background levels (0.1–0.2 TU) by ~2045 (ref 62.) (one tritium unit, TU, is defined as one tritium atom per 1018 hydrogen atoms in water, equivalent to 0.119 Bq L−1). Continued observations are needed to assess how ongoing climate-driven changes in circulation influence the distribution of the 3H tracer in the Pacific Ocean.

Cesium−137 (137Cs)

137Cs is among the most widely studied marine radionuclides due to its well-characterized source history, radiological relevance, half-life (30.17 years), and relatively straightforward analysis. In seawater, cesium occurs both in the dissolved form and as attached to particles. However, due to the very low concentrations of suspended material and biomass in open ocean seawater (5–100 μg L-1), the vast majority ( > 95%) of 137Cs exists in the dissolved phase63. In coastal regions, the particulate fraction is considerably higher, commonly reaching 5–15% and occasionally exceeding 30% in areas with elevated suspended loads. As a result, 137Cs is largely transported by physical processes (advection and mixing). Vertical profiles typically show low surface concentrations, a subsurface maximum within a few hundred meters, and low concentrations below the thermocline (around 1 km).

Although 137Cs activity concentrations in the NW Pacific decreased exponentially in surface waters, decadal apparent environmental half-life (Tap) values varied over the last five decades. The longer Tap observed in the 2000s likely reflects a climate-driven slowdown of regional circulation associated with climate change, as no significant new 137Cs sources were introduced in the NW Pacific Ocean. (Supplementary Fig. 4).

The distribution of 137Cs from atmospheric nuclear weapons testing and later nuclear accidents has evolved in complex ways across the Pacific Ocean. Surface water measurements from 1960 to 2020 reveal strong spatial and temporal variability, which captured inputs mainly during the 1960s from global fallout and during 2011 from the F1NPP accident. Contributions from European and Japanese nuclear reprocessing plants and the Chornobyl accidents were in the NW Pacific Ocean negligible64 (Fig. 2 and Supplementary Fig. 5). The availability of dense observational datasets, combined with well-constrained estimates of radioactive inputs, has allowed the development and validation of ocean general circulation models (OGCMs)62,65,66,67,68,69. These models reconstruct how 137Cs spread through the ocean (Fig. 3) and demonstrate its value as a tracer of large-scale ocean circulation.

Fig. 2: 137Cs activity concentrations in the world ocean by decade (observed values as colored circles; modeled decadal means as contours).
Fig. 2: 137Cs activity concentrations in the world ocean by decade (observed values as colored circles; modeled decadal means as contours).The alternative text for this image may have been generated using AI.
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By the 1980s, the 137Cs concentration became more uniform throughout the ocean, but the number of observations decreased until the F1NPP accident in March 2011. This accident triggered many expeditions in the North Pacific. After 2020 the accident influence is largely confined to regions adjacent to the F1NPP. Data from the University of Tsukuba ERAN database23,81 (https://eran-database.jp/list/00085.html) and from the IAEA MARIS database (https://maris.iaea.org). The global maps were produced in Python using Matplotlib and Cartopy (https://scitools.org.uk).

Fig. 3: ¹³⁷Cs profiles across the South Pacific (30°S), Indian (20°S) and Atlantic (30°S) Oceans during the BEAGLE (Blue Ocean Global Expedition (2003–2004).
Fig. 3: ¹³⁷Cs profiles across the South Pacific (30°S), Indian (20°S) and Atlantic (30°S) Oceans during the BEAGLE (Blue Ocean Global Expedition (2003–2004).The alternative text for this image may have been generated using AI.
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The Indian Ocean exhibits pronounced subsurface 137Cs maxima at 100–200 m (notably near 100°E and 55°E), representing the highest 137Cs concentrations observed in the Southern Hemisphere, which are comparable to maxima in the North Pacific. These data confirm the transport of surface waters from the North Pacific through Indonesian seas into the South Indian Ocean and then to the subtropical gyre, which serves as a long-term reservoir of anthropogenic pollutants on decadal timescales. Similar upper-ocean concentrations in the SW Indian Ocean and the South Atlantic Ocean indicate inter-basin exchange via the Agulhas system and the broader overturning circulation associated with the GMOC. The North Pacific therefore acts as a source of anthropogenic radionuclides and other pollutants to the South Indian and South Atlantic Oceans. Adapted from ref. 20, licensed under CC BY).

In the Pacific region, the majority of fallout-derived 137Cs was deposited in the NW Pacific (Supplementary Fig. 1). It was then transported eastward to the NE Pacific, before being redistributed by large-scale gyre circulation and carried downward into subsurface layers. From the western equatorial Pacific, 137Cs was transported to the Indian Ocean through the Indonesian Throughflow, as well as from the North Pacific to the South Pacific and the Indian Ocean via equatorial currents. Labeled waters were subsequently transported into the South Atlantic via inter-basin exchange pathways65,70,71 (Box Fig. 1). The North Pacific thus acts as a major source of anthropogenic radionuclides for other ocean basins, allowing long-term changes in ocean circulation and climate to be traced in seawater and sediments, and captured by circulation models.

The F1NPP accident injected the short-lived radioisotope cesium-134 (134Cs, half-life of 2.07 years) together with 137Cs, providing a fingerprint that clearly separates recent inputs from older global fallout. Observations and model simulations show that 137Cs concentrations in the upper 200 m declined steadily from the mid-1960s until 2011, when the F1NPP accident-related input became evident. The 137Cs signal was clearly detectable in both surface and intermediate waters (200–1500 m) across the western and eastern North Pacific, but concentrations have declined over time as the signal dispersed and mixed72,73,74,75,76,77,78,79,80 (Supplementary Fig. 6). Model results suggest that 137Cs from the F1NPP accident will become indistinguishable from background levels by around 2030 (ref 77).

Plutonium isotopes

Plutonium has four long-lived radioisotopes whose ratios provide distinctive fingerprints that reflect their source in the ocean (Supplementary Table 1). In particular, the 240Pu/239Pu atom ratio is widely used to distinguish global fallout (typical ratio ~0.18) from close-in fallout from Pacific Proving Grounds (PPG) tests (ratios of ~0.21–0.36)80,81,82. These signatures allow mapping of plutonium origins and pathways throughout the ocean. It has been shown that PPG-derived plutonium contaminated the North Pacific subtropical gyre, whereas much of the global fallout-derived plutonium entered the subarctic gyre83 (Supplementary Fig. 2).

Plutonium entered the North Pacific through close-in fallout mainly in the 1950s and global fallout mainly in the 1960s. In 1980, PPG-derived Pu accounted for ~60% of the inventory in the North Pacific, while the contribution from the F1NPP accident was negligible72. Although Pu of global fallout origin was injected into surface waters in the mid-latitude region, Pu of PPG origin contaminated surface and bottom water of the tropical North Pacific Ocean, reaching surface waters of the subarctic Pacific in the 1980s83. The high 239,240Pu levels observed in the surface and bottom waters of the North Pacific documented this transport70. The flow rate of the bottom water in the North Pacific appears to be faster (decadal time scale) than that estimated from the 14C age (centennial and millennial timescales between the South and North Pacific)84. Therefore, Pu isotopes in the ocean have been important tracers for deciphering oceanic biogeochemical processes and ocean circulation, and for studying the impact of climate change.

Plutonium exhibits strong particle reactivity: typically, a measurable fraction is particle-bound in the water column, allowing efficient vertical transport by sinking material, while 137Cs is mostly dissolved in seawater85. Plutonium is therefore scavenged from surface waters onto sinking particles and can later be released back into solution at depth as the particles degrade15,30 (Box 2).

Multidecadal variability and lateral export

Measurements over the past five decades show pronounced temporal and spatial variability in 239,240Pu activity concentrations across the North Pacific30,31. In the mid-latitude NW Pacific, intermediate-depth plutonium maxima have declined over the past 30 years without a corresponding increase at depths greater than 1,000 m. This decline indicates that plutonium at these depths has been exported laterally, primarily toward the Southern Hemisphere. On the contrary, vertical plutonium profiles in the equatorial North Pacific remained relatively stable over the same period, likely reflecting a balance between the inflow of plutonium-rich waters from the North Pacific and the outflow through equatorial currents82.

When particle-scavenging models, simulating the removal of plutonium by sinking particles and its subsequent regeneration as particles degrade at depth, are combined with OGCMs and biogeochemical processes, they reproduce the observed vertical profiles reasonably well. Together, these approaches indicate that changes in plutonium distributions arise from a combination of altered ocean circulation and particle production and degradation.

Plutonium isotope signatures and inter-basin exchange

Advances in analytical techniques, particularly ICP–MS and AMS, now allow highly precise measurements of plutonium isotope ratios86,87. These data have been crucial for tracing plutonium sources and transport pathways in the ocean. Elevated 240Pu/239Pu atom ratios, characteristic of close-in fallout from Pacific Proving Grounds (PPG) fallout, have been used to track its dispersal from the Bikini region to the South China, East China and Indonesian seas, and onward to the subarctic, documenting the presence of PPG-derived Pu in both surface and deep waters of the North Pacific80,83,84. Representative datasets suggest that this transport has been sensitive to climate variability32,33.

Plutonium isotopes have also been used to investigate variability in the Indonesian Throughflow (ITF), a key conduit connecting the Pacific and Indian Oceans. Sediment records from the Lombok Basin show a smaller decline in plutonium deposition over time than the faster decrease recorded in high-resolution coral archives from the NW Pacific88. This discrepancy suggests changes in transport pathways and a weakening of the ITF during the latter half of the 20th century, consistent with climate model projections of a long-term weakening of the ITF89.

The 239,240Pu/137Cs activity ratio in seawater provides an additional tracer of changes in ocean processes associated with climate variability. Because plutonium is approximately two orders of magnitude more particle reactive than cesium, this ratio increases with depth as plutonium is scavenged onto sinking particles and subsequently regenerated during particle remineralisation15,89,90. The observed exponential decrease in this ratio over time in the NW Pacific indicates reduced vertical mixing91. The depth of plutonium regeneration closely corresponds to the depth at which sinking organic carbon is remineralized back to CO₂. When remineralization occurs deeper in the water column, a greater fraction of atmospheric CO₂ is sequestered in the deep ocean; conversely, shallower remineralization reduces this drawdown. Therefore, variations in remineralization depth directly link plutonium cycling to the efficiency of the ocean’s biological carbon pump, since shallower carbon regeneration results in less effective transfer of carbon to the deep ocean. A comparable shoaling of the remineralisation depth has also been observed in the South Pacific Ocean92. Together, the changes observed in both the North and South Pacific suggest weaker carbon sequestration and reduced nutrient resupply to surface waters, consistent with enhanced upper-ocean stratification under ongoing climate change.

Radionuclide constraints on a slowdown of the GMOC in the Pacific Ocean

Although climate models project a potential slowdown of the global meridional overturning circulation (GMOC) with climate change, the magnitude and timing remain uncertain1,2. A weaker GMOC would reduce deep-water formation and storage, alter the distribution of anthropogenic radionuclides, and provide observational constraints on the rate and extent of circulation change.

Climate-driven weakening of the GMOC suppresses deep-water formation in the North Atlantic and Antarctic regions, which contributes to the warming of North Pacific bottom waters92. Reduced formation decreases the upward transport of abyssal waters in the North Pacific, including those influencing the subarctic region and the subarctic current system (Supplementary Fig. 2). Therefore, plutonium isotope signatures provide sensitive tracers of changes in bottom-water ventilation, the associated decline in the supply of cold, nutrient-rich deep waters, and changes the pathways and timescales by which labeled waters are transported between basins31,82,83.

Circulation changes are also recorded in the distribution of 137Cs in the Pacific Ocean. Water-column sections from the SHOTS project, obtained during BEAGLE 2003–2004 round-the-world cruises (South Atlantic 30°S, Indian 20°S, Pacific 30°S), show elevated 137Cs concentrations in the Tasman Sea, consistent with cross-equatorial transport of global-fallout 137Cs from the NW Pacific, and very low concentrations in the eastern South Pacific, where upwelling of low-137Cs deep waters occurs20 (Fig. 3). Complementary 239,240Pu measurements along the same transects show the lowest activities in the South Atlantic, reflecting progressive particle scavenging during transport from the western North Pacific. Increased Antarctic glacial meltwater input has further reduced Antarctic Bottom Water formation, leading to warmer and older abyssal waters. When interpreted together with circulation models, these patterns provide tracer-based constraints on pathways linking the Pacific, Indian and Atlantic Oceans, and on the timescales of exchange relevant to the GMOC45,69.

A marginal-sea testbed: climate impacts in the Sea of Japan

The Sea of Japan (SOJ; also known as the East Sea) is among the most intensively studied marginal seas with respect to anthropogenic radionuclides, with time series dating back to the 1970s93,94,95. The SOJ is a semi-enclosed marginal sea (area 0.98 × 10⁶ km²; maximum depth 3,742 m) connected to the East China Sea, North Pacific, and Okhotsk Sea through four shallow straits: Tsushima Strait ( ~ 130 m), Tsugaru Strait ( ~ 130 m), Soya Strait ( ~ 50 m), and Tatarskiy (Mamiya) Strait ( ~ 30 m). These shallow sills largely isolate the deep waters of the SOJ, resulting in a deep-water residence time of approximately 1,000 years84. A warm and saline (salinity 34.2–34.5) Tsushima Warm Current, a branch of the Kuroshio Current (KC), enters the SOJ through the Tsushima Strait. In contrast, the cold and less saline Liman Cold Current flows southward through the Tatarskiy Strait, contributing to a cyclonic circulation in the northern SOJ. The SOJ maintains a self-contained thermohaline circulation system with active deep-water formation96,97,98 (Fig. 4).

Fig. 4: Bathymetry of the Sea of Japan, the main currents, and the vertical structure of the Japan Sea Proper Water (JSPW) and Bottom Water (BW) during summer and fall.
Fig. 4: Bathymetry of the Sea of Japan, the main currents, and the vertical structure of the Japan Sea Proper Water (JSPW) and Bottom Water (BW) during summer and fall.The alternative text for this image may have been generated using AI.
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Most of the JSPW and BW water masses are very cold and homogeneous, resulting in a very small water density gradient. The Upper JSPW, Lower JSPW, and Bottom Water are characterized by the following potential temperature (PT) ranges: PT > 0.5 °C (large PT gradient), 0.5–0.1 °C (small PT gradient), 0.1–0.07 °C (very small PT gradient), and PT < 0.07 °C (no PT gradient), respectively. A shows the PT structure north of the Subpolar Front, while B shows the structure south of the Subpolar Front. Green ellipses indicate anticyclonic eddies. An anticyclonic eddy north of the Subpolar Front extends to depths greater than 2000 m (ref.99). The orange ellipse marks the area of Bottom Water formation near Vladivostok. The map was produced using the Global Mapping Tool (www.altova.com).

Below the thermocline, the deeper layers are occupied by cold (0.0–0.6 °C), highly oxygenated (200–230 µmol kg⁻¹ dissolved oxygen), and relatively homogeneous water with a salinity of 34.06–34.08, known as the Japan Sea Proper Water (JSPW). This water mass is formed by deep convection in the offshore region near Vladivostok during the winter monsoon96. It overlies the very cold (potential temperature <0.001 °C) and vertically homogeneous Bottom Water (BW), which occupies the layer below 2,000–2,500 m depth in the Japan Basin (JB). Due to the shallow sills of the four straits, both the JSPW and BW are largely isolated from the surrounding seas and the Northwest Pacific. Newly formed JSPW sinks into the deep layers in the northwestern region during winter, then gradually upwells and spreads throughout the basin over the following year as part of the large-scale circulation94.

The sea-surface temperature (SST) in the central SOJ has increased between 1901 and 2022 by 1.94 °C, approximately three times the global mean ocean warming rate93,94. The deep-water formation region off Vladivostok has also become warmer and fresher in recent decades, strengthening upper-ocean stratification and slowing deep-water formation. Since the 1970s, bottom-water formation off Vladivostok has declined markedly, as surface waters no longer cool enough to reach the seafloor ( ~ 3000 m) and instead sink only to 1000–2000 m96. These changes suppress deep convection and reduce vertical exchange, making the SOJ a sensitive setting for detecting climate-driven circulation changes using radionuclide tracers96. Plutonium, because of its sensitivity to both vertical mixing and particle cycling, has proven particularly effective for quantifying exchange between deep and surface layers, including the influence of mesoscale eddies. Because the SOJ is among the most eddy-active seas globally, it also provides an ideal basin for assessing climate impacts on eddy dynamics97,98,99 (Box 3).

Temporal variations of 239,240Pu, 137Cs and 90Sr in the Sea of Japan

Deep waters in the SOJ, specifically JSPW ( < 1500 m), exhibit elevated 239,240Pu levels due to biogeochemical regeneration and long residence times ( ~ 1,000 yr), related to the basin’s restricted topography. Surface 239,240Pu activity concentrations show larger horizontal variability than the corresponding 137Cs (and 90Sr). In 1994, elevated surface 239,240Pu activities and vertically homogeneous profiles were observed from the surface to the upper JSPW. This variability was strongly influenced by eddy dynamics and biogeochemical processes85. After 2000, recirculation of Pu from the JSPW to the surface weakened, and surface 239,240Pu activity concentrations decreased owing to reduced mixing, enhanced stratification and diminished cyclonic activity associated with weaker deep-water formation. This trend may reflect increasing winter surface temperatures that suppress convection and limit the upward supply of subsurface Pu98.

In contrast, 137Cs (and 90Sr) activity concentrations in bottom-water ( > 3000 m) increased due to their conservative behavior97. Their vertical distributions to ~2000 m were governed primarily by physical transport and vertical eddy diffusion. After 2010, surface 239,240Pu activity concentrations increased, whereas 137Cs concentrations decreased in the SOJ, differing from trends in the North Pacific, where both radionuclide concentrations decreased monotonically (Supplementary Fig. 8). Overall, vertical profiles indicate that eddy diffusion dominates radionuclide transport from the surface to the upper JSPW (500−1500 m) depths85,95.

Winter convection, cyclonic upwelling, particle scavenging, remineralization and sediment–water interactions together regulate radionuclide cycling and the physical–biogeochemical state of the SOJ. Owing to this sensitivity, the SOJ responds rapidly to increases in seawater temperature, producing subsurface temperature anomalies and episodic heatwaves associated with cyclonic and anticyclonic eddies. Observations of anthropogenic radionuclides in the SOJ provide some of the strongest evidence of ongoing climate-driven impacts on the marine environment.

Conclusions and perspectives

Anthropogenic radionuclides provide powerful observation-based constraints on the ocean’s response to climate change. Their well-characterized input histories and largely conservative behavior in seawater make them effective time markers for quantifying water-mass formation, transport and vertical exchange. Across the Pacific, 3H and 137Cs document multidecadal ventilation and transport pathways in the upper ocean and thermocline, while plutonium isotopes add sensitivity to particle-mediated export, regeneration, and source attribution. Therefore, they provide independent, observation-based constraints on circulation changes that are difficult to resolve with physical measurements alone. Beyond retrospective reconstruction, anthropogenic radionuclides also provide critical benchmarks for evaluating and improving ocean general circulation models.

Long-term investigations of anthropogenic radionuclides have great potential to improve our understanding of current and future climate change impacts on the world ocean and adjacent seas. The emerging picture from radionuclide tracer observations, interpreted in conjunction with circulation and scavenging models, is broadly consistent with a warming ocean that is increasingly stratified in the upper layers, with implications for reduced vertical exchange and potentially altered overturning circulation and inter-basin connectivity100,101.

Robust detection of radionuclides in oceans and attribution of long-term climate trends require sustained sampling and careful separation of long-term climate forcing from decadal variability and episodic radionuclide inputs. Despite substantial existing datasets, sustained time-series observations and improved spatial coverage, particularly in under-sampled regions of the South Pacific, South Atlantic and Southern Ocean, radionuclide tracers can detect and attribute long-term, climate-driven changes in ocean circulation, supported by recent developments in modeling Earth systems102. Continuing advances in high-sensitivity analytical techniques (ICP-MS, AMS, and underground low-background gamma spectrometry) will enable smaller-volume sampling and higher-resolution vertical profiling.

Integrating anthropogenic radionuclide measurements within coordinated international observation frameworks will strengthen their value not only as historical markers of past circulation but also as practical indicators for detecting and quantifying ongoing processes in the world ocean, associated with climate change.