Paper A MILLENNIUM PERSPECTIVE ON THE CONTRIBUTION OF GLOBAL FALLOUT RADIONUCLIDES TO OCEAN SCIENCE Hugh D. Livingston and Pavel P. Povinec* Following entry to the ocean, the behavior of these Abstract—Five decades ago, radionuclides began to enter the radio-elements was determined by their physical and ocean from the fallout from atmospheric nuclear weapons chemical properties and their fate by oceanic physical tests. The start of the 21st century is an appropriate vantage and biogeochemical processes. As in the terrestrial envi- point in time to reflect on the fate of this unique suite of ronment, the post-delivery behavior of these fallout manmade radionuclides—of which more than two-thirds ar- rived at the surface of the oceans of the planet. During these radionuclides provided a unique opportunity to gain five decades much has been learned of the behavior and fate of insights into the nature of the processes controlling their these radionuclides and, through their use as unique tracers, of fate. The great depth and dynamic nature of the oceans how they have contributed to the growth of basic knowledge of have resulted in substantially greater dispersion and complex oceanic physical and biogeochemical processes. Some dilution than on land. This review is intended to trace the of the highlights of the ways in which fallout radionuclides key events and trends of studies during the second half of have given new insights into these processes are reviewed in the 20th century associated with the entry of global the historical context of technological and basic ocean science fallout to the world oceans. Initially these studies were developments over this period. The review addresses major driven by the need to assess the impact and fate of fallout processes involved, such as physical dispersion and mixing, radioactivity on marine life and associated transfer to particle association and transport of reactive nuclides, biolog- ical interactions, and mixing and burial within ocean sedi- man through marine exposure pathways. Subsequently, ments. These processes occur over a range of scales ranging fallout radionuclides were widely used as oceanic pro- from local to global. Finally, an account is given of the present cess tracers, due to their unique time-marker labeling of spatial distribution within the oceans of the various compo- sea water, marine organisms, and sediments according to nents of the fallout radionuclide suite. their individual chemical properties. In this sense they Health Phys. 82(5):656–668; 2002 contributed in no small way to the broad growth of Key words: fallout; radionuclide; radioactivity, environmen- knowledge of the intricate physical, chemical, biological, tal; National Council on Radiation Protection and Measure- and sedimentological processes which occur in the ments oceans. So this review will also try to present a broad account of the major research areas where fallout radio- nuclide studies have contributed to advances in ocean INTRODUCTION science—with some illustrative examples where appro- priate. AS DISCUSSED in detail elsewhere in this issue, the fallout from the atmospheric nuclear weapon’s tests in the mid-20th century has by now largely been transferred CHARACTERISTICS OF FALLOUT INPUT TO from the atmosphere to the surface of Earth. Since about THE OCEANS AND THE STUDY TIME FRAME 70% of the surface of the planet is ocean, much of this mixture of artificial radionuclides has ended up there. The course of ocean fallout studies, by the large Despite the fact that most atmospheric nuclear weapons body of international scientists and institutions involved, tests took place in the Northern Hemisphere, it can be followed a series of phases and trends. They were based shown from the proportions of land and ocean in each on a common set of physical and chemical characteristics hemisphere, and from the latitudinal fallout patterns over and were accompanied by a variety of perturbations from time, that over two-thirds of the fallout entered the ocean. other sources of artificial radionuclides which impacted the oceans. An account of these provides the context in which the whole range of ocean fallout research was * International Atomic Energy Agency, Marine Environment Laboratory, 4 Quai Antoine 1er, B.P. 800, MC 98000, Monaco Cedex. framed. For correspondence or reprints contact: H. D. Livingston, Inter- The primary time-frame of ocean fallout studies was national Atomic Energy Agency, Marine Environment Laboratory, 4 set by the history of nuclear weapons testing and the Quai Antoine 1er, B.P. 800, MC 98000, Monaco Cedex, or email at resultant transfer to the atmosphere of debris which was [email protected]. (Manuscript received 8 November 2001, accepted 18 December 2001) delivered to land and sea at rates documented elsewhere. 0017-9078/02/0 The main periods of delivery of fallout to the oceans Copyright © 2002 Health Physics Society were the years following the two intense series of 656 Contribution of global fallout radionuclides to ocean science ● H. D. LIVINGSTON AND P. P. POVINEC 657 atmospheric tests in the mid-1950’s and early 1960’s. As a result of this behavioral setting premise above, The scale of testing by countries that continued atmo- it has been common practice (Livingston and Jenkins spheric tests after the 1962 Test Ban Treaty was much 1982) to classify the oceanic behavior of fallout radio- diminished and represented a relatively small continued nuclides as broadly “conservative” or “non- input. The majority of the fallout produced in atmo- conservative.” The conservative group includes radionu- spheric tests was injected into the stratosphere and clides having a very small affinity for particle association subsequently returned to Earth’s surface as global fallout. and whose behavior consequently is controlled by the physics of ocean circulation and mixing. Examples of Some tests, especially in the earlier test series in the 3 90 85 137 1950’s time frame, were conducted at or near ground these include H, Sr, Kr, and (in most situations) Cs. level and produced tropospheric fallout with a regional The nonconservative group includes radionuclides with a impact. The oceanic impact of ground level tests was range of strengths of particle association and whose ultimate behavior is linked to these affinities. Examples significant from testing at oceanic locations such as those 55 147 on Pacific islands—especially the United States test of these include Fe, Pm, plutonium and americium series at the Pacific Test Site in the Marshall Islands. So isotopes, etc. It is important to note that, despite this classification, the time scales over which such behavioral this regional impact was largely confined to the Western differences may be observed are also important to keep in North Pacific. Minor traces of tropospheric fallout from mind. For example, it is well established that the asso- the U.S. Nevada Test Site were identified in the Atlantic ciation of nonconservative radionuclides with plankton Ocean and Mediterranean Sea (Buesseler and Sholkovitz blooms provides a mechanism by which reactive fallout 1987) but were not quantitatively very significant. nuclides can be removed from surface sea water and The pattern of global fallout delivery to the world released through remineralization processes below the oceans was anticipated to be very similar to that recorded euphotic zone. This can occur over seasonal time scales on land networks of sampling stations. This would follow and produces separation of nonconservative and conser- the now classic depositional pattern with maxima at vative fallout nuclides. But as even reactive radionu- mid-latitudes, minima towards the equator and the poles clides can have oceanic residence times of hundreds to and with much larger input rates in the Northern Hemi- thousands of years, physical circulation processes can sphere. In fact, the patterns of surface sea water concen- dominate in the short term—at least in the open ocean trations observed during peak fallout years [summarized which received the major part of the fallout input by far. by latitude and time in the comprehensive RIME report Different considerations apply in productive coastal shelf (Volchok et al. 1971)] proved rather complex due to areas, and in estuaries, where particle association pro- seasonal fluctuations in delivery, ocean mixing, and cesses can exert substantial controls on reactive radionu- nonstandard observational networks. Sometimes the clides over much shorter time scales. overland depositional pattern was observed, e.g., in the mean 90Sr surface values reported in the North Atlantic in HISTORICAL PERSPECTIVES ON OCEAN 1960 and 1961, but often the data were difficult to FALLOUT STUDIES interpret. A more convincing demonstration of oceanic distributions came more than a decade later, and relied on Inevitably, studies of the fate of fallout from nuclear water column inventory patterns of 137Cs (Kupferman et weapon’s tests and its use as a tool for ocean science al. 1979; Livingston and Jenkins 1982). The interesting have been limited by the availability and state of devel- point about these inventory patterns was that not only did opment of analytical measurements and sampling tech- they show a latitudinal pattern similar to overland fallout nologies. They have also been limited by the contempo- patterns, but that these patterns still were preserved rary level of the state of knowledge of the oceans and its substantially later than the major inputs—despite the processes. Ocean fallout studies span a half century, which has been marked by a staggering growth of dynamic nature of the ocean, with the active gyral scientific and technical knowledge. The period began on circulation of surface and sub-surface currents. the nuclear side at a time of slide rules, gas filled A further general comment needs to be made con- detectors and the first generation of radiochemical tech- cerning physico-chemical properties of global fallout niques. It ended in an age of computers, sophisticated entering the ocean and which explains subsequent obser- semi-conductor detectors, advanced radiochemistry, and vations of its fate and movements.