Detonation Site Research Articles

Investigation of the distribution of transuranic radionuclides in marine sediment at the Montebello Islands, Western Australia

Three nuclear weapons tests were conducted in the 1950s at the Montebello Islands, Western Australia. The detonations were of different yields and configurations (two tower tests, one ship test), and led to substantial radionuclide contamination within the surrounding terrestrial and marine ecosystems. The region possesses great ecological and recreational significance, particularly within the marine environment. However, studies conducted so far have largely neglected the marine ecosystem which makes up the majority of the Montebello Island Marine Park and in which most test fallout would have deposited. Here we investigated the distribution of the transuranic radionuclides 238Pu, 239,240Pu and 241Am in marine sediment from the Montebello Islands. Marine sediment samples near Operation Mosaic G2 and Operation Hurricane were collected and analysed by gamma and alpha spectrometry. Activity concentrations of 239,240Pu across both series ranged from 45 to 2900 Bq kg−1, while 241Am levels ranged from 2.8 to 70 Bq kg−1. Higher activity concentrations were observed in sediment near the land-based, higher yield Mosaic G2 test, compared with the ship-based, lower yield Hurricane test. Sediment samples located closer to the detonation site were also observed to have higher activity concentrations. Radioactive particles of 0.94 mm and 1.5 mm in diameter were identified by analysis of size-fractioned sediment via investigation of 152Eu levels, photostimulated autoradiography and point gamma spectroscopy. Particles were confirmed to have transuranic radionuclide interiors, with surface coatings which were dominated by vitrified CaCO3. Their long-term resistance to weathering and subsequent persistence in the marine environment can therefore be attributed to their coated structural form. Our study confirms the persistence of transuranic radionuclides in Montebello Island marine sediment and highlights the need for additional studies to improve our understanding of the nuclear legacy in this region.

Fire in the Hole: Modeling the Thermal Effects of a Nuclear Detonation in an Urban Environment

Introduction:In general, models for thermal effects of nuclear weapons are not as well developed as models for blast and radiation effects, yet casualties resulting from fires and burns in a nuclear detonation would significantly impact civil defense and emergency healthcare. Previous studies have conducted in-depth analysis of the various atmospheric conditions that affect the thermal radiation transmissivity. However, such models have yet to consider the role that buildings play in the urban environment to estimate the casualties from the thermal effect more accurately.Method:A three-dimensional model of the area within a three-mile radius of the detonation site in Atlanta, Georgia, USA was created in Blender. To represent the thermal energy resulting from a 15 kiloton, near-surface burst, a point light was created with a power of 96,725 gigawatts and a radius of 81 meters. Using the Cycles render engine, the resulting light/shadow was orthographically captured directly above the scene.Results:The rendered model demonstrated the attenuating effects of the built, urban environment. Nearly half (46.82%) of the pixels in the resulting raster were black, or regions that were not exposed to any thermal energy. Slightly less than a quarter (22.32%) of the pixels were white or light gray, or regions that received mostly direct thermal energy. The remaining regions (30.86% of the pixels) were dark gray, or regions that were initially in shadow from the thermal pulse but received thermal energy via reflection from nearby buildings.Conclusion:As the thermal pulse travels at the speed of light, it arrives at a location before the blast wave. As such, the built urban environment offers protection from the thermal energy released during a nuclear detonation. Future studies that incorporate this thermal model may more accurately determine the quantity and geospatial distribution of burn casualties in the aftermath of a nuclear detonation.

Results and evaluation of US Navy shock trial environmental mitigation of marine mammals and sea turtles

A shock trial of a US Navy Destroyer, the USS Winston S. Churchill, was conducted offshore of northern Florida in May and June 2001. The shock trial consisted of three underwater detonation tests, spaced approximately one week apart. Environmental mitigation to minimise the impact of the shock trial on marine mammals and sea turtles was based on a Safety Range of 3.7km (2 n.miles) radius around the detonation site, and a Buffer Zone of an additional 1.85km (1 n.mile) radius beyond the Safety Range. Mitigation included site selection surveys, pre-detonation aerial, vessel and bio-acoustic monitoring, and post-detonation aerial and vessel monitoring. Six species of odontocete and two species of sea turtle were identified during mitigation monitoring, as well as several sightings that could not be identified by species. Site selection aerial surveys were implemented to select a test site with the lowest abundance of marine mammals and sea turtles. Nearly 300 animals were seen during site selection surveys. Pre-detonation aerial and vessel monitoring was implemented to sight any marine mammal and sea turtle within the Safety Range on designated test days, and track the animals until they could be verified to be outside the Safety Range. Approximately 1,200 marine mammals and 32 sea turtles were sighted during pre-detonation monitoring. Pre-detonation bio-acoustic monitoring was implemented to detect large cetaceans within the Safety Range and Buffer Zone; the only calls heard were from dolphins that could not be localised. Post-detonation monitoring was implemented to determine the effectiveness of mitigation procedures. No injured or dead marine mammals or turtles were detected during approximately 185 hours of post-detonation aerial and vessel visual monitoring. Post-detonation monitoring resulted in observations of 767 marine mammals and 42 sea turtles. With only two exceptions, the same marine mammal and sea turtle species were observed prior to, during and after the shock trial test time period. Factors leading to the success of this environmental mitigation effort are summarised, and recommendations for improvements to mitigation efforts of this size and scope are suggested. These recommendations include the use of a simultaneous second aircraft for improved coverage during pre-detonation surveys, increased post-detonation aerial monitoring, equitable survey data for all test sites under consideration during planning stages, and reassessment of bio-acoustic monitoring need and purpose.

Evolution of Nuclear Environments: From Forbidden Gardens to Nuclear Landscape Monuments

The dawn of Anthropocene saw the birth of nuclear landscapes (NL): places heavily contaminated by radioactivity, left behind by human interventions. From nuclear weapon production to detonation sites and atomic power plants, unfortunate events had resulted in environmental catastrophes, turning these NLs into forbidden gardens - off-limits frontiers of waste. Human absence promoted NL to metamorphose into post-nuclear landscapes, characterized by a primal image of nature: pristine and spontaneous. It is an unreleased kind of wilderness, a living archive of human ecocides. Later, governmental interventions gradually transformed these sites into Nuclear Landscape Monuments (NLM), making them embodiments of degradation and redemption. The essay investigates the evolution of these nuclear environments and their wild ambivalent nature. It further elucidates the shift in humans’ attitudes towards nature, through an atomic narrative: from production and destruction to recovery and reconciliation. The essay also highlights the role of anthropogenic and natural agencies in establishing this intricate co-existing relationship between humans and non-humans.

Characterization of underwater soundscape variations pre- and post-ship shock trial underwater detonations

During Full Ship Shock Trials (FSSTs), a ship is subjected to a series of underwater detonations conducted at various distances to assess its ability to withstand shock waves that simulate near misses during combat. In the summer of 2021, the US Navy successfully conducted a FSST of the USS Gerald R. Ford, in the Atlantic Ocean off the coast of Jacksonville, FL. We collected passive acoustic recordings during each of the three underwater detonations, using an array of bottom-moored autonomous underwater recorders—Rockhoppers and SoundTraps—that were deployed at 15 vantage positions around the three sites. While the acoustic recordings of the underwater detonations were aimed at providing the Naval Sea Systems Command valuable data for updating their underwater acoustic propagation models, it also provided opportunistic data to assess possible impacts on prevailing biota. Our analyses of the recordings included an assessment of the recorded soundscape pre- and post-trial and a characterization of relative vocal activity by large faunal groups (baleen whales, delphinids, and fish). The results provide an insight into behavioral changes in response to the underwater detonations at different distances from the detonation sites as well as valuable information for improving impact mitigation considerations during future FSSTs.

Dose Coefficients for Internal Dose Assessments for Exposure to Radioactive Fallout.

This paper presents values as well as the bases for calculating internal dose coefficients suitable for estimating organ doses from the exposure to radioactive fallout that could result from the detonation of a nuclear fission device. The 34 radionuclides discussed are the same as those given in a priority list of radionuclides for fallout dose assessments presented in a companion overview paper. The radionuclides discussed are those that are believed to account for a preponderance of the organ doses that might be received by intake by persons of all ages (including in utero and via breast feeding for infants) following exposure to radioactive fallout. The presented dose coefficients for ingestion account for age and include modifications for variations in solubility with distance as discussed previously in the literature, and those for inhalation similarly account for age, solubility, and particle sizes that would be relevant at various distances of exposure as discussed in a companion paper on ingestion dose methods. The proposed modifications peculiar to radioactive fallout account for systematic changes in solubility and particle sizes with distance from the site of detonation, termed here as the region of "local fallout" and the region "beyond local fallout." Brief definitions of these regions are provided here with more detailed discussion in a companion paper on estimating deposition of fallout radionuclides. This paper provides the dose coefficients for ingestion and inhalation (for particle sizes of 1 μm, 5 μm, 10 μm, and 20 μm) for the region "local fallout." These dose coefficients for "local fallout" are specific for particles formed in a nuclear explosion that can be large and have radionuclides, particularly the more refractory ones, distributed throughout the volume where the radionuclide has reduced solubility. The dose coefficients for the region "beyond local fallout" are assumed to be the ones published by the International Commission on Radiological Protection (ICRP) in 1995. Comparisons of the presented dose coefficients are made with values published by the ICRP.

A Methodology for Estimating External Doses to Individuals and Populations Exposed to Radioactive Fallout from Nuclear Detonations.

A methodology of assessment of the doses from external irradiation resulting from the ground deposition of radioactive debris (fallout) from a nuclear detonation is proposed in this paper. The input data used to apply this methodology for a particular location are the outdoor exposure rate at any time after deposition of fallout and the time-of-arrival of fallout, as indicated and discussed in a companion paper titled "A Method for Estimating the Deposition Density of Fallout on the Ground and on Vegetation from a Low-yield Low-altitude Nuclear Detonation." Example doses are estimated for several age categories and for all radiosensitive organs and tissues identified in the most recent ICRP publications. Doses are calculated for the first year after the detonation, when more than 90% of the external dose is delivered for populations close to the detonation site over a time period of 70 y, which is intended to represent the lifetime dose. Modeled doses in their simplest form assume no environmental remediation, though modifications can be introduced. Two types of dose assessment are considered: (1) initial, for a rapid but only approximate dose estimation soon after the nuclear detonation; and (2) improved, for a later, more accurate, dose assessment following the analysis of post-detonation measurements of radiation exposure and fallout deposition and the access of information on the lifestyle of the exposed population.

Gamma and Decay Energy Spectroscopy Measurements of Trinitite

We report gamma-ray spectroscopy measurements of trinitite samples and analogous samples obtained from detonation sites in Nevada and Semipalatinsk, as well as in situ measurements of topsoil at the Trinity site. We also report the first isotopic composition measurements of trinitite using the novel forensics technique of decay energy spectroscopy (DES) as a complement to traditional forensics techniques. Our gamma spectroscopy and DES measurements are compared to other published results.

Estimated Radiation Doses Received by New Mexico Residents from the 1945 Trinity Nuclear Test

The National Cancer Institute study of projected health risks to New Mexico residents from the 1945 Trinity nuclear test provides best estimates of organ radiation absorbed doses received by representative persons according to ethnicity, age, and county. Doses to five organs/tissues at significant risk from exposure to radioactive fallout (i.e., active bone marrow, thyroid gland, lungs, stomach, and colon) from the 63 most important radionuclides in fresh fallout from external and internal irradiation were estimated. The organ doses were estimated for four resident ethnic groups in New Mexico (Whites, Hispanics, Native Americans, and African Americans) in seven age groups using: (1) assessment models described in a companion paper, (2) data on the spatial distribution and magnitude of radioactive fallout derived from historical documents, and (3) data collected on diets and lifestyles in 1945 from interviews and focus groups conducted in 2015-2017 (described in a companion paper). The organ doses were found to vary widely across the state with the highest doses directly to the northeast of the detonation site and at locations close to the center of the Trinity fallout plume. Spatial heterogeneity of fallout deposition was the largest cause of variation of doses across the state with lesser differences due to age and ethnicity, the latter because of differences in diets and lifestyles. The exposure pathways considered included both external irradiation from deposited fallout and internal irradiation via inhalation of airborne radionuclides in the debris cloud as well as resuspended ground activity and ingestion of contaminated drinking water (derived both from rivers and rainwater cisterns) and foodstuffs including milk products, beef, mutton, and pork, human-consumed plant products including leafy vegetables, fruit vegetables, fruits, and berries. Tables of best estimates of county population-weighted average organ doses by ethnicity and age are presented. A discussion of our estimates of uncertainty is also provided to illustrate a lower and upper credible range on our best estimates of doses. Our findings indicate that only small geographic areas immediately downwind to the northeast received exposures of any significance as judged by their magnitude relative to natural radiation. The findings presented are the most comprehensive and well-described estimates of doses received by populations of New Mexico from the Trinity nuclear test.

Evaporative loss of moderately volatile metals from the superheated 1849 Ma Sudbury impact melt sheet inferred from stable Zn isotopes

The retention of moderately volatile elements on the growing Earth remains a major uncertainty in models of terrestrial accretion. Large impactors were the main carriers of accreted material but their mutual energetic collisions and impacts onto the Earth also caused chemical fractionation for which limited experimental data exist. The objective of this work was to study several moderately volatile elements in the third-largest impact basin preserved on Earth at Sudbury, Ontario. We conducted a new chemostratigraphic transect (n=41) of Zn isotope ratios and concentrations by analysing melt sheet and basin fill samples. The data were compared to common Pb, Cs, Cd and Sb concentration systematics. Within the crystallised melt sheet there are strong trends in the extent of moderately volatile element deficits, Zn isotope composition (δ66/64ZnJMC-L from 0.18 to 0.47‰) and initial Pb isotope composition. The combined evidence suggests that these trends reflect footwall contamination of a melt sheet that had experienced evaporative Zn-loss of up to 75–80%. Accounting for plausible isotopic signatures of target rocks, the maximum mass-dependent Zn isotope fractionation ε was 0.29 ± 0.04‰ (1 s.d.), which translates to modest fractionation factors α=0.99986 to 0.99975. This is comparable to melt fallout-glass and fused sands from nuclear detonation sites. We attribute the observed Zn loss and isotope fractionation to the formation of the impact melt. The rapid formation of a solid lid of breccias upon seawater ingress may have prevented stronger evaporative loss and isotope fractionation. Within the crater fill, there is an up-stratigraphy increase in Zn isotope variability (δ66/64ZnJMC-L from 0.29 to 1.05‰). Combined with evidence for biogenic reduced C, this suggests sedimentation of authigenic particulates within an enclosed crater sea.In the melt sheet, the Zn-Pb and Rb-Cs pairs experienced different extents of maximum evaporative loss (Pb up to 98.4% vs. Zn 78%; and Cs ∼90% vs. Rb ∼30%). The relative loss pattern could reflect evaporation from superheated silicate melt at ∼1,450°C and 1 atm. Loss from super-liquidus melts formed by bolide impacts could have been a significant process shaping the Earth's volatile and moderately volatile inventory.

Simulated nuclear contamination scenario, solid cancer risk assessment, and support to decision

Abstract The detonation of an (hypothetical) improvised nuclear device (IND) can generate atmospheric release of radioactive material in the form of particles and dust that ultimately contaminate the soil. In this study, the detonation of an IND in an urban area was simulated, and its effects on humans were determined. The risk of solid cancer development due to radiation was calculated by taking into account prompt radiation and whole-body exposure of individuals near the detonation site up to 10 km. The excess relative risk (ERR) of developing solid cancer was evaluated by using the mathematical relationships from the Radiation Effects Research Foundation (RERF) studies and those from the HotSpot code. The methodology consists of using output data obtained from simulations performed with the HotSpot health physics code plugging in such numbers into a specific given equation used by RERF to evaluate the resulting impact. Such a preliminary procedure is expected to facilitate the decision-making process significantly.

Investigation of fissile materials collected from a non-critical nuclear explosion site using non-destructive analytical techniques

In this work, we identify, isolate, and characterize hot particles from bulk soil samples collected from a non-critical nuclear detonation site. Several non-destructive techniques were used to identify and isolate the nuclear fuel particles from soil samples. Digital autoradiography was used to indicate and localize individual “hot particles”, while γ-spectroscopy was employed to confirm plutonium presence. In addition, scanning electron microscopy and energy dispersive spectroscopy were used to study the surface morphology and elemental composition of the hot particles. Initial sample analyses indicate that explosively dispersed non-critical fuel morphology may be markedly different from other non-critical accidents.

3D reconstructions of a controlled bus bombing

Abstract Objectives to demonstrate the usefulness of 3D reconstructions to better understand the dynamic of a controlled bus bombing. Materials and methods 3D models of the victims (pigs) were created from post-mortem CT scanning using Mimic software; 3D models of the crime scene (bus) were generated by means of photogrammetry and modelling techniques, using Photomodeler and 3Ds max software applications. Combinations and visualizations of the scene before and after the explosion were created using 3Ds max. Results several 3D reconstructions of the scene before and after the explosion were re-created to better visualize the dynamic of the incident and the blast injuries. All the pigs suffered extensive blast injuries. The bomb pig and the green pig (adjacent to the detonation site) underwent traumatic amputation and destruction due to a combination of blast injuries. Primary blast injuries and secondary injuries were found in all the other pigs. Fractures both due to tertiary or combined blast injuries were found in all the pigs except the yellow one (far from the detonation side). Three different “injury zones” could be identified based on the severity of the injuries of the pigs, as evaluated from autopsy and CT scanning. Conclusion 3D reconstructions give a clearer and more comprehensive view of the scene and the victims, allowing a better understanding of the entire scene. CT scanning has proved to be an important tool in evaluating blast injuries in dead victims.

Effects of local and global meteorological data on predicting blast noise levels

Predicting noise levels due to military blasts is important near training installations, since surrounding communities may be adversely affected. Prediction models, however, rely on atmospheric or weather data that is not readily available or easily attainable. In this study, blast noise level predictions 1-8 km from the source are made using two sources of meteorological data: publicly available data derived from weather stations that are tens of kilometers from a detonation site, and near-ground weather stations that are 2-6 km from the source. To predict the blast noise levels, statistical learning regression models are trained on each set of meteorological data and over 1000 measured blast events. Predictions from both meteorological data sources are compared to actual blast noise levels recorded during a field experiment by Valente et al. [2012], which were captured under a wide variety of atmospheric conditions. The root-mean-square errors for each model are compared, and the most important input pa...

Point measurements of ambient biological noise before, during, and after multiple underwater detonation events over coral substrate off the southern coast of Oahu, Hawai

Measurements were taken of background noise at the U.S. Navy's Pu‘uloa Underwater Detonation Range off the south coast of O‘ahu during a U.S. Navy underwater explosive training exercise in shallow water (10-18 m) where distance from the detonation site, charge size, and explosive composition were controlled. The autonomous recording unit was deployed approximately 1500 m from the detonation location. The combined data from the paired low (-220 dB re 1 V /μPa) and high (-170 dB re 1 V /μPa) sensitivity hydrophones gives an approximately 100 dB dynamic range view of each event. Each trial consisted of three replicate events per charge weight (2.2, 4.5, and 8.6 kg) for a total of nine explosive events over the coral substrate. An analysis of the background noise environment immediately before and after each replicate is presented using metrics such as spectral probability density. Results are considered in the context of typical coral reef noise including discussion of snapping shrimp noise (one of the dominant features of the soundscape) before and after each event. [Work supported by U.S. Pacific Fleet.]

Chromosome aberrations in Japanese fishermen exposed to fallout radiation 420-1200km distant from the nuclear explosion test site at Bikini Atoll: report 60years after the incident.

During the period from March to May, 1954, the USA conducted six nuclear weapon tests at the "Bravo" detonation sites at the Bikini and Enewetak Atolls, Marshall Islands. At that time, the crew of tuna fishing boats and cargo ships that were operating approximately 150-1200km away from the test sites were exposed to radioactive fallout. The crew of the fishing boats and those on cargo ships except the "5th Fukuryu-maru" did not undergo any health examinations at the time of the incident. In the present study, chromosome aberrations in peripheral blood lymphocytes were examined in detail by the G-banding method in 17 crew members from 8 fishing boats and 2 from one cargo ship, 60years after the tests. None of the subjects examined had suffered from cancer. The percentages of both stable-type aberrations such as translocation, inversion and deletion, and unstable-type aberrations such as dicentric and centric ring in the study group were significantly higher (1.4- and 2.3-fold, respectively) than those in nine age-matched controls. In the exposed and control groups, the percentages of stable-type aberrations were 3.35% and 2.45%, respectively, and the numbers of dicentric and centric ring chromosomes per 100 cells were 0.35 and 0.15, respectively. Small clones were observed in three members of the exposed group. These results suggest that the crews were exposed to slightly higher levels of fallout than had hitherto been assumed.

Stable isotope probing reveals the importance of Comamonas and Pseudomonadaceae in RDX degradation in samples from a Navy detonation site.

This study investigated the microorganisms involved in hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) degradation from a detonation area at a Navy base. Using Illumina sequencing, microbial communities were compared between the initial sample, samples following RDX degradation, and controls not amended with RDX to determine which phylotypes increased in abundance following RDX degradation. The effect of glucose on these communities was also examined. In addition, stable isotope probing (SIP) using labeled ((13)C3, (15)N3-ring) RDX was performed. Illumina sequencing revealed that several phylotypes were more abundant following RDX degradation compared to the initial soil and the no-RDX controls. For the glucose-amended samples, this trend was strong for an unclassified Pseudomonadaceae phylotype and for Comamonas. Without glucose, Acinetobacter exhibited the greatest increase following RDX degradation compared to the initial soil and no-RDX controls. Rhodococcus, a known RDX degrader, also increased in abundance following RDX degradation. For the SIP study, unclassified Pseudomonadaceae was the most abundant phylotype in the heavy fractions in both the presence and absence of glucose. In the glucose-amended heavy fractions, the 16S ribosomal RNA (rRNA) genes of Comamonas and Anaeromxyobacter were also present. Without glucose, the heavy fractions also contained the 16S rRNA genes of Azohydromonas and Rhodococcus. However, all four phylotypes were present at a much lower level compared to unclassified Pseudomonadaceae. Overall, these data indicate that unclassified Pseudomonadaceae was primarily responsible for label uptake in both treatments. This study indicates, for the first time, the importance of Comamonas for RDX removal.

Avian Community Composition in Response to High Explosive Testing Operations at Los Alamos National Laboratory in Northern New Mexico

Breeding bird abundance, species richness, evenness, diversity, composition, productivity, and survivorship were determined near a high-explosive detonation site at Los Alamos National Laboratory, New Mexico, USA, during pre-operation (1997-1999) and operation (2000-2014) periods. The operation periods consisted of detonations ( 0.05) in mean avian abundance and species evenness in any of the operation periods as compared with the pre-operation period. Species richness and diversity were significantly higher (p < 0.05) during the vessel containment period (2007-2014) than the pre-operation period. The time period of this study coincided with a wildfire (2000), a bark beetle infestation (2002), and two periods of drought (Nov 1999-Mar 2004 and Dec 2005-Dec 2014) that affected the study area. Analysis of aerial photos determined that the average percent canopy cover of mature ponderosa pines (Pinus ponderosa) within 100 feet of mist net sites declined from 12% to 3% between 1991 and 2014 and the percent cover of shrubs slightly increased. The percent similarity in presence/abundance between the pre-operation avian community and avian community during the open air, foam containment, and vessel containment periods were 59%, 63% and 68% respectively. Two bird species associated with large trees became less common over the study period (capture rate dropped below 2.0 adults per 600 net-hours relative to the pre-operations period), and four bird species associated

Ground Truth Determinations of Detonation Sites of Peaceful Nuclear Explosions in the Sakha Republic (Yakutia), Russia

Abstract Although ground truth (GT) locations for peaceful nuclear explosions (PNEs) conducted in the former Soviet Union are generally thought to be well known, they can be erroneous by up to tens of kilometers. Information (maps, photographs, descriptions) published in radionuclide contamination and environmental studies, combined with satellite imagery, allow for the improvement of the coordinates of these detonations. We are able to link disturbed areas visible in satellite imagery to probable detonation sites for all PNEs conducted in the Sakha Republic (Yakutia), Russia, and improve on, or confirm, previously published solutions; taking possible uncertainties and alternative sites into consideration, all of them can be located to GT0 or GT1. The improvements are most pronounced for Crystal, where a mound over the detonation site can be identified, and for detonations conducted for the purpose of enhanced petroleum recovery near Tas‐Yuryakh.

Early Bomb Radiocarbon Detected in Palau Archipelago Corals

EARLY BOMB RADIOCARBON DETECTED IN PALAU ARCHIPELAGO CORALS Danielle Glynn 1,2 • Ellen Druffel 1 • Sheila Griffin 1 • Robert Dunbar 3 • Michael Osborne 3 • Joan Albert Sanchez-Cabeza 4 ABSTRACT. In order to evaluate the variability in surface water masses in the Western Pacific Warm Pool, we report high- precision radiocarbon measurements in annual and seasonal bands from Porites lutea corals collected from the Palau Archi- pelago (7N, 134E). Annual coral bands from 1945 to 2008 and seasonal samples from 1953 to 1957 were analyzed to cap- ture the initial early input of bomb 14 C from surface thermonuclear weapons testing in the Marshall Islands. Results show a pre-bomb average  14 C value of –54.9‰ between 1945 and early 1953. Beginning early in 1954, there is a rapid increase to a maximum of –23.1‰ at the start of 1955. Values continued to rise after 1957 to a post-bomb peak of 141‰ by 1976. The large initial rise in  14 C cannot be accounted for by air-sea CO 2 exchange. Results therefore suggest that the primary cause of this increase is the lateral advection of fallout-contaminated water from the Marshall Islands to Palau via the North Equa- torial Current and then to the North Equatorial Countercurrent. INTRODUCTION Atmospheric 14 CO 2 exchanges with CO 2 in the ocean to become part of the pool of dissolved inor- ganic carbon (DIC) in seawater. It takes approximately 10 yr for atmospheric 14 CO 2 to exchange with the surface ocean through air-sea gas exchange. The  14 C values then decrease with depth in the ocean due to decay and isolation from the atmospheric source of 14 C. Hermatypic, or reef-build- ing, corals that grow in the surface ocean incorporate DIC into their aragonitic skeletons and record the  14 C value of the surrounding seawater at the time of formation. Annual density bands, which are discernable when a coral slab is X-rayed, can be dated and used as proxies of past changes in water mass (Druffel and Linick 1978) and climate (Guilderson and Schrag 1998). Human-induced atmospheric injection of 14 C by thermonuclear bomb testing occurred during the 1950s and early 1960s (Yang et al. 2000). Global, regional, and local nuclear fallout sources have been observed in coral records. Ground-based nuclear explosions at Bikini Atoll (1135N, 16523E) turned coral, seawater, and other materials at the detonation site into reactive dust parti- cles containing higher  14 C values, known as “close-in fallout.” The surface nuclear explosion on 1 March 1954 at Bikini Atoll liberated and contaminated particles that eventually condensed into droplets that fell out of the atmosphere (Glasstone and Dolan 1977). Fallon and Guilderson (2008) observed a rapid rise of  14 C values in an Indonesian coral approximately 11 months after this nuclear detonation, suggesting that nuclear close-in fallout led to a more rapid absorption of 14 C into the water and coral than air-sea gas exchange alone. However, the processes by which this fallout was incorporated back into the environment are not well known. The Palau Archipelago lies in a complicated region of oceanic currents between the Philippine Sea and the Pacific. Tradewinds drive surface currents from east to west in the tropical Pacific, forming the North Equatorial Current (NEC). In the western Pacific, these waters then turn into the eastward- flowing North Equatorial Countercurrent (NECC), which brings water to Palau. The strength and position of both currents has been found to vary seasonally and show large interannual variability 1 Earth System Science Department, University of California, Irvine, Irvine, California 92697-3100, USA. author. Email: dglynn@uci.edu. 3 Department of Environmental Earth System Science, Stanford University, Stanford, California 94305-2115, USA. 4 Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autonoma de Mexico, 04510 Ciudad de Mexico, Mexico. 2 Corresponding © 2013 by the Arizona Board of Regents on behalf of the University of Arizona Proceedings of the 21st International Radiocarbon Conference edited by A J T Jull & C Hatte RADIOCARBON, Vol 55, Nr 2–3, 2013, p 1659–1664