In this project, researchers at the Lamont-Dougherty Earth Observatory of Columbia University will obtain the total concentrations of the anthropogenic radionuclides 239Pu, 240Pu, 237Np, and 137Cs in selected full-depth profiles from the various Atlantic GEOTRACES cruises occurring in 2010 (US, Netherlands, and Germany). This will allow the broad brush characterization of these anthropogenic radionuclides some 37 years after GEOSECS. The isotopes of interest, in addition to being transient tracers, exhibit a range of Kd values (sediment water distribution coefficients, Pu > Np ,Cs), and geochemical behaviors as well as provide a means to resolve different sources of radioactive contamination. This will allow the team to address processes such as advection (new water mass tracers), sources and sinks (characteristic isotopic signatures), as well as processes related scavenging and particle dynamics across a range of contrasting regions. The planned US, German, and Dutch GEOTRACES cruises transect regions that represent a full suite of physical and biogeochemical processes that affect TEIs. By comparing radionuclide distributions, isotopic composition, inventories, and inventory ratios of particle reactive (Pu) to conservative (Cs and Np), we will learn first order information about rates of scavenging and transport of these nuclides that is complimentary to that gained through study of other TEIs by GEOTRACES.
Since GEOSECS, -- the first and only study to produce a view of the global oceanic distributions of plutonium and 137Cs -- methods have improved requiring less volume and providing not only better sensitivity but additional information (i.e., mass spectrometric techniques are capable of measuring the 240Pu/239Pu atom ratio; traditional alpha spectrometry is not). Where possible, the team will compare distributions measured in 2010 to those measured in 1972-1973 providing a ca. 40-year perspective on processes that control the fate and transport of these contaminants. In addition to providing updated baselines for Pu and 137Cs concentrations in the Atlantic, they will also provide information on 237Np as well as the 240Pu/239Pu isotopic composition, which have not been measured at this scale.
Broader Impacts: This project will be an international collaboration and will yield valuable environmental radionuclide data from a number of locations throughout the Atlantic that would not be obtained otherwise. An important problem related to both anthropogenic climate change and sustainability is societal heavy reliance on fossil fuels as the main source of energy. Although the solution will certainly be complex, an increase in the proportion of energy derived from nuclear power is likely. The information gained from this proposal will be useful in assessing the fate of nuclides that might be released as a result of increased reliance on nuclear power in the future.
Introduction The main goal of our RAPID project is to elucidate the key factors that govern the behavior of anthropogenic radionuclides in the marine environment. Significant contamination on global, regional, and local scales has occurred as a result of nuclear industrial activities such as weapons production and testing, fuel reprocessing (e.g., Sellafield and La Hague), and accidents (e.g., Chernobyl and Fukushima). Contaminants like plutonium, cesium and neptunium are potentially harmful to both the natural and human environment, due to their toxicity and/or radioactivity. Improving our knowledge of their behavior is important with respect to our stewardship of the ocean environment. While we generally understand these contaminants with respect to broad classification (e.g., particle reactive, conservative, etc.), our knowledge is limited with respect to their present spatial distributions and complete characterization of how physical, chemical, and biological processes affect their transport and fate. Further, we can use specific contaminants as tools to learn about the processes that regulate the distributions of trace elements themselves, such as removal from the ocean on settling particles and transport by ocean currents. The isotopes of interest (137Cs, 237Np, 239Pu, and 240Pu) have several characteristics that make them useful and unique tools for studying oceanic processes. They are transient tracers with well-constrained release histories making them capable of providing independent estimates of process rates. Where possible, we compare distributions measured in 2010 to those measured during the 1972-1973 GEOSECS Program, providing a ~40 year perspective on processes that control the fate and transport of these contaminants. In addition to providing updated baselines for Pu and 137Cs concentrations in the Atlantic Ocean, we are also providing information on 237Np as well as the 240Pu/239Pu isotopic composition, which have not previously been measured at this level of detail. In 2010, we were presented with an opportunity to participate in several GEOTRACES cruises occurring in the Atlantic Ocean (Fig 1). Given the timing of the cruise departures, we secured support via a RAPID award. GEOTRACES is a global study of the marine biogeochemical cycles of trace elements and their isotopes, representing an important and sustained focus within the international oceanographic community. Among its strengths are the measurement of multiple TEIs and the inclusion of several specialized sampling systems on each cruise. A key aspect of the program is the truly collaborative nature of the work and the assembly of a critical mass of knowledge and expertise being brought to bear on the study of marine biogeochemical processes. This approach provides important insights and allows questions to asked and answered that would be difficult if not impossible on a single project cruise. With dependence on nuclear power likely to rise, our participation in GEOTRACES is timely and provides an unprecedented opportunity learn about the behavior of anthropogenic radionuclides in the ocean. Objectives Determine the vertical distributions of 239Pu, 240Pu, 237Np, and 137Cs and compute water column inventories for each nuclide. Determine the horizontal distributions of the radionuclides of interest; combined stations from the different cruises allow zonal and meridional sections. Characterize the isotopic composition of the different water masses where they intersect the different cruise tracks and in areas where important processes are occurring. Activities International collaboration and coordination of sample collection for the anthropogenic radionuclides on several Atlantic GEOTRACES. Optimization of analytical methods and development of efficient sample processing protocols. Analysis of >200 samples including seawater samples and large volume filtered particulate samples. Creation of an important sample archive given the large amount of ancillary information and their place in the long-term time series of the fate and transport of anthropogenic radionuclides in the marine environment. Major Findings Comparison to historical data - Where GEOSECS data (~1972) are available for comparison, Pu inventories have increased slightly or remained approximately the same. Conversely, in all cases where comparisons could be made, Cs inventories have increased dramatically from those measured in the 1970s (Fig. 2). Pu, 137Cs, and 237Np sections along GA02 Leg 1 - Elevated levels of Pu are observed at mid-water depth. The pattern is generally similar to that of dissolved phosphate which supports export from the surface on particles and re-mineralisation at depth as the key process. Elevated levels of 137Cs and 237Np in the northern deep waters provide evidence of additional input present in the Denmark Strait overflow water, likely originating from European fuel reprocessing facilities. (Fig. 3). At equatorial stations (GA11), we observe elevated 137Cs without elevated 237Np, suggesting a different source or process. Pu-particulate fluxes in the equatorial Atlantic (GA11) - Assuming that vertical processes dominate, deviations of the water column 237Np/239Pu inventory ratios from the average global fallout value can be used to estimate Pu-particulate fluxes in the equatorial regions, which are comparable to Pu-particulate fluxes obtained from sediment trap studies (Fig. 4).
