Aims and background

This project describes the application of the radioactive isotopes ¹²⁹I and ¹⁰Be as tracers of ocean water circulation, with the aims of:

1. Finding the temporal and spatial pattern of water mass exchange within the Nordic Seas (Iceland, Norwegian and Greenland Seas) and between the North Atlantic and Arctic Oceans.
2. Quantifying the magnitude of radio-active contamination from anthropogenic sources, including atomic weapon tests, nuclear accidents and discharges from nuclear reprocessing facilities.

The extent of water mass circulation between the North Atlantic and Arctic Ocean is an important issue for 1) understanding of the link between climatic variability and ocean circulation and 2) recognition of the fate (residence time and ventilation pathways) of pollutants that enter the ecologically sensitive marine waters of the Arctic Ocean. The exchange of water between these oceans occurs via a complex pattern of surface and deep currents that have their main paths along the Nordic (Iceland, Greenland and Norwegian) and the Labrador Seas.

Presently, we have only limited data on the magnitude and residence time of water circulation in the Nordic Seas. For example, further knowledge is needed on the East Greenland Current and its branches (Jan Mayen Current, East Iceland Current) and the details of the inflow and outflow of the Nordic Seas, mainly in the eastern Fram Strait, the Denmark Strait and the Iceland-Scotland sill. Therefore, one main objective of the Arctic Ocean 2002 expedition was to obtain new and comprehensive data on water condition within the Nordic Seas, particularly during spring. Such data have rarely been sampled or measured before.

Radioactive isotopes (natural and anthropogenic) have been extensively used as tracers in establishing circulation patterns and ventilation rates in the oceans. For example, the distributions of radioactive ¹⁴C, ¹³⁷Cs, ⁹⁰Sr and ¹²⁹I have been valuable tools for estimating the age and ventilation in the Arctic and North Atlantic, among other oceans. Elaborate time-series data are, however, not available, which means that most oceanic processes are currently far from fully understood. Several problems are generally faced when applying radioactive tracer  techniques, including: 1) half-life of the isotope; 2) sensitivity of the analytical techniques; 3) origin of the isotope; 4) oceanic residence time and 5) time span or time window considered. The choice of a suitable radioactive isotope or isotopes to fulfil a specific goal in solving a scientific problem is thus limited by the points mentioned above.

In this project we have selected two radioactive isotopes for our tracer approach. These isotopes are iodine-129 (half-life 15.7 million years) and beryllium-10 (half-life 1.51 million years). ¹²⁹I has both natural and anthropogenic sources and has an oceanic residence time of about 10 000 years. Naturally, the isotope is produced either in the atmosphere by bombardment of mainly xenon (Xe)-129 molecules by cosmic rays, or in the earth’s crust by fission of uranium atoms. Anthropogenic production has added huge amounts of the isotope to the earth’s surface environments (see table below) which has in practice led to overprinting of all the naturally produced signal.

Presently, radioactive releases from nuclear reprocessing plants in western Europe constitute the main source of the ¹²⁹I found in the oceans. The main marine pathway of the isotope to the Arctic Ocean and back ventilation into the North Atlantic is via the Nordic Seas (figure 1). Concentration of ¹²⁹I in these water masses depends on the source, path, residence time and rate of mixing, features that can be evaluated once concentrations of the isotope are measured. The biophilic nature of iodine adds a further concern to monitoring of the environmental levels and impacts. Presently ¹²⁹I may not constitute an environmental health hazard. This situation may, however, change due to continuous discharge from nuclear facilities worldwide, and due to the long half-life of ¹²⁹I , which assures its presence in nature essentially forever compared to human life times.

The isotope ¹⁰Be has no anthropogenic source and its natural production occurs to 99% in the atmosphere through cosmic ray bombardment of oxygen and nitrogen molecules. The variability of ¹⁰Be concentrations in ocean water mainly reflects changes in water mass conditions and sources (deep vs. surface water and residence time). Data on the distribution of ¹⁰Be in the ocean are rather limited and thus this study will provide both new data sets and application in assessing ocean circulation dynamics.

Sampling and analytical techniques

The sampling campaign on the ice-breaker Oden mainly involved water and ice. Ice cores from five stations with ice thicknesses of 1.5–3 m were sliced at about 20 cm intervals and the slices were put into polyethylene bottles after measurement of temperature. The ice samples (a total of 35 samples) were left to melt for measurement of salinity and then stored in a cold room.

Water samples (0.25–2 litres) were collected from stations covering Storfjorden, the section between Svalbard and north Greenland, the Greenland Sea and the Denmark Strait (figure 1). The samples (a total of 310) were taken from water containers combined with a CTD (Conductivity, Temperature and Depth) measurement instrument. Samples were taken in polyethylene bottles which were then securely closed and stored in a cold dark room.

¹²⁹I and ¹⁰Be isotopes occur in ocean water in minute amounts (10^-15 g/litre). Accordingly detection and measurements of these isotopes are performed using ultra-sensitive accelerator mass spectrometry. Before measuring the isotopic content of a sample, I and Be must be chemically extracted from the samples. After extraction as silver iodide (AgI) and beryllium oxide (BeO), the material is ionised and atoms are accelerated to energies of millions of electron volts (MeV), where discrimination between the ions of interest is achieved. Finally, ions are individually counted at the detector and identified by their atomic number and mass.

We shall start measurement of ¹²⁹I and ¹⁰Be from the Arctic Ocean 2002 expedition during 2003.

Scientific co-operation

The objectives of this project will be best achieved if all regions of the Nordic Seas are sampled. This has been accomplished by co-operation with the Knorr expedition, which was organised by the US National Science Foundation and sampled the ice-free water regions of the Nordic Seas. We had one of our scientists on the Knorr expedition and we have sampled several sections (about 400 surface and deep water samples), which will be used in our investigation. At the same time, French scientists (led by Dr. Jean-Claude Gascard from the Université Pierre et Marie Curie) have collected about another 200 samples during the Knorr expedition for analysis of the isotope ¹²⁹I. Co-operation with the French scientists will enlarge the resolution of samples to be analysed and accordingly the details of water circulation in the Nordic Seas.