Using radium and actinium isotopes as tracers of ocean dynamics:
A Franco-British partnership (LEGOS-Univ. Edinburgh)
Mixing in the ocean is an important mechanism that controls heat transport and the distribution of chemical elements. Quantification of the mixing is, however, problematic. We proposed to use a multi-tracer approach to better constrain this process. To achieve this goal, a collaboration between LEGOS and the university of Edinburgh, UK was established in the framework of the ALLIANCE programme.
Mixing of ocean waters is an important way of transport for substances in the marine system. Mixing processes control heat and gas transport, but also the fate of hazardous substances at the sea floor or nutrient inputs. Naturally occurring radiosotopes can be used trace ocean mixing and to quantify fluxes in the ocean. Unique tools for studying the -still poorly understood- mixing in the ocean on a variety of time scales are four radium isotopes (224Ra, 223Ra, 228Ra and 226Ra) with half-lives ranging from 3.7 days to 1600 years. Actinium (227Ac), one of the rarest and least studied elements in the ocean, has important similarities with some of the Ra isotopes, but also clear differences, which can help to clarify processes that are not mirrored in Ra isotopes.
In the framework of the Franco-British partnership ALLIANCE programme (Egide/ Bristish council), we proposed to combine our capabilities in Ra and Ac analysis, through a collaboration between LEGOS (P. van Beek) and the University of Edinburgh, UK (W. Geibert). The collaborating institutes have unique strengths. While LEGOS Toulouse provides some of the world leading facilities for low-level gamma spectrometry and ultra-trace analysis in marine samples, the UK group has exclusive expertise in the radiochemical analysis of actinium. The combined capabilities will enable the institutes to tackle questions in the context of the release of substances from the sea floor and subsequent transport to the open ocean. This is of importance for understanding/ modelling the release of hazardous substances from the sea floor, for the distribution of growth-limiting micronutrients in the ocean, or for ocean mixing, which is one of the controlling factors of the earth's climate. The variety of half-lives provided by the radium and actinium isotopes ranges from 3.7 days to 1600 years, giving insight to transport processes from near-shore coastal release, to coastal/open ocean mixing and global circulation.
We therefore aim at combining our knowledge to form a larger research unit that can tackle research questions which are out of reach for the single institutions.The goal of this collaborative project was to provide mutual exchange of samples and analytical techniques that could strengthen both institutions for future applications. We also wanted to establish networks amongst the PhD-students, as a way to improve their future chances for international collaboration. In particular, the network that we started to build here will allow us to build group of geochemists who will be able to participate efficiently to cruises that will take place in the framework of the GEOTRACES program.
Each partner spent time either at LEGOS or at the University of Edinburgh (3 trips funded by Egide-British Council per year for each partner). PhD-students were trained on the different methods (RaDeCC, low-background gamma-spectrometry at Toulouse; alpha spectrometry at Univ. Edinburgh). Analyses of samples collected in different projects could thus be analyzed by the consortium. Standard solutions that were shared between the institutions were also made. Interaction between the scientists also favoured discussions of the data thus produced and production of collaborative papers.
Ra and actinium were analyzed in collaboration with each partner in samples that were collected by the phD students in the framework of their research studies:
- in Loch Etive (Scotland; Alan Hsieh, Univ. Oxford)
- off Antarctica (Amber Annett and Sian Henley, Univ. Edinburgh)
- off Kerguelen Islands (KEOPS 2 project).
Unique radium and actinium data could thus be obtained and published as collaborative papers (Annett et al., 2013; Hsieh et al., 2013; Cockenpot et al., in prep.). As an example, we report a vertical profile of 227Ac determined in the wake of the Kerguelen Plateau (Figure 1). The comparison of the 227Ac activities with the 231Pa activities allows us to calculate excess 227Ac activities. We postulate that these excess 227Ac activities have been released by the deep-sea sediments and were brought upward by the vertical mixing. This pattern makes of 227Ac a useful tracer to quantify vertical mixing.
Fig. 1 : 227Ac activities determined at station B11 off the Kerguelen Plateau (Southern Ocean). These activities are shown in comparison with the 231Pa activities (231Pa being the mother of 227Ac) determined by Venchiarutti et al.(2011).
Very few data of excess 227Ac have been reported until now. 227Ac is rare in the ocean and is thus difficult to analyze. The vertical profile of excess 227Ac thus constitutes a unique dataset, which was acquired thanks to a fruitful collaboration between LEGOS-Univ. Edinburgh.
Hsieh, Y.-T., Geibert, W., van Beek, P., Stahl, H., Aleynik, D., and Henderson, G.M., 2013. Using the radium quartet (228Ra, 226Ra, 224Ra, and 223Ra) to estimate water mixing and radium inputs in Loch Etive, Scotland. Limnology and Oceanography 58: 1089-1102
Annett A., Henley S., van Beek P., Souhaut M., Ganeshram R., Venables H., Meredith M., Geibert W., 2013. Use of radium isotopes to estimate mixing rates and trace sediment inputs to surface waters in northern Marguerite Bay (Antarctic Peninsula), Antarctic Science 25(3), 445–456, doi:10.1017/S0954102012000892
Participants: Pieter van Beek, Marc Souhaut/ Coll. Walter Geibert (Univ. Edinburgh, UK)
LEGOS : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Toulouse)
University of Edinburgh
University of Oxford
Fundings: ALLIANCE Programme; EGIDE (France) - BRITISH COUNCIL (UK)