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Natural radioisotopes as tracers of the fate of organic carbon in the oceans
Shimmield, G.B. (1996). Natural radioisotopes as tracers of the fate of organic carbon in the oceans, in: Baeyens, J. et al. (Ed.) Integrated Marine System Analysis. European Network for Integrated Marine System Analysis. FWO Vlaanderen: minutes of the first network meeting (Brugge, 29.02.96-02.03.96). pp. 203-204
In: Baeyens, J. et al. (Ed.) (1996). Integrated Marine System Analysis. European Network for Integrated Marine System Analysis. FWO Vlaanderen: minutes of the first network meeting (Brugge, 29.02.96-02.03.96). Vrije Universiteit Brussel. Laboratorium voor Analytische Chemie: Brussel. 217 pp., meer
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| Abstract |
The ubiquitous distribution of cosmogenic uranium isotopes (238U and 235U) in seawater, and thorium (232Th) in sediments, gives rise to the natural decay series comprising a number of isotopes having distinctive biogeochemical affinities. With a range of useful half-lives, from days to thousands of years, these radionuclides have become valuable tools in chemical oceanography. Knowing the source function (i.e. the distribution of a parent isotope), and the half-lives of both parent and daughter, the measured distribution of a daughter isotope yields information on oceanic residence time and the time constant of "scavenging" for particle reactive nuclides. In many cases, efficient removal of the daughter nuclide onto sinking particulate phases results in an "excess" accumulation in sediments whose distribution down core yields information on bioturbation rates and sediment accumulation rates. Furthermore, the known production rate of certain nuclides (e.g. 230Th) makes them suitable for the calibration of sediment traps to examine trapping efficiency of sinking particles. Recently, studies have shown the similarity in radionuclide and carbon fluxes in the open ocean. This has prompted the use of radionuclides as tracers of export flux from the euphotic zone after employing a suitable conversion factor from nuclide activity to carbon wt. %. Such studies offer tremendous advantages in terms of improved spatial and temporal resolution for flux studies when compared to more traditional trapping methods. In this presentation, the current knowledge on radionuclide biogeochemical cycling is presented with reference to work on dissolved and particulate 210Po, 210Pb and 234Th in the Atlantic and Southern Oceans measured at the University of Edinburgh. New methodologies have been employed (shipboard gamma spectroscopy) and their advantages and disadvantages will be discussed. Water column profiles provide a means of estimating residence time of particulate matter and fluxes. These are compared to sediment trap estimates, and the problems of non - steady state dynamics in the models employed, are identified. Deep water column sediment traps provide valuable samples for the measurement of radionuclide and carbon fluxes over the annual productivity cycle. Examples are taken from the northeast Atlantic. At the benthic interface, radioisotope distribution may be used to calculate bioturbation rates, and average carbon fluxes. This methodology is compared to other estimates of benthic carbon flux. The importance of the distribution of oceanic primary production, and the seasonality of export flux, is shown to have a major impact on processes affecting carbon cycling in the benthic realm. New advances in sample collection and measurement are described. In conclusion, some recommendations for future work in identifying shortcomings in radionuclide flux measurements are discussed, as well as a request for the incorporation of such studies in many biogeochemical cycling experiments. |
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