Marine tracers

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

Although there are three Rji isotopes in the U- and Th-decay series, only is sufficiently long lived tm= 3.8 days) to be a useful estuarine tracer. Radioactive decay of Ra continuously produces Rn, which because of its short half-life is generally in secular equilibrium in seawater. Being chemically non-reactive except for very weak Van der Waals bonding makes this isotope a unique marine tracer in that it is not directly involved in biogeochemical cycles. 

Aarkrog A, Carlson L, Chen QJ, et al. (1988) Origin of Tech-nerium-99 and its use as a marine tracer Nature 335 338. 

In the marine environment, the numerous radionuclides can be classified into three broad categories based on their production or origin (1) those derived from the weathering of continental rocks, the primordial radionuclides, (2) those formed from cosmic radiation, the cosmogenic radionuclides, and (3) those artificially introduced into nature, the anthropogenic or transient radionuclides and tracers. The primordial radionuclides (e.g. Th, and U) were... 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

Uses. The main use for Sr nitrate is for producing red colors in pyrotechnics, railroad fusees, marine signals, tracer compositions and in matches (Refs 2, 4 7). A high degree of purity is required, for the contamination of Sr nitrate with... 

Miller RJ, Smith CR, DeMaster DJ, Pomes WL (2000) Feeding selectivity and rapid particle processing by deep-sea megafaunal deposit feeders A " Th tracer approach. J Mar Res 58 653-573 Moore RM, Hunter KA (1985) Thorium adsorption in the ocean - reversibility and distribution amongst particle sizes. Geochim Cosmochim Acta 49 2253-2257 Moore RM, Millward GE (1988) The kinetics of reversible Th reactions with marine particles. Geochim Cosmochim Acta 52 113-118... 

Nozaki Y, Yamada M, Nikaido H (1990) The marine geochemistry of actinium-227 evidence for its migration through sediment pore water. Geophys Res Lett 17 1933-1936 Nozaki Y (1993) Actinium-227 a steady state tracer for the deep-se basin wide circulation and mixing studes. In Deep Ocean Circulation, Physical and Chemical Aspects. Teramoto T (ed) Elsevier p 139-155... 

Broecker WS, Peng T-H (1982) Tracers in the Sea. Lamont-Doherty Geological Observatory, New Yoik Buesseler KO, Bauer J, Chen R, Eghnton T, Gustafsson O, Landing W, Mopper K, Moran SB, Santschi P, Vernon Clark R, Wells M (1996) Sampling marine colloids using cross-flow filtration Overview and results from an intercomparison stndy. Marine Chem 55 1-31 Buffle J, van Leeuwen H (eds) (1992) Environmental Particles. Lewis Publishers, Boca Raton Florida... 

Cochran JK (1984) The fates of U and Th decay series nuclides in the estuarine environment. In The Estuary as a Filter. Kennedy VS (ed) Academic Press, London, p 179-220 Cochran JK (1992) The oceanic chemistry of the uranium - and thorium - series nuclides. In Uranium-series Disequilibrium Applications to Earth, Marine and Environmental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 334-395 Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cochran JK, Carey AE, Sholkovitz ER, Surprenant LD (1986) The geochemistry of uranium and thorium in coastal marine-sediments and sediment pore waters. Geochim Cosmochim Acta 50 663-680 Corbett DR, Chanton J, Burnett W, Dillon K, Rutkowski C. (1999) Patterns of groundwater discharge into Florida Bay. Linrnol Oceanogr 44 1045-1055... 

Santschi PH, Guo LD, Asbill S, Allison M, Kepple AB, Wen LS (2001) Accumulation rates and sources of sediments and organic carbon on the Palos Verdes shelf based on radioisotopic tracers ( Cs, " °Pu, 240pu, 2i0pb, 234xh, 238u j d C). Marine Chem 73 125-152 Sarin MM, Church TM (1994) Behavior of uranium during mixing in the Delaware and Chesapeake Estuaries. Estuar Coastal Shelf Sci 39 619-631... 

---