Thorium Time study

Kharkar DP, Thomson J, Turekian KK, Forster WO (1976) Uranium and thorium series nuclides in plankton from the Caribbean. Limnol Oceanogr 21 294-299 Krishnaswami S, Lai D, Somayajulu BLK, Weiss R, Craig H (1976) Large-volume in situ filtration of deep Pacific waters mineralogical and radioisotope studies. Earth Planet Sci Lett 32 420-429 Livingston HD, Cochran JK (1987) Determination of transuranic and thorium isotopes in ocean water in solution and in filterable particles. J Radioanal Nucl Chem 115 299-308 Masque P, Sanchez-Cabeza JA, Braach JM, Palacios E, Canals M (2002) Balance and residence times of °Pb and 4 o in surface waters of the northwestern Mediterranean Sea. Cont Shelf Res 22 2127-2146 Matsumoto E (1975) Th-234-U-238 radioactive disequilibrium in the surface layer of the oceans. Geochim Cosmochim Acta 39 205-212... 

The common-lead method looks at the isotopic evolution of lead in systems with U/Pb and Th/Pb ratios similar to or less than the ratios in bulk solar system materials. The original formulation, by Holmes and Houtermans, is a single stage model that accounts for the isotopic composition of any sample of common lead in terms of primordial lead plus radiogenic lead produced in the source up to the time that lead was separated from uranium and thorium. Multistage models that more accurately describe the evolution of natural systems have been developed. The common-lead method is used in cosmochemistry primarily to study the time of differentiation and reservoir evolution in differentiated bodies... 

The first studies of thorium alkoxides were carried out by Bradley [223] in the mid-950s in connection with the problem of the separation of rare elements of Group IV and Ce(IV). They were devoted at the same time to the solution of certain theoretical problems such as the influence of the size of the central... 

In a study of the long-lived protactinium isotopes produced from thorium bombarded by high energy deuterons or helium ions (175), pieces of thorium metal of 25 mil thickness were used to increase the total yield of the protactinium. On the other hand, when the time for chemical separations had to be shortened in order to study the short-lived protactinium Isotopes, thinner pieces of thorium, 5 mils or less in thickness, were used to ensure rapid dissolution. In some cases, thorium nitrate powders wrapped in aluminum were used as the target in order to reduce further the time for dissolution of the target. 

Slater was invited to work with J. J. Thomson57 at the Cavendish Laboratory in Cambridge, where she studied the decay products of thorium from 1903 to 1905. That year, she accepted a position as Science Teacher at KEVI, staying there until 1909, when she took up an offer of Science Teacher at Cheltenham Ladies College. In 1913, she returned to Newnham, this time as Assistant Lecturer in Physics and Chemistry, being promoted to Lecturer in 1914. 

Holland and Gottfried (1955) studied the changes in density and unitcell dimensions as a function of the total a-particle dose D per milligram of zircon. D was estimated from the present a-activity and the Th/U ratio, together with the age T of the sample, on the assumption that equilibrium in the uranium and thorium decay series had been established in a time that was short compared with T. T was determined from the present a-activity and lead content (see Faure 1977). It was found that for )< 10, the density was approximately 4.7 g/cm. As the dose was increased, the density decreased slowly at first and then more rapidly, and finally approached asymptotically a value of approximately 3.95 g/cm. ... 

At about the time that Honeyman and Santschi published their colloidal pumping hypothesis, lab and field studies showed that thorium and other metals in seawater are, indeed, associated with colloids (Baskaran et al., 1992 Moran and Buesseler, 1992 Moran and Moore, 1989 Niven and Moore, 1988). Subsequently, many more studies have demonstrated that colloids are abundant in seawater and that they play an important role in the marine biogeochemical cycles of organic carbon and of many trace elements (Guo and Santschi, 1997). 

Regardless of the size cutoff used to define the colloid fraction (1 kDa or 10 kDa), turnover times of colloidal material were found to be short (ranging from about a day to a few weeks). In studies where COM was further divided into size fractions of >1 kDa and >10 kDa, the larger size fraction was found to have a shorter turnover time (about a week in the Mid-Atlantic Bight and in the Gulf of Mexico) than the smaller size fraction (about a month Guo et al., 1997). These residence times represent lower limits, because desorption of thorium and disaggregation of particles was not considered. Nevertheless, even allowing for substantial errors due to these... 

Bruland and Coale (1986) and Coale and Bruland (1985) applied a steady-state irreversible scavenging model (Equations (l)-(5) above) to measured profiles of dissolved and particulate " Th in order to evaluate the rate constant for sorption (fci) and the mean residence time of particulate thorium (Equation (5)). They reasoned that desorption could be neglected because the calculated residence time of particulate thorium (2-18 d) was much less than the residence time of particulate thorium with respect to desorption (3 months Bacon and Anderson, 1982). Therefore, an irreversible scavenging model could be applied, at least to surface waters of the productive California Current, which served as their study region. Similar reasoning has been applied to the Southern Ocean (Rutgers van der Loeff et al., 1997). 

The two principal disadvantages of using Po as an export tracer are its half-hfe and the labor-intensive nature of its measurements. The half-life of °Po (138 d) is substantially greater than the timescale for seasonal phytoplankton blooms. Consequently, the amplitude of the seasonal cycle of scavenging and export will be damped by the time constant for °Po production and decay. Application of the °Po method requires independent determination of °Pb and °Po concentrations, both of which are labor intensive. Typically, °Po is determined by isotope dilution alpha spectrometry, and °Pb isolated from the sample is set aside for several months to permit ingrowth of °Po, which is measured a second time to evaluate the concentration of °Pb. Despite these limitations, if further studies confirm that Po is a more specific tracer than thorium for POC, and if it can be demonstrated... 

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