Other
Scientific paper
Apr 1978
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978e%26psl..39..235k&link_type=abstract
Earth and Planetary Science Letters, Volume 39, Issue 2, p. 235-249.
Other
4
Scientific paper
A fiber extraction technique is used to concentrate Ra and Th isotopes from 1000 liters or more of seawater. Natural 226Ra and 234Th are used as yield tracers. In the equatorial Pacific the 228Ra activity of surface water varies from 20 to 1 dpm/1000 kg and generally decreases away from continental shelf areas. Across the Peru Current System, this decrease is modeled as one-dimensional diffusion and indicates the possibility of two flow regimes with distinct characteristic mixing lengths and apparent eddy diffusivities of 105 and 107 cm2/s. The perturbing effects of advection and equatorial upwelling west of the Galapagos Islands are noted. Off the coast of Southern California a vertical 228Ra distribution gives an apparent diffusivity of 1.6 cm2/s for the upper thermocline. 226Ra concentrations near the coast appear to be higher than the open ocean values at comparable depths, which may reflect supply of this isotope from continental shelf sediments and/or upwelling. The insoluble daughter/soluble parent activity ratios 228Th/228Ra and 234Th/238U in the equatorial Pacific surface water display latitudinal trends which may be correlated with productivity variations. Near the coast of California these ratios reflect the differing oceanographic conditions north and south of Pt. Conception indicating a mean chemical removal time constant on the order of 4 months for Th and other highly reactive elements within the Southern California Bight. The 232Th content of seawater sampled is less than 0.1 μg/1000 1; most of the published values for seawater 232Th could well be too high. A comparison of the two methods of determining 228Ra (via 228Ac and via 228Th) made on 64 seawater samples shows that the time delay required by the 228Th method is more than compensated by its better analytical simplicity and precision.
Knauss Kevin G.
Ku Teh-Lung
Moore Willard S.
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