Uranium-series isotopic studies of marine phosphorites and associated sediments from the East Australian continental margin

Details Uranium-series isotopic studies of marine phosphorites and associated sediments from the East Australian continental margin Uranium-series isotopic studies of marine phosphorites and associated sediments from the East Australian continental margin

Oct 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..80...19o&link_type=abstract

Earth and Planetary Science Letters, Volume 80, Issue 1-2, p. 19-35.

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The uranium-series disequilibria of phosphorites, sediments, and coexisting solitary corals from the outer continental shelf and upper slope off eastern Australia have been determined in an attempt to relate the genesis of phosphorites in this area to an absolute time scale. The 234U/238U, 230Th/234U, and 231Pa/235U activity ratios in the phosphorites are internally consistent, and indicate that the apatite component of the phosphorites has remained a closed system with respect to U, Th, and Pa, except for very minor losses of 234U. These losses do not affect the 230Th/234U-derived ages, and comprise only 1-2% of the total 234U activity, significantly less than the losses observed in phosphorite nodules from off Peru-Chile.
A morphological subdivision of the phosphorites into either non-ferruginous or ferruginous is supported by the isotopic data. Earthy, friable non-ferruginous phosphatic nodules range in age from 2.5 to > 250 kyr; Holocene ages were obtained from five different locations in water depths ranging from 365 to 450 m. In contrast, the uranium-series isotopes in well-indurated ferruginous nodules occurring in the same general area, but in shallower ( < 350 m) water, are in radioactive equilibrium, indicating that their ages exceed 800 kyr. Solitary corals (Caryophyllia planilamellata Dennant 1906) are associated with non-ferruginous nodules on the upper slope, and commonly contain internal phosphatic molds. Several corals and their internal molds from one location were dated by uranium-series methods; the coral ages ranged from 17 to 20 kyr, and were typically 10-15 kyr older than their respective phosphatic internal molds. These results are consistent with the sedimentological observations, and provide independent evidence that geologically meaningful ages can be obtained from marine phosphorites.
Phosphate deposition off eastern Australia appears to have been largely continuous throughout the late Quaternary, rather than being restricted to times of high sea-level, as reported for phosphorites off Peru-Chile.

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