Elsevier

Geochimica et Cosmochimica Acta

Volume 56, Issue 9, September 1992, Pages 3375-3388
Geochimica et Cosmochimica Acta

Scavenging of thorium isotopes by colloids in seawater of the Gulf of Mexico

https://doi.org/10.1016/0016-7037(92)90385-VGet rights and content

Abstract

A suite of surface-water samples from the Gulf of Mexico was analyzed to ascertain the extent of association of Th isotopes (232Th, 234Th) with colloids and the role of colloidal material in scavenging Th from the water column. These are the first measurements of naturally occurring colloidal Th. The fraction of 232Th, 234Th associated with colloids (i.e., 10,000 Dalton < colloids < 0.4 μm) ranged from 10 to 78% of the Th passing 0.4 μm Nucleopore cartridge filters. Colloid mass concentrations were much larger than the corresponding 0.4 μm filter-retained particle concentrations. The conditional partitioning constants for 234Th with colloids, Kc, and filter-retained particles, Kf, are comparable in magnitude. Thorium scavenging rate constants decreased in value with increasing distance from the coast (0.164 to 0.007 d−1), and this is attributed to the decreasing particulate-matter concentration from coastal to deeper Gulf waters. In addition, there exists a highly significant, positive correlation between values of the Th scavenging rate constant and the fraction of 0.4 μm filter-passing Th associated with colloids. An average residence time of 6 days, with a range of 4–26 days, was calculated for the characteristic time scale of colloid transfer through the particle size spectrum, including sedimentation. The large fraction of 234Th which was found to be associated with colloids suggests that Th isotopes can be used as in-situ “coagulometers,” tracing the aggregation of colloidal material with, or into, large aggregates of filter-retained sizes.

References (52)

  • K.G Knauss et al.

    Radium and thorium isotopes in the surface waters of the East Pacific and coastal S. California

    Earth Planet. Sci. Lett.

    (1978)
  • S Krishnaswami et al.

    The simultaneous determination of Th, Pu, Ra isotopes, 210Pb, 55Fe, 32Si, and 14C in marine suspended phases

    Anal. Chim. Acta

    (1976)
  • Y.H Li et al.

    Natural radionuclides in waters of the New York Bight

    Earth Planet. Sci. Lett.

    (1981)
  • E Matsumoto

    234Th-238U radioactive disequilibrium in the surface layer of the ocean

    Geochim. Cosmochim. Acta

    (1975)
  • B.A Mckee et al.

    The use of 234Th238U disequilibria to examine the fate of particlereactive species on the Yangtze continental shelf

    Earth Planet. Sci. Lett.

    (1984)
  • B.A McKee et al.

    Temporal variability in the partitioning of thorium between dissolved and particulate phases on the Amazon shelf: Implications for the scavenging of particle-reactive species

    Continental Shelf Res.

    (1986)
  • W.S Moore

    The thorium isotope content of ocean water

    Earth Planet. Sci. Lett.

    (1981)
  • S.B Moran et al.

    The distribution of colloidal aluminum and organic carbon in coastal and open ocean waters off Nova Scotia

    Geochim. Cosmochim. Acta

    (1989)
  • K.L Nash et al.

    Interaction of humic and fulvic acids with Th(IV)

    J. Inorg. Nucl. Chem.

    (1980)
  • Y Nozaki et al.

    Alpha-emitting thorium isotopes in northwest Pacific deep waters

    Earth Planet. Sci. Lett.

    (1983)
  • L.E Sahl et al.

    Transport of mud on continental shelves: evidence from the Texas shelf

    Mar. Geol.

    (1987)
  • P.H Santschi et al.

    Thorium isotopes as analogues for “particle-reactive” pollutants in coastal marine environments

    Earth Planet. Sci. Lett.

    (1980)
  • B.L.K Somayajulu et al.

    Thorium and uranium isotopes in seawater and sediments

    Earth Planet. Sci. Lett.

    (1966)
  • Y Sugimura et al.

    A high-temperature catalytic oxidation method for the determination of non-volatile dissolved organic carbon in seawater by direct injection of a liquid sample

    Mar. Chem.

    (1988)
  • M.P Bacon et al.

    Distribution of thorium isotopes between dissolved and particulate forms in the deep sea

    J. Geophys. Res.

    (1982)
  • M Baskaran et al.

    A method for rapid in-situ extraction and laboratory determination of Th, Pb, and Ra isotopes from large volumes of seawater

    Deep-Sea Res.

    (1992)
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