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GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 20, 1961, doi:10.1029/2002GL015666, 2002

Control of Pa/Th ratio by particulate chemical composition in the ocean

Laodong Guo

International Arctic Research Center, University of Alaska Fairbanks, AK, USA


Min Chen

Department of Oceanography, Xiamen University, Xiamen, China


Celine Gueguen

International Arctic Research Center, University of Alaska Fairbanks, AK, USA


Abstract

Controlled laboratory experiments were carried out using pure sorbents to examine the interactions of 234Th (or 233Pa) with particle surfaces and whether there is a fractionation between 234Th and 233Pa by particulate chemical composition. Our results show that the adsorption of 233Pa and 234Th on particle surfaces is highly selective and compound specific, resulting in a fractionation of 233Pa/234Th ratio, especially on SiO2 (Kd ratio, Kd-233Pa/Kd-234Th, = 10) and CaCO3 (Kd ratio, Kd-233Pa/Kd-234Th, = 0.01) surfaces. These new results agree well with previously published field observations that showed an increase of 231Pa/230Th ratio (or Kd of 231Pa) with increasing opal abundance and decreasing carbonate content in oceanic particles. In addition, they highlight the complications of using 231Pa/230Th ratio as a proxy and have implications for both oceanography and trace element geochemistry although more studies are needed to further examine the influence of natural organic matter on the fractionation of Th/Pa ratios.

Published 19 October 2002.

Index Terms: 4875 Oceanography: Biological and Chemical: Trace elements; 4825 Oceanography: Biological and Chemical: Geochemistry; 4866 Oceanography: Biological and Chemical: Sorptive scavenging; 4860 Oceanography: Biological and Chemical: Radioactivity and radioisotopes; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615).

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Citation: Guo, L., M. Chen, and C. Gueguen (2002), Control of Pa/Th ratio by particulate chemical composition in the ocean, Geophys. Res. Lett., 29(20), 1961, doi:10.1029/2002GL015666.