Physics
Scientific paper
Oct 1969
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969gecoa..33.1153c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 33, Issue 10, pp.1153-1167
Physics
5
Scientific paper
The factors controlling the marine geochemistry of fluorine and the behavior of fluorine in the major sedimentary cycle today have been investigated with analyses of minerals from marine and terrestrial sediments. Calcium carbonate precipitation dominates the removal of dissolved fluoride from sea water, with incorporation into calcium phosphates apparently the next most important removal mechanism. An enrichment of fluorine over chlorine by a factor of about 10 5 occurs during CaCO 3 , precipitation. The residence time of the fluoride ion in sea water is calculated from its sedimentation rate to be a few million years, similar to that of calcium. Fluorine is being removed from sea water about 100 times faster than chlorine, but not nearly as fast as elements such as Fe or Al. Calcium carbonate and phosphate precipitation together can account for at most 10-20 % of the dissolved fluoride brought annually by rivers to the oceans. Atmospheric precipitations contain enough fluorine, the major part of which has apparently come from the sea, to provide most of the fluorine in the river waters. Only a small fraction of the fluorine found in atmospheric precipitations can be explained by volcanic and industrial inputs, or by soil dust. Since most of the fluoride in river waters is therefore cyclic, the oceans can closely approach a steady state for fluorine, despite the apparent discrepancy in injection and removal rates. The F / Cl ratio in rain and snow is 10-1000 times that in sea water, so fluorine is apparently being preferentially injected into the atmosphere at the sea surface. As atmospheric precipitation weathers crustal rocks and soils it dissolves out relatively little fluorine. Most of the fluorine mobilized during weathering remains bound in the solid phases such as the clays. Upon reaching the sea, the clays remove very little, if any, fluoride from solution as they pass to the sediments. Their relatively large fluorine contents are thus unreactive in the major sedimentary cycle.
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