Journal
NATURE
Volume 433, Issue 7024, Pages 399-402Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/nature03217
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Silicon is the second most abundant element on Earth. It is an important nutrient for phytoplankton(1) and is readily absorbed by terrestrial vegetation(2); it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates(3). But the continental cycle of silicon is not well known, and only a few studies have attempted to use silicon stable isotopes (Si-28, Si-29 and Si-30)(4-13) to quantify the continental silicon reservoirs. Dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 11.1parts per thousand (refs 7, 10). It is enriched in Si-30 with respect to the igneous rocks reservoir, which has a mean isotopic value of -0.3parts per thousand (refs 4, 9). This enrichment can only be produced by a major fractionation during weathering, and should result in the formation of a continental Si-30-depleted reservoir. Such a reservoir, however, has not been identified to date. Here we analyse silicon isotopes of in situ quartz from a sandstone series in France, using a new-generation secondary ion mass spectrometry apparatus. We show that quartz that precipitates as siliceous cements forms a strongly Si-30-depleted reservoir with isotopic values down to -5.7parts per thousand, a more negative value than any previously published for terrestrial samples. Our findings suggest that quartz re-precipitation plays an important role in the biogeochemical cycle of silicon.
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