Another continental pool in the terrestrial silicon cycle

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.