Another continental pool in the terrestrial silicon cycle

• Published in: Nature 2005-01, Vol.433 (7024), p.399-402
• Main Authors: Parron, Claude, Meunier, Jean Dominique, Basile-Doelsch, Isabelle
• Format: Article
• Language: English
• Subjects: Bacillariophyceae; Biogeochemical cycles; Carbon dioxide removal; Chemical Precipitation; Climate; Continental interfaces, environment; Crystallization; Diatoms - metabolism; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fractionation; France; Geochemistry; Geologic Sediments - chemistry; Geology; Humanities and Social Sciences; Igneous rocks; Isotopes; letter; Mass spectrometry; multidisciplinary; Phytoplankton; Phytoplankton - metabolism; Plantae; Quartz; Quartz - chemistry; Quartz - metabolism; Reservoirs; Sandstone; Science; Science (multidisciplinary); Sciences of the Universe; Sediments; Silicates; Silicon; Silicon - chemistry; Silicon - metabolism; Silicon cycle; Soil and rock geochemistry; Stable isotopes; Water - chemistry; Water - metabolism; Water analysis; Weathering
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature03217

Silicon is the second most abundant element on Earth. It is an important nutrient for phytoplankton and is readily absorbed by terrestrial vegetation; it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates. But the continental cycle of silicon is not well known, and only a few studies have attempted to use silicon stable isotopes (28Si, 29Si and 30Si) to quantify the continental silicon reservoirs. Dissolved silicon in sea and river waters forms a reservoir of mean isotopic value +1.1‰ (refs 7, 10). It is enriched in 30Si with respect to the igneous rocks reservoir, which has a mean isotopic value of -0.3‰ (refs 4, 9). This enrichment can only be produced by a major fractionation during weathering, and should result in the formation of a continental 30Si-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 30Si-depleted reservoir with isotopic values down to -5.7‰, 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.