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Silicon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

The silicon isotope composition (³⁰Si) of biogenic opal provides a view of the silica cycle at times in the past. Reconstructions require the knowledge of silicon isotope fractionation during opal biomineralization. The ³⁰Si of specimens of hexactinellid sponges and demosponges growing in the modern ocean ranged from –1.2 to –3.7 (n = 6), corresponding to the production of opal that has a ³⁰Si value 3.8 ± 0.8 more negative than seawater silicic acid and a fractionation factor () of 0.9964. This is three times the fractionation observed during opal formation by marine diatoms and terrestrial plants and is the largest fractionation of silicon isotopes observed for any natural process on Earth. The ³⁰Si values of sponge spicules across the Eocene-Oligocene boundary at Ocean Drilling Program Site 689 on Maud Rise range from –1.1 to –3.0, overlapping the range observed for sponges growing in modern seawater.

Key Words: silicon isotopes • sponge spicules • silica cycle • ODP Site 689 • ³⁰Si • fractionation