SILICON ISOTOPE FRACTIONATION BY MARINE SPONGES AND THE RECONSTRUCTION OF THE SILICON ISOTOPE COMPOSITION OF ACIENT DEEP WATER

Abstract

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.