Silicon isotopic fractionation in marine sponges: A new model for understanding silicon isotopic variations in sponges

The silicon (Si) isotope (δ30Si) composition of deep-sea sponges from near Antarctica, subantarctic waters (Tasmania Seamounts) and subtropical waters north of New Zealand vary widely between + 0.87‰ and − 3.40‰ (vs. NBS28). Depth profiles show that sponge δ30Si compositions trend to lower values with increasing depth. This is exemplified by sponges from the Tasmania Seamounts where δ30Si varies from + 0.87‰ to − 3.13‰ over a depth range from 100 m to 1200 m. These changes in δ30Si of sponges are inconsistent with a Rayleigh type isotope fractionation model requiring constant δ30Si fractionation between sponge and seawater. We conclude that overall Si isotope fractionation Δ30Si (δ30Si sponge – δ30Si seawater) is influenced by seawater Si concentration, with more fractionated (lower) isotope values being associated with sponges collected from waters high in Si. We invoke and fit a model whereby the Δ30Si fractionation varies as a function Si influx and efflux. Using this model it appears that Δ30Si fractionation during transport into the sponge is constant at − 1.34‰. The model also shows asymptotic behaviour with Δ30Si trending towards a maximum of − 6.02‰ at very high Si concentrations. These results suggest that the δ30Si composition of fossil spicules may be useful for reconstruction paleo-Si concentrations during the past.