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 100m to 1200m. 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.