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Abstract: The silicon (Si) isotope (δ 30Si) 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 δ 30Si compositions trend to lower values with increasing depth. This is exemplified by sponges from the Tasmania Seamounts where δ 30Si varies from +0.87‰ to −3.13‰ over a depth range from 100m to 1200m. These changes in δ 30Si of sponges are inconsistent with a Rayleigh type isotope fractionation model requiring constant δ 30Si fractionation between sponge and seawater. We conclude that overall Si isotope fractionation Δ 30Si (δ 30Si sponge – δ 30Si 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 Δ 30Si fractionation varies as a function Si influx and efflux. Using this model it appears that Δ 30Si fractionation during transport into the sponge is constant at −1.34‰. The model also shows asymptotic behaviour with Δ 30Si trending towards a maximum of −6.02‰ at very high Si concentrations. These results suggest that the δ 30Si composition of fossil spicules may be useful for reconstruction paleo-Si concentrations during the past. [Copyright &y& Elsevier]