text: According to empirical calibrations, the difference in δSi (denoted as ΔSi) between sponges and their hosting water is correlated with the Si concentration of the hosting solution. Therefore, it has been suggested that the Si concentrations in bottom waters of ancient oceans can be interpreted from the δSi of coexisting sponge spicules, which are preserved in the rock record. It has been proposed that this relation is determined by the growth rate and the Si uptake kinetics of sponges, but the current understanding of sponge biomineralization pathways is limited. Although the mechanism behind this relation is yet to be clear, it appears consistent among various laboratory experiments, modern environments, and core top sediments. However, there is also evidence that the δSi of carnivorous sponges may differ significantly from the expected correlation.