Figure 1.Iceberg near Antarctica in the Southern Ocean.

In this study, relationships were investigated between coupled sponge and seawater samples for Si isotope fractionation compared with equivalent Si concentrations from the water column.  The Southern Ocean was chosen for field work as it is the only oceanic area where diatom productivity is dominant and it is a useful natural laboratory for oceanic Si research with concentration gradients stratified by latitude and depth (higher concentrations at lower latitudes and depths).  Accordingly, siliceous sponges from both the Hexactinellid and Demosponge classes were sampled at a variety of latitudes and depths. Results were compared to a new model that assumes variable Si isotope fractionation dependent on the Si concentration of seawater.  The new model offers a new perspective on what controls biological fractionation in biogenic opal and in turn, will yield a novel interpretation of the paleo-oceanic distribution of Si.