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. The silicon isotopic composition ( δ30Si ) ofdeep sea sponges' skeletal element – spicules – reflects the silicic acid(DSi) concentration of their surrounding water and can be used as naturalarchives of bottom water nutrients. In order to reconstruct the past silicacycle robustly, it is essential to better constrain the mechanisms ofbiosilicification, which are not yet well understood. Here, we show that theapparent isotopic fractionation ( δ30Si ) during spiculeformation in deep sea sponges from the equatorial Atlantic ranges from − 6.74 ‰ to − 1.50 ‰ in relatively low DSi concentrations(15 to 35  µ M). The wide range in isotopic composition highlightsthe potential difference in silicification mechanism between the two majorclasses, Demospongiae and Hexactinellida. We find the anomalies in theisotopic fractionation correlate with skeletal morphology, whereby fusedframework structures, characterised by secondary silicification, exhibitextremely light δ30Si signatures compared with previousstudies. Our results provide insight into the processes involved duringsilica deposition and indicate that reliable reconstructions of past DSi canonly be obtained using silicon isotope ratios derived from sponges withcertain spicule types.

Silicon isotopes of deep sea sponges: new insights into biomineralisation and skeletal structure