A new tool to decipher the past silica cycle? The Si isotope signatures of radiolarians: taxa-specific isotope difference and their potential for studying past biogeochemistry

Dissolved silica (DSi) is an essential nutrient for marine silicifiers, and its cycling is closely linked to the biological pump and carbon cycle. The biological uptake of DSi and subsequent formation of biogenic opal (BSi) discriminate against heavier 30Si isotopes, resulting in BSi with lighter isotope compositions than surrounding seawater. The resulting stable isotope compositions (δ30Si) of BSi have proven reliable tools to reconstruct the DSi concentrations of bottom waters (sponges) or surface ocean nutrient utilization (diatoms). However, crucial information from the subsurface and intermediate waters is missing to fully reconstruct silica cycling.

A new tool to close this gap is the δ30Si of radiolarians (δ30Sirad), marine zooplankton dwelling in shallow to deep depths. Radiolarian δ30Si is thought to reflect DSi concentrations of the surrounding seawater, but estimations of a fractionation factor indicating how radiolarians discriminate against the 30Si are difficult due to the inability to culture radiolarians: Accordingly, estimates are limited to analyses to comparisons of radiolarians from surface sediment and DSi concentrations from their habitat depths. So far, only two studies have estimated an apparent fractionation factor (Δ30Si= δ30Sirad - δ30SiDSi) for radiolarians, ranging between -0.0‰ and -1.2‰. The differences in Δ30Si could now be attributed to different radiolarian taxa, likely caused by taxa-specific Si isotope fractionation during DSi uptake.

Significant inverse relationships have also shown the assumed relationship between δ30Sirad and DSi concentrations. However, these relationships still have large uncertainties, and δ30Sirad cannot identify changes below 10 to 15 µM DSi, highlighting the need for more investigations.