Sylvain Richoz

The silicon isotope composition (δ³⁰Si) of biogenic silica is often used as an archive of past environmental conditions. For example, sponge spicule δ³⁰Si is known to be related to seawater-dissolved Si concentrations. Such a proxy application requires that the δ³⁰Si is not diagenetically altered—or at least that any alteration can be identified and accounted for. Yet the preservation of pristine isotope signals during (early) diagenesis is challenged by observations of structural changes to the amorphous silica (opal-A) of biogenic silica toward a more stable amorphous silica phase (opal-CT). This transformation is known to be associated with a resetting of oxygen isotope (δ¹⁸O) values but with unclear implications for the preservation of other geochemical signatures. This was investigated using modern and Cretaceous siliceous sponge spicules. Modern spicules collected from different ocean basins were uniformly transparent opal-A, whereas Cretaceous spicules exhibited two preservation states: visually similar to modern or clearly altered toward a milky, translucent composition. A comparison of δ³⁰Si and δ¹⁸O values of spicules from both categories within single samples reveals the milky, translucent individuals are offset from the transparent individuals and thus presumably unsuitable for palaeoenvironmental applications. A suite of geochemical and structural analyses (XRD, Raman spectroscopy, and FT-IR spectroscopy) demonstrate that even visually clear Cretaceous spicules are subtly different from their modern counterparts, implying caution is required when interpreting δ³⁰Si values or other geochemical proxies in ancient biogenic silica.