The influence of sediment diagenesis and aluminium on oxygen isotope exchange of diatom frustules

The oxygen isotope composition of diatom frustules, δ¹⁸O_diatom, is thought to reflect the isotopic composition of the ambient seawater at the time of biomineralization. However, the δ¹⁸O_diatom can be overprinted due to the susceptibility of silanol groups (both external and internal) to isotope exchange. Here, using high-resolution imaging, we investigate what factors may influence this post-mortem isotopic alteration during the initial stages of diagenesis in the sediment. A diatomaceous clay was incubated with ¹⁸O-enriched seawater with fresh diatom detritus placed at the sediment–water interface (SWI) and at depth in the sediment. NanoSIMS analysis showed that the fresh diatom detritus as well as fossil frustules became significantly enriched in ¹⁸O, and that a relationship between Al-content and ¹⁸O-exchange could be observed. To further study the potential role of Al as an inhibitor of oxygen exchange, we measured Al on the surface of fossil frustules and performed additional incubations of diatom detritus in seawater with various concentrations of dissolved Al. The presence of Al-rich material bound to the surface of fossil frustules did not reduce the extent of ¹⁸O-enrichment in the underlying silica. However, exposure of diatoms detritus to dissolved Al, which led to a significant increase in frustule Al/Si ratio and a homogenously distributed Al in the frustule valve, significantly lowered the amount of ¹⁸O-enrichment. We hypothesize that Al incorporated into the silica structure can slow down ¹⁸O exchange while Al present as surface contaminants (clays or other aluminosilicates) has no inhibitory role.