Rapid post-mortem oxygen isotope exchange in biogenic silica

The oxygen isotope composition of diatom frustules is thought to reflect the isotopic composition of the ambient seawater at the time of biomineralisation. However, significant concerns exist surrounding the degree of post-mortem alteration that might occur. Here, we study post-mortem overprinting of the δ¹⁸O signal in various forms of silica by incubating the samples in ¹⁸O-enriched seawater and analysing them with nanoSIMS and Raman micro-spectroscopy. The nanoSIMS data show that significant ¹⁸O-exchange occurs throughout diatom frustules (fresh as well as fossil) over days to weeks when placed in ¹⁸O-enriched seawater. This is a time-scale similar to that of the sinking diatom detritus in marine systems. Similarly, rapid ¹⁸O-exchange, although to a lesser degree, also occurs in sponge spicules. In contrast, no significant ¹⁸O-exchange occurs on this time-scale in crystalline silica, most likely due to the absence of silanol (Si-OH) groups as observed in Raman measurements. Together these results confirm that the oxygen signature of external silanol groups can be overprinted on short timescales, but also show that internal silanol groups are similarly susceptible. These internal silanols form a large pool of oxygen in the silica structure and are difficult to remove when cleaning. This results in the presence of an internal pool directly influenced by changes in the external water signature which may potentially be incorporated into the silica structure during maturation. The final measured δ¹⁸O in biogenic silica, therefore, most likely, represents a mixture of signals from the original growing stage, water-column settling, and sediment pore waters.