Calibrating the triple oxygen isotope signature of cultured diatoms

Abstract

The 18 O/ 16 O of precipitated silica in natural environments is posited to record the temperature of the ambient environment. Measurements of 17 O/ 16 O in silica (SiO 2 ), when paired with 18 O/ 16 O, provide an opportunity to further constrain the environmental conditions at the time of mineral precipitation and ripening. On Cenozoic timescales, siliceous frustule precipitation by diatoms has served as the primary sink of silica from the ocean. Frustule precipitation is a biologically catalyzed process from an ocean undersaturated in SiO 2 . Here, we measure the 17 O/ 16 O and 18 O/ 16 O in diatom frustules from semicontinuous cultures with two different diatom species and across a range of growth rates and temperatures. To further enable these measurements, we developed a laser fluorination method to reproducibly measure the triple oxygen isotope composition of silica frustules. While the δ 13 C of the organic matter scaled predictably with growth rate, neither the δ 18 O or Δ 017 O of the frustule silica varied significantly with respect to variations in growth rate or species. Temperature, however, does carry a control on the product δ 18 O composition. Conversely, the measured Δ 017 O compositions were systematically negative with respect to the corresponding theoretical equilibrium isotope prediction, consistent with other empirical work on the SiO 2 system and only weakly temperature-dependent. This isotopic offset may be caused by the silica concentrating mechanisms diatoms use to precipitate their frustules. These results are consistent with literature arguments regarding the utility of diatom δ 18 O in reconstructing temperature, but suggest that Δ 017 O may not be a suitable temperature proxy for diatomaceous sediment.

In‐Situ and Triple Oxygen Isotope Characterization of Seafloor Drilled Cherts: Marine Diagenesis and Its Bearing on Seawater Reconstructions

“…While temperature and marine pore water fluids control the δ′ 18 O—Δ′ 17 O values of final diagenetic silica, the values of primary silica (e.g., diatoms or radiolaria) play an important role in the final chert value (Figure 10). Currently, a limited number of studies addresses the biogenically mediated silica‐water triple O‐isotope fractionation (see Liljestrand et al., 2021). The observed Δ′ 17 O fractionation yields values very close to the empirical calibration at low temperatures (Sharp et al., 2016; Wostbrock et al., 2018).…”