Diatom Frustule-Bound δ13c Measurements and Reconstruction of Εp

Abstract:

Diatom frustules contain embedded organic matter such as amino acids, which may offer a potential phase for measuring the carbon isotopic fractionation during marine photosynthesis (εp). We describe optimal sample preparation methods for cleaning external organic matter and separating frustules into various size classes while reducing significance of opal from radiolaria and silicoflagellates. Analysis of discrete separated fractions of diatom opal from each sediment sample is facilitated by our new method for analysis of very small sample sizes using NanoEA with cryotrapping of evolved CO₂. We evaluate the fidelity with which frustule-bound organic matter captures the variation in εp by comparing δ¹³C of frustule-bound organic matter with that of total cellular carbon in diatoms grown in culture, and by comparing the δ¹³C of frustule-bound organic matter in core tops with that of δ¹³C of diatom-produced biomarkers in the overlying water column. In core-top transects, frustule-bound δ¹³C is consistent with strong growth rate control over fractionation during photosynthesis (εp). Finally, we evaluate the temporal trends in carbon isotopic fractionation during photosynthesis since the Late Miocene in samples from the Eastern Equatorial Pacific.