Cleaning methods for the isotopic determination of diatombound nitrogen in non‐fossil diatom frustules

Ground‐truthing the stable isotope ratio of diatom frustule‐bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical differences from sedimentary fossil diatom, for analysis. We tested and compared the performance of five oxidative treatment protocols in cleaning fresh diatom frustules of external organic material for isotopic analysis of diatom frustule‐bound N. We applied each treatment to diatom material from a T. weiss‐flogii culture and measured the diatom‐bound N content and isotopic composition (δ15N) of samples. A favored treatment is described that includes a multi‐step, progressive, oxidative cleaning. The results suggest that the application of H2O2 to fresh biogenic opal for more than 20 min increased the accessibility of the N within the opal matrix to subsequent chemical treatment, an effect not observed for diatom frustules from deep ocean sediments. We attribute this effect to structural alteration and dissolution of the diatom opal by the H2O2, consistent with previous observations regarding the high reactivity of fresh diatom opal and its subsequent chemical hardening in the sedimentary environment. In addition, the presence of high levels of organic matter upon treatment with H2O2 may exacerbate the chemical alteration of frustules. We find that the δ15N of diatom‐bound N is ~3 ‰ higher than that of the bulk culture biomass, consistent with expectations from surface sediment and down‐core measurements.