Chaetoceros resting spores record low diatom-bound nitrogen isotope values: Evidence from laboratory culture and marine sediment

Abstract. The nitrogen isotopic composition of diatom frustule-bound organic matter (δ¹⁵N_DB) is often used to study changes in high latitude biological pump efficiency across glacial-interglacial cycles, but the proxy may be biased by species-specific effects. The genus Chaetoceros is of particular interest because of its abundance throughout ocean basins, its shifting biogeography during glacial periods, and many species’ ability to form heavily silicified resting spores. Here we investigate how Chaetoceros resting spores (CRS) record surface nitrate conditions in their nitrogen isotopic composition using both laboratory culture experiments and assemblage-specific sedimentary δ¹⁵N_DB measurements. We find that CRS record δ¹⁵N_DB values 7.0 ± 2.6 ‰ lower than vegetative Chaetoceros in culture and 5.6 ± 1.9 ‰ lower than non-CRS diatoms in sediment. Low values are attributed to assimilation of isotopically light ammonium, heavy silicification, and internal fractionation during sporulation. Despite the large δ¹⁵N_DB difference observed in CRS versus non-CRS diatoms, increased CRS relative abundance in open ocean glacial sediments does not significantly bias δ¹⁵N_DB records due to the spores’ small size.