Foraminifera-Bound Nitrogen Isotope Evidence for Reduced Nitrogen Fixation in the Atlantic Ocean During the Last Ice Age

In the organic matter bound within planktonic foraminifera shells in Caribbean Sea sediments, the 15N/14N ratio decreases from the last ice age to the current interglacial, while bulk sediment 15N/14N shows minimal glacial/interglacial change. The foraminiferal data are best explained by less nitrogen (N) fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline. In addition, during the last ice age, coherent 15N/14N differences among species with different depth preferences appear to record a sharper depth gradient in particulate N 15N/14N, also consistent with less N fixation. The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification, in the sense that would have worked to balance the ocean N budget and reduce variability in the ocean N reservoir, global productivity, and atmospheric carbon dioxide.