Testing models of past ocean chemistry using foraminifera 15N/14N

Previous models have attempted to explain glacial to interglacial changes in atmospheric pCO2 by invoking changes in the ocean's nutrient concentration or regional changes in nutrient utilization. Nitrogen is limiting to primary production and has a residence time in the ocean compatible with glacial to interglacial variations. But up to now, there has been no geological indicator for glacial to interglacial changes in the ocean's nitrogen cycle. We propose that 15N/14N ratios for the organic matrix of preserved foraminifera yield an interpretable δ15N record. We have developed models to study the relationship between plausible changes in the ocean's nitrogen cycle and resulting changes in the 15N/14N ratio of dissolved NO3−. Our model results demonstrate that 15N/14N ratios exhibit excursions during periods in which the oceans are accumulating or losing nitrogen regardless of the means by which these changes occur. Foraminifera 15N/14N results for a Pacific and an Atlantic core provide evidence against wide spread glacial anoxia, a consequence of many models of ocean chemistry. We suggest that increasing shelf‐sediment denitrification upon deglaciation reduced nitrogen concentrations in the modern ocean. Ambiguities between the two foraminifera 15N/14N records indicate that local effects associated with changes in hydrography or ecology need to be studied further.