Geophys. Res. Lett. http://dx.doi.org/10.1029/2011GL048325 (2011)

Nitrogen dynamics in the eastern tropical Pacific Ocean are controlled by the position of the southern equatorial upwelling zone across glacial–interglacial cycles, according to an analysis of marine sediments.

Nathalie Dubois and Markus Kienast of Dalhousie University, Canada, analysed the organic carbon content and nitrogen isotopic composition of marine sediments from cores located throughout the eastern equatorial Pacific that span the past 150,000 years. During glacial periods, all the cores showed the same general trends in nitrogen utilization and denitrification. In contrast, the sites from the lowest latitudes exhibited more isotopically heavy nitrate than the northernmost site during the last interglacial period and the Holocene epoch. This suggests a different local pattern of nitrogen cycling during warm intervals, probably reflecting changes in the equatorial upwelling system.

Specifically, the pattern could be explained by a more intense and southern-located meridional atmospheric circulation cell during interglacials, which would push the southern equatorial upwelling zone farther from the core sites during interglacial periods and lengthen the amount of time the waters reaching the sites were exposed to nitrate utilization.