Depth sections of nitrate δ15N (upper left; ‰), nitrate δ18O (upper right; ‰), nitrate concentration (lower left; μM) and nitrate δ15N minus nitrate δ18O (lower right; ‰) versus latitude (°N) over the upper 1000 m (top part of each panel), and full water column (bottom part of each panel). The map inset indicates the location of data shown in the section as well as published and unpublished measurements.