Fig. 1.

Study sites and global occurrence of NATZ in marine sediments. (A) Bathymetry map of the Arctic Mid-Ocean Ridge in the Norwegian-Greenland Sea. Cores GC08 and GC09 (orange) were sampled during summer 2014. Cores GC04 and GC05 (red) were sampled during summer 2016. (B) Location of marine sediments bearing an observed NATZ, identified based on previously published profiles of nitrate and ammonium. Maps in A and B were created in GeoMapApp version 3.6.10 using the default Global Multi-Resolution Topography Synthesis basemap. (C) Plot of NATZ depth against water depth of sediment locations included in B. (D) Plot of diffusive fluxes of nitrate and ammonium into NATZ against water depths for sediment locations included in B.