Fig. 3: δ¹⁵N_NO3arc values modeled by (1) across East Antarctica based on surface mass balance (SMB).

The spatial variability of δ¹⁵N_NO3arc values across East Antarctica are modeled by applying the field data regression of ln(δ¹⁵N_NO3arc + 1) vs. SMB⁻¹ to the 1979–2015 mean SMB output (35 km resolution) from the MAR¹³, adjusted for dry site bias (see Methods). Values of δ¹⁵N_NO3arc are undefined (gray) at some locations near the coast with very low or negative SMBs due to high sublimation and wind scouring. Preservation of NO₃⁻ is not expected in these locations, which often correspond to blue ice zones (blue polygons, zones with >100 km² extent shown)⁷⁶. Samples of δ¹⁵N_NO3arc from the field database are illustrated by colored circles with the same color gradient as the modeled δ¹⁵N_NO3arc values. Regions with SMB less than or greater than 40–200 kg m⁻² a⁻¹ (i.e., the SMB range targeted by the δ¹⁵N_NO3arc proxy described here) are illustrated with hatching and crosses, respectively. Presently occupied stations in the CONMAP database are shown as triangle icons for spatial reference, and the Aurora Basin North (ABN) site is indicated with a red star.