enrichment_of_15N_in_particulate_nitrogen_in_particles_in_sea_water_expressed_as_lowercase_delta_15N_relative_to_atmospheric_nitrogen 1 [UUUU] Isotopic enrichment of 15N, often called d15N or delta 15N (lower case delta), is a measure of the ratio of stable isotopes 15N:14N. Reference: Karl, D. (2018) Sediment trap flux measurements from the Hawaii Ocean Time-Series (HOT) project at station ALOHA. Biological and Chemical Oceanography Data Management Office (BCO-DMO). doi:10.1575/1912/bco-dmo.737393.1.