Nitrate isotopic composition between Bermuda and Puerto Rico: Implications for N 2 fixation in the Atlantic Ocean

N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low‐ δ 15 N nitrate throughout the Sargasso Sea thermocline, which we attribute primarily to the input of recently fixed N. Although previous work suggests southward increases in N 2 fixation and ventilation age, no meridional trend in nitrate δ 15 N is apparent. In the upper 200 m, the algal uptake‐driven increase in nitrate δ 18 O is greater than in δ 15 N, because of (1) a higher fraction of nitrate from N 2 fixation at shallower depths and/or (2) cycling of N between nitrate assimilation and nitrification. A mean depth profile of newly fixed nitrate estimated from the nitrate isotope data is compared with results from an ocean circulation model forced with different Atlantic fields of N 2 fixation. The nitrate from N 2 fixation is communicated between the model's North and South Atlantic and suggests a whole Atlantic N 2 fixation rate between 15 and 24 Tg N a −1 . One important caveat is that fixed N in atmospheric deposition may contribute a significant proportion of the low‐ δ 15 N N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N 2 fixation would still be too high.