Nitrate isotopic composition between Bermuda and Puerto Rico: Implications for N2 fixation in the Atlantic Ocean

Abstract

N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low‐δ15N nitrate throughout the Sargasso Sea thermocline, which we attribute primarily to the input of recently fixed N. Although previous work suggests southward increases in N2 fixation and ventilation age, no meridional trend in nitrate δ15N is apparent. In the upper 200 m, the algal uptake‐driven increase in nitrate δ18O is greater than in δ15N, because of (1) a higher fraction of nitrate from N2 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 N2 fixation. The nitrate from N2 fixation is communicated between the model's North and South Atlantic and suggests a whole Atlantic N2 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‐δ15N N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N2 fixation would still be too high.