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

N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low-delta N-15 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 delta N-15 is apparent. In the upper 200 m, the algal uptake-driven increase in nitrate delta O-18 is greater than in delta N-15, 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- delta N-15 N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N-2 fixation would still be too high.