The flux and isotopic composition of reduced and total nitrogen in Bermuda rain

Abstract

The concentration and 15N/ 14N ratio of total nitrogen (TN) were measured in precipitation samples collected at Bermuda between January and December 2000. By correcting for nitrate, analyzed previously, the concentration and δ 15N of “reduced” N (RN, i.e., ammonium + organic N) were also determined. The TN precipitation flux (∼ 10–19 mmol N m − 2 yr − 1 ) is twice the NO 3 − precipitation flux, and the mass-weighted annual average δ 15N of TN, − 2.3‰, is higher than the δ 15N of NO 3 − in the same samples (− 4.5‰), indicating that RN has an annual average δ 15N of − 0.6‰. While neither the concentration nor the flux of RN (6.8 µM and 5.2 mmol N m − 2 yr − 1 , respectively) in precipitation shows statistically significant seasonal variation, the δ 15N of RN varies significantly from − 2.7‰ in the cool season to 1.5‰ in the warm season. This seasonality in the δ 15N of RN is similar to that of NO 3 −, implying that RN and NO 3 − in precipitation may have related sources or, more speculatively, mechanisms of inter-conversion. Additionally, the seasonality of the RN δ 15N at Bermuda is similar to that of typical ammonium concentrations in precipitation at Bermuda, both showing maxima in the spring and late summer, raising the possibility that the maxima in the RN δ 15N derives from ammonium at those times. Finally, the low δ 15N of the TN flux will cause it to have an effect on the δ 15N of Sargasso Sea thermocline NO 3 − that is in the same sense as the effect of N 2 fixation, with slightly greater isotopic leverage. If the atmospheric TN flux is not marine-derived, it could explain a substantial fraction of the previously documented upward decrease in NO 3 − δ 15N from deep water into the thermocline of the Sargasso Sea, for example, ∼ 20 to 35‰ of it, assuming a N2 fixation rate of 45 mmol N m-2 yr-1 as estimated by Hansell et al. [Hansell, D.A., Bates, N.R., and Olson, D.B., 2004. Excess nitrate and nitrogen fixation in the North Atlantic Ocean. Mar. Chem., 84:243–265.].