The flux and isotopic composition of reduced and total nitrogen in Bermuda rain
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 δ15N 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 NO3- precipitation flux, and the mass-weighted annual average δ15N of TN, - 2.3‰, is higher than the δ15N of NO3- in the same samples (- 4.5‰), indicating that RN has an annual average δ15N 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 δ15N of RN varies significantly from - 2.7‰ in the cool season to 1.5‰ in the warm season. This seasonality in the δ15N of RN is similar to that of NO3-, implying that RN and NO3- in precipitation may have related sources or, more speculatively, mechanisms of inter-conversion. Additionally, the seasonality of the RN δ15N 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 δ15N derives from ammonium at those times. Finally, the low δ15N of the TN flux will cause it to have an effect on the δ15N of Sargasso Sea thermocline NO3- that is in the same sense as the effect of N2 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 NO3- δ15N 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.]. © 2008 Elsevier B.V.