TY - JOUR
T1 - The flux and isotopic composition of reduced and total nitrogen in Bermuda rain
AU - Knapp, Angela N.
AU - Hastings, Meredith G.
AU - Sigman, Daniel M.
AU - Lipschultz, Fredric
AU - Galloway, James N.
N1 - Funding Information:
We are grateful to Dr. K. Simmons at BIOS for his permission and assistance collecting the rain samples in Bermuda. We thank Dr. A. Peters at BIOS for providing data from the ongoing BIOS Air Quality Program, which is funded by the Bermuda Ministry of the Environment. We thank S. Seitzinger for discussions and W. Keene for a comprehensive review of the manuscript. Financial support was provided by an ASEE/NDSEG graduate fellowship and a NOAA Climate and Global Change Postdoctoral Fellowship to ANK, the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement No. NA17RJ1232 to MGH, NSF Biocomplexity grant OCE-9981479 (the MANTRA project) and NASA IDS grant NNG04G091G to DMS. This is BBSR/BIOS contribution # 1705, and JISAO contribution # 1445. E. A. Boyle and M. A. Altabet separately suggested to us that the Sargasso Sea NO 3 − δ 15 N minimum may be, at least partially, a transient feature associated with deposition of anthropogenic fixed N.
PY - 2010/6/20
Y1 - 2010/6/20
N2 - 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.].
AB - 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.].
KW - Atmospheric precipitation
KW - Bermuda
KW - Nitrogen cycle
KW - Nitrogen fixation
KW - Nitrogen isotopes
KW - North Atlantic
KW - Sargasso Sea
KW - Total nitrogen
KW - Wet deposition
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U2 - 10.1016/j.marchem.2008.08.007
DO - 10.1016/j.marchem.2008.08.007
M3 - Article
AN - SCOPUS:77953233574
SN - 0304-4203
VL - 120
SP - 83
EP - 89
JO - Marine Chemistry
JF - Marine Chemistry
IS - 1-4
ER -