Assessment of excess nitrate development in the subtropical North Atlantic

Dennis A Hansell, Donald Olson, F. Dentener, L. M. Zamora

Research output: Contribution to journalArticle

43 Citations (Scopus)

Abstract

Geochemical estimates of N2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 1011 mol N yr- 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 1011 mol N yr- 1 for ∼ 38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 1011 mol N yr- 1 for ∼ 28% of the total), and, calculated by mass balance of the excess N field, N2 fixation (adding excess N at 2.6 ± 2.2 × 1011 mol N yr- 1 for ∼ 33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2 fixation in the North Atlantic subtropical gyre is estimated at ∼ 4 × 1011 mol N yr- 1. This relatively low rate of N2 fixation suggests that i) the rate of N2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2 fixation.

Original languageEnglish (US)
Pages (from-to)562-579
Number of pages18
JournalMarine Chemistry
Volume106
Issue number3-4
DOIs
StatePublished - Aug 2007

Fingerprint

Nitrates
fixation
Temperature distribution
nitrate
thermocline
Biological materials
Nitrogen fixation
accumulation rate
Nutrients
Ventilation
Nitrogen
nitrogen fixation
gyre
atmospheric deposition
repository
dissolved organic matter
ventilation
rate
mass balance
Testing

Keywords

  • Atmospheric deposition
  • Dissolved organic matter
  • Nitrogen cycle
  • Nitrogen fixation
  • North Atlantic

ASJC Scopus subject areas

  • Chemistry(all)
  • Oceanography

Cite this

Assessment of excess nitrate development in the subtropical North Atlantic. / Hansell, Dennis A; Olson, Donald; Dentener, F.; Zamora, L. M.

In: Marine Chemistry, Vol. 106, No. 3-4, 08.2007, p. 562-579.

Research output: Contribution to journalArticle

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abstract = "Geochemical estimates of N2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 1011 mol N yr- 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 1011 mol N yr- 1 for ∼ 38{\%} of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 1011 mol N yr- 1 for ∼ 28{\%} of the total), and, calculated by mass balance of the excess N field, N2 fixation (adding excess N at 2.6 ± 2.2 × 1011 mol N yr- 1 for ∼ 33{\%} of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2 fixation in the North Atlantic subtropical gyre is estimated at ∼ 4 × 1011 mol N yr- 1. This relatively low rate of N2 fixation suggests that i) the rate of N2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2 fixation.",
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N2 - Geochemical estimates of N2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 1011 mol N yr- 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 1011 mol N yr- 1 for ∼ 38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 1011 mol N yr- 1 for ∼ 28% of the total), and, calculated by mass balance of the excess N field, N2 fixation (adding excess N at 2.6 ± 2.2 × 1011 mol N yr- 1 for ∼ 33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2 fixation in the North Atlantic subtropical gyre is estimated at ∼ 4 × 1011 mol N yr- 1. This relatively low rate of N2 fixation suggests that i) the rate of N2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2 fixation.

AB - Geochemical estimates of N2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 1011 mol N yr- 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 1011 mol N yr- 1 for ∼ 38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 1011 mol N yr- 1 for ∼ 28% of the total), and, calculated by mass balance of the excess N field, N2 fixation (adding excess N at 2.6 ± 2.2 × 1011 mol N yr- 1 for ∼ 33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2 fixation in the North Atlantic subtropical gyre is estimated at ∼ 4 × 1011 mol N yr- 1. This relatively low rate of N2 fixation suggests that i) the rate of N2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2 fixation.

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