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Nitrogen Isotopes & the biological pump 11/8/12

Nitrogen Isotopes & the biological pump 11/8/12. Lecture outline: the nitrogen cycle 2) δ 15 N overview ocean applications 4) the sedimentary record. A massive Trichodesmium (a nitrogen fixer) bloom off Australia, photographed by the shuttle. The nitrogen cycle. 0 ‰. 5 ‰.

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Nitrogen Isotopes & the biological pump 11/8/12

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  1. Nitrogen Isotopes & the biological pump 11/8/12 • Lecture outline: • the nitrogen cycle • 2) δ15N overview • ocean applications • 4) the sedimentary record A massive Trichodesmium (a nitrogen fixer) bloom off Australia, photographed by the shuttle

  2. The nitrogen cycle 0‰ 5‰

  3. Nitrogen isotope basics • two stable isotopes: • 14N: 99.64% • 15N: 0.36% • standard = N2 in air (15N/14N = 0.00368) • measured as: N2 • Isotopic composition of environmental • sources of N: • Nitrate, rain: -13 to +2 • Organic N, soil: 0 to +9 • Fertilizer: -5 to +5 • Animal waste: +8 to +22 • N Reactions and Fractionations: • nitrogen fixation N2 NO3 (subtropical oceans) -2 to 0‰ NO3(aq) • nitrification NH3 NO3 (soils) -20‰ • denitrification NO3  NO2N2 -30‰ • primary production NO3  particulate organic matter -2 to -5 • Seawater δ15N depends on sources of water (deep = 5‰, surface gyre = 0‰)

  4. Ex: Nitrogen isotope depth variations in temperate soil profile Vervaet et al., 2002 What’s going on here?

  5. Nitrogen isotopes in the food web Data: • 1st order control : you • are what you eat • -trophic enrichment • generally borne out model: Minigawa & Wada, 1984; McCutchan et al., 2003

  6. Kolber et al., 2006

  7. Karl et al., 2002 0‰ 0.6‰ -1 to 0‰ -1 to 0‰ 20-30‰ light heavy

  8. Sigman et al., 2009

  9. Nitrogen isotope effects in the ocean • Depend on: • starting d15N of NO3 • completion of process (complete = 0 fractionation); denitrification and • nitrification can go to • completion Sigman et al., 2009

  10. Primary production in a closed system – Raleigh fractionation Bottom line: if you use all the NO3, biomass will be heavier (closer to seawater) if you use only a small fraction, biomass will be lighter (depleted wrt seawater)

  11. sinking particles @300m Sigman et al., 2009

  12. Nitrogen isotope depth variations in a coastal upwelling zone oxygen is consumed by raining C(org) NO3 deficit What’s going on here? Voss et al., 2001

  13. 15N tracer studies in oceanography – quantify N cycling 1. Estimate productivity: Feed labelled 15N solutions to biology, quantify uptake of 15N analogous to 14C-labelled experiments 2.Estimate nitrogen fixation, denitrication rates: Inject labelled 15N into incubation bottle containing nitrogen fixers, measure 15N over time (Montoya and Co.) Eppley and Peterson, 1979

  14. Sigman et al., 1999 δ15N is observed to be anti-correlated to [NO3] Interpretation?

  15. NO3-δ15N in the equatorial Pacific . . . and this signal is preserved in the sedimentary record

  16. Sedimentary δ15N in denitrification zone records rapid climate change events What’s going on here? Altabet et al., 2002

  17. Why do we care? Higher glacial sediment δ15N values mean? What do you need to know to tell a more definitive story? Francois et al., Nature, 1997

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