Presentation on theme: "Nitrogen Isotopes & the biological pump11/8/12 Lecture outline: 1)the nitrogen cycle 15 N overview 3)ocean applications 4)the sedimentary record A."— Presentation transcript:
Nitrogen Isotopes & the biological pump11/8/12 Lecture outline: 1)the nitrogen cycle 15 N overview 3)ocean applications 4)the sedimentary record A massive Trichodesmium (a nitrogen fixer) bloom off Australia, photographed by the shuttle
Nitrogen isotope basics two stable isotopes: 14 N: 99.64% 15 N: 0.36% standard = N 2 in air ( 15 N/ 14 N = 0.00368) measured as: N 2 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: 1.nitrogen fixationN 2 NO 3 (subtropical oceans)-2 to 0‰ NO 3 (aq) 2.nitrificationNH 3 NO 3 (soils)-20‰ 3.denitrificationNO 3 NO 2 N 2 -30‰ 4.primary productionNO 3 particulate organic matter-2 to -5 Seawater 15 N depends on sources of water (deep = 5‰, surface gyre = 0‰)
Ex: Nitrogen isotope depth variations in temperate soil profile Vervaet et al., 2002 What’s going on here?
Nitrogen isotopes in the food web Minigawa & Wada, 1984; McCutchan et al., 2003 model: Data: -1 st order control : you are what you eat -trophic enrichment generally borne out
Nitrogen isotope effects in the ocean Depend on: 1)starting 15 N of NO 3 2)completion of process (complete = 0 fractionation); denitrification and nitrification can go to completion
Primary production in a closed system – Raleigh fractionation Bottom line: if you use all the NO 3, biomass will be heavier (closer to seawater) if you use only a small fraction, biomass will be lighter (depleted wrt seawater)
Voss et al., 2001 Nitrogen isotope depth variations in a coastal upwelling zone oxygen is consumed by raining C(org) NO3 deficit What’s going on here?
15 N 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 15 N over time (Montoya and Co.) Eppley and Peterson, 1979
Sigman et al., 1999 15 N is observed to be anti-correlated to [NO3] Interpretation?
NO 3 - 15 N in the equatorial Pacific... and this signal is preserved in the sedimentary record
Altabet et al., 2002 Sedimentary 15 N in denitrification zone records rapid climate change events What’s going on here?
Francois et al., Nature, 1997 Why do we care? Higher glacial sediment 15 N values mean? What do you need to know to tell a more definitive story?