Nitrogen Isotopes & the biological pump11/8/12 Lecture outline: 1)the nitrogen cycle 15 N overview...
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Transcript of Nitrogen Isotopes & the biological pump11/8/12 Lecture outline: 1)the nitrogen cycle 15 N overview...
Nitrogen Isotopes & the biological pump 11/8/12
Lecture outline:1) the nitrogen cycle
δ15N overview
3) ocean applications
4) the sedimentary record
A massive Trichodesmium (a nitrogen fixer) bloom offAustralia, photographedby the shuttle
The nitrogen cycle
0‰
5‰
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 environmentalsources of N:Nitrate, rain: -13 to +2Organic N, soil: 0 to +9Fertilizer: -5 to +5Animal 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 NO2N2 -30‰• primary production NO3 particulate organic matter -2 to -5
Seawater δ15N depends on sources of water (deep = 5‰, surface gyre = 0‰)
Ex: Nitrogen isotope depth variations in temperate soil profile
Vervaet et al., 2002What’s going on here?
Nitrogen isotopes in the food web
Minigawa & Wada, 1984; McCutchan et al., 2003
model:
Data:
-1st order control : youare what you eat
-trophic enrichment generally borne out
Kolber et al., 2006
light heavy
Karl et al., 2002
0‰
0.6‰
-1 to 0‰
-1 to 0‰
20-30‰
Sigman et al., 2009
Sigman et al., 2009
Nitrogen isotope effects in the ocean
Depend on:1)starting δ15N of NO3
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 NO3, biomass will be heavier
(closer to seawater)if you use only a small fraction, biomass
will be lighter (depleted wrt seawater)
sinkingparticles@300m
Sigman et al., 2009
Voss et al., 2001
Nitrogen isotope depth variations in a coastal upwelling zone
oxygen is consumedby rainingC(org)
NO3 deficitWhat’s
going on here?
15N tracer studies in oceanography – quantify N cycling
1. Estimate productivity:Feed labelled 15N solutionsto biology, quantifyuptake of 15N
analogous to 14C-labelledexperiments
2. Estimate nitrogen fixation,denitrication rates:Inject labelled 15N into incubation bottle containingnitrogen fixers, measure15N over time(Montoya and Co.)
Eppley and Peterson, 1979
Sigman et al., 1999
δ15N is observed to beanti-correlated to [NO3]
Interpretation?
NO3-δ15N in theequatorial Pacific
. . . and this signalis preserved inthe sedimentaryrecord
Altabet et al., 2002
Sedimentary δ15N in denitrification zone recordsrapid climate change events
What’s going on here?
Francois et al., Nature, 1997
Why do we care?
Higher glacial sediment δ15N values mean?
What do you need to know to tell a moredefinitive story?