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?