Nitrogen Isotopes & the biological pump
11/8/12
Lecture outline:
1) the nitrogen cycle
2)
15N overview
3)
ocean applications
4)
the sedimentary record
A massive
Trichodesmium (a
nitrogen fixer) bloom off
Australia, photographed
by 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 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 fixation
N2 NO3 (subtropical oceans)-2 to 0‰ NO3(aq)
2. nitrification
NH3  NO3 (soils)
-20‰
3. denitrification
NO3  NO2N2
-30‰
4. primary production
NO3  particulate organic matter
-2 to -5
Seawater 15N depends on sources of water (deep = 5‰, surface gyre = 0‰)
Ex: Nitrogen isotope depth variations in temperate soil profile
What’s going on here?
Vervaet et al., 2002
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
Kolber et al., 2006
Karl et al., 2002
0‰
0.6‰
-1 to
0‰
-1 to
0‰
light
20-30‰
heavy
Sigman et al., 2009
Nitrogen isotope effects in the
ocean
Depend on:
1) starting d15N of NO3
2) completion of process (complete = 0
fractionation); denitrification and
nitrification can go to
completion
Sigman et al., 2009
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)
sinking
particles
@300m
Sigman et al., 2009
Nitrogen isotope depth variations in a coastal upwelling zone
oxygen is
consumed
by raining
C(org)
What’s
going
on here?
NO3
deficit
Voss et al., 2001
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
15N is observed to be
anti-correlated to [NO3]
Interpretation?
Sigman et al., 1999
NO3-15N in the
equatorial Pacific
. . . and this signal
is preserved in
the sedimentary
record
Sedimentary 15N in denitrification zone records
rapid climate change events
What’s going on here?
Altabet et al., 2002
Why do we care?
Higher glacial sediment 15N values mean?
What do you need to know to tell a more
definitive story?
Francois et al., Nature, 1997