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Lecture 18 The Ocean Nitrogen Cycle
Denitrification Reactions Distributions
Nitrogen Fixation Reactions Distributions
Why is this important for chemical oceanography?What controls ocean C, N, P?g ≈ 1.0
Mass Balance for whole ocean:C/ t = VRCR – f B
CS = 0; CD = CD
VU = VD = VMIX
Negative Feedback Control:if VMIX ↑VUCD ↑B ↑f B ↑ (assumes f will be constant!)assume VRCR then CD ↓ (because total ocean balanceVUCD ↓ has changed; sink > source)B ↓
CS
CD
if VMIX = m y-1 and C = mol m-3
flux = mol m-2 y-1
The nutrient concentration of the deep ocean will adjust so thatthe fraction of B preserved in thesediments equals river input!
Nitrogen species:NO3
- ; NO2- ; N2O; N2 ; NH4
+
(V) (III) (I) (0) (-III)
Nitrogen Isotopes:14N 99.634%15N 0.366%
Isotopic Composition:15 15
14 14 tan15 315
14 tan
( ) ( )10
( )[ ]sample s dard
s dard
N NN NN
NN
‰
The standard is atmospheric N2
Fractionation factors , where is the isotopic enrichment factor
1000 1
FractionationHeavier stable isotope forms stronger bond.
Microbial Enzymes break light isotope bonds more easily.
Reactants become heavier (enriched) (e.g. NO3- → N2)
Products become lighter (depleted)
Partial versus total reaction (products have same values as reactants)
The Global Nitrogen Budget-one example(Brandes et al, 2002)
Main Ocean Source of N
Nitrogen FixationEnzyme catalyzed reduction of N2
N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16Pi
Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase
Inactivated when exposed to O2
Main Ocean Sink of N
Fixed Nitrogen (NO3-, NO2
-, NH4+) is
converted to N2 in low oxygen zones of the ocean
Two Pathways
Denitrification ( <2 to 10 M O2):
2 NO3- + organic matter → N2
Anammox (<2 M O2) NH4
+ + NO2- → N2 + H2O
Schematic of Ocean Nitrogen Cycle
Gruber (2005) Nature 436, 786
% of Export Production (as N) at HOT derived from N2 Fixation
(N-P mass balance model of Karl et al (1997) Nature 388, p. 533)
Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163)
Capone and Knapp (2007) Nature, 445, 159
N* is defined asN* = [NO3] – 16 x [PO4] +2.9
Global distribution of O2 at the depth of the oxygen minimumGruber and Sarmiento, 1997
N* at 200m in the Pacific (Gruber and Sarmiento, 1997)
Kuypers (2003) Nature 422: 608-611.
Nitrogen Cycle w/ anammox
PO4 versus Nitrate (GEOSECS data)Insert shows the effect of nitrification, photosynthesis, N2 fixationand denitrification.
The solid line shows thelinear equationP = 1/16 N + 0.345(equivalent to N* = 0)
Values to the right havepositive N*, to the left have negative N*
What is N*?
N* is defined asN* = [NO3] – 16 x [PO4] +2.9
The Global Nitrogen Budget-one example(Brandes et al, 2002)
Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)
Deutsch et al, 2004)
Downcore records of 15N-orgN from several sitesHigh values of 15N-OrgN suggest more extensive denitrification
Deutsch et al, 2004