Lecture 19
The Ocean Nitrogen Cycle
Sinks/Sources
The Global Oxygen Cycle
Sink - Denitrification
Reactions
Distributions
Source - Nitrogen Fixation
Reactions
Distributions
Source/Sinks
Source - Organic Carbon Burial in
sediments
Sink - Weathering
The Global Carbon Cycle
Source from rivers via weathering
Sink = CaCO3 and org C burial
Need Urey reaction
Main Ocean Source of N
Nitrogen Fixation
Enzyme 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
An excellent example of how paradigms change with time
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 mM O2):
2 NO3- + organic matter → N2
Anammox (<2 mM O2)
NH4+ + NO2- → N2 + H2O
Schematic of Ocean Nitrogen Cycle
Gruber (2005) Nature 436, 786
Where are low oxygen zones?
Global distribution of O2 at the depth of the oxygen minimum
Gruber and Sarmiento, 1997
Spatial Coupling of N sources and sinks
(Deutsch et al, 2007, Nature, 445, 163)
Also, Capone and Knapp (2007) Nature, 445, 159
Spatial coupling of N2 fixation and denitrification (Model results; Deutsch et al, 2007)
What is N*?
How to calculate
excess or deficient
NO3The solid line shows the
linear equation
P = 1/16 N + 0.345
(equivalent to N* = 0)
Values to the right have
positive N*, to the left have
negative N*
N* is defined as
N* = [NO3] – 16 x [PO4] +2.9
PO4 versus Nitrate (GEOSECS data)
Insert shows the effect of nitrification, photosynthesis, N2 fixation
and denitrification.
Vertical distribution of N*
N2-Fix
denitrif
excess
deficit
N* is defined as
N* = [NO3] – 16 x [PO4] +2.9
Map View of N*
N* at 200m in the Pacific (Gruber and Sarmiento, 1997)
N* on density of 26.5
Ryabenko 2013 Topics in Oceanography
Why is N* negative – two sinks
Nitrogen Cycle w/ anammox and denitrification
Kuypers (2003) Nature 422: 608-611.
Nitrogen species:
NO3- ; NO2- ; N2O; N2 ; NH4+
(V) (III) (I) (0) (-III)
Nitrogen Isotopes:
14N
99.634%
15N
0.366%
Isotopic Composition:
15
15
( N 14 ) sample  ( N 14 ) s tan dard
3
N
N
‰
 15 N  [
]

10
15
( N 14 ) s tan dard
N
The standard is atmospheric N2
Fractionation factors  1000e 1
,
where e is the isotopic enrichment factor
Fractionation
Heavier 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)
Ocean could be at SS or not!
Why is this important for
chemical oceanography?
What controls ocean C, N, P?
The nutrient concentration of
the deep ocean will adjust so that
the fraction of B preserved in the
sediments equals river input!
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 balance
VUCD ↓ has changed; sink > source)
B↓
CS
CD
if VMIX = m y-1 and C = mol m-3
flux = mol m-2 y-1
Atm Input (25)
N2 Fix (110-330)
Nitrogen Balance
VRCR (25)
B
Denitrification
sed = 200-280
wc = 75
fB (25)
Fluxes in Tg N y-1
Brandes et al 2002
Net fluxes
= -200 to 0
(sink > source; non-SS??)
The Global Oxygen Balance
Large O2 linked to Small C
As P = R, O2 not affected by DP
Earth is overall reducing
Separate O2; sequester reducing material
tO2 = 4 my
solar UV
only non-cyclic
only w/o biology
P and R in balance
accelerated
weathering
tC = 20yr
tC = 108 yr
Small imbalance in P-R
marine org C only, not terrestrial
80% in hemipelagic sediments
where %orgC = 0.5%
orgC includes H2S and Fe(II)
Present is key to past
stoichiometric so use moles
Walker (1974) AJS
1) If P ceased and R continued
org C would be consumed in 20 yr
O2 would decrease by 1%
2) If the only sink is weathering, O2 would go to 0
in 4 my. This is a short time geologically so controlling
balance must to strong.
3) Control on O2 = org C burial (O2 source) vs weathering (O2 sink)
4) Feedback mechanism
if atm O2
anoxic ocean
org C burial
atm O2
5) Control is with source rather than sink
Sedimentary org C reservoir has not changed with time
Hemipelagic sediments (org C > 0.5%)
200m to 3000m
80% of sediment orgC
CO2  and O2 
The long-term global carbon balance
weathering
CaCO3(s) + CO2(g) + H2O = 2HCO3- + Ca2+
2HCO3- + Ca2+ = CaCO3(s) + CO2(g) + H2O
deposition
A better example of reverse weathering!
Fig. 2.5 Emerson and Hedges
Chemical Weathering, the Geological Carbon Cycle, Control on CO2
1. CO2 is removed by weathering of silicate and carbonate rocks on land.
2. The weathering products are transported to the ocean by rivers where they
are removed to the sediments as CaCO3 and SiO2.
3. When these sediments are subducted and metamorphosed at high T and P,
CaCO3 and SiO2 are converted into CaSiO3 and CO2 is returned to the atmosphere.
Ittekkot (2003) Science 301, 56
For more detail see Berner (2004) The Phanerozoic Carbon Cycle: CO2 and O2. Oxford Press, 150pp.
TABLE 2 Oceanic fluxes of carbon
Demand
Flux
Atmospheric
River input
Derived from atmosphere
Derived from carbonates
43.2
35.0
8.7
-35.0
Hydrothermal input
0.5
Carbonate deposition
Deposited as carbonates
Lost to atmosphere
49.4
24.7
24.7
Net atmospheric demand
+24.7
-12.4
Units: 1012 mol/y
1. Some CO2 produced by carbonate deposition, but not enough!
2. The rest must come from the Urey reaction.
From McDuff and Morel (1980)
There must be a tight feedback control on atm CO2
1. The problem of the cool sun (Sagan and Mullen, 1972).
Solar luminosity has increased by 25% over the age of the solar system.
But liquid water has existed for 3.8 byr!
There must be a temperature buffer!
2. Was it NH3?? No. Most likely the greenhouse gas CO2.
3. CO2 is produced to the atmosphere by volcanoes and metamorphism.
Such as The “Urey Reaction” CaCO3(s) + SiO2(s) = CaSiO3(s) + CO2(g)
4. The important sink of CO2 is weathering of silicate minerals. Weathering
of silicate rocks consumes CO2 and produces Ca2+ and Mg2+ to rivers. In
the ocean this Ca2+ and Mg2+ is removed by formation of carbonate rocks
which produces CO2. The rate of weathering is influenced by rock type,
slope, temperature and runoff
5. The weathering and deposition of carbonate rocks alone is not
sufficient. Need the Urey Reaction!
7. A negative feedback. If the earth became cooler, silicate weathering
would decrease, atmospheric CO2 would increase and the earth would
warm!
% 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)
Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)
The Global Nitrogen Budget-one example
(Brandes et al, 2002)
Downcore records of 15N-orgN from several sites
High values of 15N-OrgN suggest more extensive denitrification
Deutsch et al, 2004)
Deutsch et al, 2004