Lecture 19 The Ocean Nitrogen Cycle

Sinks/Sources

Sink - Denitrification Reactions Distributions

Source - Nitrogen Fixation Reactions Distributions

The Global Oxygen CycleSource/Sinks

Source - Organic Carbon Burial in sediments

Sink - Weathering

The Global Carbon CycleSource from rivers via weathering

Sink = CaCO3 and org C burial

Need Urey reaction

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

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

Global distribution of O2 at the depth of the oxygen minimumGruber and Sarmiento, 1997

Where are low oxygen zones?

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)

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*?How to calculateexcess or deficientNO3

-

N* is defined asN* = [NO3] – 16 x [PO4] +2.9

N* is defined asN* = [NO3] – 16 x [PO4] +2.9

Vertical distribution of N*

deficit

excess

N2-Fix

denitrif

N* at 200m in the Pacific (Gruber and Sarmiento, 1997)

Map View of N*

N* on density of 26.5

Ryabenko 2013 Topics in Oceanography

Kuypers (2003) Nature 422: 608-611.

Nitrogen Cycle w/ anammox and denitrification

Why is N* negative – two sinks

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 e 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)

Ocean could be at SS or not!

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!

VRCR (25)

N2 Fix (110-330)

Denitrification sed = 200-280 wc = 75

B

fB (25)Fluxes in Tg N y-1

Brandes et al 2002

Net fluxes = -200 to 0(sink > source; non-SS??)

Nitrogen BalanceAtm Input (25)

Walker (1974) AJS

The Global Oxygen Balance

solar UVonly non-cycliconly w/o biology

Earth is overall reducingSeparate O2; sequester reducing material

Present is key to past

P and R in balance

Small imbalance in P-Rmarine org C only, not terrestrial80% in hemipelagic sedimentswhere %orgC = 0.5%

orgC includes H2S and Fe(II)

stoichiometric so use moles

Large O2 linked to Small CAs P = R, O2 not affected by DP

acceleratedweathering

tC = 20yr

tO2 = 4 my

tC = 108 yr

1) If P ceased and R continuedorg C would be consumed in 20 yrO2 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 controllingbalance 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 sinkSedimentary 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

2HCO3- + Ca2+ = CaCO3(s) + CO2(g) + H2O

CaCO3(s) + CO2(g) + H2O = 2HCO3- + Ca2+

A better example of reverse weathering! Fig. 2.5 Emerson and Hedges

weathering

deposition

Chemical Weathering, the Geological Carbon Cycle, Control on CO21. 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 Flux Atmospheric Demand

River input 43.2 Derived from atmosphere 35.0 -35.0 Derived from carbonates 8.7

Hydrothermal input 0.5

Carbonate deposition 49.4 Deposited as carbonates 24.7 Lost to atmosphere 24.7 +24.7

Net atmospheric demand -12.4

Units: 1012 mol/y

From McDuff and Morel (1980)

1. Some CO2 produced by carbonate deposition, but not enough!2. The rest must come from the Urey reaction.

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!

There must be a tight feedback control on atm CO2

% 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)

Deutsch et al, 2004)

Downcore records of 15N-orgN from several sitesHigh values of 15N-OrgN suggest more extensive denitrification

Deutsch et al, 2004