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Oxygen in the Ocean

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Oxygen in the OceansO2 (M)

0 50 100 150 200 250 300

DE

PT

H (

m)

0

1000

2000

3000

4000

5000

NorthPacific

NorthAtlantic

photosynthesis

OMZ oxygen minimum zonerespiration

respiration and mixing

O2 (M)

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1. Equilibrating or stripping with an inert gas and measure by G.C. , M.S. or I.R.

Dissolved oxygen determinations

He

To gas chromatograph, Mass spectrometer or I.R. spectrometer

frit

septum

Sample introduction

Signal

CO2

N2

O2

Elution time3/28

Dissolved oxygen determinations2. Diffusion of oxygen through a Teflon membrane

and polarographic determination.

Dissolved oxygen is reduced to hydroxide at the cathode, while the silver anode is oxidised:

O2 + 2H2O + 4 4OH-4Ago + 4Cl- 4AgCl + 4

and the resulting current is proportional to the concentration of oxygen.

Oxygen

Seabird SBE-43 O2 probe

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3. Quenching of a fluorescent dye trapped within a sensing foil (optode).

Dissolved oxygen determinations

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4. Direct measurement in solution (O2 by Winkler Method).Mn2+ + 2 OH- Mn(OH)2 (Winkler reagent)Mn(OH)2 + 0.5 O2 MnO(OH)2 2 Mn(OH)2 + 0.5 O2 2 Mn(OH)3 MnO(OH)2 + 4H+ + 3I- Mn2+ + I3- + 3H2O2Mn(OH)3 + 6H+ + 3I- 2Mn2+ + I3- + 6H2OI3- + 2S2O32- 3 I- + S4O62-

The thiosulfate titrant is standardized daily with KIO3:

IO3- + 6H+ + 8I- 3I3- + 3H2OI3- + 2S2O32- 3 I- + S4O62-

Dissolved oxygen determinations

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Dissolved oxygen determinations

NS2O32- = NIO3- VIO3-/(VS2O32- - Vblank)Vblank = volume of titrant needed to titrate the reagent blank (KI + NaOH, H2SO4)

[O2] = (VS2O32- - Vblank) NS2O32- - Vreagent [O2]reagent(Vbottle Vreagent)

[O2]reagent = oxygen concentration in the combined reagents

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Dissolved oxygen determinations

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Dissolved oxygen determinations

y = 0.3886x + 59.941R = 0.9859

62.0

63.0

64.0

65.0

66.0

6 8 10 12 14Dis

solv

ed o

xyge

n (m

icro

M)

Titrant volume (mL)

Reagent blank

y = 0.927x - 0.9407R = 0.9994

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

0.0 100.0 200.0 300.0 400.0

Prob

e D

O (m

icro

M)

Measured/Winkler DO (microM)

Probe calibration

n = 47

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Oxygen solubility = f (temperature & salinity)

From: Benson and Krause (1984) Limnol. Oceanogr. 29: 620-632

ln C = -135.29996 + 1.572288 x 105/T 6.637149 x 107/T2 + 1.243678 x 1010/T3 8.621061 x 1011/T4 Sp (0.020573 12.142/T + 2363.1/T2)

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Oxygen in the OceansO2 (M)

0 50 100 150 200 250 300

DE

PT

H (

m)

0

1000

2000

3000

4000

5000

NorthPacific

NorthAtlantic

photosynthesis

OMZ oxygen minimum zonerespiration

respiration and mixing

O2 (M)

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Oxygen in surface waters

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TEMPERATURE ( oC)

20 21 22 23 24 25 26

DE

PT

H (

m)

0

20

40

60

80

100

120

140

O2 (M)

210 220 230 240

O2 SATURATION

100 105 110 1150

20

40

60

80

100

120

140

Oxygen supersaturation in the surface ocean

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Oxygen in surface watersOxygen saturation (%) = 100 [G]/[G] = 100 [G]/(PG/KG )

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1. N2 time N2 O2

Due to GreaterSolubility of O2

2. Diffusion Coefficients are ApproximatelyDouble in Bubbles

3. Bubbles are Pushed to Depths of 50 m

4. Air Injection- the total dissolution of the Air in a Bubble due to Hydrostatic Pressure

Gas N2 O2 Ar CO2 Ne He KrAir % +7.7 +3.8 +3.5 +0.1 +11.6 +13.8 +1.8

From the total dissolution of a bubble (1cm3)of air at STP (15oC and S =35)

Bubble injection

Air (%) = (([G]meas/[G]equil)-1) 10015/28

1. N2

time

N2

O2

Due to Greater Solubility of O2

2. Diffusion Coefficients are Approximately Double in Bubbles

3. Bubbles are Pushed to Depths of 50 m

4. Air Injection- the total dissolution of the Air in a Bubble due to Hydrostatic Pressure

Gas N2 O2 ArCO2 Ne He Kr

(Air%+7.7 +3.8 +3.5 +0.1 +11.6 +13.8 +1.8

From the total dissolution of a bubble (1cm3) of air at STP (15oC and S =35)

Gas supersaturation in the Ocean

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Inert gases correct biological oxygen production for physical processes

Ar has very similar physical properties to O2O2:Ar ratios are a qualitative measure of biological oxygen production

Need to account for physical processes using inert gases

Contribution of productivity/photosynthesis to oxygen supersaturation in the surface ocean

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Oxygen distribution in the oceans

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LATITUDE

[O2] M

EAS -

[O2] C

ALC

(m

ol k

g-1 )

-15

-10

-5

0

5

10

SurfaceWaters

5oS 0 5oN

Effect of upwelling on surface ocean oxygen concentrations

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Apparent Oxygen Utilization

C106H263O110N16P + 138 O2 106 CO2 + 16 NO3- + HPO42- + 122 H2O + 18 H+ + (trace elements)

AOU = amount of dissolved oxygen used for respiration= [O2]satn [O2]meas

where [O2]satn = PO2(atm)/KO2, with KO2 being a function of T and S

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LATITUDE

[O2] M

EAS -

[O2] C

ALC

(m

ol k

g-1 )

-15

-10

-5

0

5

10

SurfaceWaters

5oS 0 5oN

Effect of upwelling on surface-ocean oxygen concentrations

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Apparent Oxygen Utilization and water age

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Dissolved oxygen and Apparent Oxygen Utilization

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Apparent Oxygen UtilizationAOU = measure of the amount of dissolved oxygen used for respiration

= [O2]satn [O2]meas

C106H263O110N16P + 138 O2 106 CO2 + 16 NO3- + HPO42- + 122 H2O + 18 H+ + (trace elements)

[NO3-]o.o. = AOU * 16/138; [SRP] o.o. = AOU/138

Redfield called the nitrate and phosphate produced in this way as nutrients of oxidative origin, as opposed to the preformed nutrients present in the body of water before it left the surface.

[NO3-]meas = [NO3-]preformed + [NO3-]o.o. = [NO3-]preformed + AOU*16/138

[SRP]meas = [SRP]preformed + [SRP]o.o. = [SRP]preformed + AOU/138

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Preformed and oxidative nitrate

[NO3-]meas = [NO3-]preformed + [NO3-]o.o. = [NO3-]preformed + AOU*16/138

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Organic matter respirationC106H263O110N16P + 138 O2

106 CO2 + 16 NO3- + HPO42- + 122 H2O + 18 H+ + (trace elements)

Hence, when phytodetritus is oxidized, 1.3 moles of oxygen are required to remineralize an amount of organic material containing one mole of carbon.

Whereas this ratio applies to the average organic matter formed in surface water, it does not necessarily apply to the organic matter falling to the deep sea. This material is partially decomposed, some is encapsulated in feacal pellets, and it does not necessarily have the same composition as the plant material synthesized in the euphotic zone.

Furthermore, in some regions of the deep sea, where oxygen becomes severely depleted, organisms use nitrate as an oxidant instead of O2.

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From: Broecker and Peng (1982) Tracers in the Sea, Eldigio Press

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Stoichiometry of organic matter respiration

CaCO3 = 0.5 (Alk + NO3-) = contribution of CO2 from CaCO3 dissolution

Org = CO2 - CaCO3 = change in CO2 from organic matter oxidation

(4)

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