Lecture 10: Ocean Carbonate Chemistry: Ocean Distributions

Ocean DistributionsControls on Distributions

What is the distribution of CO2 added to the ocean?

See Section 4.4 Emerson and Hedges

Sarmiento and Gruber (2002) Sinks for Anthropogenic CarbonPhysics Today August 2002 30-36

CO2

CO2 → H2CO3 → HCO3- → CO3

2-

+ H2O = CH2O + O2

BorgC

+ Ca2+ = CaCO3

BCaCO3

Atm

Ocn

Biological Pump

Controls:pH of oceanSediment diagenesis

CO2

Gas Exchange

Upwelling/Mixing

River FluxCO2 + rocks = HCO3

- + clays

Influences on pCO2

Ko: Solubility of CO2

K1, K2: Dissociation constants

Function of Temperature, Salinity

Depends on biologyand gas exchange

Depends on biology only

Influence of Nitrogen Uptake/Remineralization on Alkalinity

NO3- assimilation by phytoplankton

106 CO2 + 138 H2O + 16 NO3- → (CH2O)106(NH3)16 + 16 OH- + 138 O2

NH3 assimilation by phytoplankton106 CO2 + 106 H2O + 16 NH4

+ → (CH2O)106(NH3)16 + 16 H+ + 106 O2

NO3- uptake is balanced by

OH- productionAlk ↑

NH4+ uptake leads to

H+ generationAlk ↓

Alk = HCO3- + 2 CO3

2- + OH- - H+

See Brewer and Goldman (1976) L&OGoldman and Brewer (1980) L&O Experimental Culture

Air-Sea CO2 Disequilibrium

Emerson and Hedges Plate 8

-2

-1

0

1

2

3

4

1985 1990 1995 2000

E

NS

O IN

DE

X (M

EI)

year

Effect of El Nino on ∆pCO2 fieldsHigh resolution pCO2 measurements in the Pacific since Eq. Pac-92

Eq Pac-92 process study

Cosca et al. in press

El Nino Index

PCO2sw

Always greater than atmospheric

Expression of Air -Sea CO2 Flux

k-transfer velocity

From Sc # & wind speed

From CMDLCCGG network

S – Solubility

From SST & Salinity

From measurements and proxies

F = k s (pCO2w- pCO2a) = K ∆ pCO2

pCO2apCO2w

MagnitudeMechanismApply over larger space time domain

Global Map of Piston Velocity (k in m yr-1) times CO2 solubility (mol m-3) = Kfrom satellite observations (Nightingale and Liss, 2004 from Boutin).

Overall trends known:

* Outgassing at low latitudes (e.g. equatorial)

* Influx at high latitudes (e.g. circumpolar)

* Spring blooms draw down pCO2 (N. Atl)

* El Niños decrease efflux

∆pCO2 fields

Monthly changes in pCO2w

∆pCO2 fields:Takahashi climatology

JGOFS Gas Exchange Highlight #4 -

Fluxes: JGOFS- Global monthly fluxes

Combining pCO2 fields with k: F = k s (pCO2w- pCO2a)

On first order flux and ∆pCO2 maps do not look that different

Do different parameterizations between gas exchange and wind matter?

Global uptakes Liss and Merlivat-83: 1 Pg C yr-1

Wanninkhof-92: 1.85 Pg C yr-1

Wanninkhof&McGillis-98: 2.33 Pg C yr-1

Zemmelink-03: 2.45 Pg C yr-1

Yes!

CO2 Fluxes: Status

Global average k (=21.4 cm/hr): 2.3 Pg C yr-1

We might not know exact parameterization with forcing but forcing is clearly important

Compare with net flux of 1.3 PgCy-1 (1.9 - 0.6)in Sarmiento and Gruber (2002), Figure 1

What happens to the CO2 that dissolves in water?

CO2 is taken up by ocean biology to produce a flux of organic mater to the deep sea (BorgC)

CO2 + H2O = CH2O + O2

Some carbon is taken up to make a particulate flux of CaCO3 (BCaCO3)

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

The biologically driven flux is called the “Biological Pump”.

The sediment record of BorgC and BCaCO3 are used to unravel paleoproductivity.

The flux of BorgC to sediments drives an extensive set of oxidation-reduction reactions that are part of sediment diagenesis.

Carbonate chemistry controls the pH of seawater which is a masterVariable for many geochemical processes.

Ocean Distributions – versus depth, versus ocean

Atlantic

Pacific

Points:1. Uniform surface concentrations2. Surface depletion - Deep enrichment3. DIC < AlkDIC > Alk

See Key et al (2004)GBC

Q?

The main features are:1. uniform surface values2. increase with depth3. Deep ocean values increase from the Atlantic to the Pacific4. DIC < Alk DIC > Alk5. Profile of pH is similar in shape to O2.6. Profile of PCO2 (not shown) mirrors O2.

Ocean Distributions of, DIC, Alk, O2 and PO4 versus Depth and Ocean

Inter-Ocean Comparison

Carbonate ion (CO32-) and pH decrease from Atlantic to Pacific

x 10-3 mol kg-1 x 10-6 mol kg-1

Alk DIC CO32- pH

Surface Water 2.300 1.950 242 8.30

North Atlantic 2.350 2.190 109 8.03 Deep Water

Antarctic 2.390 2.280 84 7.89 Deep Water

North Pacific 2.420 2.370 57 7.71 Deep water

Deep Atlantic to Deep PacificAlk = 0.070DIC = 0.180

SoAlk/DIC = 0.40

CO32- decreases from

surface to deep Atlanticto deep Pacific. These CO3

2- are from CO2Sys.Can Approximate as CO3

2- ≈ Alk - DIC

Q? CO2Sys

Controls on Ocean DistributionsA) Photosynthesis/RespirationOrganic matter (approximated as CH2O for this example) is produced and consumed as follows:

CH2O + O2 CO2 + H2OThen:

CO2 + H2O H2CO3*

H2CO3* H+ + HCO3

-

HCO3- H+ + CO3

2-

As CO2 is produced during respiration we should observe:pH DIC Alk PCO2

The trends will be the opposite for photosynthesis.

B) CaCO3 dissolution/precipitation

CaCO3(s) Ca2+ + CO3 2-

Also written as:CaCO3(s) + CO2 + H2O Ca2+ + 2 HCO3

-

As CaCO3(s) dissolves, CO32- is added to solution. We should observe:

pH DIC Alk PCO2

Photosynthesis/respiration (shown as apparent oxygen utilization or AOU = O2,sat – O2,obs) and CaCO3 dissolution/precipitation vectors (from Park, 1969)

CH2O + O2 → CO2 + H2O as O2↓ AOU ↑ CO2 ↑

Composition of Sinking Particles and Predicted Changes

Ocean Alkalinity versus Total CO2 in the Ocean(Broecker and Peng, 1982)

Emerson and Hedges Color Plate

DIC/Alk ≈ 1.5/1

Work Backwards

Alk / DIC ≈ 0.66 = 2/3

= 2 mol Org C / 1 mol CaCO3

From Klaas and Archer (2002) GBC

Data from annual sediment traps deployments

5 g POC g m-2 y-1 / 12 g mol-1 = 0.4 mol C m-2 y-1

40 g CaCO3 g m-2 y-1 / 105 g mol-1 = 0.38 mol C m-2 y-1

What is composition of sinking particles?

Org C / CaCO3 ~ 1

PIC/POC in sediment trap samples

POC and CaCO3 Export Fluxes

  This Study Previous StudiesPOC (Gt a−1)

Global export 9.6 ± 3.6 11.1–12.9 [Laws et al., 2000]b

9.2 [Aumont et al., 2003]c

8.6 [Heinze et al., 2003]c

8.7–10.0 [Gnanadesikan et al., 2004]c

9.6 [Schlitzer, 2004]d

5.8–6.6 [Moore et al., 2004]c

CaCO3 (GtC a−1)

Global export 0.52 ± 0.15 0.9–1.1 [Lee, 2001]b

1.8 [Heinze et al., 1999]c

1.64 [Heinze et al., 2003]c

0.68–0.78 [Gnanadesikan et al., 2004]c

0.38 [Moore et al., 2004]c

0.84 [Jin et al., 2006]c

0.5–4.7 [Berelson et al., 2007]b

Based on Global Model results of Sarmiento et al (2992) GBC; Dunne et al (2007) GBC

Revelle FactorThe Revelle buffer factor defines how much CO2 can be absorbed by homogeneous reaction with seawater. = dPCO2/PCO2 / dDIC/ DIC

B = CT / PCO2 (∂PCO2/∂CT)alk = CT (∂PCO2/∂H)alk

PCO2 (∂CT/∂H)alk

After substitution

B ≈ CT / (H2CO3 + CO32-)

For typical seawater with pH = 8, Alk = 10-2.7 and CT = 10-2.7

H2CO3 = 10-4.7 and CO32- = 10-3.8; then B = 11.2Field data from GEOSECS

Sundquist et al., Science (1979)

dPCO2/PCO2 = B dDIC/DIC

A value of 10 tells you that a change of 10%in atm CO2 is required to produce a 1% change in total CO2 content of seawater, By this mechanism the oceans can absorb about half ofthe increase in atmospheric CO2

B↑ as T↓

CO2

CO2 → H2CO3 → HCO3- → CO3

2-

Atm

Ocn

350ppm + 10% = 385ppm

11.3 M

+1.2 (10.6%)

12.5

1640.5 M

+27.7 (1.7%)

1668.2

183.7

-11.1 (-6.0%)

174.2

Revelle Factor Numerical Example (using CO2Sys)

CO2 + CO32- = HCO3

-

1837

+17.9 (+0.97%)

1854.9

DIC

The total increase in DIC of +17.9 M is mostly due to a big changein HCO3

- (+27.7 M) countering a decrease in CO32- (-11.1 M).

Most of the CO2 added to the ocean reacts with CO32- to make HCO3

-.The final increase in H2CO3 is a small (+1.2 M) portion of the total.