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 ocean (controlled by DIC and Alk)Sediment diagenesis

CO2

Gas Exchange

Upwelling/Mixing

River FluxCO2 + rocks = HCO3

- + clays

Influences on pCO2

Ko: Solubility of CO2 (same as KH)K1, K2: Dissociation constants

Function of Temperature, Salinity

Depends on biologyand gas exchange

Depends on biology only

Derive starting with: CO2(g) + CO32- = 2 HCO3

-

And use alk – DIC ~ CO32- and 2DIC – alk ~ HCO3

-

Ocean Distributions – versus depth, versus ocean

Atlantic

Pacific

Points:1. Uniform surface concentrations2. Surface depletion - Deep enrichment3. DIC < Alk4. DDIC > DAlk

See Key et al (2004)GBC

Q?

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 CO2 is an acid

The trends will be the opposite for photosynthesis.

B) CaCO3 dissolution/precipitation

CaCO3(s) Ca2+ + CO3 2-

Also written as: CO32- is a base

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

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

pH DIC Alk PCO2 Summary: DIC is from both organic matter and CaCO3

Alk is only from CaCO3

Influence of Nitrogen Uptake/Remineralization on AlkalinityNO3

- assimilation by phytoplankton106 CO2 + 138 H2O + 16 NO3

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

NH4 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

The main features are:1. uniform surface values2. increase with depth3. Deep ocean values increase from the Atlantic to the Pacific4. DIC < Alk DDIC > DAlk5. 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 1.950 246 8.12

North Atlantic 2.350 2.190 128 7.75 Deep Water

Antarctic 2.390 2.280 101 7.63 Deep Water

North Pacific 2.420 2.370 72 7.46 Deep water

Deep Atlantic to Deep PacificDAlk = 0.070DDIC = 0.180

SoDAlk/DDIC = 0.40

CO32- decreases from

surface to deep Atlanticto deep Pacific. These CO3

2- are from CO2Sys.Can Approximate as CO3

2- ≈ Alk - DICQ? CO2Sys/CO2Calc

S = 35T = 25C

Composition of Sinking Particles and Predicted Changes

Composition of Sinking Particles and Predicted ChangesAssume the following average elemental composition of marine particulate matter

P N C Ca SiSoft Parts 1 15 105 0 0Hard Parts 0 0 26 26 50Composite 1 15 132 26 50

Implies Org C / CaCO3 ~ 105/26 ~4/1

The impact of this material dissolving

CH2O + O2 CO2 + H2O DDIC = 1 DAlk = 0 CaCO3 Ca2+ + CO3

2- DDIC = 1 DAlk = 2

1 mol CaCO3 4 mol orgC CompositeDDIC 1 4 5DCa 1 0 1Dalk 2 0 2

Consequences: 1) DAlk/DDIC = 2/5 = 0.40 (DIC changes more than Alk) 2) Dalk – DDIC ~ DCO3

2- = 2 – 5 = -3 (CO3 2- decreases)

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

Emerson and Hedges Color Plate

DDIC/DAlk ≈ 1.5/1

Work Backwards

DAlk / DDIC ≈ 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 = 0.42 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 ~ 1.1

Q. What does this imply?

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) GBCPOC/CaCO3 = 9.6 / 0.52 = 18.5

Revelle FactorThe Revelle buffer factor defines how much CO2 can be absorbed by homogeneous reaction with seawater. B = 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↓ as CT↑

CO2

CO2 → H2CO3 → HCO3- → CO3

2-

Atm

Ocn

350ppm + 10% = 385ppm

11.3 mM

+1.2 (10.6%)

12.5

1640.5 mM

+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 mM is mostly due to a big changein HCO3

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

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

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

at constant alkalinity

Emerson and Hedges Plate 8

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

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 ↑