Lecture 10: Ocean Carbonate Chemistry: Ocean Distributions Controls on Distributions What is the distribution of CO 2 added to the ocean? See Section 4.4 Emerson and Hedges

Sarmiento and Gruber (2002) Sinks for Anthropogenic Carbon Physics Today August

CO 2 CO 2 → H 2 CO 3 → HCO 3 - → CO H 2 O = CH 2 O + O 2 B orgC + Ca 2+ = CaCO 3 B CaCO3 Atm Ocn Biological Pump Controls: pH of ocean (controlled by DIC and Alk) Sediment diagenesis CO 2 Gas Exchange Upwelling/ Mixing River Flux CO 2 + rocks = HCO clays

Influences on pCO 2 K o : Solubility of CO 2 (same as K H ) K 1, K 2 : Dissociation constants Function of Temperature, Salinity Depends on biology and gas exchange Depends on biology only Derive starting with: CO 2 (g) + CO 3 2- = 2 HCO 3 - And use alk – DIC ~ CO 3 2- and 2DIC – alk ~ HCO 3 -

Ocean Distributions – versus depth, versus ocean Atlantic Pacific Points: 1. Uniform surface concentrations 2. Surface depletion - Deep enrichment 3. DIC < Alk  DIC >  Alk See Key et al (2004) GBC Q?

Controls on Ocean Distributions A) Photosynthesis/Respiration Organic matter (approximated as CH 2 O for this example) is produced and consumed as follows: CH 2 O + O 2  CO 2 + H 2 O Then: CO 2 + H 2 O  H 2 CO 3 * H 2 CO 3 *  H + + HCO 3 - HCO 3 -  H + + CO 3 2- As CO 2 is produced during respiration we should observe: pH  DIC  Alk  P CO2  CO 2 is an acid The trends will be the opposite for photosynthesis. B) CaCO 3 dissolution/precipitation CaCO 3 (s)  Ca 2+ + CO 3 2- Also written as:CO 3 2- is a base CaCO 3 (s) + CO 2 + H 2 O  Ca HCO 3 - As CaCO 3 (s) dissolves, CO 3 2- is added to solution. We should observe: pH  DIC  Alk  P CO2  Summary: DIC is from both organic matter and CaCO 3 Alk is only from CaCO 3

Influence of Nitrogen Uptake/Remineralization on Alkalinity NO 3 - assimilation by phytoplankton 106 CO H 2 O + 16 NO 3 - → (CH 2 O) 106 (NH 3 ) OH O 2 NH 4 assimilation by phytoplankton 106 CO H 2 O + 16 NH 4 + → (CH 2 O) 106 (NH 3 ) H O 2 NO 3 - uptake is balanced by OH - production Alk ↑ NH 4 + uptake leads to H + generation Alk ↓ Alk = HCO CO OH - - H + See Brewer and Goldman (1976) L&O Goldman and Brewer (1980) L&O Experimental Culture

The main features are: 1. uniform surface values 2. increase with depth 3. Deep ocean values increase from the Atlantic to the Pacific 4. DIC < Alk  DIC >  Alk 5. Profile of pH is similar in shape to O Profile of P CO2 (not shown) mirrors O 2. Ocean Distributions of, DIC, Alk, O 2 and PO 4 versus Depth and Ocean

Inter-Ocean Comparison

Carbonate ion (CO 3 2- ) and pH decrease from Atlantic to Pacific x mol kg -1 x mol kg -1 AlkDICCO 3 2- pH Surface Water North Atlantic Deep Water Antarctic Deep Water North Pacific Deep water Deep Atlantic to Deep Pacific  Alk =  DIC = So  Alk/  DIC = 0.40 CO 3 2- decreases from surface to deep Atlantic to deep Pacific. These CO 3 2- are from CO 2 Sys. Can Approximate as CO 3 2- ≈ Alk - DIC Q? CO 2 Sys/CO2Calc S = 35 T = 25C

Composition of Sinking Particles and Predicted Changes

Assume the following average elemental composition of marine particulate matter P NCCaSi Soft Parts Hard Parts Composite Implies Org C / CaCO3 ~ 105/26 ~4/1 The impact of this material dissolving CH 2 O + O 2  CO 2 + H 2 O  DIC = 1  Alk = 0 CaCO 3  Ca 2+ + CO 3 2-  DIC = 1  Alk = 2 1 mol CaCO 3 4 mol orgCComposite  DIC  Ca  alk Consequences: 1)  Alk/  DIC = 2/5 = 0.40 (DIC changes more than Alk) 2)  alk –  DIC ~  CO 3 2- = 2 – 5 = -3 (CO 3 2- decreases)

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 CaCO 3

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 g CaCO 3 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 / CaCO 3 ~ 1.1 Q. What does this imply?

PIC/POC in sediment trap samples

POC and CaCO 3 Export Fluxes This StudyPrevious Studies POC (Gt a −1 ) Global export9.6 ± –12.9 [Laws et al., 2000] b b 9.2 [Aumont et al., 2003] c c 8.6 [Heinze et al., 2003] c c 8.7–10.0 [Gnanadesikan et al., 2004] c c 9.6 [Schlitzer, 2004] d d 5.8–6.6 [Moore et al., 2004] c c CaCO 3 (GtC a −1 ) Global export0.52 ± –1.1 [Lee, 2001] b b 1.8 [Heinze et al., 1999] c c 1.64 [Heinze et al., 2003] c c 0.68–0.78 [Gnanadesikan et al., 2004] c c 0.38 [Moore et al., 2004] c c 0.84 [Jin et al., 2006] c c 0.5–4.7 [Berelson et al., 2007] b b Based on Global Model results of Sarmiento et al (2992) GBC; Dunne et al (2007) GBC POC/CaCO 3 = 9.6 / 0.52 = 18.5

Revelle Factor The Revelle buffer factor defines how much CO 2 can be absorbed by homogeneous reaction with seawater.  = dP CO2 /P CO2 / dDIC/ DIC B = C T / P CO2 (∂P CO2 /∂C T ) alk = C T (∂P CO2 /∂H) alk P CO2 (∂C T /∂H) alk After substitution B ≈ C T / (H 2 CO 3 + CO 3 2- ) For typical seawater with pH = 8, Alk = and C T = H 2 CO 3 = and CO 3 2- = ; then B = 11.2 Field 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 CO 2 is required to produce a 1% change in total CO 2 content of seawater, By this mechanism the oceans can absorb about half of the increase in atmospheric CO 2 B↑ as T↓ as C T ↑

CO 2 CO 2 → H 2 CO 3 → HCO 3 - → CO 3 2- Atm Ocn 350ppm + 10% = 385ppm 11.3  M +1.2 (10.6%)  M (1.7%) (-6.0%) Revelle Factor Numerical Example (using CO 2 Sys) CO 2 + CO 3 2- = HCO (+0.97%) DIC The total increase in DIC of  M is mostly due to a big change in HCO 3 - (+27.7  M) countering a decrease in CO 3 2- (-11.1  M). Most of the CO 2 added to the ocean reacts with CO 3 2- to make HCO 3 -. The final increase in H 2 CO 3 is a small (+1.2  M) portion of the total. at constant alkalinity

Emerson and Hedges Plate 8

Effect of El Nino on ∆pCO 2 fields High resolution pCO 2 measurements in the Pacific since Eq. Pac-92 Eq Pac-92 process study Cosca et al. in press El Nino Index P CO2sw Always greater than atmospheric

Photosynthesis/respiration (shown as apparent oxygen utilization or AOU = O 2,sat – O 2,obs ) and CaCO 3 dissolution/precipitation vectors (from Park, 1969) CH 2 O + O 2 → CO 2 + H 2 O as O 2 ↓ AOU ↑ CO 2 ↑