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AN = 6 (6P/6N) The Ocean Carbon Cycle AW = 12.011 Carbon dioxide (+4) Oxidation: -4 to +4 Marine Microplankton Ecology Isotopes: 11C, 12C, 13C, 14C OCN 626 Forms: oxides, hydrides, sulfides and halides WHY CARBON? Methane (-4)

1 GtC = 1 Pg C = 1 x 1015 g C

The oceans carbon cycle Carbon pools and fluxes

• Carbon reservoirs: - 2- • The main components: – Inorganic carbon system : DIC (H2CO3, HCO3 , CO3 ); total alkalinity; pH; pCO2 – DIC, DOC, PC (includes POC and PIC) • Coulometry (DIC), potentiometric titration (total alkalinity), spectrophotometric

(pH), pCO2 equilibrator • The major biologically mediated – Dissolved organic carbon (DOC) processes involved in carbon • High temperature combustion – Particulate carbon (POC and PIC) transformations (photosynthesis, • High temperature combustion (POC); acidification/ IR detection (PIC) respiration, calcification) • Carbon fluxes: – Biological carbon production (POC and DOC) • Reactivity of ocean components • 14C-bicarbonate assimilation (primary production), changes in carbon and oxygen inventories • The importance of ocean biology – Particulate carbon export • Sediment trap particle collections

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Vertical variations in carbon Particulate Carbon (mol L-1) concentrations in 0246810 Dissolved Organic Carbon (mol L-1) the open sea 20 40 60 80 100 0.01 g C in a 0 sugar cube The ocean holds Concentrations of 1000 50 grams of CO2 inorganic carbon for every 1 gram of increase with depth. 2000 CO2 in the atmosphere Concentrations of Depth (m) 3000 organic carbon decrease DIC 4000 DOC CO2 in oceans ~39,120,000,000,000,000,000 g C with depth. POC CO2 in the atmosphere ~750,000,000,000,000,000 g C 5000 Dissolved organic carbon ~700,000,000,000,000,000 g C 1900 2000 2100 2200 2300 2400 Living and dead particles ~3,000,000,000,000,000 g C Dissolved Inorganic Carbon (mol L-1)

What processes control inorganic carbon gradients in the sea? • Both abiotic and biotic processes Strong control carbon seasonality in distributions in the high the sea. latitude Northern • Atmospheric hemisphere CO2 levels are directly linked Weak to feedbacks seasonal patterns in the involving mid to low oceanic latitudes inorganic and organic carbon reservoirs.

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Dissolved inorganic carbon in seawater K Concentration of inorganic carbon in seawater is •HO + CO 0 H CO 2 2(g) K 2 3 partly controlled by thermodynamics •HCO 1 H+ + HCO - 2 3 K 3 - 2 + 2- • HCO3  H + CO3 )

-1 •Solubility of CO2 in •K0 = [H2CO3]/pCO2 seawater and speciation + - •K1 = [H ][HCO3 ]/[H2CO3] of the different inorganic + 2- - •K2 = [H ][CO3 ]/[HCO3 ] carbon components

s (mmol C L s (mmol - 2- n (H2CO3, HCO 3 ,CO, CO3 ) depends on seawater temperature, pressure, salinity, pH, and alkalinity. DIC concentratio DIC

- 2- [H2CO3] : [HCO3 ] : [CO3 ] 0.5% : 88.6% : 10.9% Temperature dependent relationship Surface seawater (pH~8.2): of DIC at atmospheric pCO2 280 ppm (preindustrial levels)

Processes influencing carbon cycling in the upper ocean Solubility pump: •Cooler water gains CO -high latitude regions are sinks for CO ATMOSPHERE 2 2 CO2 •Cooler, CO2 rich waters sink •Maintains vertical gradient in CO2 Gas exchange E-P balance •Air-sea heat fluxes drive air-sea CO2 fluxes

Net community Organic MIXED Poles Equator DIC Carbon LAYER production (DOC, POC) Mixed layer Entrainment boundary Diffusion Diffusion THERMOCLINE DIC

Particle export Adapted from Keeling et al. (2004)

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Temporal variability in mixed layer Interannual inorganic carbon variability in inorganic 2020 carbon 2000 ) concentrations Interannual -1 variations in 1980  Annual

DIC DIC

concentrations L C mol 1960 accumulation (0-  closely coincide ( 150 m) of nDIC ~ with changes to 1940 0.1 mol C m-2 yr-1 upper ocean 35.5 salinity. Salinity Interannual 35.0 variations in the E- P balance and 34.5 mixing important controls on 88 90 92 94 96 98 00 02 04 06 08 10 carbon inventories Year Dore et al. (2003) Winn et al. 1994, Dore et al. 2003, 2009

The oceanic laboratory: World Ocean Circulation Experiment hydrographic transects

Opportunity to evaluate distributions and inorganic pCO2 = pCO2(W) – pCO2(A) carbon throughout the world’s oceans Regions of negative pCO2 flux are regions where the ocean is a net sink for atmospheric CO2

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• Penetration of anthropogenic carbon partly reflects ocean 1 Gt = 1 billion tonnes = 1 Petagram = 1015 g C conveyor belt model of thermohaline circulation ssessment(2007) Report A

Measurements of DIC concentrations combined with IPCC Fourth carbon isotope ratios (13C/12C) and stoichiometry of

O2/CO2 makes it possible to reconstruct how much anthropogenic carbon has penetrated into the ocean. Note that the Southern Ocean remains poorly Arrows show the fluxes (in gigatonnes of carbon per year) between the atmosphere and its characterized. In addition, note that anthropogenic two primary sinks, the land and the ocean, averaged over the 1990s. Anthropogenic fluxes carbon has penetrated significantly below about 2000 are in red; natural fluxes in black. The net flux between reservoirs is balanced for natural meters of the water column only in the North Atlantic, processes but not for the anthropogenic fluxes. Within the boxes, black numbers give the where surface waters sinks to depth on time-scales preindustrial sizes of the reservoirs and red numbers denote the changes resulting from relevant to anthropogenic CO emission. 2 human activities since preindustrial times.

Carbonate chemistry plays a Long-term record major role in controlling of seawater pCO2 seawater pH (surface ocean)

DIC (mol L-1) and pH at 3 depth 1900 2000 2100 2200 2300 2400 strata at Station 0 ALOHA in the subtropical North 1000 “Ocean acidification due to increasing atmospheric carbon dioxide” Pacific. Physical The Royal Society 2005. www.royalsoc.ac.uk and biological 2000

Depth (m) Depth processes combine to alter 3000 pH DIC the rate of acidification along 4000 6.0 6.5 7.0 7.5 8.0 8.5 a depth pH continuum. Vertical profiles of DIC and pH at Station ALOHA Dore et al. (2009) PNAS 106: 12235-12240

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Increasing oceanic DIC has two important implications

•H2O + CO2(g)  H2CO3 + - •H2CO3  H + HCO3 - + 2- • HCO3  H + CO3 Saturation state:

2+ 2- 2+ 2- Ω = ([Ca ]seawater X [CO3 ]seawater) / ([Ca ]saturated X [CO3 ]saturated) • Increased H2CO3 (lowers pH) 2- When Ω > 1, CaCO3 supersaturated, shell formation favored. • Decreased CO3 (increases solubility of CaCO3) When Ω < 1, CaCO3 undersaturated, dissolution occurs.

Aragonite: Pteropods and corals Calcite: Coccolithophores and foraminifera

Dissolution and formation of shells 2+ 2- changes [Ca ] <1%; thus changes in [CO3 ] largely control Ω

Line 5

“Ocean acidification due to Feely et al. (2008); Science 320: 1490 - 1492 increasing atmospheric carbon dioxide” Distribution of the depths of the undersaturated water (aragonite saturation < 1.0; pH < 7.75) on the The Royal Society 2005. continental shelf of western North America. On transect line 5, the corrosive water reaches all the www.royalsoc.ac.uk way to the surface in the inshore waters near the coast. The black dots represent station locations.

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Biologically mediated carbon transformations in the sea

• Photosynthesis and respiration Sunlight Photosynthesis:

6CO2 + 12H2OC6H12O6 + O2 + 6H2O + heat

Respiration:

C6H12O6 + 6O2 6CO2 + 6H2O + heat

Calcification: 2+ - Biology also plays an important role in controlling carbon Ca + 2HCO3 CaCO3 + H2O + CO2 exchange between the ocean and atmosphere

Organic Carbon Pump Calcium carbonate pump Photosynthesis CO CO Plankton and carbon 2 2 : CO2

2+ - Ca 2HCO3 → CaCO3 + H2O + CO2 CO2 + 2H2O → CH2O + O2

+ CO2 +H2O =

+ O2 Decomposition :

CO2 + O2

= CO2 +H2O

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CO2 Carbon is the currency of life The biological carbon pump

0

Protein 1000 Carbohydrates ) Biology m 2000 CO2 Depth ( Depth

3000 Nucleic acids

4000 % dry 1900 2000 2100 2200 2300 2400 Substrate Element weight of Cellular Components Source Total inorganic carbon cell -1 (mol C kg ) C55DOC, CO2 Main constituent of cellular material

Biology establishes direct linkages between carbon cycling and the cycling of other elements

Gruber and Galloway (2008) Nature 451: 293-296

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