Ocean AcidificationOcean Acidification

The other COThe other CO22 problem problem

Keith HunterKeith HunterDepartment of ChemistryDepartment of Chemistry

University of OtagoUniversity of Otago

CO2 concentrations in dated polar ice cores show that the atmospheric increase began in the mid-18th century (about 1750).

It has been constant around 280 µatm for at least the last 7,000 years.

1200 1400 1600 1800

Year

270

280

290

300

310

320

330

340

pC

O2 (

atm

)

CO2 in polar ice cores

About half of the fossil fuel CO2 is “missing” from the atmosphere…

1960 1970 1980 1990

Year

320

340

360

380C

O2 (

ppm

v)

CO2 at Mauna Loa

+ Fossil fuel CO2

Fossil fuels combusted +240

Atmosphere +160Ocean +120Land - 40

Penetration of Penetration of anthropogenic COanthropogenic CO22

into the oceaninto the ocean

Feely et al. Feely et al. Science Science 20042004

As atmospheric pCO2 and free CO2 increase, pH and carbonate decrease

CO2 + CO32- + H2O → 2 HCO3

-

pH

Free CO2

Carbonate

400 600 800 1000

pCO2

7.7

7.8

7.9

8.0

8.1

pH

50

100

150

200

250

Forms of CO2 in waterFree CO2 beer, soda waterBicarbonate HCO3

- baking sodaCarbonate CO3

2- limestone, washing soda

CO2(g) + H2O + CO32- 2 HCO3

-

H2CO

3

CO32-

HCO3-

CT

400 600 800 1000

pCO2 (atm)

-100

0

100

200

300

Ab

solu

te c

han

ge

(m

ol

kg-1

)

Initially, carbonate is consumed

CO2(g) + H2O + CO32- 2 HCO3

-

Then calcium carbonate is dissolved

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

H2CO

3

CO32-

HCO3-

CT

400 600 800 1000

pCO2 (atm)

-100

0

100

200

300

Ab

solu

te c

han

ge

(m

ol

kg-1

)

The changeover occurs when CaCO3 becomes undersaturated

[Ca2+][CO32-] < Ks

The saturation state controls whether calcium carbonate is stable or not (i.e. whether it dissolves)

pH 8.2 pH 7.4pH 8.2 pH 7.4

Corals subjected to acidified water lose their CaCO3

When transferred to normal seawater, they calcify again

Fine and Tchernov, Science 2007

Observed and predicted Southern Ocean surface acidification conditions for the 21st century

Predicts first “tipping point” for aragonitic organisms by about 2030

McNeil B. I., Matear R. J. PNAS 2008;105:18860-18864

Otago Coastal Transect forpH and pCO2 measurements

Otago Shelf TransectOtago Shelf TransectSubantarctic watersSubantarctic waters

Annual pH change

Equilibrium -0.0017 (Baring Head CO2 data)

Subantarctic -0.0020 0.0017 (standard error, n = 28, probability 80%)

Strong seasonal cycle driven by plankton productivity

Water is fully acidified in winter

Much less so in summer

Spatial VariabilitySpatial Variability

3 different water masses: Neritic (0-18 km), Subtropical (18-32 km) and Subantarctic (> 32 km).

Subantarctic water seems to be close to equilibrium with atmospheric CO2 as assumed in global models of CO2 uptake by the ocean. Neritic and Southland Current waters are out of equilibrium, less affected by anthropogenic CO2, i.e. less acidified.

Waikouaiti River

Dunedin

Cape Saunders

Blueskin Bay

10 km

Taiaroa Head

183 m

Otago Harbour3

1

5 7 911

24

6 8 10

Neritic

Southland Current

Subantarctic

7.8

7.85

7.9

7.95

8

0 10 20 30 40 50 60 70

pH

Distance from Taiaroa head

Otago Shelf Transect, September 2005

Equilibrium with air

I n 1750