1. Ocean Acidification The other CO 2 problem Keith Hunter Department of Chemistry University of Otago

2. CO 2 concentrations in dated polar ice cores show that the atmospheric increase began in the mid-18 th century (about 1750). It has been constant around 280 µatm for at least the last 7,000 years.

3. About half of the fossil fuel CO 2 is “missing” from the atmosphere…

5. Fossil fuels combusted+240 Atmosphere+160 Ocean+120 Land - 40

6. Penetration of anthropogenic CO 2 into the ocean Feely et al. Science 2004

8. As atmospheric pCO 2 and free CO 2 increase, pH and carbonate decrease CO 2 + CO 3 2- + H 2 O → 2 HCO 3 - Forms of CO 2 in water Free CO 2 beer, soda water Bicarbonate HCO 3 - baking soda Carbonate CO 3 2- limestone, washing soda

9. CO 2 (g) + H 2 O + CO 3 2-  2 HCO 3 -

10. Initially, carbonate is consumed CO 2 (g) + H 2 O + CO 3 2-  2 HCO 3 - Then calcium carbonate is dissolved CO 2 (g) + H 2 O + CaCO 3  2 HCO 3 - + Ca 2+ The changeover occurs when CaCO 3 becomes undersaturated [Ca 2+ ][CO 3 2- ] < K s

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

12. pH 8.2 pH 7.4 pH 8.2 pH 7.4 Corals subjected to acidified water lose their CaCO 3 When transferred to normal seawater, they calcify again Fine and Tchernov, Science 2007

13. 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

14. Otago Coastal Transect for pH and pCO 2 measurements

15. Otago Shelf Transect Subantarctic waters Annual pH change Equilibrium -0.0017 (Baring Head CO 2 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

16. Spatial 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 CO 2 as assumed in global models of CO 2 uptake by the ocean. Neritic and Southland Current waters are out of equilibrium, less affected by anthropogenic CO 2, i.e. less acidified.