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Ocean Acidification The other CO 2 problem Keith Hunter Department of Chemistry University of Otago
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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.
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About half of the fossil fuel CO 2 is “missing” from the atmosphere…
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Fossil fuels combusted+240 Atmosphere+160 Ocean+120 Land - 40
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Penetration of anthropogenic CO 2 into the ocean Feely et al. Science 2004
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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
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CO 2 (g) + H 2 O + CO 3 2- 2 HCO 3 -
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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
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The saturation state controls whether calcium carbonate is stable or not (i.e. whether it dissolves)
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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
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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
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Otago Coastal Transect for pH and pCO 2 measurements
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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
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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.
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