Ocean Acidification The other CO 2 problem
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Ocean AcidificationOcean Acidification
The other COThe other CO22 problem problem
Keith HunterKeith HunterDepartment of ChemistryDepartment of Chemistry
University of OtagoUniversity of Otago
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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
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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
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Fossil fuels combusted +240
Atmosphere +160Ocean +120Land - 40
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Penetration of Penetration of anthropogenic COanthropogenic CO22
into the oceaninto the ocean
Feely et al. Feely et al. Science Science 20042004
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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
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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
)
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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
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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 7.4pH 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
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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 forpH and pCO2 measurements
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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
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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
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