Ocean acidification Lessons from the geologic past

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Ocean acidificationLessons from the geologic
past
Surface Ocean CO2 Variability and Vulnerability
Workshop Paris, France 11-14 April 2007

• Ken Caldeira
• Carnegie Institution Department of Global Ecology
• Stanford, CA 94305 USA
• kcaldeira_at_globalecology.stanford.edu

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CO2 was high in the past.Why worry about the
future?
Figure from Wikipedia Dragons flight
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The long-term global carbon cycle
WEATHERINGOF SILICATEROCKS
CO2
CO2
IONS CARRIED BY RIVERS TO OCEAN
ORGANISMS USE IONS TO BUILD CALCIUM CARBONATE
SHELLS
Image source unknown
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Greenhouse weathering feedback
CO2
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Greenhouse weathering feedback
Climate Temp precip.

CO2
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Greenhouse weathering feedback
Weathering Ca-Mg Silicates
Climate Temp precip.


CO2
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Greenhouse weathering feedback
Weathering Ca-Mg Silicates
Climate Temp precip.



CO2
CO2 CaSiO3 CaCO3 SiO2
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Figure from Wikipedia Dragons flight
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Controls on CO2(aq)

• On long time scales (gtgt 100 kyr), atmospheric
  pCO2 is controlled by relations between geologic
  CO2 degassing and chemical weathering of silicate
  rocks
• On these time scales, ocean CO2(aq) is
  controlled by atmospheric pCO2

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Ridgwell, Kennedy and Caldeira (Science,
2003)Cartoon by Andy Ridgwell
Both shallow-water and deep-water carbonate
accumulation increase with increasing CO32-
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To change the depth of the calcite lysocline by 3
km, CO32- would need to change by only 40 µmol
kg-1.
If this were occur according to the
reaction CO2 CO32- H2O ? 2 HCO3- only 625
PgC carbon would be required ( 5 1016 molC)
From Broecker and Peng 1982
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WRE CO2 stabilization pathways
Prescribed atmospheric CO2
Computed CO2 emissions
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WRE CO2 stabilization pathways
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CCD over past 50 myr
Tripati et al., 2005
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Laboratory results for growth of coral skeletons
at different CO2 levels
of year 1750 growth rate
Saturation in ocean of coral skeletal
mineral(aragonite CaCO3)
Courtesy Chris Langdon
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Corals and past, present, and future ocean
chemistry
Atmospheric CO2 785 ppm
Atmospheric CO2 280 ppm
NOAA (Feely et al. 2006)
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Reef distribution through time
Recent
0
500
400
300
200
100
Millions of years ago
Kiessling
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Future ocean chemistry
NOAA (Feely et al. 2006)
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Negative feedback stabilizingocean chemistry
CaCO3formation(dissolution)

CaCO3saturation inocean
-
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Negative feedback stabilizingocean chemistry
CO2release
-
CaCO3formation(dissolution)

CaCO3saturation inocean
-
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Negative feedback stabilizingocean chemistry
CO2release
This feedback stabilizes ocean chemistry on the
time scale of 1 kyr lt t lt 10 kyr
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CaCO3formation(dissolution)

CaCO3saturation inocean
-
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Controls on CaCO3 saturation

• The carbonate sedimentary system controls the
  aragonite and calcite saturation state of the
  ocean
• These saturation state has been relatively
  constant over the past 500 million years
• perhaps decreasing over the past 100 million
  years

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Ocean Ca2 over past 570 million years
Mg2
Horita et al, 2002
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Controls on CO32-

• If, the carbonate sedimentary system controls the
  aragonite and calcite saturation state of the
  ocean, then CO32- is controlled by
• variation in Ca2
• variation in dominant lithology (influenced by
  Ca2 and Mg2)

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Ocean chemistry controls on geologic time scales

• The atmosphere controls CO2(aq)
• The carbonate sedimentary system controls Ca2
  CO32-
• From CO2(aq) and CO32-, we can calculate all
  carbon species and pH

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An example of ocean pH estimatestaking into
consideration ?T, ?pCO2, ?Ca2, etc.
Assumes constant surface ocean carbonate
saturation
100 million years ago, carbonate saturation
states may have been higher but pH may have been
lower
pH
No pretense that this is correct, but illustrates
scale of possible pH variation.
Time (Ma)
Caldeira and Portner, in prep.
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Calcitic and aragonitic seas over the past 500 Ma
Mg/Ca
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Glacial-interglacial rates of CO2 change

• Typical rates are hundredths of a ppm per year

Calculated from Petit et al (1999)
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Recent rates of CO2 change

• Typical rates are several ppm per year

Calculated from Keeling and Whorf (2005)y
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Rates of atmospheric CO2 change
Recent
Glacial-Interglacial
Atmospheric CO2 is changing roughly 100 times
more rapidly than natural variation
Calculated from Petit et al (1999)
Calculated from Keeling and Whorf (2005)
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Amounts include total fossil-fuel plus net land
biosphere emissions to the atmosphere
Caldeira and Wickett (2005)
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Anthropogenic CO2 emissions exceed natural
emissions by a factor of about 50
CO2 emissions from human activities gt 7 PgC / yr
Volcanic, metamorphic, hydrothermal CO2 degassing
0.1 PgC / yr
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Predicted future CO2 concentration exceed those
inferred for past 25 million years
Even if most fossil-fuel carbon is never
released to the atmosphere, we will produce
geologically unusual conditions
Paleo-CO2 lines (Pagani et al., 1999 Pearson
and Palmer, 2000) Year 2300 atmospheric CO2
predictions for scenarios involving fossil-fuel
plus net biomass release over several centuries
colors(Caldeira and Wickett, 2005)
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Site 690 55 myr ago

• Remained above CCD throughout entire PETM
• paleodepth 1900 m

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65 Ma
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What happened at the K/T boundary?

• Asteroid or comet impact into CaSO4-rich platform
  at Chicxulub
• CO2 releases
• 50 PgC impact metamorphism
• Land biomass burning
• Reduced organic C pump in ocean

NASA
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Similar results obtained from leaf stomatal
evidence (Beerling et al., 2002)
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What happened at the K/T boundary?

• To produce calcite undersaturation in upper 100 m
  of ocean
• 5 x 1015 mol SO42-
• Impact related SO2 releases(Sigurdsson et al,
  1991)
• Low end 38 x 1015 mol SO2
• High end 133 x 1015 mol SO2

NASA
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Extinction of calcareous plankton at the K/T
boundary (66.4 Ma)
Planktonic and benthic isotopes from DSDP Site
577Zachos et al., 1988
CaCO3 accumulation ratesZachos and Arthur, 1986
Strangelove Ocean
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Mel Pollinger
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Time scale for recovery after mass extinction
(lt 2 million years)
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Conclusion

• If current trends in fossil-fuel use continue, we
  may produce
• pCO2 and pH that have not been experienced in
  tens of millions of years
• Carbonate saturation states that occur only in
  rare unusual events
• Probably more extreme than PETM (55 Ma)
• Probably less extreme than K/T boundary (65 Ma)

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Some (weak) evidence for decreasing CaCO3
saturation
Iryu and Yamada, 1999
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Some evidence for decreasing CaCO3 saturation
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