Ben McNeil Climate Change Research Centre, University of New South Wales, Sydney

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Ben McNeilClimate Change Research
Centre,University of New South Wales, Sydney
Southern Ocean Acidification Earlier Impacts
via Seasonal Amplification
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Importance of the Ocean for Atmospheric CO2
Observed CO2
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Unfortunate Consequences of Oceanic Anthropogenic
CO2 Absorption
CO2
Increase
Increase from air-sea exchange
CO2 H2O ? HCO3- H ? CO32- 2H
Decrease
pH -logH More CO2 Uptake More Acidic
Less CO32-
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Ocean Carbon Chemistry Primer
CO2(gas)
280 matm
560 matm
CO2 H2O ?? H2CO3 H2CO2 ?? H HCO3- HCO3-
?? H CO32-
8 mmol kg-1
15 mmol kg-1
Carbonic acid
1617 mmol kg-1
1850 mmol kg-1
Bicarbonate
268 mmol kg-1
176 mmol kg-1
Carbonate
1893 mmol kg-1
2040 mmol kg-1
TCO2 (DIC)
100 increase in pCO2 8 increase in DIC
100 increase pCO2 33 decrease in CO32-
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Changing Carbonate Equilibrium in Seawater -
Graphically
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Calcium Carbonate (CaCO3) Cycle
Stable CaCO3 ? gt1
Unstable CaCO3 ? lt1
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Two forms of calcium carbonate - very different
stability
Aragonite (plankton and corals)
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? lt 1 Geochemically Dissolves Aragonite Shells
Peteropod, Clio pyramidata after 24 hours Orr et
al (2005)
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Will ? lt1 this century?
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Southern Ocean Vulnerable
Orr et al (2005)
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But averages arent the problem! Natural Cycles
amplify the mean trend
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How do natural carbonate cycles impact annual
trend in Southern Ocean acidification?

• Estimate the seasonal cycle of carbonate in the
  Southern Ocean using sparse measurements of
  Dissolved Inorganic Carbon (DIC) and Alkalinity
  (ALK)
• Combine with future oceanic CO2 uptake to better
  estimate the onset of Southern Ocean
  acidification
• Any in-situ evidence?
• Implications for Southern Ocean marine calcifiers

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How to Estimate Surface Seasonality in the
Southern Ocean?
Step 1) Gather all available DIC/ALK surface
measurements up to 55m
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Empirical fits for surface DIC / ALK
DIC f(temp, sal, nitrate, silicate, oxygen) S.E
8umol/kg n1200 ALKf(temp,sal) S.E
9umol/kg

• No change to the fit with season
• No strong residual pattern indicating potential
  biases

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World Ocean Atlas - Hydrographic Data

• 96000 stations

100 times the amount of data than carbon
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First ever seasonal carbon climatology
Step 2) Extrapolate using World Ocean Atlas
climatology
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Step 3) Calculate seasonal pH and Carbonate ion
from DIC/ALK
DIC (umol/kg)
ALK (umol/kg)
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Seasonal Variability in Carbon
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Independent Winter Analysis
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IS92a Atmospheric CO2 Scenario
Future Projections Over the Annual Cycle
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Onset of Southern Ocean Aragonite Undersaturation
(?lt1)
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450ppm is the tipping point
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Southern Ocean Acidification Divide
Acidification divide set-up by the Polar Front
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Southern Ocean Pteropods Vulnerable

• Pteropods, particularly Limacina species, are an
  abundant group, with regionally very high
  densities
• Pteropods can make a substantial contribution to
  both meso-and macrozooplankton communities
• Up to 65 of zooplankton biomass
• Feed for higher trophic species (fish, whales
  etc)
• Pteropods have long life-cycles (1-3years),
  vulnerable to peridodic undersaturation

• Limacina helicina
• Limacina retroversa

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Knowns and Unknowns of Ocean Acidification

• Flow on effects to fisheries
• Yet unknown?

• Organisms that require calcium carbonate will be
  vulnerable
• Phytoplankton and corals

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Conclusions

• Considerable seasonal amplification of the
  carbonate ion in the Southern Ocean
• Very low winter-time values coupled with future
  anthropogenic CO2 will induce corrosive
  seawater conditions decades before previously
  though (2030 in the open Southern Ocean)
• Suggests that 450ppm is the tipping point for
  these corrosive conditions
• The Ross Sea was found to have a huge seasonal
  cycle, and the rapid decline in ? between summer
  and autumn can not be ruled out in causing the
  Pteropod die-off collected in sediment traps
• The very low winter ? brings forward the onset of
  aragonite saturation to the year 2015 in the Ross
  Sea
• Coinciding biological and geochemical time-series
  studies are needed to understand the implications
  for Southern Ocean calcifiers