What Caused Pleistocene Cycles in Atmospheric CO2

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What Caused Pleistocene Cycles in Atmospheric CO2

• The Carbonate Mechanism

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DIC ALK

• CO2(g)? CO2(aq)
• ? H2CO3 ?HHCO3- ? 2HCO32-
• DIC- Dissolved inorganic carbon Sum off all
  these species
• ALK- Alkalinity the total equivalence of ions
  that must be protonated in order to lower pH of
  seawater to the pKa of carbonic acid

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Changing ALK to Deep Sea

• Inputs of ALK to ocean are weathering of
  carbonates and weathering of silicates
• Outputs are precipitation of CaCO3 and
  consumption of ALK by hydrothermal alteration
• If increased weathering of carbonates in a
  glacial, or lessened coral reef growth, the deep
  sea inventories of DIC ALK would increase by
  12 ratio ? decrease CO2(atm)
• Coral reef hypothesis- shift in locus of CaCO3
  deposition to deep sea (Berger 82 Opdyke and
  Walker 92)

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The Lysocline

• Increasing ALK to the deep sea increases CO32-
  which pushes the calcite saturation depth and the
  lysocline deeper
• More area to precipitate CaCO3 over
• Burial comes into equilibrium with increased ALK
  supply CO2 stabilises at lower value
• Lysocline would deepen by 1km for 25ppmv
  decrease in CO2 associated with increased ALK
• To match pCO2 changes calcite burial rate must
  have been 2-3 times today

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Calcite Respiratory Dissolution

• Significant dissolution of calcite occurs in
  response to the addition of CO2 to the pore water
  as oxic organic carbon degredation (Archer
  Maier-Reimer 1994)
• Produces a minimum in CO32- at a depth of a few
  cm in sediment allowing dissolution, despite
  supersaturated overlying waters
• Represents separation of saturation horizon and
  lysocline
• Glacial times might have higher Corg/CaCO3 ratio
  of material reaching deep sea sediments more
  calcite respiratory disasolution

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• From Archer and Maier-Reimer 1994
• Calcite respiratory dissolution allows lysocline
  and saturation horizon seperation, allows CO32-
  build up

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Support

• Glacial increased Corg production suggested by
• Increased benthic-planktonic d13C gradients,
  increased sediment Corg from cores change in
  foraminiferal species
• In general calcite organisms replaced by silicic
  diatoms
• Front between Si CaCO3 production moved
  equatorwards
• pH

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Sanyal et al 1995

• Boron isotopes from forams
• Tropical surface waters pH 0.20.1pH units higher
  than Holocene
• Tropical deep waters pH 0.30.1pH units higher
  than Holocene
• If pH difference due to CaCO3 accumulation, ALK
  10 higher? CO32- 100µmolkg-1 higher ?
  deepening of calcite saturation by several km
• Lysocline estimated only 700m deeper

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Problems

• If lysocline and saturation horizon was offset by
  several km in glacial times, would expect some
  significant separation today due to Corg rain
• Transitition- pH estimates suggest 100µmolkg-1
  CO32- more than today, when excess Corg shut off
  lysocline drop below abyssal plains- mass calcite
  accumulation everywhere- CO32- drops lysocline
  rises and redissolves. Lack of evidence hard to
  believe

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CaCO3 Compensation Model

• At start of glacial excess metabolic CO2
  transferred from upper to deep ocean
• Decrease ALKDIC ? less CO32- and dissolution of
  seafloor CaCO3
• Raises ALKDIC in 21 ratio until new steady
  state
• As initial CO2 addition was simple rearrangement
  within ocean, net ALKDIK gain hence lower pCO2

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• CO2 shift could be due to
• i) Reduced ventilation of deep ocean around
  Antarctica
• ii) Formation of low nutrient Glacial Atlantic
  Intermediate Water at expense of NADW
• iii) More efficient Corg export due to increased
  nutrient supply
• Results in upper ocean enriched in 13C and deep
  ocean depleated