What role does marine biology play on glacial interglacial CO2 cycles

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What role does marine biology play on glacial
interglacial CO2 cycles

• Lap Ting Cheung
• 4 February 2008

Kohfeld et al., Role of Marine Biology in
Glacial-Interglacial CO2 cycles, Science, 308,
74-78
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Background

• Causes to 80 100 ppm atmos CO2 fluctuation
  remained uncertain
• Physical mechanisms alone was not enough to lower
  atmos CO2 during glaciation
• Changes in marine biological pump could be a
  possible mechanism

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Various hypotheses proposed on the impact of
marine biology on atmos CO2

• Iron fertilisation of marine plankton
• Increases in ocean nutrient content
• Increases in ocean nutrient utilisation
• Changes to dominant plankton type
• Rain ratio
• Silica leakage
• ? They affect different parts of the ocean and
  leave information on the marine sediment record

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1. Iron fertilisation hypothesis

• Increase in aeolian Fe dust blown onto ocean
  during glaciation(higher dust deposition)?
  reduces Fe limitation in HNLC zone increases C
  fixation
• Increase in biogenic fluxes to marine sediments
• Affects N. Pacific / Southern Oceans /
  equatorial Pacific

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2. Increased-nutrient hypothesis

• Higher amt of C fixed per unit of NO3 and PO4
• Whole ocean nutrient increases
• ? carbon fixation and removal of surface water
  will increase
• Increases C and biogenic fluxes to marine
  sediments (export production)
• Affects Nutrient limited regions

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Tests on the 2 hypotheses via sediment record
Kohfeld et al. 2005

• Use of various export-production indicators (to
  avoid environmental influences on productivity
  and fluxes)
• e.g. biogenic components (Organic C / opal /
  alkenone ) Radionuclides (231Pa and 10Be) Ba /
  U / Cd

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4 different time periods for comparison

• Late Holocene (last 5000 years) modern
• Low dust flux
• LGM (18kyrs 22kyrs BP)
• High dust flux
• MIS 5a-5d (80kyrs 110kyrs BP)
• Beginning of glacial times
• Atmos CO2 50 ppm lower than late Holocene
• Dust composition comparable to modern times (i.e.
  low) ? allow study of export production on atmos
  CO2 w/o Fe fertilisation enhancement
• MIS 5e (130kyrs BP)
• Analog to modern times, with similar forcings and
  insolation patterns operating like today

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A. Comparison between Stage 5a 5d with late
Holocene
Kohfeld et al. 2005

• Overall global export production was lower than
  late Holocene
• Increased production at E Atlantic and present
  day APF

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B. Comparison between LGM with Stage 5a 5d
Kohfeld et al. 2005

• Overall global export production was higher than
  Stages 5a 5d
• Exception at southern ocean (near APF)

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C. Comparison between LGM with late Holocene
Kohfeld et al. 2005

• Overall global export production was higher than
  late Holocene
• Exception at area S of modern APF, in which
  global export production was lower in both LGM
  and Stages 5a-5d than late Holocene

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Test Implications

• Export production changes
• start from Stages 5a-5d (decreases)
• then fully increased at LGM.
• The 50 ppm drop in atmos CO2 during Stages 5a-5d
  cannot be explained by the 2. increased nutrients
  hypothesis
• Part of the drop can be explained by 1. Fe
  fertilisation hypothesis
• Timing of high export production (LGM) coincide
  with high dust influx
• sediment record supports the Fe fertilisation
  hypothesis during glaciation

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3. Increase in ocean nutrient utilisation
hypothesis

• Particularly affects Southern Oceans due to the
  strong connections between the surface (with
  unused nutrients) and the bottom CO2 rich waters
• To drive nutrient utilisation
• Reduction in vertical mixing S of APF ? reduce
  supply of CO2 and nutrients to the surface water
• Removes CO2 contact with surface ? drive down
  atmos CO2
• Record from LGM

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3. Increase in ocean nutrient utilisation
hypothesis
Author argues the reliability of d15N data hence
does not support this hypothesis will drive CO2
down S of APF
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Kohfeld et al. 2005
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4. Changes to dominant plankton typeA. Rain
ratio hypothesis

• Changes in rain ratio (CaCO3 / Corg) via shifting
  to non-carbonate producing species ? result in
  reduced CaCO3 export from surface to deep oceans
• This increases water alkalinity CO2 will be
  more soluble on surface waters ? reduce CO2 atmos
• At deep waters, overall carbonate ion content
  should increase ? resulting in more carbonate
  preservation

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4. Changes to dominant plankton typeA. Rain
ratio hypothesis

• Evidences for rain-ratio hypothesis is not
  observed
• Only small changes in carbonate ion level1. no
  deepened lysocline, 2. minor forams size
  changes, 3. minor changes to forams assemblage
  dissolution indices

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4. Changes to dominant plankton typeB.
Silica-leakage hypothesis

• Southern Ocean, Increase in d30Si and d15N
• At high Fe situations, diatoms take on less
  silicic acid relative to nitrate
• Unused silicic acid is transferred to Si-limited
  regions via SAMW ? increases diatoms production
  and reduces coccoliths production
• Si boosts - Rain ratio drops
• Lead to increase in export production in Southern
  Oceans
• Affects Low latitude areas where SAMW mixes with
  thermocline

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4. Changes to dominant plankton typeB.
Silica-leakage hypothesis

• Export production S of APF was low during stages
  5a-d
• Dust supply were low as well
• Silica leakage hypothesis cannot explain the
  initial 50-ppm drop in atmos CO2
• Change in dominant plankton type cannot explain
  the CO2 drawdown

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Limitations

• Poor data coverage
• Nutrient utilisation can occur at places other
  than the S oceans
• Vertical fluxes can be affected by other factors
  like sediment redistribution
• Changes on carbonate-ion concentration in
  deepwater is poorly determined
• Physical and Biological processes may interact

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Conclusions

• Low export production at Stages 5a -5d, but
  increases and peaks at LGM.
• Observations does not support the marine
  biological pump as a dominant element on
  influencing atmosphere CO2 changes during
  glaciation