1. Palaeoclimate Change SOES 3015
Lecture 7:
Mechanisms of change-3:
Phanerozoic CO2 & global geochemical carbon cycle (PAW)
Lecture outline:
• Patterns of long-term climate change
- Precambrian, Palaeozoic & Lower Mesozoic
- Upper Mesozoic & Cenozoic
• Geochemical carbon cycle
- Doomsday Life on Earth
- How does the geochemical carbon cycle work?
- Introduction to the BLAG model
Introduction
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2. (1) Patterns of long-term climate change
Precambrian, Palaeozoic & Lower Mesozoic
Upper Mesozoic & Cenozoic
(i)  Precambrian, Palaeozoic & Lower Mesozoic
Precambrian: very few constraints on changes in climate:
one is the date of first appearance of Life on Earth
~3,500 Ma - structurally complex microfossils
- implies earlier origins ~3.850 Ma*
* time elapsed = 1 calendar year
Cambrian ‘explosion’ ~ mid. Sept.
K/T boundary ~ Christmas day

3. Earth History (1982) National Academy of Sciences Press.
Phanerozoic:
Two Phanerozoic ‘supercycles’ (~ 700 Ma; Fischer ‘81)
• first order eustatic SL sequence stratigraphy (Exxon)
= ‘greenhouse’ - ‘icehouse’ -- global volcanism (atms. PCO2)
Fischer, A.G., Long term climatic oscillations recorded in stratigraphy, in: geophysics study committee, (eds) Studies in geophysics, Climate in
Earth History (1982) National Academy of Sciences Press.

4. (ii) Upper Mesozoic & Cenozoic
Mid-Cretaceous
• Widespread evidence for global warmth:
• poles free of continental-scale ice-caps
…. leaf physiognomy to drop stones
• poleward spread of flora & fauna (terrestrial & marine):
… dinosaurs to breadfruit trees
• marine foraminiferal calcite 18O:

5. Aptian/Albian vs. Turonian (~110 vs. 90 Ma)
• thermal maximum:
Aptian/Albian vs. Turonian (~110 vs. 90 Ma)
• cooling to Maastrichtian (K/Pg = 65 Ma)
Courtesy of GSA: Barrera, E., (1994) Global environmental changes preceding the Cretaceous-Tertiary boundary: Early-late Maastrichtian transition, Geology, v. 22, p

6. & significantly warmer: • intermediate & deep waters
• surface mid- & high latitude waters
= reduced equator to pole temperature gradient
Data sourde from: Huber, B.T., Hodell, D.A., Hamilton, C.P., Middle–Late Cretaceous climate of the southern high latitudes: Stable isotopic evidence for minimal equator-to-pole thermal gradients Geological Society of America Bulletin October, 1995, v. 107, p ,

7. • Explanations for global warmth:
• ‘geography’
• less high latitude continentality
less ice formation dec. albedo
less barriers to poleward ocean heat transport
• higher sea levels dec. albedo
Application of General Circulation Models: GCMs--
• atmospheric: Ocean boundary & simulate atmosphere
(cf. Met Office)
• oceanic: Atmosphere boundary & simulate oceans
• coupled: simulate both
GCM Results:
Problem:
• warming insufficient cf. geological data (esp. high lat.)
Solution = another forcing factor
• higher atms. POC2: Min. 4x present (Barron & Washington ‘85)
ie. ‘greenhouse’, seems obvious to us in C21st
But: where did all this CO2 come from?

8. Courtesy of GSA: Larson, R. L
Courtesy of GSA: Larson, R.L., (1991) Geological consequences of superplumes, Geology, v. 19, p

9. (2) Geochemical Carbon Cycle
• Doomsday Life on Earth
• How does the geochemical carbon cycle work?
• Introduction to the BLAG model
(i) • Doomsday Life on Earth
• wipe out all life
• combust all Corg
this would add less CO2 to the atmosphere than we have already added by anthropogenic burning of fossil fuels
Que: What does this tell us?
Ans:
• that sedimentary rocks represent by far the largest reservoir of C on
the planet
• that the biological carbon cycle represents a mere sub-routine of the
geochemical carbon cycle
 on geological (Ma) timescales: (large) s atms. PCO2 will be caused by (small) perturbations to the geochemical carbon cycle

10. What is the geochemical carbon cycle?
Paul Wilson, University of Southampton

11. (ii) How does the geochemical cycle work?
Let us consider chemical weathering on the continents
Continents:
• kerogens (shales) ‘organic C’
• carbonates (limestones & dolostones) ‘inorganic C’
• silicates (granites & basalts) no C
CO2 + H2O = H2CO3 + CaCO3 = Ca2+ + 2HCO3- (1)
2CO2 + H2O = H2CO3 + CaSiO3 = Ca2+ + 2HCO3- + SiO2 (2)
carbonates: one bicarbonate from carbonate & one bicarbonate from atmosphere
silicates: all C in bicarbonate from the acid
Oceans:
Formation & burial new carbonate (eg. Forams & corals)
2HCO3- + Ca2+ = CaCO3 + CO2 + H2O (3)
Bicarbonate-C’s produced during limestone weathering:
• one buried as new carbonate
• other returned to the atmosphere
No net consumption of atmospheric CO2 (compare 1 & 3)

12. Same is not true of silicate weathering:
• returning only one bicarbonate-C to atmosphere
Net (50%) consumption of atmospheric CO2 (compare 2 & 3)
CO2 + CaSiO3 = CaCO3 + SiO2 (4)
But this cannot be the whole story
‘cos unchecked, would consume atms. CO2
(doomsday life on Earth very quickly, ~0.3 Ma)
Que: So how do we close the cycle?
Ans: Volcanic zones & metamorphic belts:
Magmatic outgassing & decarbonation
CaCO3 + SiO2 = CaSiO3 + CO2 (5)

13. (2) Introduction to the ‘BLAG’ model
Berner, Lasagna & Garrels (Am. J. Sci. 1983)
Plate tectonics formulation-- quantification of fluxes:
Outputs:
• palaeoatms. pCO2 (100 Ma)
• mean surface air temperature
Inputs:
• continental land area (rocks weathered)
-- CO2 draw-down
• sea floor spreading rates (sediments metamorph.)
-- CO2 supply
-ve feedback:
• inc. pCO inc. T inc. weathering
check ‘run away greenhouse’

14. To view these figures please visit the following:
Figure 1 (Carbonate-Silicate Cycle in modern oceans)
Figure 5 (Imputs of CO2 and spreading rates)
Figure 12 (Outputs, time vs. temperature)
Click the following link: Berner, R.A., Lasaga, A.C., Garrels, R.M., (1983) The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. American Journal of Science, v. 283, no.7, p

15. Since publication BLAG has faced two challenges:
• ‘black shale crowd’ - s Org C burial vs. weathering:
(BLAG = assumed balance 100 Ma)
• K OAEs CO2 drawdown
cooling (Arthur et al. ‘88)
burial dead org. matter (not photosynthesis)
CO2 + H2O = CH2O + O2 (7)
Metamorphism, oxidative weathering (not respiration)
= BLAG agreement  Model adjusted - GEOCARB
• ‘uplift crowd’ - weathering ≠ temperature
instead mountain building events
= BLAG non-agreement - huge debate

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