Palaeoclimate Change SOES 3015 Lecture 7 Mechanisms of

Palaeoclimate Change SOES 3015 Lecture 7: Mechanisms of change-3: Phanerozoic CO 2 & global geochemical carbon cycle (PAW) Lecture outline: • Patterns of long-term climate change - Precambrian, Palaeozoic & Lower Mesozoic Upper Mesozoic & Cenozoic - Doomsday Life on Earth How does the geochemical carbon cycle work? Introduction to the BLAG model • Geochemical carbon cycle www. oceanography. ac. uk

(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

Phanerozoic: Two Phanerozoic ‘supercycles’ (~ 700 Ma; Fischer ‘ 81) • first order eustatic SL sequence stratigraphy (Exxon) = ‘greenhouse’ - ‘icehouse’ -- global volcanism (atms. PCO 2) 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.

(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 18 O:

• 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. 877 -880.

& 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. 1164 -1191,

• 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. POC 2: Min. 4 x present (Barron & Washington ‘ 85) ie. ‘greenhouse’, seems obvious to us in C 21 st But: where did all this CO 2 come from?

Courtesy of GSA: Larson, R. L. , (1991) Geological consequences of superplumes, Geology, v. 19, p. 963 -966.

(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 CO 2 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. PCO 2 will be caused by (small) perturbations to the geochemical carbon cycle

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

(ii) How does the geochemical cycle work? Let us consider chemical weathering on the continents Continents: • kerogens (shales) • carbonates (limestones & dolostones) • silicates (granites & basalts) ‘organic C’ ‘inorganic C’ no C CO 2 + H 2 O = H 2 CO 3 + Ca. CO 3 = Ca 2+ + 2 HCO 3(1) 2 CO 2 + H 2 O = H 2 CO 3 + Ca. Si. O 3 = Ca 2+ + 2 HCO 3 - + Si. O 2 (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) 2 HCO 3 - + Ca 2+ = Ca. CO 3 + CO 2 + H 2 O (3) Bicarbonate-C’s produced during limestone weathering: • one buried as new carbonate • other returned to the atmosphere No net consumption of atmospheric CO 2 (compare 1 & 3)

Same is not true of silicate weathering: 3) • returning only one bicarbonate-C to atmosphere Net (50%) consumption of atmospheric CO 2 (compare 2 & CO 2 + Ca. Si. O 3 = Ca. CO 3 + Si. O 2 (4) But this cannot be the whole story ‘cos unchecked, would consume atms. CO 2 (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 Ca. CO 3 + Si. O 2 = Ca. Si. O 3 + CO 2 (5)

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

To view these figures please visit the following: Figure 1 (Carbonate-Silicate Cycle in modern oceans) Figure 5 (Imputs of CO 2 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. 641 -683.

Since publication BLAG has faced two challenges: • ‘black shale crowd’ - s Org C burial vs. weathering: (BLAG = assumed balance 100 Ma) • K OAEs (Arthur et al. ‘ 88) CO 2 drawdown cooling burial dead org. matter (not photosynthesis) CO 2 + H 2 O = CH 2 O + O 2 (7) Metamorphism, oxidative weathering (not respiration) GEOCARB = BLAG agreement • ‘uplift crowd’ - weathering ≠ temperature instead mountain building events Model adjusted -

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