Palaeoclimate Change SOES 3015 Lecture 8 Tools

Palaeoclimate Change SOES 3015 Lecture 8: Tools & Insights-5: Sr isotopes, global weathering & subduction vs. mountain building (PAW) Lecture outline: • CO 2: Subduction vs. Mountain-building - BLAG & ocean crust cycling - Himalayan uplift • Sr isotopes - Isotope systematics - Consequences & uses • Global weathering debate - Critique of uplift hypothesis www. oceanography. ac. uk

(1) CO 2: Subduction vs. Mountain-building • BLAG & ocean crust cycling • Mountain building as alternative CO 2 drawdown (i) BLAG & ocean crust cycling Key lesson from last Lec: At long (multi-million yr) timescales atms p. CO 2 is controlled by the balance between: • CO 2 production @spreading centres (outgassing) & subduction boundaries (metamorphic decarbonation) • CO 2 consumption by continental weathering of silicate rocks Paul Wilson, University of Southampton

Question: Does the system ever get out of balance? Answer: Yes - to an extent- look @Cretaceous spreading rates; high eustatic sea level-maximum; no ice Courtesy of Geological Society of America: Larson, R. L. , (1991), Geological consequences of superplumes. Geology October, v. 19, p. 963 -966.

But: ~ 3. 5 Billion year record of Life on Earth something must act as a -ve feedback to keep climate within reasonable limits (compare Mars & Venus) Recall, BLAG’s -ve feedback: • inc. p. CO 2 inc. T inc. weathering check ‘run away greenhouse’ University of Southampton

For figure please follow link to full article below: Problems: • lack of evidence for -ve feedback (cold regions intense weathering) From: Huh, Y. , Edmond, J. M. , (1999), The fluvial geochemistry of the rivers of Eastern Siberia: III. Tributaries of the Lena and Anabar draining the basement terrain of the Siberian Craton and the Trans-Baikal Highlands, Geochimica et Cosmochimica Acta, v. 63, p. 967 -987. (Figure 10 a)

OBSERVED COOLING LAGS BLAG PREDICTION • p. CO 2 & Temp • CCD (predicted vs. observed) GREEN HOUSE • mismatch in Cenozoic shifts ICE HOUSE Problems: From: Zachos, J. C. , Pagani, M. , Sloan, L. , Thomas, E. , Billups, K. , (2001) ‘Trends, rhythms, and aberrations in global climate 65 Ma to present’, Science, v. 292, p. 686– 693. Reprinted with permission from AAAS. This figure may be used for non-commercial, classroom purposes only. Any other uses requires the prior written permission from AAAS. Largest in p. CO 2, global temp For Composite of CCD curves for Indian, Pacific, & CCD: and Atlantic Oceans, Please see figure 4 in • BLAG- 100 to 50 Ma (pre-Eoc) • Palaeoceanog evidence- 50 to 0 Ma (post-Eoc) Andel, V. , (1975) Mesozoic and Cenozoic calcite compensation depth and the global distribution of calcarious sediments. Earth and Planetary Science letters, v. 26 p 187 -194.

(ii) Mountain building as alternative CO 2 draw-down Mountain uplift: Physical effects: • Phanerozoic-- glacials & orogenies associated • Cenozoic-- Himalayan Tibetan Plateau Uplift Courtesy of Geological Society of America: Raymo, M. E. , (1991), Geochemical evidence supporting T. C. Chamberlin's theory of glaciation, Geology, v. 19, p. 344 -347.

Mountain uplift: Physical effects: • Topographic AGCM experiment: • middle lat. Jet stream blocked • onset Asian monsoon circulation Courtesy of Chicago Publishers: J. E. Kutzbach, J. E. , Prell, W. L. , Ruddiman, W. F. , (1993), Sensitivity of Eurasian Climate to Surface Uplift of the Tibetan Plateau, The Journal of Geology, v. 101, p. 177 -190.

• Results: Qualitative fit w/ palaeotemp. record but too warm (esp high lat summer) HTP signif. but not whole story • Mountain uplift: Chemical effects: (MIT - Raymo & Edmond) • Uplift intense rains; fresh rocks; steep slopes inc. weathering rates CO 2 draw-down & global cooling • Evidence: • HTP ~25% global Total Dissolved Load • ‘transport-limited’ • Debate: • Yale school (BLAG): slow-down ocean crust cycling predicts: Temp Weathering CCD • MIT school: speed-up continental crust weathering predicts: Temp Weathering CCD Question: How can we test? Answer: We need a proxy for global weathering rates– …sea water 87 Sr/86 Sr?

(2) Sr isotopes • Isotope systematics • Consequences & uses (i) Isotope systematics 88 Sr 38 82. 53 87 Sr 86 Sr 38 7. 04 87 Rb 84 Sr 38 9. 87 37 87 Sr 38 + 0. 56 - + v + Q T 1/2 Rb ~ 48. 8 Ga Sr ~ 5 Ma • Ocean mixing: ~ 1. 5 ka • No fractionation pptn. CO 3’s Sr (Å = 1. 13) Ca (Å = 0. 99) Ca 0. 99 Sr 0. 1 CO 3 Rb (Å = 1. 48) Ca (Å = 0. 99) marine CO 3’s Rb incompatible 38 (all stable) %

87 Sr/86 Sr high low old (T 1/2 Rb) acidic (Rb K) continental marine CO 3’s young ultramafic oceanic (ii) Consequences & uses • Consequences • 87 Sr/86 Sr uniform oceans world wide (S ≥ 18‰) • marine CO 3’s equilibrium • 87 Sr/86 Sr does not degrade • sea water 87 Sr/86 Sr controlled by mixing 3 sources: • young volcanics (JH) • old sialics (JR) • marine carbonates (JB) (DSr; diagenetic suscept. )

• Uses Integrated proxy weathering (oceanic vs. continental crust) Sr isotope tests of uplift-driven CO 2 draw-down • Phanerozoic correlation w/ orogenies & icehouses but: • JR vs. JH? • ’s in JR 87 Sr/86 Sr ≠ weathering rate … Gangese-Brahamaputra. • JRCO 3 ≠ p. CO 2 drawdown.

Courtesy of Geological Society of America: Raymo, M. E. , (1991), Geochemical evidence supporting T. C. Chamberlin's theory of glaciation, Geology, v. 19, p. 344 -347.

Figure is available by accessing the full article here: Richter, F. M. , Rowley, D. B. , De. Paolo D. J. , (1992), Sr isotope evolution of seawater: the role of tectonics, Earth and Planetary Science Letters, v. 109, p. 11 -23. (Figure 3 a)

From: Edmond, J. M. , (1992), Himalayan Tectonics, Weathering Processes, and the Strontium Isotope Record in Marine Limestones, Science, v. 258, p. 1594 -1597. Reprinted with permission from AAAS. This figure may be used for non-commercial, classroom purposes only. Any other uses requires the prior written permission from AAAS.

Figure is available by accessing the full article here: Richter, F. M. , Rowley, D. B. , De. Paolo D. J. , (1992), Sr isotope evolution of seawater: the role of tectonics, Earth and Planetary Science Letters, v. 109, p. 11 -23. (Figure 3 b)

(3) Global weathering debate • Critique of the uplift hypothesis (i) Critique of the uplift hypothesis Chicken or Egg? • Evidence for uplift: • Geomorphology (high, incised terraine) • Sedimentation (Bay of Bengal) • Palaeobotany-- taxonomy vs. physiognomy is actually evidence of climate & climate D erosion uplift (isotasy) Need -ve feedback: • BLAG: dec. PCO 2 dec. T dec. weathering (check ‘run away icehouse’) • MIT: alternatives- imbalance in C cycle • SL erode shelves • nutrient supply • Himalayan uplift Tethyan Corg • Metamorphic decarbonation in orogenic belts Question: are these alternative -ve feedback mechs convincing? Answer: BLAG: No - re-state CO 2 -dependence silicate weathering -ve feedback must work globally & long time-scales

we need: • other weathering proxies (Ge/Si; Nd; Os) • better constraints on ’s in JR vs. JR 87 Sr/86 Sr Cenozoic sea water Sr/Ca • ~1. 5 x modern in late K • min = ~40 Ma • steady inc to Present Lear, C. , Pers. Comm. (2010), Data publicised in: Lear, C. H. , Elderfield, H. , Wilson, P. A. , (2003), A Cenozoic seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes, Earth and Planetary Science Letters, v. 208, p. 69 -84.

• combine w/ sea water 87 Sr/86 Sr • calculate ’s in JR & JR 87 Sr/86 Sr inc by ~ 0. 001 • 35 to 0 Ma assoc w/ 2 fold inc JR • 75 to 35 Ma assoc w/ 2 -3 fold dec JR Lear, C. , Pers. Comm. (2010), Data publicised in: Lear, C. H. , Elderfield, H. , Wilson, P. A. , (2003), A Cenozoic seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes, Earth and Planetary Science Letters, v. 208, p. 69 -84.

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