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Lecture 7: Back into the Icehouse: Last 55 Myr (Chapter 6)
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CO2 evolution in the last 50 Myr
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Did climate cool?
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Cooling inferred from terrestrial evidences Antarctic Arctic
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Northern mid-lattiude cooling inferred from leave shape Problem with land records: incomplete, sporadic, regional 10-15 o C
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Cooling trend inferred from benthic foraminifera δ 18 O Abyssal Time Machine (continuous!) Δδ 18 O shell = Δδ 18 O W + Δδ 18 O T Paleothermometer i) Δδ 18 O T =ΔT/4.2 o C Cooler (temperature dependence) increases ii) Δδ 18 O W More continental ice increases Δδ 18 O W (fractionation)
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standard (SMOW) heavylight Heavy (minor) isotope ( 18 O) Light isotope( 16 O) 0 ‰ O 18 + ‰ O 18 - ‰ O 18 水样 Evaporation leads to fractionation Heavy molecular more stable
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Isotope fractionation thermodynamic fractionation proportional to temperature -16 -14 -12 -10 -8 -6 -4 -2 ‰ 18 O Equilibrium Fractionation
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δ 18 O fractionation Antarctica and Greenland all melting reduces Δδ 18 O W from 0 -1
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IAEA/WMO/GNIP Stations (183 stations in 53 countries)
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Long term mean seasonal cycle
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Cooling trend inferred from benthic foraminifera δ 18 O i) Δδ 18 O T =ΔT/4.2 o C Cooler (temperature dependence) increases ii) Δδ 18 O W More continental ice increases Δδ 18 O W (fractionation) Abyssal Time Machine (continuous!) Δδ 18 O shell = Δδ 18 O W + Δδ 18 O T Paleothermometer Δδ 18 O T =1.5, Δδ 18 O W =0 Δδ 18 O c =1.5 Δδ 18 O T =1.75 Δδ 18 O W =1.0 Δδ 18 O c =2.75 13 o C
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An independent paleothermometer: Ma/Ca
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Two independent paleothermometers => ice sheet 15 o C13 o C14 o C
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Cooling trend inferred from benthic foraminifera d 18 O 1) Cooler (temperature dependence) 2) More continental ice (fractionation) Consistent evidence Deep ocean/high latitude cools by 15 o C over 55 myr Abyssal Time Machine (continuous!)
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Why does the climate cools?
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Ocean Gateway Hypotheses Hypothesis:Opening of Drake’s Passage (20myr) cools the Antarctic Hypothesis: Closing of Panama isthmus (10-4 myr) redirects warm/salty water northward, preventing sea ice formation, more evaporation to help glaciation
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Problem: earlier by 2 myr for glacial cycle Modeling: opposite due to heat transport Opening of Panama Isthmus
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Hypothesis:Opening of Drake’s Passage (20myr) cools the Antarctic Problem: timing 10 myr before intense glaciation, 10 myr after first glaciation Modeling: not too much effect, Combined A+O heat transport not much change Drake Passage Lesson: needs to be more quantitative and comprehensive!
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Ocean Gateway Hypotheses Hypothesis:Opening of Drake’s Passage (20myr) cools the Antarctic Problem: timing 10 myr before intense glaciation, 10 myr after first glaciation Modeling: not too much effect Hypothesis: Closing of Panama isthmus (10-4 myr) redirects warm/salty water northward, preventing sea ice formation, more evaporation to help glaciation Problem: earlier by 2 myr for glacial cycle Modeling: opposite due to heat transport Contradictory to each other! Too much handwavering
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Why does the climate cools? The Role of CO 2 Reduced CO2: Slower input (BLAG) Faster removal (uplifting weathering) Increased coastal upwelling buried enough organic carbon
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Testing spreading (BLAG) hypothesis OK No ??
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Testing uplifting weathering hypothesis
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Tibetan Plateau the unusually large uplifting in the last 20 myr No major high topography like this in the last 150 myr
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Tibetan Plateau suspended particles, evidence of unusual physical weathering from Tibet
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Himalayan sediments in the Indian Ocean: Evidence of strong physical weathering 1)Steep terrain along the southern Himalaya 2)Uplift intensified monsoon (why?)
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Negative feedback on uplifting due to chemical weathering Positive feedback on uplifting due to ice rock fragmentation Feedbacks on uplifting weathering
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References for Reading Tibet Uplift: Climate Impact An, Z., Kutzbach, J. Prell, W. & Porter, S., 2001: Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times. Nature, 411, 62-66 Boos and Kuang, 2010: Dominant control of the South Asian monsoon by oragrphic insolation versus plateau heating. Science, 463, 218-222 Asian monsoon Potential Impact on global thermohaline Emile-Geay J., et al., 2003: Warren revisited: Atmospheric freshwater fluxes and “Why is no deep water formed in the North Pacific”, Journal of Geophysical Research, Vol.108(C6), 3178, doi:10.1029/2001JC001058
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Early Pliocene Climate: An Analogue for Future Global Warming Climate? (move to later orbital…) Fedorov A. et al., 2006: The Pliocene Paradox (Mechanisms for a permanent El Nino). Science, 312, 1485-1489
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Strong gradient Weak gradient West East ∆SST Tropical Pacific SST changes (Wara et al. 2005) SST 18 O
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End of Chapter 6
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Volcanic aerosol cooling
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