# Lecture 7: Back into the Icehouse: Last 55 Myr (Chapter 6)

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Lecture 7: Back into the Icehouse: Last 55 Myr (Chapter 6)

CO2 evolution in the last 50 Myr

Did climate cool?

Cooling inferred from terrestrial evidences Antarctic Arctic

Northern mid-lattiude cooling inferred from leave shape Problem with land records: incomplete, sporadic, regional 10-15 o C

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)

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

Isotope fractionation thermodynamic fractionation proportional to temperature -16 -14 -12 -10 -8 -6 -4 -2 ‰  18 O Equilibrium Fractionation

δ 18 O fractionation Antarctica and Greenland all melting reduces Δδ 18 O W from 0  -1

IAEA/WMO/GNIP Stations (183 stations in 53 countries)

Long term mean seasonal cycle

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

An independent paleothermometer: Ma/Ca

Two independent paleothermometers => ice sheet 15 o C13 o C14 o C

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!)

Why does the climate cools?

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

Problem: earlier by 2 myr for glacial cycle Modeling: opposite due to heat transport Opening of Panama Isthmus

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!

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

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

Testing spreading (BLAG) hypothesis OK No ??

Testing uplifting weathering hypothesis

Tibetan Plateau the unusually large uplifting in the last 20 myr No major high topography like this in the last 150 myr

Tibetan Plateau suspended particles, evidence of unusual physical weathering from Tibet

Himalayan sediments in the Indian Ocean: Evidence of strong physical weathering 1)Steep terrain along the southern Himalaya 2)Uplift intensified monsoon (why?)

Negative feedback on uplifting due to chemical weathering Positive feedback on uplifting due to ice rock fragmentation Feedbacks on uplifting weathering

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

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

Strong gradient Weak gradient West East ∆SST Tropical Pacific SST changes (Wara et al. 2005) SST  18 O

End of Chapter 6

Volcanic aerosol cooling

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