GEO101 Spring 2023 Contents Growth of glaciers in the Cenozoic Era Glacial cycles The astronomical theory for glacial cycles Cenozoic Glaciers Cenozoic era started cooling 50 million years ago ID: 1022561
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1. Pleistocene glaciations and glacial cyclesGEO101Spring 2023
2. ContentsGrowth of glaciers in the Cenozoic EraGlacial cyclesThe astronomical theory for glacial cycles
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5. Cenozoic GlaciersCenozoic era started cooling 50 million years ago.18O isotopes in marine shellsCooling due to the formation of Himalayan MountainsGlaciersGreenland: 2.6 million years.Antarctica: 34 million years. [Note: intermittent glaciers between 26 and 14 million years ago due to volcanic activity of the formation of Sierra Nevada and Andes mountains]
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7. Cenozoic GlaciersAntarctica:40 million years ago: close to S. America and Australia34 million years ago: shifted to S. Pole, creating ocean passageways and cold currentsThe cooling decreased snowmelt and formed ice sheets.
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9. Cenozoic glaciersGreenland:40 million years ago: slightly further south, ocean passages with N. America narrowernow, warm Equatorial current provides moisture.By 2 million years ago, weak NE current because of small ocean passageways around Greenland and open channel connecting the Atlantic and Pacific Oceans.Greenland not glaciated until wide ocean passageway and the creation of the Isthmus of Panama, to receive enough snowfall
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11. Glacial CyclesIce-sheets regularly expand or contract on Antarctica for 34 million years, and in the N. hemisphere (including Greenland) for 2.6 million years.Glacier size change = snowfall − snowmeltif snowfall > snowmelt, expansionAntarctica permanently glaciated as decreased snowmeltGreenland because of increased snowfallGlacial deposits on land (tills) provide evidence.
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14. Red areas indicate the terminal moraines, where glacial tills deposited.14,000-18,000 YBP10,000-14,000 YBP6,000-10,000 YBP
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16. The process of pluckingThe formation of the terminal moraine
17. Glacial cyclesComplete record of glacial cycles found in oxygen isotopes in marine fossils glacial events: more 18O in ocean~20 glacial cycles during the last 2.6-million-year Pleistocene glaciationsglacial event (glaciers advanced) + interglacial event (glaciers retreated)Currently, interglacial event
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19. Astronomical Theory for Pleistocene Glacial CyclesInsolation (Incoming solar radiation) drives cyclesParticularly, changes in N. hemisphere summer insolationWhy North? More land massWhy summer? Glacier melting in summerChanges in insolation due to Earth’saxial tiltorbital shapewobble of rotation axis [precession]
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23. JuneSeptemberDecemberMarch
24. MarchJuneSeptemberDecember
25. Vertical rays of sun All locations on Earthstriking equator experience 12 hours of daylightEarth at September Equinox
26. 24 hours of daylight 24 hours of darkness north of Arctic Circle south of Antarctic CircleVertical rays of Sun striking Tropic of Cancer (23.5 N) Equator has 12 hours of daylightEarth at June Solstice
27. The Astronomical TheoryAxial tilt:Currently, 23.5°, causes seasonal variation.Without tilt, 12 hours of sunlight the entire year.Ranges between 22.5° and 24.5° over 41,000 yr. cycleLow tilt (22.5°)shorter northern hemisphere summer/winterless summer meltHigh tilt (24.5°) means more summer melt.
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29. The Astronomical TheoryEccentricity of orbit:Eccentricity: oval shapedOrbit ranges between very eccentric and perfectly circular (100,000 years).Currently, nearly circular.“aphelion”: furthest from sunCurrently, during our summerThus, it helps glaciers survive the summer.
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32. NH winter
33. The Astronomical TheoryPrecession:Axis of rotation wobbles.Therefore, axis points toward different places.Wobble determines when aphelion occurs.Glaciers favored when aphelion in Northern hemisphere summer because summer is cooler.
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35. NH summerNH winter
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37. The Astronomical TheoryMost severe glacial events:Earth’s tilt is at 22.5°, orbit is very eccentric, and aphelion during northern hemisphere summer.Currently, aphelion during NH summer but tilt is large enough and orbit is near circular, so we are interglacial.Marine isotopes match NH summer insolation.NH summer insolation is low: glacial eventNH summer insolation is high: interglacialNext topic: Role of Oceans in Glacial Cycles
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