Pleistocene Glaciation (Ch.14) Geologic evidence Milankovitch cycles Glacial climate feedbacks

Transcription

1 Pleistocene Glaciation (Ch.14) Geologic evidence Milankovitch cycles Glacial climate feedbacks

2 End of last ice-age rise of human civilization Modern ice-ages begin Asteroid impact end of dinosaurs Cambrian explosion of life beginning of fossil record Earth freezes over? life survives in small pockets? Rise of atmospheric oxygen Earliest evidence of life prokaryotic bacteria Formation of earth

3 Pleistocene Glaciations Earth climate in last ~2.5 m.y. was characterized by regular cycles of growth and decay of Northern Hemisphere continental ice sheets --- Glacial & interglacial cycles Although polar ice has existed on Antarctica for tens of millions of years, only during the past 2.5 m.y. have ice sheets extended from the Arctic into the northern mid-latitudes regularly.

4 Erratics Pipestone National Monument, Minnesota

5 Chapter 12 Chapter 14 Geologic time scale (last 4b.y.) Few data Low temporal resolution last 3 m.y. rich data high temporal resolution S, ΔT g, A subtle forcing becomes important Big pictures detailed structures

6 What happens during a glacial period? -- Temperature drops -- Ice cover on land (and maybe ocean, but we don t know) extends equatorward -- Sea level drops (increased water mass on land); 130 m (400 ft) for last ice age -- Amount of land mass increases -- Land compresses (due to huge weight of ice)

7 How do we know about ice ages? Glacial deposits document major glaciations Ocean sediment cores (A continuous record of climate change was being deposited on the sea floor. It was contained in the isotopic composition of the skeletons of marine organisms) 18 O fraction accurate measure of ice amount Other climate markers (temperature) Very precise dating Ice cores 18 O fraction record of local temperature CO2 and other gases in ice bubbles

8 O18 / O16 Climate Signal O16 is normal oxygen; O18 has two extra neutrons => heavier Water (H2O) can have either O18 or O16 Takes energy to evaporate water, more energy to evaporate O18 than O16 Takes less energy to condense O18 than O16 Atmosphere Less O18 Atmosphere Less O18 More O18 Ocean More O18 Ocean

9 O18 / O16 Climate Signal Most evaporation and precipitation occurs in tropics => tends to concentrate O18 in warmer tropical and subtropical ocean Air moving poleward is constantly losing O18 (by precipitation) so polar air is more depleted in O18 Evaporation from cold water has less O18 than that from warm water Tropical Atmosphere Less O18 (than ocean) Polar Atmosphere Even Less O18

10 Schematic of O18 Loss

11 Current data on O18 ratios in precipitation Less O18

12 Okay, so what? (1) Glacial ice cores O18 ratio in ice records LOCAL conditions Less O18 in ice => less O18 in polar air => colder temperatures globally Why? Colder waters in tropics => reduced amount of O18 in evaporated water vapor Precipitation over long transit time to pole further reduces O18 ratio

13 Okay, so what? (2) Ocean sediment cores O18 ratio in ocean sediment records ice amount Why? Ice on poles is reduced in amount of O18 => ocean water is enriched (more O18 relative to O16) As ice at poles accumulates, ratio of O18 increases in ocean Sediments (shells, corals, etc) become enriched in O18 The higher the ratio of O18 to O16, the more ice on land

14 The message O18 / O16 ratio is an indicator of global temperature In polar ice cores, lower ratio means colder global temperature In ocean sediments (and shells) lower ratio means less global ice coverage This means warmer ocean / warmer global temperature (actually calcium carbonate skeletons precipitated from a glacial-age ocean has a larger ratio, not only because the sea-water is enriched in the heavier isotope but also because the water is cold and plankton incorporates more O18)

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16 3.5 million years record of δ 18 O signal In-class activity: 1. Which way does time go in this graph? 2. Is this an ocean sediment record or a polar ice-core record? 3. There are two major transitions that can you see in the record. Please identify them by approximate year of occurrence. 4. Was sea-level higher or lower than now during the three warmest interglacials?

17 3.5 million years record of δ 18 O signal Oscillations get longer Ice ages begin TIME 1. Which way does time go in this graph? 2. Is this an ocean sediment record or a polar ice-core record? 3. There are two major transitions that can you see in the record. Please identify them by approximate year of occurrence. 4. Was sea-level higher or lower than now during the three warmest inter-glacials? Less ice mass => higher sea level (warmer than today?

18 The major intervals of Northern Hemisphere glaciation, glacial, occurred every 100,000 years during the past 700,000 years. Before that time, they occurred every 40,000 years. During the glaciation, significant amount of water are removed from the ocean (during the last glaciation, sea level drops 130 m). During glacials, T s ~9-10 o C; CO 2 200ppm During interglacials, continental glacials mainly occurs at Greenland and Antarctic; T s ~15 o C; CO ppm. The Holocene Epoch (the past 10,000 years) represents one such interglacial.