The Ice Ages-Cenozoic Climatic History
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Outline 23: The Ice Ages-Cenozoic Climatic History Continental Glacier in Antarctica Valley Glaciers in Alaska, note the moraines Valley Glaciers in Alaska, note the moraines Mendenhall Glacier, Juneau, Alaska Prince William Sound, Alaska Glacier Bay National Park, Alaska Ice Ages in Earth History • Late Cenozoic: last 5 MY • Pennsylvanian and Permian: 300 MY ago • Ordovician: 450 MYago • Late Proterozoic: 700 MY ago • Early Proterozoic: 2200 MY ago Phanerozoic Climate Change Late Proterozoic Glacial Deposits and Striated Bedrock, North Africa Glacial Striations Pleistocene Glacial striations on bedrock, Alaska Definitions • Ice Age or Ages: whenever there are glaciers at one or both poles during earth history. • Glacial stage: when glaciers are very extensive during an ice age. • Interglacial stage: when glaciers are less extensive during an ice age. Cenozoic Climatic History • Paleocene and Eocene: warm, tropical climate, no ice at the poles. • Oligocene: ice starts to form on Antarctica as the Circum-Antarctic Current forms. This leads to Albedo changes causing more cooling. Circum-Antarctic Current • Flows clockwise around Antarctica; driven by westerly winds. • Formed as South America, Africa, and Australia moved away from Antarctica. • It is the fundamental cause of the Cenozoic ice ages. Eocene Oligocene Earth’s Albedo • Albedo refers to total reflectance of sunlight from the Earth. • Ice: >90% reflectance • Land: 30-70% reflectance • Water: 20% Cenozoic Climatic History • Miocene: glaciers on Antarctica reach maximum size 5 MY ago. Sea level falls 150 ft. causing Mediterranean Sea to dry up. Deep oceans cool to near freezing causing cooling of Arctic waters. Earth’s albedo continues to change. Cenozoic Climatic History • Pliocene: first glaciers in Arctic; earth’s albedo continues to change. • Pleistocene: maximum glaciation in Northern Hemisphere. About 20 cycles of glacial-interglacial stages. We are currently in an interglacial stage. Ice Age Facts • Ice Cover of Land –10% today –30% during last glacial stage • Ice Thickness –up to 9,000 ft. on Greenland –up to 11,000 ft. on Antarctica Coastline changes between glacial and interglacial stages Eastern Beringia during the last glacial maximum with eustatic sea level lowering of 120 m. The region was largely unglaciated with the exception of local uplands that supported alpine glaciers. (GSA Today, August 2009) Animals, including humans, walked from Asia to North America during the Pleistocene glacial stages. Two Routes: Sea starting 16,000 years ago. Land starting 15,000 years ago. Scientific American, Nov. 2011 Modern Environments Glacial Environments Ice Age Facts • Sea Level Changes: –If all modern ice melted, sea level would rise 200 ft. to pre-Ice Age levels. –During maximum glaciation sea level was 400 ft. lower. –Range of possible sea level changes = 600 ft. Ice Age Facts • Isostatic Depression and Rebound –Currently, the center of Greenland is 1,000 ft. below sea level. –Hudson Bay and Baltic Sea areas have rebounded 800 ft. since last ice age ended 10,000 years ago. Glacial rebound in Canada. Meters of uplift since the glaciers melted 6000 years ago. Glacial rebound in Scandinavia. Mm per year during the 20th Century. Lakes that formed in the western U.S. during the last glacial stage when the climate was wetter. Bonneville Salt Flats: Dried up Pleistocene lake bed Ice Age Facts • The Great Lakes were carved by glaciers. • The ice sheets were larger in North America than in Eurasia. Size of sheets depends on amount of snowfall and rate of melting. Terminal morraines in North America WVU Ice-carved lakes: Finger Lakes, NY 14,000 yrs 12,000 yrs 10,000 yrs Exit Glacier, Seward, Alaska is receding End moraine of Exit Glacier, Seward, Alaska Maximum North American ice coverage and dates of melting. Causes of Glacial Cycles: The Milankovitch Theory • Changes in the Earth’s orbital parameters causes climate extremes of northern hemisphere summers. • Cool northern hemisphere summers lead to glaciation as less winter ice melts. • Cold winters by themselves don’t cause glaciation. Causes of Glacial Cycles: The Milankovitch Theory • Orbital Parameters: –eccentricity: the shape of the orbit –tilt of Earth’s axis: ranges from 22.1-24.5 degrees, currently 23.5; also called obliquity –perihelion: time of the year when the Earth is closest to the sun. Changes due to precession of the earth’s axis. Variation in the Earth’s orbital parameters is caused by the gravitational pull of Jupiter. Cooler Warmer (Exaggerated) More tilt, more seasonal extremes. If the Earth is closer to the sun at the Equinoxes, it will be cooler at the Solstices because of the elliptical orbit. Deep Sea Record of Temperatures • The ratio of O-18 to O-16 in deep water sea shells indicates cycles of temperature change. • These cycles match the predicted Milankovich cycles. The three cycles overlap to create a complicated pattern. Oxygen isotopic 1 composition of 2 foraminifera shells 3 during the Ice Age 4 5 6 1 mm 7 8 Glacial stages Cold Warm Foram shells recovered from oceanic sediments Antarctic Ice Core Glacial stages 4 3 2 1 The greenhouse gas, CO2, is increasing rapidly. Data from ice cores. 4 3 2 1 The greenhouse gas, CO2, is increasing rapidly. Data from ice cores during the last 1000 years. Glaciers are retreating worldwide due to global warming. 1978 margin (2001 photo) .