Lecture_20_Ice_Ages

Lecture_20_Ice_Ages - Periods with significant glacial...

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1 1 Long-term cooling driven by CO 2 Periods with significant glacial activity 2 Evidence for Glaciation • Grooved and polished surfaces • Unsorted (boulders, rocks, sand, clay) sediments (“till”) • Dropstones (ice rafted) in marine sediments
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2 3 Evidence of Recent Glacial Advance 20,000 kyr BP 4 Today Last glacial maximum
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3 5 A continuous record of glaciation from oxygen isotopes • H 2 16 O evaporates more readily from the ocean than H 2 18 O • Snow (which forms when vapor condenses) will have a lower 18 O/ 16 O than the ocean • Glaciers will have a lower 18 O/ 16 O than the ocean • The more water locked up in glaciers, the higher the 18 O/ 16 O of the seawater left behind 6 Sea level 120 m lower Today Last Ice Age (20,000 years ago) Ice sheet - low 18 O/ 16 O 18 O/ 16 O .1 % higher
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7 During the Cretaceous there were no continental ice sheets. The ratio of 18 O/ 16 O in sea water was 1. Higher than today 2. Lower than today 8 Oxygen isotopes in benthic foraminifera from sediment cores • Last week: – cold T higher 18 O/ 16 O in shell – warm T lower 18 O/ 16 O in shell given the same seawater • This week: – higher 18 O/ 16 O in seawater, higher
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Lecture_20_Ice_Ages - Periods with significant glacial...

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