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112lect23 - GY 112 Lecture Notes D Haywick(2006 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Proterozoic Glaciations Lecture Goals : A) Glaciations: What happens and why B) Proterozoic glaciations C) Snowball Earth– the most up to date info Textbook reference: Levin 7 th edition (2003) Chapter 7; Levin 8 th edition (2006), Chapter 9 A) Why Glaciations Well, that is a good question. Geologists think that they understand why glaciations occurred in the “recent past” (i.e., the last few million years), but there is a lot of speculation about why it occurred earlier in the Phanerozoic and the Proterozoic. First let’s state some obvious facts. Conditions have to be pretty cold for glaciations to occur. Cold temperatures occur in mountainous areas due to the altitude effect (a drop of 3 degrees C for every 1000 m of elevation), and the result is localized or alpine glaciers . Alpine glaciers occur in valleys between mountains around the world (they are also called valley glaciers ) and even occur in tropical latitudes if the mountain ranges are high enough. This type of glaciation is impressive, but is not all that important to the world as a whole. The really impressive glaciations occur when temperatures across the planet decrease to the point where glaciers begin to grow at low elevations in high latitude areas (e.g., near the poles). We presently live at a time where there are significant continental glaciers on Antarctica and Greenland (see image to left from http://badc.nerc.ac.uk/community/highlighted_sites_old/satim/Greenland-landsat.jpg ), but in the not too-distant pass, the glaciers covered much, much larger regions. We will talk more about these “recent” glaciations in an upcoming lecture, but it is worthwhile at this time to address the major effects. As ice builds up on a continent, the volume of sea water deceases and sea level will drop around the world. This is known as a eustatic sea level fall . At the same time, the stable isotopic composition of the sea water will change (refer to Lecture 9) and the seas will become more saline (slightly). As ice melts, the volume of
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GY 112 Lecture Notes D. Haywick (2006) 2 fresh water flowing into the oceans increases and changes back to “normal conditions” and we experience a eustatic sea level rise . The reasons why continental glaciers advance and retreat have been considered for well over 100 years. Most geologists believe that the Earth undergoes many changes that ultimately affect the climate of our planet. By themselves, no one “effect” is probably sufficient to induce a glaciation, but collectively, they could. The flipping between icehouse conditions (times when there is continental ice at the poles) and greenhouse c onditions (when there is no continental ice at the poles) is still pretty much a mystery. However, we have a pretty good handle on why glacial ice advances and retreats during icehouse conditions. Times of maximum continental glacier advance are considered glacial periods (do not confuse this period with geological time periods!) and times when the ice has retreated are called
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