Lecture #18 notes: Geology 3950 Spring 2006; CR Stern Timing and causes of ice ages (text pages th ed and th ed)

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1 Lecture #18 notes: Geology 3950 Spring 2006; CR Stern Timing and causes of ice ages (text pages th ed and th ed) The last ice age, which began to end 18,000 years ago, was only the most recent of numerous advances of continental ice sheets. The most recent episode of Quaternary northern hemisphere ice ages began about 4 million years ago. By between 2 and 1 million years ago ice advances were taking place regularly in approximately 100,000 year cycles. The last advance and retreat were just the most recent of more than 10 such cycles. Ice ages also took place very long ago in the Paleozoic (fig 1) and Precambrian. The Precambrian (570 Ma) ice ages were among the most severe and much of the earth s oceans were frozen over producing what is called the snowball earth. A summary of the main features of the timing of ancient and recent ices ages that need to be explained in understanding the causes of ice ages is as follows: LONG-TERM cycles (100 s of millions of years) 1) Ice ages occurred in the late Precambrain (570 Ma) and middle Paleozoic (450 Ma) and late Paleozoic (300 Ma) as well as the Quaternary (5 Ma to recent) SHORT-TERM cycles (the more recent Tertiary and Quaternary glacial cycles) 1) Ice began to build up on Antarctica around 30 Ma and covered Antarctica by around 15 Ma 2) Continental ice sheets formed in the northern hemisphere starting around 4 Ma 3) Ice ages basically northern hemisphere advances of continental ice sheets - began to come and abruptly go in 100,000 year cycles between 1-2 Ma. The last

2 such ice age reached a maximum 18,000 years ago after which the ice rapidly retreated and we entered the current inter-glacial period. 4) During the 100,000 year cycles of an ice age, the continental ice masses do not grow continuously, but advance and retreat in 20,000 year cycles. Long-term cycles: The late Paleozoic ice ages involved cyclic advances and retreats of a continental ice cap that covered the southern continents of South America, Antarctica, Africa, India and Australia (fig 2). However, the geologic evidence for the late Paleozoic ice ages, which occurs on the 5 continents shown in figure 2, were not fully understood until the theory of continental drift and plate tectonics provided other evidence that these continents were all assembled into the single large continent of Gondwanaland during the late Paleozoic (fig 3) The long-term cycles suggest that one very important factor involved in creating ice ages is that large areas of land must be present close to the polar regions. Ice caps do not form in the sea, since ice floats and is broken to pieces by storms and waves. In the late Paleozoic, the Gondwana continent was located close to the South Pole. Since then Antarctic has not moved much, but South America, Africa, India and Australia have all

3 moved north. Also North America, Europe and Asia have also moved further north far enough north to support the current episode of (Quaternary <5 million years) northern hemisphere ice ages. However, even though Antarctic was close to the South Pole, and North America, Europe and Asia close to the north pole as early as the Cretaceous (100 Ma) and early Tertiary, no ice ages occurred on any of these continents during this time. That is because the world was tropical, with average temperatures of approximately 25 C during the Cretaceous, more that 10 C warmer than the average global temperature today (see figure 4 and lecture #19). The world was tropical at this time because CO 2 in the atmosphere was high (as much as three times the CO 2 in todays atmosphere see lecture #19). High CO 2 in the Cretaceous atmosphere was a result of the high sea levels at that time, which were 250 meters above todays sea-level (see figure 4 above) so that much of the continental coastlines were under water. When sea levels are high, less land is exposed and the area of rock weathering is decreased, so more CO 2 remains in the atmosphere and the earth is warmer (see lecture #16). When sea levels are low, more land is exposed and the area of rock-atmosphere interaction is greater so that CO 2 is more rapidly removed from the atmosphere by rock weathering. As sea levels and CO 2 in the atmosphere decreased the earth cooled beginning after the mid Tertiary (30 Ma see figure 4 above). So, a second important factor for producing glaciations is low CO 2 in the atmosphere resulting from low sea level. In the late Paleozoic, when the continents were all together, (fig 3 above) sea level was low, the global climate was cool (like that of today) and this allowed the late Paleozoic ice ages to occur.

4 Recent glacial cycles: As sea levels lowered in the Teritary and global climate cooled (fig 4 above) ice began to first cover Antarctica, initially at 30 Ma and completely by 15 Ma. Then, because of the even lower sea-level and cooler temperatures resulting from the growth of the Antarctic ice sheet, ice sheet began to develop in the northern hemisphere by about 4 Ma. Ice developed in Antartica before the northern hemisphere because Antarctica is located directly at the South Pole, while the northern hemisphere continents are at high latitudes, near, but not directly over the North Pole. The evidence for the age of the first northern hemisphere continental ice sheets comes from ice-rafter boulders, carried by ice bergs, found in deep sea sediments far from any continental coast. The short term 100,000 year cycles of advances and retreats of the northern hemisphere ice sheets was determined by analyzing the oxygen isotopes ( 16 O and 18 O) of marine plankton fossils in oceanic sediments obtained by deep-sea drilling into the floor of the ocean. Seawater contains oxygen of both these isotopes, but glaciers are almost pure 16 O. This is because the heavier 18 O isotope is harder to evaporate from seawater, and enters precipitation near the equator in the tropic, so that water vapor producing the snow that falls at the poles is almost pure 16 O, so that the glaciers formed from this snow are almost pure 16 O (figs 5 and 6).

5 When continental glaciers are at their maximum size, as they were 18,000 years ago, 18 O is more enriched (by 0.2%) in seawater then they are when glaciers are smaller in size such as today (fig 7 below) Marine plankton draw oxygen from sea-water as they grow, and their fossils, which settle to the sea-floor, preserve a record of changing oxygen isotope ratios through time as continental glaciers have grown and retreated during the ice age cycles. These changes indicate that the continental glaciers have cycled between maximum size every 100,000 years and then disappear during periods referred to as inter-glacial events which also occur every 100,000 (fig 8). The figure indicates that the last glacial maximum ended 18,000 years ago and the continental ice sheet disappeared very rapidly as we entered the current interglacial. The previous glacial maximum was 130,000 years ago, which was followed by an interglacial time of low ice volume between 132,000 to 118,000 years ago. The figure also indicates that after the last interglacial, which lasted about 15,000 years, the ice began to again to build up again, but slowly, and it took 100,000 years for it to

6 reach it maximum size 18,000 years ago. Also, as it built up, it advanced and retreated (but not completely) at cycles of approximately 20,000 year intervals. The small 20,000 year retreat of the ice as it is growing in size, and the large retreats every 100,000 year after the ice sheets have reached a maximum size, have different explanations. The 20,000 year cycles are related to changes in the orbital geometry of the earth, in particularly the tilt of the earth axis (fig 9) that make summers in the northern hemisphere either a little warmer or a little cooler. Warmer summers in the northern hemisphere cause snow that falls during winter to melt during the summer, so glaciers do not advance, while cooler summers allow. If the tilt of the earth s orbit is a maximum (24.5 ), northern hemisphere summers will be warmer, and it is a minimum (21.5 ) they will be cooler. The differences, along with differences in the location of the earth during northern hemisphere summers (either closer or further from the sun) amount of up to 20% change in the northern hemisphere sunlight (fig 10). Note that the 20,000 year cycles of warmer and cooler northern hemisphere (65 ) summers (July) correlate with the 20,000 year cycles of advances and retreats, but do not explain the 100,000 year cycles of maximum advances being replaced by interglacial events.

7 The 100,000 year cycles are though to reflect the fact that when the continental ice sheets are large enough, they depress the crust beneath them below sea-level and during the next 20,000 year warm-up they are destroyed by rising sea-level which essentially floats them away. The it takes 10,000 years for the land to rebound above sea-level and for the orbital cycles to again create cooler northern hemisphere summers, and the ice sheets begin to grow again. It takes them another 100,000 years to again reach their maximum size before they collapse under their own weight and disappear again. A summary of the timing of glacial cycles and their causes is LONG-TERM cycles (100 s of millions of years cycles) 1) Need a continental land mass near the poles 2) Need relatively cool global temperatures which are a function of sea level and CO2 in the atmosphere SHORT TERM cycles (20,000 and 100,000 year cycles) 1) 20,000 year cycles reflects small changes in the angle of the earth s axis of rotation, which make northern hemisphere summers either a little cooler (so winter snow does not melt) or a little warmer (so winter snow melts) 2) 100,000 cycles reflect the fact that as the ice sheets get bigger and bigger, they depress the underlying continental crust until there base is below sea level and they float away