Pleistocene Ice Age

PLEISTOCENE ICE AGE For more than 200 million years from the end of the Permian to about 40 million years ago, no major ice sheets covered the world. This suggests that the presence of ice sheets at both poles during the Pleistocene was a unique event in Earth's history. Analysis of deep-sea sediments and glacial ice cores provides a historical record of ice ages. They began about 3 million years ago, when the development of glaciers spread across the northern continent. During this time, sea surface water cools dramatically. Diatoms (Fig. 192), a species of algae with shells made of silica, declined sharply in Antarctica.This may have occurred when sea ice reached its maximum northern limit, thus overshadowing the algae below. In the absence of sunlight for photosynthesis, diatoms disappear. Their disappearance marked the initiation of the Pleistocene glacial epoch in the Northern Hemisphere. The last glaciation period began about 115,000 years ago. It increased about 75,000 years ago, probably due to the massive eruption of Mount Toba in Indonesia. It peaked about 18,000 years ago. During the height of the Ice Age, glaciers 2 miles or more thick blanketed Canada, Greenland, and Northern Europe (Fig. 193). In North America, the largest ice sheet, called Laurentide, blankets an area of 5 million square miles. It stretches from Hudson Bay, reaching north to the Arctic Ocean and south to eastern Canada, New England, and the upper midwestern United States. The smaller ice sheet, called the Cordilleran, comes from the Canadian Rockies. It engulfs western Canada and the northern and southern parts of Alaska. It left an icefree corridor in the middle, used by people migrating from Asia to North America. Glaciers also invaded a small part of the northwestern United States. Europe's largest ice sheet, called the Fennoscandians, spread from northern Scandinavia. It covers most of Great Britain as far south as London and most of northern Germany, Poland, and European Russia. A smaller ice sheet, called Alpine, centered on the Swiss Alps, covered parts of Austria, Italy, France, and southern Germany. In Asia, an ice sheet covers the Himalayas and covers parts of Siberia. In the southern hemisphere, only Antarctica has a major ice sheet. It grew about 10 percent larger than its current size and extended as far as the tip of South America. A small ice sheet covers the mountains of Australia, New Zealand, and the Andes in South America. Elsewhere, alpine glaciers are on mountain tops that are currently ice-free. Excess ice with nowhere to go except to the sea gives birth to forming icebergs. During the height of the last ice age, icebergs covered half the ocean area. Ice floating in the ocean reflected sunlight back into space, thus maintaining a cold climate with average global temperatures about 5 degrees Celsius cooler than today. About 10 million cubic miles of water are tied up in continental ice sheets. They covered about a third of the land surface, with the volume of ice three times larger than its current size. The accumulated ice dropped sea level about 400 feet, advancing the coastline tens of miles toward the sea. Sea level decline exposes connected land and continental bridges, spurring migration of species, including humans, to different parts of the world. The weight of large ice sheets causes the continental crust to sink deeper into the upper mantle. Even today, northern lands are rebounding by as much as half an inch per year long after the weight of glaciers has lifted. Lower temperatures reduce the rate of evaporation of seawater and lower the average amount of rainfall, leading to the expansion of deserts in many parts of the world (Table 11). The fierce desert winds produce tremendous dust storms. Thick dust suspended in the atmosphere blocks sunlight, lowering the temperature even further. Most windblown sand deposits, called loess (Fig. 194), in the central United States were laid during the Pleistocene Ice Age. Cold weather and approaching ice force species to migrate to warmer latitudes. In front of the ice sheet, which is advancing perhaps several hundred feet per year, lush deciduous forests give way to evergreen forests that produce grassland. These grasslands became barren tundra and rough periglacial areas on the periphery of the ice sheet.