The Pleistocene Ice Age
The Pleistocene Glaciers
Today, most of the land surface in New York City lies hidden beneath concrete and steel, but in Central Park it’s still possible to see land in a seminatural state. If you stroll through the park, you’ll find that the top surfaces of outcrops are smooth and polished, and in places have been grooved and scratched. Here and there, glacial erratics rest on the bedrock. You are seeing evidence that an ice sheet once scraped along this now-urban ground. Geologists estimate that the ice sheet that overrode the New York City area may have been 250 m thick, enough to bury a 75-story building.
The fact that glaciated landscapes still decorate the surface of the Earth means that the last ice age occurred fairly recently during Earth’s history. Otherwise, the landscape features would have been either eroded away or buried. The ice age responsible for the glaciated landscapes of North America, Europe, and Asia happened mostly during the Pleistocene Epoch, which began about 2.6 Ma, so as we’ve noted earlier, it is commonly known as the Pleistocene Ice Age.
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| Pleistocene ice sheets of the northern hemisphere. |
Based on studying patterns of glacial striations and of the sources of erratics, geologists have developed an approximate idea of where the great Pleistocene ice sheets originated, and the directions in which the ice sheets flowed. In North America, major ice sheets appear to have initiated in at least three locations (figure above). The Labrador ice sheet formed over north-eastern Canada, the Keewatin ice sheet originated in north-western Canada, and the Baffin ice sheet formed over
Baffin Bay. These sheets, together with one or more smaller ones, merged to form the giant Laurentide ice sheet that covered all of Canada east of the Rocky Mountains, and spread southward over the northern portion of the United States. The Cordilleran ice sheet, which originated in the mountains of western Canada, spread westward to the Pacific coast and eastward until it merged with the Laurentide ice sheet. Other ice sheets formed in Greenland, Scandinavia, northern Russia, and Siberia.
In addition to continental ice sheets, sea ice in the northern hemisphere expanded to cover all of the Arctic Ocean and parts of the North Atlantic during the Pleistocene. Sea ice surrounded Iceland and approached Scotland and also fringed most of western Canada and southeastern Alaska.
Life and Climate in the Pleistocene World
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| Climate belts during the Pleistocene. |
During the Pleistocene Ice Age, all climatic belts shifted southward (figure above a, b). Geologists can document this shift by examining fossil pollen, which can survive for thousands of years if preserved in the sediment of bogs.
Fossils also tell us that numerous species of now-extinct large mammals inhabited the Pleistocene world (figure above c). Giant mammoths and mastodons, relatives of the elephant, along with woolly rhinos, musk oxen, reindeer, giant ground sloths, bison, lions, saber-toothed cats, giant cave bears, and hyenas wandered forests and tundra in North America. Early human-like species were already foraging in the woods by the beginning of the Pleistocene Epoch, and by the end modern Homo sapiens lived on every continent except Antarctica, and had discovered fire and invented tools.
Timing of the Pleistocene Ice Age
Louis Agassiz assumed that only one ice age had affected the planet. But close examination of the stratigraphy of glacial deposits on land revealed that paleosols (ancient soil preserved in the stratigraphic record), as well as beds containing fossils of warmerweather animals and plants, lay between distinct layers of glacial sediment. This observation suggested that between episodes of glacial deposition, glaciers receded and temperate climates prevailed. In the second half of the 20th century, when modern methods for dating geological materials became available, the difference in ages between the different layers of glacial sediment could be confirmed. Clearly, glaciers advanced and then retreated more than once during the Pleistocene. Times during which the glaciers grew and covered substantial areas of the continents are called glacial periods, or glaciations, and times between glacial periods are called interglacial periods, or interglacials.
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| Pleistocene glacial deposits in the north-central United States. Curving moraines reflect the shape of glacial lobes. |
Using the on-land sedimentary record, geologists recognized five Pleistocene glaciations in Europe and, traditionally, four in the mid-western United States (Wisconsinan, Illinoian, Kansan, and Nebraskan, named after the southernmost states in which their till was deposited; figure above). Since the mid-1980s, geologists no longer recognize Nebraskan and Kansan; they are lumped together as “pre-Illinoian.”
The chronology of glaciations was turned on its head in the 1960s, when geologists began to study submarine sediment. They found that some layers contained glacially transported grains, while others did not. Similarly, they found that at a given location, some layers contained fossils of cold-water plankton and other fossils of warm-water plankton. Researchers found that in sediment of the last 2.6 million years, there is evidence for 20 to 30 glaciations during the Pleistocene Epoch. The traditionally recognized glaciations of Europe and the United States might represent only the largest of these.
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| The timing of glaciations. Ice ages have occurred at several times in the geologic past. |
Geologists refined their conclusions about the frequency of Pleistocene glaciations by examining the isotopic composition of fossil shells. Shells of many plankton species consist of calcite (CaCO3). The oxygen in the shells includes two isotopes, a heavier one (18O) and a lighter one (16O). The ratio of these isotopes tells us about the water temperature in which the plankton grew; this is because as water gets colder, plankton incorporate a higher proportion of 18O into their shells. The isotope record confirms that 20 to 30 of these events occurred during the last 2.6 million years (figure above a).
Older Ice Ages during Earth History
So far, we’ve focused on the Pleistocene Ice Age because of its importance in developing Earth’s present landscape. Was this the only ice age during Earth history, or do ice ages happen frequently? To answer such questions, geologists study the stratigraphic record and search for ancient glacial deposits that have hardened into rock. These deposits, called tillites, consist of larger clasts distributed throughout a matrix of sandstone and mudstone. In many cases, tillites are deposited on glacially polished surfaces.
By using the stratigraphic principles, geologists have determined that tillites were deposited during the Late Paleozoic; these are the deposits Alfred Wegener studied when he argued in favour of continental drift (figure above b). Tillites were also deposited between 850 and 630 Ma (at the end of the Proterozoic Eon), about 2.4 to 2.1 Ga (near the beginning of the Proterozoic), and perhaps about 2.9 Ga (in the Archean Eon). Strata deposited at other times in Earth history do not contain tillites. Thus, it appears that glacial advances and retreats have not occurred steadily throughout Earth history, but rather are restricted to specific time intervals, or ice ages, of which there were four or five: Pleistocene, Permian, late Proterozoic, early Proterozoic, and perhaps Archean.
Of particular note, some tillites of the late Proterozoic event were deposited at equatorial latitudes, suggesting that, for at least a short time, the continents worldwide were largely glaciated, and the sea may have been covered worldwide by ice. Geologists refer to the ice-encrusted planet as snowball Earth.
Credits: Stephen Marshak (Essentials of Geology)
