At that time all the continental land areas were larger. The Bering Shelf was exposed at this time and Siberia was connected to Alaska by a land bridge, thus allowing intercontinental migration of animals, including early humans.
The last Pleistocene ice age reached a maximum 22,, yBP. During this period two large ice sheets have been identified in the Northern Hemisphere: the Laurentide Ice Sheet covering parts of Eastern North America, and the Scandinavian Ice Sheet covering parts of northern Europe. The cordilleran Ice Sheet over western North America achieved a maximum somewhat later than these, about 14, years ago.
The figure above shows the temperature changes over Antarctica blue line over the last , years based on ice core information. Notice how abruptly one ice age ends around , years ago and how abruptly the next cool period or ice age begins around , years ago.
The Antarctic climate remained cool for over , years, reaching its coldest around 14, years ago. Again note how abruptly this most recent ice age ended 14, years ago. Also shown in the figure are the inferred concentrations of two greenhouse gases carbon dioxide and methane. These seem to correlate quite well with the temperature changes, however, a cause and effect mechanism has not been proven.
In other words, it is not known whether increases in greenhouse gases caused the warming or whether they increased as a result of the warming. This is quite important because Al Gore uses this relationship as proof that as we increase greenhouse gases temperatures will follow. The glacials in the following tables show historical usages, are a simplification of a much more complex cycle of variation in climate and terrain, and are generally no longer used. These names have been abandoned in favor of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognized since the historical terminology was established.
Corresponding to the terms glacial and interglacial, the terms pluvial and interpluvial are in use Latin: pluvia, rain. A pluvial is a warmer period of increased rainfall; an interpluvial, of decreased rainfall.
Formerly a pluvial was thought to correspond to a glacial in regions not iced, and in some cases it does. Rainfall is cyclical also. Pluvials and interpluvials are widespread. There is no systematic correspondence of pluvials to glacials, however.
Moreover, regional pluvials do not correspond to each other globally. Any coincidence is an accident of regional factors. Only a few of the names for pluvials in restricted regions have been strategraphically defined. The waveform response comes from the underlying cyclical motions of the planet, which eventually drag all the transients into harmony with them.
The repeated glaciations of the Pleistocene were caused by the same factors. Glaciation in the Pleistocene was a series of glacials and interglacials, stadials and interstadials, mirroring periodic changes in climate.
The main factor at work in climate cycling is now believed to be Milankovitch cycles. Milankovitch cycles cannot be the sole factor responsible for the variations in climate since they explain neither the long term cooling trend over the Plio-Pleistocene, nor the millennial variations in the Greenland Ice Cores.
Milankovitch pacing seems to best explain glaciation events with periodicity of ,, 40,, and 20, years. Such a pattern seems to fit the information on climate change found in oxygen isotope cores. The timing of our present interglacial interval known as the Holocene, Postglacial, or the Present Interglacial to that of the previous interglacial, beginning about , years ago The Eemian Interglacial , suggests that the next glacial would likely begin in about 3, years.
In oxygen isotope ratio analysis, variations in the ratio of O 18 to O 16 two isotopes of oxygen by mass measured by a mass spectrometer present in the calcite of oceanic core samples is used as a diagnostic of ancient ocean temperature change and therefore of climate change.
Cold oceans are richer in O 18 , which is included in the tests of the microorganisms foraminifera contributing the calcite. A more recent version of the sampling process makes use of modern glacial ice cores. Although less rich in O 18 than sea water, the snow that fell on the glacier year by year nevertheless contained O 18 and O 16 in a ratio that depended on the mean annual temperature.
Temperature and climate change are cyclical when plotted on a graph of temperature versus time. This sort of graph is based on another of isotope ratio versus time. Ratios are converted to a percentage difference from the ratio found in standard mean ocean water SMOW.
The graph in either form appears as a waveform with overtones. One half of a period is a Marine isotopic stage MIS. It indicates a glacial below zero or an interglacial above zero. Overtones are stadials or interstadials.
According to this evidence, Earth experienced MIS stages beginning at about 2. Early Pleistocene stages were shallow and frequent. The latest were the most intense and most widely spaced. By convention, stages are numbered from the Holocene, which is MIS1.
Glacials receive an even number; interglacials, odd. The largest glacials were 2, 6, 12, and 16; the warmest interglacials, 1, 5, 9 and For matching of MIS numbers to named stages, see under the articles for those names. Both marine and continental faunas were essentially modern and many animals, specifically, mammals were much larger than their modern relatives. The severe climatic changes during the ice age had major impacts on the fauna and flora.
With each advance of the ice, large areas of the continents became totally depopulated, and plants and animals retreating southward in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food supply.
A major extinction event of large mammals megafauna , which included mammoths, mastodons, saber-toothed cats, glyptodons, ground sloths, Irish elk, cave bears, and short-faced bears, began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period. Although many vertebrates became extinct during this period, mammals that are familiar to us today — including apes, cattle, deer, rabbits, kangaroos, wallabies, bears, and members of the canine and feline families — could be found during this time.
Other than a few birds that were classified as dinosaurs, most notably the Titanis , there were no dinosaurs during the Pleistocene Epoch. They had become extinct at the end of the Cretaceous Period , more than 60 million years before the Pleistocene Epoch began. Birds flourished during this period, including members of the duck, geese, hawk and eagle families.
There were also some flightless birds such as ostriches, rheas and moas. The flightless birds did not fare as well, as they had to compete with mammals and other creatures for limited supplies of food and water, as a good portion of the water was frozen.
As for vegetation, it was fairly limited in many areas. There were some scattered conifers, including pines, cypress and yews, along with some broadleaf trees such as beeches and oaks. On the ground, there were prairie grasses as well as members of the lilly, orchid and rose families. About 13, years ago, more than three-fourths of the large Ice Age animals, including woolly mammoths , mastodons, saber-toothed tigers and giant bears, died out. Scientists have debated for years over the cause of the extinction, with both of the major hypotheses — human overhunting and climate change — insufficient to account for the mega die-off.
0コメント