During the Pleistocene, mountain glaciers formed on all the continents and vast glaciers, in places as much as several thousand feet thick, spread across North America and Eurasia. In the eastern U. The Cenozoic [Recent Life] Era is divided into two main sub-divisions: Most of the Cenozoic is the Tertiary, from 65 million years ago to 1.
Stratigraphy Pliocene—Pleistocene boundary Definition of the base of the Pleistocene has had a long and controversial history.
Because the epoch is best recognized for glaciation and climatic changemany have suggested that its lower boundary should be based on climatic criteria—for example, the oldest glacial deposits or the first occurrence of a fossil of a cold-climate life-form in the sediment record.
Other criteria that have been used to define the Pliocene—Pleistocene include the appearance of humans, the appearance of certain vertebrate fossils in Europe, and the appearance or extinction of certain microfossils in deep-sea sediments. These criteria continue to be considered locally, and some workers advocate a climatic boundary at about 2.
Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles related to a marine sequence of strata. Following studies by a series of international working groups, correlation programs, and stratigraphic commissions, agreement was reached in to place the lower boundary of the Pleistocene series at the base of marine claystones that conformably overlie a specific marker bed in the Vrica section in Calabria.
The boundary occurs near the level of several important marine biostratigraphic events and, more significantly, is just above the position of the magnetic reversal that marks the top of the Olduvai Normal Polarity Subzone, thus allowing worldwide correlation.
Since evidence of Cenozoic glaciation was discovered in rocks laid down earlier than those of the Vrica section, some geologists proposed that the base of the Pleistocene be moved to an earlier time.
To many geologists, the most reasonable time coincided with the type section for the Gelasian Stage, the rock layer laid down during the Gelasian Age, found at Monte San Nicola near Gela, Sicily. The base marker for the Gelasian—that is, the global stratotype section and point GSSP —was placed in rock dated to 2, years ago a notable point because it is within 20, years of the Gauss-Matuyama geomagnetic reversal.
In addition, the date of the rock is closely correlated with the timing of a substantial change in the size of granules found in Chinese loess deposits. Changes in loess grain size suggest regional climate changes. The Pleistocene is subdivided into four ages and their corresponding rock units: Of these, only the Gelasian and Calabrian are formal intervals, whereas others await ratification by the ICS.
The Calabrian, which was previously known as the early Pleistocene, extends to the Brunhes—Matuyama paleomagnetic boundary atyears ago. The Ionian, also known as the middle Pleistocene, extends to the end of the next to the last glaciation at aboutyears ago.
The Tarantian, also known as the late Pleistocene, includes the last interglacial—glacial cycle ending at the Holocene boundary about 11, years ago.
Chronology and correlation The chronology of the Pleistocene originally developed through observation and study of the glacial succession, which in both Europe and the United States was found to contain either soils that developed under warm climatic conditions or marine deposits enclosed between glacial deposits.
From these studies, as well as studies of river terraces in the Alpsa chronology was developed that suggested the Pleistocene consisted of four or five major glacial stages which were separated by interglacial stages with climates generally similar to those of today.
Beginning with studies in the s, a much better chronology and record of Pleistocene climatic events have evolved through analyses of deep-sea sediments, particularly from the oxygen isotope record of the shells of microorganisms that lived in the oceans.
Marine oxygen isotope record The isotopic record is based on the ratio of two oxygen isotopesoxygen 16O and oxygen 18Owhich is determined on calcium carbonate from shells of microfossils that accumulated year by year on the seafloor. The ratio depends on two factors, the temperature and the isotopic composition of the seawater from which the organism secreted its shell.
Shells secreted from colder water contain more oxygen relative to oxygen than do shells secreted from warmer water. The isotopic composition of the oceans has proved to be related to the storage of water in large ice sheets on land.
Because molecules of oxygen evaporate less readily and condense more readily, an air mass with oceanic water vapour becomes depleted in the heavier isotope oxygen as the air mass is cooled and loses water by precipitation. When moisture condenses and falls as snow, its isotopic composition is also dependent on the temperature of the air.
Snow falling on a large ice sheet becomes isotopically lighter i. As a result, large ice sheets store water that is relatively light has more oxygenand so during a major glaciation the ocean waters become relatively heavier contain more oxygen than during interglacial times when there is less global ice.
Accordingly, the shells of marine organisms that formed during a glaciation contain more oxygen than those that formed during an interglaciation. Although the exact relationship is not known, about 70 percent of the isotopic change in shell carbonate is the result of changes in the isotopic composition of seawater.
Because the latter is directly related to the volume of ice on land, the marine oxygen isotope record is primarily a record of past glaciations on the continents. Long core samples taken in portions of the ocean where sedimentation rates were high and generally continuous and where water temperature changes were relatively small have revealed a long record of oxygen isotope changes that indicate repeated glaciations and interglaciations going back to the Pliocene.
The record is relatively consistent from one core sample to the next and can be correlated throughout the oceans. Warmer periods interglacials are assigned odd numbers with the current warm interval, the Holocene, being 1, while the colder glacial periods are assigned even numbers.
Subdivisions within isotopic stages are delineated by letters. The ages of the stage boundaries cannot be measured directly, but they can be estimated from available radiometric ages of the cores and from position with respect to both paleomagnetic boundaries and biostratigraphic markers, and also by using sedimentation rates relative to these data.
An isotopic record from the North Atlantic suggests the first major glaciation in that region occurred about 2, years ago. It also suggests that the first glaciation likely to have covered extensive areas of North America and Eurasia occurred aboutyears ago during oxygen isotope stage The largest glaciations appear to have taken place during stages 2, 6, 12, and 16; the interglacials with the least global ice, and thus possibly the warmest, appear to be stages 1, 5, 9, and The Current or Quaternary Ice Age is the last of five known major glacial periods, or ice ages, during Earth's history, the preceding ones being the Karoo Ice Age (– Ma), Andean-Saharan (– Ma), Cryogenian (– Ma) and Huronian (2,–2, Ma).
Introduction. Dinosaurs bring wonder to children and adults alike. That such great beasts once roamed the earth is hard to imagine. Even harder to imagine is that some dinosaurs such as Tyrannosaurus rex were probably giant killing machines (after the Fall, anyway).
Of all the many questions related to dinosaurs, their disappearance from the earth is the most mysterious of all. Within the past 3 million years, glaciers—in the form of icecaps and repeatedly subjected to glaciation many times more extensive than the areal distribution of present-day glaciers.
More recent evidence, from Glaciers: Clues . The Pleistocene Epoch ( million to 11, years ago) is the epoch of hominin (protohuman) expansion. Knowledge of nonhuman primates, except for some selected Old World monkeys, is surprisingly sketchy.
No ape fossils are known until relatively recent . acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large.
Jul 07, · Any Chance of an Ice Age in our Lifetime? An Ice age COULD begin within the next centruy and cold weather could set in rather quickly. However, Large extensive glaciation would not be able to set in quickly as it is a function of time to build up enough ice for the glaciers to expand like they did in the last ice age.
Even in Status: Resolved.