The Quaternary sub-era (or period) is the historically designated geologic time period extending from the end of the Pliocene epoch roughly 1.8–1.6 million years ago to the present. The Quaternary is one subdivision of the Cenozoic era. It also includes two geologic subdivisions: The Pleistocene (1.6 million years ago to 10,000 years ago) and the Holocene epochs (10,000 years ago to present). The Quaternary is marked by cycles of glaciations.
In a recent revision of the international classification of geological time periods, the Quaternary was subsumed into the Neogene. This move by the International Commission on Stratigraphy (ICS) has met with some resistance from geologists, since, among other reasons, there is a substantial body of published work under the heading of "Quaternary," and there is a tradition of maintaining the Quaternary, a term that has existed since 1759, as our present geological time period (Hinton 2006).
At one time, there were "Primary," "Secondary," "Tertiary," and "Quaternary" periods. The Primary and Secondary periods have long since been done away with, and there are efforts to discontinue the use of "Tertiary" or to consider it a "sub-era" (Hinton 2006). Likewise, there are efforts to consider the "Quaternary" a sub-era. In this classification, the Quaternary sub-era encompasses the Pleistocene and the Holocene epochs, and the Tertiary sub-era encompasses the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. At the same time, the Cenozoic is divided into two "periods," the Neogene, which encompasses the Miocene, Pliocene, Pleistocene, and Holocene, and the Paleogene, which encompasses the Paleocene, Eocene, and Oligocene. Clague (2006) argues on behalf of the International Union for Quaternary Research that there is no scientific justification or historical precedence for the changes the ICS proposes and that the placement of the Quaternary as a "sub-era" will lead to its demise as a term (Clague et al. 2006). Some have proposed that the Cenozoic be divided into three periods: The Paleogene, the Neogene, and the Quaternary.
The openness of the scientific community to consider the modification of concepts, terminology, or theories as new information is examined as one of the core values of science.
|Cenozoic era (65-0 mya)|
|Tertiary sub-era||Quaternary sub-era|
In 1821, a Swiss engineer, Ignaz Venetz, presented an article in which he suggested the presence of traces of a glacier's passage at a considerable distance from the Alps. This idea was initially disputed by another Swiss scientist, Louis Agassiz, but when he undertook to disprove it, he ended up affirming his colleague's theory. A year later Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects. This idea gained him international fame.
The term Quaternary ("fourth") was proposed by Jules Desnoyers in 1829 to address sediments in France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks. The Quaternary, which follows the Tertiary and extends to the present, roughly covers the time span of recent glaciations, including the last glacial retreat. An occasional alternative usage places the start of the Quaternary at the onset of North Pole glaciation approximately 3 million years ago and includes portions of the upper Pliocene.
The Pleistocene has been dated in 2005 by the International Commission on Stratigraphy (a body of the International Union of Geological Sciences) from 1.81 million to 11,550 years Before Present (BP), with the end date expressed in radiocarbon years and marking the beginning of the Holocene. The name Pleistocene was intended to cover the recent period of repeated glaciations; however, the start was set too late and some early cooling and glaciation are now reckoned to be in the end of the Pliocene. Some climatologists would therefore prefer a start date of around 2.5 million years BP. The name Plio-Pleistocene is in use to mean the last ice age.
The continuous climatic history from the Pliocene into the Pleistocene and Holocene was one reason for the International Commission on Stratigraphy to discourage the use of the term "Quaternary."
The 1.8–1.6 million years of the Quaternary represents the time when recognizable humans existed. Over this short a time period, the total amount of continental drift was less than 100 km, which is largely irrelevant to paleontology. Nonetheless, the geological record is preserved in greater detail than that for earlier periods, and is most relatable to the maps of today. The major geographical changes during this time period included emergence of the Strait of Bosphorus and Skaggerak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and Europe; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The Great Lakes and Hudson's Bay are also the results of the last cycle. Following every other ice age within the Quaternary, there was a different pattern of lakes and bays.
Geological and geochemical analysis of ice cores and ocean cores verified that there were several periods of forward and backward movement of the glaciers and that past temperatures on Earth were very different from today. Thick glacial advances and retreats occurred in several stages over much of North America and Europe, parts of South America and Asia, and all of Antarctica.
The occurrence of the Pleistocene glaciations are thought to have resulted, at least in part, in the cyclic variation of interception absorption of solar radiation. These cycles are called Milankovitch cycles, after the Serbian Milutin Milankovitch who described them. Milankovitch cycles influence climate by increasing or decreasing the amount of sunlight received by certain parts of the globe through time. These changes include a change in the precession of the equinoxes, the tilt of the Earth's axis, and how round versus elliptical the Earth's orbit is (eccentricity). These vary on time scales of 21,000, 41,000, and 100,000 years, respectively. The dominance of the 100,000-year time scale of the Pleistocene glaciations over the last 700,000 years leads many scientists to believe that the eccentricity cycle played a significant role in the climate of this time. Before this time, the ~41,000-year obliquity cycle appeared to dominate. Some scientists remain skeptical of these connections, but a recent paper by Huybers and Wunsch (2005) found that obliquity and eccentricity played a statistically significant role in the glacial cycles.
Oxygen isotope ratio cycles are cyclical variations in the ratio of the mass of oxygen with an atomic weight of 18 to the mass of oxygen with an atomic weight of 16 present in calcite of the oceanic floor as determined by core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio are considered to mirror climate changes in geologic history.
An oxygen molecule (chemical symbol O) has three naturally occurring isotopes: O-16, O-17, and O-18, where the 16, 17, and 18 refer to the atomic weights. The most abundant is O-16, with a small percentage of O-18 and an even smaller percentage of O-17. Oxygen isotope analysis considers only the ratio of O-18 to O-16 present in a core sample taken from limestone deposits in the ocean floor.
The calculated ratio of the masses of each sample is then compared to a standard ratio representing a standard temperature. The ancient sea water in which the limestone was deposited is then either hotter or cooler by a quantitative amount. The method becomes statistical when many samples are considered.
Limestone is deposited from the calcite shells of microorganisms. Calcite, or calcium carbonate (chemical formula CaCO3), is formed from water, H2O, and carbon dioxide (CO2) dissolved in the water. The carbon dioxide provides two of the oxygen atoms in the calcite. The calcium must rob the third from the water. The isotope ratio in the calcite is therefore the same, after compensation, as the ratio in the water from which the microorganisms of a given layer extracted the material of the shell.
O-18 is two neutrons heavier than O-16 and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is therefore required to vaporize it than for O-16, and the molecule must lose less energy to condense.
Energy adds to or takes from the vibrational motion of the molecule, expressed as temperature. At the boiling point, the vibration is sufficiently high to overcome the adhesion between water molecules and they fly into the space of the container or the atmosphere. At the dew point, the molecules adhere into droplets and fall out of the atmosphere as rain or snow. Below the boiling point, the equilibrium between the number of molecules that fly out and the number that return is a function of water temperature.
A warmer water temperature means that the molecules require less energy to vaporize, as they already have more energy. A cooler water temperature means that the water requires more energy to vaporize. As a heavier, O-18 water molecule requires more energy than an O-16 water molecule to depart from the liquid state, cooler water releases vapor that is higher in O-16 content. Cooler air precipitates more O-18 than warmer air. Cooler water therefore collects more O-18 relative to O-16 than does warmer water.
The O-18/O-16 ratio provides an accurate record of ancient water temperature. Water 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit) cooler than present represents glaciation. Precipitation and therefore glacial ice contain water with a low O-18 content. Since large amounts of O-16 water are being stored as glacial ice, the O-18 content of oceanic water is high. Water up to 5 degrees Celsius (9°F) warmer than today represents an interglacial period, when the O-18 content is lower. A plot of ancient water temperature over time indicates that climate has varied cyclically, with large cycles and harmonics, or smaller cycles, superimposed on the large ones. This technique has been especially valuable for identifying glacial maxima and minima in the Pleistocene.
Temperature and climate change are cyclical when plotted on a graph of temperature versus time. Temperature coordinates are given in the form of a deviation from today's annual mean temperature, taken as zero. This sort of graph is based on another of isotope ratio versus time. Ratios are converted to a percentage difference (δ) 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 44 MIS stages beginning at about 2.4 MYA in the Pliocene. Pliocene 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 first major glacial was MIS22 at about 850,000 YA. The largest glacials were 2, 6, and 12; the warmest interglacials, 1, 5, 9, and 11.
The Great Lakes formed and giant mammals flourished in parts of North America and Eurasia not covered in ice. These mammals became extinct when the Ice Age ended about 10,000 years ago. Modern humans evolved about 100,000 years ago.
The name Pleistocene is derived from the Greek pleistos (most) and ceno (new). The Pleistocene follows the Pliocene epoch and is followed by the Holocene epoch. The Pleistocene is the third epoch of the Neogene period, the first epoch of the Quarternary, and the sixth epoch of the Cenozoic era. It lasted from 1.8 million to 12,000 years before the present.
The end of the Pleistocene corresponds with the end of the Paleolithic age used in archaeology.
The modern continents were essentially at their present positions during the Pleistocene, probably having moved no more than 100 km since. During the Pleistocene glacial cycles, glaciers pushed to the 40th parallel latitude in some places. It is estimated that, at maximum glacial extent, 30 percent of the Earth's surface was covered by ice. In addition, a zone of permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometers in North America, and several hundred in Eurasia. The mean annual temperature at the edge of the ice was −6°C; at the edge of the permafrost, 0°C.
Each glacial advance tied up huge volumes of water in continental ice sheets 1500–3000 m thick, resulting in temporary sea-level drops of 100 m or more over the entire surface of the Earth. During interglacial times, such as we are experiencing now, drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions.
The effects of glaciation were global. Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene. The Andes were covered in the south by the Patagonian ice cap. There were glaciers in New Zealand and Tasmania. The current decaying glaciers of Mount Kenya, Mount Kilimanjaro, and the Ruwenzori Range in east and central Africa were larger. Glaciers existed in the mountains of Ethiopia and to the west in the Atlas mountains.
In the northern hemisphere, many glaciers fused into one. The Cordilleran ice sheet covered the North American northwest; the east was covered by the Laurentide ice sheet. The Fenno-Scandian ice sheet rested on north Europe, including Great Britain, and the Alpine ice sheet lay on the Alps. Scattered domes stretched across Siberia and the Arctic shelf. The northern seas were frozen.
South of the ice sheets, large lakes accumulated due to blockage of outlets and decreased evaporation in the cooler air. North-central North America was totally covered by Lake Agassiz. Over 100 basins, now dry or nearly so, were overflowing in the American west. Lake Bonneville, for example, stood where Great Salt Lake now does. In Eurasia, large lakes developed as a result of the runoff from the glaciers. Rivers were larger and had a more copious flow. African lakes were fuller, apparently from decreased evaporation.
Increased dust accumulation in ice cores from Greenland and Antarctica suggests that conditions were drier and windier, as much of the water was tied up in ice caps. A decrease in oceanic and other evaporation because of colder air temperatures, resulted in drier deserts that were far more extensive.
Four major glacial events have been identified, as well as many minor intervening events. A major event is a general glacial excursion, termed just a "glacial." Glacials are separated by "interglacials." During a glacial, the glacier experiences minor advances and retreats. The minor excursion is a "stadial"; times between stadials are "interstadials."
These events are defined differently in different regions of the glacial range, which have their own glacial history depending on latitude, terrain, and climate. There is a general correspondence between glacials in different regions. Investigators often interchange the names if the glacial geology of a region is in the process of being defined. However, it is generally incorrect to apply the name of a glacial in one region to another. You would not refer to the Mindel as the Elsterian or vice versa.
Four of the better-known regions with the names of the glacials are listed in the table below. It should be emphasized that these glacials are a simplification of a more complex cycle of variation in climate and terrain. Many of the advances and stadials remain unnamed. Also, the terrestrial evidence for some of them has been erased or obscured by larger ones, but we know they existed from the study of cyclical climate changes.
|Region||Glacial 1||Glacial 2||Glacial 3||Glacial 4|
|Midwest of U.S.||Nebraskan||Kansan||Illinoian||Wisconsin|
|Region||Interglacial 1||Interglacial 2||Interglacial 3|
|Midwest of U.S.||Aftonian||Yarmouthian||Sangamonian|
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. For example, some have used the term "Riss pluvial" in Egyptian contexts. Any coincidence is an accident of regional factors. Names for some pluvials in some regions have been defined.
There are no faunal stages defined for the Pleistocene or Holocene. Both marine and continental faunas were essentially modern. It is believed by most scientists that humans evolved into modern man during the Pleistocene. Few major new animals evolved, again presumably because of the short—in geologic terms—duration of the period. There was a major extinction of large mammals in Northern areas at the end of the Pleistocene Epoch. Many forms such as saber-toothed cats, mammoths, mastodons, glyptodonts, and so forth, became extinct worldwide. Others, including horses, camels, and cheetahs became extinct in North America.
The end of the Pleistocene is marked as the beginning of significant climate warming at around 10,000 years BP. The time period from that point forward is known as the Holocene.
During the Holocene, three distinct changes occurred.
The first of these is a significant rise in carbon dioxide (from 210 ppm to 280ppm), which was reported from trapped gas bubbles in ice cores (Neftel et al. 1982).
The second change that was seen worldwide at around this time was a change in the species assemblage of foraminifera, microscopic oceanic microorganisms, found in ocean sediments. This change around 11k BP indicates an increase in ocean temperatures (Broecker et al. 1960).
The third major change during this time (12k BP to 10k BP) was the extinction of a number of large mammals in North America (Kurten and Andersen 1980). The extinctions were especially severe in North America where native horses and camels were eliminated. Palynologists noted abrupt worldwide changes in vegetation during this time, with forests replacing tundra.
The end of the Pleistocene also marks the end of an abrupt climate reversal known as the Younger Dryas (12.7 to 11.5 ky BP), where following deglaciation and climate warming, temperatures rapidly dipped back down, turning forested landscape back into tundra. Almost as rapidly as the climate cooled, the warm temperatures were restored.
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