Ice ages

Ice ages

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  ICE AGES By Aryan Goswami Bscgeoinformatics
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  INTRODUCTION An ice ageis a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and greenhouse periods, during which there are no glaciers on the planet. Earth is currently in the Quaternary glaciation. Individual pulses of cold climate within an ice age are termed glacial periods (or, alternatively, glacial, glaciations, glacial stages, stadial, stades, or colloquially, ice ages), and intermittent warm periods within an ice age are called interglacial or interstadials.
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  EVIDENCE There are threemain types of evidence for ice ages • GEOLOGICAL • CHEMICAL • PALEONTOLOGICAL
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  GEOLOGICAL • Geological evidencefor ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratic. Successive glaciations tend to distort and erase the geological evidence for earlier glaciations, making it difficult to interpret. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacial were short compared to the long interglacial. The advent of sediment and ice cores revealed the true situation: glacial are long, interglacial short. It took some time for the current theory to be worked out.
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  CHEMICAL • The chemicalevidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores. For the most recent glacial periods ice cores provide climate proxies from their ice, and atmospheric samples from included bubbles of their ice, and atmospheric samples from included bubbles of air. Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions. This allows a temperature record to be constructed. However, this evidence can be confounded by other factors recorded by isotope ratios.
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  PALEONTOLOGICAL • The paleontologicalevidence consists of changes in the geographical distribution of fossils. During a glacial period cold adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because lower latitudes. This evidence is also difficult to interpret because it requires sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated; ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and the finding of the relevant fossils.
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  MAJOR ICE AGES •There have been at least five major ice ages in the Earth's history (the Huronian, Cryogenian, Andean-Saharan, late Paleozoic, and the latest Quaternary Ice Age). Outside these ages, the Earth seems to have been ice free even in high latitudes; such periods are known as greenhouse periods. • The first three ice ages are still rather poorly known, but the rock records of the last two, and the last in particular, are quite good.
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  THE PALEOZOIC ICEAGE • Evidences of glaciation during at least some part of the Paleozoic Era are found on all the continents of the Southern Hemisphere. In Africa, for example Gouges are found on rocks of Ordovician age in the Sahara Desert (near the present equator). These Gouges are made by the erosional action of pebbles and rocks dragged along the bottom of the glacier. Definite ice-deposited rocks of a somewhat later age are found all the way to the southern tip of Africa.
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  THE LATE CENOZOICICE AGE • The Ice Age we know best is the most recent. It appears to have begun with the formation of an ice cap in Antarctica about 20 million years ago, or in the early part of the Miocene time division of the Cenozoic Era. The evidence for this is the presence of rock fragments of this age found in cores of sediment raised from the Southern Ocean. The only explanation for these fragments so far from land is their transportation by icebergs that broke from a large, parent ice sheet in Antarctica.
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  MAJOR CAUSES OFICE AGES • CHANGES IN EARTH’S ATMOSPHERE • POSITION OF THE CONTINENTS • FLUCTUATIONS IN OCEAN CURRENTS • UPLIFT OF THE TIBETAN PLATEAU • VARIATIONS IN EARTH’S ORBIT • VOLCANISM
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  CHANGES IN EARTH’S ATMOSPHERE •There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism. • The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO2 levels in the atmosphere, mainly from volcanoes, and some supporters of Snowball Earth argue that it was caused in the first place by a reduction in atmospheric CO2. The hypothesis also warns of future Snowball Earths.
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  POSITION OF THE CONTINENTS Thegeological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase Earth's reflectivity and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a positive feedback loop. The ice age continues until the reduction in weathering causes an increase in the greenhouse effect. There are three main contributors from the layout of the continents that obstruct the movement of warm water to the poles: • A continent sits on top of a pole, as Antarctica does today. • A polar sea is almost land-locked, as the Arctic Ocean is today. • A supercontinent covers most of the equator, as Rodinia did during the Cryogenian period.
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  FLUCTUATION IN OCEAN CURRENTS •Another important contribution to ancient climate regimes is the variation of ocean currents, which are modified by continent position, sea levels and salinity, as well as other factors. They can cool (e.g., aiding the creation of Antarctic ice) and the ability to warm (e.g., giving the British Isles a temperate as opposed to a boreal climate). The closing of the Isthmus of Panama about 3 million years ago may have ushered in the present period of strong glaciation over North America by ending the exchange of water between the tropical Atlantic and Pacific Oceans.
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  VOLCANISM • Volcanic eruptionsmay have contributed to the inception and/or the end of ice age periods. At times during the paleoclimate, carbon dioxide levels were two or three times greater than today. Volcanoes and movements in continental plates contributed to high amounts of CO2 in the atmosphere. Carbon dioxide from volcanoes probably contributed to periods with highest overall temperatures. One suggested explanation of the Paleocene- Eocene Thermal Maximum is that undersea volcanoes released methane from clathrates and thus caused a large and rapid increase in the greenhouse effect. There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.
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  REFERENCES • Wikipedia.com • History.com •Geology.utah.gov • Cdm.org
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