In climatic history and geology, the cold period or cryomer is a period within an ice age with, on average, lower temperatures between two periods of time with average higher temperatures, so-called warm periods . The terms ice age and glacial are sometimes used synonymously, sometimes limited to phases with more extensive glaciation.
The same cold period is usually named differently in different regions of the world. The last glacial period with its maximum a little more than 20,000 years ago in northern Central Europe as Vistula , in the northern Alps as Würm , in northern Russia as Waldai, in Siberia as Zyryanka-, on the British Isles as Devensian-, on Ireland known as the Midlandian, in North America as Wisconsin, in Venezuela as Mérida, in Chile as Llanquihue and in New Zealand as Otira.
Sediment deposits during glacials
Material from glacial streams or meltwater from the inland ice is deposited by the flowing water; however, their formation is linked to the occurrence of glaciers. These phenomena are called glacifluvial , glacifluvial or fluvioglacial , depending on whether the action of the ice or flowing water dominates in the immediate vicinity. These include the transition cones near the glacier, the pebbles of which are generally not yet rounded. The material sorting is already there, but unclear. With increasing distance from the glacier, the glacifluvial deposits are purely fluvial, but cannot be explained without the glacier. The Sander are known in northern Germany.
Glaciolimic deposits are deposited in the basin of a lake that is dammed up by the ice. Here, too, the formation of the deposits is inconceivable without the glacier.
Material that has been deposited in the sea by glaciers and glacier rivers is called glazimarin .
The search for the causes of the almost regular alternation of glacial and interglacial stages within the current Ice Age is still one of the challenges facing paleoclimatology today . She is closely associated with the names James Croll and Milutin Milanković . Both had taken up ideas of the French Joseph-Alphonse Adhémar , according to which changes in the geometry of the earth's orbit are responsible for recurring cold periods.
The change in the geometry of the Earth's orbit is caused by reciprocal gravitational forces in the sun , planets and moon system . They change the shape of the earth's elliptical orbit ( eccentricity ) around the sun with a period of about 100,000 years, the inclination of the earth's axis to orbit with a period of about 40,000 years (obliqueness of the ecliptic ), while the equinox occurs of the elliptical orbit takes the same position on the ellipse again after about 20,930 years ( tropical apse rotation ). Through these so-called Milanković cycles , the distribution of solar energy on earth changes periodically.
Inspired by the German meteorologist Wladimir Peter Köppen , Milutin Milanković formulated the hypothesis in 1941 in his work The Canon of Earth Radiation and its Application to the Ice Age Problem that a glacial period always occurs when the summer solar radiation is minimal in high northern latitudes. According to Köppen, cool summers are more decisive for ice build-up than cold winters. So Milanković looked for the causes of cold periods where they are most obvious, in the high northern latitudes.
The variations in the Earth's orbit parameters (Milanković cycles) were the trigger and suitable boundary conditions, but their effect was reinforced by other factors. Tectonic processes and their influence on the oceanic circulation are assumed to be a cause for the beginning of both the Antarctic and the northern hemisphere glaciation. In addition, the CO 2 content of the atmosphere is a key factor, which is closely linked to temperature fluctuations, as various studies of ice cores in Antarctica and Greenland, which contain ice from the past 800,000 years, have shown. According to this, the decrease in the concentration of the greenhouse gas carbon dioxide (together with methane and nitrous oxide ) should represent about a third of the temperature change between warm and cold periods, and according to a more recent publication even for half. Other positive feedback processes such as ice-albedo feedback , vegetation cover and the variability of the water vapor content in the atmosphere played an additional role. For the fluctuations in the cold periods between so-called stadials and interstadials , feedback effects in connection with the thermohaline circulation are assumed.
According to the current state of research, the cycles demonstrated by Milanković are responsible for temperature depressions, but these only cause a decrease of a few tenths of a Kelvin, cover only one hemisphere and were also already present in the Precambrian, where, according to today's knowledge, there was not such a marked change came from warm and cold times. It is also ultimately unclear why the change between warm and cold periods occurred at a rate of 41,000 years until about a million years ago, but then every 100,000 years since the so-called Mid-Pleistocene Revolution . The period of 41,000 years is associated with the fluctuation of the inclination of the Earth's axis, while the period of 100,000 years could be related to the eccentricity of the earth's orbit, but is by far the weakest of the frequencies predicted by Milanković. It is assumed that reinforcements of the Milanković cycles through processes within the earth's climate are responsible for this. Therefore, several causes, both exogenous and endogenous, must be held responsible for the Pleistocene alternation. The interaction of tectonic, astronomical, oceanic and climatic processes must be taken into account, as each individual process is not able to cause global cold times.
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