The Preboreal is a stage of the Holocene epoch. It is succeeded by the Boreal, it lasted from 10,300 to 9,000 BP in radiocarbon years or 8350 BC to 7050 BC in Gregorian calendar years. It is the first stage of the Holocene epoch; the Holocene has not been formally divided by the IUGS. As a result the Preboreal is only a proposal, as stratigraphy and dating techniques have improved since this 1972 proposal the dates would be different if proposed today. Instead others have begun to use the terms Early and Late, which should be Lower and Upper for the Holocene. If this terminology were to be used the preboreal would be replaced by Lower Holocene which would be dated 11.7 - 8.2 ka B2K. In July 2018 the International Commission on Stratigraphy ratified Greenlandian as the globally recognised first age of the Holocene, much overlapping with the North European regional term Preboreal
Laurentide Ice Sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square kilometers, including most of Canada and a large portion of the northern United States, multiple times during the Quaternary glacial epochs, from 2.588 ± 0.005 million years ago to the present. The last advance covered most of northern North America between c. 95,000 and c. 20,000 years before the present day and, among other geomorphological effects, gouged out the five Great Lakes and the hosts of smaller lakes of the Canadian Shield. These lakes extend from the eastern Northwest Territories, through most of northern Canada, the upper Midwestern United States to the Finger Lakes, through Lake Champlain and Lake George areas of New York, across the northern Appalachians into and through all of New England and Nova Scotia. At times, the ice sheet's southern margin included the present-day sites of northeastern coastal towns and cities such as Boston and New York City and Great Lakes coastal cities and towns as far south as Chicago and St. Louis and followed the present course of the Missouri River up to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet.
The ice coverage extended as far south as 38 degrees latitude mid-continent. This ice sheet was the primary feature of the Pleistocene epoch in North America referred to as the ice age, it was up to 2 mi thick in Nunavik, Canada, but much thinner at its edges, where nunataks were common in hilly areas. It created much of the surface geology of southern Canada and the northern United States, leaving behind glacially scoured valleys, moraines and glacial till, it caused many changes to the shape and drainage of the Great Lakes. As but one of many examples, near the end of the last ice age, Lake Iroquois extended well beyond the boundaries of present-day Lake Ontario, drained down the Hudson River into the Atlantic Ocean, its cycles of growth and melting were a decisive influence on global climate during its existence. This is because it served to divert the jet stream which would otherwise flow from the warm Pacific Ocean through Montana and Minnesota to the south; that gave the Southwestern United States, otherwise a desert, abundant rainfall during ice ages, in extreme contrast to most other parts of the world which became exceedingly dry, though the effect of ice sheets in Europe had an analogous effect on the rainfall in Afghanistan, parts of Iran western Pakistan in winter, as well as North Africa.
Its melting caused major disruptions to the global climate cycle, because the huge influx of low-salinity water into the Arctic Ocean via the Mackenzie River is believed to have disrupted the formation of North Atlantic Deep Water, the saline, deep water that flows from the Greenland Sea. That interrupted the thermohaline circulation, creating the brief Younger Dryas cold epoch and a temporary re-advance of the ice sheet, which did not retreat from Nunavik until 6,500 years ago. During the Pre-Illinoian Stage, the Laurentide Ice Sheet extended as far south as the Missouri and Ohio River valleys; the ultimate collapse of the Laurentide Ice Sheet is suspected to have influenced European agriculture indirectly through the rise of global sea levels. Canada's oldest ice is a 20,000-year-old remnant of the Laurentide Ice Sheet called the Barnes Ice Cap, on central Baffin Island. During the Late Pleistocene, the Laurentide ice sheet reached from the Rocky Mountains eastward through the Great Lakes, into New England, covering nearly all of Canada east of the Rocky Mountains.
Three major ice centers formed in North America: the Labrador and Cordilleran. The Cordilleran covered the region from the Pacific Ocean to the eastern front of the Rocky Mountains and the Labrador and Keewatin fields are referred to as the Laurentide Ice Sheet. Central North America has evidence of the numerous sublobes; the Keewatin covered the western interior plains of North America from the Mackenzie River to the Missouri River and the upper reaches of the Mississippi River. The Labrador covered spread over eastern Canada and the northeastern part of the United States abutting the Keewatin lobe in the western Great Lakes and Mississippi valley. Cordilleran Ice Sheet covered up to 2,500,000 square kilometres at the Last Glacial Maximum; the eastern edge abutted the Laurentide ice sheet. The sheet was anchored in the Coast Mountains of British Columbia and Alberta, south into the Cascade Range of Washington; that is one and a half times the water held in the Antarctic. Anchored in the mountain backbone of the west coast, the ice sheet dissipated north of the Alaska Range where the air was too dry to form glaciers.
It is believed that the Cordilleran ice melted in less than 4000 years. The water created numerous Proglacial lakes along the margins such as Lake Missoula leading to catastrophic floods as with the Missoula Floods. Much of the topography of Eastern Washington and northern Montana and North Dakota was affected. Keewatin Ice flow has had four or five primary lobes identified ice divides extending from a dome over west-central Keewatin. Two of the lobes abut the adjacent Baffin ice sheets; the primary lobes flow towards Saskatchewan. Ice flowed across all of Maine and into the Gulf of St. Lawrence covering the Maritime Provinces; the Appalachian Ice Complex, flowed from the Gaspé Peninsula over New Brunswick, the Magdalen Shelf, Nova Scotia. The Labrador flow extended across the mouth of the St. Lawrence River, reaching the Gaspé Peninsula and across Chaleur Bay. From the Escuminac center on the
In the geosciences, paleosol can have two meanings. The first meaning, common in geology and paleontology, refers to a former soil preserved by burial underneath either sediments or volcanic deposits, which in the case of older deposits have lithified into rock. In Quaternary geology, sedimentology and geology in general, it is the typical and accepted practice to use the term "paleosol" to designate such "fossil soils" found buried within either sedimentary or volcanic deposits exposed in all continents as illustrated by Rettallack and other published papers and books. In soil science, paleosols are soils formed long periods ago that have no relationship in their chemical and physical characteristics to the present-day climate or vegetation; such soils form on old continental cratons and as small scattered localities in outliers of ancient rock. Because of the changes in the Earth's climate over the last fifty million years, soils formed under tropical rainforest have become exposed to arid climates which cause former oxisols, ultisols or alfisols to dry out in such a manner that a hard crust is formed.
This process has occurred so extensively in most parts of Australia as to restrict soil development - the former soil is the parent material for a new soil, but it is so unweatherable that only a poorly developed soil can exist in present dry climates when they have become much drier during glacial periods in the Quaternary. In other parts of Australia, in many parts of Africa, drying out of former soils has not been so severe; this has led to large areas of relict podsols in quite dry climates in the far southern inland of Australia and to the formation of torrox soils in southern Africa. Here, present climates allow the maintenance of the old soils in climates under which they could not form if one were to start with the parent material on which they developed in the Mesozoic and Paleocene. Paleosols in this sense are always exceedingly infertile soils, containing available phosphorus levels orders of magnitude lower than in temperate regions with younger soils. Ecological studies have shown that this has forced specialised evolution amongst Australian flora to obtain minimal nutrient supplies.
The fact that soil formation is not occurring makes ecologically sustainable management more difficult. However, paleosols contain the most exceptional biodiversity due to the absence of competition. Paleosols record certain aspects of the climate. Quaternary paleosols in particular have been used to determine past environmental conditions, such as paleo-precipitation and mean temperatures. Paleosols are an important archive of information about ancient ecosystems and various components of fossil soils can be used to study past plant life. Paleosols contain ancient plant materials such as pollen grains and phytoliths, a biomineralized form of silica produced by many plants such as grasses. Both pollen and phytolith fossils from different plant species have characteristic shapes that can be traced back to their parent plants. Over long geological time scales, phytoliths may not be preserved in paleosols due to ability of the poorly crystalline silica to dissolve. Another indicator of plant community composition in paleosols is the carbon isotopic signature.
The ratio of different carbon isotopes in organic matter in paleosols reflects the proportions of plants using C3 photosynthesis, which grow in cooler and wetter climates, versus plants using C4 photosynthesis, which are better adapted to hotter and drier conditions. Other methods for detecting past plant life in paleosols are based on identifying the remains of leaf waxes, which are slow to break down in soils over time. Paleopedology Paleopedological record Pedogenesis Pedology Korshov Commission on Paleopedology of the International Union of Soil Science, Subcommission on Paleopedology of the International Union for Quaternary Research
Till or glacial till is unsorted glacial sediment. Till is derived from the entrainment of material by the moving ice of a glacier, it is deposited some distance down-ice to form terminal, lateral and ground moraines. Till is classified into primary deposits, laid down directly by glaciers, secondary deposits, reworked by fluvial transport and other processes. Glacial drift is the coarsely graded and heterogeneous sediment of a glacier. It's content may vary from clays to mixtures of clay, sand and boulders; this material is derived from the subglacial erosion and entrainment by the moving ice of the glaciers of available unconsolidated sediments. Bedrock can be eroded through the action of glacial plucking and abrasion and the resulting clasts of various sizes will be incorporated to the glacier's bed; the sedimentary assemblage forming this bed will be abandoned some distance down-ice from its various sources. This is the process of glacial till deposition; when this deposition occurs at the base of the moving ice of a glacier, the sediment is called lodgement till.
Eroded unconsolidated sediments can be preserved in the till along with their original sedimentary structures. More these sediments lose their original structure through the mixture processes associated with subglacial transport and they contribute to form the more or less uniform matrix of the till. Till is deposited at the terminal moraine, along the lateral and medial moraines and in the ground moraine of a glacier; as a glacier melts a continental glacier, large amounts of till are washed away and deposited as outwash in sandurs by the rivers flowing from the glacier, as varves in any proglacial lakes which may form. Till may contain detectable concentrations of gems or other valuable ore minerals picked up by the glacier during its advance, for example the diamonds found in the U. S. states of Wisconsin, in Canada. Prospectors use trace minerals in tills as clues to follow the glacier upstream to find kimberlite diamond deposits and other types of ore deposits. In cases where till has been indurated or lithified by subsequent burial into solid rock, it is known as the sedimentary rock tillite.
Matching beds of ancient tillites on opposite sides of the south Atlantic Ocean provided early evidence for continental drift. The same tillites provide some support to the Precambrian Snowball Earth glaciation event hypothesis. There are various types of classifying tills: primary deposits – laid down directly by glacier action secondary deposits – reworked by fluvial transport, etc. Traditionally a further set of divisions has been made to primary deposits, based upon the method of deposition. Lodgement tills – sediment, deposited by plastering of glacial debris from a sliding glacier bed. Deformation tills – Sediment, disaggregated and homogenised by shearing in the sub glacial deformed layer. Melt out tills – Released by melting of stagnant or moving debris-rich glacier ice and deposited without subsequent transport or deformation. Split up into sub glacial melt out supraglacial melt-out till. Sublimation till – similar to melt out till, except the ice is lost through sublimation rather than melt.
Occurs only in cold and arid conditions in Antarctica. Van der Meer et al. 2003 have suggested that these till classifications are outdated and should instead be replaced with only one classification, that of deformation till. The reasons behind this are down to the difficulties in classifying different tills, which are based on inferences of the physical setting of the till rather than detailed analysis of the till fabric or particle size. Boulder clay – A deposit of clay full of boulders, formed from the ground moraine material of glaciers and ice-sheets Diamictite – A lithified sedimentary rock of non- to poorly sorted terrigenous sediment in a matrix of mudstone or sandstone
Last Glacial Maximum
The Last Glacial Maximum was the most recent time during the Last Glacial Period when ice sheets were at their greatest extent. Vast ice sheets covered much of North America, northern Europe, Asia; the ice sheets profoundly affected Earth's climate by causing drought, a large drop in sea levels. The ice sheets reached their maximum coverage about 26,500 years ago. Deglaciation commenced in the Northern Hemisphere at 20 ka and in Antarctica at 14.5 ka, consistent with evidence for an abrupt rise in the sea level at about 14.5 ka. The LGM is referred to in Britain as the Dimlington Stadial, dated by Nick Ashton to between 31 and 16 ka. In the archaeology of Paleolithic Europe, the LGM spans the Gravettian, Magdalenian and Périgordian; the LGM was followed by the Late Glacial. According to Blue Marble 3000, the average global temperature around 19,000 BC was 9.0 °C. This is about 6.0 °C colder than the 2013-2017 average. The figures given by the Intergovernmental Panel On Climate Change estimate a lower global temperature than the figures given by the Zurich University of Applied Sciences.
However, these figures are open more to interpretation. According to the IPCC, average global temperatures increased by 5.5 ± 1.5 °C since the last glacial maximum, the rate of warming was about 10 times slower than that of the 20th Century. It appears that they are defining the present as sometime in the 19th Century for this case, but they don’t specify exact years, or give a temperature for the present. Berkeley Earth puts out a list of average global temperatures by year. If you average all of the years from 1850 to 1899, the average temperature comes out to 13.8 °C. When subtracting 5.5 ± 1.5 °C from the 1850-1899 average, the average temperature for the last glacial maximum comes out to 8.3 ± 1.5 °C. This is about 6.7 ± 1.5 °C colder than the 2013-2017 average. This figure is open to interpretation because the IPCC does not specify 1850-1899 as being the present, or give any exact set of years as being the present, it does not state whether or not they agree with the figures given by Berkeley Earth.
According to the United States Geological Survey, permanent summer ice covered about 8% of Earth's surface and 25% of the land area during the last glacial maximum. The USGS states that sea level was about 125 meters lower than in present times; when comparing to the present, the average global temperature was 15.0 °C for the 2013-2017 period. About 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice. The formation of an ice sheet or ice cap requires both prolonged precipitation. Hence, despite having temperatures similar to those of glaciated areas in North America and Europe, East Asia remained unglaciated except at higher elevations; this difference was. These anticyclones generated air masses that were so dry on reaching Siberia and Manchuria that precipitation sufficient for the formation of glaciers could never occur; the relative warmth of the Pacific Ocean due to the shutting down of the Oyashio Current and the presence of large'east-west' mountain ranges were secondary factors preventing continental glaciation in Asia.
All over the world, climates at the Last Glacial Maximum were cooler and everywhere drier. In extreme cases, such as South Australia and the Sahel, rainfall could be diminished by up to 90% from present, with florae diminished to the same degree as in glaciated areas of Europe and North America. In less affected regions, rainforest cover was diminished in West Africa where a few refugia were surrounded by tropical grasslands; the Amazon rainforest was split into two large blocks by extensive savanna, the tropical rainforests of Southeast Asia were affected, with deciduous forests expanding in their place except on the east and west extremities of the Sundaland shelf. Only in Central America and the Chocó region of Colombia did tropical rainforests remain intact – due to the extraordinarily heavy rainfall of these regions. Most of the world's deserts expanded. Exceptions were in what is now the western United States, where changes in the jet stream brought heavy rain to areas that are now desert and large pluvial lakes formed, the best known being Lake Bonneville in Utah.
This occurred in Afghanistan and Iran, where a major lake formed in the Dasht-e Kavir. In Australia, shifting sand dunes covered half the continent, whilst the Chaco and Pampas in South America became dry. Present-day subtropical regions lost most of their forest cover, notably in eastern Australia, the Atlantic Forest of Brazil, southern China, where open woodland became dominant due to drier conditions. In northern China – unglaciated despite its cold climate – a mixture of grassland and tundra prevailed, here, the northern limit of tree growth was at least 20° farther south than today. In the period before the Last Glacial Maximum, many areas that became barren desert were wetter than they are today, notably in southern Australia, where Aboriginal occupation is believed to coincide with a wet period between 40,000 and 60,000 years Before Present. During the Last Glacial Maximum, much of the world was cold and inhospitable
The Pleistocene is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and with the end of the Paleolithic age used in archaeology; the Pleistocene is the first epoch of the Quaternary Period or sixth epoch of the Cenozoic Era. In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Middle Pleistocene and Upper Pleistocene. In addition to this international subdivision, various regional subdivisions are used. Before a change confirmed in 2009 by the International Union of Geological Sciences, the time boundary between the Pleistocene and the preceding Pliocene was regarded as being at 1.806 million years Before Present, as opposed to the accepted 2.588 million years BP: publications from the preceding years may use either definition of the period. Charles Lyell introduced the term "Pleistocene" in 1839 to describe strata in Sicily that had at least 70% of their molluscan fauna still living today.
This distinguished it from the older Pliocene epoch, which Lyell had thought to be the youngest fossil rock layer. He constructed the name "Pleistocene" from the Greek πλεῖστος, pleīstos, "most", καινός, kainós, "new"; the Pleistocene has been dated from 2.588 million to 11,700 years BP with the end date expressed in radiocarbon years as 10,000 carbon-14 years BP. It covers most of the latest period of repeated glaciation, up to and including the Younger Dryas cold spell; the end of the Younger Dryas has been dated to about 9640 BC. The end of the Younger Dryas is the official start of the current Holocene Epoch. Although it is considered an epoch, the Holocene is not different from previous interglacial intervals within the Pleistocene, it was not until after the development of radiocarbon dating, that Pleistocene archaeological excavations shifted to stratified caves and rock-shelters as opposed to open-air river-terrace sites. In 2009 the International Union of Geological Sciences confirmed a change in time period for the Pleistocene, changing the start date from 1.806 to 2.588 million years BP, accepted the base of the Gelasian as the base of the Pleistocene, namely the base of the Monte San Nicola GSSP.
The IUGS has yet to approve a type section, Global Boundary Stratotype Section and Point, for the upper Pleistocene/Holocene boundary. The proposed section is the North Greenland Ice Core Project ice core 75° 06' N 42° 18' W; the lower boundary of the Pleistocene Series is formally defined magnetostratigraphically as the base of the Matuyama chronozone, isotopic stage 103. Above this point there are notable extinctions of the calcareous nanofossils: Discoaster pentaradiatus and Discoaster surculus; the Pleistocene covers the recent period of repeated glaciations. The name Plio-Pleistocene has, in the past, been used to mean the last ice age; the revised definition of the Quaternary, by pushing back the start date of the Pleistocene to 2.58 Ma, results in the inclusion of all the recent repeated glaciations within the Pleistocene. The modern continents were at their present positions during the Pleistocene, the plates upon which they sit having moved no more than 100 km relative to each other since the beginning of the period.
According to Mark Lynas, the Pleistocene's overall climate could be characterized as a continuous El Niño with trade winds in the south Pacific weakening or heading east, warm air rising near Peru, warm water spreading from the west Pacific and the Indian Ocean to the east Pacific, other El Niño markers. Pleistocene climate was marked by repeated glacial cycles in which continental glaciers pushed to the 40th parallel in some places, it is estimated. In addition, a zone of permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometres in North America, several hundred in Eurasia; the mean annual temperature at the edge of the ice was −6 °C. Each glacial advance tied up huge volumes of water in continental ice sheets 1,500 to 3,000 metres thick, resulting in temporary sea-level drops of 100 metres or more over the entire surface of the Earth. During interglacial times, such as at present, 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 Tasmania; the current decaying glaciers of Mount Kenya, Mount Kilimanjaro, the Ruwenzori Range in east and central Africa were larger. Glaciers existed 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 Fenno-Scandian ice sheet rested including much of Great Britain. Scattered domes stretched across Siberi
Loess is a clastic, predominantly silt-sized sediment, formed by the accumulation of wind-blown dust. Ten percent of the Earth's land area is covered by similar deposits. Loess is an aeolian sediment formed by the accumulation of wind-blown silt in the 20–50 micrometer size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate, it is homogeneous and porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs. The word loess, with connotations of origin by wind-deposited accumulation, came into English from German Löss, which can be traced back to Swiss German and is cognate with the English word loose and the German word los, it was first applied to Rhine River valley loess about 1821. Loess is homogeneous, friable, pale yellow or buff coherent non-stratified and calcareous. Loess grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar and other minerals.
Loess can be described as a dust-like soil. Loess deposits may become thick, more than a hundred meters in areas of China and tens of meters in parts of the Midwestern United States, it occurs as a blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess stands in either steep or vertical faces; because the grains are angular, loess will stand in banks for many years without slumping. This soil has a characteristic called vertical cleavage which makes it excavated to form cave dwellings, a popular method of making human habitations in some parts of China. Loess will erode readily. In several areas of the world, loess ridges have formed that are aligned with the prevailing winds during the last glacial maximum; these are called "paha ridges" in "greda ridges" in Europe. The form of these loess dunes has been explained by a combination of tundra conditions. Loess comes from the German Löss or Löß, from Alemannic lösch meaning drop as named by peasants and masons along the Rhine Valley.
The term "Löß" was first described in Central Europe by Karl Cäsar von Leonhard who reported yellowish brown, silty deposits along the Rhine valley near Heidelberg. Charles Lyell brought this term into widespread usage by observing similarities between loess and loess derivatives along the loess bluffs in the Rhine and Mississippi. At that time it was thought that the yellowish brown silt-rich sediment was of fluvial origin being deposited by the large rivers, it wasn't until the end of the 19th century that the aeolian origin of loess was recognized the convincing observations of loess in China by Ferdinand von Richthofen. A tremendous number of papers have been published since focusing on the formation of loess and on loess/palaeosol sequences as archives of climate and environment change; these water conservation works were carried out extensively in China and the research of Loess in China has been continued since 1954. Much effort was put into the setting up of regional and local loess stratigraphies and their correlation.
But the chronostratigraphical position of the last interglacial soil correlating to marine isotope substage 5e has been a matter of debate, owing to the lack of robust and reliable numerical dating, as summarized for example in Zöller et al. and Frechen, Horváth & Gábris for the Austrian and Hungarian loess stratigraphy, respectively. Since the 1980s, thermoluminescence, optically stimulated luminescence and infrared stimulated luminescence dating are available providing the possibility for dating the time of loess deposition, i.e. the time elapsed since the last exposure of the mineral grains to daylight. During the past decade, luminescence dating has improved by new methodological improvements the development of single aliquot regenerative protocols resulting in reliable ages with an accuracy of up to 5 and 10% for the last glacial record. More luminescence dating has become a robust dating technique for penultimate and antepenultimate glacial loess allowing for a reliable correlation of loess/palaeosol sequences for at least the last two interglacial/glacial cycles throughout Europe and the Northern Hemisphere.
Furthermore, the numerical dating provides the basis for quantitative loess research applying more sophisticated methods to determine and understand high-resolution proxy data, such as the palaeodust content of the atmosphere, variations of the atmospheric circulation patterns and wind systems, palaeoprecipitation and palaeotemperature. According to Pye, four fundamental requirements are necessary for the formation of loess: a dust source, adequate wind energy to transport the dust, a suitable accumulation area, a sufficient amount of time. Periglacial loess is derived from the floodplains of glacial braided rivers that carried large volumes of glacial meltwater and sediments from the annual melting of continental icesheets and mountain icecaps during the spring and summer. During the autumn and winter, when melting of the icesheets and icecaps ceased, the flow of meltwater down these rivers either ceased or was reduced; as a consequence, large parts of the submerged and unvegetated floodplains of these braided rivers dried out and were exposed to the wind.
Because these floodplains consist of sediment containing a high content of glacial