193-712: The African humid period (AHP; also known by other names ) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, trees and lakes was caused by changes in the Earth's axial tilt ; changes in vegetation and dust in the Sahara which strengthened the African monsoon ; and increased greenhouse gases . During
386-422: A beta particle (an electron , e ) and an electron antineutrino ( ν e ), one of the neutrons in the C nucleus changes to a proton and the C nucleus reverts to the stable (non-radioactive) isotope N . During its life, a plant or animal is in equilibrium with its surroundings by exchanging carbon either with the atmosphere or through its diet. It will, therefore, have
579-611: A change from low-amplitude glacial cycles with a dominant periodicity of 41,000 years to asymmetric high-amplitude cycles dominated by a periodicity of 100,000 years. However, a 2020 study concluded that ice age terminations might have been influenced by obliquity since the Mid-Pleistocene Transition, which caused stronger summers in the Northern Hemisphere . Glaciation in the Pleistocene
772-527: A decade. It was revised again in the early 1960s to 5,730 ± 40 years, which meant that many calculated dates in papers published prior to this were incorrect (the error in the half-life is about 3%). For consistency with these early papers, it was agreed at the 1962 Radiocarbon Conference in Cambridge (UK) to use the "Libby half-life" of 5568 years. Radiocarbon ages are still calculated using this half-life, and are known as "Conventional Radiocarbon Age". Since
965-574: A deviation from today's annual mean temperature, taken as zero. This sort of graph is based on another isotope ratio versus time. Ratios are converted to a percentage difference 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 102 MIS stages beginning at about 2.588 Ma BP in
1158-612: A dominance of grasses over humid tropics trees. The tree Lophira alata and others may have spread out of the African forests during the AHP, and the Lactuca plants may have split into two species under the effects of the AHP and other climate changes in Africa during the Holocene. The Sahara climate did not become entirely homogeneous; its central-eastern parts were probably drier than
1351-443: A dry to a "green" Sahara and back have threshold behaviour, with the change occurring once a certain level of insolation is exceeded; likewise, a gradual drop of insolation often leads to a sudden transition back to a dry Sahara. This is due to various feedback processes which are at work, and in climate models there is often more than one stable climate-vegetation state. Sea surface temperature and greenhouse gas changes synchronized
1544-421: A few regions had been studied and the names were relatively few. Today the geologists of different nations are taking more of an interest in Pleistocene glaciology. As a consequence, the number of names is expanding rapidly and will continue to expand. 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 evidence remains from
1737-423: A few years, but the surface waters also receive water from the deep ocean, which has more than 90% of the carbon in the reservoir. Water in the deep ocean takes about 1,000 years to circulate back through surface waters, and so the surface waters contain a combination of older water, with depleted C , and water recently at the surface, with C in equilibrium with the atmosphere. Creatures living at
1930-408: A given sample stopped exchanging carbon – the older the sample, the less C will be left. The equation governing the decay of a radioactive isotope is: N = N 0 e − λ t {\displaystyle N=N_{0}\,e^{-\lambda t}\,} where N 0 is the number of atoms of the isotope in the original sample (at time t = 0, when
2123-483: A gradual advance is supported by tephrochronological data. Likewise, in the Sahara there might have been a delay of about a millennium between the onset of the AHP and the full establishment of humid conditions, as vegetation growth and the filling of river systems took time. Lake Victoria reappeared and overflowed; Lake Albert also overflowed into the White Nile 15,000–14,500 years ago and so did Lake Tana , into
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#17327653455622316-400: A higher δ C than one that eats food with lower δ C values. The animal's own biochemical processes can also impact the results: for example, both bone minerals and bone collagen typically have a higher concentration of C than is found in the animal's diet, though for different biochemical reasons. The enrichment of bone C also implies that excreted material
2509-531: A large number of icebergs in the North Atlantic; the discharge of large amounts of such icebergs between 11,500 and 21,000 years before present coincided with droughts in the subtropics . Before the onset of the AHP, it is thought that Lake Victoria , Lake Albert , Lake Edward , Lake Turkana and the Sudd swamps had dried out. The White Nile had become a seasonal river whose course along with that of
2702-413: A mass of less than 1% of those on land and are not shown in the diagram. Accumulated dead organic matter, of both plants and animals, exceeds the mass of the biosphere by a factor of nearly 3, and since this matter is no longer exchanging carbon with its environment, it has a C / C ratio lower than that of the biosphere. The variation in the C / C ratio in different parts of
2895-472: A northward movement of the ITCZ and increasing moisture gradients between land and sea. Two temperature gradients, one between a cooler Atlantic during spring and an already warming African continent, the other between warmer temperatures north of 10° latitude and cooler south, may have assisted in this change. In Eastern Africa, ITCZ changes had relatively little effect on precipitation changes. The past position of
3088-640: A number of dry/wet periods have been defined for the Central Africa region. In general, these types of climate fluctuations between wetter and drier periods are known as " pluvials " and " interpluvials ", respectively. The term "Green Sahara" is frequently used to describe the AHPs. Because the AHP did not affect all of Africa and is thus not technically accurate, some scientists have instead used and recommended "North African humid period" and "Northern African humid period". Other terms that have been applied to
3281-869: A paper in Science in 1947, in which the authors commented that their results implied it would be possible to date materials containing carbon of organic origin. Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings, Zoser and Sneferu , independently dated to 2625 BC plus or minus 75 years, were dated by radiocarbon measurement to an average of 2800 BC plus or minus 250 years. These results were published in Science in December 1949. Within 11 years of their announcement, more than 20 radiocarbon dating laboratories had been set up worldwide. In 1960, Libby
3474-498: A role in vegetation changes. Isotope ratios such as the hydrogen / deuterium ratio that have been used to reconstruct past precipitation values likewise are under the influence of various physical effects, which complicates their interpretation. Most records of Holocene precipitation in eastern Africa come from low altitudes. The term "African humid period" (AHP) was coined in 2000 by Peter B. de Menocal et al. Earlier humid periods are sometimes known as "African humid periods" and
3667-492: A sample is, the less C there is to be detected, and because the half-life of C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to approximately 50,000 years ago (in this interval about 99.8% of the C will have decayed), although special preparation methods occasionally make an accurate analysis of older samples possible. In 1960, Libby received
3860-578: A team of Russian scientists in collaboration with Princeton University announced that they had brought two female nematodes frozen in permafrost , from around 42,000 years ago, back to life. The two nematodes, at the time, were the oldest confirmed living animals on the planet. The evolution of anatomically modern humans took place during the Pleistocene. At the beginning of the Pleistocene Paranthropus species were still present, as well as early human ancestors, but during
4053-602: A widespread settlement of the Sahara and the Arabian Deserts, and had a profound effect on African cultures, such as the birth of the Ancient Egyptian civilization . People in the Sahara lived as hunter-gatherers and domesticated cattle, goats and sheep. They left archaeological sites and artifacts such as one of the oldest ships in the world , and rock paintings such as those in the Cave of Swimmers and in
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#17327653455624246-653: Is also known as " aqualithic " although substantial differences between the cultures of various places have been found. The greening of the Sahara led to a demographic expansion and especially in the Eastern Sahara human occupancy coincides with the AHP. Conversely occupation decreased along the Nile valley, perhaps due to the expansion of wetlands there and frequent large-scale flooding of the Nile delta. Humans were hunting large animals with weapons that have been found in archaeological sites and wild cereals occurring in
4439-429: Is also referred to individually as a carbon exchange reservoir. The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the C generated by cosmic rays to fully mix with them. This affects the ratio of C to C in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir. The atmosphere, which
4632-405: Is an important factor in the precipitation increase. Specifically, increased precipitation increases the amount of vegetation; vegetation absorbs more sunlight and thus more energy is available for the monsoon. In addition, evapotranspiration from vegetation adds more moisture, although this effect is less pronounced than the albedo effect. Heat fluxes in the soil and evaporation are also altered by
4825-414: Is assumed to have originally had the same C / C ratio as the ratio in the atmosphere, and since the size of the sample is known, the total number of atoms in the sample can be calculated, yielding N 0 , the number of C atoms in the original sample. Measurement of N , the number of C atoms currently in the sample, allows the calculation of t , the age of the sample, using
5018-440: Is contaminated so that 1% of the sample is modern carbon, it will appear to be 600 years younger; for a sample that is 34,000 years old, the same amount of contamination would cause an error of 4,000 years. Contamination with old carbon, with no remaining C , causes an error in the other direction independent of age – a sample contaminated with 1% old carbon will appear to be about 80 years older than it truly is, regardless of
5211-428: Is depleted in C because of the marine effect, C is removed from the southern atmosphere more quickly than in the north. The effect is strengthened by strong upwelling around Antarctica. If the carbon in freshwater is partly acquired from aged carbon, such as rocks, then the result will be a reduction in the C / C ratio in the water. For example, rivers that pass over limestone , which
5404-400: Is depleted in C relative to the diet. Since C makes up about 1% of the carbon in a sample, the C / C ratio can be accurately measured by mass spectrometry . Typical values of δ C have been found by experiment for many plants, as well as for different parts of animals such as bone collagen , but when dating a given sample it is better to determine
5597-426: Is done by calibration curves (discussed below), which convert a measurement of C in a sample into an estimated calendar age. The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: an age quoted in radiocarbon years means that no calibration curve has been used − the calculations for radiocarbon years assume that
5790-729: Is estimated that, at maximum glacial extent, 30% 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 kilometres in North America , and several hundred in Eurasia . The mean annual temperature at the edge of the ice was −6 °C (21 °F); at the edge of the permafrost, 0 °C (32 °F). Each glacial advance tied up huge volumes of water in continental ice sheets 1,500 to 3,000 metres (4,900–9,800 ft) thick, resulting in temporary sea-level drops of 100 metres (300 ft) or more over
5983-459: Is less CO 2 available for the photosynthetic reactions. Under these conditions, fractionation is reduced, and at temperatures above 14 °C (57 °F) the δ C values are correspondingly higher, while at lower temperatures, CO 2 becomes more soluble and hence more available to marine organisms. The δ C value for animals depends on their diet. An animal that eats food with high δ C values will have
African humid period - Misplaced Pages Continue
6176-415: Is mostly composed of calcium carbonate , will acquire carbonate ions. Similarly, groundwater can contain carbon derived from the rocks through which it has passed. These rocks are usually so old that they no longer contain any measurable C , so this carbon lowers the C / C ratio of the water it enters, which can lead to apparent ages of thousands of years for both the affected water and
6369-459: Is no systematic correspondence between 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. Only a few of the names for pluvials in restricted regions have been stratigraphically defined. The sum of transient factors acting at
6562-598: Is one of the first places where pottery was developed probably under the influence of increased availability of resources during the AHP. The humid period also favoured its development and spread in West Africa during the 10th millennium BC ; the so-called "wavy line" or "dotted wavy-line" motif was widespread across Northern Africa and as far as Lake Turkana . Pleistocene The Pleistocene ( / ˈ p l aɪ s t ə ˌ s iː n , - s t oʊ -/ PLY -stə-seen, -stoh- ; referred to colloquially as
6755-1066: Is possible that giant lakes only formed in the southern part of the Sahara. Other lakes are known from Adrar Bous in Niger , Era Kohor and Trou au Natron in the Tibesti Mountains , I-n-Atei in the Hoggar , at Ine Sakane and in Taoudenni in Mali , the Garat Ouda and Takarkori Lakes in the Acacus Mountains , Chemchane in Mauretania , at Guern El Louläilet in the Great Western Erg and Sebkha Mellala close to Ouargla , both in Algeria , at Wadi Shati and elsewhere in
6948-456: Is sometimes called) percolates into the rest of the reservoir. Photosynthesis is the primary process by which carbon moves from the atmosphere into living things. In photosynthetic pathways C is absorbed slightly more easily than C , which in turn is more easily absorbed than C . The differential uptake of the three carbon isotopes leads to C / C and C / C ratios in plants that differ from
7141-416: Is substantially longer than the time it takes for its C to decay below detectable levels, fossil fuels contain almost no C . As a result, beginning in the late 19th century, there was a noticeable drop in the proportion of C in the atmosphere as the carbon dioxide generated from burning fossil fuels began to accumulate. Conversely, nuclear testing increased the amount of C in
7334-453: Is the main pathway by which C is created: n + 7 N → 6 C + p where n represents a neutron and p represents a proton . Once produced, the C quickly combines with the oxygen ( O ) in the atmosphere to form first carbon monoxide ( CO ), and ultimately carbon dioxide ( CO 2 ). C + O 2 → CO + O CO + OH → CO 2 + H Carbon dioxide produced in this way diffuses in
7527-511: Is the most profound climate change of the low latitudes during the last 100,000 years and stands out within the otherwise relatively climatically stable Holocene. It is part of the so-called Holocene climatic optimum and coincides with a global warm phase, the Holocene Thermal Maximum . Liu et al. 2017 subdivided the humid period into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward, with
7720-571: Is unclear. The Dufuna canoe , one of the oldest known ships in the world, appears to date to the Holocene humid period and implies that the waterbodies of that time were navigated by humans. The cultural units "Masara" and "Bashendi" existed in Dakhleh Oasis during the AHP. In the Acacus Mountains, several cultural horizons known as Early and Late Acacus and Early, Middle, Late and Final Pastoral have been identified while in Niger
7913-565: Is usually needed to determine the size of the offset, for example by comparing the radiocarbon age of deposited freshwater shells with associated organic material. Volcanic eruptions eject large amounts of carbon into the air. The carbon is of geological origin and has no detectable C , so the C / C ratio in the vicinity of the volcano is depressed relative to surrounding areas. Dormant volcanoes can also emit aged carbon. Plants that photosynthesize this carbon also have lower C / C ratios: for example, plants in
African humid period - Misplaced Pages Continue
8106-399: Is where C is generated, contains about 1.9% of the total carbon in the reservoirs, and the C it contains mixes in less than seven years. The ratio of C to C in the atmosphere is taken as the baseline for the other reservoirs: if another reservoir has a lower ratio of C to C , it indicates that the carbon is older and hence that either some of
8299-525: The C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly. The development of radiocarbon dating has had a profound impact on archaeology . In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across great distances. Histories of archaeology often refer to its impact as
8492-402: The C has decayed, or the reservoir is receiving carbon that is not at the atmospheric baseline. The ocean surface is an example: it contains 2.4% of the carbon in the exchange reservoir, but there is only about 95% as much C as would be expected if the ratio were the same as in the atmosphere. The time it takes for carbon from the atmosphere to mix with the surface ocean is only
8685-489: The C / C ratio in the atmosphere. This increase in C concentration almost exactly cancels out the decrease caused by the upwelling of water (containing old, and hence C -depleted, carbon) from the deep ocean, so that direct measurements of C radiation are similar to measurements for the rest of the biosphere. Correcting for isotopic fractionation, as is done for all radiocarbon dates to allow comparison between results from different parts of
8878-592: The Ice Age ) is the geological epoch that lasted from c. 2.58 million to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations . Before a change was finally confirmed in 2009 by the International Union of Geological Sciences , the cutoff of the Pleistocene and the preceding Pliocene was regarded as being 1.806 million years Before Present (BP). Publications from earlier years may use either definition of
9071-404: The δ C value for that sample directly than to rely on the published values. The carbon exchange between atmospheric CO 2 and carbonate at the ocean surface is also subject to fractionation, with C in the atmosphere more likely than C to dissolve in the ocean. The result is an overall increase in the C / C ratio in the ocean of 1.5%, relative to
9264-486: The Acacus Mountains . Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the Nile Valley and Mesopotamia , where they gave rise to early complex societies . Herodotus in 440 BC and Strabo in 23 AD discussed
9457-652: The Air Mountains . Ephemeral lakes developed between dunes, and a "freshwater archipelago" appears to have existed in the Murzuq basin. All these lake systems left fossils such as fish, limnic sediments and fertile soils that were later used for agriculture (El Deir, Kharga Oasis ). Finally, crater lakes formed in volcanic fields such as Trou au Natron and Era Kohor in the Tibesti, and sometimes survive to this day as smaller remnant lakes such as Malha crater in
9650-659: The Alpine ice sheet on the Alps . Scattered domes stretched across Siberia and the Arctic shelf. The northern seas were ice-covered. South of the ice sheets large lakes accumulated because outlets were blocked and the cooler air slowed evaporation. When the Laurentide Ice Sheet retreated, north-central North America was completely covered by Lake Agassiz . Over a hundred basins, now dry or nearly so, were overflowing in
9843-575: The Blue Nile . The White Nile flooded part of its valley and reconnected to the main Nile. In Egypt widespread flooding by the "Wild Nile" took place; this "Wild Nile" period led to the largest recorded floods on this river, sedimentation in floodplains, and probably also impacted human populations along the river. Even earlier, 17,000–16,800 years ago, meltwater from glaciers in Ethiopia – which were retreating at that time – may have begun to increase
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#173276534556210036-556: The Bodélé Depression and perhaps as much as 8% of the present-day Sahara desert. It influenced the climate itself; for example rainfall would have been reduced at the centre of the lake and increased at its margins. Lake Chad was possibly fed from the north by rivers draining the Hoggar (Taffassasset drainage) and Tibesti Mountains, from the Ennedi Mountains in the east through the "Eastern palaeorivers" and from
10229-609: The Congo Air Boundary or increased convergence along this boundary may have contributed; the Congo Air Boundary would have been shifted east by the stronger westerly winds directed by lower atmospheric pressure over Northern Africa, allowing additional moisture from the Atlantic to reach East Africa. The parts of East Africa that were isolated from Atlantic moisture did not become significantly wetter during
10422-579: The Holocene AHP or correlative climate phases are "Holocene humid period", which also covers an analogous episode in Arabia and Asia; "early to mid-Holocene humid episode"; "Holocene Pluvial"; "Holocene Wet Phase"; " Kibangien A " in Central Africa; "Makalian" for the Neolithic period of northern Sudan; "Nabtian Pluvial", "Nabtian Wet Phase" or "Nabtian period" for the 14,000–6,000 humid period over
10615-529: The ICS timescale, the Pleistocene is divided into four stages or ages , the Gelasian , Calabrian , Chibanian (previously the unofficial "Middle Pleistocene"), and Upper Pleistocene (unofficially the "Tarantian"). In addition to these international subdivisions, various regional subdivisions are often used. In 2009 the International Union of Geological Sciences (IUGS) confirmed a change in time period for
10808-604: The Isthmus of Panama , causing a faunal interchange between the two regions and changing ocean circulation patterns, with the onset of glaciation in the Northern Hemisphere occurring around 2.7 million years ago. During the Early Pleistocene (2.58–0.8 Ma), archaic humans of the genus Homo originated in Africa and spread throughout Afro-Eurasia . The end of the Early Pleistocene is marked by
11001-470: The Kiffian culture has been related to the beginning of the AHP. Ancient civilizations thrived, with farming and animal husbandry taking place in Neolithic settlements. Possibly, the domestication of plants in Africa was delayed by the increased food availability during the AHP, it only took place around 2,500 BC . Humans created rock art such as petroglyphs and rock paintings in the Sahara, perhaps
11194-677: The Laurentide Ice Sheet . 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 originally thought to be the youngest fossil rock layer. He constructed the name "Pleistocene" ('most new' or 'newest') from the Greek πλεῖστος ( pleīstos ) 'most' and καινός ( kainós ( Latinized as cænus ) 'new'). This contrasts with
11387-675: The Mayo Kebbi and the Benue River , eventually reaching the Gulf of Guinea . Older dune systems were submerged by Lake Chad. Among the large lakes which may have formed in the Sahara are Lake Megafezzan in Libya and Lake Ptolemy in Sudan. Quade et al. 2018 raised some doubts about the size and existence of some of these lakes such as Lake Ptolemy, Lake Megafezzan, Lake Ahnet-Mouydir ; it
11580-515: The Meidob volcanic field . Potentially, the increased availability of water during the AHP may have facilitated the onset of phreatomagmatic eruptions such as maar formation in the Bayuda volcanic field , although the chronology of volcanic eruptions there is not well known enough to substantiate a link to the AHP. Increased precipitation resulted in the formation or reactivation of river systems in
11773-615: The Mid-Pleistocene Transition , with the cyclicity of glacial cycles changing from 41,000-year cycles to asymmetric 100,000-year cycles, making the climate variation more extreme. The Late Pleistocene witnessed the spread of modern humans outside of Africa as well as the extinction of all other human species. Humans also spread to the Australian continent and the Americas for the first time, co-incident with
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#173276534556211966-505: The Nile crocodile and the fish Clarias gariepinus and Tilapia zillii . It is possible that the name Tassili n'Ajjer , which means "plateau of the rivers" in Berber , is a reference to past river flows. On the other hand, intense flows of these rivers may have made their shores dangerous to humans and thus created additional impetus for human movement. Now-dry river valleys from the AHP in
12159-527: The Nobel Prize in Chemistry for his work. Research has been ongoing since the 1960s to determine what the proportion of C in the atmosphere has been over the past 50,000 years. The resulting data, in the form of a calibration curve , is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age. Other corrections must be made to account for
12352-728: The North Atlantic and the Mediterranean Sea towards the Mediterranean coasts of Africa. There were complex interactions with the atmospheric circulation of the extratropics and between moisture coming from the Atlantic Ocean and the Indian Ocean , and an increased overlap between the areas wetted by the monsoon and those wetted by extratropical cyclones . Climate models indicate that changes from
12545-579: The Northern Hemisphere while decreasing it in the Southern Hemisphere . According to climate modelling , orbital changes by themselves cannot increase precipitation over Africa enough to explain the formation of the large desert lakes such as 330,000 square kilometres (130,000 sq mi) Lake Megachad , the climate proxies for precipitation, or the northward expansion of vegetation unless ocean and land surface changes are factored in. Decreasing albedo resulting from vegetation changes
12738-662: 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 . The Fenno-Scandian ice sheet rested on northern Europe , including much of Great Britain;
12931-563: The Ténéré desert a cemetery has been found, which has been used to reconstruct the lifestyle of these former inhabitants of the Sahara, and at Lake Ptolemy in Nubia humans settled close to the lake shore, using its resources and perhaps even engaging in leisure activities. At that time, many humans appear to have depended on water-bound resources, seeing as many of the tools left by the early humans are associated with fishery ; hence this culture
13124-409: The last glacial maximum , the latter is equivalent to the "Kanemian"; "Kanemian dry period" refers to a dry period between 20,000 and 13,000 years before present in the Lake Chad area. The African humid period took place in the late Pleistocene and early-middle Holocene , and saw increased precipitation in Northern and Western Africa due to a northward migration of the tropical rainbelt. The AHP
13317-477: The last glacial period ended about 10,000 years ago. Over 11 major glacial events have been identified, as well as many minor glacial events. A major glacial event is a general glacial excursion, termed 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
13510-940: The rain shadows of mountains and could have supported arid climate vegetation, explaining the presence of its pollen in sediment cores . In addition, north–south gradations in vegetation patterns have been reconstructed from charcoal and pollen data. Fossils record changes in the animal fauna of the Sahara. This fauna included antelopes , baboons , cane rats , catfish , clams , cormorants , crocodiles, elephants, frogs, gazelles , giraffes , hartebeest , hares , hippos , molluscs , Nile perches , pelicans , rhinoceroses , snake-eagles , snakes, tilapia , toads , turtles and many more animals, and in Egypt there were African buffaloes , spotted hyenas , warthogs , wildebeest and zebra . Additional birds include brown-necked raven , coot , common moorhen , crested grebe , glossy ibis , long-legged buzzard , rock dove , spur-winged goose and tufted duck . Large herds of animals lived in
13703-484: The woolly rhinoceros , various giraffids , such as the Sivatherium ; ground sloths , Irish elk , cave lions , cave bears , Gomphotheres , American lions , dire wolves , and short-faced bears , began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period. At the end of the last ice age, cold-blooded animals, smaller mammals like wood mice , migratory birds, and swifter animals like whitetail deer had replaced
13896-621: The "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age , and the beginning of the Neolithic and Bronze Age in different regions. In 1939, Martin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research. They synthesized C using
14089-543: The AHP although at one site in Somalia the seasonality of precipitation may or may not have decreased. Various contributing factors may have led to the increased humidity in East Africa, not all of which were necessarily operating simultaneously during the AHP. That the "African humid period" reached this part of Africa has been doubted. Finally, increased greenhouse gas concentrations may have been involved in directing
14282-496: The AHP has been described as a mosaic between various vegetation types of semi-desert and humid origin rather than a simple northward displacement of plant species, and some brown or yellow vegetation communities persisted. There was no southward displacement of Mediterranean plants during the Holocene and on the Tibesti Mountains cold temperatures may have restricted the expansion of tropical plants. Pollen data often show
14475-540: The AHP has been used as an experiment in the Paleoclimate Modelling Intercomparison Project . Most recently, the effects of the Sahara greening on other continents has drawn scientific attention. The concept of a Sahara significantly different than today, and the rich record it left, has driven the imagination of the public and scientists alike. While the precipitation changes since the last glacial cycle are well established,
14668-663: The AHP, but both their expansion and subsequent retreat were slower than that of lakes. The Niger River , which had been dammed by dunes during the LGM, formed a lake in the Timbuktu region that eventually overflowed and drained at some point during the AHP. In some parts of the Sahara ephemeral lakes formed such as at Abu Ballas , Bir Kiseiba , Bir Sahara , Bir Tarfawi and Nabta Playa in Egypt, which may relate to later Egyptian religions, or swamp -lakes such as at Adrar Bous close to
14861-475: The AHP. In Egypt, some rivers active during the AHP are now gravel ridges. In the Air , Hoggar and Tibesti Mountains, the so-called "Middle Terrace " was emplaced at this time. The rivers of the Sahara, lakes and their watersheds may have acted as pathways for the spread of humans and animals; the rivers were often connected to each other by alluvial fans . Proposed examples of animals that spread through rivers are
15054-569: The Early Pleistocene Gelasian . Early Pleistocene 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 and interglacials receive an odd number. The first major glacial was MIS2-4 at about 85–11 ka BP. The largest glacials were 2, 6, 12, and 16. The warmest interglacials were 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under
15247-409: The Earth's surface is cyclical: climate, ocean currents and other movements, wind currents, temperature, etc. The waveform response comes from the underlying cyclical motions of the planet, which eventually drag all the transients into harmony with them. The repeated glaciations of the Pleistocene were caused by the same factors. The Mid-Pleistocene Transition , approximately one million years ago, saw
15440-602: The Eastern Mediterranean and Levant ; " Neolithic pluvial "; "Neolithic Subpluvial"; "Neolithic wet phase"; " Nouakchottien " of the Western Sahara 6,500 – 4,000 years before present; "Subpluvial II" and " Tchadien " in the Central Sahara 14,000 – 7,500 years before present. The terms "Big Dry", " Léopoldvillien " and Ogolien [ fr ] have been applied to the dry period in
15633-674: The Fezzan in Libya , at Bilma, Dibella, Fachi and Gobero in the Ténéré , Seeterrassental in Niger and at "Eight Ridges", El Atrun, Lake Gureinat, Merga, "Ridge", Sidigh, at Wadi Mansurab, Selima and Oyo in Sudan. The lakes of Ounianga merged into two large lakes and overflowed, either above surface or underground. Mosaics of small lakes developed in some regions, such as the Grand Erg Occidental . Wetlands also expanded during
15826-480: The ITCZ in Arabia is also contentious. The African humid period that took place in East Africa appears to have been caused by different mechanisms. Among the proposed mechanisms are decreased seasonality of precipitation due to increased dry season precipitation, shortening of the dry season, increased precipitation and increased inflow of moisture from the Atlantic and Indian Oceans. The Atlantic moisture inflow
16019-773: The Jebel Marra and Tibesti Mountains between 15,000 and 14,000 years ago and the youngest stage of glaciation in the High Atlas mountains took place at the same time as the Younger Dryas and early African humid period. Around 14,500 years ago, lakes started to appear in the arid areas. The humid period began about 15,000–14,500 years ago. The onset of the humid period took place almost simultaneously over all of Northern and Tropical Africa, with impacts as far as Santo Antão on Cape Verde . In Arabia, wet conditions apparently took about two millennia to advance northward,
16212-686: The Mediterranean coast and the Nile Valley. The aridity during the Last Glacial Maximum appears to have been the consequence of the colder climate and larger polar ice sheets , which squeezed the monsoon belt to the equator and weakened the West African Monsoon. The atmospheric water cycle and the Walker and Hadley circulations were weaker as well. Exceptional dry phases are linked to Heinrich events when there are
16405-479: The North 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, had a more copious flow, and were braided . African lakes were fuller, apparently from decreased evaporation. Deserts, on the other hand, were drier and more extensive. Rainfall was lower because of the decreases in oceanic and other evaporation. It has been estimated that during
16598-434: The Pleistocene to 2.58 Ma, results in the inclusion of all the recent repeated glaciations within the Pleistocene. Radiocarbon dating is considered to be inaccurate beyond around 50,000 years ago. Marine isotope stages (MIS) derived from Oxygen isotopes are often used for giving approximate dates. Pleistocene non-marine sediments are found primarily in fluvial deposits , lakebeds, slope and loess deposits as well as in
16791-436: The Pleistocene's overall climate could be characterised 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, and 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
16984-503: The Pleistocene, changing the start date from 1.806 to 2.588 million years BP, and accepted the base of the Gelasian as the base of the Pleistocene, namely the base of the Monte San Nicola GSSP . The start date has now been rounded down to 2.580 million years BP. The IUGS has yet to approve a type section , Global Boundary Stratotype Section and Point (GSSP), for the upper Pleistocene/Holocene boundary ( i.e.
17177-805: The Pleistocene, the East Antarctic Ice Sheet thinned by at least 500 meters, and that thinning since the Last Glacial Maximum is less than 50 meters and probably started after ca 14 ka. During the 2.5 million years of the Pleistocene, numerous cold phases called glacials ( Quaternary ice age ), or significant advances of continental ice sheets, in Europe and North America, occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter interglacials which lasted about 10,000–15,000 years. The last cold episode of
17370-411: The Pleistocene, the plates upon which they sit probably having moved no more than 100 km (62 mi) relative to each other since the beginning of the period. In glacial periods, the sea level would drop by up to 120 m (390 ft) lower than today during peak glaciation, exposing large areas of the present continental shelf as dry land. According to Mark Lynas (through collected data),
17563-586: The Sahara and consisted of an open grass savannah with shrubs and trees, with a moist savanna vegetation getting established in the mountains. In general, the vegetation expanded northward to 27 – 30° northern latitude in West Africa with a Sahel boundary at about 23° north, as the Sahara was populated by plants that today often occur about 400–600 kilometres (250–370 mi) farther south. The northward movement of vegetation took some time and some plant species moved faster than others. Plants that perform C3 carbon fixation became more common. The fire regime of
17756-486: The Sahara during the AHP such as brachiaria , sorghum and urochloa were an additional source of food. Humans also domesticated cattle , goats and sheep . Cattle domestication may have occurred especially in the more environmentally variable Eastern Sahara, where the lack of lakes (cattle having high requirements of drinking water ) may however have limited the occurrence of cattle. Animal husbandry picked up in earnest around 7,000 years ago when domestic animals came to
17949-611: The Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event . The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel , Arabia and East Africa, the end of the period appears to have taken place in several steps, such as the 4.2-kiloyear event . The AHP led to
18142-447: The Sahara, and a population boom may be linked to this change in cultural practice; cattle and goats spread southwestwards from northeasternmost Africa from 8,000 years before present. Dairying has been demonstrated in some locations and cattle-husbandry is supported by the frequent depiction of cattle in rock paintings . The relative importance of hunter-gatherer practices and pastoralism, and whether people were sedentary or migratory,
18335-415: The Sahara. Some animals expanded over the whole desert, while others were limited to places with deep water. Earlier humid periods in the Sahara may have allowed species to cross the now-desert. A reduction in open grasslands at the beginning of the AHP may explain the decline of the populations of some mammals during and a population bottleneck in cheetahs at the start of the humid period, while leading to
18528-636: The Sahara. The large Tamanrasset River flowed from the Atlas Mountains and Hoggar westward towards the Atlantic and entered it in the Bay of Arguin in Mauritania . It once formed the 12th largest watershed in the world and left a submarine canyon and riverine sediments. Together with other rivers it formed estuaries and mangroves in the Bay of Arguin. Other rivers in the same area also formed submarine canyons, and sediment patterns in marine sediment cores and
18721-775: The Senegal River expanded its riverbed, breached dunes and re-entered the Atlantic Ocean. During the African humid period, lakes, rivers, wetlands and vegetation including grass and trees covered the Sahara and Sahel creating a "Green Sahara" with a land cover that has no modern analogues. Evidence includes pollen data, archaeological sites, evidence of faunal activity such as diatoms , mammals , ostracods , reptiles and snails , buried river valleys , organic-rich mats , mudstones , evaporites as well as travertines and tufas deposited in subaqueous environments. The vegetation cover then extended over almost all of
18914-399: The actual calendar date, both because it uses the wrong value for the half-life of C , and because no correction (calibration) has been applied for the historical variation of C in the atmosphere over time. Carbon is distributed throughout the atmosphere, the biosphere, and the oceans; these are referred to collectively as the carbon exchange reservoir, and each component
19107-706: The appearance of Homo sapiens about 300,000 years ago. Artifacts associated with modern human behavior are unambiguously attested starting 40,000–50,000 years ago. According to mitochondrial timing techniques, modern humans migrated from Africa after the Riss glaciation in the Middle Palaeolithic during the Eemian Stage , spreading all over the ice-free world during the late Pleistocene. A 2005 study posits that humans in this migration interbred with archaic human forms already outside of Africa by
19300-418: The appropriate correction for the location of their samples. The effect also applies to marine organisms such as shells, and marine mammals such as whales and seals, which have radiocarbon ages that appear to be hundreds of years old. The northern and southern hemispheres have atmospheric circulation systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between
19493-527: The articles for those names. Both marine and continental faunas were essentially modern but with many more large land mammals such as Mammoths , Mastodons , Diprotodons , Smilodons , tigers , lions , Aurochs , short-faced bears , giant sloths , species within Gigantopithecus and others. Isolated landmasses such as Australia , Madagascar , New Zealand and islands in the Pacific saw
19686-438: The atmosphere might be expected to decrease over thousands of years, but C is constantly being produced in the lower stratosphere and upper troposphere , primarily by galactic cosmic rays , and to a lesser degree by solar cosmic rays. These cosmic rays generate neutrons as they travel through the atmosphere which can strike nitrogen-14 ( N ) atoms and turn them into C . The following nuclear reaction
19879-547: The atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis . Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere . The ratio of C to C is approximately 1.25 parts of C to 10 parts of C . In addition, about 1% of the carbon atoms are of the stable isotope C . The equation for the radioactive decay of C is: 6 C → 7 N + e + ν e By emitting
20072-399: The atmosphere, which reached a maximum in about 1965 of almost double the amount present in the atmosphere prior to nuclear testing. Measurement of radiocarbon was originally done with beta-counting devices, which counted the amount of beta radiation emitted by decaying C atoms in a sample. More recently, accelerator mass spectrometry has become the method of choice; it counts all
20265-421: The atmospheric C / C ratio has not changed over time. Calculating radiocarbon ages also requires the value of the half-life for C . In Libby's 1949 paper he used a value of 5720 ± 47 years, based on research by Engelkemeir et al. This was remarkably close to the modern value, but shortly afterwards the accepted value was revised to 5568 ± 30 years, and this value was in use for more than
20458-474: The beginning of the AHP across Africa. The African humid period has been explained by increased insolation during Northern Hemisphere summer. Due to precession , the season at which Earth passes closest to the Sun on its elliptical orbit – the perihelion – changes, with maximum summer insolation occurring when this happens during Northern Hemisphere summer. Between 11,000 and 10,000 years ago, Earth passed through
20651-425: The biosphere, gives an apparent age of about 400 years for ocean surface water. Libby's original exchange reservoir hypothesis assumed that the C / C ratio in the exchange reservoir is constant all over the world, but it has since been discovered that there are several causes of variation in the ratio across the reservoir. The CO 2 in the atmosphere transfers to the ocean by dissolving in
20844-454: The calibration curve (IntCal) also reports past atmospheric C concentration using this conventional age, any conventional ages calibrated against the IntCal curve will produce a correct calibrated age. When a date is quoted, the reader should be aware that if it is an uncalibrated date (a term used for dates given in radiocarbon years) it may differ substantially from the best estimate of
21037-435: The carbon exchange reservoir means that a straightforward calculation of the age of a sample based on the amount of C it contains will often give an incorrect result. There are several other possible sources of error that need to be considered. The errors are of four general types: In the early years of using the technique, it was understood that it depended on the atmospheric C / C ratio having remained
21230-500: The carbon exchange reservoir, but because of the long delay in mixing with the deep ocean, the actual effect is a 3% reduction. A much larger effect comes from above-ground nuclear testing, which released large numbers of neutrons into the atmosphere, resulting in the creation of C . From about 1950 until 1963, when atmospheric nuclear testing was banned , it is estimated that several tonnes of C were created. If all this extra C had immediately been spread across
21423-470: The concept of a Green Sahara in the 1930s. Later in the 20th century, conclusive evidence of a past greener Sahara, the existence of lakes and higher Nile flow levels was increasingly reported and it was recognized that the Holocene featured a humid period in the Sahara. The idea that changes in Earth's orbit around the Sun influence the strength of the monsoons was already advanced in 1921, and while
21616-729: The continent. A similar wet episode took place in the tropical Americas, China, Asia, India , the Makran region, the Middle East and the Arabian Peninsula and appears to relate to the same orbital forcing as the AHP. An early Holocene monsoonal episode extended as far as the Mojave Desert in North America. In contrast, a drier episode is recorded from much of South America where Lake Titicaca , Lake Junin ,
21809-435: The date of the sample. Samples for dating need to be converted into a form suitable for measuring the C content; this can mean conversion to gaseous, liquid, or solid form, depending on the measurement technique to be used. Before this can be done, the sample must be treated to remove any contamination and any unwanted constituents. This includes removing visible contaminants, such as rootlets that may have penetrated
22002-705: The desert such as highlands like the Red Sea Hills were reached by the westerlies or weather systems associated with the subtropical jet stream —and thus received precipitation—is contentious. It is only clearly supported for the Maghreb in northwestern Africa and parts of northeastern Africa, though river flow/ terrace formation and lake development in the Tibesti and Jebel Marra mountains and residual Nile flow may be explained in this way. The highlands of Africa appear to have been less affected by drought during
22195-580: The discharge of the Amazon River and water availability in the Atacama were lower. The discharge of the Congo , Niger , Nile , Ntem , Rufiji , and Sanaga rivers increased. Runoff from Algeria , equatorial Africa, northeastern Africa and the western Sahara was also larger. Changes in the morphology of the river systems and their alluvial plains occurred in response to the increased discharge, and
22388-494: The early 20th century hence gives an apparent date older than the true date. For the same reason, C concentrations in the neighbourhood of large cities are lower than the atmospheric average. This fossil fuel effect (also known as the Suess effect, after Hans Suess, who first reported it in 1955) would only amount to a reduction of 0.2% in C activity if the additional carbon from fossil fuels were distributed throughout
22581-706: The east ( Nile Valley ). The human population in the Sahara rapidly increased during the AHP, interrupted by a brief decline between 7,600 and 6,700 years ago. Traces of human activity have been found in the Acacus Mountains where caves and rock shelters were used as basecamps for humans, such as the Uan Afuda cave and the Uan Tabu and Takarkori rock shelters. The first occupation in Takarkori took place between 10,000 and 9,000 years ago; about five millennia of human cultural evolution are recorded there. At Gobero in
22774-489: The eastern Sahara have been used as analogues for former river systems on Mars . Conditions and resources were ripe for first hunter-gatherers , fishermen and, later, pastoralists ; the exact chronology – when humans returned in the Sahara after the onset of the AHP – is disputed. They may have come either from the north ( Maghreb or Cyrenaica ) where the Capsian culture was located, the south ( Sub-Saharan Africa ), or
22967-401: The entire carbon exchange reservoir, it would have led to an increase in the C / C ratio of only a few per cent, but the immediate effect was to almost double the amount of C in the atmosphere, with the peak level occurring in 1964 for the northern hemisphere, and in 1966 for the southern hemisphere. The level has since dropped, as this bomb pulse or "bomb carbon" (as it
23160-618: 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 Zealand and Tasmania . The current decaying glaciers of Mount Kenya , Mount Kilimanjaro , and
23353-424: The equation above. The half-life of a radioactive isotope (usually denoted by t 1/2 ) is a more familiar concept than the mean-life, so although the equations above are expressed in terms of the mean-life, it is more usual to quote the value of C 's half-life than its mean-life. The currently accepted value for the half-life of C is 5,700 ± 30 years. This means that after 5,700 years, only half of
23546-413: The equator, and rainforests had retreated in favour of afromontane and savannah landscapes as temperatures, rainfall, and humidity decreased. There is little and often equivocal evidence of human activity in the Sahara or Arabia at that time, reflecting its drier nature; in the Acacus Mountains the last human presence was recorded 70,000–61,000 years ago and by the LGM humans had largely retreated to
23739-471: The errors caused by the variation over time in the C / C ratio. These curves are described in more detail below . Coal and oil began to be burned in large quantities during the 19th century. Both are sufficiently old that they contain little or no detectable C and, as a result, the CO 2 released substantially diluted the atmospheric C / C ratio. Dating an object from
23932-782: The evolution of large birds and even reptiles such as the Elephant bird , moa , Haast's eagle , Quinkana , Megalania and Meiolania . The severe climatic changes during the Ice Age had major impacts on the fauna and flora. With each advance of the ice, large areas of the continents became depopulated, and plants and animals retreating southwards in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food supply. A major extinction event of large mammals ( megafauna ), which included mammoths , mastodons , saber-toothed cats , glyptodons ,
24125-650: The existence of a greener Sahara, although their reports were at first questioned owing to their anecdotal nature. In 1850 the researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering petroglyphs in the Murzuq Desert , as did Ahmed Hassanein following his 1923 exploration of the Libyan Desert when he saw depictions of savanna animals at Gabal El Uweinat . Further discoveries of petroglyphs led desert explorer László Almásy to coin
24318-496: The expansion of the population of other animals such as Hubert's multimammate mouse and Natal multimammate mouse . A number of lakes formed or expanded in the Sahara and the Hoggar and Tibesti Mountains . The largest of them was Lake Chad which increased to at least ten times its present-day size to form Lake Megachad. This enlarged Lake Chad reached dimensions of 1,000 by 600 kilometres (620 mi × 370 mi) in north–south and east–west direction respectively, covering
24511-444: The extinction of most large-bodied animals in these regions. The aridification and cooling trends of the preceding Neogene were continued in the Pleistocene. The climate was strongly variable depending on the glacial cycle, with the sea levels being up to 120 metres (390 ft) lower than present at peak glaciation, allowing the connection of Asia and North America via Beringia and the covering of most of northern North America by
24704-506: The fertilizing effects of increased carbon dioxide concentrations in the atmosphere. Other sources of albedo changes: Warmer extratropics during summer may have drawn the Intertropical Convergence Zone (ITCZ) northward by about five or seven degrees latitude , resulting in precipitation changes. Sea surface temperatures off North Africa warmed under orbital effects and through weaker trade winds , leading to
24897-525: The flow of water and sediment in the Nile. In the East African Rift water levels in lakes began to rise by about 15,500/15,000-12,000 years ago; Lake Kivu began overflowing into Lake Tanganyika by about 10,500 years ago. About the same time that the AHP started, the cold glacial climate in Europe associated with Heinrich event 1 ended with climate changing as far as Australasia . A warming and retreat of sea ice around Antarctica coincides with
25090-468: The former being wetter than the latter. The African humid period was not the first such phase; evidence for about 230 older such "green Sahara"/wet periods exist going back perhaps to the first appearance of the Sahara 7–8 million years ago, for example during Marine Isotope Stage 5 a and c. Earlier humid periods appear to have been more intense than the AHP of the Holocene, including the exceptionally intense Eemian humid period. This humid period provided
25283-908: The former is much easier to measure, and the latter can be easily derived: the depletion of C relative to C is proportional to the difference in the atomic masses of the two isotopes, so the depletion for C is twice the depletion of C . The fractionation of C , known as δ C , is calculated as follows: δ C 13 = ( ( C 13 C 12 ) sample ( C 13 C 12 ) standard − 1 ) × 1000 {\displaystyle \delta {\ce {^{13}C}}=\left({\frac {\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{sample}}}{\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{standard}}}}-1\right)\times 1000} ‰ where
25476-414: 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. For most of the 20th century, only
25669-450: The historical terminology was established. 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 is 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
25862-445: The immediately preceding Pliocene ("newer", from πλείων ( pleíōn , "more") and kainós ) and the immediately subsequent Holocene ("wholly new" or "entirely new", from ὅλος ( hólos , "whole") and kainós ) epoch , which extends to the present time. The Pleistocene has been dated from 2.580 million (±0.005) to 11,700 years BP with the end date expressed in radiocarbon years as 10,000 carbon-14 years BP. It covers most of
26055-454: The initial C will remain; a quarter will remain after 11,400 years; an eighth after 17,100 years; and so on. The above calculations make several assumptions, such as that the level of C in the atmosphere has remained constant over time. In fact, the level of C in the atmosphere has varied significantly and as a result, the values provided by the equation above have to be corrected by using data from other sources. This
26248-404: The inner tree rings do not get their C replenished and instead only lose C through radioactive decay. Hence each ring preserves a record of the atmospheric C / C ratio of the year it grew in. Carbon-dating the wood from the tree rings themselves provides the check needed on the atmospheric C / C ratio: with a sample of known date, and a measurement of
26441-673: The laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer than had been previously thought. This was followed by a prediction by Serge A. Korff , then employed at the Franklin Institute in Philadelphia , that the interaction of thermal neutrons with N in the upper atmosphere would create C . It had previously been thought that C would be more likely to be created by deuterons interacting with C . At some time during World War II, Willard Libby , who
26634-622: The large amounts of material moved about by glaciers. Less common are cave deposits, travertines and volcanic deposits (lavas, ashes). Pleistocene marine deposits are found primarily in shallow marine basins mostly (but with important exceptions) in areas within a few tens of kilometres of the modern shoreline. In a few geologically active areas such as the Southern California coast, Pleistocene marine deposits may be found at elevations of several hundred metres. The modern continents were essentially at their present positions during
26827-775: The largest density of such creations in the world. Scenes include animals and everyday life such as swimming which supports the presence of past wetter climates. One well-known such petroglyph location is the Cave of Swimmers in the Gilf Kebir mountains of Egypt; other well known sites are the Gabal El Uweinat mountains also of Egypt, Arabia and the Tassili n'Ajjer in Algeria where rock paintings from this time have been discovered. Humans also left artifacts such as Fesselsteine and ceramics in what today are inhospitable deserts. North Africa together with East Asia
27020-519: The last glacial maximum. The end of the glacial drought occurred between 17,000 and 11,000 years ago, with an earlier beginning noted in the Acacus and Saharan mountains 26,500–22,500 and (possibly) 18,500 years ago, respectively. In southern and central Africa earlier starts 17,000 and 17,500 years ago, respectively, may be linked to Antarctic warming, while Lake Malawi appears to have been low until about 10,000 years ago. High lake levels occurred in
27213-399: The late Pleistocene, incorporating archaic human genetic material into the modern human gene pool. Radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating ) is a method for determining the age of an object containing organic material by using the properties of radiocarbon , a radioactive isotope of carbon . The method
27406-415: 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 9700 BCE (11,700 calendar years BP). 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 significantly different from previous interglacial intervals within the Pleistocene. In
27599-470: The lower Palaeolithic they disappeared, and the only hominin species found in fossilic records is Homo erectus for much of the Pleistocene. Acheulean lithics appear along with Homo erectus , some 1.8 million years ago, replacing the more primitive Oldowan industry used by A. garhi and by the earliest species of Homo . The Middle Paleolithic saw more varied speciation within Homo , including
27792-407: The magnitude and timing of the changes are unclear. Depending on how and where measurements and reconstructions are made, different beginning dates, ending dates, durations and precipitation levels have been determined for the African humid period. The amounts of precipitation reconstructed from paleoclimate records and simulated by climate modelling are often inconsistent with each other; in general,
27985-463: The main Nile may have been dammed by dunes. The Nile Delta was partially dry, with sandy plains extending between ephemeral channels and exposed seafloor, and it became a source of sand for ergs farther east. Other lakes across Africa, such as Lake Chad and Lake Tanganyika , also had shrunk during this time, and both the Niger River and Senegal River were stunted. Whether some parts of
28178-478: The megafauna and migrated north. Late Pleistocene bighorn sheep were more slender and had longer legs than their descendants today. Scientists believe that the change in predator fauna after the late Pleistocene extinctions resulted in a change of body shape as the species adapted for increased power rather than speed. The extinctions hardly affected Africa but were especially severe in North America where native horses and camels were wiped out. In July 2018,
28371-462: The monsoon is unclear. During summer, solar heating is stronger over the North African land than over the ocean, forming a low pressure area that draws moist air and precipitation in from the Atlantic Ocean. This effect was strengthened by the increased summer insolation, leading to a stronger monsoon that also reached farther north. The effects of these circulation changes reached as far as
28564-510: The neighbourhood of the Furnas caldera in the Azores were found to have apparent ages that ranged from 250 years to 3320 years. Any addition of carbon to a sample of a different age will cause the measured date to be inaccurate. Contamination with modern carbon causes a sample to appear to be younger than it really is: the effect is greater for older samples. If a sample that is 17,000 years old
28757-480: The northward expansion of the monsoon. There is not universal agreement on the effects of dust on precipitation in the Sahel, however, in part because the effects of dust on precipitation may be dependent on its size. In addition to raw precipitation changes, changes in precipitation seasonality such as the length of dry seasons need to be considered when assessing the effects of climate change on vegetation, as well as
28950-597: The occurrence of submarine landslides in the area have been related to the activity of these rivers. Rivers such as the Irharhar in Algeria , Libya and Tunisia and the Sahabi and Kufra rivers in Libya were active during this time although there is some doubt that they had perennial flow; they appear to have been more important in earlier humid periods. Small watersheds, wadis and rivers discharging into endorheic basins such as Wadi Tanezzuft also carried water during
29143-420: The ocean surface have the same C ratios as the water they live in, and as a result of the reduced C / C ratio, the radiocarbon age of marine life is typically about 400 years. Organisms on land are in closer equilibrium with the atmosphere and have the same C / C ratio as the atmosphere. These organisms contain about 1.3% of the carbon in the reservoir; sea organisms have
29336-407: The onset of the AHP in tropical southeastern Africa; there, orbital changes would be expected to lead to climate variations opposite to those in the Northern Hemisphere. The pattern of humidity changes in south-eastern Africa are complex. The African humid period extended over most of Africa: The Sahara and eastern, southeastern and equatorial Africa. In general, forests and woodlands expanded through
29529-642: The onset of the African humid period as well as to increased humidity in Arabia. Later, in the Blytt–Sernander sequence the humid period coincides with the Atlantic period . During the Last Glacial Maximum , the Sahara and Sahel had been extremely dry with even less precipitation than today as reflected by the extent of dune sheets and water levels in closed lakes . The Sahara was much larger, extending 500–800 kilometres (310–500 mi) farther south to about 12° northern latitude. Dunes were active much closer to
29722-635: The organism from which the sample was taken died), and N is the number of atoms left after time t . λ is a constant that depends on the particular isotope; for a given isotope it is equal to the reciprocal of the mean-life – i.e. the average or expected time a given atom will survive before undergoing radioactive decay. The mean-life, denoted by τ , of C is 8,267 years, so the equation above can be rewritten as: t = ln ( N 0 / N ) ⋅ 8267 years {\displaystyle t=\ln(N_{0}/N)\cdot {\text{8267 years}}} The sample
29915-523: The original description was partly inaccurate, later widespread evidence for such orbital controls on climate was found. At first it was believed that humid periods in Africa correlate with glacial stages (" pluvial hypothesis ") before radiocarbon dating became widespread. The development and existence of the African humid period has been investigated with archaeology , climate modelling and paleoproxies , with archaeological sites , dunes and deposits left by lakes, aeolian deposits and leaf wax in
30108-571: The pathways for early humans to cross Arabia and Northern Africa and which, together with later moist periods, has been linked to expansions of the Aterian populations and the speciation of insect species. Such humid periods are usually associated with interglacials , while glacial stages correlate to dry periods; they occur during precession minima, unless large ice sheets or insufficient greenhouse gas concentrations suppress their onset. The Bølling–Allerød warming appears to be synchronous with
30301-433: The perihelion at the time of summer solstice , increasing the amount of solar radiation by about 8%, resulting in the African monsoon becoming both stronger and reaching farther north. Between 15,000 and 5,000 years ago, summer insolation was at least 4% higher than today. The obliquity also decreased during the Holocene but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on
30494-478: The period. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology . The name is a combination of Ancient Greek πλεῖστος ( pleîstos ) 'most' and καινός ( kainós ; Latinized as cænus ) 'new'. At the end of the preceding Pliocene, the previously isolated North and South American continents were joined by
30687-440: The plants and freshwater organisms that live in it. This is known as the hard water effect because it is often associated with calcium ions, which are characteristic of hard water; other sources of carbon such as humus can produce similar results, and can also reduce the apparent age if they are of more recent origin than the sample. The effect varies greatly and there is no general offset that can be applied; additional research
30880-430: The plants. When the animal or plant dies, it stops exchanging carbon with its environment, and thereafter the amount of C it contains begins to decrease as the C undergoes radioactive decay . Measuring the proportion of C in a sample from a dead plant or animal, such as a piece of wood or a fragment of bone, provides information that can be used to calculate when the animal or plant died. The older
31073-413: The pre-existing Egyptian chronology nor the new radiocarbon dating method could be assumed to be accurate, but a third possibility was that the C / C ratio had changed over time. The question was resolved by the study of tree rings : comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years. (Since that time
31266-538: The preceding Last Glacial Maximum , the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, and its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1 , simultaneously to the Bølling–Allerød warming . Rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and
31459-400: The proportion of C in different types of organisms (fractionation), and the varying levels of C throughout the biosphere (reservoir effects). Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests performed in the 1950s and 1960s. Because the time it takes to convert biological materials to fossil fuels
31652-417: The ratio of O to O (two isotopes of oxygen ) by mass (measured by a mass spectrometer ) present in the calcite of oceanic core samples is used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in O , which is included in the tests of the microorganisms ( foraminifera ) contributing the calcite. A more recent version of
31845-405: The ratios in the atmosphere. This effect is known as isotopic fractionation. To determine the degree of fractionation that takes place in a given plant, the amounts of both C and C isotopes are measured, and the resulting C / C ratio is then compared to a standard ratio known as PDB. The C / C ratio is used instead of C / C because
32038-461: The recent period of repeated glaciations. The name Plio-Pleistocene has, in the past, been used to mean the last ice age. Formerly, the boundary between the two epochs was drawn at the time when the foraminiferal species Hyalinea baltica first appeared in the marine section at La Castella, Calabria, Italy. However, the revised definition of the Quaternary , by pushing back the start date of
32231-415: The same over the preceding few thousand years. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. Neither
32424-456: The same proportion of C as the atmosphere, or in the case of marine animals or plants, with the ocean. Once it dies, it ceases to acquire C , but the C within its biological material at that time will continue to decay, and so the ratio of C to C in its remains will gradually decrease. Because C decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since
32617-412: The sample since its burial. Alkali and acid washes can be used to remove humic acid and carbonate contamination, but care has to be taken to avoid removing the part of the sample that contains the carbon to be tested. Particularly for older samples, it may be useful to enrich the amount of C in the sample before testing. This can be done with a thermal diffusion column. The process takes about
32810-407: The sampling process makes use of modern glacial ice cores. Although less rich in O than seawater, the snow that fell on the glacier year by year nevertheless contained O and O in a ratio that depended on the mean annual temperature. Temperature and climate change are cyclical when plotted on a graph of temperature versus time. Temperature coordinates are given in the form of
33003-457: The sea and wetlands playing an important role. Pollen , lake deposits and former levels of lakes have been used to study the ecosystems of the African humid period, and charcoal and leaf impressions have been used to identify vegetation changes. Questions in AHP research are its beginning, cause, intensity, end, land feedbacks and fluctuations during the AHP. The time 6,000 years ago has received particular attention, especially since that period of
33196-496: The simulation of the Green Sahara is considered a problem for earth system models. There is more evidence of the late phase of the AHP than its beginning. Erosion of lake sediments and carbon reservoir effects make it difficult to date when they dried up. Vegetation changes by themselves do not necessarily indicate precipitation changes, as changes in seasonality, plant species composition and changes in land use also play
33389-649: The sole factor responsible for the variations in climate since they explain neither the long-term cooling trend over the Plio-Pleistocene nor the millennial variations in the Greenland Ice Cores known as Dansgaard-Oeschger events and Heinrich events. Milankovitch pacing seems to best explain glaciation events with periodicity of 100,000, 40,000, and 20,000 years. Such a pattern seems to fit the information on climate change found in oxygen isotope cores. In oxygen isotope ratio analysis, variations in
33582-599: The south by the Chari - Logone and Komadugu Rivers . The Chari River was the main tributary while the rivers draining the Tibesti formed alluvial fans /the Angamma river delta at their entry into northern Lake Chad. Skeletons of elephants, hippos and hominins have been found in the Angamma delta, which is the dominant shoreline feature of northern Lake Chad. The lake overflowed into the Niger River during highstand through
33775-500: The start of the African humid period, although the Antarctic Cold Reversal also falls into this time and may relate to a drought interval recorded in the Gulf of Guinea . The African humid period was caused by a stronger West African Monsoon directed by changes in solar irradiance and in albedo feedbacks. These led to increased moisture import from both the equatorial Atlantic into West Africa, as well as from
33968-421: The study of cyclical climate changes. The glacials in the following tables show historical usages, are a simplification of a much more complex cycle of variation in climate and terrain, and are generally no longer used. These names have been abandoned in favour of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognised since
34161-482: The subtropics. Obliquity and precession are responsible for two of the foremost Milankovich cycles and are responsible not only for the onset and cessation of ice ages but also for monsoon strength variations. Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, as the insolation changes are reversed; this observation is borne out by data from South America. The precession change increased seasonality in
34354-401: The surface water as carbonate and bicarbonate ions; at the same time the carbonate ions in the water are returning to the air as CO 2 . This exchange process brings C from the atmosphere into the surface waters of the ocean, but the C thus introduced takes a long time to percolate through the entire volume of the ocean. The deepest parts of the ocean mix very slowly with
34547-590: The surface waters, and as a result water from some deep ocean areas has an apparent radiocarbon age of several thousand years. Upwelling mixes this "old" water with the surface water, giving the surface water an apparent age of about several hundred years (after correcting for fractionation). This effect is not uniform – the average effect is about 400 years, but there are local deviations of several hundred years for areas that are geographically close to each other. These deviations can be accounted for in calibration, and users of software such as CALIB can provide as an input
34740-422: The surface waters, and the mixing is uneven. The main mechanism that brings deep water to the surface is upwelling, which is more common in regions closer to the equator. Upwelling is also influenced by factors such as the topography of the local ocean bottom and coastlines, the climate, and wind patterns. Overall, the mixing of deep and surface waters takes far longer than the mixing of atmospheric CO 2 with
34933-478: The tree-ring data series has been extended to 13,900 years.) In the 1960s, Hans Suess was able to use the tree-ring sequence to show that the dates derived from radiocarbon were consistent with the dates assigned by Egyptologists. This was possible because although annual plants, such as corn, have a C / C ratio that reflects the atmospheric ratio at the time they were growing, trees only add material to their outermost tree ring in any given year, while
35126-404: The two. The atmospheric C / C ratio is lower in the southern hemisphere, with an apparent additional age of about 40 years for radiocarbon results from the south as compared to the north. This is because the greater surface area of ocean in the southern hemisphere means that there is more carbon exchanged between the ocean and the atmosphere than in the north. Since the surface ocean
35319-648: The upper 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 (C2r) chronozone , isotopic stage 103. Above this point there are notable extinctions of the calcareous nannofossils : Discoaster pentaradiatus and Discoaster surculus . The Pleistocene covers
35512-434: The value of N (the number of atoms of C remaining in the sample), the carbon-dating equation allows the calculation of N 0 – the number of atoms of C in the sample at the time the tree ring was formed – and hence the C / C ratio in the atmosphere at that time. Equipped with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct
35705-439: The vegetation changed; in the desert the expansion of vegetation facilitated fire activity, while in the savanna the increased prevalence of woody vegetation reduced fire activity. Forests and plants from the humid tropics were concentrated around lakes, rivers and the Atlantic Ocean coast of Senegal ; waterbodies were also settled by aquatic and partially aquatic plants and the Senegalese coast by mangroves . The landscape during
35898-458: The vegetation. Reduced dust generation from a wetter Sahara, where major dust-generating regions were submerged by lakes, influences the climate by reducing the amount of light absorbed by dust. Decreased dust emissions also modify cloud properties, making them less reflective and more efficient at inducing precipitation. In climate models, reduced amounts of dust in the troposphere together with vegetation changes can often but not always explain
36091-421: The western and central sectors and the Libyan sand sea was still a desert although pure desert areas retreated or became arid / semiarid . An arid belt may have existed north of 22° latitude and towards the Nile Delta, or the vegetation and the African monsoon might have reached 28–31° northern latitude; in general conditions between 21° and 28° northern latitude are poorly known. Dry areas may have persisted in
36284-448: The ‰ sign indicates parts per thousand . Because the PDB standard contains an unusually high proportion of C , most measured δ C values are negative. For marine organisms, the details of the photosynthesis reactions are less well understood, and the δ C values for marine photosynthetic organisms are dependent on temperature. At higher temperatures, CO 2 has poor solubility in water, which means there
36477-543: Was a series of glacials and interglacials, stadials and interstadials, mirroring periodic climate changes. The main factor at work in climate cycling is now believed to be Milankovitch cycles . These are periodic variations in regional and planetary solar radiation reaching the Earth caused by several repeating changes in the Earth's motion. The effects of Milankovitch cycles were enhanced by various positive feedbacks related to increases in atmospheric carbon dioxide concentrations and Earth's albedo. Milankovitch cycles cannot be
36670-411: Was awarded the Nobel Prize in Chemistry for this work. In nature, carbon exists as three isotopes . Carbon-12 ( C ) and carbon-13 ( C ) are stable and nonradioactive; carbon-14 ( C ), also known as "radiocarbon", is radioactive. The half-life of C (the time it takes for half of a given amount of C to decay ) is about 5,730 years, so its concentration in
36863-429: Was developed in the late 1940s at the University of Chicago by Willard Libby , based on the constant creation of radiocarbon ( C ) in the Earth's atmosphere by the interaction of cosmic rays with atmospheric nitrogen . The resulting C combines with atmospheric oxygen to form radioactive carbon dioxide , which is incorporated into plants by photosynthesis ; animals then acquire C by eating
37056-431: Was in part triggered by a stronger West African and Indian monsoon, perhaps explaining why the effects of the AHP extended into the Southern Hemisphere. The behaviour of the easterly trade winds is unclear; increased moisture transport by easterly trade winds may have aided in the development of the AHP but alternatively a stronger Indian Monsoon that draws easterly winds away from East Africa may have occurred. Changes in
37249-728: Was then at Berkeley, learned of Korff's research and conceived the idea that it might be possible to use radiocarbon for dating. In 1945, Libby moved to the University of Chicago , where he began his work on radiocarbon dating. He published a paper in 1946 in which he proposed that the carbon in living matter might include C as well as non-radioactive carbon. Libby and several collaborators proceeded to experiment with methane collected from sewage works in Baltimore, and after isotopically enriching their samples they were able to demonstrate that they contained C . By contrast, methane created from petroleum showed no radiocarbon activity because of its age. The results were summarized in
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