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97-397: The Hangenberg Event can be recognized by its unique multi-phase sequence of sedimentary layers, representing a relatively short interval of time with extreme fluctuations in the climate, sea level, and diversity of life. The entire event had an estimated duration of 100,000 to several hundred thousand years, occupying the upper third of the 'Strunian' (latest Famennian), and a small portion of

194-463: A fact noted very early in the study of the extinction. One major Famennian group, the clymeniids , were already suffering smaller extinctions just prior to the event. Although clymeniids survived the extinction event itself, they became a dead clade walking and died out shortly after it. Ammonoid extinction rates were highest near the base of the Postclymenia evoluta zone, in the early part of

291-554: A flood-prone property to qualify for government-subsidized insurance, a local community must adopt an ordinance that protects the floodway and requires that new residential structures built in Special Flood Hazard Areas be elevated to at least the level of the 100-year flood. Commercial structures can be elevated or floodproofed to or above this level. In some areas without detailed study information, structures may be required to be elevated to at least two feet above

388-445: A floodplain. The quantity of sediments in a floodplain greatly exceeds the river load of sediments. Thus, floodplains are an important storage site for sediments during their transport from where they are generated to their ultimate depositional environment. When the rate at which the river is cutting downwards becomes great enough that overbank flows become infrequent, the river is said to have abandoned its floodplain. Portions of

485-496: A meter long remained, and it would take 40 million years before they started to increase in size again. Four-limbed vertebrates ( stegocephalians , a.k.a. " tetrapods " in the broad sense of the term) evidently survived, eventually leading to the earliest true amphibians , who gave rise to the fully terrestrial ( amniote ) sauropsids and synapsids in the Carboniferous. However, no known Famennian "tetrapod" persisted into

582-768: A minor marine regression (sea level fall). This may have been caused by a small glacial phase, but other evidence suggests a warm and wet climate at the time. The uppermost part of the Wocklum Limestone and the Drewer Sandstone occupy the LE spore zone. They also belong to the praesulcata conodont zone (named after Siphonodella/Eosiphonodella praesulcata ) and the DFZ7 foraminifera zone (characterized by Quasiendothyra kobeitusana ). The last pre-extinction ammonoid faunas are dominated by wocklumeriids, forming

679-656: A rapid increase in plant cover at the end of the Famennian is lacking. Chemical analysis of cores in the Bakken shale suggest that while it was being formed, successive eras of higher sea level corresponded with euxinic (high in toxic hydrogen sulfide and low in oxygen) water in the shallow ocean basin, which could kill animals in the ocean and near the shoreline. As oceans flooded terrestrial basins, water would have entered areas with high nutrient levels, leading to an algal bloom, removing oxygen and then creating hydrogen sulfide as

776-546: Is a problem in freshwater systems. Much of the phosphorus in freshwater systems comes from municipal wastewater treatment plants and agricultural runoff. Stream connectivity controls whether phosphorus cycling is mediated by floodplain sediments or by external processes. Under conditions of stream connectivity, phosphorus is better able to be cycled, and sediments and nutrients are more readily retained. Water in freshwater streams ends up in either short-term storage in plants or algae or long-term in sediments. Wet/dry cycling within

873-406: Is advantageous for the rapid colonization of large areas of the floodplain. This allows them to take advantage of shifting floodplain geometry. For example, floodplain trees are fast-growing and tolerant of root disturbance. Opportunists (such as birds) are attracted to the rich food supply provided by the flood pulse. Floodplain ecosystems have distinct biozones. In Europe, as one moves away from

970-401: Is an area of land adjacent to a river . Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high discharge . The soils usually consist of clays, silts , sands, and gravels deposited during floods. Because of regular flooding, floodplains frequently have high soil-fertility since nutrients are deposited with

1067-498: Is any area subject to inundation by a 100-year flood. A problem is that any alteration of the watershed upstream of the point in question can potentially affect the ability of the watershed to handle water, and thus potentially affects the levels of the periodic floods. A large shopping center and parking lot, for example, may raise the levels of 5-year, 100-year, and other floods, but the maps are rarely adjusted and are frequently rendered obsolete by subsequent development. In order for

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1164-460: Is described as vertical accretion , since the deposits build upwards. In undisturbed river systems, overbank flow is frequent, typically occurring every one to two years, regardless of climate or topography. Sedimentation rates for a three-day flood of the Meuse and Rhine Rivers in 1993 found average sedimentation rates in the floodplain of between 0.57 and 1.0 kg/m . Higher rates were found on

1261-641: Is just above this and was used as a base for the Carboniferous in the past. The GSSP for the Tournaisian is near the summit of La Serre hill, in the Lydiennes Formation of the commune of Cabrières , in the Montagne Noire (southern France). The GSSP is in a section on the southern side of the hill, in an 80 cm deep trench, about 125 m south of the summit, 2.5 km southwest of the village of Cabrières and 2.5 km north of

1358-565: Is largely a result of flood control, hydroelectric development (such as reservoirs), and conversion of floodplains to agriculture use. Transportation and waste disposal also have detrimental effects. The result is the fragmentation of these ecosystems, resulting in loss of populations and diversity and endangering the remaining fragments of the ecosystem. Flood control creates a sharper boundary between water and land than in undisturbed floodplains, reducing physical diversity. Floodplain forests protect waterways from erosion and pollution and reduce

1455-454: Is less clear-cut. Some neritic taxa expanded after the initial extinction pulse but died out at the end of the crisis with other members of the 'survivor' fauna. Crinoids survived relatively unscathed, and instead used the extinction as an opportunity to drastically increase their diversity and body size. The two remaining orders of trilobites , Phacopida and Proetida , were strongly affected. The order Phacopida completely died out during

1552-402: Is most common in sections of rivers where the river bed is accumulating sediments ( aggrading ). Repeated flooding eventually builds up an alluvial ridge, whose natural levees and abandoned meander loops may stand well above most of the floodplain. The alluvial ridge is topped by a channel belt formed by successive generations of channel migration and meander cutoff. At much longer intervals,

1649-729: Is now Europe, North America, and China, represent a low-latitude paleobiogeographical realm known as the Amerosinian realm. Divaricating (widely branching) trunks of Lepidodendropsis lycophytes are by far the most abundant and widespread plant fossils of the Tournaisian, yet there was some minor variation in other flora through time and space. In eastern North America, lyginopterids and probable progymnosperms were also common, as indicated by leaf form genera such as Adiantites , Rhodeopteridium , and Genselia . The progymnosperm leaf Triphyllopteris may be more common in Europe while

1746-549: Is poorly studied, but appears to have been significant as well. Bivalves were barely affected, even in anoxic deep-water environments. Bivalves in the family Naiaditidae apparently took advantage of the Famennian glaciation to expand from polar regions towards the equator, sparking diversification in the Carboniferous tropics. Brachiopod diversity was somewhat impacted by the event, with survival largely based on ecology. Deep-water rhynchonellids and chonetids completely died out, but extinction among neritic (shallow-water) taxa

1843-654: The Gattendorfia genozone, LC I-A2) and the MFZ1 foraminifera zone. Along with the Givetian and Frasnian stages, the Famennian was qualitatively acknowledged as having elevated extinction rates as early as Raup and Sepkoski 's 1982 landmark paper on mass extinctions. However, late Famennian extinction rates were typically considered to be of lesser taxonomic severity than those in the Kellwasser Event, one of

1940-467: The Postclymenia zone (UD VI-E), an ammonoid genozone based on massive extinctions within the group, rather than new occurrences. This is also the case for the costatus – kockeli Interregnum ( ck I) conodont zone. Foraminifera disappear from the fossil record during the black shale interval. Uranium-Lead dating of ash beds in Poland provide dates of 358.97 ± 0.11 Ma and 358.89 ± 0.20 Ma below and above

2037-803: The Mississippian , the oldest subsystem of the Carboniferous . The Tournaisian age lasted from 358.9 Ma to 346.7 Ma . It is preceded by the Famennian (the uppermost stage of the Devonian ) and is followed by the Viséan . In global stratigraphy, the Tournaisian contains two substages: the Hastarian (lower Tournaisian) and Ivorian (upper Tournaisian). These two substages were originally designated as European regional stages. The Tournaisian

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2134-602: The Wocklumeria genozone (also known as the UD VI-D zone). A very short subzone (UD VI-D2) diagnosed by Epiwocklumeria occurs in the first few layers of the lower crisis interval. The main marine extinction pulse begins abruptly with the subsequent deposition of the Hangenberg Black Shale , a layer of organic material deposited in anoxic deep-water environments. This is correlated with the beginning of

2231-557: The Yellow River in China – see list of deadliest floods . The worst of these, and the worst natural disaster (excluding famine and epidemics), was the 1931 China floods , estimated to have killed millions. This had been preceded by the 1887 Yellow River flood , which killed around one million people and is the second-worst natural disaster in history. The extent of floodplain inundation depends partly on flood magnitude, defined by

2328-414: The miospore index fossil Waltzispora lanzonii . The floral diversity of Tournaisian southern tundra consists almost entirely of relict Devonian genera; this suggests that Late Devonian land plant extinctions in lower latitudes were mostly driven by competition from new tropical species, rather than global environmental pressures. Trilobites experienced their final substantial diversification event in

2425-747: The return period . In the United States, the Federal Emergency Management Agency (FEMA) manages the National Flood Insurance Program (NFIP). The NFIP offers insurance to properties located within a flood-prone area, as defined by the Flood Insurance Rate Map (FIRM), which depicts various flood risks for a community. The FIRM typically focuses on the delineation of the 100-year flood inundation area, also known within

2522-611: The "big five" mass extinctions. Depending on the method used, the Hangenberg Event typically falls between the fifth and tenth deadliest post- Cambrian mass extinctions, in terms of marine genera lost. Most estimates of proportional extinction have low resolution, only as fine as the stages in which the extinctions occur. This can lead to uncertainty in differentiating between the Hangenberg Event and other Famennian extinctions in broad-scale extinction trackers. Benton (1995) estimated that 20–23.7% of all families went extinct in

2619-528: The Angaran realm. The most common plant fossils in this region were shrub-sized lycophytes such as Ursodendron and Tomiodendron , shorter than their arborescent tropical relatives. Gondwanan plant fossils are uncommon: southernmost Gondwana was covered by dwarf lycophytes, even smaller than those of the Angaran realm. Subtropical and temperate lycophytes such as Lepidodendropsis , Archaeosigillaria , and Frenguellia could be found in some parts of

2716-574: The Carboniferous, with " ichthyostegalian "- grade stegocephalians such as Ichthyostega and Acanthostega disappearing from the fossil record. A distinct gap in time traditionally separated the Famennian "tetrapod" faunas from their successors in the Early Carboniferous. This fossil hiatus, known as " Romer's Gap ", has been linked to the Hangenberg Event. However, recent and continued discovery of many Visean and Tournaisian "tetrapods" has helped to close in this gap, suggesting that

2813-764: The Chinese Tangbagouan regional stage. In the British Isles, where the Hastarian and Ivorian are difficult to distinguish, the entire Tournaisian was equivalent to the Courceyan regional stage. The base of the Tournaisian (which is also the base of the Carboniferous system ) is at the first appearance of the conodont Siphonodella sulcata within the evolutionary lineage from Siphonodella praesulcata to Siphonodella sulcata . The first appearance of ammonite species Gattendorfia subinvoluta

2910-634: The D–C boundary. Nevertheless, the general trend was sea level rise, with the melting of the glaciers which formed in the middle crisis interval. In the early Tournaisian, the crisis finally ends at the base of the Hangenberg Limestone , a fossiliferous limestone superficially similar to the pre-crisis Wocklum Limestone. The base of the Hangenberg Limestone is characterized by the first occurrence of gattendorfiine ammonoids (making up

3007-404: The Early Carboniferous. These survivors were generally small and fast-breeding, resulting in a decrease in average vertebrate body size across the extinction. Even so, few Devonian chondrichthyan and actinopterygian species survived into the Carboniferous, indicating that these groups also experienced extinctions. Sharks that survived the extinction were greatly reduced in size; only sharks less than

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3104-546: The Famennian, lowered to ~28% considering only "multiple interval" genera which appeared prior to the stage. The Famennian extinction(s) would be the eighth worst mass extinction by the latter metric. He also found that the percentage loss of "well-preserved" (hard tissue) marine genera in the last substage of the Famennian was around 21%, nearly as large as the rate in the last substage of the Frasnian. The Famennian-wide extinction rate for "multiple interval" marine animal families

3201-466: The Famennian, with marine families at a proportion of 1.2–20.4%. About 27.4–28.6% of continental families appear to have died out, but the early and low-diversity nature of Devonian continental life makes this estimate very imprecise. Sepkoski (1996) plotted extinction rates for marine animal genera and families throughout the Phanerozoic . This study found that >45% of genera were lost during

3298-399: The Givetian crisis was ranked as the eighth. Even in areas with oxygenated seabeds, such as parts of Morocco, the ecospace of Hangenberg event communities was restricted to only a few ecological groups, particularly slow-moving pelagic predators (i.e. ammonoids and acanthodians ) and epifaunal sessile filter feeders (i.e. bivalves and bryozoans ). Reef ecosystems disappeared from

3395-462: The Hangenberg Event affected some vertebrates less severely than previously thought. Coprolitic evidence from early Tournaisian deposits in eastern Greenland has also supported the notion that tetrapods were not as affected by the Hangenberg Event as previously thought. During the Famennian, the world was covered by a fairly homogenous and low-diversity land plant flora, dominated by giant Archaeopteris trees. The palynomorph Retispora lepidophyta

3492-562: The Hangenberg Event was the joint seventh-worst post-Cambrian mass extinction, tied with the poorly-studied early Serpukhovian extinction in the Carboniferous. McGhee et al . 2013 attempted to tackle extinction rates via a new resampling protocol designed to counter biases in biodiversity estimates, such as the Signor–Lipps effect and Pull of the Recent . They found a significantly higher extinction rate, with 50% of marine genera lost during

3589-470: The Hangenberg Event. Conodonts were moderately affected by the event, with different regions varying in the number of species lost. Pelagic conodonts had a total species extinction rate of about 40%, with some areas have a local rate as high as 72%. About 50% of neritic conodont species died out, with survivors characterized by their wide distribution and versatile ecology. Species diversity rebounded soon afterwards, returning close to pre-extinction levels by

3686-535: The Hangenberg event than by the Kellwasser event. The Hangenberg event was an anoxic event marked by a layer of black shale , and has been suggested to have been linked to an increase in terrestrial plant cover. That would have led to increased nutrient supply in rivers and may have led to eutrophication of semi-restricted epicontinental seas and could have stimulated algal blooms . However, support for

3783-552: The Hangenberg event. An asteroid impact has been suggested as a possible cause of the Hangenberg event. However, most impact craters, such as the Hangenberg-aged Woodleigh Crater , cannot be dated precisely enough to determine any causal relationship between the impact and the extinction event. Tournaisian The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of

3880-574: The Kellwasser Event, and all remaining subgroups ( arthrodires , antiarchs , phyllolepids , and ptychodontids ) died out abruptly at the end of the Devonian. Sarcopterygians (lobe-finned fish) were also strongly affected: onychodontidans , porolepiforms , tristichopterids , and most other " osteolepidids " went extinct. Some large fish, namely rhizodonts , megalichthyids , and a few acanthodians , survived but failed to significantly increase their ecological disparity, eventually dying out later in

3977-531: The LE and/or LN spore zones, which are difficult to distinguish outside of Europe. Less well-constrained glacial deposits have also been found in Peru , Libya , South Africa , and central Africa . The Late Famennian glacial phase , along with other short glacial phases in the Tournaisian and Visean , acted as a prelude to the far larger and more prolonged Late Paleozoic Ice Age which stretched across much of

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4074-492: The LN spore zone, indicated by the first occurrence of Verrucosisporites nitidus . However, in some areas the boundary between the LE and LN zones is unclear and possibly based on geography more than chronology. The black shale was deposited during a large marine transgression (sea level rise), as indicated by flooding reducing the input of terrestrial spores and increasing eutrophication . The Hangenberg Black Shale corresponds to

4171-721: The Late Carboniferous and Early Permian. The upper crisis interval begins with the return of prominent carbonate rocks: a marly unit, the Stockum Limestone , spans the Devonian–Carboniferous (D–C) boundary. Foraminifera reappear in the fossil record within the Stockum Limestone, forming the DFZ8 zone characterized by Tournayellina pseudobeata . The base of the Stockum Limestone also sees

4268-560: The NFIP as the Special Flood Hazard Area. Where a detailed study of a waterway has been done, the 100-year floodplain will also include the floodway, the critical portion of the floodplain which includes the stream channel and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters. Another commonly encountered term is the Special Flood Hazard Area, which

4365-458: The Paleozoic. Dipnoans (lungfish) persisted through the extinction more easily than other sarcopterygians, though they were apparently extirpated from marine environments. Among the most major ecological changes associated with the extinction are the rise of chondrichthyans ( sharks and kin) and actinopterygians (ray-finned fish), which took over in diversity and relative abundance during

4462-451: The Tournaisian. Bryozoans maintained high rates of both speciation and extinction during the late Famennian, with only a small drop in overall richness. The Hangenberg event effectively "refreshed" bryozoan diversity, eliminating old clades and allowing new forms to radiate and eventually reach a diversity peak in the Visean stage. Ammonoids were nearly wiped out by the Hangenberg Event,

4559-520: The Viséan, and river systems of the Tournaisian were more similar to those of the Late Devonian . The Tournaisian saw a new diversification of arborescent (tree-sized) lycophytes and giant sphenophytes (horsetails). They coexisted alongside ferns and lignophytes ( wood -bearing plants), including early seed plants such as lyginopteridalean pteridosperms ("seed ferns"). The Tournaisian

4656-400: The abandoned floodplain may be preserved as fluvial terraces . Floodplains support diverse and productive ecosystems . They are characterized by considerable variability in space and time, which in turn produces some of the most species-rich of ecosystems. From the ecological perspective, the most distinctive aspect of floodplains is the flood pulse associated with annual floods, and so

4753-511: The advantages provided by the richness of the alluvial soil of the floodplain are severely offset by frequent floods brought on by cyclones and annual monsoon rains. These extreme weather events cause severe economic disruption and loss of human life in the densely-populated region. Floodplain soil composition is unique and varies widely based on microtopography. Floodplain forests have high topographic heterogeneity which creates variation in localized hydrologic conditions. Soil moisture within

4850-508: The algae decayed. Evidence such as glacial deposits in northern Brazil (near the Devonian South Pole) suggests widespread glaciation at the end of the Devonian, as a broad continental mass covered the polar region. The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall, and thus a cause of the extinctions may have been an episode of severe global cooling and glaciation at

4947-756: The atmosphere, reacting to inorganic chlorine compounds and producing ClO , an ozone-depleting compound. However, this mechanism has been criticized for its slow and weak effect on ozone concentrations, as well as its suspect rejection of volcanic influences. Alternatively, cosmic rays from a nearby supernova would be capable of a similar degree of ozone depletion. The impact of a nearby supernova can be supported or refuted by testing for trace amounts of Plutonium-244 in fossils, but these tests have yet to be published. Ozone depletion could just as easily be explained by an increase in greenhouse gas concentrations resulting from an intense period of arc volcanism . The spore malformations may not even be related to UV radiation in

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5044-560: The beginning of the Protognathodus kockeli conodont zone and further ammonoid diversification within the upper Acutimitoceras (Stockumites) genozone (LC I-A1). A major extinction among land plants and palynomorphs indicates the beginning of the VI spore zone shortly before the D–C boundary. 'Survivor' faunas of marine invertebrates, such as the last cymaclymeniid ammonoids and phacopid trilobites, also die out at this time, making it

5141-501: The black shale. This constrains the main marine extinction pulse to a duration of 50,000 to 190,000 years. In the middle crisis interval, the black shale grades into a thicker deposit of more oxygenated shallow-water sediment. It may be represented by shale ( Hangenberg Shale ) or sandstone ( Hangenberg Sandstone ), and fossils are still rare. These layers are still within the ck I conodont zone and LN spore zone, and foraminifera are still absent. However, ammonoid fossils switch over to

5238-467: The channel shifts varies greatly, with reported rates ranging from too slow to measure to as much as 2,400 feet (730 m) per year for the Kosi River of India. Overbank flow takes place when the river is flooded with more water than can be accommodated by the river channel. Flow over the banks of the river deposits a thin veneer of sediments that is coarsest and thickest close to the channel. This

5335-436: The crisis. Plankton suffered severe losses. Acritarchs declined strongly in the late Famennian and were very rare in the Tournaisian. Foraminifera also experienced very high extinction rates which devastated their formerly high diversity. Surviving forms were low in diversity and small in size, an example of the ' Lilliput effect ' often seen after mass extinctions. The flask-shaped chitinozoans completely died out during

5432-517: The crisis. 75% of remaining families, 86% of genera, and 87% of species died out at this time. A few cymaclymeniids (including Postclymenia ) briefly expanded into a cosmopolitan 'survivor' fauna, but ultimately died out at the end of the crisis. Only one ammonoid family, the Prionoceratidae , survived the full extinction interval and went on to rediversify into later goniatite groups. Extinction in non-ammonoid nautiloids and gastropods

5529-729: The early Tournaisian . It is named after the Hangenberg Black Shale, a distinctive layer of anoxic sediment originally found along the northern edge of the Rhenish Massif in Germany . This layer and its surrounding geological units define the "classic" Rhenish succession, one of the most well-studied geological examples of the extinction. Sequences equivalent to the Rhenish succession have been found at over 30 other sites on every continent except Antarctica , confirming

5626-577: The end of the Famennian, marking the dawn of the Late Palaeozoic Ice Age . One hypothesis for the cause of the last pulse of the extinction notes the abundance of malformed plant spores at the Devonian-Carboniferous boundary. This could implicate increased UV-B radiation and ozone depletion as the kill mechanism, at least for terrestrial organisms. Intense warming may lead to increased convection of water vapor in

5723-436: The end-Famennian, these were chitinozoans within the pelagic filter-feeder megaguild, and stromatoporoids within the attached epifaunal (seabed-living) filter-feeder megaguild. Other taxa impacted by the extinction rediversified or their niches were filled rather quickly, but these communities were exceptions. By comparison, the end-Frasnian extinction was ranked as the fourth most ecologically-severe mass extinction, and

5820-593: The event. Deep-water phacopids were eradicated at the start of the crisis, while widespread shallow-water phacopids went extinct slightly later, alongside the cymaclymeniid ammonoids. Proetids were also hit hard, but several families in the group survived and rediversified quickly in the Tournaisian. Ostracods experienced notable faunal turnover, with groups such as leperditicopids dying out. At least 50% of pelagic ostracod species went extinct, with some areas having extinction rates up to 66%. Shallow-water species were less affected, with newer taxa replacing older ones late in

5917-497: The event. This estimate would rank the end-Famennian extinction as the fourth-deadliest mass extinction, ahead of the end-Frasnian extinction. They also ranked the end-Famennian mass extinction as the seventh most ecologically severe extinction, tied with the Hirnantian (end-Ordovician) mass extinction . This was justified by the fact that two whole communities within an ecological megaguild went extinct with no replacements. For

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6014-579: The first place, and could simply be a result of volcanism-related environmental pressures such as acid rain . Evidence of coronene and mercury spikes occurring in the Tien Shan Mountains of southern Uzbekistan near the Devonian–Carboniferous boundary has led some researchers to hypothesise a volcanic cause for the Hangenberg event. The activities of the Kola and Timan-Pechora magmatic provinces have been proposed as other hypothetical causes for

6111-472: The flood waters. This can encourage farming ; some important agricultural regions, such as the Nile and Mississippi river basins , heavily exploit floodplains. Agricultural and urban regions have developed near or on floodplains to take advantage of the rich soil and freshwater. However, the risk of inundation has led to increasing efforts to control flooding . Most floodplains are formed by deposition on

6208-448: The floodplain ecosystem is defined as the part of the river valley that is regularly flooded and dried. Floods bring in detrital material rich in nutrients and release nutrients from dry soil as it is flooded. The decomposition of terrestrial plants submerged by the floodwaters adds to the nutrient supply. The flooded littoral zone of the river (the zone closest to the river bank) provides an ideal environment for many aquatic species, so

6305-462: The floodplain has a big impact on phosphorus availability because it alters water level, redox state, pH, and physical properties of minerals. Dry soils that were previously inundated have reduced availability of phosphorus and increased affinity for obtaining phosphorus. Human floodplain alterations also impact the phosphorus cycle. Particulate phosphorus and soluble reactive phosphorus (SRP) can contribute to algal blooms and toxicity in waterways when

6402-599: The floodplain. Other smaller-scale mitigation efforts include acquiring and demolishing flood-prone buildings or flood-proofing them. In some floodplains, such as the Inner Niger Delta of Mali , annual flooding events are a natural part of the local ecology and rural economy , allowing for the raising of crops through recessional agriculture . However, in Bangladesh , which occupies the Ganges Delta ,

6499-517: The fossil record during the Hangenberg Event, not returning until the late Tournaisian. Metazoan ( coral and sponge ) reefs had already been devastated by the Frasnian–Famennian event, and were still recovering during the Famennian. The end of the Famennian not only eliminated the metazoan reef community, but also many calcimicrobial reefs which were previously unscathed. Nevertheless, in the absence of pressures from metazoan communities, there

6596-403: The global nature of the Hangenberg Event. Below the Hangenberg Event strata is the Wocklum Limestone, a pelagic unit rich in fossils (especially ammonoids). In some places the Wocklum Limestone grades into the Drewer Sandstone, a thin turbidite deposit which initiates the lower crisis interval. Increased erosion and siliciclastic input indicates that the Drewer Sandstone was deposited during

6693-454: The hamlet of Fontès . The top of the Tournaisian (the base of the Viséan) is at the first appearance of the fusulinid species Eoparastaffella simplex ( morphotype 1/morphotype 2). The Tournaisian contains eight conodont biozones : The Tournaisian coincides with Romer's gap , a period of remarkably few terrestrial fossils, thus constituting a discontinuity between the Devonian and

6790-465: The impact of floodwaters. The disturbance by humans of temperate floodplain ecosystems frustrates attempts to understand their natural behavior. Tropical rivers are less impacted by humans and provide models for temperate floodplain ecosystems, which are thought to share many of their ecological attributes. Excluding famines and epidemics , some of the worst natural disasters in history (measured by fatalities) have been river floods, particularly in

6887-462: The impact of the Kellwasser Event. Furthermore, because the vertebrate fossil record of the Famennian is sparse, many extinctions attributed to the Kellwasser event may have actually been caused by the Hangenberg event. Among vertebrates, 44% of high-level clades and over 96% of species were lost during the Hangenberg Event, which occurred globally and did not discriminate between freshwater and marine species. Placoderm diversity had already decreased in

6984-448: The inside of river meanders and by overbank flow. Wherever the river meanders, the flowing water erodes the river bank on the outside of the meander. At the same time, sediments are simultaneously deposited in a bar on the inside of the meander. This is described as lateral accretion since the deposition builds the point bar laterally into the river channel. Erosion on the outside of the meander usually closely balances deposition on

7081-417: The inside so that the channel shifts in the direction of the meander without changing significantly in width. The point bar is built up to a level very close to that of the river banks. Significant net erosion of sediments occurs only when the meander cuts into higher ground. The overall effect is that, as the river meanders, it creates a level flood plain composed mostly of point bar deposits. The rate at which

7178-457: The levees (4 kg/m or more) and on low-lying areas (1.6 kg/m ). Sedimentation from the overbank flow is concentrated on natural levees, crevasse splays , and in wetlands and shallow lakes of flood basins. Natural levees are ridges along river banks that form from rapid deposition from the overbank flow. Most of the suspended sand is deposited on the levees, leaving the silt and clay sediments to be deposited as floodplain mud further from

7275-524: The lower Acutimitoceras (Stockumites ) genozone (UD VI-F), indicating that post-Devonian ammonoids were beginning to diversify after the main extinction pulse. A major marine regression occurred during the middle crisis interval, as indicated by the increased amount of erosion and river-supplied siliciclastic material. Some areas even show deep incised valley fill deposits , where rivers have cut into their former floodplains . Strata in Morocco suggest that

7372-556: The lycophyte Sublepidodendron characterizes Tournaisian China. Late Devonian seed plants like Rhacopteris also persisted into the Tournaisian tropics. Lepidodendron , a massive arborescent lycophyte which would dominate coal forests through the rest of the Carboniferous, first appeared near the Tournaisian-Viséan boundary. Northern Asia ( Kazakhstan and Siberia ) was positioned within subtropical or temperate northern latitudes, and developed its own endemic floras,

7469-617: The makeup towards ash (49%) with maple increasing to 14% and oak decreasing to 25%. Semiarid floodplains have a much lower species diversity. Species are adapted to alternating drought and flood. Extreme drying can destroy the ability of the floodplain ecosystem to shift to a healthy wet phase when flooded. Floodplain forests constituted 1% of the landscape of Europe in the 1800s. Much of this has been cleared by human activity, though floodplain forests have been impacted less than other kinds of forests. This makes them important refugia for biodiversity. Human destruction of floodplain ecosystems

7566-734: The mid-late Tournaisian, briefly regaining a level of diversity not seen since the Middle Devonian . Almost all new species belonged to the recently-evolved family Phillipsiidae , while the few surviving Devonian-type trilobites declined. Most early Tournaisian trilobites were widespread deep-water species. By the late Tournaisian, they had recolonized shallower environments and divided into three different biogeographic zones corresponding to North America, Europe, and East Asia. 43°33′20″N 3°21′26″E  /  43.5556°N 3.3572°E  / 43.5556; 3.3572 Floodplain A floodplain or flood plain or bottomlands

7663-421: The middle of the Tournaisian. The Hangenberg Event has also been implicated in the final extinction of several agnathan (jawless fish) groups. Other vertebrates apparently experienced a major ecological turnover across the Devonian-Carboniferous boundary. The Hangenberg Event's impact on vertebrate evolution approaches "Big Five" events such as the end-Cretaceous and end-Permian extinctions, and far exceeds

7760-458: The more modern terrestrial ecosystems of the Carboniferous. The middle of the Tournaisian is marked by a southern glaciation event, of a slightly lesser extent than the glaciations which swept over Gondwana in the later Carboniferous and the very end of the Devonian. During the Tournaisian, South America was located at south polar latitudes and formed the westernmost part of the supercontinent Gondwana . The southwestern coastline of Gondwana

7857-762: The nitrogen-to-phosphorus ratios are altered farther upstream. In areas where the phosphorus load is primarily particulate phosphorus, like the Mississippi River, the most effective ways of removing phosphorus upstream are sedimentation, soil accretion, and burial. In basins where SRP is the primary form of phosphorus, biological uptake in floodplain forests is the best way of removing nutrients. Phosphorus can transform between SRP and particulate phosphorus depending on ambient conditions or processes like decomposition, biological uptake, redoximorphic release, and sedimentation and accretion. In either phosphorus form, floodplain forests are beneficial as phosphorus sinks, and

7954-580: The river may abandon the channel belt and build a new one at another position on the floodplain. This process is called avulsion and occurs at intervals of 10–1000 years. Historical avulsions leading to catastrophic flooding include the 1855 Yellow River flood and the 2008 Kosi River flood . Floodplains can form around rivers of any kind or size. Even relatively straight stretches of river are capable of producing floodplains. Mid-channel bars in braided rivers migrate downstream through processes resembling those in point bars of meandering rivers and can build up

8051-511: The river, the successive plant communities are bank vegetation (usually annuals); sedge and reeds; willow shrubs; willow-poplar forest; oak-ash forest; and broadleaf forest. Human disturbance creates wet meadows that replace much of the original ecosystem. The biozones reflect a soil moisture and oxygen gradient that in turn corresponds to a flooding frequency gradient. The primeval floodplain forests of Europe were dominated by oak (60%) elm (20%) and hornbeam (13%), but human disturbance has shifted

8148-418: The river. Levees are typically built up enough to be relatively well-drained compared with nearby wetlands, and levees in non-arid climates are often heavily vegetated. Crevasses are formed by breakout events from the main river channel. The river bank fails, and floodwaters scour a channel. Sediments from the crevasse spread out as delta -shaped deposits with numerous distributary channels. Crevasse formation

8245-525: The sea level fell by more than 100 meters (328 feet) during the middle crisis interval. This regression was caused by a cooling episode, and time-constrained glacial deposits have been found in Bolivia and Brazil (which would have been high- latitude areas), as well as the Appalachian Basin (which would have been a tropical alpine environment). These are known to have been deposited within

8342-507: The second largest extinction pulse of the Hangenberg Crisis. Conodont zones (usually characterized by Protognathodus kuehni or Siphonodella/Eosiphonodella sulcata ) define the D–C boundary, but difficulty in finding reliable and universal index taxa has complicated study of the boundary in many areas. The sea level fluctuated during the upper crisis interval, as several minor regressions and transgressions continued to occur around

8439-408: The spawning season for fish often coincides with the onset of flooding. Fish must grow quickly during the flood to survive the subsequent drop in water level. As the floodwaters recede, the littoral experiences blooms of microorganisms, while the banks of the river dry out and terrestrial plants germinate to stabilize the bank. The biota of floodplains has high annual growth and mortality rates, which

8536-622: The start of the VI zone, the last spore zone of the Devonian. This land plant extinction, which wiped out most or all of the Archaeopteris and R. lepidophyta floras, is correlated with the extinction of 'survivor' faunas in the latter part of the Hangenberg Event. Spore taxa that went extinct include specialized forms with divided spines (likely from an early form of lycopod ) as well as widespread tiny spores ( Retispora , Diducites , Rugospora) which were probably from fast-growing fern-like plants . Plants were significantly more affected by

8633-623: The supercontinent, such as Argentina and Australia . In the middle Tournaisian glaciation, species-poor frigid tundra developed in western Argentina. These south polar tundras hosted the oldest known seed plants in Gondwanan territories, which likely spread south across a land bridge once the Rheic Ocean closed between Laurussia and Gondwana. Tournaisian terrestrial sediments in South America are additionally characterized by

8730-459: The surrounding grade. Many State and local governments have, in addition, adopted floodplain construction regulations which are more restrictive than those mandated by the NFIP. The US government also sponsors flood hazard mitigation efforts to reduce flood impacts. California 's Hazard Mitigation Program is one funding source for mitigation projects. A number of whole towns such as English, Indiana , have been completely relocated to remove them from

8827-672: The upper 30 cm of the soil profile also varies widely based on microtopography which affects oxygen availability. Floodplain soil stays aerated for long stretches of time in between flooding events, but during flooding, saturated soil can become oxygen-depleted if it stands stagnant for long enough. More soil oxygen is available at higher elevations farther from the river. Floodplain forests generally experience alternating periods of aerobic and anaerobic soil microbe activity which affects fine root development and desiccation. Floodplains have high buffering capacity for phosphorus to prevent nutrient loss to river outputs. Phosphorus nutrient loading

8924-442: Was a brief resurgence of microbial carbonate in the early Tournaisian, a similar pattern to other mass extinctions. The last true stromatoporoid sponges, a major group of Devonian reef builders, completely died out in the Hangenberg Event. Conversely, tabulate corals were apparently not strongly impacted. Rugose corals , which were already fairly rare, experienced a large extinction and ecological turnover before rediversifying in

9021-479: Was a transitional stage for lignophyte evolution: Devonian progymnosperm taxa such as Archaeopteris had gone extinct, but new types of woody trees such as Pitus and Protopitys set the stage for even greater morphological diversity. There is still much debate over the proportion of spore-bearing (progymnosperm) to seed-bearing (spermatophyte) woody plants, but both were evidently major parts of Tournaisian ecosystems. Tropical and subtropical swamps, in what

9118-420: Was abundant in most spore zones used to define the terrestrial ecosystems of the Famennian. The major marine extinction pulse of the Hangenberg Event occurred at the boundary between the LE and LN zones, the third- and second-to-last spore zones of the Devonian, respectively. Plants were unaffected at this time. However, they started to decline near the end of the LN zone and the terrestrial ecosystem collapsed at

9215-470: Was around 16%. All of these estimates approached, but did not surpass, the end-Frasnian extinction, and the Givetian extinction(s) also surpassed the Famennian extinction(s) in the "multiple-interval" and "well-preserved" full-stage categories. Using an updated biodiversity database, Bambach (2006) estimated that a total of 31% of marine genera died out in the last substage of the Famennian. By this metric,

9312-403: Was bustling with distinctive cold-water brachiopod and bivalve faunas. Coal is less common in the Tournaisian than in the rest of the Carboniferous, and forests and swamps were at low-density despite some trees reaching heights of up to 40 meters (131 feet). Anabranching channels and anastomosing rivers (with permanent channels splitting around large vegetated islands) would not develop until

9409-484: Was named after the Belgian city of Tournai . It was introduced in scientific literature by Belgian geologist André Hubert Dumont in 1832. Like many Devonian and lower Carboniferous stages, the Tournaisian is a unit from West European regional stratigraphy that is now used in the official international time scale. The Tournaisian correlates with the regional North American Kinderhookian and lower Osagean stages and

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