Hangenberg event

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 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 the global nature of the Hangenberg Event. The Hangenberg Black Shale corresponds to 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 (ckI) conodont zone. Foraminifera disappear from the fossil record during the black shale interval. Glaciation – the middle crisis interval of Ohio, a valley fill deposit equivalent to the Hangenberg Sandstone.|left 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 ckI conodont zone and LN spore zone, and foraminifera are still absent. However, ammonoid fossils switch over to 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. 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 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 the Late Carboniferous and Early Permian. Aftershocks – the upper crisis interval 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 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 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 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 the Gattendorfia genozone, LC I-A2) and the MFZ1 foraminifera zone. == Extinction severity ==

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 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 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 the Famennian, 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. 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 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 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 the Givetian crisis was ranked as the eighth. == Impact on life ==

Reef builders Reef ecosystems disappeared from 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. 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 the Tournaisian. Other invertebrates Ammonoids were nearly wiped out by the Hangenberg Event, 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 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. 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 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. The two remaining orders of trilobites, Phacopida and Proetida, were strongly affected. The order Phacopida completely died out during 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 the crisis. 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. Dipnoans (lungfish) persisted through the extinction more easily than other sarcopterygians, though they were apparently extirpated from marine environments. Even so, few Devonian chondrichthyan and actinopterygian species survived into the Carboniferous, indicating that these groups also experienced extinctions. However, recent and continued discovery of many Visean and Tournaisian "tetrapods" has helped to close in this gap, suggesting that 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. Plants During the Famennian, the world was covered by a fairly homogeneous and low-diversity land plant flora, dominated by giant Archaeopteris trees. The palynomorph Retispora lepidophyta 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. Plants were significantly more affected by the Hangenberg event than by the Kellwasser event. ==Probable causes==

Anoxia The Hangenberg event was a global anoxic event (with oxygen deprivation in sea water), marked by a layer of black shale. The first seed plants evolved during the Famennian and would have been more capable of exploiting and weathering upland environments away from river edges. However, support for a rapid increase in plant cover at the end of the Famennian is lacking. Global cooling 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. and thus a cause of the extinctions may have been an episode of severe global cooling and glaciation at the end of the Famennian, Volcanism The Tian Shan Mountains of southern Uzbekistan record spikes in the concentration of coronene and mercury near the Devonian–Carboniferous boundary. This has led some researchers to hypothesize a volcanic cause for the Hangenberg event. == Other potential factors ==

Ozone depletion 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 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 could lead to 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 not yet been undertaken. 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 the first place, and could simply be a result of volcanism-related environmental pressures such as acid rain. ==See also==