The end-Devonian was a time of significant changes in the global climate and biosphere, including the biodiversity crisis known as the Hangenberg Event. This event occurred roughly. 13.5 million years after the Frasnian-Famennian Mass Extinction, and was linked with globally widespread deposition of the anoxic Hangenberg Black Shale. The Hangenberg Extinction (with 50% marine genera loss) significantly affected the pelagic realm, especially Ammonoids, Conodonts, many Vertebrates, and benthic reef biotas, such as Trilobites and Ostracods, and had an ecological impact similar to the end-Ordovician Mass Extinction. Moreover, a drastic reduction of Phytoplankton diversity is also observed at the Devonian/Carboniferous boundary. Deposition of the Hangenberg Black Shale was a short-term event that lasted between about 50 and 190 thousand years, while the extended crisis interval encompassed a time span of one to several hundred thousand years. The postulated factors responsible for this global event, such as high productivity and anoxia, a calcification crisis caused by ocean acidification, perturbation of the global carbon cycle, glacio-eustatic sea-level changes driven by orbital forcing, volcanic and hydrothermal activity, and evolution of Land Plants, are still vividly discussed. In fact, extensive volcanism has been implicated in all ‘Big Five’ mass extinctions and other biotic crises in the Phanerozoic, including the Hangenberg Crisis. As the main source of mercury in the geological past was volcanic and submarine hydrothermal activity, and mercury anomalies in the sedimentary record have recently been used as a proxy for volcanic activity in relation to global events and palaeoenvironmental perturbations, including for the Devonian/Carboniferous boundary from different palaeogeographical domains.
In a paper published in the journal Scientific Reports on 30 April 2020, Michał Rakociński, Leszek Marynowski, and Agnieszka Pisarzowska of the Faculty of Natural Sciences at the University of Silesia in Katowice, Jacek Bełdowski and Grzegorz Siedlewicz of the Institute of Oceanology of the Polish Academy of Sciences, Michał Zatoń, also of the Faculty of Natural Sciences at the University of Silesia in Katowice, Maria Cristina Perri and Claudia Spalletta of the Department of Biological, Geological and Environmental Sciences at the University of Bologna, and Hans Peter Schönlaub of the Commission for Geosciences of the Austrian Academy of Sciences, report very large, anomalous mercury spikes in two marine Devonian/Carboniferous successions of the Carnic Alps, supporting volcanism as the driving mechanism (ultimate cause) of the Hangenberg Event. Furthermore, They also detected methylmercury, a strong neurotoxin that bioaccumulates in the food chain, in sedimentary rocks for the first time. Thus, Rakociński et al. claim that volcanic-driven methylmercury poisoning in otherwise anoxic seas could be an another proximate (direct) kill mechanism of the end-Devonian Hangenberg extinction.
Rakociński et al. examined two successions of deep-water, pelagic sedimentary rocks, encompassing the uppermost Devonian and Devonian/Carboniferous boundary intervals: Kronhofgraben (Austria) and Plan di Zermula A (Italy) in the Carnic Alps. The Kronhofgraben and Plan di Zermula A sections consist of organic-rich Hangenberg Black Shale and micritic limestone.
Late Devonian (360 million years ago) palaeogeographic map. showing the studied localities and the location of prominent areas of Late Devonian magmatism and associated volcanism, as well as (Al) giant mercury deposits reactivated by Variscan magmatic and tectonic activity in Almadén (Spain). Rakociński et al. (2020).
The Kronhofgraben section in the central Carnic Alps of Austria is situated in a gorge of the Aßnitz Creek, about 7 km east of Plöckenpass and 1 km northwest of the Kronhof Törl pass at the Austrian–Italian border. The Devonian/Carboniferous boundary beds crop out in the eastern side of the Kronhofgraben gorge at an altitude of 1390 m. The Plan di Zermula A section in the southern Carnic Alps of Italy appears on the western slope of the Mount Zermula massif, along the road from Paularo to Stua di Ramaz. Grey limestones and black shales represent the studied interval in both sections. The Hangenberg Black Shale horizon is assigned to the upper part of the Bispathodus ultimus Conodont Biozone (equivallent to the Middle-Upper Siphonodella praesulcata zones) in Kronhofgraben (40 cm thick) and in Plan di Zermula A (15 cm thick) is underlain by Cephalopod limestones of the lower part of the Bispathodus ultimus Zone (equivallent to the Upper Apsotreta expansa- Lower Siphonodella praesulcata zones). The first carbonate bed above the Hangenberg Black Shale belongs to the Siphonodella sulcata Zone (equivallent to the Protognathodus kockeli Zone).
The Devonian/Carboniferous boundary in both sections is situated directly above the Hangenberg Black Shale. The Devonian/Carboniferous boundary may be somewhat problematic and needs redefinition (caused by problems with discrimination of Siphonodella sulcata from its supposed ancestor Siphonodella praesulcata). The new criterion for definition of the base of the Carboniferous System proposed by the Working Group on the boundary is: identification of the base of the Protognathodus kockeli Zone, beginning of radiation and top of major regression (top of Hangenberg Black Shale) and end of mass extinction. In the limestone overlying the Hangenberg Black Shale, Conodonts of the species Protognathodus kockeli were found in both sections. Therefore, the position of the Devonian/Carboniferous boundary did not changed in comparison to previous studies.
The Devonian/Carboniferous boundary successions in the Plan de Zermula A and Kronhofgraben were deposited in deeper palaeoenvironment. In the late Devonian, Carnic Alps represented the northern tips of Gondwana and belonged to the Gondwana-derived Bosnian–Noric Terrane accreted to the intra-Alpine Mediterranean terrane during the Carboniferous. The investigated rocks outcropped in the Carnic Alps reflected strong thermal alteration.
The Hangenberg Black Shale intervals in the sections investigated display extremely high mercury values, with maxima of 20216 and 9758 parts per billion in Kronhofgraben and Plan di Zermula, respectively. The Hangenberg Black Shale from the Plan di Zermula A section contains mercury anomalies that are roughly 13–100 times higher than the 100 parts per billion background, whereas in the Kronhofgraben section the anomalies are roughly 12–84 times higher than the background values.
Interestingly, significant concentrations of methylmercury were found in the whole Kronhofgraben section, where methylmercury is in the range 13–348 picograms per gram, dry weight. Additionally, we found 55 picograms per gram, dry weight of methylmercury in the Novchomok section in Uzbekistan and 72.72 picograms per gram, dry weight of methylmercury sampled from the uppermost Devonian part of the Woodford Shale from the Arbuckle Anticline in Oklahoma, USA. Traces of methylmercury were also found in the Hangenberg Black Shale interval at Kowala Quarry, Poland (20.66 picograms per gram, dry weight of methylmercury).
In comparison to methylmercury levels found in modern sediments (reaching from 1000 to 700000 picograms per gram, dry weight in polluted basins), those detected in sedimentary rocks studied, are relatively low. However, the original amounts of methylmercury in the investigated sediments would have been higher but impoverished during diagenesis. The mercury enrichments are observed in organic-rich Hangenberg equivalent intervals such as Kronhofgraben (from 0.51 to 13.28% total organic carbon) and Plan di Zermula A (from 0.7 to 12.53% total organic carbon). The values of the mercury/total organic carbon ratio in the Hangenberg Black Shale at Kronhofgraben range from 815 to 8096.5 (parts per billion/%), while the background samples show a range from 387.5 to 985 (parts per billion/%). In Plan di Zermula A, the values of mercury/total organic carbon ratios in the Hangenberg Black Shale range from 779 to 3269 (parts per billion/%) and are higher than those from the background samples (ranging from 84.5 to 676.8 parts per billion/% mercury/total organic carbon).
Volcanic and hydrothermal activities are considered to be the main sources of elevated mercury in sedimentary. Besides mercury delivery to the atmosphere by volcanic activity, other processes can produce mercury spikes in the sedimentary record, including widespread wildfires, terrestrial input, magmatic emplacement or thermogenic processes related to bolide impact rocks. Additionally, some authors have suggested that mercury enrichments can be sulphide-hosted in euxinic (high sulphur/low oxygen) facies, and high mercury spikes not necessary would be connected with volcanic activity. However, in such a case, the Hg enrichments would be well-correlated with total sulphur, which is not observed in our sections. Although extensive wildfires on land were confirmed during the Hangenberg event, based on the co-occurrence of charcoal and high concentrations of polycyclic aromatic hydrocarbons in sedimentary rocks, these, however, could have also been induced by volcanism, as evidenced by the co-occurrence of charcoals and ash layers. No conclusive evidence for bolide impact at the Devonian/Carboniferous boundary has been detected thus far. In fact, at the Devonian/Carboniferous boundary, volcanic activity has frequently been documented, mainly on the basis of the presence of ash layers below, above and within the Hangenberg Black Shale (e.g. in the Holy Cross Mountains, Iberian Pyrite Belt, and Rhenish Massif), mercury spikes, as well as the presence of abnormal or strongly altered spores (tetrads), which could reflect the mutagenic effect of regional acidification caused by explosive volcanism. The most plausible sources of very large amounts of mercury during the end-Devonian interval are the massive Magdalen silicic large igneous province and the Siberian (Yakutsk–Viluy) and/or the Kola–Dnieper large igneous provinces; however, the interval also overlaps with formation of the Almaden mercury deposit (last mineralisation pulse episodes), which constitutes one of the largest geochemical anomalies on Earth and coincided with the first phase of the Variscan Orogeny (mountain-building episode associated with the formation of the supercontinent of Pangea), as considered for the Hangenberg Crisis. According to current knowledge, three large igneous provinces encompass the Late Devonian interval (380–360 million years ago): Yakutsk-Viluy (Siberia; continental type with an area of 0.8 million km²), Kola-Dnieper (Baltica; continental type with area of 3 million km²) and Magdalen (Laurussia, continental-silic type). Moreover, Rakociński et al. cannot exclude other additional mercury sources, for instance connected with explosive eruptions which could overlap with large igneous province activity. Mercury has a strong affinity to organic matter and to a minor extent can also be associated with sulphides and clay minerals; therefore, mercury is normalized to total organic carbon content. Importantly, the mercury spikes in Rakociński et al.'s sections are also evident when normalized to total organic carbon content, which can be interpreted as an effect of increased input of mercury to the basins independently of the potential influence of reducing depositional conditions. The mercury vs. aluminium oxide correlation in the investigated successions is very weak, indicating no correlation of mercury with the clay fraction. However, mercury exhibits a good correlation with molybdenum in the all sections. This could indicate that some mercury was associated with sulphides as a result of its intensified precipitation in a sulphide-rich (euxinic) water column. In the sections investigated, mercury vs. total sulphur correlation is very weak, which does not confirm sulphides as host of mercury. However, the mercury vs. total organic carbon correlation in the Devonian/Carboniferous boundary at Novchomok section is very low, which confirm that mercury enrichments are facies independent and thus are indicative of volcanic activity during this time. For the Kronhofgraben and Plan di Zermula A sections this correlation is good, suggesting possible different sources of this element. However, as already emphasised, there are a number of lines of evidence for volcanic and hydrothermal activities, as well as widespread wildfires, during this time allowing for a firm statement that increased mercury input to the basins was connected with diverse volcanic activities and related combustion of biomass on land. Moreover, the Hangenberg Event took place during an interglacial period; therefore, some mercury could have originated from permafrost melting but even if this process had taken place, mercury would have previously accumulated in the permafrost as a result of volcanic or pyrogenic processes. To summarise, based on all the available data, Rakociński et al. state that the main sources of mercury were volcanism and related hydrothermal activities. In fact, volcanic processes are main sources of mercury in atmosphere.
Schematic model of deposition, mercury sources and mercury methylation during the Hangenberg Event. Rakociński et al. (2020).
The organic form of mercury (methylmercury) is a strong neurotoxin that is bioconcentrated in aquatic food chains and is able to cross the blood–brain barrier; thus, this form of mercury is much more toxic to living organisms than inorganic mercury. In modern environments, methylmercury is generated predominantly by anaerobic microorganisms, such as sulphate-reducing Bacteria (e.g., Geobacter sulfurreducens). Despite widespread mercury pollution, annual emissions of mercury have recently been higher from natural sources than anthropogenic ones, constituting as much as 70% of all mercury emissions. However, the concentrations of mercury detected in all the end-Devonian sections are surprisingly high, similar to the present-day mercury concentrations found in highly polluted basins, e.g., some parts of the Baltic Sea. The mercury concentrations of up to 20 000 parts per billion in Kronhofgraben and 1000–10 000 parts per billion in the Plan di Zermula A, and mercury spikes determined in Germany, south Vietnam, the Czech Republic and south China sections suggest, that global mercury concentrations were highly elevated during the Hangenberg event. This finding implies that, during favorable sedimentary conditions, very high concentrations of methylmercury can be produced on the global scale. In the investigated samples Rakociński et al. measured relatively minor amounts of methylmercury in comparison with the methylmercury levels in modern sediments. In polluted basins, concentrations of methylmercury vary from 1000 to 700 000 picograms per gram, dry weight of methylmercury and are much higher relative to total methylmercury concentration from Rakociński et al.'s sections. However, the original amounts of methylmercury in the investigated sediments would have been higher, assuming large enrichment of total mercury in anomalous samples. It is very probable that methylmercury could have been demethylated during diagenesis as a result of the common diagenetic process of demethylation, which is influenced by temperature. Because of the strong thermal alteration of the investigated rocks, the occurrence of demethylation seems to be very likely.
Therefore, regardless of the mercury source, its high level in the end-Devonian water column, subsequent trapping in sediment and biomethylation to the more toxic methylmercury form by anaerobic Bacteria, would have had an additional devastating impact on aquatic life during the Hangenberg Event. This can be produced under conditions of extended anoxia/euxinia during this time and the occurrence of rich sulphate-reducing Bacteria communities which can change mercury to its methyl form. Additionally, blooms of Green Algal phototrophs (prasinophytes) during black shale events would have contributed, mostly indirectly, to methylmercury production. However, indisputable evidence for Bacterial mercury methylation is the occurrence of notable concentrations of methylmercury in the sediments investigated and the similarities in the distributions of mercury and methylmercury in the Kronhofgraben section.
Observation of modern marine environments has confirmed that methylmercury is highly toxic to animals at higher trophic levels (such as Fish, Birds and Mammals). In this light it seems to be evident that severe extinction of marine and nonmarine Fish and Tetrapods, as well as pelagic Conodont Animals, during the Hangenberg event may also have resulted from methylmercury poisoning that could have affected different aquatic habitats. Although the effect of methylmercury on benthic invertebrates is regarded as minimal, these organisms were significantly affected by concomitant, globally widespread anoxia. Such anoxia asphyxiation–methylmercury poisoning may have also been kill mechanisms in other mass extinctions, but this should be tested by searching for traces of methylmercury in other sedimentary rocks.
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