Understanding the plasma environment around Mercury.

As the closest planet to the Sun, has the most direct exposure to the solar winds of any planet in the Solar System. The planet has a weak magnetic field, with a magnetosphere strongly linked to the surface and exosphere (a zone around its solid surface within which individual atoms and molecules can be found moving freely, but seldom, if ever, interacting) by a range of processes. These are largely driven by the exchange of energy with the solar winds, and loss of material from the planetary surface. The magnetosphere contains a mixture of ions derived from the solar winds, primarily hydrogen⁺ and helium²⁺ ions, and others derived from the planet's surface via ionization of material in the exosphere. 

The Mariner 10 spacecraft made three flybys of Mercury in 1974 and 1975, detecting heavy ions thought to be derived from the planet's exosphere. Subsequent observations of Mercury by Earth-based telescopes were able to identify ions such as sodium⁺, potassium⁺, and calcium⁺. Between 2011 and 2015 the MESSENGER spacecraft orbited Mercury, during which time its Fast Imaging Particle Spectrometer was able to detect a variety of ions in the planet's magnetosphere, including hydrogen⁺ and helium²⁺ derived  from the solar winds, as well as heavier ions such as helium⁺, oxygen⁺, water group ions (hydroxide⁺, water⁺, and hydrogen peroxide⁺), sodium⁺, magnesium⁺, aluminium⁺, and silicon⁺, derived from the planet. The helium⁺ ions showed a relatively even distribution around the planet, but the other planetary-derived ions were concentrated around the planetary cusp (i.e. directly facing the Sun) and in the equatorial band of the nightside of the planet, although it was not possible to further define the distribution of individual types of ion.

The BepiColombo spacecraft is a joint project by the European Space Agency and the Japan Aerospace Exploration Agency which was launched in 2018, on aa trajectory which would lead it to make close flybys of Mercury in October 2021, June 2022, June 2023, September 2024, December 2024, and January 2025, before finally entering the planet's orbit in November 2026. 

In a paper published in the journal Communications Physics on 3 October 2024, a team of scientists led by Lina Hadid of the Observatoire de Paris, Sorbonne Université, Université Paris Saclay, École polytechnique, and Institut Polytechnique de Paris, present the result of a study of ion plasma observations made by BepiColombo's Mercury Plasma Particle Experiment instruments, during the flyby of Mercury made on 19 June 2023.

The 19 June 2023 flyby took BepiColombo to about 235 km above the surface of Mercury, enabling sampling of ions within the magnetosphere plasma at low altitudes along the spacecraft's trajectory. During the flyby BepiColombo approached Mercury from its dusk-nightside, passing through the post-midnight magnetosphere close to the equator of the planet, and moving away towards dawn-dayside. 

BepiColombo’s journey through Mercury’s magnetosphere. European Space Agency.

BepiColombo crossed the bow-shock of the planet inbound at 6.44 pm GMT, and outbound at 7.52 pm. Outside of this bowshock region, both before and after the crossing, ions with energies of about 10 and about 20 electronvolts were constantly observed, with mass-per-charge ratios of 1 and 16. This is consistent with hydrogen⁺ and oxygen⁺ ions, derived from water molecules outgassed from the planet.

Projections of BepiColombo’s third Mercury flyby trajectory in the aberrated Mercury–Sun magnetospheric (aMSM) coordinate system. (a) X'–Z' and (b) X'–Y' planes, all expressed in Mercury radii (The radius of Mercury is 2440 km). Note the displacement in (a) of the magnetopause relative to the planetary centre because of the northward offset of the magnetic dipole by approximately 0.2 of the radius of Mercury. In traditional MSM coordinates, the X-axis and Z-axis point to the sun and north pole, respectively, and the Y-axis completes a right-hand system. In the aberrated coordinates, Mercury’s orbital velocity is considered. The X-axis is anti-parallel to the solar wind direction in the rest of the reference frame of Mercury. The aberration angle varies between about 5.5° and about 8.4° assuming a solar wind speed of 400 km per second. The black arrows indicate the viewing direction of instrument during this flyby. The magenta and cyan crosses represent the observed inbound (and outbound) bow shock and magnetopause crossings, respectively. The red dot highlights the closest approach of BepiColombo to Mercury. The black solid and dashed lines represent the modelled dayside bow shock and magnetopause that are obtained from the statistical distribution of observed crossing points, respectively. Hadid et al. (2024).

As BepiColombo entered the dusk magnetosphere of Mercury, it encountered ions with energies of around 20 000 electronvolts, but following this the energy of the ions fell to a few tens of electronvolts. Hadid et al. interpret this area as the low latitude boundary layer, the area along the magnetospheric side of a planet's low-latitude magnetopause where plasmas form the magnetosheath and magnetosphere are mixed. This kind of energy dispersion within the plasma mantle is typically seen at high latitudes; its presence close to the equator of Mercury suggests a relationship between the plasma mantle and the low latitude boundary layer, with convection carrying ions deep into the magnetosphere. 

Hydrogen⁺ ions were detected in the low latitude boundary layer region which Hadid et al. interpret as having derived from the duskside magnetosphere (i.e. the part of the magnetosphere where the Sun is setting). The low latitude boundary layer region is also likely to contain heavy ions derived from the dayside exosphere of the planet and transported over the polar caps, although Hadid et al. are careful to emphasise that determining the origin of the low latitude boundary layer is beyond the scope of the current study.

Model of the hydrogen⁺ ion trajectories. (a) Shows various particle trajectory projections in the equatorial plane traced backward in time. (b) Shows the particle kinetic energy versus time. The ions are launched from different locations (closed circles) along BepiColombo’s orbit, and their trajectories are traced backward in time. The colour code depicts the different magnetospheric regions, viz., the Low- Latitude Boundary Layer in green, the umbra in blue, the Plasma Sheet Horns in yellow, and ring current in red. The test hydrogen⁺ ion trajectories were computed using a modified Luhmann–Friesen model for the magnetic field combined with a two-cell convection pattern for the electric field. The full equation of motion was integrated backward in time using a fourth-order Runge–Kutta technique. Hadid et al. (2024).

As BepiColombia enetered the umbra (shadow) of Mercury and inner part of the low latitude boundary layer at 7.24 pm it encountered 'cold' ions with energies as low as 30-100 electronvolts. Hadid et al. suggest that this might be because negatively charged, causing low-energy ions from the exosphere to become attracted towards it. The ions encountered around the low latitude boundary layer include oxygen⁺ and calcium⁺ and/or potassium⁺ ions thought to have originated from the dayside of the planet and lighter hydrogen⁺ and helium²⁺ ions, probably of solar origin.

At 7.28 pm, shortly after leaving the low latitude boundary layer, BepiColombo began to encounter 'hot' ions with energies in the kiloelectronvolt range, in an area corresponding to the 'plasma sheet horns' detected by the MESSENGER spacecraft a decade previously. These ions are thought to originate from the tail of the magnetosphere, and to be accelerated towards the planet by convection currents.

After passing through this region at 7.32pm, BepiColombo encountered a region with intense ion fluxes, with ion energies in the 5-40 kiloelectronvolt range. Because this layer is present at low altitudes in the equatorial region, Hadid et al. interpret this as a tenuous ring current, in which charged particles could become trapped in orbits of the planet at altitudes of 1.3-1.5 times its radius. At this altitude a hydrogen⁺ could orbit Mercury in about four minutes, bouncing back and forth on either side of the equatorial plane throughout its motion around the planet. The presence of such a ring current had been suggested from the MESSENGER data, but the data was rather limited, with particles with energies of no more than 13 kiloelectronvolts being detected. The greater energy range detected in the BepiColombo data provides much better support for the presence of such a ring current, although again it is not sufficient to state definitively that this is what is being detected. 

The high energy particles within this band appear to be hydrogen⁺ and helium²⁺ ions, but BepiColombo also encountered larger particles. The most common of these have energies of around 2 kiloelectronvolts, and are interpreted as oxygen⁺ ions, while more energetic particles, with energies of around 10 kiloelectronvolts, and are interpreted as being predominantly calcium⁺ and potassium⁺ ions, with some sodium⁺ ions also present. BepiColombo also encountered cold ions, with energies of about 15 electronvolts. These cold ions are presumed to have originated from the surface of Mercury, and peaked at the closest to the planet, an altitude of 332 km.

After passing through the post-midnight magnetosphere of Mercury, BepiColombo re-entered the planet's magnetosheath and then moved back into the solar wind. This solar wind comprises a compressed and heated stream of hydrogen⁺ and helium²⁺ ions, although outgasses water group ions could again be detected in this region.

Mercury’s magnetosphere during BepiColombo’s third flyby. European Space Agency.

The 19 June 2023 flyby of BepiColombo has provided us with our first reasonably detailed view of the structure of Mercury's magnetosphere, demonstrating that it is not greatly different from that of the Earth. Both low and high energy ions were observed in the planet's magnetosphere, including the deepest parts encountered, suggesting that ion sputtering (the dislodging of low energy ions from the surface of the planet by the impact of high energy ions from the Sun) plays a significant role in the system. The evidence collected by the spacecraft supports the presence of a ring current encircling Mercury, and for the first time demonstrate the presence of a low latitude boundary layer. 

The magnetosphere of Mercury will remain a subject of study for the remainder of the BepiColombo mission, including the planned further flybys and orbital stage, which should serve to greatly enhance our understanding of the planet's magnetic environment.

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Methylmercury poisoning as a possible cause of the end-Devonian Mass Extinction.

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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Fluctuations in mercury and organic carbon in the peatlands of southwest China before the End Permian Extinction.

Carbon has two stable isotopes, carbon¹² and carbon¹³, of which plants preferentially incorporate carbon¹² into their tissues as it requires less energy to fix; this means that sediments with a high plant-derived carbon content (such as coal bed) will tend to be enriched in carbon¹² relative to sediment without (such as marine limestones). Furthermore, an increase in carbon in the atmosphere from burning plant matter, either as forests or coal beds, will tend to lead to an increase in the relative amount of carbon¹² in all sediments, known to geochemists as a positive organic carbon isotope excursion, whereas an increase in atmospheric carbon from other sources, such as volcanic eruptions, will tend to lead to a a drop in carbon¹² in all sediments, or a negative organic carbon isotope excursion.The Permian-Triassic mass extinction was the most severe extinction event of the Phanerozoic, both in marine and terrestrial settings, but the relative timing of these crises is debated. A negative carbon isotope excursion in both carbonate and organic matter is seen at the main extinction horizon and is usually attributed to release of volcanic carbon. Most proposed kill mechanisms for the Permian-Triassic mass extinction are linked to the effects of Siberian Traps eruptions. A spike in mercury concentrations observed at the onset of the Permian-Triassic mass extinction, thought to be derived from Siberian eruptions, provides a chemostratigraphic marker in marine records. A similar mercury enrichment event has also been documented in contemporaneous terrestrial sediments. Marine records show widespread environmental instability prior to the Permian-Triassic mass extinction, and a new study of the Sydney Basin (New South Wales, Australia), suggests that the collapse of southern high-latitude floras occurred significantly before the onset of marine extinctions roughly coincident with onset of northern high latitude marine stress.

In a paper published in the journal Geology on 3 January 2020, Daoliang Chu of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Stephen Grasby of the Geological Survey of Canada, Haijun Song, also of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Jacopo Dal Corso of the School of Earth and Environment at the University of Leeds, Yao Wang, again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Tamsin Mather of the Department of Earth Sciences at the University of Oxford, Yuyang Wu, Huyue Song, Wenchao Shu, and Jinnan Tong, once again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, and Paul Wignall, also of the School of Earth and Environment at the University of Leeds, evaluate the timing and nature of the terrestrial crisis at the End of the Permian in southwest China by examining variations in fossil charcoal abundance from paleo–tropical peatlands to explore changes in wildfire occurrence and the carbon-isotope composition of land plant cuticles, charcoal, and bulk organic matter to track changes in the isotopic composition of atmospheric carbon dioxide. In addition they investigated sedimentary mercury concentrations, and the integration of their record with carbon isotope values permits chemostratigraphic correlation of terrestrial and marine records.

Chu et al. examined the continental Permian-Triassic transition in cored borehole ZK4703, drilled 15 km south of Fuyuan County in Yunnan Province, China, and the Chinahe outcrop section, 30 km southeastern of Xuanwei City, both from the border area between western Guizhou and eastern Yunnan in southwestern China. Latest Permian to earliest Triassic terrestrial strata in this region include, in ascending order, the fluvial-coastal swamp facies of the Xuanwei and Kayitou Formations. The former consists of sandstone, mudstone, and common coal beds. The associated plant fossils belong to the Gigantopteris flora and include Pecopterids (Tree Ferns), Gigantopterids (a morphologically advanced group of Permian Vascular Plants that disappeared in the End Permian Extinction), Lycopsiales (Giant Club Mosses), and Equisetales (Horsetails) taxa, collectively regarded as tropical rainforest-type vegetation. The Kayitou Formation (latest Permian to earliest Triassic age) is similar to the underlying Xuanwei Formation, but lacks coal and is shale dominated. Previous studies showed that the loss of the Gigantopteris flora occurred in the lowest Kayitou Formation.

Late Permian to Early Triassic palaeogeographic map showing locations of the ZK4703 core (25.54151°N, 104.28994°E) and the Chinahe section (26.13077°N, 104.35637°E) in southwestern China, and the Meishan section, south China. Chu et al. (2020).

Organic carbon isotopes, charcoal abundance, fossil plant ranges, total organic carbon, total sulphur concentrations, and aluminium and Mercury contents were assessed through the Permian-Triassic transition in the ZK4703 core and at the Chinahe outcrop. To avoid facies variation issues, only mudstone samples were processed for charcoal. Some charcoal was examined under scanning electron microscope to confirm identification. To ensure that the charcoal concentrations were not affected by variations in the nature or abundance of organic material, its abundance was normalised to phytoclast abundance and total organic carbon.

 Location map of the studied section. Chinahe section (26.13077°N, 104.35637°E) is located in the Chinahe Viliage of the Tianba town, Xuanwei City. ZK4703 core (25.54151°N, 104.28994°E), drilled in Anzichong Viliage of Dahe Town, Qujing City. Chu et al. (2020).

The proportion of carbon¹² drops sharply in the lower part of the Kayitou Formation at Chinahe, both in organic matter and charcoal (a negative organic carbon isotope excursion). At the same time bulk organic matter and palynomorphs (pollen fossils) from the ZK4703 core section also show a drop in carbon¹² values.

Cuticle and charcoal particles under binocular microscope and scanning electron microscope. Chu et al. (2020).

The abundant, peat-forming Gigantopteris flora is seen at six levels in the Xuanwei Formation at Chinahe, and is dominated by well-preserved, large leaves. Both diversity and abundance of this flora decline drastically at the very top of the formation at a level that corresponds to the onset of the negative negative organic carbon isotope excursion. Thereafter, the flora consists of a monotonous assemblage of small plants, mostly Annalepis and Peltaspermum.

 Typical Gigantopteris flora from the Xuanwei Formation of the Chinahe section. (A) Gigantopteris dictyophylloides; (B) Annularia pingloensis; (C) Lobatannularia sp.; (D) Pecopteris marginata; (E) Gigantonoclea guizhouensis; (F) Pecopteris sp.; (G) Compsopteris contracta; (H) Abundant plant leaf fossils preserved on the same bedding surface. Chu et al. (2020).

At Chinahe, the charcoal abundance is less than 300 particles per 100 g rock prior to the a negative organic carbon isotope excursion, but rises briefly above background levels during the onset of the excursion (1524 particles per 100 g at 25 m log height), and ranges from 400 to 1600 particles per 100 g in the 4 m interval of the uppermost part of the Xuanwei Formation to lower part of the Kayitou Formation. Similarly, in the ZK4703 record there is a sharp increase in charcoal abundance, from under 400 particles per 100 g below 15 m, to over 2400 particles per 100 g above 16.5 m height at the base of the Kayitou Formation. Scanning electron microscope observation shows that the charcoal preserves anatomical details and has similar preservation and structures with variable size, indicating minimal transport sorting. The reported variations in charcoal abundance do not appear to be an artifact of preservation or changes in terrestrial organic delivery, because variations in preserved phytoclasts (microscopic plant fragments) and total organic carbon do not vary with charcoal abundance.

The plant fossil ranges and species richness of the Chinahe section. (1) Peltaspermum sp.; (2) Annalepis sp.; (3) Compsopteris contracta; (4) Fascipteris densata; (5) Cladophlebis permica; (6) Annularia pingloensis; (7) Compsopteris sp.; (8) Lobatannularia heianensis; (9) Pecopteris marginata; (10) Lobatannularia cathaysiana; (11) Pecopteris guizhouensis; (12). Rajahia guizhouensis; (13) Gigantonoclea sp.; (14) Stigmaria sp.; (15) Gigantonoclea guizhouensis; (16) Gigantopteris dictyophylloides; (17) Pecopteris sp.. Chu et al. (2020).

Mudstone total organic carbon concentrations are relatively high in the Xuanwei Formation and modestly enriched concentrations persist into the lower part of Kayitou Formation before dropping at the 27 m log height at Chinahe. Both overall mercury levels and the mercury-total organic carbon ratio rise above background levels immediately above the interval with the onset of the negative organic carbon isotope excursion and increased charcoal abundance. High overall mercury levels and the mercury-total organic carbon ratios can also be observed at higher stratigraphic levels, with a peak value at 19.75 m in the ZK4703 core, which is about 50 times background levels. Overall mercury levels and the mercury-total organic carbon ratio drop to the previous baseline values above 37 m at Chinahe and 25 m in ZK4703. The weak correlation between overall mercury levels and the mercury-total organic carbon ratios suggests that the mercury fluctuations are not affected by changes in total organic carbon. Additionally, the ZK4703 core has low total sulphur contents which show no significant covariation with mercury values. Correlation between Aluminium and Mercury concentrations is also weak, indicating that mercury fluctuations are not controlled primarily by clay content, even if some mercury is probably adsorbed onto clay. Nonetheless, there is secular variability in the mercury/aluminium ratio, with very low background mercury/aluminium values below and above the mercury anomaly and enriched mercury/aluminium values within the interval.

Volcanic emissions represent one of the largest natural inputs of mercury to the atmosphere, and the mercury enrichment seen in many marine Permian-Triassic boundary sequences is thought to record large-scale Siberian Traps eruptions. Volcanic mercury emissions from this source may have been up to 10 000 milligrammes per year (roughly 14 times natural background levels). Thermogenic release of mercury from baking of organic-rich sediments on contact with Siberian Traps intrusions is another potential source of mercury. Terrestrial plants constitute a large mercury reservoir, and so wildfires can also contribute significantly to mercury fluxes to the atmosphere and freshwater environments such as those studied by Chu et al., who propose that the mercury spikes observed in terrestrial and marine successions provide a useful correlative tool between terrestrial and marine records along with carbon isotope ratios.

The onset of the main phase of marine extinctions in South China, at the top of the Clarkina yini Zone, correlates with a peak in mercury concentrations and mercury/total organic carbon ratios, while a second phase of extinctions at the top of the Isarcicella staeschi Zone corresponds to a rise in mercury concentrations and mercury/total organic carbon ratios that peaks in the following Isarcicella isarcica Zone. The relative magnitude of these peaks varies between sections: at Meishan, South China, the lower mercury/total organic carbon ratios peak is the largest, whereas at Guryul Ravine, Kashmir, the second peak is larger. Levels of organic carbon began to decline somewhat before the marine extinctions in the Clarkina changxingensis Zone.

In the terrestrial sections of southwestern China, the floral mass extinction (and charcoal peak) starts with the onset of the negative organic carbon isotope excursion. Mercury concentrations begin to slightly rise at the same time, while mercury/total organic carbon ratios shows a sharp spike at the minimum of the negative organic carbon isotope excursion. The mercury and mercury/total organic carbon ratios peak 4–6 m above the terrestrial extinction level and likely correlate with the rise in mercury/total organic carbon ratios values seen at the end–Isarcicella staeschi Zone that saw diverse taxa disappear from Triassic oceans. Thus, the terrestrial crisis seen in equatorial sections of China appears to predate the main marine extinction phase (which occurred near the low point of negative organic carbon isotope excursion), and likely dates to the late Clarkina changxingensis Zone.

Chu et al.'s results demonstrate that a synchronous onset of the negative organic carbon isotope excursion is present in the bulk organic matter, cuticle, and charcoal carbon isotope records from terrestrial settings. Changes in organic carbon values of land plant cuticles record changes in atmospheric CO₂. Chu et al. suggest that the observed negative organic carbon isotope excursion in cuticles and fossil charcoal reflects an injection of carbon¹³-depleted emissions associated with the Siberian Traps. Interestingly, in our study, the peak in mercury concentrations and mercury/total organic carbon ratios also occurred after the onset of the negative carbon isotope excursion, suggesting a decoupling between the carbon and the mercury records that could result from the source of these two elements, be it volcanic, thermogenic, continental runoff, wildfire, or a combination of different reservoirs. Such decoupling deserves further investigation because it suggests different mechanisms of carbon and mercury release and/or processing in End Permian environments.

Studies have shown insignificant or constant fractionation of carbon isotopes during the burning process, and so charcoal carbon isotope ratios are a direct record of the original wood tissue carbon isotope ratios. The gradual decrease in the proportion of carbon¹³ in terrestrial plant-derived charcoal in the studied successions indicates, as discussed also for cuticle carbon isotope ratios, a change in the proportion of carbon¹³ in the original peat and vegetation due to changes in the proportion of carbon¹³ in the atmospheric CO₂. The charcoal carbon¹³ negative shift is coeval with an increase in charcoal abundance, i.e., with increased wildfire activity suggesting that the latest Permian forests experienced recurring wildfires and regrowth while the atmosphere became more carbon¹³ depleted. Additionally, burning of terrestrial plant biomass can also increase the emission of mercuty into the atmosphere, and then this mercury can be scavenged and buried in sediments.

The intensification of wildfire activity at the time of terrestrial mass extinction provides evidence of the harmful climatic changes in the lead-up to terrestrial crisis. The Gigantopteris coastal swamp flora thrived in humid, warm equatorial locations and was unlikely to have been adapted to intense levels of wildfire, as evidenced by the low charcoal abundance prior to the extinction interval. Thus, the increased wildfires suggest a transition to more unstable conditions punctuated by dry periods that would have been detrimental to coastal swamp floras. 

Chu et al.'s study sheds new light on the temporal links between the deterioration in the terrestrial environment and floral extinction, and the geochemical changes that mark the Permian-Triassic mass extinction. Their terrestrial mercury record from the Permian-Triassic transition shows a sharp peak contemporaneous with the disappearance of Permian flora that correlates with marine mercury records. Carbon isotope data from cuticles and fossil charcoal, thought to reflect changes in the carbon isotope composition of the atmosphere, show a negative carbon isotope excursion during the terrestrial flora mass extinction interval. However, this was prior to the increase in mercury concentrations. Charcoal abundance shows that the floral extinctions coincided with an increase of wildfire activity and the carbon-cycle disruption. This likely reflects a change from persistent humidity to an unstable climate with frequent drought episodes. The temporal relationships between the events show that terrestrial disruption occurred shortly (but measurably) before the marine crisis.

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Collapse at illegal gold mine kills six in Jambi Province, Sumatra.

Six people have been confirmed dead following a collapse at an unlicensed gold mine in Jambi Province, Sumatra, on Sunday 22 December 2019. The incident happened at about  1.00 pm local time in the village of Pulau Baru following heavy rains in the area. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. One of the deceased has been identified as a local man, the other five are reported to have come from Central Java. Unlicensed mining operations in Indonesia are notoriously unsafe, and also cause a variety of environmental problems associated with indiscriminate digging, with some mining haven taken place inside national parks.

Rescue workers at the scene of a mine collapse in Jambi Province, Sumatra, on 22 December 2019. Basarnas.

The site on which the mine was operated had apparently been leased from a local landowner for the purpose if mining by two businessmen, who's identities have not been released for legal reasons while there is an ongoing investigation. The mineworkers were apparently using mercury to extract gold from ore at the site, a process in which ore is mixed with mercury to remove the gold, then the resulting amalgam of the two metals burned to remove the mercury, effectively evaporating it off, resulting in large amounts of fumes. There are means of extracting gold from ore without mercury, but this is the simplest method on a low technology base. Mercury is a highly potent neurotoxin, and can cause a variety of other health and developmental problems in children; it is considered to be particularly harmful to infants and fetuses. The fumes are persistent in the atmosphere and can travel long distances, which places those not directly involved in the amalgamation process at risk, since the process is often carried out in residential areas, and close to streams from which drinking water is extracted and fish are caught and eaten. On this occassion waste from the process was being dumped into a local river, which had already caused local villages to stop using it as a sourse of drinking water. Local authiorities are extremely concerned about the potential impact of this on settlements further downstream, including the city of Jambi, which has a population of about 600 000.

Sumatra has a wet tropical climate, with a rainy season that lasts from October to April, when rainfall typically reaches 200-300 mm per month and a dry season from May to September, when rainfall is usually below 200 mm per month (though the area is never truly dry. This is driven by the Southeast Asian Monsoon Seasons, with the Northeast Monsoon driven by winds from  the South China Sea fuelling the wetter rainy season and the Southwest Monsoon driven by winds from the southern Indian Ocean the drier dry season. Such a double Monsoon Season is common close to the equator, where the Sun is highest overhead around the equinoxes and lowest on the horizons around the solstices, making the solstices the coolest part of the year and the equinoxes the hottest.

The winds that drive the Northeast and Southwest Monsoons in Southeast Asia. Mynewshub.

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