Saturday 29 February 2020

Canadian Supreme Court allows British Columbian mining company to be sued in Canada for actions in Eritrea.

The Canadian Supreme Court in Ottawa has ruled that a Vancouver-based company can be sued in British Columbia for actions carried out at a mining project in Eritrea, East Africa. The mining company, Nevsun Resources, had objected to the case being brought in a Canadian court on the basis that Canadian law usually requires cases to be brought in the jurisdiction where the offence occurred. However the court rejected this argument, agreeing with three Eritrean men, currently living in Canada as refugees and attempting to gain recompence from Nevsun, that the nature of the allegations comprises a 'breach of customary international law', and that the Canadian courts therefore have a duty to hear the case.

The Bisha Mine site. Bisha Mining.

Customary international law is a legal principle introduced after the Second World War that holds certain matters are breaches of Human rights so serious that they should be policed internationally, in effect giving courts in all jurisdictions both the right and the duty to try cases involving such breaches regardless of where they occurred. In this case it is alleged that the men's rights were breached when they were forced to work as conscript labourers during the construction of facilities at the Bisha Mine site in Eritrea, and that they were forced to work twelve hour shifts, six days per week, and endured punishments including being beaten with sticks and being tied up and left out in the sun.

The Eritrean National Service Program enables the government to conscript citizens for both military service and as workers on projects deemed to be of public worth. It has been widely condemned internationally, with an enquiry carried out by the United Nations in 2016 describing the program as a widespread and systematic abuse of Human rights amounting to a system of modern slavery, with about 400 000 people trapped in a system of lifelong service.

This system has caused many people to flee the country, resulting in a wave of refugees arriving on countries in the Middle East and Europe. Nevertheless, many governmentss still regard Eritrea as an ally in the region, with the effect that companies operating in Eritrea face little if any sanction in their home nations for actions that take place there.

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Craspedotropis gretathunbergae: A new species of terrestrial Caenogastropod from the Kuala Belalong rainforest of Brunei

Although Mollusca is, after Arthropoda, the second-most species-rich animal phylum on land (at least in terms of described species), their species numbers are only moderately high compared with the Arthropods. Even in hyperdiverse forested regions in the humid tropics, such as northern Borneo, the numbers of known species are in the hundreds, not thousands and trends of discovery appear to be levelling off towards the true number of species. This means that, with relatively little fieldwork effort, it is possible to obtain a more or less complete inventory of the terrestrial malacofauna of a certain area. 

In a paper published in the Biodiversity Data Journal on 20 February 2020, a team of taxonomic experts and lay people led by Menno Schilthuizen of Taxon Expeditions, the Naturalis Biodiversity Center, Universiti Malaysia Sabah, and the Faculty of Science & Institute for Biodiversity and Environmental Research at University Brunei Darussalam, describe a new species of terrestrial Caenogastropod discovered during a citizen science project in the Kuala Belalong rainforest of Brunei Darussalam (Borneo).

Schilthuizen et al. are using field courses to several biodiversity hotspots in the world for such a purpose and they use the term 'taxon expeditions' for this. Taxon expeditions are a new concept in which a group of taxonomic experts and lay people work together in a hybrid work form of field course and biodiversity discovery expedition to discover unknown species from a given area. On their annual taxon expedition to the Ulu Temburong lowland rainforest in Brunei Darussalam, surveys of terrestrial snails have so far yielded over 25 species. The expected diversity for this type of forest would be around 85 species, but already our inventories are turning up previously unknown species.

Schilthuizen et al. describe a new species from the large Caenogastropod family Cyclophoridae. In the Southeast Asian tropics, non-Stylommatophora (i.e. Neritopsina and Caenogastropoda, not Pulmonates, the most common and familiar group of terrestrial Gastropods) comprise nearly half of the total malacofauna. These Snails are, however, more sensitive to habitat disturbance than the Stylommatophora. Studies of  malacofauna conservation biology on limestone hills in Sabah have previously shown that these non-Stylommatophora groups are the first to disappear after forest degradation, presumably as a result of lower tolerance ranges for temperature and humidity.

The description of the new Cyclophorid is the concerted effort of untrained ‘citizen scientists’ working together in a field lab. The specific epithet was decided upon during a voting session, in which participants could suggest and vote for scientific names.

Schilthuizen et al. collected living snails by hand, at night, amongst vegetation along the left (south) bank of the Belalong river, 50 m downstream of Kuala Belalong Field Studies Centre. Specimens were sorted to putative morphospecies and then further examined morphologically with a dissection microscope with 10× and 20× eye pieces. Photographs and video were made either with a smartphone through the eyepiece of a microscope or on a translucent white acrylic sheet with a Nikon D800e fitted with a Laowa 25 mm ultra-macro lens. Drawings were made based on the photographs. Specimens were deposited in the collection of the University of Brunei Darussalam Museum.

The new species is placed in the genus Craspedotropis, and given the specific name gretathunbergae, in hounour of the young climate activist Greta Thunberg, because Caenogastropod Microsnails from tropical rainforests, like this new species, are very sensitive to the droughts and temperature extremes that are likely to be more frequent as climate change continues. Via mutual contacts, Schilthuizen et al. approached Ms. Thunberg and learned that she would be 'delighted' to have this species named after her.

The protoconch (the larval shell of a Snail, which forms the tip of the adult shell) lacks distinct sculpture and the juncture with the teleoconch (adult shell) not visible. The spire of the shell is highly conical, consisting of 5.25 to 5.75 convex whorls, The radial sculpture comprises growth lines and densely placed riblets (30-70 per mm on the body whorl), while the spiral sculpture comprises 4 high, prominent, here and there somewhat crenellated radial ribs, which start after 1.5 to 2.0 whorls. The first spiral rib (from the top) is located between the periphery and the suture with the previous whorl. The second sits at the periphery. The third is located near the suture with the next whorl and is visible on the body whorl in some, but not all, individuals. The fourth is located on the basal side and only visible on the edge of the umbilicus. Between the ribs are fine spiral lines. The umbilicus (gap between the whorls, seen on the underside of an upturned shell) is wide, without additional spiral ribs.

Craspedotropis gretathunbergae, shell in apertural view. Pierre Escoubas in Schilthuizen et al. (2020).

The aperture of the shell is slightly wider than high, the peristome (shell around the apature) slightly thickened, with angularities coinciding with the four spiral ribs, but without sinuosities; the basal side is horizontal. The apature is 0.86 - 0.90 mm high and 0.92 - 0.97 mm wide. The peristome carries about 10 tightly packed accretions, coloured dark brown because of the thickened periostracum (thin organic coating or 'skin' of the shell).

Craspedotropis gretathunbergae. (a) Shell with operculum in apertural view. (b) Detail of the body whorl in lateral view. (c) Detail of the shell in umbilical view. Schilthuizen et al. (2020).

The operculum ('lid' used to seal the shell) is thin and horn-like. The whorl margins not raised but flattened into a smooth concave dish, of which the outer part is greenish-brown (presumably because of a thickly applied periostracum), the central part nearly clear. No calcareous matter visible.

The species is known only from the Ulu Temburong lowland rainforest of Temburong District in Brunei Darussalam. It is found in tropical mixed Dipterocarp lowland rainforest. All individuals were found alive at the foot of a steep hill-slope, next to a river bank, foraging at night on the upper surfaces of green leaves of understorey plants, up to 1 m above ground level.

Active individual of Craspedotropis gretathunbergae. Pierre Escoubas in Schilthuizen et al. (2020).

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Friday 28 February 2020

Atractosteus spatula: Alligator Gar found in Welsh canal.

Investigators from RSPCA Cymru have begun an enquirey after an Aligator Gar, Atractosteus spatula, was found dead in the Monmouthshire and Brecon Canal at Risca in Caerphilly on Thursday 27 February 2020. The Fish was found by local resident Tyrese Martin, 16, while on his way to school. Alligator Gars are large predatory Fish native to North America, capable of reaching over 3 m in length, but quite unsuited to the Welsh climate, leading to speculation that the animal may have been kept as a pet but been released into the canal when it outgrew it's aquarium. Aligator Gar are considered to be large exotic animals in Wales, and as such their are restrictions on keeping them, leading the RSPCA to speculate that the Fish may have been being kept (and possibly brought into the country and traded) illegally.

 An Aligator Gar, Atractosteus spatula, found in the Monmouthshire and Brecon Canal at Risca in Caerphilly on Thursday 27 February 2020. Tyrese Martin.

Aligator Gars are the largest freshwater Fish in North America, and the largest living members of the Lepisosteiformes (Gars), an ancient lineage of Ray-finned Fish, which branched off as a separate evolutionary lineage during the Mesozoic. They lack the expandable jaws of Teleosts, and have a heavily vascularised swim-bladder which can be used as a lung. Gars today are found only in the Americas, but fossil specimens are known from Europe and India.
 An Aligator Gar, Atractosteus spatula, found in the Monmouthshire and Brecon Canal at Risca in Caerphilly on Thursday 27 February 2020. Tyrese Martin.
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Asteroid 2020 DD3 passes the Earth.

Asteroid 2020 DD3 passed by the Earth at a distance of about 1 017 000 km (2.65 times the average  distance between the Earth and the Moon, or 0.68% of the distance between the Earth and the Sun), slightly before 10.05 am GMT on Friday 21 February 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2020 DD3 has an estimated equivalent diameter of 20-62 m (i.e. it is estimated that a spherical object with the same volume would be 20-62 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere between 23 and 5 km above the ground, with only fragmentary material reaching the Earth's  surface.

The calculated orbit of 2020 DD3. JPL Small Body Database.

2020 DD3 was discovered on 24 February 2020 (three days after its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2020 DD3 implies that the asteroid was the 76th object (object D3 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, so that D3 = (24 x 3) + 4 = 76) discovered in the second half of February 2020 (period 2020 D).

2020 DD3 has a 772 day (2.11 year) orbital period and an eccentric orbit tilted at an angle of 5.60° to the plane of the Solar System, which takes it from 0.56 AU from the Sun (i.e. 56% of he average distance at which the Earth orbits the Sun, insdie the orbit of the planet Venus) to 2.74 AU from the Sun (i.e. 274% of the average distance at which the Earth orbits the Sun, and considerably outside the orbit of the planet Mars). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This means that close encounters between the asteroid and Earth occasionally occur, with the next predicted for March 2058.

2020 DD3 also has frequent close encounters with the planets Venus, which it is next predicted to pass in January 2094, and Mars, which it is predicted to pass in March 2058. Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet.

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Thursday 27 February 2020

Preserved gum in an Early Cretaceous Welwitschiacean.

A wide variety of vascular plants produce fluid exudates such as resins and gums, with each group differing in chemical definitions. Due to similarity in physical appearance distinguishing exudates based on chemistry is vital, for example gums and resins are visually similar resulting in these terms being used interchangeably. However, their chemical definitions are very different; resins are composed of lipid-soluble terpenoids, while gums are complex, highly branched (non-starch) water-soluble polysaccharides. A common example of this misunderstanding is the Eucalyptus, which is known as a Gum Tree, but nuclear magnetic resonance analysis of the Eucalyptus exudate shows its composition to be polyphenolic and is therefore actually a kino (i.e. neither a gum nor a resin, bt closer in composition to resins). Differences between gum and resin can also be seen in the functional roles within the plant. The main roles of resins are to respond to wounding, as a defence against pathogens and to dissuade herbivory by Insects and other organisms. Gum is involved in food storage, structural support, and also for wound sealing, but there is no common role across taxa. Further confusion arises as some plants, e.g. Boswellia and Commiphora species, even produce exudates with a mixture of polysaccharide and resin components (the gum resins Myrrh and Frankincense respectively). Until now only fossilised plant resin (ambers) and latex filaments have been reported preserved in the fossil record. While the fossilisation of fluid exudates might seem unlikely, the fossilisation of resin is relatively common, and extends back some 320 million years to the Carboniferous, but chemically confirmed gums have never been reported.

In a paper published in the journal Scientific Reports on 25 February 2020, Emily Roberts of the School of the Environment, Geography and Geosciences at the University of Portsmouth, and the Department of Palaeontology at the University of Vienna, Leyla Seyfullah, also of the Department of Palaeontology at the University of Vienna, Robert Loveridge, also of the School of the Environment, Geography and Geosciences at the University of Portsmouth, Paul Garside of Conservation Research, at the British Library, and David Martill, again of the School of the Environment, Geography and Geosciences at the University of Portsmouth, describe the first known incidence of preserved gum in the fossil record, in a preserved Welwitschiacean leaf from the Early Cretaceous Crato Formation of Brazil.

The Early Cretaceous (approximately 120 million year old) Crato Formation utcrops on the northern flanks of the Chapada do Araripe, a plateaux on the border between Ceará, Pernambuco and Piauí States in northern Brazil. In is noted for the exceptionally well preserved fossils from the Nova Olinda Member, which include Dinosaurs, Crocodiles, Fish, Pterosaurs, Crustaceans, Arachnids, Plants and most notably Insects, which are present in large numbers, often showing exceptional preservation hese Insects are of particular interest as they date from a time in the Early Cretaceous when Flowering Plants were rapidly diversifying, and relationships between Insect and Plant groups that would come to dominate the Earth’s terrestrial biology were being formed.. Investigations of different groups of fossilized Animals from the Crato Formation show that they are preserved as various mineral replacements, and their preservation was microbially-mediated.

Stratigraphy of the Crato Formation. Stratigraphy of the Araripe Basin indicating the fossil bearing laminated limestones of the Nova Olinda Member of the Crato Formation. Roberts et al. (2020).

Amber has also previously been reported from the Crato Formation Lagerstätte, inside fossil plant remains and as isolated clasts, and is attributed to Conifers. The fossil leaves occur as compressions showing at least some three-dimensionality. An amber-coloured substance is visible in some of the fossil leaves of Welwitschiophyllum brasiliense from the Crato Formation.

Fossil Welwitschiophyllum leaves from the Crato Formation, Brazil showing three- dimensional preservation. (a) Complete Welwitschiophyllum leaf (UOP-PAL-MC0002). Showing three dimensionality. (b) A magnified view of Welwitschiophyllum (UERJ 14-P1) showing that the leaf is preserved three dimensionally with distinct parallel leaf tissues. Scale bars, (a) 10 mm (b) 5 mm. Roberts et al. (2020).

Welwitschiophyllum is considered a relative of the extant Gymnosperm Welwitschia mirabilis, the sole member of this Gnetalean genus. The Welwitschiaceae have a sparse macrofossil record, fossils assigned to this family, including Welwitschiophyllum, derive solely from the Crato Formation. However, the pollen record shows Welwitschiaceae were once a diverse and prevalent group that saw a decline with increasing Angiosperm pollen. Today, Welwitschia is restricted to the Namib Desert in Namibia and Southern Angola and has chemically confirmed gum in both the cone and in abaxial ducts within leaves.

Roberts et al. investigated this amber-coloured substance inside fossil Welwitschiophyllum leaves to test whether Welwitschiophyllum produced a resin (now fossilised as amber), or a gum like its presumed extant relative Welwitschia, using Fourier-transform infrared spectroscopy and Attenuated total reflectance spectroscopy. They report the first geochemical evidence for fossilised gum preserved inside Welwitschiophyllum leaves, and suggest areas for future investigation to understand how a roughly 120 million year old gum may have survived.

Fossils of Welwitschiophyllum occur as long detached leaves up to 850 mm in length with thin bands of an amber-like substance. These are particularly visible where the fossil surface has been abraded or removed. This substance in the Welwitschiophyllum leaves resembles amber in ducts, lying parallel to the long axis of the leaves. These ducts are inferred here as adaxial (upper leaf surface) due to the curvature of the leaf base. However, the absence of preserved cuticle and other anatomical features, means that their precise orientation cannot be confirmed. This constituent arrangement contrasts with the traumatic formation of gum in its presumed relative Welwitschia. Slight compaction of the specimens gives these ducts an ellipsoidal cross section, but they appear to have a repeating pattern showing a principal duct followed by a secondary duct ranging in diameter from 75 μm to 200 μm.

Fossil Welwitschiophyllum leaves with gum ducts from the Crato Formation, Brazil. (a) Complete elongate fossil leaf (UERJ 13-P1) showing a curved base and degraded fibrous apex, with a partially abraded surface (lighter part of fossil) exposing the internal leaf tissue and linear gum duct arrangement. (b) Detail from (a) (UERJ 13-P1) where the gum ducts appear as amber-brown structures within Welwitschiophyllum leaf tissue. (c) Transverse thin section through the fossil leaf (UERJ 14-P1) with arrowheads indicating the orange coloured gum ducts within the brown leaf tissue. The black line of tissue may be compressed remains of vascular tissue (below the leaf is the preserving sediment). (d) An oblique thin section of the leaf (UERJ 14-P1) showing the repeating pattern of the amber-coloured gum ducts. Scale bars, (a) 20 mm (b) 3 mm, (c) and (d) 500 μm. Roberts et al. (2020).

Analysis using Fourier-transform infrared spectroscopy and Attenuated total reflectance spectroscopy are commonly used on both living and fossil plants showing that complex biomolecules survive and are identifiable in the fossil record Fourier-transform infrared spectroscopy analyses compare living and fossil resin and gum samples. Additionally, Attenuated total reflectance spectroscopy analysis confirms that the amber-coloured substance in the fossil leaves, which was extracted and purified for testing, generated a spectrum closely matching those of published gum signatures and is remarkably similar to that of Welwitschia gum.

Thin section of a Welwitschiophyllum leaf. Thin section through the fossil leaf (UERJ 13-P1) showing amber-coloured gum ducts within brown leaf tissue. Scale bar, 500 μm. Roberts et al. (2020).

The discovery of in situ preserved plant gum is unusual because of its solubility in water. This is particularly striking in a formation thought to be deposited in a hypersaline lagoon setting. Solubility experiments were undertaken on Welwitschia gum to determine whether the increased salinity of the lagoon may have affected the solubility of the gum in any way. In the freshwater, brackish, normal marine, and hypersaline water tests the extracted gum dissolved within 49–59 minutes, showing that salinity does not affect solubility, and therefore the preservation (or not) of exposed gum.

Roberts et al.'s analyses of the amber-coloured substance inside the fossil Welwitschiophyllum leaves shows a distinct chemical spectrum that clearly differs from those of ambers and resins, but which closely compares to plant gum spectra. This means that the recovered substance from the Crato Formation fossil Welwitschiophyllum leaves is a preserved gum and not an amber formed from resin. The chemically detected presence of gum in ducts inside two separate fossil leaves confirms that this is not an isolated occurrence within these Crato Formation fossils.

Due to the soluble nature of gum, its preservation in the fossil record is unexpected. This is particularly notable here as the leaves containing gum were firstly deposited in a hypersaline lagoon, then later this deposit was exposed to continental weathering. Thus, water featured in both the formation and weathering of the Crato Formation, yet the gum persisted. The gum solubility experiments showed that in each case of differing salinities the extracted Welwitschia gum dissolved, so saline levels appear to have no bearing on gum preservation.

How the gum came to be preserved is currently not understood and further investigation is needed into the taphonomic and diagenetic processes surrounding these gum-preserving fossil leaves. We can speculate that there are at least two factors involved. Firstly, the nature of the microbially-mediated taphonomy and diagenesis in the Crato palaeo-lake setting has been shown to be critical in the preservation of labile structures in animals from the Crato Formation Konservat-Lagerstätte. Secondly, perhaps only in part, the coriaceous nature of the fossil leaves played a role. Both the surrounding duct tissue and the large amount of resistant embedding leaf tissues would have provided some protection from dissolution in water. In extant Welwitschia the outer walls of the epidermal cells are specialised with three layers, thickening and strengthening the epidermis, but the preservation of the fossil leaves prohibits epidermal comparison. The regular arrangement of ducts in Welwitschiophyllum suggests that they were formed through duct initiation, i.e. constituent, as opposed to the stress initiated response known as gummosis. Their formation was likely to be used for food storage or structural support, signifying that the hydrophilic gum was constituent within the fossil leaves.

Despite the very low preservation potential of a highly water-soluble exudate, the first preserved gum was recovered from the Early Cretaceous. This fossil gum presents a chemical signature remarkably similar to gum in extant Welwitschia and distinct from those of fossil resins. This shows that gum production in plants extends back into the fossil record by at least 120 million years. This is then the first report of a highly soluble biomolecule recovered from the Crato Formation and future work should focus on how this preserved gum survived. Furthermore, fossilised plants with observed internal ‘resins’ should be chemically confirmed in case further instances of gums or other types of plant exudate can be identified from the fossil record.

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Wednesday 26 February 2020

Resolving the age of the Middle Devonian basin-scale precious metal enrichment event in the northern Yukon, Canada.

Hyper-enriched Black Shales, or polymetallic shales, are an important global resource for Zinc, Nickel, Copper, Molybdenum, Selenium, Uranium, Vanadium, Chromium, Cobalt, Silver, Gold, Platinum Group Elements, and Rare Earth Elements. These deposits are thin (genrally under 10 cm) and laterally extensive. The Nick Prospect, one of at least four Hyper-enriched Black Shale localities in Yukon, Canada, is the best-known Hyper-enriched Black Shale in Canada and contains an average of 5.3% Nickel by weight, 0.72% Zinc by weight, 2400 parts per million Molybdenum, 670 parts per million Platinum Group Elementss, 90 parts per billion Au, and 24.5 parts per million Rhenium. The Hyper-enriched Black Shale is conspicuously situated at a regional stratigraphic contact between the Road River Group and the Canol Formation. Recent work has shown that the ambient paleoenvironment in which Hyper-enriched Black Shale formed was redox stratified and that pyrite is the mineralogical host of economically important metals; however, the age of mineralization with respect to host rock deposition is not known. Precise age constraints are required for mineralisation to test the favored model that elemental enrichments in Hyper-enriched Black Shales originate from ambient seawater. and they also can provide a clearer understanding of the broader environmental parameters that may have facilitated mineralisation.

In a paper published in the journal Geology on 3 January 2020, Michael Gadd and Jan Peter of the Geological Survey of Canada, Danny Hnatyshin and Rob Creaser of the Department of Earth and Atmospheric Sciences at the University of Alberta, Sofie Gouwy, also of the Geological Survey of Canada, and Tiffani Fraser of the Yukon Geological Survey, present the results of a study which aimed to resolve the age of the Hyper-enriched Black Shale mineralising event(s) in northern Yukon by applying Rhenium-Osmium geochronology to Hyper-enriched Black Shales from the Nick Prospect and the Peel River Showing.

Map of Middle Devonian paleogeographic elements (light blue represents carbonate platforms; dark blue, basins) on the Laurentian continental margin with Hyper-enriched Black Shale localities from the current study (red stars) and other Hyper-enriched Black Shale localities (red circles). NWT, Northwest Territories. Gadd et al. (2020).

Rhenium is one of the rarest elements on Earth and the second highest stable element on the periodic table, after Hafnium. There are two naturally occuring isotopes of Rhenium, the stable Rhenium¹⁸⁵, and the 'unstable' Rhenium¹⁸⁷. Unusually, the 'unstable' form of Rhenium is the more common; this is because it has an extremely long half-life, about 4160 million years, which means that a significant proportion of the Rhenium¹⁸⁷ present during the formation of the Earth is still in that form (like all elements heavier than Iron, Rhenium is formed during supernova events at the end of the life-cycle of massive stars). When Rhenium¹⁸⁷ does decay it does so into the stable radiogenic isotope Osmium¹⁸⁷. Osmium also has two stable isotopes, the non-radiogenic Osmium¹⁸⁸, and the radiogenic Osmium¹⁸⁷. This means that over time ths amount of Rhenium¹⁸⁷ in a rock sample will fall at a predictable rate, while the amount of Osmium¹⁸⁷ rises at the same rate, and the amount of Osmium¹⁸⁸ remains constant, making it possible to date the rock by comparing the ratios of these elements.

An early Rhenium-Osmium geochronologic study on Hyper-enriched Black Shale mineralisation at the Nick Prospect determined a Devonian age for the Hyper-enriched Black Shale, which only broadly agreed with established stratigraphic relationships. Since then, sulphide Rhenium-Osmium geochronology has been refined and is suited for dating Hyper-enriched Black Shale mineralisation and other sediment-hosted base-metal sulphide deposits.

Gadd et al. combined data from the Rhenium-Osmium geochronology with Conodont biostratigraphic ages that further develop the chronological framework in the context of global episodic changes in the Devonian marine environment. The combined age data allowed Gadd et al. to assess the possible role of global-scale eustatic and biotic events that punctuated the Devonian, and may have been climacteric in forming Hyper-enriched Black Shale mineralisation along the western Laurentian continental margin.

The northern Yukon Hyper-enriched Black Shale showings are located within Paleozoic basinal strata of the Richardson Trough. This tectonic depression has been interpreted as a failed rift. It is bounded to the east by the Mackenzie Platform and to the west by the Yukon Stable Block; the latter a stable, offshore shelf during the pre-Carboniferous Paleozoic. Sedimentary strata within the Richardson Trough are characterized by over 1000 m of fine-grained, carbonaceous siliciclastic rocks of the Upper Cambrian to Middle Devonian Road River Group. The Canol Formation overlies the Road River Group and consists of as much as 220 m of Middle Devonian to lower Upper Devonian cherty, carbonaceous shale. It is at this contact that a thin (1–10 cm thick) Hyper-enriched Black Shale layer is documented in several localities throughout northern Yukon. These Hyper-enriched Black Shale layers consist of about 60% by volume sulphides and 40% by volume non-sulphides.and sulphides are interbedded with black, siliceous shales. Samples typically contain a number of sedimentary features that include laminar bedding disrupted by softsediment deformation. The upper and lower contacts with the enclosing host rocks are typically sharp, but minor slumping into the underlying siliceous shales may be locally present. There is minor to abundant biogenic debris, which includes Conodont elements and pyrite-permineralized plant matter and a relatively low abundance of terrigenous clastic detritus.

Hand‐sample specimen from Peel River (17‐ POA‐049C) showing semi‐massive sulphides. Gadd et al. (2020).

The stratigraphic sections for the Peel River Showing and the Nick Prospect are similar lithologically, with local variations in the thicknesses of units. The lithologies comprise (1) black shale with 0.5–1.5-mdiameter calcareous concretions; (2) siliceous black shale with minor calcareous intercalations; (3) a 1–10-cm-thick stratiform semi-massive Nickel-Zinc-Iron-sulphide Hyper-enriched Black Shale layer; and (4) siliceous to cherty black shale that is in sharp contact with the underlying Hyper-enriched Black Shale mineralisation. The significant difference between the stratigraphic sections of the two locations is that three distinct Hyper-enriched Black Shale layers have been documented at Peel River, whereas only one is identified at Nick Prospect Gadd et al's study focuses on the Hyper-enriched Black Shale layer that is common to both localities (i.e. at the regional stratigraphic contact).

Devonian stratigraphic sections of the Richardson trough, northern Yukon, Canada, using the Devonian time scale (A). Note the position of hyper-enriched black shale (red lines) at the regional contact between the Road River Group and Canol Formation at Peel River showing (B) and Nick Prospect (C). Black stars denote locations of Conodonts for which biostratigraphic information is available. PGE, Platinum Group Element. Gadd et al. (2020).

The contact between the Road River Group and the overlying Canol Formation is conformable. The terminal Road River Group (i.e. the Hyper-enriched Black Shale) is a regionally extensive condensed section. Condensed sections form in response to shoreline transgressions that form during late transgressive to early highstand systems tracts. They are clastic sediment-starved, such that thin (centimeter-scale) beds represent relatively long durations. Several criteria are used to identify condensed sections, and relevant ones present within the Hyper-enriched Black Shale are high abundances of authigenic minerals (such as sedimentary sulphides), abundant biogenic debris, high abundances of metals, and widespread geographic distribution. Moreover, Hyper-enriched Black Shales are starved of clastic sediments, especially in comparison to the immediately underlying Road River Group. 

Hand‐sample specimen from Nick Prospect (18‐POA‐077) showing semi‐massive sulfides. Gadd et al. (2020).

Pyrite is the most abundant sulfide in the Yukon Hyper-enriched Black Shale, and there are several textural varieties, including framboidal, microcrystalline, and nodular A key mineralogical difference
between the localities is that at Nick Prospect, the predominant Nickel sulfide is vaesite (NiS₂), whereas millerite (NiS) predominates at Peel River. Another difference is that vaesite and pyrite host Platinum Group Elements and Rhenium at Nick Prospect, but pyrite is the primary host for these elements at Peel River. The sulphide paragenetic sequence is broadly the same at both places, wherein framboidal pyrite is the earliest sulphide. The large range in framboid diameters (0.003–0.1 mm) suggests that precipitation was initiated within reducing pore waters rather than in the water column. Subsequent pyrite growth overprints framboids or consists of aggregated microcrystals within nodules. Significant Nickel-sulphide postdates pyrite, as evidenced by Nickel-sulphides encrusting or replacing pyrite.

Photomicrograph showing complex, intergrown relationships among sulfide minerals in a siliceous shale matrix from Peel River. Gadd et al. (2020).

Samples of the Hyper-enriched Black Shale mineralization were collected from the regional stratigraphic contact at both Peel River (65.893°N, 135.931°W) and Nick (64.728°N, 135.235°W) and were processed at the University of Alberta. Mineral separates (9) from Peel River sample 17-POA-049c have Rheniu, and Osmium abundances of 30.1–44.2 parts per million and 179–325 parts per billion, respectively. Mineral separates (6) from the Nick Prospect sample 18-POA-077 have Rhenium and Osmium abundances of 8.8–42.2 parts per million and 46.3–311 parts per billion, respectively. The host shale at Peel River contains 1.12 parts per million Rhenium and 5.82 parts per billion Osmium, whereas the host shale at Nick Prospect contains 1.42 parts per million Rhenium and 8.87 parts per billion Osmium.

Photomicrograph of pyrite‐vaesite mineralisation in a specimen from Nick Prospect wherein pyrite is overgrown and replaced by vaesite. Gadd et al. (2020).

Isochron ages were calculated using IsoplotR software with the Model 3 regression, which accounts for potential variability in initial Osmium¹⁸⁷/Osmium ¹⁸ ratios. Sulphide mineralisation from the Peel River sample yields an isochron age of 387.5 million years, plus or minus 4.4 million years, which overlaps within error with the isochron age of sulphide mineralization from the Nick Prospect (390.7 million years, give or take 5.1 million years). Unmineralised host shale isochrons coincide with the sulphide isochrons at Peel River and Nick Prospect. Including the latter data modifies the isochron ages to 386.6 give or take 3.1 million years and and 390.6 plus or minus 4.8 million years for Peel River and Nick Prospect, respectively.

Isochron ages for the Peel River and Nick Prospect localities, which are about 130 km apart, reveal that mineralization is coeval within a reasonable margin of error. Ages of Conodonts extracted from shales 0.9 m above and 1.1 m below the Hyper-enriched Black Shale at Peel River indicate that the age of sedimentation is within the Polygnathus eiflius (388.6–388.2 million year old) to the Polygnathus ansatus (386.3–385.4 million year old) biostratigraphic zones. These ages bracket the Peel River isochron age. Carbonate concretions 1.2–1.9 m below the Hyper-enriched Black Shale at the Nick Prospect yield Conodonts from the Polygnathus australis (389.2–389.0 million year old) to
Polygnathus ensensis (388.2–387.7 million year old) biozones. The Nick Prospect isochron age is within this range, but must be younger than the age of the underlying sediments from which the Conodont elements were extracted. The close agreement between Peel River and Nick Prospect isochron ages for the semi-massive sulphides and the Conodont ages signifies that mineralisation was synchronous with sedimentation.

Conodont specimens from the studied samples. (A) Polygnathus parawebbi, upper oblique view (O‐255899); (B) Polygnathus eiflius, upper view (C‐626870); (C) Polygnathus linguiformis,  upper view (C‐626871); (D) Polygnathus rhenanus, right lateral, upper and left lateral views of one specimen (C‐626871). Gadd et al. (2020).

The radiogenic nature of the Hyper-enriched Black Shale samples limits the precision of the projected Osmium isotope ratio and precludes unequivocal determination of the origin of the Osmium-bearing fluids. However, the Osmium isotope ratio of the Hyper-enriched Black Shale at Peel River and Nick Prospect are comparable to Middle Devonian seawater values. Temporally, the best estimate for the Osmium isotope composition of seawater comes from Middle Devonian organic matter. The Osmium isotope ratio for Peel River and Nick Prospect overlap with this value, which would be expected if Osmium was scavenged from seawater. Furthermore, the host black shales, which derived Osmium from ambient seawater, produce a statistically indistinguishable isochron when added to the sulphide isochron indicating that the host black shales and sulphides share an Osmium source (seawater). A seawater source for metals is also supported by Rare Earth Element distributions, Thallium, and Molybdenum isotopes, Sulphur isotopes, and mineral chemical compositions. Due to imprecision of the Osmium isotope data, a magmatic-hydrothermal Osmium source cannot be definitively discounted; however, the geochemical data presented in this and previous studies indicate a seawater source for the metals is the most probable.

Hyper-enriched Black Shale at the Peel River showing and Nick Prospect occurs at the Road River Group–Canol Formation stratigraphic contact, as does Hyper-enriched Black Shale at the Moss and Monster River showings. Based on the synsedimentary mineralisation ages and shared stratigraphic relationships between Peel River and Nick, Gadd et al. speculate that Hyper-enriched Black Shale at Moss and Monster River also formed during deposition at about 390–386 million years ago. Metal enrichments in black shales from this time period are documented elsewhere in western Canada and globally. At Trail River shales at the stratigraphic contact between the Road River Group and Canol Formation are enriched in a similar suite of elements (Nickel, Molybdenum, Selenium, etc.) to those in the Hyper-enriched Black Shale, albeit at lower abundances. Recently discovered Nickel-Zinc Hyper-enriched Black Shale occurs at the Akie Property in northeastern British Columbia in the Kechika Trough (1100 km southeast of the Yukon Hyper-enriched Black Shale), a southeast-striking, finger-like extension of the Selwyn Basin. Here, Crinoid-bearing bioclastic wackestone collected 6 m above the Hyper-enriched Black Shale yields Conodont elements from the Polygnathus australis to the Polygnathus ansatus zones, suggesting that the pre-Polygnathus ansatus zone age for Hyper-enriched Black Shale in the Kechika Trough correlates with that of Hyper-enriched Black Shale in the Richardson Trough. Interestingly, the Eifelian-Givetian stage boundary at Jebel Mech Irdane, Morocco, contains pyritic beds with anomalously high (albeit lower than Hyper-enriched Black Shale in Yukon and British Columbia) abundances of redox-sensitive trace elements (e.g., Nickel, Vanadium, Molybdenum, Arsenic) and Platinum Group Elements.

Sea-level instability is a well-established aspect of the Middle Devonian, and the radiometric and biostratigraphic age constraints of the Hyper-enriched Black Shale overlap with the ages of the Kačák, pumilio, and Taghanic biotic events. These events were of global extent, and are defined by abrupt faunal changes, eustatic sea-level rise, continental shelf flooding, and black shale deposition. Some of the salient controls on Hyper-enriched Black Shale formation are: nutrient-rich upwelling, effective trapping of these nutrients due to basin architecture, highly efficient organic matter remineralization, and condensed sedimentation. Additionally, Gadd et al. suggest that metal enrichment may be a hallmark feature of Middle Devonian eustaticbiotic events. Eustatic sea-level rise associated with the Middle Devonian event(s) may have facilitated basin-scale Hyper-enriched Black Shale mineralization by providing steady-state input of nutrient and other trace elements in seawater over long durations in a setting with little clastic input in favorable basinal positions (e.g., semi-restricted sub-basins) on the western Laurentian continental margin.

Comparison of radiometric Rhenium-Osmium isochron ages (white bar is age and blue bar is 2σ uncertainty) of hyper-enriched black shale (HEBS) at Peel River locality and Nick Prospect (northern Yukon, Canada) with Conodont biostratigraphic ages. Light-blue vertical bars correspond with globally recognized biotic events: (K), Kačák; (P), pumilio; (T), Taghanic. (L. Dev), Lower Devonian; (U. Dev.), Upper Devonian; (Frasn.), Frasnian; (rhen.-v.), rhenanus-varcus; (semi.), semialternans. Conodont genus is Polygnathus. Gadd et al. (2020).

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