Aphrastura subantarctica: A new species of Ovenbird from the Diego Ramírez Archipelago.

Ovenbirds, Furnariidae, are a group of small-to-medium sized Passarine Birds distributed from Mexico to the southernmost islands of South America (the North American Ovenbird, Seiurus aurocapilla, is not actually a true Ovenbird), which get their name from their construction of elaborate clay nests by some members of the group - although most species do not actually do this. 

The genus Aphrastura contains two species of South American Overnbirds known colloquially as Rayaditos. The Thorn-tailed Rayadito, Aphrastura spinicauda, is found in temperate forests from north-central Chile to the islands off the southern tip of South America, while the Masafuera Rayadito, Aphrastura masafuerae, is found only in the misty Tree Fern forests of the remote Alejandro Selkirk  Island, 670 km off the west coast of Chile. 

The southernmost population assigned to Aphrastura spinicauda is found in the Diego Ramírez Archipelago, a group of continental islands forming the southernmost part of South America, separated from Cape Horn by the 100 km wide Drake Passage. The Diego Ramírez Archipelago is a group of small islands, rocks, and reefs, with a total exposed area of about 79 km². None of the islands there have any trees, let alone forests, the islands are constantly buffered by strong winds, average annual rainfall is about 1500 mm, distributed throughout the year, with annual temperatures being 5.6°C, falling to 3.3°C in the coolest month (August) and rising to 7.5°C in the warmest month (March).

Despite this apparently hostile environment, the second largest landmass in the group, Gonzalo Island, which is covered entirely by Tussock Grass, Poa flabellata, and otherwise populated only by burrow-nesting Seabirds, hosts a population of Rayaditos, birds otherwise known only from dense forest interiors, which otherwise are usually reluctant to cross wooded scrublands and never found in open environments.

In a paper published in the journal Scientific Reports on 26 August 2022, a team of scientists led by Riccardo Rozzi of the Cape Horn International Center of the Universidad de Magallanes, and the Sub-Antarctic Biocultural Conservation Program, Department of Philosophy and Religion and Department of Biological Sciences at the University of North Texas, examine the Gonzalo Island population of Aphrastura, looking at its ecology and genetic heritage, and formally describe it as a new species.

Genetic analysis of DNA obtained from blood samples has already proven a useful tool in the study of the genus Aphrastura, establishing that populations of Aphrastura spinicauda found on Chiloé Island, the Chonos Archipelago, and on Mocha Island, all off the west coast of Chile, are genetically distinct enough to be classified as different subspecies.

Applying the same methodology to the Gonzalo Island population, Rozzi et al. found that the population was significantly diverged from other populations assigned to Aphrastura spinicauda, and that there were no signs of ongoing interbreeding between the Gonzalo Island population and any other population, including other populations on islands off the southern coast of South America.

Based upon this conclusion, Rozzi et al. describe this population as a new species, which they name Aphrastura subantarctica, in reference to its distribution, being found only on a remote, Subantarctic Island. Morphologically, this species resembles other species of Aphrastura, most notably in sharing their distinctive thornlike tail-feathers, but has a larger body, larger beak, larger tarsus (lower leg), and shorter tail than other members of the genus.

Habitat characteristics and individual appearance of two populations of Aphrastura. (A) Forest habitat on Navarino Island. (B) An individual of the Thorn-tailed Rayadito, Aphrastura spinicauda, from Navarino Island. (C) Tussock Grass, Poa flabellata, habitat on Gonzalo Island, Diego Ramírez Archipelago, with a mist net. (D), (E) Individuals of the proposed new species Aphrastura subantarctica from Gonzalo Island. Omar Barroso in Rozzi et al. (2022).

Other members of the genus Aphrastura typically nest in holes in trees. However, trees are entirely absent on Gonzalo Island, and the dominant form of vegetation, Tussock Grass, offers no equivalent nesting spaces. Observation of the nesting behaviour of Aphrastura subantarctica on Gonzalo Island found that about half of nesting sites were on the ground, in locations sheltered by Tussock Grasses or geographical features, while the remainder were in burrows abandoned by nesting Seabirds or natural soil cavities.

The adult Birds typically forage for invertebrates on the ground no more than 20-30 m from the nest, and fly less often, and lower, than other members of the genus. Both increased body-size and reduced flying compared to close relatives are common in Bird species inhabiting small islands, and are considered to be adaptations to living in environments where being swept out to sea by high winds is a more serious threat than predation, something certainly true for the Aphrastura population on Gonzalo Island.

Gonzalo Island is more than 100 km from its nearest neighbour, Horn Island, a distance probably impossible for any member of the genus Aphrastura to cross, particularly given the prevalent high winds in the area. Rozzi et al. suggest that these Birds probably reached the island during the Last Glacial Maximum, between 28 000 and 18 000 years ago, when lower sea levels would have meant the islands would have been larger, possibly with now submerged smaller islands between, enabling a small population to reach the new environment.

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Abandoned mine discovered in Redruth, Cornwall.

A disused mine tunnel, thought to be over a hundred years old, has been found in the former mining town of Redruth in Cornwall, UK. The tunnel was found beneath land attached to a house by local expert Stuart Dann of survey company Mining Eye Ltd., beneath a manhole cover on the property, and is thought to present no threat to the house or any other nearby property. The accessible tunnel is about 1.6 km in length, although it was presumably once much longer as it has partially collapsed preventing further below-ground exploration. 

Previously unknown mining tunnel discovered in Redruth, Cornwall. Stuart Dann/Cornwall Live.

Old mine-workings are hardly unusual in Redruth, the town was once the richest centre of tin and copper mining in the world. However, the new tunnel appears to be part of a fairly large mining operation, the discovery of which is unusual. Stuart Dann, who has now bought the property on which the mine tunnel is situated, intends to carry out exploratory drilling work on the site to determine the extent of the tunnelling.

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The Nadir Crater: A possible second End Cretaceous Impact Structure off the coast of West Africa.

High velocity impacts by large asteroids and comets are a known, but poorly understood, threat to life on Earth. Impactors with diameters of about 50 m (the estimated size of the object which caused the Tunguska explosion) are thought to hit the Earth roughly once every 900 years, and objects with diameters of 1 km or greater, likely to cause devastation on a regional or global scale, to impact the Earth roughly every million years. The largest impacts cause devastation on a global scale, with the best known of these being the Chicxulub impact 66 million years ago, an event thought to have caused the End Cretaceous Extinction Event. Despite the importance of such events, very little evidence of their having occurred is known, with only about 200 large impact craters known on Earth, of which only 15-20 are interpreted as having happened in a marine environment, despite 71% of the Earth's surface being covered by water. This lack of impact craters, and in particular well preserved impact craters, severely hampers our ability to understand the effects of the events which cause them.

In addition to this general lack of impact craters in the rock record, there is a proportional lack of double impact craters. About 15% of the asteroids in near-Earth orbits are thought to be binaries (i.e. pairs of gravitationally locked asteroids, whereas only 2-4% of known impact craters on Earth have ever been suggested as potentially being parts of pairs. Furthermore, most of these suggestions have either been disproven or are disputed. On Venus, where meteor impacts are generally recorded as 'dark splotches' rather than true craters, due to the much more destructive nature of the Venusian atmosphere, about 14% of impacts appear to be binary. Even allowing for a considerable degree of inaccuracy in these numbers, binary impacts seem to be significantly under-represented on Earth.

Another phenomenon seen elsewhere in the Solar System but largely absent on Earth is impact clustering, i.e. the formation of a series of impact craters over a relatively short period of time. This is typically caused by the breakup of a large object due to a gravitational encounter with a planet, followed by the debris from this breakup remaining on an orbit which intersects that of the planet, resulting in a series of impacts over a drawn out period of time. The only observed instance of such an event was the breakup of Comet Shoemaker-Levy 9 in 1992, which was followed by a series of impacts in 1994 as the debris from this breakup collided with Jupiter, but it is thought that the breakup of larger bodies can result in strings of impacts which could continue for several million years. Only a single such event has been recorded on Earth; a series of linked craters in the Ordovician, which is combined in a shift in the ratio of argon isotopes in the Earth's atmosphere at that time, and evidence for an increase in the amount of extraterrestrial dust raining down on the Earth at that time. Other proposals have been made for clusters of impacts in the Cretaceous and Eocene, but the evidence in support of such events is sparse. This is despite the Moon showing clear evidence of a number of periods of impact clustering, events which must surely also have affected the Earth.

In a paper published in the journal Science Advances on 17 August 2022, Uisdean Nicholson of the School of Energy, Geoscience, Infrastructure and Society at Heriot-Watt University, Veronica Bray of the Lunar and Planetary Laboratory at the University of Arizona, Sean Gulick of the Institute for Geophysics, Department of Geological Sciences and Center for Planetary Systems Habitability at the University of Texas at Austin, and Benedict Aduomahor, also of the School of Energy, Geoscience, Infrastructure and Society at Heriot-Watt University, announce the possible discovery of a second End-Cretaceous impact structure on the Guinea Plateau of the coast of West Africa, based upon evidence from two-dimensional seismic surveys.

The Guinea Plateau is an area of submerged continental crust extending southwest from the coasts of Guinea and Guinea Bissau for about 400 km, where it meets with the Guinea Fracture Zone, beyond which lies the deep ocean. The plateau can be divided roughly into two parts, an inner zone which is about 400 m deep across most of its extent, and an outer zone, known as the Guinea Terrace, which deepens gradually from about 400 m to about 1200 m. The Guinea Fracture Zone is home to a series of seamounts (inactive submarine volcanoes), including the Nadir Seamount.

Map and regional seismic sections showing location of Nadir Crater. (A) Regional bathymetry map of the Guinea Plateau and Guinea Terrace showing location of 2D seismic reflection and well data used in the study. JS, Jane Seamount; NS, Nadir Seamount; PS, Porter Seamount. The white dashed line shows the NE extent of high-amplitude discontinuous seismic facies at the top Maastrichtian interpreted as ejecta deposits and associated tsunami deposits. The north-east limit of this facies closely corresponds with the Maastrichtian shelf-slope break at the landward margin of the Guinea Terrace. Inset map shows a palaeogeographic reconstruction of the Atlantic near the end of the Cretaceous, about 66 Ma ago. Ch, Chicxulub Crater; Nd, Nadir Crater; Bo, Boltysh Crater. (B). Regional composite 2D seismic reflection profile extending from the GU-2B-1 well in the east to the deep Atlantic basin in the west, showing the structural and stratigraphic character of the Guinea Plateau and Guinea Terrace. (C) North-South seismic profile from the salt basin in the north to the Nadir Seamount, south of the Guinea Fracture Zone. Nicholson et al. (2022).

The Guinea Plateau formed by extensional rifting in the Triassic-Jurassic, as the opening of the Atlantic Ocean pulled the North American landmass away from the west coast of Africa. The plateau remained part of the passive (i.e. seismically inactive) margin of the Central Atlantic until about 100 million years ago, when the its southern margin underwent further rifting, as South America separated from Africa.

The crater which Nicholson et al. propose is located on the southwestern part of the Guinea Terrace, roughly 60 km to the north of the Guinea Fracture Zone and 100 km northwest of the Nadir Seamount. The crater is covered by about 400 m of sediment, and a water column about 900 m deep.

The stratigraphy of the area has been determined from boreholes and seismic surveys. Cretaceous deposits here can be divided into a Lower Cretaceous sequence, roughly 145-100 million years old, and an Upper Cretaceous sequence, 100-66 million years old, separated by an unconformity known as the Top Albian. The deposits making up the Upper Cretaceous sequence are largely undeformed, and extend laterally across the Guinea Terrace. These deposits can be further subdivided by a series of (seismically) reflective surfaces, numbered KU1-KU4, which are interpreted as representing major flooding events. The Upper Cretaceous sequence can be divided into a Lower Unit, which is about 300 m thick and contains a series of high amplitude reflective surfaces, and an Upper Unit, which is about 500 m thick and contains moderate-to-low amplitude reflective surfaces.

Seismic characteristics of the Nadir Crater. (A) Seabed depth map of crater showing seismic line locations and the mapped extent of the crater rim and damage zone.  (B)  West-east  seismic  section  (pre-stack  depth  migration  –  depth  domain)  across  the  crater,  highlighting  the  crater  morphology  and  damage  zone,  and  the  extent  of  subsurface deformation. (C) Detailed seismic stratigraphic and structural elements of the crater. KP, Cretaceous-Paleogene sequence (KP1 equivalent to Top Maastrichtian); KU, Upper Cretaceous seismic horizons. KU1 and KP1 'regionals' are schematic reconstructions of these seismic horizons before formation of the crater at the end of the Cretaceous and have been used to reconstruct a conceptual model of crater formation. (D) Southwest-northeast seismic section (pre-stack time migration – time domain) across the crater, showing crater morphology and seismic facies outside the crater, including high-amplitude seismic facies sitting above a roughly 100-ms-thick unit of chaotic reflections, interpreted to have formed as a result of seismic shaking following the impact event. Nicholson et al. (2022).

The end of the Cretaceous sequence here, identified as the Top Maastrichtian, is clearly identifiable, and well defined, but the age of other reflective surfaces in the sequence is less certain. The KU-1 reflective surface may mark the Top Turonian, making it about 90 million years old, and the high amplitude surfaces between this and the underlying Top Albian surface may represent black shale deposits, with elevated levels of organic carbon. The lower amplitude reflective surfaces above KU-1 probably represent marine shales and marls.

The Top Maastrichtian Surface lies at the top of another set of distinctive, high amplitude reflective surfaces, and is marked by an erosional unconformity on the southwest part on the Guinea Terrace, which is not present to the north and east. The reflective surfaces at this level extend laterally across the Guinea Plateau, although they are eroded away on both the continental slope and the inshore shallows. On the Guinea Terrace these surfaces overlie a chaotic seismic surface about 100 m deep.

Overlying the Top Maastrichtian, the Cainozoic deposits begin with a thick sequence of carbonates and clastic sediments, although these are considerably thinner on the Guinea Terrace than elsewhere on main Guinea Plateau. Precise dating of these sediments is again difficult, though sediments younger than about 23 million years old are apparently absent on the Guinea Terrace; strat here comprise a Palaeocene-Eocene sequence about 300 m thick, overlain by a series of Oligocene mass transport deposits (debris from submarine landslides).

The feature which Nicholson et al. identify as the Nadir Crater is a depression at least 8.5 km wide in the Top Maastrichtian. This can be seen where two separate seismic profiles cross one-another, suggesting a rounded or elliptical shape. Since there is no particular reason to believe that the intersection of the two seismic profiles lies at the centre of the depression, 8.5 km wide must be taken as a minimum measurement. The crater bed is 200 m below the 'seafloor' represented by the Top Maastrichtian, while the crater rim is raised 20-40 m above this level. There appears to be a slight uplift in the centre of the crater at the Top Maastrichtian level; this is more pronounced at the Top Albian level, where it rises 350 m above the surrounding terrain. On either side of this central uplift the strata below the crater floor and rime are intensely deformed, with a column of extensively folded and fractured material extending downwards 700 m from the Maastrichtian surface. A wider area of faulting extends 10-12 km from the crater in all directions, with all faults dipping towards the crater.

All of these phenomena are consistent with large impact craters elsewhere on Earth and other Solar System bodies. The rim is raised  above the surrounding terrain approximately one tenth as much as the crater floor is depressed beneath it. The crater is approximately one twentieth as deep as it is wide. The damage zone around the crater is approximately twice as wide as the crater itself. The crater has a terraces structure and a raised central peak.

The deformation beneath the crater is also consistent with the predictions for large craters of this sort. The central uplift seen in such craters is caused by the elastic rebound of shock waves passing through the rock, causing material beneath the crater to flow upwards, initially flowing freely the brecciating as the rock regains its brittleness. The deformations seen in the zone 10-12 km from the crater are consistent with these areas being subjected to a significant shockwave, then flowing back towards the crater to fill a partial void.

Based upon examination of other impact craters on Earth, the zone of uplift beneath a crater 8.5 km wide would be expected to extend downwards to between 630 and 780 m beneath the impact surface. That of the Nadir Crater extends down about 800 m, with some deformation extending downwards into underlying Jurassic Sediments. However, this does not necessarily imply a much larger crater; rather it may be a consequence of a bolide impacting soft, unconsoidated marine sediments. The Mjølnir Crater, in the Barents Sea off the coast of Norway, shows a similar pattern of deformation beneath the crater, although here there is less deformation outside the crater rim, as is the case at Chicxulub. This deeper pattern of damage at sites where large, high velocity objects impact shallow marine sediments may be caused by the sudden decrease in the porosity of the strata impacted, resulting greater brecciation, and the formation of hydrothermal systems which could potentially last for millions of years.

The Top Maastrichtian layer within the crater is overlain by a series of high amplitude reflective layers identified as KP-1 to KP-3, with KP-1 marking the boundary between the Maastrichtian and the Palaeocene. Between the Top Maastrichtian and KP-1 is a layer about 100 m thick and relatively transparent to seismic waves, which Nicholson et al. interpret as a possible suevite layer (i.e melt materia), similar to that seen as the Chicxulub Crater. Above this, between KP-1 and KP-2 is a unit made up of a series of low-amplitude reflective surfaces onlapping onto the inner crater walls, which Nicholson et al. interpret as reworked ejecta material. 

The crater is surrounded by a blanket of material made up of a series of high amplitude reflective surfaces, which Nicholson et al. also interpret as reworked ejecta, combined with material deposited by tsunamis triggered by the impact. Beneath this is a layer of chaotic material, which may represent older strata re-organised by the shock wave from the impact.

The Nadir Crater a deep inner crater with in inner peak and a flat outer zone, surrounded by a raised rim, features also seen in confirmed marine craters such as the Chesapeake Crater (off the east coast of North America), Lockne Crater (in northern Sweden), and Flynn Creek Crater (in Tennessee), strongly supporting the idea that this is an impact crater, although this hypothesis will only be confirmed by drilling into the structure.

There are other processes which can form structures resembling impact craters, such as the dissolution of salt deposits, escape of gas or fluids, volcanic activity, deformation due to tectonic stresses, or any combination of these. However, none of these possible alternatives appears consistent with the geology of the Guinea Terrace,

Salt diapers (domes formed when salt intrudes into overlying rocks) can collapse as the salt within them dissolves into the water column, leaving a circular depression surrounded by radial faults. However, such structures do not have uplifted central zones, nor would they be surrounded by a ring of high amplitude material of the type Nicholson et al. interpret as ejecta at the Nadir Crater. Furthermore, while there are salt diapers on the Guinea Terrace, these are in a zone far to the northwest of the Nadir Crater, with the closest diaper being over 250 km from the crater. 

Craters left by fluids escaping from mud layers as they are buried and then subjected to pressure, are common features on the seafloor, being particularly common on the continental margins, where large amounts of methane are generated. Most of these are a few metres to a few hundred metres across, but the largest can exceed 10 km, making ruling our such an origin for the Nadir Crater impossible on the basis of size alone. However, such structures are almost always found in clusters, and no structures similar to the Nadir Crater can be seen anywhere else on the Guinea Plateau, nor are there any other signs of gas or fluid escape in this region. Furthermore, craters formed by fluid escape do not typically produce crater rings, central uplift, or ejecta, all of which can be seen at the Nadir Crater. 

A variety of depressions, typically bounded by fault zones, can be formed when slip-strike deformation leads to areas of seafloor being pulled apart. However, such depressions are rarely circular, typically being more than twice as long as they are wide. Nor do such structures have rims or centrally uplifted zones. More importantly, the Nadir Crater is located on an area of the African continental margin where slip strike activity is thought to have ceased about 44 million years before it formed, making such an origin highly unlikely.

The most plausible alternative hypothesis for the origin of the Nadir Crater is volcanism. Phreatic explosions, which occur when hot magma comes into contact with water, can form maars (broad, low relief volcanic craters) similar in form to the Nadir Crater. Furthermore, there clearly was volcanic activity nearly contemporary to the formation of the crater in the area; the Nadir Seamont is only 100 km away from the Nadir Crater, and 7.4 million years younger, i.e. close enough to be part of the same volcanic complex, along with the nearby Grimaldi Seamounts. However, maar structures seldom exceed 2-3 km in diameter, and their three dimensional structure has a distinctive funnel-shape unlike anything revealed by the seismic surveys at the Nadir Crater. Furthermore, maars do not have a central uplift structure, making it highly unlikely that the Nadir Crater is a volcanic maar.

Next Nicholson et al. used a hydrocode model to simulate the impact of a 400 m wide object (the estimated size of the Nadir impactor) into soft sediments covered by a range of water depths. All of the models produced a crater between seven and nine kilometres in diameter, with elevated rims and significant central uplift. The amount of subsequent rim collapse varied with water depth, with impacts into deeper water showing more collapse. This lead to more infill into the crater, resulting in a flatter profile and a less prominent central uplift zone. At depths of greater than 1 km, the rim completely collapsed into the crater, creating a very flat profile. In all cases, regardless of depth, the impact resulted in a crater surrounded by a network of faults.

Numerical model results of iSALE hydrocode simulations of final crater architecture for different water depths (200, 500, 800, 1100, and 1500 m, as indicated on the figure) Models assume a 400 m diameter impactor with an impact angle of 90°. The water layer has been removed from each image to better highlight the final crater morphology. Model outcomes show total plastic strain on the left and deformed lithologies on the right - the gray unit represents the assumed Cenomanian-Turonian black shale deposits as a marker horizon. Nicholson et al. (2022).

All models show the development of a central uplift, although the extent of this varies with depth, with larger central uplift zones in impacts at greater depth, possibly representing the rocks rebounding more when the weight of the overlying water is removed by the flash-vaporization cause by the impact. These simulations showed material from as deep as 2 km below the surface being moves upwards, with material from up to 1 km deep reaching the subsurface within the central peak; based upon the available data Nicholson et al. estimate that material from as deep as 750 m beneath the Nadir Crater is present within its central peak.

Snapshots of numerical model results of an iSALE hydrocode simulation for 800 m target water depth. Considered this to be the most likely water depth for the impact, based on crater morphology. Model snapshots show transient crater formation and generation of a rim-wave tsunami after 3 seconds; rebound of central uplift and propagation of a rim-wave tsunami away from crater at 38 seconds; crater collapse, resurge, and central jet formation at 85 seconds; and further resurge at 245 seconds. Nicholson et al. (2022).

All of the simulated impacts produced zones of brecciation beneath the crater, with the depth to which this extended ranging from about 1 km up to about 2.5 km. Areas of damage were also present around all the simulated craters, with faults and fractures extending at least 5 km from the craters and reaching depths of at least 1 km.

The models are consistent with the observed features at the Nadir Crater, insofar as all of the craters forming at depths shallower than 1 km produced raised rims, uplifted central areas, terraces made up of debris, and areas of damage outside the crater. The models which produced craters most similar to the Nadir Crater had water depths of about 800 m.

Based upon the data gained by running these simulations, Nicholson et al. developed a conceptual model for the chain of events around the impact. It is assumed that the initial stratigraphy of the area comprised a series of flat, horizontal bedding plains, overlain by about 800 m of seawater. The initial impact would have formed a transient crater about 1 km deep, followed rapidly by the formation of the central uplifted zone, and the collapse of the initial crater rims. This would produce the current observed structure; a crater with a marked central uplift and observable rims surrounded by terraces of collapsed material, and a wider area of fallen ejecta, a hypothesis which they propose could by tested by drilling in the area, in order to confirm the actual nature of the deposits observed in the seismic profiles.

Conceptual model of the impact sequence at the Nadir impact site, based on seismic observations and analog models.  (A) At time zero, the impactor hits the water surface at a velocity of about 20 km per second, initiating a rim-wave tsunami in its wake. (B) Several seconds later, the transient crater forms, as the impactor and a substantial body of water are vaporised. Impactites (melt rock and breccias) line the transient cavity. Tsunami waves propagate away from the evacuated crater. Shock waves cause substantial damage below and around the impact site, and seismic waves propagate across the plateau; (C) major uplift (roughly 400 m relative to pre-impact regional) occurs in the 'rebound' crater modification phase, resulting in the formation of a raised crater peak; (D) radial collapse of the subsurface damage zone results in further modification of the crater, including the formation of terraces at the surface. Resurge of water transports substantial volume of ejecta and other sediment into the crater, deposited above the impactites. Nicholson et al. (2022).

Taking the data from the simulations, Nicholson et al. used the Earth Impacts Effect Program to assess the environmental damage caused by the event which formed the Nadir Crater. The initial impact would vaporise vast amounts of seawater and seafloor sediment and release an amount of energy roughly equivalent to 5000 megatons of TNT. This in turn would lead to the formation of a fireball over 5 km in diameter, an an air blast which would be travelling at about 470 km per hour at a distance of 50 km from the impact site. 

The event would release seismic waves equivalent to a Magnitude 7.0 Earthquake, fluidising seafloor sediments for hundreds of kilometres in all directions. Following fluidisation of the sediments they would likely be reworked extensively by the tsunami waves caused by the impact. The seismic waves would also be likely to trigger massive landslides along the Guinea Terrace.

The initial impact would cause an 'ejecta curtain', made up of water and sediment, over 2 km high, which would then collapse back onto the sea surface, triggering the formation of a tsunami wave over 500 m high, which would spread at about 400 m per second. As well as spreading out from the rim of the crater, this tsunami would spread inwards, forming a central water jet rising to over 2 km. This would then collapse back into the crater, forming a new tsunami, a process which might repeat itself several times. These repeated tsunamis would cause extensive reworking of sediments along the Guinea Terrace, at depths of up to 800 m. They would also cause extensive scouring of the West African coastline, and to a lesser extent that of South America, which, despite being about 1000 km away, would still be hit by waves about 5 m high. 

The impact would lead to large volumes of water entering the atmosphere, as well as black carbon dust derived from the black shale deposits laid down on the Guinea Plateau during the Cenomanian  and  Turonian.  The high temperatures and pressures unleashed by the impact would turn much of the organic material present in the area directly into methane, with global climatic implications, albeit short lived ones. The seismic shock wave could potentially release a lot more methane from gas hydrate deposits on the Guinea Plateau, and adjacent areas of the deep sea floor.

The impact which caused the Nadir Crater appears to have happened at, or very close to, the Cretaceous-Palaeocene boundary. This places the Nadir Impact very close to the Chicxulub Impact, chronologically speaking, raising the question as to whether the two events were connected. Potentially the Nadir and Chicxulub impactors might have been two parts of a binary object, two fragments of an object which broke up as it came within the Earth's gravitational field, two objects from a longer impact cluster, or two unrelated objects which happened to fall at about the same time. 

Impacts by binary asteroids have previously been suggested as the cause of other crater pairs, including the Lockne and Målingen craters in  Sweden, the East and West Clearwater craters in Quebec, and the Suvasvesi craters in Finland, although more refined dating has shown that neither the Clearwater and Suvasvesi craters are in fact binaries, and no there is no evidence in favour of the Lockne and Målingen craters being related, and therefore no evidence of a binary impact anywhere on Earth. 

Reconstructions show that the Nadir Crater site would have been about 5500 km from the Chicxulub Crater site at the end of the Cretaceous; significantly less than the 8000 km that separates them today, but still to far apart to have been caused by the impacts of two parts of a single object which broke up within the Earth's Roche limit (the minimum distance to which a large satellite can approach a larger body without tidal forces overcoming the internal gravity holding the satellite together), or within the Earth's atmosphere. 

This does not, however, rule out two fragments of a single object which was broken into fragments by an earlier encounter with the Earth's tidal field, then impacted the Earth on a subsequent encounter, in the way that Comet Shoemaker-Levy 9 was torn apart by Jupiter's gravitational field, then impacted Jupiter in several fragments on a subsequent pass several years later. There is some evidence of other large impacts around the End of the Cretaceous, most notably at Boltysh in Ukraine (currently dated to 65.4 million years ago, or 650 000 years younger than the Chixulub impact), as well as preserved fossil meteorites from Poland and the North Pacific. This could imply that the Earth was not just hit by a single large impact, but a protracted asteroid shower, lasting from months to one or two million years, with each impact triggering earthquakes and tsunamis, and contributing to rising atmospheric levels of greenhouse gasses and aerosols.

Estimates as to how often objects the size of the Nadir impactor hit the Earth vary from about once every 100 000 years to about once every 700 000 years. Thus even if it can be demonstrated that the Nadir Impactor fell within a million years of the Chicxulub impactor, it would be impossible to demonstrate that the two events were part of a single asteroid shower (Near Earth Asteroid (101995) 1999 RQ36 is roughly equivalent in size to the Nadir impactor, and is estimated to have a 1 in 1750 chance of hitting the Earth within the next few hundred years). However, if it can be demonstrated that the Chicxulub, Nadir, and Boltysh events all happened within a relatively short period of time, then it would present a much stronger case for the events being related, with important implications for our understanding of events at the End of the Cretaceous.

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(594913) 2020 AV2 'Ayló'chaxnim: The first discovery of an asteroid orbiting entirely inside the orbit of Venus.

Close to a million asteroids have been discovered to date, almost all of them outside the orbit of the Earth. Of the tiny majority that break this rule, the vast majority are Apollo Asteroids, which have Earth-orbit crossing orbits, but spend most of their time outside the orbit of the Earth. Much rarer are the Aten Asteroids, which have Earth-orbit crossing orbits, but are closer to the Sun than the Earth is for the majority of the time. Rarer still, are the Atira Asteroids, which orbit entirely inside of the orbit of the Earth. Theoretically, a small subset of these asteroids will orbit entirely inside the orbit of Venus, although to date no such asteroid has been discovered.

In a paper published on the online arXiv database at Cornell University and accepted for publication in the Monthly Notices of the Royal Astronomical Society, Bryce Bolin of the Division of Physics, Mathematics and Astronomy and Infrared Processing and Analysis Center at the California Institute of Technology, Tomas Ahumada of the Department of Astronomy at the University of Maryland, Pieter van Dokkum of the Department of Astronomy at Yale University, Christoffer Fremling, also of the Division of Physics, Mathematics and Astronomy at the California Institute of Technology, Mikael Granvik of the Department of Physics at the University of Helsinki and the Asteroid Engineering Lab at the Luleå University of Technology, Kevin Hardegree-Ullman of the Steward Observatory of the University of Arizona, Yuichi Harikane of the Institute for Cosmic Ray Research at the University of Tokyo and the National Astronomical Observatory of Japan, Josiah Purdum of the Caltech Optical Observatories of the California Institute of Technology, Eugene Serabyn, of the Jet Propulsion Laboratory at the California Institute of Technology, John Southworth of the Astrophysical Research Institute at Liverpool John Moores University, and Chengxing Zhai, also of the Jet Propulsion Laboratory at the California Institute of Technology, describe the discovery of the first known asteroid orbiting entirely within the orbit of Venus.

The object was discovered by the Zwicky Transient Facility at the Palomar Observatory, which used the Samuel Oschin Schmidt Telescope to detect transients (short lived phenomena) in the Northern Hemisphere sky. Specifically, a portion of the Zwicky Transient Facility's activities, the Twilight Survey, is dedicated to looking at objects and events close to the Sun, alternating between evening and morning observing twilight on clear nights.

This survey covered approximately 40 000 square degrees of sky between September 2019 and January 2020, covering an area with a solar elongation of between 35 and 60 degrees. Objects orbiting entirely within the orbit of Venus should have solar elongations of less than 46 degrees, and calculations suggest that about 85% of such objects should have a maximum solar elongation which falls within the range covered by the Zwicky Transient Facility's Twilight Survey.

On 4 January 2020 the Zwicky Transient Facility detected an object about 40 degrees from the Sun. This object was given the designation 2020 AV2, which implies that it was the 71st asteroid (asteroid V2 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that V2 = (25 x 2) + 21 = 71) discovered in the first half of January 2020 (period 2020 A; the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

Sun-centric sky distribution of Twilight Survey coverage between 2019 September 19 and 2020 January 30. The blue star indicates the first observed location of 2020 AV2 and the yellow star indicates the Sun. The colour scale is the number of Zwicky Transient Facility visits per square degree. Bolin et al. (2020).

Following this initial discovery follow-up observations were made with the Kitt Peak Electron Multiplying Charge Coupled Device Demonstrator on 9 January 2020, and the Southern Astrophysical Research Telescope and Magellan Telescope on 17-19 July 2020. These observations enabled the calculation of the orbit of 2020 AV2, establishing that it has an aphelion distance (the point on its orbit which is furthest from the Sun) of 0.65 AU (65% of the distance at which the Earth orbit's the Sun). This places the orbit of 2020 AV2 entirely inside the orbit of Venus, which has a perihelion distance (point on its orbit closest to the Sun) of 0.71 AU.

This conformation led to the International Astronomical Union's Minor Planet Centre giving the asteroid the designation (594913), indicating that it is the 594 913rd confirmed asteroid. This conformation gives its discoverers the right to name the asteroid (a right which is not always exercised, given the very large number of asteroids discovered, and the fact that the majority of these are discovered by automated surveys), with the International Astronomical Union's Working Group on Small Body Nomenclature accepting the name 'Ayló'chaxnim, which means 'Venus girl' in the Luiseño language indigenous to southern California, on 8 November 2021. Bolin et al. further propose that the term 'Ayló'chaxnim be used as a generic name for all asteroids orbiting inside the orbit of Venus.

Zwicky Transient Facility images of 'Ayló'chaxnim. (A) One of the Zwicky Transient Facility discovery 30 second r-band images of 'Ayló'chaxnim taken on 4 January 2020. 'Ayló'chaxnim is indicated by the red circle. (B) Composite image of the first four Zwicky Transient Facility 30 second r-band exposures over a 22 minute time interval. The images were aligned on the background stars before being co-added. 'Ayló'chaxnim is indicated by the red circles, with the first observation labelled with a red arrow. The apparent faintness of the third detection is due to variations in sky transparency over the 22 minute sequence. The asteroid was moving about 130 arcseconds per hour in the northeast direction resulting in a roughly 10 arcseconds spacing between the detections of 'Ayló'chaxnim. The spatial scale in (B) is the same as in (A). (C) Recovery image of 'Ayló'chaxnim made using the 6.5 m Magellan Baade telescope and Four Star infrared camera on 2021 July 18. The asteroid was moving in the northeast direction at 100 arcseconds per hour. The cardinal directions are indicated. Bolin et al. (2022).

Bolin et al. calculate that (594913) 2020 AV2 'Ayló'chaxnim will enter a 3:2 orbital resonance with Venus (orbit locked to that of Venus on which it will orbit three times for every two orbits of Venus) in approximately 60 000 years time, and that 10 000 years later repeated close encounters with Mercury will have caused it to have lost much of the variation in its orbital path. This resonant orbit will protect the asteroid from close encounters with Venus, in the same way that the resonant orbit between Pluto and Neptune protects Pluto from close encounters with Neptune. (594913) 2020 AV2 'Ayló'chaxnim should remain locked into this orbital resonance for 10 000 years, then move in and out of it for another 100 000 years.

Orbital configuration of 'Ayló'chaxnim. (A) A side view of the plane of the Solar system. The orbits and locations of Earth (purple), Venus (blue), Mercury (red) and 'Ayló'chaxnim (yellow) on 4 January 2020 are shown looking from the side. (B) The same as panel (A) but looking from above the orbital plane of the inner Solar System. The perihelion and aphelion directions of 'Ayló'chaxnim and the planets are plotted with dotted and solid lines respectively. The perihelion distance of Venus, 𝑞Venus, and the aphelion distance of 'Ayló'chaxnim, 𝑄'Aylo'chaxnim are indicated with labelled barred lines. The heliocentric cartesian coordinates 𝑥, 𝑦 and 𝑧 are indicated with the position of the Sun as the origin. Bolin et al. (2022).

Bolin et al. further estimate that (594913) 2020 AV2 'Ayló'chaxnim has migrated to within the orbit of Venus within the past million years, and is likely to remain within this area for about 2 million years from now; earlier calculations of this residence time were shorter, however Bolin et al.'s is based on more data, and therefore presumably more accurate.

The currently available data prevents the accurate calculation of the evolution of (594913) 2020 AV2 'Ayló'chaxnim's orbit into the tens of millions of years scale. However, a model developed by Bolin et al. suggests that (594913) 2020 AV2 'Ayló'chaxnim has a 50% chance of colliding with the Sun or a planet within the next 10 million years, and a 90% chance of undergoing such a collision in the next 90 million years. The model also suggests that (594913) 2020 AV2 'Ayló'chaxnim has a 13% chance of eventually colliding with the Sun, a 52% chance of eventually colliding with Venus, a 16% chance of colliding with the Earth, a 13% chance of colliding with Mercury, and a 2% chance of colliding with Mars, leaving the asteroid with a 4% chance of either surviving on approximately its current orbit for more than 50 million years (the limit of the simulation) or being ejected from the Solar System.

Spectroscopic data on (594913) 2020 AV2 'Ayló'chaxnim collected by the Keck telescope suggests that it has a reddish surface, probably indicative of a silicate S-type asteroid-like composition. Such asteroids are most abundant in the Main Asteroid Belt, the probable point of origin of (594913) 2020 AV2 'Ayló'chaxnim; Bolin et al. calculate an approximately 77% chance that the asteroid originated in the inner Main Asteroid Belt, an 18% chance that it derives from the Hungaria Asteroid Group, which orbit just inside the Main Asteroid Belt, and a 4% chance that it derives from a group of asteroids within the Main Asteroid Belt which have a 3:1 orbital resonance with Jupiter.

Based upon the albedo of (594913) 2020 AV2 'Ayló'chaxnim (i.e. the amount of light it reflects), Bolin et al. calculate that it has a diameter of about 1.6 km.

There are estimated to be about 1000 Near Earth Asteroids with diameters in excess of 1 km, although only about 0.3% of these (i.e. three km-scale asteroids) are estimated to remain permanently inside the orbit of Venus. Thus (594913) 2020 AV2 'Ayló'chaxnim appears to be a significant (if predicted) discovery of an unusual Solar System body. However, this region of the Solar System is relatively underexplored, leaving us with no independent test of the models of this area at this time.

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Glasgow Life formerly hands back seven items to India.

Glasgow Life, the body which, amongst other things, overseas museums in the Scottish city, has agreed to return seven items taken from India during the colonial period. Six of the objects are believed to have been stolen from shrines and temples in Kanpur, Kolkata, Gwalior, Bihar and Hyderabad, while the seventh, a 14th century ceremonial tulwar (sword), was stolen from the Nizam of Hyderbad by his prime minister in 1905, and sold to General Sir Archibald Hunter, the then Commander-in-Chief of the Bombay Command of the Indian Army.

A fourteenth century ceremonial tulwar returned to the Government of India in a ceremony in Glasgow this week. Glasgow Life.

The objects, currently held by the Kelvingrove Art Gallery and Museum, are the latest in a series of items returned to other countries from Glasgow, which started in 1998 with the return of of a Ghost Shirt to the Lakota Nation. The latest items are being returned following following a meeting of Glasgow City Council in April this year (2022), in which a recommendation by the Working Group for Repatriation and Spoliation was accepted. The objects were formerly given to the acting Indian High Commissioner, Sujit Gosh, at a ceremony at the Kelvingrove Museum and Art Gallery on 19 August 2022. Other objects due to be repatriated following the April meeting include 19 'Benin Bronzes' (items looted from the city of Benin in modern Edo State, Nigeria, during a British military action in 1895), and 25 items belonging to the Cheyenne River Sioux and Oglala Sioux tribes of South Dakota, believed to have been taken from the dead following the Wounded Knee Massacre.

A ceremony at the Kelvingrove Museum and Art Gallery in Glasgow on 19 August 2022, in which items in the museum's collection believed to have been stolen from sites in India were formally returned to the Indian High Commissioner. BBC.

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Pagrus major: A Red Seabream from the Late Pleistocene of Taiwan.

Although the island of Taiwan has been extensively studied by geologists interested in its tectonic structure, it has been largely overlooked by palaeontologists. This is unfortunate, as the subduction of the South China Sea basin beneath the Philippine Sea Plate here has created an accretionary wedge on the Hengchun Peninsula on the south of the island that records the evolving marine faunas of the South China Sea biodiversity hotspot the Late Pleistocene.

In a paper published in the journal Zoological Studies on 5 May 2022, Chien-Hsiang Lin of the Biodiversity Research Center of Academia Sinica, Hsin-Yueh Ou of the Department of Life Science at Tunghai University, Chia-Yen Lin, also of the Biodiversity Research Center of Academia Sinica, and Hong-Ming Chen of the Department of Aquaculture at the National Taiwan Ocean University, describe a fossil Fish from a concretion assigned to the Late Pleistocene Szekou Formation of the Hengchun Peninsula.

The specimen comes from a sandstone block collected some decades ago by amateur collectors, and donated to the Biodiversity Research Museum at Academia Sinica by Cheng-Kai Chiang. Only half of the block is present, exposing a Fish skeleton, and this appears does not appear to have been broken open by Human intervention, making it likely that the counterpart was never found. Although the exact point of origin of the fossil is unknown, its collectors did record where it was found, and a visit to this site suggests that the Szekou Formation is the only lithological unit here which matches the rock type of the specimen. Since nodules from the Szekou Formation have produced a variety of other Late Pleistocene Fish, Crustaceans, and Molluscs, this does not seem an unreasonable assumption.

Pagrus major, ASIZF0100141, from the Late Pleistocene Szekou Formation, Hengchun Peninsula, southern Taiwan. (A) Lateral view of the nodule. (B) Dorsal view of the anterior part of the nodule, note that fragments of the right jaws are exposed. (C) Drawing of the recognised skull parts. Abbreviations: al.p.pm, alveolar process of the premaxilla; An, angular. ar.p.pm, articular process of the premaxilla; as.p.pm, ascending process of the premaxilla; Art, articular; Brr, branchiostegal ray; can., caniniform teeth; Ce, ceratohyal. con., conical teeth; cp, coronoid process of dentary; cr.s, superior crest on the articular; Den, dentary; d.p.art, descending process of the articular; Enp, endopterygoid; Ino, interoperculum; mdc, dorsal crest on maxilla; mol., molariform teeth; Mpt, metapterygoid; Mx, maxilla; Op, opercle; Pl, palatine; Pmx, premaxilla; Pop, preopercle; Q, quadrate; Sop, subopercle. Lin et al. (2022).

Exposed on the nodule are an almost complete lower part (splanchnocranium) of a left skull, plus a fragmentary right premaxilla and dentary, and the anterior part of the body, largely covered by scale fragments. Fragments of the coracoid and cleithrum are visible just behind the opercle. 

Based upon examination of the teeth and maxilla, the remains can clearly be assigned to the Family Sparidae (Seabream). It can further be excluded from the genus Dentex by the presence of molariform teeth (which Dentex lacks). The specimen has villiform teeth along the proximal ring of its jaws, which suggests that it belongs to the genera Pagrus, Chrysophrys, or Evynnis. Furthermore, the specimen has large anterior caniform teeth, which would appear to rule out membership of the genus Evynnis. 

The genera Pagrus and Chrysophrys each contain a single species; the Red Seabream, Pagrus major and the Australasian Snapper, Chrysophrys auratus. These species can only be differentiated morphologically by the presence of a bump on the supraoccipital bone (forehead) of Chrysophrys auratus. It has been suggested that these are in fact different populations of the same species, but recent genetic analysis has revealed that they are separate species, with their most recent common ancestor probably living in the Pliocene or Early Pleistocene. The two species are now often considered to be members of the same species.

Today, Pagrus major is found in the western Pacific, around the coasts of Japan, South China, Taiwan and the Philippines, whereas Chrysophrys auratus is found further east, around the coasts of Australia, New Zealand, and the islands of the South Pacific. However, it does not automatically follow that either species would have had the same distribution in the Pleistocene, when the distribution of climate bands was quite different.

The specimen from the Szekou Formation lacks any sign of a supraoccipital bump, which leads Lin et al. to conclude that it is a specimen of Pagrus major, and that therefore this species was present in the waters around Taiwan in the Late Pleistocene.

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