Neoponera vejestoria: A Ponerine Ant from Miocene Dominican Amber, and its implications for the evolution of the ecological niches occupied by Ants in the Greater Antilles.

The makeup of modern ecological communities is determined by three factors, speciation, migration, and extinction. The first two of these can be largely understood through the examination of extant species, but our knowledge of past extinction events is entirely derived from the fossil record. Extinction events can create opportunities for surviving species to adapt into vacant niches, but can also lead to the vanishing of those niches, and of co-dependent niches and the organisms that occupy them. Island ecosystems are particularly vulnerable to extinction events, and their ecological communities are therefore particularly prone to being modified in this way. This makes the fossil history of island ecosystems, which are likely to have been shaped by numerous extinction events. of particular interest to evolutionary ecologists, although very few islands have fossil deposits that record past faunas in sufficient detail to be useful.

The island of Hispaniola in the Greater Antilles is home to 126 indigenous Ant species, drawn from a wide range of Neotropical groups which occupy different ecological roles. The island is also home to an excellent Fossil Lagerstätte, the Dominican Amber deposits, from which over 1100 species of Early Miocene Insects, including 86 species of Ants. The fauna recorded in this amber deposit is very similar to that of the modern Neotropics, with 84 of the described Ant species being placed in extant Neotropical genera. However, about a third of the genera found in the fossil fauna, while still present in the wider Neotropics, are absent from not just Hispaniola, but the wider Greater Antilles island group, implying that they have suffered local extinctions over the 16 million years since the amber was deposited. The ancient and modern faunas were studied by biologist Edward O. Wilson in the 1980s, who concluded that larger species and those with more specialize ecologies were more likely to have gone extinct. Since Wilson carried out his study, considerably more has been learned about both the fossil and living Hispaniolan Ant faunas, but Wilson's observations remain unchallenged.

In a paper published in the journal BMC Biology on 8 February 2023, Gianpiero Fiorentino of the Federated Department of Biological Sciences at the New Jersey Instituteof Technology, John Lattke of the Departamento de Zoologia at the Universidade Federal Do Paraná,  Adrian Troya of the Departamento de Biología at the Escuela Politécnica Nacional, Christine Sosiak, also of the Federated Department of Biological Sciences at the New Jersey Institute of Technology, Minsoo Dong of the Applied Biology Program at Kangwon National University, and Phillip Barden, again of the Federated Department of Biological Sciences at the New Jersey Institute of Technology, and of the Division of Invertebrate Zoology at the American Museum of Natural History, describe a new species of Ponerine Ant from Miocene Dominican Amber, and discuss its implications for the evolution of the ecological niches occupied by Ants in the Greater Antilles.

The new species is assigned to the Subfamily Ponerinae, one of the largest and most diverse Ant families, both in ecological and morphological terms, and to the genus Neoponera, which is one of the more diverse genera of Neotropical Ponerine Ants, with 58 extant species, divided among seven species groups. The majority of Neoponera species are arboreal in habit, which is unusual in Ponerine Ants, although the genus is quite ecologically variable, with individual species having lifestyle strategies which vary from generalist ground-dwelling hunters, to specialist Termite raiders, to mutuaslistic relationships with certain Trees, which they protect from herbivores in return for food and nesting spaces. Despite this ecological adaptability, the genus Neoponera does not appear to do well in island ecosystems, with a few species in the Lesser Antilles but none in the Greater Antilles or on other Neotropical islands.

The new species is named Neoponera vejestoria, where 'vejestoria' derives from the Spanish 'vejestorio' meaning an old person or thing. The new species is placed in the genus Neoponera on the basis of its eyers, which are convex eyes placed at about head mid-length (found in all members of the genus), its well developed aroliae (a lobe on the leg to which the pretarsus attaches, again well developed in all members of the genus), and its slit-shaped propodeal spiracle (breathing opening, found in most members of the genus). It resembles members of the foetida and aenescens species groups, being assigned to the former group on the basis of its well developed malar carinae (ridges on its 'cheeks') and the positioning of its eyes.

Photomicrographs of Neoponera vejestoria (Holotype, MNHNSD FOS 18.01). (A) Head in frontal view. (B) Body in dorsal view. (C) Body in lateral view. Scale bars: (A) 1 mm; (B), (C) 2 mm. Fiorentino et al. (2023).

Based upon its morphology, Neoponera vejestoria is thought to have been a ground-dwelling generalist predator. It is the first known fossil species within the genus Neoponera, as well as the first known fossil member of the Pachycondyla genus group of Ponerine Ants from the Neotropics. It is also the largest predatory Ant known from the island of Hispaniola, either in the Miocene Amber deposits or today, although it would only be considered a medium-sized compared to a wider sampling of Ants; most modern Ants on Hispaniola are quite small, and while the size-range present in the Miocene Ants is larger, none of them are exceptionally large.

Neoponera vejestoria (Holotype BALDR0443): (A) Lateral view of mesosoma; (B) Dorsal view of posterior mesosoma and propodeum; (C) Lateral view of gaster; (D) Ventral view of mesosoma; (E) Metatarsi 1-5; (F) Arolium and metatarsal claws; (G) Metapleural gland. Scale bars: (A) 2 mm, (B) 1 mm, (C) 2 mm, (D) 2 mm, (E) 0.25 mm, (F) 0.125 mm, (G) 0.25 mm. Fiorentino et al. (2023).

Living Ponerine Ants are less morphologically variable and, on average, smaller than fossil species. This is less true if only Ants from Hispaniola are considered, where the largest Ponerine Ants other than Neoponera vejestoria, livening or fossil, all belong to the genus Odontomachus, which is the only Ant genus on the island today containing medium-sized species. However, it is likely that the Dominican Amber deposits do not represent the full range of Miocene Ants on the island, and that other larger Ants may have existed on the island at that time.

Computerized tomographic reconstruction of Neoponera vejestoria to illustrate difficult to view characters. (A) Head in front view; (B) Profile view of mesosoma and gaster; Dorsal view of head, posterior mesosoma, and propodeum as a computerized tomographic reconstruction (C) and as a photograph of the fossil (D). Fiorentino et al. (2023).

Fiorentino et al.'s study suggests that the fauna of Hispaniola has been shaped not just by the extinction of lineages of Ants, but also the loss of the ecological niches which they once occupied. Predatory Ants on the island have always tended to be on the small side, but the range of sizes was clearly greater in the Early Miocene than it is today. Furthermore, Ants with more specialized feeding strategies, such as the Blind Army Ants of the genus Neivamyrmex, the Trap-jaw Ants, Acanthognathus spp., and the subterranean predatory Ants, Acanthostichus spp., have also died out on the island. The discovery that the island was once occupied by the Ponerine Ant Neoponera vejestoria provides further evidence of the modification of the island's ecological community through the extinction of certain ecological traits. Neoponera vejestoria is at least a third larger than any living predatory Ant on the island, which is a conspicuous difference in size, and likely to have been a ground-nesting generalist predator, which is not in itself unusual.

Artistic reconstruction of Neoponera vejestoria. Minsoo Dong in Fiorentino et al. (2023).

The Ant fauna of the Miocene of the island of Hispaniola is likely to be severely under-represented in Dominican Amber. The amber was formed from a resin extruded by a Leguminous Tree, and therefore best preserves the Insects which lived on, or at least visited the canopy of these trees, as well as, to a lesser extent, species which lived on the ground beneath. Insects which lived elsewhere on the island are highly unlikely to have been preserved at all. This could well imply that the Miocene size-range of Ants on the island was wider than has been preserved in the fossil record. Other lineages of larger Ants besides Neoponera appear to have been lost on Hispaniola, such as the large-bodied genus Paraponera, which is found in Dominican amber, but absent from the Caribbean region today.

Mammals, and other Vertebrates, often face selective pressures against larger sizes when groups become isolated on islands, but this is much harder to demonstrate in Insects, although it has previously been demonstrated in Carabid Beetles on European islands.

Excluding Neoponera vejestoria, the largest Ant species found on Hispaniola in both the Miocene and modern faunas belong to the genus Odontomachus. These Ants are ground dwelling generalist predators, something they share with Ants of the genus Neoponera. Today, these two Ant genera are seldom encountered on the same island, with Odontomachus being found on Hispaniola and the other islands of the Greater Antilles, as well as Cocos, Barbados, and Tobago, while Neoponera is only found on the island of Margarita. Fiorentino et al. therefore consider it possible that the two types of Ant compete for similar niches, which may have been a driver of the extinction of Neoponera on Hispaniola.

The discovery of Neoponera in Dominican Amber highlights the fact that these deposits still have much to tell us about the Miocene fauna of Hispaniola, and the way in which the island's ecology has changed over time, despite over five decades of study on the subject.

Neoponera vejestoria is clearly a distinct species, but nevertheless is morphologically very similar to extant species in the same genus, something which has been observed in other Ants from Miocene Dominican Amber. Most modern members of the genus Neoponera inhabit specialist niches, and the group is predominantly arboreal today, but some members are still ground-dwelling generalist predators, and it has previously been suggested that this is likely to be the ancestral state for the genus, a hypothesis supported by the discovery of Neoponera vejestoria, although the alternative hypothesis, that Neoponera vejestoria had secondarily adopted a ground-dwelling generalist lifestyle as an adaptation to island life, cannot be excluded.

The discovery of Neoponera vejestoria also sheds light on the history of the wider  Pachycondyla genus group (which includes Pachycondyla, Neoponera, and Dinoponera, among others), which are important members of Neotropical ecosystems, being the first fossil Ant confidently placed within the group, and the first fossil member of an extant genus in the group. Other fossils have previously been referred to this group, but none of them with confidence, and  Fiorentino et al. suggest that all of these specimens need re-examination, to shed better light on the history of the group. Molecular clock estimates of the age of the genus Neoponera have suggested that it first appeared between 26 and 12 million years ago, while the foetida species group has been thought to be no more than 12 million years old. A better understanding of the fossil record of the Pachycondyla genus group would clearly lead to better calibration of these molecular estimates.

The discovery of Neoponera vejestoria provides a rare example of island extinction in the fossil record. This is almost certainly linked to the body size of the species, which is likely to have placed it at greater risk of extinction in an island ecosystem. As the ecological niche occupied by Neoponera vejestoria is still in existence, this demonstrates that a large size in itself is a threat to Ant species on islands.

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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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Understanding ocean chemistry in the Western Interior Seaway during the Cenomanian–Turonian Extinction Event.

The boundary between the Cenomanian and Turonian stages of the Cretaceous Period is marked by a mass extinction event that saw the demise of a quarter of the marine invertebrates present at the onset of the crisis, combined with carbon, oxygen, and sulphur isotope levels, the deposition of a thick (up to 3 m in places), organic-rich, black shale in many ocean basins, and the onset of a greenhouse climate, known as the Cretaceous Climatic Maximum, which peaked in the early Turonian, then gradually cooled off over the remaining 24 million years of the Cretaceous. Numerous causes have been proposed for the Cenomanian–Turonian Extinction, but the most likely is thought to be massive volcanic emplacements, possibly in the Caribbean Large Igneous Province, which injected large amounts of carbon dioxide, hydrogen sulphide, and sulphur dioxide, as well as a variety of metal compounds, into the ocean-atmosphere system. Increasing atmospheric carbon dioxide would have led to higher global temperatures and higher precipitation on land, which in turn would have led to higher erosion on land, more nutrients being washed into the oceans, and vast Algal Blooms, which would be recorded as a higher burial rate for organic carbon, causing the global carbon isotope excursion, which can be observed from both black shales and carbonate rocks spanning the Cenomanian–Turonian boundary. At the time much of the world's ocean system was dominated by shallow, epicontinental seas (i.e. seaways covering continents in the already warm Cretaceous world), which would have quickly become stagnant when these Algal Blooms were combined with a combination of an injection of oxygen consuming metals and a break-down in ocean circulation caused by the rising temperatures, resulting in large portions of the global ocean becoming anoxic and hostile to multicellular life. The widespread occurrence of black shales at the Cenomanian–Turonian boundary is thought to be a reflection of this. Curiously, however, these phenomena are not recorded in all sequences spanning the Cenomanian–Turonian boundary, with many shallow marine environments (which would be predicted to be the most severely impacted by such events) seemingly unaffected. This variability, with the event leaving a strong signal in some sequences, a light one in others, and being totally absent in some places, leads to the conclusion that the 'global event' may in fact have been a series of overlapping local occurrences, driven by multiple factors rather than a single change in global atmospheric composition.

In a paper published in the journal Scientific Reports on 30 June 2021, Rob Forkner of the Deep Time Institute, Jeremy Dahl of Biomarker Technologies, Inc., and the Stanford University Institute for Materials and Energy Sciences, Andrea Fildani, also of the Deep Time Institute, Silvana Barbanti, also of Biomarker Technologies, Inc., Inessa Yurchenko of the Department of Geological Sciences at Stanford University, and Mike Moldowan, again of the Deep Time Institute, present the results of a study of the USGS Portland-1 core, which was drilled in Colorado, and which includes a section of the Greenhorn Formation including the Cenomanian–Turonian boundary.

 

Palaeogeographic map of North America during Oceanic Anoxic Event 2. The location of the Portland-1 core as well as active volcanic centres are shown. Forkner et al. (2021).

Forkner et al. sampled the core through the Cenomanian–Turonian boundary interval (as determined by the isotope excursion), as well as on either side, for organic geochemical analyses. They initially targeted layers with high organic carbon which were thick enough to determine if reworking or bioturbation had occurred, although this severely limited the number of suitable layers, with the effect that samples were taken at intervals of between 3 and 12 cm across the boundary interval, and 20 cm or more outside this interval. The samples were round segments 2-3 cm in diameter and 1 cm thick taken from the larger core, which were first tested for rock richness and maturity (the extent to which rocks have been heated, altering organic molecules preserved within them), before the most suitable samples were selected for analysis by gas chromatography–mass spectrometry.

Molecular fossils, or biomarkers, are recognisable fragments of molecules synthesised by biological organisms, which can be used to determine the presence and abundance of groups of organisms. Forkner et al. analysed biomarkers from the Portland-1 core across the Cenomanian–Turonian boundary, thereby obtaining a series of snapshots of the water column ecology, which were used to develop a new molecular stratigraphy for the boundary, thereby deriving a wealth of new information with regard to the biota, depositional environment, and the multiple drastic environmental changes that occurred before, during, and after the Cenomanian–Turonian Extinction Event.

A geological examination was used to establish a lithological sequence of events (i.e. changes in the rock type being laid down over time, which would have related to local environmental conditions), using photographs to cover those sections of the core which have previously been heavily sampled by previous workers. This enabled the comparison of similar facies ( specified characteristics, which can be any observable attribute of rocks), in order to correlate changes in the biota in intervals with similar climatic and environmental conditions. This selection process meant that effectively only the finest grained, dark mudrocks were sampled, as these gave the greatest opportunity to detect changes in the water column biota uninfluenced by sedimentary conditions.

The data obtained from the Portland-1 core indicates that, in this area of the Cretaceous Western Interior Seaway at least, conditions in the water column during the Cenomanian–Turonian boundary event (sometimes known as the Cenomanian–Turonian Ocean Anoxic Event) conditions do not appear to have been particularly anoxic. In fact, the sediments laid down across the boundary appear to have been laid down in a more oxygenated environment than wither the sediments above the boundary or those below it, something which has been noted at other locations in the Western Interior Seaway, and coeval shallow water deposits from the Tethys Ocean. The sediments laid down before the boundary layers are predomanenty finely laminated, whereas those across the boundary interval are mostly heavily bioturbated, suggesting a thriving benthic community living within them.

 

USGS Portland-1 core lithologic section, carbon isotope profile and RockEval data. Facies Explanation: (1) Peloidal/foraminiferal, packstone/grainstone; (2) Bioturbated peloidal packstone; (3) Bioturbated peloidal wackestone; (4) Skeletal grainstone; (5) Rippled mudstone; (6) Silty laminated mudstone; (7) Diffusely laminated mudstone; (8) Massive mudstone; (9) Bentonite. Samples were limited to facies (7) and (8). The occurrence of bioturbated peloidal carbonates during the Oceanic Anoxic Event positive carbon isotope excursion indicates that the environment at the time of deposition was oxygenated and supported a diversity of tropical marine life. Note that the core is measured in imperial units as the Portland-1 core and core photos are curated with imperial measurements. This reference is preserved here in the case that the reader wishes to cross-reference these results to the Portland-1 core. Radio-isotopic measurements from bentonites A, B, and C, along with biostratigraphy and correlation of depositional cycles to orbital timescales have produced an average sedimentation rate of 0.93 cm/per thousand years during the Oceanic Anoxic Event positive carbon isotope excursion. The interval of samples with the greatest flux in measured biomarker concentration occurs from about 473 feet (144 m) to about 479 feet (146 m), in the central portion of the Oceanic Anoxic Event positive carbon isotope excursion. Sample spacing in this interval is somewhat irregular in order to stay within the same depositional facies, but varies between 3 and 12 cm indicating that rapid flux in organic geochemical composition of analysed sediments over periods approximately 3–15 thousand years. The carbon isotopic excursion (CIE) that defines the Oceanic Anoxic Event is shown on the carbon¹³ as a proportion of total carbon (δ¹³C) track and highlighted in blue on all compound tracks. Hydrogen Index (HI) is generally negatively correlated with depositional environment oxygen concentrations, thus supporting the trend of Oceanic Anoxic Event positive carbon isotope excursion oxygenation. Oxygen Index (OI) generally correlates positively with depositional environment oxygen concentrations, and again provides evidence for oxygenation during the Oceanic Anoxic Event positive carbon isotope excursion. Forkner et al. (2021).

The geochemical analysis of samples extracted from the core supports the geological analysis. The 'Hydrogen Index', which derives from the proportion of total organic carbon made up of hydrocarbons, is generally negatively correlated with the oxygen concentration in the depositional environment, i.e. the Hydrogen Index tends to go up when there is less oxygen and down when there is more oxygen. In the Portland-1 core the Hydrogen Index above the Cenomanian–Turonian boundary layer averages at 509, during the boundary the average fell to 177, and below the boundary the average rose again, to 423, supporting the idea that oxygen levels in the water column rose rather than fell during the boundary interval. The 'Oxygen Index', derived from the purporting of carbon dioxide to total organic carbon, is positively correlated with the level of oxygen in depositional environment (i.e. the Oxygen Index goes up when the amount of oxygen present in the depositional environment goes up). In the Portland-1 core the Oxygen Index above the boundary layer averages 16, in the boundary layer averages 28, and below the boundary layer averages 15, again suggesting a rise in oxygen levels across the boundary interval.

A number of biomarkers also strongly imply a rise in oxygen levels during the Cenomanian–Turonian boundary interval. The Gammacerane Index is derived from the ratio of the biomarker gammacerane (derived from bacterivorous Ciliates) to hopane (derived from Bacteria), is associated with stratification in the water column, with high levels of gammacerane typically indicating highly saline or reducing conditions. In the Portland-1 core the Gammacerane Index drops to its lowest level during the Cenomanian–Turonian boundary interval, implying conditions became less reducing (generally a sign of higher oxygen levels). The Homohopane Index is derived from the proportion of C₃₅ hopanes (hopane molecules with 35 carbon atoms) to the total C₃₁-C₃₅ hopanes (hopanes with between 31 and 35 carbon atoms). This idex also tends to rise with reducing conditions, and again has its lowest valuse in the Cenomanian–Turonian boundary interval in the Portland-1 core, again suggesting that the enviroment became less reducing during this interval. The proportions of A-oleanane relative to oleanane and 17α-diahopane relative to 17α-hopane are also thought to be indicative of higher oxygen levels, since both A-oleanane and 17a-diahopane require oxygen for their producers (Bacteria and Flowering Plants, respectively), to form them from their precursors, oleanene and hopane. The levels of A-oleanane and 17a-diahopane remain constant throughout the section, but the levels of oleanene and hopane fall during the Cenomanian–Turonian boundary layers, so that the proportion of A-oleanane and 17a-diahopane rise, presumably indicative of a rise in oxygen. 

 

Compound tracks through the Cenomanian–Turonian boundary layers relating to oxygenation before, during, and after the event. Gammacerane and Homohopane Indexes, which are affected by sediment redox conditions, show a significant decrease and the ratios related to 17α-Diahopane exhibit an increase with striking fluctuations within the Cenomanian–Turonian boundary layers. These broad scale changes reflect an overall increase in oxygenation during the Cenomanian–Turonian boundary interval, with periods of reducing conditions punctuated through the event. The relative preservation of des-A-oleanane revealed by the des-Aoleanane/oleanane ratio could be a function of oxidation. Forkner et al. (2021).

Given the generally healthy ecosystem recorded in the sediments of the Portland-1 core across the Cenomanian–Turonian boundary, and the geochemical evidence for a healthy, well-oxygenated water column, biomarkers associated with primary Algal production would be expected to increase across the Cenomanian–Turonian boundary layer. However, a range of such biomarkers, including cholestanes, ergosteranes and C₂₇, C₂₈, and C₃₀ steranes, undergo fluctuations in the boundary layer, with a general decrease in levels. This would appear to represent a deteriorating environment, with repeated stressful intervals.

While this decrease in steranes implies a drop in Algal productivity across the Cenomanian–Turonian boundary, levels of hopanes, indicative of Bacterial productivity, remain relatively high, although again they undergo some severe fluctuations during the boundary interval, suggesting that environmental conditions were fluctuating in the water column. 

Fluctuating biomarker ratios are highly indicative of an unstable environment, with fluctuating primary productivity. In order to further explore this, Forkner et al. examined three further examples. The hopane/sterane ratio reflects the levels of both heterotrophic and photosynthetic Bacteria to primary producers including marine Algae and terrestrial Plants. The 3β-methylhopane/dinosteranes ratio compares the ratio of 3β-methylhopane, derived from aerobic methanotrophs and fermentative Bacteria, to the ratio of dinosteranes, which are almost exclusively derived from Dinoflagellates. The 3β-methyl-24-ethylcholestane/4α-methyl-24-ethylcholestane ratio compares a reworked sterane to one produced by marine Algae. All of these ratios record a significant drop in primary production across the Cenomanian–Turonian interval, but with rapid fluctuations which appear to be unrelated to any change in the lithology, and which Forkner et al. suggest might be related to short-term anoxia events not recorded in the rock record.

 

Compound tracks through the Cenomanian–Turonian boundary interval relating to productivity before, during, and after the event. For the main the Cenomanian–Turonian boundary interval section, the concentration of biomarkers derived from algae such as C₂₇, C₂₈, and C₃₀steranes (24-n-propylcholestanes), and 4α-methyl-24-ethylcholestane 20R decreases significantly relative to concentrations of those derived from bacteria, which increase moderately with variations. This suggests that the record of productivity variability we interpret is reliable and is not simply a record of poor preservation of the organic fraction. Of note is the interval in all track from about 473 feet (144 m) to about 479 feet (146 m) where organic geochemical measurements return the most erratic results. By applying the most recently published timescales through this interval, it is possible to calculate that the productivity cycles of biomarker decline and recovery can vary from about 26 thousand years at the shortest to about 130 thousand years at the longest. These productivity cycles occur within individual lithofacies internal to single lithocycles. Fornkner et al. (2021).

Isoprenoids can be derived from chlorophyll side chains produced by photosynthetic Algae, although other organisms do produce them, including Cyanobacteria and some Archaeans. Notably, head-to-head isoprenoids such as biphytane are produced by marine Archaea. Unfortunately, isoprenoids are found at very low concentrations throughout the core, although some fluctuations can be observed during the Cenomanian–Turonian boundary layers, and biphytane and its diagenetic products are only ever found at trace levels, and often drop below detectability levels. Studies of the Cenomanian–Turonian layer in other sections have suggested that there might have been blooms of opportunistic Archaea during this interval, but this cannot be detected in the Portland-1 core.

A number of biomarkers remain relatively constant on either side of the Cenomanian–Turonian boundary layers, but fluctuate greatly during the boundary interval. Previous work on the Portland-1 core and other cores from the same area have been studied extensively to determine sedimentation rates. These studies have led to a calculated sediment accumulation rate of 0.93 cm per thousand years in the upper part of the Cenomanian–Turonian boundary, using calculations that include radiometric dates from bentonite layers, and orbital time scales. Calibrating this with the observed fluctuations in biomarker ratios suggests that productivity fluctuations were occuring on cyclical scales of between 26 and 90 thousand years, with individual biological collapses happening in as little as 3200 years.

One possible cause of influxes into the shallow, enclosed, Western Interior Seaway during the Cenomanian–Turonian boundary period that has been previously suggested is sea level rises, Evidence for such changes has been found in the Tethys Ocean, where repeated cycles of carbonate platforms being replaced by deeper sediments have been observed. Such an increase in water volume would provide relief from the effects of stagnation, but cannot explain the drastic changes in water column ecology observed by Forkner et al., which appear to happen on a much finer scale. Forkner et al. were unable to find any references to previous examples of such rapid changes in organic composition of a rock sequence, particularly one one independent of changes in the lithology, and conclude that the drivers these changes were clearly independent of the drivers of lithology changes.

Studies of the lithology and astronomical forcing cycles recorded in the Portland-1 core have previously concluded that the carbonate-mudstone cycle here (which would have been driven by changes in sealevel) would have lasted about 100 000 years. The biological productivity cycle, however, is clearly working on a much shorter, more erratic, and independent timescale. Forkner et al. cannot rule out the possibility that some shorter Milankovitch-band processes was occurring in the Western Interior Seaway, but the irregular nature of the cycles makes this unlikely, as does the fact that they are not seen outside the Cenomanian–Turonian boundary interval.

Forkner et al. suggest that these biomarker cycles may have been driven by changes in primary productivity (i.e. photosynthesis), and the subsequent decay of organic matter. During the intervals on either side of the boundary, more reducing conditions prevailed, probably due to higher rates of organic decay. During times of extreme stress within the boundary period, the amount of primary production dropped, and there was a drop in the amount of organic matter in the water column, and therefore the amount of decay that could occur. However, the driver of this stress, and the cause of the erratic cycle it was following, remain unclear.

A cause of environmental stress unrelated to sealevel changes or orbital forcing and operating on rapid and erratic timescales could plausibly be volcanism. Volcanism has the potentially to disrupr Algal productivity rapidly, and would not cause any change in the depositional environment (which changes in sealevel related to Milankovitch forcing would do). A number of other cores from the Western Interior Seaway (including Eagle Ford, Boquillas, and Bouldin Flags) contain numerous bentonite layers (caused by volcanic ash falling on water then settling to the bottom), which have been used to determine the age of the sediments and the rate at which they were accumulating. Volcanism can impact Algal productivity in a number of ways, from lowering light levels to altering the pH of the water. While this cannot be proven from Forkner et al.'s current work, it would potentially have left detectable signs which could be revealed by future investigations. Either way, these findings appear to support the idea that the Cenomanian–Turonian Extinction Event was caused by a prolonged breakdown in environmental conditions rather than a single catastrophic event.

The Cenomanian–Turonian boundary reflects a profound change in environmental conditions on a global level, although the record of this varies from location to location. Geochemical examination of organic molecular fossils preserved in the Portland-1 core, a nearly-continuous core of sediment recovered from the Western Interior Seaway of North America, shows a record of extreme environmental variability not previously observed, with a generally higher level of oxygenation than before the crisis, prior to previous assumptions about a single, massive, anoxia event driving the crisis. Instead, the environment seems to have undergone a series of smaller, but still significant, crises, operating on an irregular cycle divorced from Milancovich forcing and sealevel changes. Forkner et al. hypothesise that this could have been driven by volcanic episodes, which tend to be erratic in their timing. A single, massive, volcanic event has previously been suggested as a possible cause of the Cenomanian–Turonian Extinction Event, injecting vast amounts of material into the atmosphere and oceans, and leading to significant changes in seawater pH, global temperature, and the hydrological cycle, but Forkner et al.'s evidence points towards a more prolonged period of change, with periods of high organic productivity punctuated by sudden collapse. This repeated stressing of the marine environment would have challenged the biota of these ecosystems, potentially causing the observed extinctions.

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Evidence for marine anoxia during the end-Triassic Mass Extinction.

The end-Triassic Mass Extinction is one of the largest known biological crises of the Phanerozoic and is regarded as one of the 'Big Five'. This extinction has been linked with voluminous volcanism during the emplacement of Central Atlantic Magmatic Province and its associated environmental effects. These effects include global warming and ocean anoxia. Existing evidence suggests that basinal marine anoxia was widespread on the northern Panthalassan margin of Pangaea and that intense shelf euxinia also became widespread in the latest Triassic–earliest Jurassic of Western Europe, but some of these conditions developed, some roughly150 thousand years after the onset of the end-Triassic Mass Extinction. Additional findings from seawater uranium isotopes in the Lombardy basin of western Tethys suggest an increase in the extent of anoxic deposition through the Triassic-Jurassic boundary. However, in other oceans, clear evidence for widespread anoxia in the latest Rhaetian that directly coincides with the beginning of end-Triassic Mass Extinction has not been recorded, leaving its role as the cause of the marine component of the end-Triassic Mass Extinction questionable.

Carbonate-associated sulphate in bulk marine carbonate and biogenic calcite is widely used to reconstruct the primary seawater sulphate sulphur isotope composition during major redox perturbations of the Earth surface system. The proportion of sulphur³⁴ in seawater is dynamically controlled by variations in the fluxes and isotopic compositions of riverine sulfate sources and marine pyrite burial. The removal of sulfate from the oceans via gypsum precipitation does not impart an isotopic fractionation, but this removal makes the global sulfate reservoir smaller and, therefore, more isotopically susceptible to changes in other fluxes. The production and burial of pyrite represent a primary redox-sensitive pathway in the marine sulphur cycle, which drives a large offset between the sulphur isotopic composition of the seawater sulphate and sedimentary pyrite pools, and thus may control variations in the sulphur isotope composition of oceanic sulphate through time. Large and rapid global-scale sulphur isotope perturbations, as well as the small ocean sulphate reservoirs needed to produce them, seem to be a feature of major deoxygenation events of the Phanerozoic. Although there is some evidence in the sedimentary pyrite isotope record that suggests the regional development of marine anoxia at the end-Triassic Mass Extinction, direct records of changes in the marine sulphate pool and therefore impacts on the global sulphur cycle are undocumented. 

In a paper published in the journal Science Advances on 9 September 2020, Tianchen He of the School of Earth and Environment at the University of Leeds, Jacopo Dal Corso, also of the School of Earth and Environment at the University of Leeds, and of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Robert Newton, Paul Wignall, and Benjamin Mills, again of the School of Earth and Environment at the University of Leeds, Simona Todaro and Pietro Di Stefano of the Department of Earth and Marine Sciences at the University of Palermo, Emily Turner and Robert Jamieson, again of the School of Earth and Environment at the University of Leeds, Vincenzo Randazzo, also of the Department of Earth and Marine Sciences at the University of Palermo, Manuel Rigo of the Department of Geosciences at the University of Padova, Rosemary Jones of the Department of Earth Sciences at the University of Oxford, and Alexander Dunhill, once again of the School of Earth and Environment at the University of Leeds, report three open marine sulphur isotope composition of oceanic sulphate profiles from Sicily (Mount Sparagio Section), Northern Ireland (Cloghan Point Section), and British Columbia (Black Bear Ridge Section). These derive from both Tethyan and Panthalassan locations; the first two sections archive well-preserved, shallow-water, peritidal, micritic, and shelly limestones and shell materials; and the last section consists of open-shelf, organic-rich, and Bivalve-rich marly limestone. The sections span the Norian to lower Hettangian and record the major losses of the end-Triassic Mass Extinction. Therefore, they provide a window into the possible links between the ecosystem response and marine redox variations in Late Triassic oceans over a broad area.

 

Simplified paleogeographical map for Triassic-Jurassic transition showing localities for all three studied sections. Yellow filled triangles indicate the location of studied sections. He et al. (2020).

All sulphur isotope composition of oceanic sulphate profiles from three different localities show similar trends, although the absolute values vary between the European and North American sections. In all sections, a large positive sulphur³⁴ isotope shift with a magnitude of more than 10 per mil (‰) is seen in the latest Rhaetian (roughly 201.5 million years ago) and coincides precisely with the extinction horizon. Two consecutive positive sulphur³⁴ excursions are shown at the Mount Sparagio Section, while only a single spike is seen at the other two sections. At the Cloghan Point Section, only the falling limb of the positive excursion was recovered because of the absence of suitable bulk carbonate or shell material below this level. The pre- and postexcursion baseline values for the two Tethyan sections are between 15 and 20‰, which are close to the existing global sulphur isotope composition of oceanic sulphate and evaporite dataset for the Late Triassic. By contrast, the sulphur isotope composition of oceanic sulphate record at the Panthalassa Black Bear Ridge Section generally yields more positive baseline values and a slightly larger positive swing. This is likely due to the development of sulphate isotopic and concentration heterogeneity between Tethyan and Panthalassan sites under low sulfate conditions. He et al. note also that the positive sulphate sulphur³⁴ excursion at the Black Bear Ridge Section is mirrored by synchronous positive sulphur³⁴ shifts in sedimentary pyrite at a deeper site (Kennecott Point section) in eastern Panthalassa, suggesting a coupled behavior in both marine oxidised and reduced sulphur sinks.

 

Sulphur isotope composition of oceanic sulphate profiles from Late Triassic to Early Jurassic for the three studied sites of the Tethys and Panthalassa oceans. R., Rhaetian. The orange shadowed field indicates the extended extinction interval following the major mass extinction horizon. The light green field indicates a hiatus between Norian and Rhaetian at the Black Bear Ridge Section. Dark green bars represent the fossil occurrence ranges. Vertical dash lines indicate pre- or postexcursion average baseline values. He et al. (2020).

He et al. calculated the age model at the most stratigraphically complete Tethyan Mount Sparagio Section. The duration of the shift from the baseline value (about 16 to 17‰) to the first peak value (roughly 31‰) is estimated to take about 50 000 years with the assumption of a constant sedimentation rate and a Rhaetian duration of 4.1 million years. This time frame is broadly in agreement with the equally short-lived major phase of the extinction, which was proposed to last for about 40 00 years. Thus, the observed sulphur³⁴-positive excursion event in the latest Triassic appears to represent an extreme and short-lived perturbation when compared to other similar positive sulphur³⁴ isotope events during, for example, the end-Permian Extinction (roughly 100 000 years), Toarcian Oceanic Anoxic Event (roughly one million years), and Cretaceous Oceanic Anoxic Event (roughlu 500 000 years).

The observed positive swing in the sulphur isotope composition of seawater sulphate in the latest Triassic could have been driven by an increase in the net burial of sedimentary pyrite under expanded anoxic/euxinic conditions. These conditions result in enhanced microbial sulphate reduction, leading to an enhanced pyrite burial flux on the continental shelves and slopes when there is sufficient supply of available iron and organic matter. Because pyrite is depleted in the heavier isotope sulphur³⁴, elevated burial fluxes on a global scale would drive the seawater sulphate sulphur³⁴ to more positive values. The oxidative biotic pathway of the global sulpjur cycle may also have the potential to drive seawater sulfate sulphur³⁴ enrichment to some extent via microbial sulphide oxidation by some sulphide-oxidising microorganisms. However, the contribution of this oxidative metabolic pathway to the oceanic sulfate pool remains unclear, and there is no obvious mechanism for it to have driven a prolonged positive sulphur isotope excursion in the global seawater sulphate inventory. On a larger scale, it may be possible to drive sulphur isotope variations by altering the weathering rates of continental pyrite and gypsum; here, a geologically sudden increase in seawater sulphur³⁴ might represent a cessation of pyrite weathering and a switch to an isotopically heavy riverine flux.

To investigate the response of seawater sulphate sulphur³⁴ to the variations of oceanic sulphate inventory and the degree of change in the net pyrite burial flux, He et al. applied a time-dependent sulphur cycle single-box model. The model assumes that the isotopic composition of the pyrite and gypsum weathering fluxes remain constant, and experiments then alter the pyrite input and output fluxes through either weathering or burial. In the model, a substantial increase in pyrite burial by approximately a factor of 5 and a very small marine sulphate reservoir (less than 1 mM) is required to replicate the magnitude and timing of the sulphur isotope composition of oceanic sulphate shift. This version of the model fixes the isotopic enrichment of buried pyrite at 30‰ more negative than contemporaneous seawater sulphate, but the expansion of euxinia may have increased this enrichment factor; thus, He et al. also experiment with a scenario in which this is increased to 40‰ during the event. This experiment has a very similar requirement for a large increase in pyrite burial and very low seawater sulphate concentration. Note that it is the size, direction, and duration of change that are the important foundations of our modeling approach. Differences in regional sulphate isotope baselines have no impact on the conclusions from the modeling work, as a similar sized isotope excursion is present in all records. Replicating the change in sulphur³⁴ by reducing pyrite weathering rates while maintaining the same gypsum weathering flux is much more difficult and requires a complete cessation of pyrite weathering and extremely low ocean sulphate (about 0.1 mM). Even then, the shape of the excursion is not readily reproducible, as the very low sulphate concentrations mean that the system rapidly recovers from the perturbation.

 

Sulphur cycle box model outputs. (A) and (B) Increased in the pyrite burial rate under different values for the starting oceanic sulfate inventory, with tests of 1 mM (A) (yellow) and 0.33 mM (B) (red). For both scenarios, a step increase in pyrite burial is assumed to occur at t = 0 over a period of 50 ka, which represents the end-Triassic Mass Extinction. Both models assume the same increase in pyrite burial rates, which ranges from 2- to 10-fold to create the shaded area, with the centerline showing a fivefold increase. The best fit to the data occurs for marine sulphate concentration (SO₄) = 0.33 mM (B). (C) Attempts to fit the sulphur isotope composition of oceanic sulphate profiles data by instead reducing the pyrite weathering rate to zero over the same 50 000 year time frame. Here, regardless of (SO₄), the shape of the curve cannot be fit. This is because creating the large excursion this way requires extremely low (SO₄), and, in these circumstances, the system is quick to regain isotopic stability. (D) to (F) Repetition of these experiments with the addition of a change in the enrichment factor sulphur³⁴ between oceanic sulfate and sedimentary pyrite and continuation to produce a better fit when (SO₄) = 0.33 mM. He et al. (2020).

The maximum marine sulphate concentrations can be independently estimated using the maximum rate of change in sulphur isotope composition of oceanic sulphate. The 'rate method' model gives an upper estimate for marine sulphate of about 0.2 to 1.1 mM for the interval through the Late Triassic–positive isotope excursion event. The lower end of these maximum estimates is consistent with the calculations inferred from our sulfur cycle box model. Therefore, the intervals predating and during the positive sulphur isotope excursion event appear to be characterised by a scarcity of oceanic sulphate when compared to a higher fluid inclusion–based estimate of at least 13 mM during the Carnian, although this was about 20 million years earlier. The development of a low sulphate ocean in the later Triassic was likely caused by substantial evaporite deposition. As shown in global compilations for this interval, minimum estimates of global halite deposition suggest a 16-fold increase from the Middle to Late Triassic. By contrast, the earlier part of the Triassic experienced a low level of evaporite occurrence following the end-Permian extinction. Late Triassic evaporites were deposited in newly formed rift basins that developed in an arid climate as Pangaea began to break up. When examined on a regional scale, for example, evaporite deposition became widespread surrounding the North Atlantic rift (northeastern Grand Banks, Oranian meseta, and Western Europe) during the Late Triassic and subsequently peaked in the Earliest Jurassic.

He et al.'s finding of low marine sulphate concentrations preceding an episode of massive pyrite burial in the latest Triassic adds to an increasing number of studies that link low seawater sulphate with the expansion of anoxic waters in the oceans. He et al. propose a conceptual model to link these observations. Marine sulphate and organic carbon availability exert a major control over the balance between three microbially mediated biogeochemical pathways in marine sediments: Microbial sulphate reduction (sulphate + formaldehyde → hydrogen sulphite + bicarbonate), methanogenesis (acetate + hydrogen → methane + carbon dioxide and carbon dioxide + hydrogen → methane + water), and the anaerobic oxidation of methane (methane + sulphate → bicarbonate + hydrogen sulphite + water). Under high sulphate conditions such as the modern ocean, microbial sulphate reduction consumes large amounts of organic carbon, while methane is produced deeper in the sediment where sulphate has been depleted. The overlying sulphate-rich pore water fuels anaerobic oxidation of methane and prevents substantial benthic methane escape, therefore limiting bottom-water oxygen consumption. In contrast, under conditions of low sulphate availability, the balance of processes oxidising organic matter in marine sediments shifts in favor of methanogenesis, as occurs widely in freshwater sediments (e.g. lakes), where sulphate supply is usually limited. Lower sulfate concentrations bring the sulphate-methane transition zone closer to the sediment-water interface and reduce the amount of organic matter consumed by microbial sulphate reduction, ultimately increasing the organic carbon flux to methanogens and limiting the capacity for anaerobic oxidation of the resulting methane. The organic matter reaching the zone of maximum methanogenesis will also have increased reactivity. The result is a greater flux of methane from the sediment, leading to increased aerobic respiration of methane close to the  sediment-water interface placing an increased burden on bottom-water oxygen levels.

In the modern system, around 98% of all buried organic carbon in the ocean is stored in continental margin sediments. On average, around 20% of the global organic carbon flux (roughly 191 Tmolof carbon per year) to the seafloor is processed via microbial sulphate reduction, and about 3 to 4% is converted to methane, giving an annual methane flux from seafloor of about 5.7 to 7.6 Tmol of methane per  year. He et al. calculate that a drawdown in oceanic sulphate concentration by roughly 97% from 29 mM (modern value) to 1 mM will reduce the rate of microbial sulphate reduction by a similar amount and that the excess organic matter will all be used by methanogens (i.e. they now process about 22 to 23% of the organic carbon), then the methane flux would rise to around 42 to 44 Tmol methane per year. This calculation is conservative, since it does not take into account any increase in reactivity of the organic matter reaching the methanogenic zone. Furthermore, suppression of anaerobic oxidation of methane under these low sulphate conditions would make it easier for this methane to reach the water column and consume free oxygen. Making more detailed calculations on the expected impact of low sulfate conditions on water column oxygen demand requires further modeling, which is beyond the scope of He et at.'s study, but their calculations demonstrate that there is clear potential for at least a six- or sevenfold elevation in the methane flux at the sediment-water interface and a concomitant increase in the global consumption of benthic oxygen. Note that these elevated demands on bottom-water oxygen exist where sulphate concentrations are low and before any additional drivers from the release of volcanic carbon dioxide.

Finding evidence for elevated aerobic methane oxidation under low sulphate conditions in the sedimentary record is not simple because the resulting dissolved inorganic carbon flux, while large when considered in the context of dissolved oxygen uptake, is small compared to the abundance of ocean dissolved inorganic carbon, especially when oxidation takes place in the water column as proposed. Isotopically depleted carbonate cements form from pore waters and are a common feature of the sedimentary record and so do not provide definitive evidence. Calcifying organisms living at the sediment-water interface are likely to provide the best archive for recording this process, evidence for which has been recognized in high-latitude late Cretaceous Bivalves, 

A key feature of our conceptual model is that sulphate poor conditions are established before volcanic perturbation, likely by widespread evaporite deposition. Previously, authors have explained the link between the expansion of marine anoxia during large igneous province-driven warming and extinction events via the decreased solubility of oxygen in warmer waters and increased productivity and oxygen demand driven by increased weathering fluxes of nutrients from land and the recycling of phosphorus once euxinic water column conditions are established. The higher bottom-water oxygen demand of a steady-state Earth system with a small marine sulphate reservoir will predispose the oceans to the rapid expansion of anoxic conditions via these mechanisms. In addition, a low sulphate ocean is likely to impose some additional feedbacks once warming has been initiated: The rate of methanogenesis is highly temperature sensitive, so methane production will increase with sediment temperature, a situation amplified by the reduced depth to the methanogenic zone under low sulphate conditions. Increased marine organic matter production will increase the delivery of organic matter and its reactivity to the methanogenic zone in sediments, again adding to increased methane fluxes across the sediment-water interface and oxygen consumption from methane oxidation. Pyrite burial will increase as anoxic conditions expand, creating downward pressure on marine sulphate concentrations, although this may be countered by bigger fluxes of weathered sulphate from land. Elevated global marine methane production may also promote methane release to the atmosphere and thereby contribute to warming trends initiated by the large-scale release of volcanic carbon dioxide, although much of the additional methane production is likely to be oxidised in the water column. These additional feedbacks may explain why the expansion of anoxic conditions is more severe under low sulphate conditions and why not all large igneous province-driven warming events create widespread oxygen depletion.

 

Conceptual model of the methane-oxygen link under high and low sulphate conditions. (A) The fate of organic carbon in the modern high sulphate ocean: More organic carbon and methane are oxidised by sulphate with negligible benthic methane flux, which limits water column oxygen demand. (B) The effect of enhanced methanogenesis in a low sulphate setting: The proportion of organic carbon available for methane production is increased, sulfate-driven anaerobic oxidation of methane is suppressed, and methane production moves closer to the sediment surface producing a high benthic oxygen demand. Red arrows in (B) indicate acceleration of biogeochemical pathways relative to modern, whereas dotted black arrows indicate retardation. (C) and (D) The envisaged oxygen depletion responses of the ocean to the same carbon dioxide forcing under high and low sulfate conditions. Sulphate is thought to be removed by evaporite deposition. Marine anoxia is exacerbated by the increased oxygen demand as net seafloor methane fluxes increase during warming. MG, methanogenesis. SMTZ, sulfate-methane transition zone. He et al. (2020).

Although anoxia may not have developed on the deep ocean floor during the Triassic-Jurassic transition, other geochemical evidence, in the form of enrichment of redox-sensitive elements (e.g. maganese and molybdenum) and nitrogen isotope fluctuations, suggests that there was a major intensification of the mid-water oxygen minimum zone in the Panthalassa Ocean at the time. Tangible evidence for this is seen where the oxygen minimum zone impinged on the western margin of the Pangean supercontinent, leading to extensive black shale deposition in Western Canada. Euxinia also became extensive in the latest Triassic shelf seas of Western Europe, both during and at the termination of the mass extinction phase. Uranium isotope data from marine carbonates provide a possible measure of ocean redox conditions with negative excursions of the proportion of uranium²³⁸ values signifying enhanced reduction from uranium (VI) to uranium (IV). Such a signal, seen at the start of the mass extinction, suggests a major increase in the area of anoxic deposition that lasted for about 50 000 years.

He et al.'s sulphur isotope composition of oceanic sulphate excursions reveal a similar link between the onset of mass extinction and an anoxia-driven isotopic excursion. The link is most clearly seen in western Tethys where Megalodont Bivalves and the Foraminifer Triasina hantkeni are suddenly lost at the onset of the positive shift. Although there is no direct evidence for anoxia at this peritidal location, some contemporaneous anoxic sedimentary matrices are seen at a neighboring site that was also connected to the western Tethys. There is a hiatus in the Panthalassan section (Black Bear Ridge Section), but the extinction level is still recorded. This occurs in the dysoxic strata of the basal Fernie Formation, where the last Rhaetian Conodonts disappear, and is coincident with the sulphur³⁴ isotope excursion. The extinction of Monotid Bivalves at Black Bear Ridge Section marks an earlier crisis at the end of the Norian, several million years before the end-Triassic event. The end-Triassic extinction is also seen at the Cloghan Point Section, where several Bivalve species, including the Rhaetian marker Rhaetavicula contorta, disappear at the base of the Cotham Member. The lack of limestones at this level precludes measurement of the ratio of sulphur³⁴, but the lowest data point obtained in this section, a short distance above, displays a strongly positive value. In summary, the major sulphur³⁴ isotope excursion found here is best explained by a major pyrite burial event driven by a large-scale, increase in anoxia in the late Rhaetian. He et al.'s age model for the Mount Sparagio Section suggests a 50 000 year duration for the initial positive shift in sulphur³⁴, a time span in remarkable accord with the 50 000 year estimate for the main anoxia intensification during latest Rhaetian based on the contemporary uranium isotope record. Subsequently, the gradual falling limb of the  sulphur³⁴ excursion corresponds with the second phase of limited anoxia that extended into the Hettangian. The event also saw the intensification of the Panthalassan mid-water oxygen minimum zone and the deposition of black shales on the Pangean margin and in the shelf seas of Europe. Shallowest water locations, such as the Mount Sparagio Section, remained oxygenated. The coincidence of the sulphur³⁴ excursion with the extinction losses implicates anoxia as an important factor in the crisis.

The late Permian and the Mesozoic Era were punctuated by recurring oceanic anoxic events accompanied by hyperthermal events and enhanced weathering that coincide with the eruption of large igneous provinces. Large positive sulphur isotope shifts in seawater sulphate provide evidence of a greatly reduced marine sulphate reservoir and enhanced pyrite burial for many of these oceanic anoxic events. He et al. explain this generalised coincidence via a mechanistic linkage between low dissolved sulphate, enhanced sedimentary methane generation, and consequent elevated bottom-water oxygen consumption. Hence, He et al. propose that a low sulphate boundary condition before volcanically driven greenhouse warming events makes the expansion of anoxic conditions more likely and that associated feedbacks during the event extend the geographic reach and intensity of anoxia. Many of these events are preceded by increased evaporite burial fluxes, suggesting that this is the mechanism for sulphate removal from the ocean. Hence, the development of widespread anoxia during rapid warming may ultimately trace some of its origins to widespread rifting or other circumstances that create favorable conditions for evaporite deposition.

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