Target Earth: how asteroids made an impact on Australia

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Gosses Bluff impact crater in the Northern Territory.
NASA’s Earth Observatory

Andrew Glikson, Australian National University

Our planet has had a few close encounters with asteroids of late.

Asteroid 2018 CC came within about 184,000km of Earth on February 6 this year. A few days later asteroid 2018 CB came within 64,000km, which is less than one-fifth the distance of Earth to the Moon.

Animation showing how asteroid 2018 CB passed closely by Earth on February 9. (NASA/JPL-Caltech)

Thankfully, both asteroids were relatively small (estimated between 15m and 40m). Neither posed a risk to Earth (this time), but Earth has not been so lucky in the past.

Read more:
Ancient asteroid impacts yield evidence for the nature of the early Earth

Research in Australia and other countries indicates that, in the distant geological past, asteroids as large as Eros (about 34.4km long and 11.2km wide) have impacted Earth. These have triggered major changes in the structure and evolution of the crust and mantle, as I’ve written about before.

Asteroid Eros.

The impact of asteroids on the Australian continent and marine shelves is examined more closely in my new book, Asteroid impacts, crustal evolution and mineral systems, with special reference to Australia, co-authored by Franco Pirajno.

In the firing line

The terrestrial planets of the inner solar system – Mars, Earth, Venus and Mercury – are all affected by asteroids deflected from the asteroid belt, located between Mars and Jupiter, and by comets falling off the Kuiper belt beyond Neptune.

The cratered surface of Mercury.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Many of these impact craters are clearly seen on Mars and Mercury as well as on our Moon. Venus too has its craters, but its thick atmosphere obscures these.

When Earth is viewed from space, it displays little or no cratering despite also being located in the trajectory of these asteroids and comets.

Earth doesn’t look very cratered from space.

But this impression is apparent rather than real. Many of the impact scars are covered or masked due to the dynamic nature of Earth and the oceans that extend over some two-thirds of the planet’s surface. The masking processes include the accretion and subduction of tectonic plates as well as intensive erosion processes.

It was not until about 1981 that the scientific community began to recognise the significance of extraterrestrial impacts for the mass extinction of species about 66 million years ago, which wiped out the dinosaurs and many other groups.

The American scientists Louis and Walter Alvarez and their colleagues had unearthed a telltale iridium-rich sedimentary layer around the 66 million-year-old Cretaceous-Tertiary boundary at Gubbio, Italy. The element iridium, typically enriched in asteroids, is a signature within sediments for material from a meteorite impact.

The discovery re-established the idea that catastrophes shaped much of Earth’s history, a theory originally promoted by the French zoologist Georges Cuvier.

Impacts on the Earth

Beyond forming craters, the impact of large asteroids on Earth resulted in the formation of structural domes due to elastic rebound of the crust. Examples include the Vredefort dome in South Africa and the buried Woodleigh dome under and east of Shark Bay in Western Australia.

The impacts also caused major seismic activity and faulting, large tsunami events, ejection of masses of particles and dust, and – as mentioned earlier – in some instances the mass extinction of species due to rapid environmental changes.

The asteroid impact record on Earth is thus to a large extent concealed and the subject of an extensive search using structural, geophysical, geochemical and other methods.

Since many impact records are covered by the oceans or were eroded, old stable parts of the Earth crust, named “cratons”, are the best places to look. This is where the scars of ancient asteroid impacts are preserved and can be found, including craters and their deep-seated roots and rebound dome structures.

Australian impacts

The criteria applied for recognition of asteroid impact structures and meteorite craters allowed the identification of at least 38 confirmed impact structures on the Australian continent and surrounding continental shelf.

There are an additional 43 examples of exposed and buried circular ring and dome features, many of which are of possible or probable impact origin.

The red circles show confirmed impact structures, green circles are impact craters, the yellow circles are possible-to-probable ring structures, red outer rings are impact structures larger than 100km in diameter, and outer white rings are impact structures less than 50km.
Google Earth/Andrew Glikson, Author provided

Examples of exposed confirmed impact structures include Gosses Bluff in the southern Northern Territory, Shoemaker in central Western Australia, and Acraman and Lawn Hill in northwestern Queensland.

A Google Earth image of the central ring of the Gosses Bluff impact structure, central Australia, Northern Territory. It is 14km in diameter and was formed around 142 million years ago.

A Google Earth image of the Shoemaker impact structure, Nabberu Basin, in Western Australia. It is 30km in diameter and was likely formed around 1,630 million years ago.

A Google Earth image of the centre of the Acraman impact structure in southern South Australia. It is 90km in diameter and was formed around 590 million years ago.

A Google Earth image of the centre of the Lawn Hill impact structure in northwestern Queensland. It is is 18km in diameter and was formed more than 515 million years ago.

The impact record of Australia thus includes exposed impact structures, buried impact structures, meteorite craters and geophysical ring anomalies of unproven origin.

Examples of large geophysical multi-ring features – total magnetic intensity anomalies, circular gravity anomalies and seismic domes – include probable buried twin impact structures in the Warburton Basin in northeast South Australia, a confirmed buried impact structure at Woodleigh in WA, and confirmed buried impact structures at Tookoonooka and Talundilly in the Eromanga Basin in southwest Queensland.

Seismic tomographic (identified 3D images) anomalies of the Warburton twin structures, South Australia, representing probable impact structures, and the Woodleigh impact structure, Western Australia.
Saygin and Kennett 2010/Andrew Glikson, Author provided

Fallout of asteroid impacts

Structures and craters caused by asteroid impacts are not the only thing we find. In the Australian landscape there are also the rock fragments and melt drops derived from clouds ejected from the impact craters.

The melt drops, condensed from impact-ejected vapour, are termed “microkrystites”. These are recognised by their radiating quench (cooling) textures and abundance of platinum group element anomalies.

Impact condensate spherules (called microkrystites) from the 2.63 billion-years-old Jeerinah Impact layer, central Pilbara Craton, Western Australia.
Bruce Simonson, Author provided

In at least one instance the evidence suggests that an impact by a cluster of large asteroids resulted in an abrupt transformation of crustal structure on the Pilbara, northwestern Australia, as well as the Barberton greenstone belt in South Africa, from a granite/greenstone system to semi-continental crustal environment.

Read more:
World’s largest asteroid impact site could be right here in Australia

Between 3.26 and 3.24 billion years ago these impacts caused a sharp tectonic uplift and magmatic activity, leading to to an onset of semi-continental crustal conditions.

Thus, far from being free from impacts, the Australian landscape has been shaped many times over millions and billions of years by asteroids falling to Earth.

The ConversationAs studies of Australian impact structure and impact ejecta progress, the critical role of asteroid impacts in the early evolution of the Earth and in the development of the Australian continent are becoming clearer.

Andrew Glikson, Earth and paleo-climate scientist, Australian National University

This article was originally published on The Conversation. Read the original article.


Citizen scientist scuba divers shed light on the impact of warming oceans on marine life

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A volunteer diver surveys marine life at Lord Howe Island.
Rick Stuart-Smith/Reef Life Survey, Author provided

Madeleine De Gabriele, The Conversation

Rising ocean temperatures may result in worldwide change for shallow reef ecosystems, according to research published yesterday in Science Advances.

The study, based on thousands of surveys carried out by volunteer scuba divers, gives new insights into the relationship of fish numbers to water temperatures – suggesting that warmer oceans may drive fish to significantly expand their habitat, displacing other sea creatures.

Citizen science

The study draws from Reef Life Survey, a 10-year citizen science project that trains volunteer scuba divers to survey marine plants and animals. Over the past ten years, more than 200 divers have surveyed 2,406 ocean sites in 44 countries, creating a uniquely comprehensive data set on ocean life.

Reef Life Survey takes volunteers on surveying expeditions at hard-to-reach coral reefs around the world.
Rick Stuart-Smith/Reef Life Survey, Author provided

Lead author Professor Graham Edgar, who founded Reef Life Survey, said the unprecedented scope of their survey allowed them to investigate global patterns in marine life. The abundance of life in warm regions (such as tropical rainforests and coral reefs) has long intrigued naturalists. At least 30 theories have been put forward, but most studies have been based on relatively limited surveys restricted to a single continent or group of species.

By tapping into the recreational scuba diving community, Reef Life Survey has vastly increased the amount of information researchers have to work with. Professor Edgar and his colleagues provide one-on-one training to volunteers, teaching them how to carry out comprehensive scans of plants and animals in specific areas.

Dr Adriana Vergés, a researcher at the University of New South Wales specialising in the impact of climate change on ocean ecosystems, said that the Reef Life Survey has already substantially improved our understanding of the marine environment.

“For example, Reef Life Survey data has greatly contributed to our understanding of the factors that determine the effectiveness of effectiveness of marine-protected areas worldwide. The team have made all their data publicly available and more and more research is increasingly making use of it to answer research questions,” she said.

Some of the divers have been working with Reef Life Survey for a decade, although others participate when they can. One volunteer, according to Professor Edgar, was so inspired by the project that he began a doctorate in marine biology (he graduated this year).

There’s a strong link between fish numbers and water warmth, which means warming oceans are likely to change global fish distribution.
Rick Stuart-Smith/Reef Life Survey, Author provided

Warming oceans means fish on the move

One of the important insights delivered by the Reef Life Survey datatbase is the relationship between water temperature and the ratio of fish to invertebrates in an ecosystem. Essentially, the warmer the water, the more fish. Conversely, colder waters contain more invertebrates like lobster, crabs and shrimp.

Professor Stewart Frusher, director of the Centre for Marine Socioecology at the University of Tasmania (and a former colleague of Professor Edgar) told The Conversation that he believes we will see wide-scale changes in fish distribution as climate change warms the oceans.

“Species are moving into either deeper water or towards the poles. We also know that not all species are moving at the same rate, and thus new mixtures of ecosystems will occur, with the fast-moving species of one ecosystem mixing with the slower moving of another,” he said.

As species migrate or expand into newly warmed waters, according to Professor Frusher, they will compete with and prey on the species already living in that area. And while it’s uncertain exactly how disruptive this will be, we do know that small ecosystem changes can rapidly lead to larger-scale impacts.

In order to predict and manage these global changes, scientists need reliable and detailed world-wide data. Professor Frusher said that, with research funding declining, scientists do not have the resources to monitor at the scales required.

The Conversation“Well-developed citizen science programs fill an important niche for improving our understanding of how the earth is responding to change,” he said.

Madeleine De Gabriele, Deputy Editor: Energy + Environment, The Conversation

This article was originally published on The Conversation. Read the original article.

El Niño in the Pacific has an impact on dolphins over in Western Australia

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Leaping bottlenose dolphins.
Kate Sprogis/MUCRU, Author provided

Kate Sprogis, Murdoch University; Fredrik Christiansen, Murdoch University; Lars Bejder, Murdoch University, and Moritz Wandres, University of Western Australia

Indo-Pacific bottlenose dolphins (Tursiops aduncus) are a regular sight in the waters around Australia, including the Bunbury area in Western Australia where they attract tourists.

The dolphin population here, about 180km south of Perth, has been studied quite intensively since 2007 by the Murdoch University Cetacean Unit. We know the dolphins here have seasonal patterns of abundance, with highs in summer/autumn (the breeding season) and lows in winter/spring.

But in winter 2009, the dolphin population fell by more than half.

A leaping bottlenose dolphin.
Kate Sprogis/MUCRU, Author provided

This decrease in numbers in WA could be linked to an El Niño event that originated far away in the Pacific Ocean, we suggest in a paper published today in Global Change Biology. The findings could have implications for future sudden drops in dolphin numbers here and elsewhere.

Read more: Tackling the kraken: unique dolphin strategy delivers dangerous octopus for dinner

A Pacific event

The El Niño Southern Oscillation (ENSO) results from an interaction between the atmosphere and the tropical Pacific Ocean. ENSO periodically fluctuates between three phases: La Niña, Neutral and El Niño.

During our study from 2007 to 2013, there were three La Niña events. There was one El Niño event in 2009, with the initial phase in winter being the strongest across Australia.

The blue vertical line shows the decline in dolphin numbers (d) during the 2009 El Niño event.
Kate Sprogis, Author provided

Coupled with El Niño, there was a weakening of the Leeuwin Current, the dominant ocean current off WA. There was also a decrease in sea surface temperature and above average rainfall.

ENSO is known to affect the strength of the south-ward flowing Leeuwin Current.

During La Niña, easterly trade winds pile warm water on the western side of the Pacific Ocean. This westerly flow of warm water across the top of Australia through the Indonesian Throughflow results in a stronger Leeuwin Current.

During El Niño, trade winds weaken or reverse and the pool of warm water in the Pacific Ocean gathers on the eastern side of the Pacific Ocean. This results in a weaker Indonesian Throughflow across the top of Australia and a weakening in strength of the Leeuwin Current.

A chart showing sea surface temperature (SST) anomalies off Western Australia. Note the extremes for the moderate El Niño in 2009 (blue rectangle), and the strong La Niña in 2011 (red rectangle)
Moritz Wandres, Author provided

The strength and variability of the Leeuwin Current coupled with ENSO affects species biology and ecology in WA waters. This includes the distribution of fish species, the transport of rock lobster larvae, the seasonal migration of whale sharks and even seabird breeding success.

The question we asked then was whether ENSO could affect dolphin abundance?

What happened during the El Niño?

These El Niño associated conditions may have affected the distribution of dolphin prey, resulting in the movement of dolphins out of the study area in search of adequate prey elsewhere.

A surfacing bottlenose dolphin.
Kate Sprogis/MUCRU, Author provided

This is similar to what happens for seabirds in WA. During an El Niño event with a weakened Leeuwin Current, the distribution of prey changes around seabird’s breeding colonies resulting in a lower abundance of important prey species, such as salmon.

This in turn negatively impacts seabirds, including a decrease in reproductive output and changes in foraging.

In southwestern Australia, the amount of rainfall is strongly connected to sea surface temperature. When the water temperature in the Indian Ocean decreases, the region receives higher rainfall during winter.

High levels of rainfall contribute to terrestrial runoff and alters freshwater inputs into rivers and estuaries. The changes in salinity influences the distribution and abundance of dolphin prey.

This is particularly the case for the river, estuary, inlet and bay around Bunbury. Rapid changes in salinity during the onset of El Niño may have affected the abundance and distribution of fish species.

In 2009, there was also a peak in strandings of dead bottlenose dolphins in WA (between 1981-2010), but the cause of this remains unknown.

Of these strandings, in southwest Australia, there was a peak in June that coincided with the onset of the 2009 El Niño.

Specifically, in the Swan River, Perth, there were several dolphin deaths, with some resident dolphins that developed fatal skin lesions that were enhanced by the low-salinity waters.

What does all this mean?

Our study is the first to describe the effects of climate variability on a coastal, resident dolphin population.

A group of bottlenose dolphins.
Kate Sprogis/MUCRU, Author provided

We suggest that the decline in dolphin abundance during the El Niño event was temporary. The dolphins may have moved out of the study area due to changes in prey availability and/or potentially unfavourable water quality conditions in certain areas (such as the river and estuary).

Read more: Explainer: El Niño and La Niña

Long-term, time-series datasets are required to detect these biological responses to anomalous climate conditions. But few long-term datasets with data collected year-round for cetaceans (whales, dolphins and porpoises) are available because of logistical difficulties and financial costs.

Continued long-term monitoring of dolphin populations is important as climate models provide evidence for the doubling in frequency of extreme El Niño events (from one event every 20 years to one event every ten years) due to global warming.

The ConversationWith a projected global increase in frequency and intensity of extreme weather events (such as floods, cyclones), coastal dolphins may not only have to contend with increasing coastal human-related activities (vessel disturbance, entanglement in fishing gear, and coastal development), but also have to adapt to large-scale climatic changes.

Kate Sprogis, Research associate, Murdoch University; Fredrik Christiansen, Postdoctoral Research Fellow, Murdoch University; Lars Bejder, Professor, Cetacean Research Unit, Murdoch University, Murdoch University, and Moritz Wandres, Oceanographer PhD Student, University of Western Australia

This article was originally published on The Conversation. Read the original article.

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