Decoding mysterious seismic signals

For the decades since their discovery, seismic signals known as PKP precursors have challenged scientists. Regions of Earth's lower mantle scatter incoming seismic waves, which return to the surface as PKP waves at differing speeds.

The origin the precursor signals, which arrive ahead of the main seismic waves that travel through Earth's core, has remained unclear, but research led by University of Utah geophysicists sheds new light on this mysterious seismic energy.

PKP precursors appear to propagate from places deep below North America and the western Pacific and possibly bear an association with "ultra-low velocity zones," thin layers in the mantle where seismic waves significantly slow down, according to research published in AGU Advances, the American Geophysical Union's lead journal. (The AGU highlighted the research in its magazine Eos.)

"These are some of the most extreme features discovered on the planet. We legitimately do not know what they are," said lead author Michael Thorne, a U associate professor of geology and geophysics. "But one thing we know is they seem to end up accumulating underneath hotspot volcanoes. They seem like they may be the root of whole mantle plumes giving rise to hotspot volcanoes."

These plumes are responsible for the volcanism observed at Yellowstone, the Hawaiian Islands, Samoa, Iceland and the Galapagos Islands.

"These really, really big volcanoes seem to persist for hundreds of millions of years in roughly the same spot," Thorne said. In previous work, he also found one of the world's largest known ultra-low velocity zones.

"It sits right beneath Samoa, and Samoa is one of the biggest hotspot volcanoes," Thorne noted.

For nearly a century, geoscientists have used seismic waves to probe Earth's interior, leading to numerous discoveries that would not be otherwise possible. Other researchers at the U, for example, have characterized the structure of Earth's solid inner core and tracked its movement by analyzing seismic waves.

When an earthquake rattles Earth's surface, seismic waves shoot through the mantle -- the 2,900-kilometer-thick dynamic layer of hot rock between Earth's crust and metal core. Thorne's team is interested in those that get "scattered" when they pass through irregular features that pose changes in material composition in the mantle. Some of those scattered waves become PKP precursors.

Thorne sought to determine exactly where this scattering happens, especially since the waves travel through Earth's mantle twice, that is, before and after passing through Earth's liquid outer core. Because of that double journey through the mantle, it has been nearly impossible to distinguish whether the precursors originated on the source-side or receiver-side of the ray path.

Thorne's team, which included research assistant professor Surya Pachhai, devised a way to model waveforms to detect crucial effects that previously went unnoticed.

Using a cutting-edge seismic array method and new theoretical observations from earthquake simulations, the researchers developed, they analyzed data from 58 earthquakes that occurred around New Guinea and were recorded in North America after passing through the planet.

"I can put virtual receivers anywhere on the surface of the earth, and this tells me what the seismogram should look like from an earthquake at that location. And we can compare that to the real recordings that we have," Thorne said. "We're able to now back project where this energy's coming from."

Their new method allowed them to pinpoint where the scattering occurred along the boundary between the liquid metal outer core and the mantle, known as the core-mantle boundary, located 2,900 kilometers below Earth's surface.

Their findings indicate that the PKP precursors likely come from regions that are home to ultra-low velocity zones. Thorne suspects these layers, which are only 20 to 40 kilometers thick, are formed where subducted tectonic plates impinge on the core-mantle boundary in oceanic crust.

"What we've now found is that these ultra-low velocity zones do not just exist beneath the hotspots. They're spread out all across the core-mantle boundary beneath North America," Thorne said. "It really looks like these ULVZs are getting actively generated. We don't know how. But because we're seeing them near subduction, we think mid-ocean ridge basalts are getting melted, and that is how it's getting generated. And then the dynamics is pushing these things all across Earth, and ultimately they're going to accumulate beneath the hotspots."

"What we've now found is that these ultra-low velocity zones do not just exist beneath the hotspots. They're spread out all across the core-mantle boundary beneath North America," Thorne said. "It really looks like these ULVZs are getting actively generated. We don't know how. But because we're seeing them near subduction, we think mid-ocean ridge basalts are getting melted, and that may be how they're getting generated."

The dynamics is pushing these things all across Earth, and ultimately, they're going to accumulate against the boundaries of Large Low Velocity Provinces, which are compositionally distinct continent scale features beneath the Pacific and Africa, according to Thorne.

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Climate change and mercury pollution stressed plants for millions of years

The link between massive flood basalt volcanism and the end-Triassic (201 million years ago) mass-extinction is commonly accepted. However, exactly how volcanism led to the collapse of ecosystems and the extinction of entire families of organisms is difficult to establish. Extreme climate change from the release of carbon dioxide, degradation of the ozone layer due to the injection of damaging chemicals, and the emissions of toxic pollutants, are all seen as contributing factors. One toxic element stands out: mercury. As one of the most toxic elements on Earth, Hg is a metal that is emitted from volcanoes in gaseous form, and thus has the capacity to spread worldwide. A new study in Nature Communications adds new compelling evidence for the combined effects of global warming and widespread mercury pollution that continued to stress plants long after volcanic activity had ceased.

An international team of Dutch, Chinese, Danish, British, and Czech scientists studied sediments from Northern-Germany in a drill-core (Schandelah-1) that spans the uppermost Triassic to lower Jurassic for microfossils and geochemical signals. A study of pollen and spore abundances revealed a profusion of fern spores showing a range of malformations, from abnormalities in wall structure to evidence for botched meiotic divisions, leading to unseparated, dwarfed, and fused fern spores. "Seeing the sheer amount and different types of malformed fern spores in sediment samples from a coastal lagoon, dating back 201 million years ago is truly astonishing. It means there must have been very many ferns being stressed," explains Remco Bos, a PhD candidate at Utrecht University and lead author of the study. "It is also not something we see regularly during other periods that also contain many fern fossils, making it a true signal connected to the end-Triassic mass-extinction event."

Deforestation and ferns

The results from Bos and co-authors confirm earlier work by co-authors Sofie Lindström (University of Copenhagen), Hamed Sanei (Aarhus University), and Bas van de Schootbrugge (Utrecht University), who previously produced similar data obtained from cores from Denmark and from nearby outcrops in Sweden. According to Sofie Lindström: "Ferns replaced trees across the extinction interval in response to dramatic environmental changes likely driven by heat stress, strongly increased monsoonal rainfall, and increased forest fire activity. Palynological results show that a pioneering fern vegetation spread across vast swaths of coastal lowlands in Northwestern Europe from Sweden and Denmark to Germany, France, Luxemburg, and Austria in response to widespread deforestation." Ferns are hardy plants, often colonizing disturbed environments, including newly formed volcanic islands or landscapes devastated by volcanism or wildfires. "What is extraordinary here is that the ferns that produced all these malformed spores in all these different sites, did not go extinct. While other plants went extinct, ferns were apparently robust enough to continue, which could also be related to their different mercury tolerance."

Climate variability

In this new study, Bos and co-authors show that the ferns, which took advantage of the dieback of forests, themselves were subjected to stress from Hg-pollution well beyond the immediate extinction interval. "We found four more intervals with high levels of Hg concentrations and high numbers of malformed spores in the 1.3 to 2 million years following the extinction interval," explains Remco Bos. This interval, known as the Hettangian, was a time of continuing adverse conditions in the oceans, with generally low diversities among marine invertebrates, such as ammonites and bivalves. On land, however, vegetation appeared to have recovered quicker. "We now show that this forest ecosystem continued to be perturbed repeatedly for at least 1.3 million years, but perhaps as long as 2 million years," Bos explains.

The four additional episodes of high Hg concentrations and high fern spore malformations were unlikely connected to later phases of Central Atlantic Magmatic Province volcanism. Instead, Bos and co-authors show that these periods correspond closely to the long eccentricity cycle, the major variation in the shape of Earth's orbit that moves Earth closer or further away from the Sun every 405 thousand years. During eccentricity maxima Earth moves closer to the Sun allowing for more sunlight to reach the Earth surface. As the Earth's atmosphere was already supercharged with carbon dioxide from the large-scale volcanism, this cyclic modulation of the climate system repeatedly triggered forest dieback, allowing for the renewed spread of pioneer ferns. As is shown by the correlation with high Hg contents, malformations in fern spores during these episodes were also the result of mercury poisoning. But where did this Hg come from?

Hg-isotopes

A crucial data set was generated at Tianjin University (China) by Wang Zheng, a co-corresponding author and geochemist specialized in metal isotope studies, especially Hg-isotopes. Mercury has different stable isotopes that behave differently in the environment. During reactions in nature, for example the expulsion from volcanism, deposition from the atmosphere, and the uptake by organisms, Hg-isotopes can become fractionated, enriching one pool in heavier isotopes, and others in lighter isotopes. Sediments with elevated levels of Hg and malformed spores also show clear variations in Hg-isotopes. "Based on the Hg-isotope variations we were able to link an initial pulse in Hg enrichment at the Triassic-Jurassic boundary to the emission of mercury from flood basalt volcanism," Wang Zheng explains. "However, the four other pulses in mercury had a different isotopic composition, indicating they were mainly driven by Hg input from soil erosion and photochemical reduction."

Climate change and toxic pollution

The combined geochemical and microfossil data thus paint a picture of a much more complex and drawn-out sequence of events, starting with massive volcanism driving climate change and releasing toxic pollutants, followed by episodic pulses of disturbance in the aftermath of the extinction event lasting for at least 1.3 million years. Dr. Tomas Navratil from the Czech Academy of Sciences, a co-author on the paper and a specialist for modern-day mercury pollution, agrees with this scenario. "Our work on polluted sites in the Czech Republic does show evidence for episodic remobilization from forest soils, especially during hot summers, and in places that are more exposed to sunlight causing the photochemical reduction of mercury and re-release to the atmosphere of previously stored mercury."

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Diverse ancient volcanoes on Mars discovered by planetary scientist may hold clues to pre-plate tectonic activity on Earth

Volcanoes are a common feature on the surfaces of solid planets within the solar system, resulting from magmatic activity occurring within the planetary crust. On Earth, volcanism is driven primarily by heat and crustal recycling associated with plate tectonics, but Mars lacks plate tectonics and the driver of volcanism is not well understood.

Recent research by Professor Joseph MICHALSKI, a geologist in the Department of Earth Sciences at The University of Hong Kong (HKU), has revealed intriguing insights into the volcanic activity on Mars.

He proposes that Mars has significantly more diverse volcanism than previously realised, driven by an early form of crust recycling called vertical tectonics.

The findings, recently published in Nature Astronomy, shed light on the ancient crust of Mars and its potential implications for understanding early crustal recycling on both Mars and Earth.

Traditionally, Mars has been known to have large shield volcanoes similar to those in Hawaii.

However, it was not known that Mars also possessed the diverse, explosive volcanoes that form on Earth due to crustal recycling.

The recent research conducted by Professor Michalski and his international team discover a vast number of diverse volcanoes in the ancient crust of Mars.

'We have known for decades that Mars has volcanoes, but most of the recognised volcanoes correspond to large basaltic shield volcanoes similar to the ones that make up Hawaii,' he explains.

'In this work, we show that the ancient crust has many other types of volcanoes such as lava domes, stratovolcanoes, calderas and large shields of ash, not lava.

Further, most scientists see Mars as a planet composed of basalt, which has low silica content and represents little crustal evolution, but these volcanoes have high silica content which means they formed from a complex process of magma evolution not known before.'

The paper suggests that intense volcanism occurred on ancient Mars, causing the crust to collapse into the mantle, where the rocks re-melted, resulting in magmas that have high silica.

This tectonic process, called vertical tectonics, is hypothesised to have occurred on the ancient Earth, but rocks on Earth from that period (the Archean, more than 3 billion years ago) are highly modified by later geological activity, so we cannot see evidence for this process clearly on this planet.

Therefore, exploring other planets like Mars, which has volcanism but no plate tectonics, can help reveal the mysteries of early crustal recycling on both the Red Planet, and by analogy, on early Earth.

Professor Michalski concluded, 'Mars contains critical geological puzzle pieces that help us understand not only that planet, but the Earth as well.

Martian volcanism is much more complex and diverse than has been previously thought.'

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