University of Wyoming professor has used computer modeling to propose that sand and mud subducted off the coast of California around 75 million years ago returned to the Earth’s crust by rising up through the mantle as enormous diapirs, like blobs in a lava lamp.
These blobs are now found at the surface of the Earth, far inland from the coast, in places including the Mojave Desert and western Arizona.
“These rocks aren’t the prettiest to look at, but they went on an extraordinary journey and have an incredible story to tell,” says Jay Chapman, an assistant professor in UW’s Department of Geology and Geophysics who focuses on tectonics.
“The rocks started their lives as sediment eroded from the Sierra Nevada Mountains and carried by rivers and streams down to the ocean, where they ended up deposited in a subduction trench, similar to the modern-day Marianas trench,” Chapman says. “Then, they were carried about 20 miles deep into the Earth by a subducting oceanic plate, where the sediments were metamorphosed into a rock called schist. That in and of itself is pretty amazing, but the truly special thing about these rocks is that they didn’t stay subducted, but somehow made their way back up to the surface, where you can go stand on them today.”
How the subducted sediments returned to the surface of the Earth and the distribution of the sediments in the subsurface are some of the questions Chapman is trying to answer with his research.
“The prevailing theory is that the sediments were smeared against and plastered to the base of the North American tectonic plate, forming a sheet-like layer,” Chapman says. “However, the density of these sediments is much lower than rocks in the mantle or lower crust and, over millions of years, computer modeling predicts that the sediments will flow and buoyantly ascend, like hot wax in a lava lamp.”
The research has implications for understanding subduction zone processes and the distribution of natural resources.
“Geoscientists around the world are working to understand what gives continental crust its unique composition, and subduction and reincorporation of sediment are a popular hypothesis,” Chapman says. “In addition, many researchers are now wondering whether fluids and elements released from the subducted sediments may have contributed to the concentration of economically important minerals and metals.”
Featured image: UW Assistant Professor Jay Chapman teaches a winter-term field course in southern Arizona, during which students were able to investigate the Orocopia Schist in person. Chapman is the author of a new paper that suggests the rock originated as sand and mud subducted off the coast of California around 75 million years ago, then returned to the Earth’s crust by rising up through the mantle as enormous diapirs. (UW Photo)
Reference: James B. Chapman, Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction, Geology (2021). DOI: 10.1130/G48647.1
The rise of oxygen levels early in Earth’s history paved the way for the spectacular diversity of animal life. But for decades, scientists have struggled to explain the factors that controlled this gradual and stepwise process, which unfolded over nearly 2 billion years.
Now an international research team is proposing that increasing day length on the early Earth—the spinning of the young planet gradually slowed over time, making the days longer—may have boosted the amount of oxygen released by photosynthetic cyanobacteria, thereby shaping the timing of Earth’s oxygenation.
Their conclusion was inspired by a study of present-day microbial communities growing under extreme conditions at the bottom of a submerged Lake Huron sinkhole, 80 feet below the water’s surface. The water in the Middle Island Sinkhole is rich in sulfur and low in oxygen, and the brightly colored bacteria that thrive there are considered good analogs for the single-celled organisms that formed mat-like colonies billions of years ago, carpeting both land and seafloor surfaces.
The researchers show that longer day length increases the amount of oxygen released by photosynthetic microbial mats. That finding, in turn, points to a previously unconsidered link between Earth’s oxygenation history and its rotation rate. While the Earth now spins on its axis once every 24 hours, day length was possibly as brief as 6 hours during the planet’s infancy.
The team’s findings were published online Aug. 2 in the journal Nature Geoscience.
Lead authors are Judith Klatt of the Max Planck Institute for Marine Microbiology and Arjun Chennu of the Leibniz Centre for Tropical Marine Research. Klatt is a former postdoctoral researcher in the lab of University of Michigan geomicrobiologist Gregory Dick, who is one of the study’s two corresponding authors. The other co-authors are from U-M and Grand Valley State University.
“An enduring question in the Earth sciences has been how did Earth’s atmosphere get its oxygen, and what factors controlled when this oxygenation took place,” Dick said from the deck of the R/V Storm, a 50-foot NOAA research vessel that carried a team of scientists and scuba divers on a sample-collection trip from the town of Alpena, Michigan, to the Middle Island Sinkhole, several miles offshore.
“Our research suggests that the rate at which the Earth is spinning—in other words, its day length—may have had an important effect on the pattern and timing of Earth’s oxygenation,” said Dick, a professor in the U-M Department of Earth and Environmental Sciences.
The researchers simulated the gradual slowing of Earth’s rotation rate and showed that longer days would have boosted the amount of oxygen released by early cyanobacterial mats in a manner that helps explain the planet’s two great oxygenation events.
The project began when co-author Brian Arbic, a physical oceanographer in the U-M Department of Earth and Environmental Sciences, heard a public lecture about Klatt’s work and noted that day length changes could play a role, over geological time, in the photosynthesis story that Dick’s lab was developing.
Cyanobacteria get a bad rap these days because they are the main culprits behind the unsightly and toxic algal blooms that plague Lake Erie and other water bodies around the world.
But these microbes, formerly known as blue-green algae, have been around for billions of years and were the first organisms to figure out how to capture energy from sunlight and use it to produce organic compounds through photosynthesis—releasing oxygen as a byproduct.
Masses of these simple organisms living in primeval seas are credited with releasing oxygen that later allowed for the emergence of multicellular animals. The planet was slowly transformed from one with vanishingly small amounts of oxygen to present-day atmospheric levels of around 21%.
At the Middle Island Sinkhole in Lake Huron, purple oxygen-producing cyanobacteria compete with white sulfur-oxidizing bacteria that use sulfur, not sunlight, as their main energy source.
In a microbial dance repeated daily at the bottom of the Middle Island Sinkhole, filmy sheets of purple and white microbes jockey for position as the day progresses and as environmental conditions slowly shift. The white sulfur-eating bacteria physically cover the purple cyanobacteria in the morning and evening, blocking their access to sunlight and preventing them from carrying out oxygen-producing photosynthesis.
But when sunlight levels increase to a critical threshold, the sulfur-oxidizing bacteria migrate back down below the photosynthetic cyanobacteria, enabling them to start producing oxygen.
The vertical migration of sulfur-oxidizing bacteria has been observed before. What’s new is that the authors of the Nature Geoscience study are the first to link these microbial movements, and the resultant rates of oxygen production, to changing day length throughout Earth’s history.
“Two groups of microbes in the Middle Island Sinkhole mats compete for the uppermost position, with sulfur-oxidizing bacteria sometimes shading the photosynthetically active cyanobacteria,” Klatt said while processing a core sample from Middle Island Sinkhole microbial mats in an Alpena laboratory. “It’s possible that a similar type of competition between microbes contributed to the delay in oxygen production on the early Earth.”
A key to understanding the proposed link between changing day length and Earth’s oxygenation is that longer days extend the afternoon high-light period, allowing photosynthetic cyanobacteria to crank out more oxygen.
“The idea is that with a shorter day length and shorter window for high-light conditions in the afternoon, those white sulfur-eating bacteria would be on top of the photosynthetic bacteria for larger portions of the day, limiting oxygen production,” Dick said as the boat rocked on choppy waters, moored a couple hundred yards from Middle Island.
The present-day Lake Huron microbes are believed to be good analogs for ancient organisms in part because the extreme environment at the bottom of the Middle Island Sinkhole likely resembles the harsh conditions that prevailed in the shallow seas of early Earth.
Lake Huron is underlain by 400-million-year-old limestone, dolomite and gypsum bedrock that formed from the saltwater seas that once covered the continent. Over time, the movement of groundwater dissolved some of that bedrock, forming caves and cracks that later collapsed to create both on-land and submerged sinkholes near Alpena.
Cold, oxygen-poor, sulfur-rich groundwater seeps into the bottom of the 300-foot-diameter Middle Island Sinkhole today, driving away most plants and animals but creating an ideal home for certain specialized microbes.
Dick’s team, in collaboration with co-author Bopaiah Biddanda of the Annis Water Resources Institute at Grand Valley State University, has been studying the microbial mats on the floor of Middle Island Sinkhole for several years, using a variety of techniques. With the help of scuba divers from NOAA’s Thunder Bay National Marine Sanctuary—which is best known for its shipwrecks but is also home to the Middle Island Sinkhole and several others like it—the researchers deployed instruments to the lake floor to study the chemistry and biology there.
They also brought mat samples to the lab to conduct experiments under controlled conditions.
Klatt hypothesized that the link between day length and oxygen release can be generalized to any given mat ecosystem, based on the physics of oxygen transport. She teamed up with Chennu to conduct detailed modeling studies to relate microbial mat processes to Earth-scale patterns over geological timescales.
The modeling studies revealed that day length does, in fact, shape oxygen release from the mats.
“Simply speaking, there is just less time for the oxygen to leave the mat in shorter days,” Klatt said.
This led the researchers to posit a possible link between longer day lengths and increasing atmospheric oxygen levels. The models show that this proposed mechanism might help explain the distinctive stepwise pattern of Earth’s oxygenation, as well as the persistence of low-oxygen periods through most of the planet’s history.
Throughout most of Earth’s history, atmospheric oxygen was only sparsely available and is believed to have increased in two broad steps. The Great Oxidation Event occurred about 2.4 billion years ago and has generally been credited to the earliest photosynthesizing cyanobacteria. Nearly 2 billion years later a second surge in oxygen, known as the Neoproterozoic Oxygenation Event, occurred.
Earth’s rotation rate has been slowly decreasing since the planet formed about 4.6 billion years ago due to the relentless tug of the moon’s gravity, which creates tidal friction.
The study was funded by grants from the National Science Foundation, the Max Planck Society and the University of Michigan Turner Fellowship. Field operations were supported by the NOAA Great Lakes Environmental Research Laboratory and NOAA’s Thunder Bay National Marine Sanctuary.
In 2020, Colorado battled the four largest wildfires in its history, leaving residents anxious for another intense wildfire season this year.
But last week, fires weren’t the issue—it was their aftermath. When heavy rains fell over the burn scar from the 2020 Cameron Peak fire, they triggered flash flooding and mudslides northwest of Fort Collins which destroyed homes, killed at least three people and damaged major roads. Flooding along the 2020 Grizzly Creek and East Troublesome burn scars also unleashed mudslides across Interstate 70 through Glenwood Canyon and in Grand County just west of Rocky Mountain National Park.
These tragic events make it clear that the effects of wildfire don’t end when the flames go out. There can be environmental consequences for years to come—and keeping an eye on water is key.
CU Boulder Today spoke with Professor Fernando Rosario-Ortiz, an environmental chemistry expert who studies how wildfires impact water quality; and Assistant Professor and CIRES FellowBen Livneh, a hydrologist who studies how climate change affects water supplies and how fires and rain influence landslide risk, about how fire may shape the future of water in the West.
What happens to water in lakes, rivers and streams after a nearby wildfire?
Rosario-Ortiz: When you have open flames, a lot of gaseous reactions and solid phase reactions, it results in the transformation of chemicals and alterations to the soil, and we observe the effects once we look at the water quality. For example, we observe the enhancement in the concentration of nutrients in water, which is not necessarily a bad thing, but it can cause subsequent issues in the reservoirs like algae blooms. There can also be a mobilization of metals and enhanced concentration and activity of what we call organic carbon as well as turbidity, which can then impact water treatment production and formation of disinfection byproducts.
How do city water suppliers and treatment plants deal with these impacts?
Rosario-Ortiz: Ideally, you want to have a secondary water source. In Fort Collins, back in 2012 after the High Park fire, the river was impacted but the reservoir was not impacted. So they could draw from the reservoir and wait for the worst to pass.
If you don’t have that option, some of the challenges after wildfire and rain events include increased sediment mobilization, which is very challenging for water treatment operations. Those are short-term effects that might give you a headache, but they can also become long-term challenges. Never mind the fact that you may have issues with infrastructure.
How can wildfire affect water quantity and timing in a landscape?
Livneh: In the western U.S. we really rely on water that flows in rivers and streams, and that fills the reservoirs for our supply. So when we think about even small changes to the amount of water that comes off of the hill slope, or across the landscape, that can have a big impact on the total availability of water.
One of the most notable things that happens in a fire is that the texture of the soil changes. Initially, less rain will soak into the soil, and more rain will become surface runoff. There’s a lot of reason to think that you will get more total water—but it’ll be much more “flashy” when it comes.
On one hand, that can be good if you have a reservoir to collect it. But we’ve heard of water utilities actually turning off their intakes after a fire if the quality of the water is too low. And that’s tricky, because often drought is involved in some fashion. So there’s often this competing need for more water, and yet the quality is low.
What are the factors that affect the likelihood of floods or mudslides after wildfire?
Livneh: When water carries enough stuff with it, we call it a debris flow, which is a type of landslide. The bigger and bigger it gets, the more impactful it is. We have research funded by NASA where we looked at 5,000 landslide sites around the world. We found that sites that had a fire in the past three years required less precipitation to cause a landslide.
But there’s also a lot of local variability that really matters. Moderately steep, heavily vegetated areas, types of soils—especially sandier soils—increase risk. Also we now have a lot of people who have built structures on steep slopes in these areas, so there’s a human element there, too. And the time of the year that it happens can matter. A fire right before your rainy season is an important factor.
What does this all mean for the future of Colorado and the western U.S.?
Rosario-Ortiz: When homes burn, you’re not just combusting houses, you’re combusting everything inside those homes. You might now be combusting electric vehicles, for example, with a large battery.
Then what are some of the other potential concerns with exposure to air? Water? That’s going to be something that we will need to explore further over the next few years.
Livneh: Some estimates say the amount of forest area being burned each year in the western U.S. has doubled in the last 25 years. And it really poses risks to communities, especially in the wildland-urban interface (WUI). Managing it is largely a kind of a policy problem, but in the next 10 years or so we’re going to continue to have these big fires.
First and foremost, people need to be paying attention to these flood watches and to local guidance on evacuation. The most important thing is saving lives.
What can we do to prepare for the future?
Rosario-Ortiz: Utilities might have to be thinking about potential upgrades in facilities. That means we may have to also consider financing of these projects and how to improve overall resiliency.
Livneh: One of the most robust features of climate change is warming, right? As rain becomes more prevalent, we’re just going to have to continue expanding our portfolio of things we do to keep up. The more open-minded we can be about managing for these things is important. I’m kind of an optimist. As humans, we’ve overcome so many technical challenges; it’s not going to be something we can’t solve our way out of.
Banner image: The aftermath of July 2021 floods in Poudre Canyon, west of Fort Collins. (Credit: Colorado Parks and Wildlife)
The increasing size of the private space industry could be a climate disaster as rockets emit vast quantities of propellant exhaust into the stratosphere and mesosphere, where it can persist for at least two to three years, warns Dr Eloise Marais (UCL Geography).
The commercial race to get tourists to space is heating up between Virgin Group founder Sir Richard Branson and former Amazon CEO Jeff Bezos. On Sunday 11 July, Branson ascended 80 km to reach the edge of space in his piloted Virgin Galactic VSS Unity spaceplane. Bezos’ autonomous Blue Origin rocket is due to launch on July 20, coinciding with the anniversary of the Apollo 11 Moon landing.
Though Bezos loses to Branson in time, he is set to reach higher altitudes (about 120 km). The launch will demonstrate his offering to very wealthy tourists: the opportunity to truly reach outer space. Both tour packages will provide passengers with a brief ten-minute frolic in zero gravity and glimpses of Earth from space. Not to be outdone, Elon Musk’s SpaceX will provide four to five days of orbital travel with its Crew Dragon capsule later in 2021.
What are the environmental consequences of a space tourism industry likely to be? Bezos boasts his Blue Origin rockets are greener than Branson’s VSS Unity. The Blue Engine 3 (BE-3) will launch Bezos, his brother and two guests into space using liquid hydrogen and liquid oxygen propellants. VSS Unity used a hybrid propellant comprised of a solid carbon-based fuel, hydroxyl-terminated polybutadiene (HTPB), and a liquid oxidant, nitrous oxide (laughing gas). The SpaceX Falcon series of reusable rockets will propel the Crew Dragon into orbit using liquid kerosene and liquid oxygen.
Burning these propellants provides the energy needed to launch rockets into space while also generating greenhouse gases and air pollutants. Large quantities of water vapour are produced by burning the BE-3 propellant, while combustion of both the VSS Unity and Falcon fuels produces CO₂, soot and some water vapour. The nitrogen-based oxidant used by VSS Unity also generates nitrogen oxides, compounds that contribute to air pollution closer to Earth.
Roughly two-thirds of the propellant exhaust is released into the stratosphere (12 km-50 km) and mesosphere (50 km-85 km), where it can persist for at least two to three years. The very high temperatures during launch and re-entry (when the protective heat shields of the returning crafts burn up) also convert stable nitrogen in the air into reactive nitrogen oxides.
These gases and particles have many negative effects on the atmosphere. In the stratosphere, nitrogen oxides and chemicals formed from the breakdown of water vapour convert ozone into oxygen, depleting the ozone layer which guards life on Earth against harmful UV radiation. Water vapour also produces stratospheric clouds that provide a surface for this reaction to occur at a faster pace than it otherwise would.
Space tourism and climate change
Exhaust emissions of CO₂ and soot trap heat in the atmosphere, contributing to global warming. Cooling of the atmosphere can also occur, as clouds formed from the emitted water vapour reflect incoming sunlight back to space. A depleted ozone layer would also absorb less incoming sunlight, and so heat the stratosphere less.
Figuring out the overall effect of rocket launches on the atmosphere will require detailed modelling, in order to account for these complex processes and the persistence of these pollutants in the upper atmosphere. Equally important is a clear understanding of how the space tourism industry will develop.
Virgin Galactic anticipates it will offer 400 spaceflights each year to the privileged few who can afford them. Blue Origin and SpaceX have yet to announce their plans. But globally, rocket launches wouldn’t need to increase by much from the current 100 or so performed each year to induce harmful effects that are competitive with other sources, like ozone-depleting chlorofluorocarbons (CFCs), and CO₂ from aircraft.
During launch, rockets can emit between four and ten times more nitrogen oxides than Drax, the largest thermal power plant in the UK, over the same period. CO₂ emissions for the four or so tourists on a space flight will be between 50 and 100 times more than the one to three tonnes per passenger on a long-haul flight.
In order for international regulators to keep up with this nascent industry and control its pollution properly, scientists need a better understanding of the effect these billionaire astronauts will have on our planet’s atmosphere.
As temperatures rise, the risk of devastating forest fires is increasing. Researchers at the Technical University of Munich (TUM) are using artificial intelligence to estimate the long-term impact that an increased number of forest fires will have on forest ecosystems. Their simulations show how Yellowstone National Park in the USA could change by the end of the century.
In many places, fire is part of the natural environment, and many tree species have become naturally adapted to recurrent fires. These adaptations range from particularly thick bark, which protects the sensitive cambium in the trunk from the fire, to the cones of certain types of pine, which open only due to the heat of fire, allowing a quick regeneration and recovery of affected woodland.
AI is accelerating ecosystem models
“The interaction between climate, forest fires, and other processes in the forest ecosystem is very complex, and sophisticated process-based simulation models are required to take account of the different interactions appropriately,” explains Prof. Seidl. A method that has been developed at TUM is using artificial intelligence to significantly expand the field of use of these complex models.
This method involves the training of a deep neural network in order to imitate the behavior of a complex simulation model as effectively as possible. The neural network learns on the basis of how the ecosystem responds to differing environmental influences, but does so using only a fraction of the computing power that would otherwise be necessary for large-scale simulation models. “This allows us to carry out spatially high-resolution simulations of areas of forest that stretch across several million hectares,” explains scientist Dr. Werner Rammer.
Forecast for the forests in Yellowstone National Park
The simulations completed by the team of scientists include simulations for the “Greater Yellowstone Ecosystem”, which has the world-famous Yellowstone National Park at its heart. This area, which is approximately 8 million hectares in size, is situated in the Rocky Mountains and is largely untouched. The researchers at the TUM have worked with American colleagues to determine how different climate scenarios could affect the frequency of forest fires in this region in the 21st century, and which areas of forest cannot regenerate successfully following a forest fire.
Depending on the climate change scenario, the study has found that by the end of the century, the current forest coverage will have disappeared in 28 to 59 percent of the region. Particularly affected were the forests in the sub-alpine zone near the tree line, where the species of tree are naturally less adapted to fire, and the areas on the Yellowstone Plateau, where the relatively flat topography is mostly unable to stop the fire from spreading.
Climate change is causing significant changes to forest ecosystems
The regeneration of the forest in the region under investigation is at threat for several reasons: If the fires get bigger and the distances between the surviving trees also increase, too few seeds will make their way onto the ground. If the climate gets hotter and drier in the future, the vulnerable young trees won’t survive, and if there are too many fires, the trees won’t reach the age at which they themselves yield seeds.
“By 2100, the Greater Yellowstone Ecosystem is expected to have changed more than it has in the last 10,000 years, and will therefore look significantly different than it does today,” explains Rammer. “The loss of today’s forest vegetation is leading to a reduction in the carbon which is stored in the ecosystem, and will also have a profound impact on the biodiversity and recreational value of this iconic landscape.”
The potential developmental trends identified in the study are also intended to help visitors to the national park understand the consequences of climate change and the urgency of the climate protection measures. In the next step, the research team will be using AI to estimate the long-term impact of the problems caused by climate change in the forests of Europe.
Featured image: The iconic landscape of Yellowstone National Park is characterized by vast forests that have been untouched by man but are threatened by increasing numbers of forest fires due to climate change.Image: R. Seidl / TUM
Earth and environmental sciences professors explain why naming new species may be a never-ending journey.
Ever since Swedish naturalist and explorer Carolus Linnaeus developed the uniform system for defining and naming species of organisms, known as binomial nomenclature (e.g. Homo sapiens for human beings), scientists have wondered if they will ever be able to predict the total number of species with whom we share the planet. At current count, there are about 54,000 known vertebrate animals, which are those having a backbone or spinal column, including mammals, birds, reptiles, amphibians and fishes. Professors in the College of Arts and Sciences explored whether or not the scientific community will ever be able to settle on a ‘total number’ of species of living vertebrates, which could help with species preservation. By knowing what’s out there, researchers argue that they can prioritize places and groups on which to concentrate conservation efforts.
Research professor Bruce Wilkinson and professor Linda Ivany, both from the Department of Earth and Environmental Sciences, recently co-authored a paper in the Biological Journal of the Linnean Society where they determined that forecasting the total number of species may never be possible.
When asking the question, ‘how many species?,’ it is important to note that only a fraction of existing species have been named. In order to make a prediction on a total number, researchers project the curve of new species descriptions each year into the future until eventually reaching a point when all species should have been found.
Wilkinson, a geologist, noticed parallels between the discovery curves of new species and the total reservoir size of nonrenewable resources like oil or mineral ores. Similar to the species curve, by extending the oil reservoir curve researchers thought they should be able to estimate the total global reservoir and how long it will take to get to it all. The theory of resource exploitation suggests that the number of discoveries over time follows a bell-shaped curve: The curve rises as production rate increases due to new discoveries and then decreases as production declines, despite all the effort continuing to go into finding the resource. The time of maximum discovery is known as Hubbert’s peak, after M. King Hubbert who predicted it. Following that time, the resource is being evermore depleted until it is used up.
“The problem with using that curve to predict how much is left is that you have to assume that the effort invested and the approach used to discover new oil, or species, is consistent and known,” says Wilkinson. “We used to think we’d gone over the peak for oil and gas around 1972, but then 15 or so years ago someone figured out how to do horizontal drilling and all of the sudden there was a new bump in the amount being discovered.”
Wilkinson and Ivany say that the discovery curve for new species of vertebrate animals shows a similar bump. Like the increase in the oil curve caused by horizontal drilling in the early 2000s, there was a surge in new species discovery beginning around 1950, when new funding was being dedicated to science after the World War II, more scientists were going into biology, and new molecular techniques were leading to an increase in the ability to distinguish species from one another.
In both cases, unforeseen changes in the effort and method of discovering new oil or species altered the way the discovery curves were playing out.
If researchers had estimated the total number of species based on data prior to 1950, their estimates would be much different from any estimate made today, and both would likely be wrong because those new advents cannot be predicted.
In some ways, this is a reflection of the scientific method, in which hypotheses stand until new facts are discovered, which lead to changes in the hypothesis.
“As much as we’d like to know ‘the number,’ the total species richness of the planet will remain an elusive target,” says Ivany.
Unusual chemistry of grain could tell scientists more about the origin of Earth’s water
Scientists have discovered a new type of star dust whose composition indicates that it formed during a rare form of nucleosynthesis (the process through which new atomic nuclei are created) and could shed new light on the history of water on Earth.
A team led by cosmochemists from Caltech and Victoria University of Wellington in New Zealand studied ancient minerals aggregates within the Allende meteorite (which fell to Earth in 1969) and found that many of them had unusually high amounts of strontium-84, a relatively rare light isotope of the element strontium that is so-named for the 84 neutrons in its nucleus.
“Strontium-84 is part of a family of isotopes produced by a nucleosynthetic process, named the p-process, which remains mysterious,” says Caltech’s François L. H. Tissot, assistant professor of geochemistry. “Our results points to the survival of grains possibly containing pure strontium-84. This is exciting, as the physical identification of such grains would provide a unique chance to learn more about the p-process.”
Tissot and collaborator Bruce L. A. Charlier of Victoria University of Wellington are co-lead authors on a study describing the findings that was published in Science Advances on July 9.
“This is really interesting,” Charlier says. “We want to know what the nature of this material is and how it fits into the mix of ingredients that went to form the recipe for the planets.”
Strontium (atomic symbol: Sr), a chemically reactive metal, has four stable isotopes: strontium-84 and its heavier cousins that have 86, 87, or 88 neutrons in their nuclei. Scientists have found that strontium is useful when attempting to date objects from the early solar system because one of its heavy isotopes, strontium-87, is produced by the decay of the radioactive isotope rubidium-87 (atomic symbol: Rb).
Rubidium-87 has a very long half-life, 49 billion years, which is more than three times the age of the universe. Half-life represents the amount of time required for the radioactivity of an isotope to drop to one-half its original value, allowing these isotopes to serve as chronometers for dating samples on varying time scales. The most famous radioactive isotope used for dating is carbon-14, the radioactive isotope of carbon; with its half-life of roughly 5,700 years, carbon-14 can be used to determine the ages of organic (carbon-containing) materials on human timescales, up to about 60,000 years. Rubidium-87, in contrast, can be used to date the oldest objects in the universe, and, closer to home, the objects in the solar system.
What is particularly attractive about using the Rb–Sr pair for dating is that rubidium is a volatile element—that is, it tends to evaporate to form a gas phase at even relatively low temperatures—while strontium is not volatile. As such, rubidium is present at a higher proportion in solar system objects that are rich in other volatiles (such as water), because they formed at lower temperatures.
Counterintuitively, Earth has an Rb/Sr ratio that is 10 times lower than that of water-rich meteorites, implying that the planet either accreted from water-poor (and thus rubidium-poor) materials or it accreted from water-rich materials but lost most of its water over time as well as its rubidium. Understanding which of these scenarios took place is important for understanding the origin of water on Earth.
In theory, the Rb–Sr chronometer should be able to tease apart these two scenarios, as the amount of Sr-87 produced by radioactive decay in a given amount of time will not be the same if Earth started with a lot of rubidium versus less of the material.
In the latter scenario, i.e., with less rubidium, the newly formed Earth would have been poor in volatiles such as water, thus the amount of Sr-87 in the earth and in volatile-poor meteorites would be similar to that observed in the oldest-known solar system solids, the so-called CAIs. CAIs are calcium- and aluminum-rich inclusions found in certain meteorites. Dating back 4.567 billion years, CAIs represent the first objects that condensed in the early solar nebula, the flattened, rotating disk of gas and dust from which the solar system was born. As such, CAls offer a geologic window into how and from what type of stellar materials the solar system formed.
“They are critical witnesses to the processes that were happening while the solar system was forming,” says Tissot.
However, the composition of CAIs has long muddled scientists’ ability to determine if Earth formed mostly dry or not. That is because CAls, unlike other solar-system materials, have anomalous ratios of the four strontium isotopes, with a slightly elevated proportion of strontium-84. Thus, they pose a challenge to the validity of the rubidium–strontium dating system. And they also raise a key question: Why are they different?
To learn more, Tissot and Charlier took nine specimens of so-called fine-grained CAls. Fine-grained CAIs have preserved their condensate (that is, snowflake-like) texture, which testifies to their pristine nature.
The team painstakingly leached out these CAIs by bathing them in gradually harsher acids to strip away the more chemically reactive minerals (and the strontium they contain), leaving a concentrate of only the most resistant fraction. The final sample contained almost pure Sr-84, while a typical sample is composed of 0.56 percent Sr-84.
“Step-leaching is a little bit of a blunt instrument because you are not entirely sure what exactly it is you are destroying at each step,” Charlier says. “But the nub of what we’ve found is, once you have stripped away 99 percent of the common components within the CAIs, what we are left with is something highly exotic that we weren’t expecting.”
“The signature is unlike anything else found in the solar system,” Tissot says. The grains carrying this signature, Tissot and Charlier concluded, must have formed prior to the birth of the solar system and survived that cataclysmic process during which stellar grains were heated to extremely high temperatures, vaporized, and then condensed into solid materials.
Given the relative abundance of strontium-84, the discovery points to the likely existence in meteorites of nanometer-sized grains containing almost pure strontium-84 that were formed during a rare nucleosynthetic process before the formation of the solar system itself. The nature of these grains is still a mystery, as only their isotopic composition in strontium reveals their existence. But the high levels of Sr-84 in the CAIs suggest that Earth and volatile-poor meteorites have more strontium-87 than CAIs, favoring the scenario in which Earth accreted with more water and volatile elements, which were subsequently lost within the first few million years after their formation.
Scientists at the University of Southampton have discovered that changes in Earth’s orbit may have allowed complex life to emerge and thrive during the most hostile climate episode the planet has ever experienced.
The researchers – working with colleagues in the Chinese Academy of Sciences, Curtin University, University of Hong Kong, and the University of Tübingen – studied a succession of rocks laid down when most of Earth’s surface was covered in ice during a severe glaciation, dubbed ‘Snowball Earth’, that lasted over 50 million years. Their findings are published in the journal Nature Communications.
“One of the most fundamental challenges to the Snowball Earth theory is that life seems to have survived,” says Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, and co-author of the study. “So, either it didn’t happen, or life somehow avoided a bottleneck during the severe glaciation.”
The research team ventured into the South Australian outback where they targeted kilometre-thick units of glacial rocks formed about 700 million years ago. At this time, Australia was located closer to the equator, known today for its tropical climates. The rocks they studied, however, show unequivocal evidence that ice sheets extended as far as the equator at this time, providing compelling evidence that Earth was completely covered in an icy shell.
The team focused their attention on “Banded Iron Formations”, sedimentary rocks consisting of alternating layers of iron-rich and silica-rich material. These rocks were deposited in the ice-covered ocean near colossal ice sheets.
During the snowball glaciation, the frozen ocean would have been entirely cut off from the atmosphere. Without the normal exchange between the sea and air, many variations in climate that normally occur simply wouldn’t have.
“This was called the ‘sedimentary challenge’ to the Snowball hypothesis,” says Professor Ross Mitchell, professor at the Chinese Academy of Sciences in Beijing, China and the lead author. “The highly variable rock layers appeared to show cycles that looked a lot like climate cycles associated with the advance and retreat of ice sheets.” Such variability was thought to be at odds with a static Snowball Earth entombing the whole ocean in ice.
“The iron comes from hydrothermal vents on the seafloor,” added Gernon. “Normally, the atmosphere oxidizes any iron immediately, so Banded Iron Formations typically do not accumulate. But during the Snowball, with the ocean cut off from the air, iron was able to accumulate enough for them to form.”
Using magnetic susceptibility – a measure of the extent to which the rocks become magnetised when exposed to a magnetic field – the team made the discovery that the layered rock archives preserve evidence for nearly all orbital cycles.
Earth’s orbit around the sun changes its shape and the tilt and wobble of Earth’s spin axis also undergo cyclic changes. These astronomical cycles change the amount of incoming solar radiation that reaches Earth’s surface and, in doing so, they control climate.
“Even though Earth’s climate system behaved very differently during the Snowball, Earth’s orbital variations would have been blissfully unaware and just continued to do their thing,” explains Professor Mitchell.
The researchers concluded that changes in Earth’s orbit allowed the waxing and waning of ice sheets, enabling periodic ice-free regions to develop on snowball Earth.
Professor Mitchell explained, “This finding resolves one of the major contentions with the snowball Earth hypothesis: the long-standing observation of significant sedimentary variability during the snowball Earth glaciations appeared at odds with such an extreme reduction of the hydrological cycle”.
The team’s results help explain the enigmatic presence of sedimentary rocks of this age that show evidence for flowing water at Earth’s surface when this water should have been locked up in ice sheets. Dr. Gernon states: “This observation is important, because complex multicellular life is now known to have originated during this period of climate crisis, but previously we could not explain why”.
“Our study points to the existence of ice-free ‘oases’ in the snowball ocean that provided a sanctuary for animal life to survive arguably the most extreme climate event in Earth history,” Dr Gernon concluded.
A unique study of ancient diamonds has shown that the basic chemical composition of the Earth’s atmosphere which makes it suitable for life’s explosion of diversity was laid down at least 2.7 billion years ago. Volatile gases conserved in diamonds found in ancient rocks were present in similar proportions to those found in today’s mantle, which in turn indicates that there has been no fundamental change in the proportions of volatiles in the atmosphere over the last few billion years. This shows that one of the basic conditions necessary to support life, the presence of life-giving elements in sufficient quantity, appeared soon after Earth formed, and has remained fairly constant ever since.
Presenting the work at the Goldschmidt Geochemistry conference, lead researcher Dr Michael Broadly said, “The proportion and make-up of volatiles in the atmosphere reflects that found in the mantle, and we have no evidence of a significant change since these diamonds were formed 2.7 billion years ago”.
Volatiles, such as hydrogen, nitrogen, neon, and carbon-bearing species are light chemical elements and compounds, which can be readily vaporised due to heat, or pressure changes. They are necessary for life, especially carbon and nitrogen. Not all planets are rich in volatiles; Earth is volatile rich, as is Venus, but Mars and the Moon lost most of their volatiles into space. Generally, a planet rich in volatiles has a better chance of sustaining life, which is why much of the search for life on planets surrounding distant stars (exoplanets) has focused on looking for volatiles.
On Earth, volatile substances mostly bubble up from the inside of the planet, and are brought to the surface through such things as volcanic eruptions. Knowing when the volatiles arrived in the Earth’s atmosphere is key to understanding when the conditions on Earth were suitable for the origin and development of life, but until now there has been no way of understanding these conditions in the deep past.
Now French and Canadian researchers have used ancient diamonds as a time capsule, to examine the conditions deep inside the Earth’s mantle in the distant past. Studies of the gases trapped in these diamonds show that the volatile composition of the mantle has changed little over the last 2.7 billion years.
Lead researcher, Michael Broadley (University of Lorraine, France) said “Studying the composition of the Earth’s modern mantle is relatively simple. On average the mantle layer begins around 30km below the Earth’s surface, and so we can collect samples thrown up by volcanoes and study the fluids and gases trapped inside. However, the constant churning of the Earth’s crust via plate tectonics means that older samples have mostly been destroyed. Diamonds however, are comparatively indestructible, they’re ideal time capsules”.
We managed to study diamonds trapped in 2.7 billion year old highly preserved rock from Wawa, on Lake Superior in Canada. This means that the diamonds are at least as old as the rocks they are found in – probably older. It’s difficult to date diamonds, so this gave us a lucky opportunity to be sure of the minimum age. These diamonds are incredibly rare, and are not like the beautiful gems we think of when we think of diamonds. We heated them to over 2000 C to transform them into graphite, which then released tiny quantities of gas for measurement”.
The team measured the isotopes of Helium, Neon, and Argon, and found that they were present in similar proportions to those found in the upper mantle today. This means that there has probably been little change in the proportion of volatiles generally, and that the distribution of essential volatile elements between the mantle and the atmosphere are likely to have remained fairly stable throughout the majority of Earth’s life. The mantle is the part between the Earth’s crust and the core, it comprises around 84% of the Earth’s volume.
Dr Broadley continued “This was a surprising result. It means the volatile-rich environment we see around us today is not a recent development, so providing the right conditions for life to develop. Our work shows that these conditions were present at least 2.7 billion years ago, but the diamonds we use may be much older, so it’s likely that these conditions were set well before our 2.7 billion year threshold”.
Commenting, Dr Suzette Timmerman (University of Alberta, Canada) said:
“Diamonds are unique samples, as they lock in compositions during their formation. The Wawa fibrous diamonds specifically were a great selection to study – being more than 2.7 billion years old – and they provide important clues into the volatile composition in this period, the Neoarchean period. It is interesting that the upper mantle already appears degassed more than 2.7 billion years ago. This work is an important step towards understanding the mantle (and atmosphere) in the first half of Earth’s history and leads the way to further questions and research”.
Dr Timmerman was not involved in this work, this is an independent comment.
The Goldschmidt Conference is the World’s main geochemistry conference. It is hosted alternately by the European Association of Geochemistry (Europe) and the Geochemical Society (USA). The 2021 conference (virtual) takes place from 4-9 July, https://2021.goldschmidt.info/. The 2022 conference takes place in Hawaii.
Featured image: One of the 2.7 billion year-old diamonds used in this work. Credit: Michael Broadley
This science news has been confirmed by us from the Goldschmidt Conference