Early Earth less hellish than previously thought

Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. -  Tamara Carley
Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. – Tamara Carley

Conditions on Earth for the first 500 million years after it formed may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates.

This alternate view of Earth’s first geologic eon, called the Hadean, has gained substantial new support from the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland, which has been proposed as a possible geological analog for early Earth.

The study was conducted by a team of geologists directed by Calvin Miller, the William R. Kenan Jr. Professor of Earth and Environmental Sciences at Vanderbilt University, and published online this weekend by the journal Earth and Planetary Science Letters in a paper titled, “Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record.”

From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”

This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form – possibly on the scale of oceans.

Accepting that the early Earth had a solid crust and liquid water (at least at times), scientists have continued to debate the nature of that crust and the processes that were active at that time: How similar was the Hadean Earth to what we see today?

Two schools of thought have emerged: One argues that Hadean Earth was surprisingly similar to the present day. The other maintains that, although it was less hostile than formerly believed, early Earth was nonetheless a foreign-seeming and formidable place, similar to the hottest, most extreme, geologic environments of today. A popular analog is Iceland, where substantial amounts of crust are forming from basaltic magma that is much hotter than the magmas that built most of Earth’s current continental crust.

“We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals – and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments,” said Miller.

In 2009, Vanderbilt doctoral student Tamara Carley, who has just accepted the position of assistant professor at Layfayette College, began collecting samples from volcanoes and sands derived from erosion of Icelandic volcanoes. She separated thousands of zircon crystals from the samples, which cover the island’s regional diversity and represent its 18 million year history.

Working with Miller and doctoral student Abraham Padilla at Vanderbilt, Joe Wooden at Stanford University, Axel Schmitt and Rita Economos from UCLA, Ilya Bindeman at the University of Oregon and Brennan Jordan at the University of South Dakota, Carley analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. She then searched the literature for all comparable analyses of Hadean zircon and for representative analyses of zircon from other modern environments.

“We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew,” said Carley.

Most importantly, their analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.

“Our conclusion is counterintuitive,” said Miller. “Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even ‘cooler’ and ‘wetter’ than those being produced today.”

How much magma is hiding beneath our feet?

Molten rock (or magma) has a strong influence on our planet and its inhabitants, causing destructive volcanic eruptions and generating some of the giant mineral deposits. Our understanding of these phenomena is, however, limited by the fact that most magma cools and solidifies several kilometres beneath our feet, only to be exposed at the surface, millions of years later, by erosion. Scientists have never been able to track the movements of magma at such great depths? that is, until a team from the University of Geneva (UNIGE) discovered an innovative technique, details of which will be published in the next issue of the journal Nature.

It is a story of three scientists: a modelling specialist, an expert in a tiny mineral known as “zircon”, and a volcanologist. Following a casual conversation, the researchers stumbled upon an idea, and eventually a new method to estimate the volume and flow of magma required for the construction of magma chambers was shaped. The technique they developed makes it possible to refine predictions of future volcanic eruptions as well as identifying areas of the planet that are rich in magma-related natural resources.

Zircon: a valuable mineral for scientists

Professor Urs Schaltegger has been studying zircon for more than ten years in his laboratory at UNIGE, one of the world’s few labs in this field. «The zircon crystals that are found in solidified magma hold key information about the injection of molten rock into a magma chamber before it freezes underground,» explains the professor. Zircon contains radioactive elements that enable researchers to determine its age. As part of the study, the team from the Section of Earth and Environmental Sciences of UNIGE paired data collected using natural samples and numerical simulation. As Guy Simpson, a researcher at UNIGE further explains: «Modelling meant that we could establish how the age of crystallised zircon in a cooled magma reservoir depends on the flow rate of injected magma and the size of the reservoir.»

Applications for society and industry

In the Nature article, the researchers propose a model that is capable of determining with unprecedented accuracy the age, volume and injection rate of magma that has accumulated at inaccessible depths. As a result, they have established that the formation of Earth’s crust, volcanic super eruptions and mineral deposits occur under very specific yet different conditions. Professor Luca Caricchi adds: «When we determine the age of a family of zircons from a small sample of solidified magmatic rock, using results from the mathematical model we have developed, we can tell what the size of the entire magma chamber was, as well as how fast the magma reservoir grew». The professor continues: «This information means that we can determine the probability of an explosive volcanic eruption of a certain size to occur. In addition, the model will be of interest to industry because we will be able to identify new areas of our planet that are home to large amounts of natural resources such as copper and gold.»

Most of the sand in Alberta’s oilsands came from eastern North America, study shows

Christine Benyon is completing her Master's degree in Geoscience in the Faculty of Science. -  Riley Brandt, University of Calgary
Christine Benyon is completing her Master’s degree in Geoscience in the Faculty of Science. – Riley Brandt, University of Calgary

They’re called the Alberta oilsands but most of the sand actually came from the Appalachian region on the eastern side of the North American continent, a new University of Calgary-led study shows.

The oilsands also include sand from the Canadian Shield in northern and east-central Canada and from the Canadian Rockies in western Canada, the study says.

This study is the first to determine the age of individual sediment grains in the oilsands and assess their origin.

“The oilsands are looked at as a Western asset,” says study lead author Christine Benyon, who is just completing her Master’s degree in Geoscience in the Faculty of Science.

“But we wouldn’t have oilsands without the sand, and some of that sand owes its origin to the Appalachians and other parts of Canada.”

The research, which also involved study sponsor Nexen Energy ULC and the University of Arizona LaserChron Center, was published last week in the Journal of Sedimentary Research.

The findings contribute to geologists’ fundamental understanding of the oilsands.

They also help oilsands companies better understand the stratigraphy, or layers, of sand and the ancient valleys where sediment was deposited, “and that could lead to better production techniques,” Benyon says.

To determine the origin of the sand, the researchers used a relatively new technique called “detrital zircon uranium-lead geochronology.”

They used a mass spectrometer to date the age of tiny and extremely durable crystallized minerals called zircons that are present in the oilsands.

“The age of those zircons tells us how long since they crystallized. And knowing that, we can infer their place of origin,” Benyon explains.

Researchers dated the zircons in nine core samples from three wells drilled into the oilsands’ McMurray Formation.

Lowermost deposits contained zircons ranging from 1,800 to 2,800 million years old. These zircons, and thereby the associated sand, originally came from the Canadian Shield, which contains rocks of those ages, Benyon says.

Most of the oilsands’ sediment contains zircons that range from 300 to 1,200 million years old – the same zircon signature found in Appalachian sources in eastern North America.

The uppermost oilsands contain zircons that are less than 250 million years old, indicating the Canadian Rockies as their most likely place of origin.

So how did so much of the sand, deposited during the Cretaceous geological time period about 145 to 66 million years ago (before the sand was emplaced with oil), get to Alberta from the other side of the continent?

No one knows for sure, but Benyon and her co-authors propose three theories, all involving sediment transported by a continental-sized river system:

  • The massive river system transported the sediment directly from eastern North America to the present-day oilsands region during the Cretaceous;

  • The river system transported the sediment from eastern North America during an earlier geological time period and deposited it in the southwestern United States; later, in the Cretaceous, rivers flowing northward ‘recycled’ the sediment and deposited it in western Canada;
  • An older (perhaps Jurassic or Permian) river system coming from the Appalachians deposited the sediment in western Canada; later, during the Cretaceous, the sediment was eroded by smaller rivers and re-deposited in northeastern Alberta.

No matter how and when the sand got to the present location of the oilsands, “the study tells us that the Appalachians were an important source of sediment in the geological history of North America,” Benyon says.

Oldest bit of crust firms up idea of a cool early Earth

A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth's crust. -  John Valley
A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth’s crust. – John Valley

With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life about 4.4 billion years ago is coming into sharper focus.

Writing today (Feb. 23, 2014) in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.

“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet. “This may also help us understand how other habitable planets would form.”

The new study confirms that zircon crystals from Western Australia’s Jack Hills region crystallized 4.4 billion years ago, building on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth’s crust. The microscopic zircon crystal used by Valley and his group in the current study is now confirmed to be the oldest known material of any kind formed on Earth.

The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock. “The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after, says Valley.

The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample. Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together, like “raisins in a pudding,” notes Valley.

The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating. “The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Valley says. “This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”

The formation, isotope ratio and size of the clumps – less than 50 atoms in diameter – become, in effect, a clock, says Valley, and verify that existing geochronology methods provide reliable and accurate estimates of the sample’s age. In addition, Valley and his group measured oxygen isotope ratios, which give evidence of early homogenization and later cooling of the Earth.

“The Earth was assembled from a lot of heterogeneous material from the solar system,” Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago “that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”

Diamonds in Earth’s oldest zircons are nothing but laboratory contamination

This image explains how synthetic diamond can be distinguished from natural diamond. -  Dobrzhinetskaya Lab, UC Riverside.
This image explains how synthetic diamond can be distinguished from natural diamond. – Dobrzhinetskaya Lab, UC Riverside.

As is well known, the Earth is about 4.6 billion years old. No rocks exist, however, that are older than about 3.8 billion years. A sedimentary rock section in the Jack Hills of western Australia, more than 3 billion years old, contains within it zircons that were eroded from rocks as old as about 4.3 billion years, making these zircons, called Jack Hills zircons, the oldest recorded geological material on the planet.

In 2007 and 2008, two research papers reported in the journal Nature that a suite of zircons from the Jack Hills included diamonds, requiring a radical revision of early Earth history. The papers posited that the diamonds formed, somehow, before the oldest zircons – that is, before 4.3 billion years ago – and then were recycled repeatedly over a period of 1.2 billion years during which they were periodically incorporated into the zircons by an unidentified process.

Now a team of three researchers, two of whom are at the University of California, Riverside, has discovered using electron microscopy that the diamonds in question are not diamonds at all but broken fragments of a diamond-polishing compound that got embedded when the zircon specimen was prepared for analysis by the authors of the Nature papers.

“The diamonds are not indigenous to the zircons,” said Harry Green, a research geophysicist and a distinguished professor of the Graduate Division at UC Riverside, who was involved in the research. “They are contamination. This, combined with the lack of diamonds in any other samples of Jack Hills zircons, strongly suggests that there are no indigenous diamonds in the Jack Hills zircons.”

Study results appear online this week in the journal Earth and Planetary Science Letters.

“It occurred to us that a long-term history of diamond recycling with intermittent trapping into zircons would likely leave some sort of microstructural record at the interface between the diamonds and zircon,” said Larissa Dobrzhinetskaya, a professional researcher in the Department of Earth Sciences at UCR and the first author of the research paper. “We reasoned that high-resolution electron microscopy of the material should be able to distinguish whether the diamonds are indeed what they have been believed to be.”

Using an intensive search with high-resolution secondary-electron imaging and transmission electron microscopy, the research team confirmed the presence of diamonds in the Jack Hills zircon samples they examined but could readily identify them as broken fragments of diamond paste that the original authors had used to polish the zircons for examination. They also observed quartz, graphite, apatite, rutile, iron oxides, feldspars and other low-pressure minerals commonly included into zircon in granitic rocks.

“In other words, they are contamination from polishing with diamond paste that was mechanically injected into silicate inclusions during polishing” Green said.

The research was supported by a grant from the National Science Foundation.

Green and Dobrzhinetskaya were joined in the research by Richard Wirth at the Helmholtz Centre Potsdam, Germany.

Dobrzhinetskaya and Green planned the research project; Dobrzhinetskaya led the project; she and Wirth did the electron microscopy.

Crystals in Picabo’s rocks point to ‘recycled’ super-volcanic magma chambers

University of Oregon geologist Ilya Bindeman, left, and graduate student Dana Drew, working in Bindeman's stable isotope laboratory say that the composition of zircon bits in igneous rocks in the Yellowstone hotspot track tell a new story on how super volcanoes recycle magma. -  University of Oregon
University of Oregon geologist Ilya Bindeman, left, and graduate student Dana Drew, working in Bindeman’s stable isotope laboratory say that the composition of zircon bits in igneous rocks in the Yellowstone hotspot track tell a new story on how super volcanoes recycle magma. – University of Oregon

A thorough examination of tiny crystals of zircon, a mineral found in rhyolites, an igneous rock, from the Snake River Plain has solidified evidence for a new way of looking at the life cycle of super-volcanic eruptions in the long track of the Yellowstone hotspot, say University of Oregon scientists.

The pattern emerging from new and previous research completed in the last five years under a National Science Foundation career award, said UO geologist Ilya N. Bindeman, is that another super-eruption from the still-alive Yellowstone volcanic field is less likely for the next few million years than previously thought (see related story, “Not in a million years, says Oregon geologist about Yellowstone eruption“). The last eruption 640,000 years ago created the Yellowstone Caldera and the Lava Creek Tuff in what is now Yellowstone National Park.

The Yellowstone hotspot creates a conveyor belt style of volcanism because of the southwest migration of the North American plate at 2-4 centimeters (about .8 to 1.6 inches) annually over the last 16 million years of volcanism. Due to the movement of the North American plate, the plume interaction with the crust leaves footprints in the form of caldera clusters, in what is now the Snake River Plain, Bindeman said.

The Picabo volcanic field of southern Idaho, described in a new paper by a six-member team, was active between 10.4 and 6.6 million years ago and experienced at least three, and maybe as many as six, violent caldera-forming eruptions. The field has been difficult to assess, said lead author Dana Drew, a UO graduate student, because the calderas have been buried by as much as two kilometers of basalt since its eruption cycle died.

The work at Picabo is detailed in a paper online ahead of publication in the journal Earth and Planetary Science Letters.

The team theorized that basalt from the mantle plume, rocks from Earth’s crust and previously erupted volcanoes are melted together to form the rhyolites erupted in the Snake River Plain. Before each eruption, rhyolite magma is stored in dispersed pockets throughout the upper crust, which are later mixed together, according to geochemical evidence. “We think that this batch-assembly process is an important part of caldera-forming eruptions, and generating rhyolites in general,” Drew said.

In reaching their conclusions, Drew and colleagues analyzed radiogenic and stable isotopic data — specifically oxygen and hafnium — in zircons detected in rhyolites found at the margins of the Picabo field and from a deep borehole. That data, in combination with whole rock geochemistry and zircon uranium-lead geochronology helped provide a framework to understand the region’s ancient volcanic past.

Previous research on the related Heise volcanic field east of Picabo yielded similar results. “There is a growing database of the geochemistry of rhyolites in the Yellowstone hotspot track,” Drew said. “Adding Picabo provides a missing link in the database.

Drew and colleagues, through their oxygen isotope analyses, identified a wide diversity of oxygen ratios occurring in erupted zircons near the end of the Picabo volcanic cycle. Such oxygen ratios are referred to as delta-O-18 signatures based on oxygen 18 levels relative to seawater. (Oxygen 18 contains eight protons and 10 neutrons; Oxygen 16, with eight protons and eight neutrons, is the most commonly found form of oxygen in nature)

The approach provided a glimpse into the connection of surface and subsurface processes at a caldera cluster. The interaction of erupted rhyolite with groundwater and surface water causes hydrothermal alteration and the change in oxygen isotopes, thereby providing a fingerprinting tool for the level of hydrothermal alteration, Drew said.

“Through the eruptive sequence, we begin to generate lower delta-O-18 signatures of the magmas and, with that, we also see a more diverse signature,” Drew said. “By the time of the final eruption there is up to five per mil diversity in the signature recorded in the zircons.” The team attributes these signatures to the mixing of diverse magma batches dispersed in the upper crust, which were formed by melting variably hydrothermally altered rocks — thus diverse delta-O-18 — after repeated formation of calderas and regional extension or stretching of the crust.

When the pockets of melt are rapidly assembled, the process could be the trigger for caldera forming eruptions, Bindeman said. “That leads to a homogenized magma, but in a way that preserves these zircons of different signatures from the individual pockets of melt,” he said. This research, he added, highlights the importance of using new micro-analytical isotopic techniques to relate geochemistry at the crystal-scale to processes occurring at the crustal-wide scale in generating and predicting large-volume rhyolitic eruptions.

“This important research by Dr. Bindeman and his team demonstrates the enormous impact an NSF CAREER award can have,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the graduate school at the University of Oregon. “The five-year project is providing new insights into the eruption cycles of the Yellowstone hotspot and helping scientists to better predict future volcanic activity.”

Geochemical ‘fingerprints’ leave evidence that megafloods eroded steep gorge

This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. -  U.S. Geological Survey
This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. – U.S. Geological Survey

The Yarlung-Tsangpo River in southern Asia drops rapidly through the Himalaya Mountains on its way to the Bay of Bengal, losing about 7,000 feet of elevation through the precipitously steep Tsangpo Gorge.

For the first time, scientists have direct geochemical evidence that the 150-mile long gorge, possibly the world’s deepest, was the conduit by which megafloods from glacial lakes, perhaps half the volume of Lake Erie, drained suddenly and catastrophically through the Himalayas when their ice dams failed at times during the last 2 million years.

“You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream,” said Karl Lang, a University of Washington doctoral candidate in Earth and space sciences.

In this case, the water moved rapidly through bedrock gorge, carving away the base of slopes so steep they already were near the failure threshold. Because the riverbed through the Tsangpo Gorge is essentially bedrock and the slope is so steep and narrow, the deep flood waters could build enormous speed and erosive power.

As the base of the slopes eroded, areas higher on the bedrock hillsides tumbled into the channel, freeing microscopic grains of zircon that were carried out of the gorge by the fast-moving water and deposited downstream.

Uranium-bearing zircon grains carry a sort of geochemical signature for the place where they originated, so grains found downstream can be traced back to the rocks from which they eroded. Lang found that normal annual river flow carries about 40 percent of the grains from the Tsangpo Gorge downstream. But grains from the gorge found in prehistoric megaflood deposits make up as much as 80 percent of the total.

He is the lead author of a paper documenting the work published in the September edition of Geology. Co-authors are Katharine Huntington and David Montgomery, both UW faculty members in Earth and space sciences.

The Yarlung-Tsangpo is the highest major river in the world. It begins on the Tibetan Plateau at about 14,500 feet, or more than 2.5 miles, above sea level. It travels more than 1,700 miles, crossing the plateau and plunging through the Himalayas before reaching India’s Assam Valley, where it becomes the Brahmaputra River. From there it continues its course to the Ganges River delta and the Bay of Benga

At the head of the Tsangpo Gorge, the river makes a sharp bend around Namche Barwa, a 25,500-foot peak that is the eastern anchor of the Himalayas. Evidence indicates that giant lakes were impounded behind glacial dams farther inland from Namche Barwa at various times during the last 2.5 million years ago.

Lang matched zircons in the megaflood deposits far downstream with zircons known to come only from Namche Barwa, and those signature zircons turned up in the flood deposits at a much greater proportion than they would in sediments from normal river flows. Finding the zircons in deposits so far downstream is evidence for the prehistoric megafloods and their role in forming the gorge.

Lang noted that a huge landslide in early 2000 created a giant dam on the Yiggong River, a tributary of the main river just upstream from the Gorge. The dam failed catastrophically in June 2000, triggering a flood that caused numerous fatalities and much property damage downstream.

That provided a vivid, though much smaller, illustration of what likely occurred when large ice dams failed millions of years ago, he said. It also shows the potential danger if humans decide to build dams in that area for hydroelectric generation.

“We are interested in it scientifically, but there is certainly a societal element to it,” Lang said. “This takes us a step beyond speculating what those ancient floods did. There is circumstantial evidence that, yes, they did do a lot of damage.”

The process in the Tsangpo Gorge is similar to what happened with Lake Missoula in Western Montana 12,000 to 15,000 years ago. That lake was more than 10,000 feet lower in elevation than lakes associated with the Tsangpo Gorge, though its water discharge was 10 times greater. Evidence suggests that Lake Missoula’s ice dam failed numerous times, unleashing a torrent equal to half the volume of Lake Michigan across eastern Washington, where it carved the Channeled Scablands before continuing down the Columbia River basin.

“This is a geomorphic process that we know shapes the landscape, and we can look to eastern Washington to see that,” Lang said.

Setting the stage for life: Scientists make key discovery about the atmosphere of early Earth

Scientists in the New York Center for Astrobiology at Rensselaer Polytechnic Institute have used the oldest minerals on Earth to reconstruct the atmospheric conditions present on Earth very soon after its birth. The findings, which appear in the Dec. 1 edition of the journal Nature, are the first direct evidence of what the ancient atmosphere of the planet was like soon after its formation and directly challenge years of research on the type of atmosphere out of which life arose on the planet.

The scientists show that the atmosphere of Earth just 500 million years after its creation was not a methane-filled wasteland as previously proposed, but instead was much closer to the conditions of our current atmosphere. The findings, in a paper titled “The oxidation state of Hadean magmas and implications for early Earth’s atmosphere,” have implications for our understanding of how and when life began on this planet and could begin elsewhere in the universe. The research was funded by NASA.

For decades, scientists believed that the atmosphere of early Earth was highly reduced, meaning that oxygen was greatly limited. Such oxygen-poor conditions would have resulted in an atmosphere filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia. To date, there remain widely held theories and studies of how life on Earth may have been built out of this deadly atmosphere cocktail.

Now, scientists at Rensselaer are turning these atmospheric assumptions on their heads with findings that prove the conditions on early Earth were simply not conducive to the formation of this type of atmosphere, but rather to an atmosphere dominated by the more oxygen-rich compounds found within our current atmosphere – including water, carbon dioxide, and sulfur dioxide.

“We can now say with some certainty that many scientists studying the origins of life on Earth simply picked the wrong atmosphere,” said Bruce Watson, Institute Professor of Science at Rensselaer.

The findings rest on the widely held theory that Earth’s atmosphere was formed by gases released from volcanic activity on its surface. Today, as during the earliest days of the Earth, magma flowing from deep in the Earth contains dissolved gases. When that magma nears the surface, those gases are released into the surrounding air.

“Most scientists would argue that this outgassing from magma was the main input to the atmosphere,” Watson said. “To understand the nature of the atmosphere ‘in the beginning,’ we needed to determine what gas species were in the magmas supplying the atmosphere.”

As magma approaches the Earth’s surface, it either erupts or stalls in the crust, where it interacts with surrounding rocks, cools, and crystallizes into solid rock. These frozen magmas and the elements they contain can be literal milestones in the history of Earth.

One important milestone is zircon. Unlike other materials that are destroyed over time by erosion and subduction, certain zircons are nearly as old as the Earth itself. As such, zircons can literally tell the entire history of the planet – if you know the right questions to ask.

The scientists sought to determine the oxidation levels of the magmas that formed these ancient zircons to quantify, for the first time ever, how oxidized were the gases being released early in Earth’s history. Understanding the level of oxidation could spell the difference between nasty swamp gas and the mixture of water vapor and carbon dioxide we are currently so accustomed to, according to study lead author Dustin Trail, a postdoctoral researcher in the Center for Astrobiology.

“By determining the oxidation state of the magmas that created zircon, we could then determine the types of gases that would eventually make their way into the atmosphere,” said Trail.

To do this Trail, Watson, and their colleague, postdoctoral researcher Nicholas Tailby, recreated the formation of zircons in the laboratory at different oxidation levels. They literally created lava in the lab. This procedure led to the creation of an oxidation gauge that could then be compared with the natural zircons.

During this process they looked for concentrations of a rare Earth metal called cerium in the zircons. Cerium is an important oxidation gauge because it can be found in two oxidation states, with one more oxidized than the other. The higher the concentrations of the more oxidized type cerium in zircon, the more oxidized the atmosphere likely was after their formation.

The calibrations reveal an atmosphere with an oxidation state closer to present-day conditions. The findings provide an important starting point for future research on the origins of life on Earth.

“Our planet is the stage on which all of life has played out,” Watson said. “We can’t even begin to talk about life on Earth until we know what that stage is. And oxygen conditions were vitally important because of how they affect the types of organic molecules that can be formed.”

Despite being the atmosphere that life currently breathes, lives, and thrives on, our current oxidized atmosphere is not currently understood to be a great starting point for life. Methane and its oxygen-poor counterparts have much more biologic potential to jump from inorganic compounds to life-supporting amino acids and DNA. As such, Watson thinks the discovery of his group may reinvigorate theories that perhaps those building blocks for life were not created on Earth, but delivered from elsewhere in the galaxy.

The results do not, however, run contrary to existing theories on life’s journey from anaerobic to aerobic organisms. The results quantify the nature of gas molecules containing carbon, hydrogen, and sulfur in the earliest atmosphere, but they shed no light on the much later rise of free oxygen in the air. There was still a significant amount of time for oxygen to build up in the atmosphere through biologic mechanisms, according to Trail.

Ancient mineral shows early Earth climate tough on continents

Pictured is a false-color microscope image of a 4-billion-year-old zircon, a tiny mineral used to study the ancient rocks in which it formed. Chemical analysis of this crystal by UW-Madison geologists Takayuki Ushikubo and John Valley suggests that rocky continents and liquid water existed on Earth at least 4.3 billion years ago. Evidence of heavy weathering by a harsh climate may help explain why no rock samples older than 4 billion years have ever been found. - Photo: courtesy Mary Diman and John Valley
Pictured is a false-color microscope image of a 4-billion-year-old zircon, a tiny mineral used to study the ancient rocks in which it formed. Chemical analysis of this crystal by UW-Madison geologists Takayuki Ushikubo and John Valley suggests that rocky continents and liquid water existed on Earth at least 4.3 billion years ago. Evidence of heavy weathering by a harsh climate may help explain why no rock samples older than 4 billion years have ever been found. – Photo: courtesy Mary Diman and John Valley

A new analysis of ancient minerals called zircons suggests that a harsh climate may have scoured and possibly even destroyed the surface of the Earth’s earliest continents.

Zircons, the oldest known materials on Earth, offer a window in time back as far as 4.4 billion years ago, when the planet was a mere 150 million years old. Because these crystals are exceptionally resistant to chemical changes, they have become the gold standard for determining the age of ancient rocks, says UW-Madison geologist John Valley.

Valley previously used these tiny mineral grains – smaller than a speck of sand – to show that rocky continents and liquid water formed on the Earth much earlier than previously thought, about 4.2 billion years ago.

In a new paper published online this week in the journal Earth and Planetary Science Letters, a team of scientists led by UW-Madison geologists Takayuki Ushikubo, Valley and Noriko Kita show that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate.

Ushikubo, the first author on the new study, says that atmospheric weathering could provide an answer to a long-standing question in geology: why no rock samples have ever been found dating back to the first 500 million years after the Earth formed.

“Currently, no rocks remain from before about 4 billion years ago,” he says. “Some people consider this as evidence for very high temperature conditions on the ancient Earth.”

Previous explanations for the missing rocks have included destruction by barrages of meteorites and the possibility that the early Earth was a red-hot sea of magma in which rocks could not form.

The current analysis suggests a different scenario. Ushikubo and colleagues used a sophisticated new instrument called an ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in western Australia. By comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to the Earth’s mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.

A timeline shows the geological context of Jack Hills zircons, ancient minerals that formed when the Earth was less than 500 million years old. - Illustration: Andree Valley
A timeline shows the geological context of Jack Hills zircons, ancient minerals that formed when the Earth was less than 500 million years old. – Illustration: Andree Valley

“At 4.3 billion years ago, the Earth already had habitable conditions,” Ushikubo says.

The zircons’ lithium signatures also hold signs of rock exposure on the Earth’s surface and breakdown by weather and water, identified by low levels of a heavy lithium isotope. “Weathering can occur at the surface on continental crust or at the bottom of the ocean, but the [observed] lithium compositions can only be formed from continental crust,” says Ushikubo.

The findings suggest that extensive weathering may have destroyed the Earth’s earliest rocks, he says.

“Extensive weathering earlier than 4 billion years ago actually makes a lot of sense,” says Valley. “People have suspected this, but there’s never been any direct evidence.”

Carbon dioxide in the atmosphere can combine with water to form carbonic acid, which falls as acid rain. The early Earth’s atmosphere is believed to have contained extremely high levels of carbon dioxide – maybe 10,000 times as much as today.

“At [those levels], you would have had vicious acid rain and intense greenhouse [effects]. That is a condition that will dissolve rocks,” Valley says. “If granites were on the surface of the Earth, they would have been destroyed almost immediately – geologically speaking – and the only remnants that we could recognize as ancient would be these zircons.”

Additional information and images are available on the authors’ Web sites Zircons Are Forever and the Wisc-SIMS ion microprobe facility.

Other co-authors on the paper include Aaron Cavosie of the University of Puerto Rico, Simon Wilde of the Curtin University of Technology in Australia and Roberta Rudnick of the University of Maryland.