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  • richardmitnick 1:44 pm on October 5, 2022 Permalink | Reply
    Tags: "What Can Zircons Tell Us About the Evolution of Plants?", , , , Events deep within Earth might chronicle the radiation of plants with roots and leaves and stems - a development that occurred about 430 million years ago., , Geochronology, , The versatile mineral could contain evidence of the evolution of land plants and their effect on the sedimentary system.   

    From “Eos” : “What Can Zircons Tell Us About the Evolution of Plants?” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    10.5.22
    Alka Tripathy-Lang

    The versatile mineral could contain evidence of the evolution of land plants and their effect on the sedimentary system.

    1
    Zircons may record the evolution of vegetation like that lining the Swiss river Kander. Credit: Adrian Michael/Wikimedia, CC-BY-3.0.

    Geologists love zircon for its ability to tell time. They’ve also used these robust, tiny time capsules in a variety of studies ranging from estimating when water first appeared on Earth to exploring the origin of plate tectonics.

    Scientists led by Chris Spencer, an assistant professor of tectonics and geochemistry at Queen’s University in Kingston, Ont., Canada, combed through data from hundreds of thousands of zircons culled from numerous studies. In a recent paper in Nature Geoscience [below], they compiled only single crystals with three kinds of analyses—the age of the zircon and two additional measurements that serve as proxies for what the melt that birthed each crystal was like.

    With this data set, the authors posit that zircons—perhaps known best for recording magmatic and metamorphic events deep within Earth might chronicle the radiation of plants with roots and leaves and stems – a development that occurred about 430 million years ago.

    2
    Zircons. Credit: Alka Tripathy-Lang.

    Elements and Isotopes

    Zircon contains zirconium, silicon, and oxygen. Other elements, like uranium and hafnium, can also sneak into its structure; uranium isotopes are radioactive and decay to lead, providing geochronologists with a way to date nearly every zircon crystal.

    Oxygen—part of zircon’s backbone—has only stable, naturally occurring isotopes. Low-temperature surface processes preferentially sort these isotopes, divvying heavy from light. For example, water with light oxygen tends to evaporate first. Water with heavy oxygen will precipitate more readily as rain. And when water interacts with rock, weathering processes partially separate heavy oxygen from its lighter counterparts, explained Brenhin Keller, an assistant professor and geochronologist at Dartmouth who was not involved with this study.

    In particular, as rocks erode, they disintegrate into sands and eventually muds made from clays. Clays tend to incorporate more heavy oxygen, explained Annie Bauer, an assistant professor and geochronologist at the University of Wisconsin–Madison who was also not involved in this study. Subducting mud and mixing it into the mantle would result in melt—and likely zircon—featuring heavier oxygen than a melt that incorporates no crustal material or crust that experienced less weathering.

    Therefore, oxygen isotopes can be used as a proxy for whether a zircon crystal’s precursor melt contained rocks that spent time at the surface, explained Spencer.

    Zircons also contain plenty of hafnium, some of which is produced by the radioactive decay of lutetium. “To a first order, the lutetium-hafnium system will tell you about the source of a magma and therefore also the source of a zircon…crystallizing from that magma,” said Keller.

    If the magma contains melt fresh from the mantle, its hafnium signature will look very different from a melt signature containing old crust that’s been recycled via subduction. In Hawaii, for instance, freshly erupted basalts weather into sediments easily identified as being “from magmas that were extracted from the mantle very, very recently,” said Spencer. The hafnium isotope signatures of these sediments will indicate their youth. Sediments in the Amazon River delta, in contrast, come from several-billion-year-old cratons. “The rocks from which those sediments are derived have a very different [hafnium isotope signature] that goes back billions of years,” he explained.

    Chemical Correlation

    “At first blush…it just looks like shotgun blasts of data,” said Spencer, referring to the relationship between oxygen and hafnium signatures. There is a general lack of correlation for pre-Paleozoic zircons older than about 540 million years, but hafnium signatures do correlate with oxygen isotopes in younger zircons.

    Taken together, these data point to zircons coming from a mantle source containing old crust (from hafnium) that was exposed to liquid water (from oxygen), said Keller.

    This relationship is surprising, said Bauer, because “there’s no reason to expect hafnium and oxygen to correlate [in zircons].” Sediments incorporated into a mantle melt might contain heavier oxygen, indicating more weathering, but they need not have a distinct hafnium signature because “it’s just random sedimentary material.”

    Pinning down just when the two signatures began to correlate took some statistical sleuthing. Nevertheless, Spencer found a shift between 450 million and 430 million years ago that suggests some rapid, irreversible change in zircon chemistry, he said.

    Around 430 million years ago, few mountains were being built, said Spencer, which led him to surmise that something else must have caused the peculiar correlation.

    Prior to about 450 million years ago, river deposits tended to have very low proportions of mud, whereas after that, muddy river deposits increased. The cause of this shift to muddy rivers, said Spencer, “is the advent of land plants.” Roots, he explained, help hold mud and other sediment on river banks, which in turn helps rivers meander. Therefore, roots control what sediment eventually arrives in subduction zones to be carried down to the mantle, melted, and returned to the surface, perhaps with zircons transcribing the tale.

    Just how land plants changed the sediment cycle, however, is still being debated, Keller pointed out. For instance, plants stabilize banks, but they can also increase the extent of weathering. “It’s a reasonable hypothesis that [plants] should maybe do something to the global cycling of sediments,” he said, “and if so, then maybe you can see it in the geochemical record.”

    Ultimately, there are only about 5,000 zircons in Spencer’s database, which he described as “paltry” compared to other zircon data repositories that reach into the hundreds of thousands of analyses. The small sample size is a result of few studies obtaining both oxygen and hafnium information from a single zircon, in addition to age.

    “The main challenges are always representativeness,” said Keller, “and preservation bias.”

    “I anxiously await the time when we have 10,000 [analyses],” said Spencer. “At this moment, this is what we have.”

    Science paper:
    Nature Geoscience

    See the full article here .

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    “Eos” is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

     
  • richardmitnick 10:46 am on March 3, 2022 Permalink | Reply
    Tags: "Scientists Can Now Trace Earth's History in Individual Grains of Sand", , , Geochronology, , New research reveals that grains of sand on a beach can tell us more than you might think about the history of the planet., , , Sedimentology   

    From Curtin University (AU) via Science Alert (AU): “Scientists Can Now Trace Earth’s History in Individual Grains of Sand” 

    From Curtin University (AU)

    via

    ScienceAlert

    Science Alert (AU)

    3 MARCH 2022
    DAVID NIELD

    1
    (Sam Mgrdichian/Unsplash)

    New research reveals that grains of sand on a beach can tell us more than you might think about the history of the planet-something to think about the next time you’re heading to the coast for a swim or splash around.

    Scientists have developed a new metric to determine what they call the “age distribution fingerprint” of the mineral zircon in sand. That fingerprint can then be used to reveal more about the evolution of the surface of the Earth across billions of years.

    Zircon is something that geologists look out for, because it can be formed when continents crash into each other. These crystals can in some cases be billions of years old, carrying a huge amount of history with them.

    The durability of zircon makes it resistant to geological erosion, and as it forms sediments, it stores information along with it.

    As the crust grinds together, forcing new rocks to congeal, a time stamp of the rock’s age is preserved in its makeup. Even once it crumbles into tiny grains, it’s possible to gather traces of this history.

    “The world’s beaches faithfully record a detailed history of our planet’s geological past, with billions of years of Earth’s history imprinted in the geology of each grain of sand, and our technique helps unlock this information,” says sedimentologist Milo Barham from Curtin University in Australia.

    By figuring out the age distribution of zircon in a sand sample – from infants to the elderly, in geological terms – the new technique enables scientists to work out what mountain-generating events were taking place in the eons leading up to the depositing of that bank of sediment.

    The approach is even able to shed light on how Earth first developed a habitable biosphere, according to the researchers, peering back further in time than other methods of geological analysis.

    Another advantage that this new research technique has over existing methods is that it can be used to understand tectonic movements even when the age of the sediment deposit itself isn’t known (a scenario that researchers often find themselves in).

    The team put their new method to the test with three case studies that highlighted how the age distribution fingerprint works, studying sediment in South America, East Antarctica, and Western Australia.

    “For example, the sediment on the west and east coasts of South America are completely different because there are many young grains on the west side that were created from crust plunging beneath the continent, driving earthquakes and volcanoes in the Andes,” says geochronologist Chris Kirkland from Curtin University.

    “Whereas, on the east coast, all is relatively calm geologically and there is a mix of old and young grains picked up from a diversity of rocks across the Amazon basin.”

    The new analysis matched what previous research had uncovered about the sites. Even individual grains of sand can reveal the tectonic forces that created them, based on the age distribution of the sediment around them, the researchers say.

    The new technique can be used to reanalyze data from older studies, the researchers suggest, as well as to tease out more details from suitable sediment in future research.

    “This new approach allows a greater understanding of the nature of ancient geology in order to reconstruct the arrangement and movement of tectonic plates on Earth through time,” says Barham.

    The research has been published in Earth and Planetary Science Letters.

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    Curtin University (AU) (formerly known as Curtin University of Technology and Western Australian Institute of Technology) is an Australian public research university based in Bentley and Perth, Western Australia. The university is named after the 14th Prime Minister of Australia, John Curtin, and is the largest university in Western Australia, with over 58,000 students (as of 2016).

    Curtin would like to pay respect to the indigenous members of our community by acknowledging the traditional owners of the land on which the Perth campus is located, the Wadjuk people of the Nyungar Nation; and on our Kalgoorlie campus, the Wongutha people of the North-Eastern Goldfields.

    Curtin was conferred university status after legislation was passed by the Parliament of Western Australia in 1986. Since then, the university has been expanding its presence and has campuses in Singapore, Malaysia, Dubai and Mauritius. It has ties with 90 exchange universities in 20 countries. The University comprises five main faculties with over 95 specialists centres. The University formerly had a Sydney campus between 2005 & 2016. On 17 September 2015, Curtin University Council made a decision to close its Sydney campus by early 2017.

    Curtin University is a member of Australian Technology Network (ATN), and is active in research in a range of academic and practical fields, including Resources and Energy (e.g., petroleum gas), Information and Communication, Health, Ageing and Well-being (Public Health), Communities and Changing Environments, Growth and Prosperity and Creative Writing.

    It is the only Western Australian university to produce a PhD recipient of the AINSE gold medal, which is the highest recognition for PhD-level research excellence in Australia and New Zealand.

    Curtin has become active in research and partnerships overseas, particularly in mainland China. It is involved in a number of business, management, and research projects, particularly in supercomputing, where the university participates in a tri-continental array with nodes in Perth, Beijing, and Edinburgh. Western Australia has become an important exporter of minerals, petroleum and natural gas. The Chinese Premier Wen Jiabao visited the Woodside-funded hydrocarbon research facility during his visit to Australia in 2005.

     
  • richardmitnick 9:57 am on February 25, 2022 Permalink | Reply
    Tags: "The Young Earth Under the Cool Sun", Astronomers look for clusters of stars in the galaxy whose members were all born at the same time-a circumstance that allows researchers to calculate the stars’ age., , , , , Evidence in zircons that are 4.4 billion years old., Geochronology, , , Our knowledge of the Sun’s rotation speed; XUV radiation; activity level and solar wind history remains incomplete., , , Sometimes solar analogues behave differently from each other and offer a range of possibilities for our Sun’s history., There is no way to look at the Sun today and know how bright it was or how intense its XUV radiation was during the Hadean or how it evolved to its present state., Venus and Mars might yield further constraints on the young Sun’s XUV radiation and solar wind.   

    From Eos: “The Young Earth Under the Cool Sun” 

    From AGU
    Eos news bloc

    From Eos

    22 February 2022
    Kimberly M. S. Cartier

    How did our planet avoid being frozen solid during the early days of our solar system?

    6
    Credit: Mihail Ulianikovi/Stock.com.

    When Earth was still in its infancy more than 4 billion years ago, it was surrounded by chaos. The planet had nearly been shattered by a giant collision whose debris would go on to form the Moon.

    The detritus of planet formation was still regularly colliding with the newly reformed Earth. Elsewhere in the solar system, the gas giants were amassing their own satellites and clearing out chunks of rocks that refused to fall in line. And for those first few hundred million years, the Sun was still waking up, with fusion by-products slowly building and causing its core to contract and glow brighter. By the end of the Hadean, when Earth was a meager half a billion years old, the Sun shone at about 75% of its current brightness.

    That poses a problem. Not much is known about what was happening on Earth at that time, but what little we do know suggests that there was some amount of liquid water present at or near the surface starting in the Hadean, and there is evidence that life itself began in the Archean (4.0–2.5 billion years ago). If modern Earth were suddenly to receive 25% less sunlight today, it would quickly freeze over, so how did early Earth manage to avoid it for 2 billion years?

    For decades this question, dubbed the “faint young Sun paradox” by Carl Sagan and George Mullen in 1972 [Science], has been an intriguing research topic for geochronologists, deep-time paleoclimatologists, and astronomers, although the scientists currently working to answer the question prefer to call it not a paradox but just a “regular ol’ problem.”

    “It isn’t really a paradox in the way that we would normally understand it,” said Colin Johnstone, an astrophysicist at the University of Vienna in Austria. A paradox describes a contradictory statement or phenomenon, and because it is somewhat naive to assume that early Earth was anything at all like modern Earth, he said, an explanation for the faint young Sun problem might not contradict settled science or the geological record at all. “It’s more just something we don’t quite understand,” Johnstone explained, “and so the problem is, How is it that the Earth was not frozen given that the Sun was less bright in the past?”

    Finding a noncontradictory answer “gets more and more difficult with every million years you go back,” said Georg Feulner, a deep-time paleoclimatologist at Potsdam Institute for Climate Impact Research in Germany. “The further back you go in time, all the uncertainties add up, and you just have to live with that. But still, I’m an optimist. By using an interdisciplinary approach, by understanding models better, by getting better isotope data, by understanding the space environment better, and by looking at the evolution of three of the four terrestrial planets in concert, I’m optimistic that we can narrow things down.”

    Solving this problem will help determine the conditions that led to life springing up on Earth and could help identify other planetary harbors of life.

    “Early Earth basically was an exoplanet,” said Claire Guimond, an exoplanet geoscientist at the University of Cambridge in the United Kingdom. “It could have been just as alien as a rocky exoplanet might be to Earth today—different atmospheric composition, different kinds of surface conditions, everything. A faint young star is always going to be something that a planet experiences. If you’re interested in a planet being temperate enough to harbor conditions for the origin of life, then you’re probably going to be interested in how likely it is that planets can overcome the lower luminosity.”

    Fire: The Young Sun

    At more than 4.5 billion years old, the Sun is just bright enough to maintain (current) Earth’s globally connected liquid ocean, and the Sun’s X-ray and ultraviolet (XUV) radiation and flares are weak enough not to strip away Earth’s protective atmosphere. But there is no way to look at the Sun today and know how bright it was or how intense its XUV radiation was during the Hadean or how it evolved to its present state. So where does that knowledge come from?

    To understand the young Sun, astronomers look for clusters of stars in the galaxy whose members were all born at the same time, a circumstance that allows researchers to calculate the stars’ age. If the cluster has stars that are of the same mass as the Sun, those stars can serve as snapshots of the Sun’s history. From this approach we know that newborn Sun-like stars shine at about 70% of the Sun’s current brightness and gradually get brighter throughout their lives. At 500 million years old (equivalent to the end of the Hadean), they reach roughly the 75% mark. With enough solar analogues of different ages to anchor solar evolution models, stellar astronomers have put together a fairly thorough timeline tracking the evolution of the Sun’s brightness, size, and mass.

    However, sometimes solar analogues behave differently from each other and offer a range of possibilities for our Sun’s history. Our knowledge of the Sun’s rotation speed; XUV radiation; activity level and solar wind history remains incomplete. “When we look at a really young cluster where all the stars are just born, so about a million years of age, there’s a big spread in the rotation rates for all the stars, and over time this spread goes away,” Johnstone said. “By the age of the current Sun, all of these stars [rotate] the same, but they weren’t the same for the first few million years. And this has a really big effect on how much [XUV] radiation the stars were emitting.” Stars that rotate faster tend to emit more XUV radiation and also have a stronger stellar wind. “Since we only see the Sun now, when this spread in the rotation rates has already disappeared, we have no way to extrapolate backward,” he said.

    Heliophysical computer models, including some that Johnstone has worked on [Earth and Planetary Science Letters], estimate these properties under different solar evolution scenarios and evaluate their potential impact on early Earth’s upper atmosphere. Geologic records rule out some possibilities, he said, like Earth having entirely lost its atmosphere at any point after the Moon-forming impact. “Any model that tells you that the atmosphere was rapidly lost in a million years, or something like that, can be discarded,” he said. (That eliminates a higher fraction of solar evolution models than you’d think.)

    Venus and Mars might yield further constraints on the young Sun’s XUV radiation and solar wind. Just like Earth, both rocky planets have likely had atmospheres for their entire histories. However harsh the early Sun’s radiation was, it spared those atmospheres, too. But because modern Venus and Mars lack plate tectonics, more evidence from 4 billion years ago might survive on their surfaces.

    Water: Hadean Zircons

    Astrophysicists reached a consensus on the probable evolution of the Sun’s brightness in the 1950s and immediately started realizing the chilly implications [Reviews of Geophysics] for Hadean Earth. “The faint young Sun problem comes when astrophysicists and deep-time geologists collide,” said Sanjoy Som, an astrobiologist at the Blue Marble Space Institute of Science in Seattle. But still very little is known about the processes that were occurring on Earth’s surface during the first tenth of its life. “We want that story, but the further back you go in time, the rarer the rocks, and the more they have been modified by post-original processes. So we have to be careful. We don’t want to be fooled by what later changes have done to them,” Som said.

    For a long time, explained Mark Harrison, geologists assumed that no Hadean rocks could possibly have survived the continuous churning of crust into mantle and back. Harrison is a geochemist at the University of California-Los Angeles.

    6
    This piece of Archean quartz pebble metaconglomerate from the Jack Hills in Western Australia contains Hadean zircons that are 4.4 billion years old. Credit: James St. John, CC BY 2.0.

    As its name suggests, Hadean Earth was initially assumed to be rather hellish, covered in a roiling magma ocean and subject to continuous impacts. During the past 2 decades, however, more evidence has cropped up suggesting that not only did Earth have a solid crust during that time but also liquid water was present.

    “The reason we don’t have very many rocks from the Archean, and none from the Hadean, is because they just got subducted by plate tectonics,” Guimond said. “There are a few places like in South Africa, Australia, and Canada where you do actually have these ancient continental cores made up of really old rocks, which just got preserved on the surface. The evidence for there being liquid water comes from zircons, which are very hard minerals that are difficult to erode.”

    Zircon grains are deep time’s record keeper, and rare examples have been discovered that have survived since the Hadean. Nearly all of the Hadean zircons analyzed thus far have come from the Jack Hills region of Western Australia (as well as a few from western Greenland and northern Canada), although 14 other locations across the world contain Hadean zircons.

    Lead isotope analyses show that the oldest Jack Hills zircons range between 4.1 billion and 4.4 billion years old, and inclusions within the crystals provide unique insight into the geochemistry of Hadean Earth.
    ===
    A few of these Hadean zircon grains can tell geologists that continent-like crust existed but not the extent of it and that liquid water was present but not how large the reservoir was. “Most everything else we see is consistent with the planet being basically frozen,” Harrison said. “It’s very likely that even in a snowball Earth scenario 4.2 billion years ago, there was still liquid water at the ice-rock interface. You don’t need an ocean of liquid water.… The whole thing could be happening under 3 kilometers of ice.”

    Hadean zircons are very rare: About 2% of all Jack Hills zircons discovered so far date from the Hadean, and the percentage is 10 or 100 times lower in other locations. Terrestrial Hadean zircons can also be found on the Moon, however, whisked away to relative safety during the impacts that formed the Moon and spared the tectonic fate of their earthly counterparts, said Harrison. Two Hadean zircons have already been found there, hidden in a rock sample brought back by Apollo 14 astronauts.

    Earth: Crust and Mantle

    Although some evidence exists that Hadean Earth had a crust and that some of it has survived to the present, how much of the surface it covered compared with the paleo-ocean is still unclear. “Continental coverage is important, because the ocean is much darker than land, by far,” Som explained. “Land reflects light back into space more than ocean does—that’s the albedo effect. If the planet Earth was much darker because it had much more extensive ocean coverage than today, that could also be a way for the planet to absorb more heat from sunlight” and remain unfrozen.

    4
    Earth’s atmospheric composition has changed dramatically over its history. During the Archean, a vertical column of air contained a high concentration (grams per square centimeter, g/cm2) of carbon dioxide (dark green) and methane (red) and small amounts of oxygen (light green) and water vapor (blue). Only within the past billion years did the atmosphere gain ozone (yellow). Here a thicker line represents more uncertainty in the measured value. Credit: Roberge et al., 2019, https://doi.org/10.1117/12.2530475 .

    We also don’t know how long it was before that crust was destroyed by plate tectonics. “In the Archean,” said Guimond, “we really can’t say for sure if we had plate tectonics happening.” It’s possible that for some time in the Hadean and the Archean, Earth had no plate tectonics at all and existed with a one-piece crust like that of present-day Mars or Venus. “When you do geodynamic modeling, the theory shows that stagnant lids might be a natural state for rocky planets,” she said.

    Whether early Earth had a stagnant-lid-type crust or today’s churning plate tectonics is key to understanding whether greenhouse gases were released from the mantle into the atmosphere in sufficient quantities to keep the planet temperate. Hadean and Archean Earth likely had a much greater quantity of carbon dioxide (CO2) in its atmosphere than modern Earth, and many deep-time paleoclimate models attempt to figure out how much CO2 or another greenhouse gas would have been needed to sufficiently warm Earth.

    All that greenhouse gas has to have come from somewhere. Although some small amount could have been deposited by the still-regular meteor strikes on early Earth, most of it would have come from magma outgassing. Scientists have extensively studied volcanic outgassing of CO2 under today’s tectonic paradigm, Guimond said, but there is no guarantee that early Earth operated under the same rules. Under a stagnant-lid regime, for example, “we found that CO2 outgassing could be about an order of magnitude lower than we have today.” That would put sharp limits on the amount of atmospheric CO2 paleoclimate models can claim existed in the Hadean and Archean.

    Air: Greenhouse Warming

    However, the solution to the faint young Sun problem is not as simple as adding more greenhouse gases to your favorite paleoclimate model: There are an incalculable number of mixtures of greenhouse gases that might provide enough warming to Hadean Earth. Luckily, there are some constraints on what atmospheric mixtures are plausible. For one, rocks provide some limits on the temperature and pressure [Nature Geoscience] of the Archean atmosphere that can translate to a limit to how much greenhouse gas our atmosphere could have physically held, Som said, “but those measurements are spotty and are unknown for the Hadean.” Earth’s atmosphere in the Hadean could have been thicker than it is today.

    Here, too, the unknown properties of the early Sun come into play. If the early Sun’s XUV radiation and solar wind were near the upper limit of what has been measured for other Sun-like stars, much of Earth’s early atmosphere would have been blasted away, necessitating an even higher output of greenhouse gases to compensate. Even in a lower-radiation scenario, such as one that Johnstone explored recently [Earth and Planetary Science Letters] , Earth’s atmosphere would need to have been at least 40% CO2 (compared with today’s 0.04% and rising).

    Moreover, there is the unknown factor of sea ice. After all, Feulner advised, the Hadean zircons can show only that liquid water was present, not whether it coexisted with ice. Surface ice, on sea or land, is a critical component of how much heat Earth absorbs or reflects: More ice reflects more sunlight away, which further cools the planet and freezes more ice. In some studies [Climate of the Past] in which paleoclimatologists modeled periods of glaciation more recent than the Hadean, the inclusion of sea ice dynamics radically altered the quantity of CO2 needed to thaw the planet.

    “When they switched off sea ice dynamics—just the fact that the sea ice gets pushed around by ocean currents and the wind—they could lower the CO2 concentration by a factor of 100…before the planet fell into a snowball regime,” Feulner said. If 3D paleoclimate models fail to include the movement of sea ice, he said, they could significantly underrepresent the amount of greenhouse gas needed to warm Hadean Earth.

    Finding the Messy Solution

    Are scientists close to answering why Earth was temperate under the faint young Sun? As more and more simulations are run—with different atmospheric greenhouse models, solar evolution scenarios, and mantle outgassing rates—many of them find at least one viable answer. So how will scientists narrow down the options?

    Ultimately, Harrison said, we need more lithic evidence from the Hadean to put better geophysical constraints on the potential solutions. And that means more zircons, especially those that don’t come from Jack Hills. “There is clear evidence that there was water at or near one location on the planet 4.3 billion years ago.… We have this one clear result from Jack Hills. There are 14 other locations that could allow us to address the question, How globally representative is Jack Hills?” he said. By analyzing Hadean zircons from across the globe in as much detail as those from Jack Hills, geochemists will start to pin down the extent of Earth’s early oceans, which will further constrain the behavior of the crust, mantle, and atmosphere.

    Beyond a boost in geophysical data, there is an almost unanimous call for better and faster 3D models of the interconnected Earth system: mantle and crust, sea ice and lower atmosphere, solar radiation and upper atmosphere. Each component of the system plays a key role in solving this early Earth puzzle. Arriving at a consensus solution will require a holistic and interdisciplinary approach that leverages the strengths of each field—paleoclimatology, geochronology, astronomy.

    “Whenever there is a paradox or a problem of this type, people look for that one glorious solution which does it all,” Feulner mused. “But there’s probably no silver bullet. [The solution] is probably a mixture of many factors contributing to the warming…just a mix of more CO2, less clouds, you name it. It’s probably messier than many people think.”

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

     
  • richardmitnick 1:08 pm on February 22, 2022 Permalink | Reply
    Tags: "Updating Dating Helps Tackle Deep-Time Quandaries", , , Cyanobacteria likely played a key role in dramatically altering Earth’s atmosphere during the Great Oxidation Event between about 2.4 billion and 2.0 billion years ago., , , Evidence for single-celled life exists as far back as the Archean eon., , Geochronology, , , , Scientists study Precambrian sedimentary rocks that have long endured the travails of tectonics and attempted erasure by erosion., Scientists tell time in the geologic record by measuring radioactive elements stored in rocks., The stage was set for the evolution of eukaryotes—organisms that encase DNA within their cellular nuclei—which eventually began to breathe oxygen and grow into bigger organisms.   

    From Eos: “Updating Dating Helps Tackle Deep-Time Quandaries” 

    From AGU
    Eos news bloc

    From Eos

    22 February 2022
    Alka Tripathy-Lang

    Geochronologists are finding fresh approaches to familiar methodologies, especially by zapping rocks with lasers to tackle classic Precambrian problems.

    1
    The archipelago of Svalbard, located in the Arctic Ocean north of Norway, includes the approximately 6-kilometer-thick Neoproterozoic to early Phanerozoic Hecla Hoek succession, shown in part here at Claravågen, on the island of Nordaustlandet. Many of the Precambrian parts of this sedimentary succession, including those shown in this image captured by drone, await radiometric age constraints. Credit: Marjorie Cantine.

    During the immense span of time that was the Precambrian—the first 88% of Earth’s 4.6-billion-year history—the planet witnessed milestone events like the dawn of life, the atmosphere’s oxygenation, and global glaciations that helped shape the world in which humanity exists today.

    To better understand such momentous processes, scientists study Precambrian sedimentary rocks that have long endured the travails of tectonics and attempted erasure by erosion. For example, they study marine shales and limestones that record the chemistry of the water column. Because gases dissolved in seawater and those in the atmosphere interact where air meets sea, understanding the geochemistry in Precambrian marine shales and limestones lets scientists tease out clues about bygone climes.

    The unique swings in geochemistry observed in Precambrian rocks are much larger in magnitude than those that occurred in more recent and familiar periods in Earth’s history, said Alan Rooney, an Earth and planetary sciences professor at Yale University. The sedimentary strata also host evidence of the evolution of complex life, from single-celled organisms to multicellular eukaryotes. “Biologically, a lot is going on” in these rocks, he said.

    However, to understand cause and effect when studying a Precambrian rock, “you need to know how old it is,” said Kaarel Mänd, a research fellow at the University of Tartu in Estonia. “Otherwise, you cannot place it in the sequence of events.”

    To this end, geochronologists—scientists who tell time in the geologic record by measuring radioactive elements stored in rocks—are now applying innovative instruments to rejuvenate isotopic dating systems that had fallen out of fashion because of their often cumbersome analytical requirements, including large sample sizes and arduous preparation and measurement. In particular, these advances help geochronologists rapidly collect data for isochron diagrams, which first revolutionized the field more than 60 years ago by providing a way to determine the ages of otherwise inscrutable ancient rocks.

    Precambrian Predicaments

    Evidence for single-celled life exists as far back as the Archean eon. But the evolution of a specific type of single-celled life-cyanobacteria-likely played a key role in dramatically altering Earth’s atmosphere during the Great Oxidation Event between about 2.4 billion and 2.0 billion years ago. During this time, the atmospheric chemistry at Earth’s surface shifted from reducing conditions, in which oxygen is rapidly consumed, to oxidizing conditions replete with the gas. The Great Oxidation Event is “perhaps the most conspicuous big event that happened in the Precambrian,” said Mänd.

    This long-term process set the stage for the evolution of eukaryotes—organisms that encase DNA within their cellular nuclei—which eventually began to breathe oxygen and grow into bigger organisms, said Annie Bauer, an assistant professor of geoscience at the University of Wisconsin–Madison. Whether this happened roughly simultaneously across the globe or in geographically isolated pockets at different times is still being studied. By comparing the timing of oxygenation from place to place, she said, scientists can determine whether these first whiffs arose together as a globally synchronous exhalation or as discrete puffs.

    Later, from about 1.8 billion to 0.8 billion years ago, atmospheric oxygen levels flattened out and stabilized, leading some scientists to dub this time the “boring billion.” Yet multicellular life emerged during this time; important ores like copper, iron, lead, and zinc—sensitive to the amount of oxygen near them—were deposited; and continents such as ancient North America grew as supercontinents assembled.

    The remainder of the geochemistry tucked into the Precambrian’s rock record features evidence of unusual climate dynamics perhaps related to Earth’s carbon cycle, said Marjorie Cantine, a postdoctoral fellow at Goethe University Frankfurt in Germany. Understanding the Earth system changes that might have led to the flowering of diverse, complex life during the Phanerozoic—the present geological eon—requires dating the rocks that hold these clues, she said.

    The Phanerozoic “[has] this really rich fossil record that you can use to tell time,” said Cantine. In contrast, Precambrian life was not mineralized. The biostratigraphy that helps geologists sort through time in the Phanerozoic is largely unavailable in Precambrian rocks, she said.

    Scientists who delve into Precambrian rocks often rely on the physical position of different rocks in relation to one another to tell their relative ages, said Mänd. Once-molten magma, for instance, will always be younger than any sedimentary rock it cuts across. For that reason, dating that crosscutting igneous rock provides a minimum age for the sedimentary strata, although the sedimentary rock could still be many millions of years older than that minimum age, he explained.

    Even in cases where scientists have tried to directly date marine Precambrian rocks, for example, they sometimes know the rocks’ ages only to within hundreds of millions of years, said Nick Roberts, a research scientist at the British Geological Survey.

    Mathematical Tricks for Dating Rocks

    Since Marie Curie first coined the term “radioactivity” in the late 1800s, the field of radiometric dating of rocks—geochronology—has emerged and matured.

    Naturally occurring elements have different isotopes, in which the number of protons is the same but the number of neutrons varies, resulting in different masses of the same element. For some elements, certain isotopes are radiogenic, meaning they exist because of radioactive decay. Geochronology focuses on measuring the decay of a radioactive “parent” isotope to a radiogenic “daughter” one, like the decay of certain isotopes of uranium to lead or rubidium to strontium. “We know pretty well how quickly that happens,” said Cantine. By measuring parent-daughter ratios in a rock or mineral, scientists can calculate when the dated material came into existence. “We’re able to do that extraordinarily well in certain special minerals” that form with parent isotopes but without any daughter products, she said.

    One of those special minerals is zircon. A hardy mineral made from zirconium, silicon, and oxygen, zircon crystals can retain their initial geochemical signatures despite being bathed in magma or doused in water. Crucially for geochronologists, zircon crystals readily incorporate uranium as they form—when the clock starts, so to speak—but they do not initially incorporate daughter isotopes of lead. Any lead found in zircon today, said Cantine, is there solely because of radioactive decay over time.

    Measuring uranium and lead in zircons can sometimes help geochronologists when they’re able to find these time capsules. For example, layers of ash belched by volcanoes often contain zircons that effectively date the time of eruption. Such layers can provide markers in successions of marine rocks, which often lack any other indicator of time. Modern laboratory techniques enable the development of very high precision dates from zircon crystals, making it so that finding zircon-bearing ashes is the dream scenario for Precambrian geologists. Unfortunately, volcanic ashes do not occur everywhere Earth scientists seek geochronology on sedimentary rocks, said Cantine. In another approach, geochronologists date many tens, or even hundreds, of zircons extracted from sandstones. Known as detrital zircons, these minerals initially formed in other rocks that were then eroded and redeposited—sometimes multiple times—before arriving at their terminal sedimentary destination. Detrital zircons can help fingerprint the source regions of the sands in a sandstone and provide a maximum age constraint for the rock, said Bauer, but they can’t tell you the actual time at which the rock formed. Recent research used a high number of detrital zircons dated at low precision using quick laser methods and then dated the youngest of those grains with more time-consuming high-precision methods. Although this process can sometimes get close to the formation age, it will still be a maximum age.

    Luckily, geochronologists have several other radiometric systems at their disposal to directly date marine sedimentary rocks like shales and carbonates. Unfortunately, as shales and carbonates form, they incorporate various radiogenic daughter products in addition to parent isotopes, meaning a single measurement is likely to yield an erroneously old age for a rock.

    To overcome such limitations, though, “we’re able to do some mathematical tricks by measuring multiple different locations within the same rock,” said Cantine.

    The ultimate trick is the isochron method, first conceived in 1961, which requires no knowledge of how much radiogenic daughter isotope a sample incorporated at the time it formed. A “device of magnificent power and simplicity,” wrote Brent Dalrymple in The Age of the Earth in 1991, “an isochron is a line of equal time.” Obtained by analyzing several minerals from the same rock or several rocks that formed together but that contain different amounts of the parent element, the simplest form of an isochron requires measurement of only a parent, its radiogenic daughter product, and a third quantity—the relative amount of a nonradiogenic isotope of the daughter element, which should remain constant over the lifetime of a sample. The inherent assumption, said Cantine, is that the rocks or minerals in question began with the same amount of all isotopes of the daughter, regardless of whether they were produced by radioactive decay, and that no later process has perturbed that balance.

    By dividing both the amount of the parent and the amount of the radiogenic daughter by the amount of that third quantity—a nonradiogenic daughter isotope—and then plotting the resulting values on the x and y axes, respectively, wrote Dalrymple, “the points will fall on a line whose slope is a function of the age of the rock.” In other words, simple division allows geochronologists to exploit the equation of a line.

    Resuscitating an Old Method

    Continental rocks exposed to water and weather at Earth’s surface deteriorate into smaller bits, including clay minerals, through physical and chemical erosion. When these flecks of former rocks end up in the seas, they eventually form layers of fine-grained sedimentary rocks called shales. That’s how shales have formed for more than 3 billion years, Mänd said.

    Often rich in organic matter, these thinly layered rocks often form in deep ocean waters. One way to date shales as old as about 2.5 billion years, said Rooney, is by using the decay of a radiogenic isotope of rhenium, a metal, to another metal, osmium. But the process of isolating rhenium from osmium is arduous, involving several days of complicated laboratory work. Dates developed through such arduous research are playing increasingly important roles in telling time in ancient sedimentary rocks lacking zircon-bearing ashes.

    When rhenium decays to osmium, it does so via a process called beta decay, in which an atom loses or gains a proton (i.e., the daughter becomes a different element) but has the same mass as the parent (i.e., the two have the same combined total number of protons and neutrons). This process holds true for any beta decay system, including rubidium’s transformation to strontium, said Mikael Tillberg, a postdoctoral fellow at Linnaeus University and the University of Gothenburg, both in Sweden. The isochron method was first demonstrated using the rubidium-strontium dating system, but other methods that often proved faster or cheaper to employ partially supplanted its use. As such, said Tillberg, rubidium-strontium dating is often viewed as antiquated. However, innovative technologies are reinvigorating this vintage timepiece that can constrain the finicky ages of those fine-grained shales.

    Laser ablation systems let geochronologists shoot holes on the scale of tens of micrometers in target materials, said Tillberg, dramatically reducing the sample size needed per measurement. The laser ablates the target rock, turning it into an aerosol that is immediately piped to a mass spectrometer.

    Because the parent and daughter isotopes used in rubidium-strontium geochronology have the same mass, attempting to measure them simultaneously in an instrument designed to measure different masses may seem counterintuitive. A triple quadrupole mass spectrometer solves this quandary, said Tillberg. (A quadrupole in this context consists of four parallel rods, with each opposing pair having a different voltage that attracts or repels charged particles.)

    When ablated, aerosolized rock enters the instrument, and a plasma ionizes it into charged particles. Then, the first quadrupole separates the particles according to mass, explained Tillberg. The next quadrupole contains a gas like nitrous oxide, which donates oxygen to strontium but not to rubidium. The strontium, now combined with oxygen, has a higher mass that is easily separated from rubidium by the third quadrupole, he said. This potent combination of laser ablation and triple quadrupole mass spectrometry allows both isotopes to be measured from the same imperceptibly small slug of sample while eliminating the complicated and time-consuming laboratory work needed to dissolve a rock and physically separate parent and daughter isotopes.

    “Honing into a single layer in the rock record…especially if samples come from drill cores that already have small sample sizes,” becomes much simpler with this updated method, said Darwinaji Subarkah, a doctoral student at the University of Adelaide in Australia. Because the measurement process is so fast, multiple spots from a single sliver of sample can be ablated and analyzed in hours, generating the data necessary for an isochron, he said. Furthermore, whereas traditional rubidium-strontium methods consume entire samples, laser ablation preserves the sample, leaving the measured rock available for future reference, said Subarkah. Moreover, because laser ablation requires much smaller amounts of material, additional sample is often available for analysis by other methods.

    However, to generate a robust isochron, the analyzed parts of a rock must be texturally equivalent. “You need to be able to assume that your initial strontium composition of all the [sample] was the same,” said Bauer. “That’s what makes sedimentary rocks really tricky.”

    Careful petrographic characterization, especially at the nanoscale, can potentially solve this problem, helping to differentiate among clays that came from eroding continents, clays that grew as the sediment became a rock, and clays that changed as the rock warmed and recrystallized, said Subarkah. By combining petrographic analyses with laser ablation, he said, “we’re actually looking at individual relationships between the different mineral phases.”

    Carbonate Conundrums

    Phanerozoic carbonate rocks—limestones and dolomites—are “often completely composed of these big pieces of fossil [animals] stuck together,” Mänd said, which helps tell time.

    But carbonates exist from more than 3 billion years ago, well into the Precambrian, when they were “completely built, as far as we know, by microbes,” according to Cantine.

    Sedimentary carbonates can be dated using uranium’s decay to lead. But carbonates don’t incorporate much uranium, and they tend to include lead as they form, said Cantine. “That means that we have problems on both the parent and the daughter side.” A variation on the simple isochron, along with lasers, has rejuvenated uranium-lead dating for carbonates.

    The first attempt at uranium-lead carbonate geochronology began in the 1980s, said Roberts, and continued through the late 1990s with studies of Precambrian rocks, mostly. Early papers describe methods that involved drilling carbonate rock samples, dissolving chunks with acids, chemically separating the uranium and lead, and making measurements on a thermal ionization mass spectrometer (TIMS) instrument, Roberts explained. This required big samples, a lot of time in the lab, and expensive equipment. However, carbonates are notoriously complicated at relatively small spatial scales, and by dissolving a large piece for analysis, any variations of uranium or lead within individual crystals or across the sample are lost as they are averaged into a single data point, he said.

    Because geochronologists can focus their lasers to zap rocks at a scale of tens of micrometers, many measurements can be obtained rapidly from a single sample, allowing researchers to observe small-scale variations. These previously inscrutable variations provide the spread in measurements needed for a good isochron, said Roberts. Although individual measurements obtained by laser ablation come with higher uncertainties than data collected by traditional methods, the sheer number of measurements made possible by using lasers means that isochron-determined ages also can be precise.

    Carbonates “are wonderfully sensitive to the environment around them,” said Cantine. Carbonate rocks can record temporally distinct processes, such as the initial deposition or precipitation of carbonate, its transformation into rock, any subsequent deformation by burial or tectonic processes, and even uplift from the ocean floor to the tops of mountains. Within a single sample, she said, “you could potentially have multiple meaningful ages preserved.” Because these different processes often leave behind texturally distinct carbonates visible only under the microscope, combining petrographic examination with laser ablation techniques is critical for connecting a date to a specific geological process, she said.

    Nevertheless, just because we have lasers doesn’t mean it’s time to leave the old methods in the past. TIMS measurements in particular are highly precise, said Cantine. In her work, she’s aiming for the best of both worlds, she said, by rapidly assessing carbonate dates using laser ablation and then following up with TIMS analyses to confirm the results.

    What Came First?

    By precisely dating sedimentary rocks with updated geochronologic techniques, said Mänd, scientists can begin to solve some long-standing chicken-and-egg problems in Precambrian geology. For example, snowball Earth glacial events recorded in sedimentary rocks that happened about 2.4 billion and 0.6 billion years ago coincide with both atmospheric oxygen fluctuations and peculiarly large swings in Earth’s carbon chemistry.

    The older snowball Earth event (the Huronian glaciations) may have been triggered by excess oxygen produced by cyanobacteria. But the widely accepted age constraints for this event come from 2.45-billion-year-old Archean rock that sits below the sedimentary rocks recording the past global freezes, along with a 2.22-billion-year-old crosscutting igneous intrusion into the sedimentary rocks, leaving a span of nearly 300 million years, said Bauer. With better time constraints, scientists parse just how many glaciations occurred, whether they were truly global, and what their relationship is to the cyanobacteria-fueled oxygen spike and the carbon swings recorded in these rocks, she explained.

    During and after the younger snowball Earth events during the Cryogenian period, Earth’s earliest animals evolved amid continued episodic glaciations and more curious carbon records. But as in the Huronian, the relations and timing of these glaciations, carbon fluctuations, and evolution of life are unclear. Understanding the time component with the help of the best available geochronologic systems and instrumentation, said Cantine, “is critical for figuring out how and in what ways these events might be connected.”

    See the full article here .

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