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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?", , , Eos, 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., , , , 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 .

    five-ways-keep-your-child-safe-school-shootings

    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 10:33 am on October 4, 2022 Permalink | Reply
    Tags: "Melting Below the Pine Island Ice Shelf Minds the Gap", , , , Eos   

    From “Eos” : “Melting Below the Pine Island Ice Shelf Minds the Gap” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    10.3.22
    Sarah Derouin

    New research shows that increased calving from West Antarctica’s Pine Island Ice Shelf will likely drive increased circulation of warm water—and melting—below the ice.

    1
    Two large cracks in the Pine Island Ice Shelf appear clearly in this image taken by the Copernicus Sentinel-2 satellite on 14 September 2019. Credit: European Space Agency, CC BY-SA 3.0 IGO

    The Pine Island Ice Shelf (PIIS) is the seaward extension of the Pine Island Glacier, a large and rapidly retreating glacier that drains part of the West Antarctic Ice Sheet. Beneath the floating PIIS is a seafloor ridge that narrows the gap through which relatively warm seawater from the open ocean can flow in and circulate beneath the ice shelf.

    This narrowing helps protect the underside of the PIIS on the landward side of the seafloor ridge from melting. But in the past decade, the ice shelf has seen large amounts of calving, causing the ice front to retreat toward the continent and approach the ridge—and the calving shows no sign of slowing.

    Bradley et al. [JRG Oceans (below)]investigated how calving affects melting of the PIIS. The team used a high-resolution ocean model to simulate ocean circulation and melt rates below the ice shelf, modeling and comparing results from both an idealized setting meant to represent the most important features of the ice shelf and ridge and real-world conditions that closely match the site characteristics for the PIIS.

    They found that ice shelf melt rates are sensitive to the thickness of the gap between the PIIS and the seafloor ridge, suggesting that the changing geometry of the gap with a retreating ice front leads to strengthening of seawater circulation beneath the ice. As calving from the PIIS ice front continues, the melt rate will increase linearly, the team found, becoming 10% higher than it is now by the time the ice front retreats to the ridgeline.

    Science paper:
    JRG Oceans

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    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 9:52 am on September 20, 2022 Permalink | Reply
    Tags: "Impact Crater off the African Coast May Be Linked to Chicxulub", A possibility is that one or more asteroids collided somewhere in deep space—most likely in the asteroid belt between Mars and Jupiter—and an ensemble of cosmic shrapnel traveled en masse to Earth, , , , Eos, Perhaps a breakup of a common parent asteroid occurred on Earth 66 million years ago resulting in the two impacts., Researchers have uncovered another crater off the coast of West Africa that might well be Chicxulub’s cousin.   

    From “Eos” : “Impact Crater off the African Coast May Be Linked to Chicxulub” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    9.19.22
    Katherine Kornei

    1
    Scientists hope to drill into a newly discovered impact crater off the west coast of Africa to explore how it’s linked—if it is—to the famous Chicxulub impact 66 million years ago. Credit: iStock.com/guvendemir.

    In the world of impact craters, Chicxulub is a celebrity: The 180-kilometer-diameter maw, in the Gulf of Mexico, was created by a cataclysmic asteroid impact at the end of the Cretaceous that spelled the demise of most dinosaurs. But researchers have now uncovered another crater off the coast of West Africa that might well be Chicxulub’s cousin. The newly discovered feature, albeit much smaller, is also about 66 million years old. That’s a curious coincidence, and scientists are now wondering whether the two impact structures might be linked. Perhaps Chicxulub and the newly discovered feature—dubbed Nadir crater—formed from the breakup of a parent asteroid or as part of an impact cluster, the team suggested. These results were published in Science Advances [below].

    Rocks of Concern

    Every day, tons of cosmic dust rain down on our planet. That microscopic debris poses no danger to life on Earth, but its larger brethren are very much cause for concern: A space rock measuring hundreds of meters in size is apt to cause regional destruction, and the arrival of something measuring kilometers in size could spell global havoc. That’s what happened 66 million years ago when a roughly 12-kilometer-wide asteroid slammed into a shallow reef in the Gulf of Mexico. That event, now known as Chicxulub after the small town that’s grown up nearby in Mexico, launched shock waves, powerful tsunamis, and blasts of superheated air that decimated life in the vicinity. And airborne particles—bits of dust, soot, and sulfate aerosols born from the sulfur-rich rocks that existed at the Chicxulub impact site—choked the atmosphere and plunged the entire planet into a sunlight-starved “impact winter” that lasted for years. When the air finally cleared, over 75% of all species had gone extinct.

    The newly discovered Nadir crater appears to have formed right around the same time as that cataclysm. Uisdean Nicholson, a sedimentary geologist at Heriot-Watt University in Edinburgh, Scotland, and his colleagues discovered the candidate crater while they were poring over observations of seafloor sediments originally collected for oil and gas exploration. The team spotted the roughly 8-kilometer-wide structure in seismic reflection imaging data obtained off the coast of West Africa. “It was pure serendipity,” said Nicholson.

    Signs of an Impact

    The putative crater is buried under roughly 300 meters of sediments topped by 900 meters of water, and its appearance strongly suggests it was created by a hypervelocity impact, said Nicholson. For starters, it’s circular in shape, with a pronounced rim. Second, it contains a small central peak, a feature that often arises in large impact craters. And perhaps most important, there’s clear evidence of deformed sediments—caused by faulting and folding—persisting hundreds of meters below what would be the crater floor. “There’s a lot of things that suggest it’s an impact,” said Gavin Kenny, a geochemist at the Swedish Museum of Natural History in Stockholm who was not involved in the research.

    3
    Fig. 1. Map and regional seismic sections showing location of Nadir Crater.
    (A) Regional bathymetry map of the Guinea Plateau and Guinea Terrace showing location of 2D seismic reflection and well data used in this study. JS, Jane Seamount; NS, Nadir Seamount; PS, Porter Seamount. The white dashed line shows the NE extent of high-amplitude discontinuous seismic facies at the top Maastrichtian interpreted as ejecta deposits and associated tsunami deposits. The north-east limit of this facies closely corresponds with the Maastrichtian shelf-slope break at the landward margin of the Guinea Terrace. Inset map shows a paleogeographic reconstruction of the Atlantic near the end of the Cretaceous, ~66 Ma ago, made using GPlates software (58). Ch, Chicxulub Crater; Nd, Nadir Crater; Bo, Boltysh Crater. (B). Regional composite 2D seismic reflection profile extending from the GU-2B-1 well in the east to the deep Atlantic basin in the west, showing the structural and stratigraphic character of the Guinea Plateau and Guinea Terrace. (C) North-South seismic profile from the salt basin in the north to the Nadir Seamount, south of the Guinea Fracture Zone. Data courtesy of the Republic of Guinea and TGS.

    4
    Fig. 2. Seismic characteristics of the Nadir Crater.
    (A) Seabed depth map of crater showing seismic line locations and the mapped extent of the crater rim and damage zone. (B) W-E seismic section (pre-stack depth migration – depth domain) across the crater, highlighting the crater morphology and damage zone, and the extent of subsurface deformation. Data courtesy of the Republic of Guinea, TGS and WesternGeco. Stratigraphic key is on Fig. 1. (C) Detailed seismic stratigraphic and structural elements of the crater. KP, Cretaceous-Paleogene sequence (KP1 equivalent to Top Maastrichtian); KU, Upper Cretaceous seismic horizons. KU1 and KP1 “regionals” are schematic reconstructions of these seismic horizons before formation of the crater at the end of the Cretaceous and are used to reconstruct a conceptual model of crater formation (Fig. 5). (D) SW-NE seismic section (pre-stack time migration – time domain) across the crater, showing crater morphology and seismic facies outside the crater, including high-amplitude seismic facies sitting above a ~100-ms-thick unit of chaotic reflections, interpreted to have formed as a result of seismic shaking following the impact event. Data courtesy of the Republic of Guinea and WesternGeco Multiclient.

    More mapping images are available in the science paper.

    Numerical simulations run by team member Veronica Bray, a planetary scientist at the University of Arizona, have suggested that the impactor was about 400 meters in diameter. The arrival of such an object moving at roughly 20 kilometers per second would have produced tsunami waves more than a kilometer in height and ground shaking equivalent to that of a magnitude 7 earthquake, Bray estimated. But the mayhem that ensued, intense as it was, was mostly limited to a regional scale, said Bray. “This wasn’t a global killer.”


    Computer Simulation of the Nadir Impact Event.
    Hydrocode simulation of the impact of a 400m asteroid into an 800m ocean, performed by Veronica Bray at the University of Arizona. This is the best-fit simulation from our Nadir Crater discovery paper. In future, we are aiming to drill into the crater. This will allow us to confirm whether the crater is due to an asteroid impact, and to determine its age. Current estimates of its age, based on its position in the rock layers, suggest it is of similar age to the Chicxulub – Dinosaur killer – impact. But we’ll only be sure when we get that all-important drill core!

    On the basis of assemblages of microfossils unearthed close to Nadir crater, Nicholson and his colleagues estimated that this feature formed at or near the end of the Cretaceous period. But it’s too simplistic to assume that a pair of gravitationally bound asteroids—a binary asteroid—formed Chicxulub and Nadir crater in a one-two punch, the authors suggested. That’s because of the extreme distance between the two sites 66 million years ago: roughly 5,500 kilometers. (They’re even farther apart now—about 8,000 kilometers—because of spreading of the Atlantic seafloor.) Binary asteroids tend to hit much closer to one another: The one example on Earth of a so-called “impact doublet” formed by a binary asteroid is characterized by craters just a little over 10 kilometers apart. “So Chicxulub and Nadir couldn’t have formed from a direct hit of a binary asteroid,” said Nicholson.

    Looking to Jupiter

    A more likely scenario, Nicholson and his collaborators suggested, is something akin to what happened to comet Shoemaker-Levy 9. In 1992, the roughly 2-kilometer-diameter comet had fragmented into more than 20 pieces after passing very close to Jupiter. Two years later, those fragments slammed into the gas giant over the course of several days, creating a series of dark scars that stretched across a wide swath of the planet.

    Perhaps a similar breakup of a common parent asteroid occurred on Earth 66 million years ago, Nicholson and his colleagues proposed. An asteroid—there’s good evidence that the Chicxulub impact was due to an asteroid rather than a comet—orbiting Earth could have been torn apart by our planet’s gravity. Those fragments could have then dispersed sufficiently in space such that they smashed into Earth within days of one another yet in very disparate locations, the researchers suggested.

    Another possibility is that one or more asteroids collided somewhere in deep space—most likely in the asteroid belt between Mars and Jupiter—and an ensemble of cosmic shrapnel traveled en masse to Earth. The result would have been an uptick in cratering that persisted not over days, as in the case of the breakup of a common parent asteroid, but over a million or so years. Scientists are aware of only one such event—known as an impact cluster—in Earth’s history, and it occurred roughly 460 million years ago. “We think an asteroid parent body broke up somewhere in the solar system and sent material flying towards Earth,” said Kenny.

    The impact cluster scenario might be more likely, Nicholson and his colleagues suggested. That’s because a third large crater—the 24-kilometer-diameter Boltysh crater in central Ukraine—also dates to around 66 million years ago. Research published last year suggested that Boltysh formed just 650,000 years after the Chicxulub impact.

    There’s also the possibility that Nadir crater was simply created by an unrelated impact, Nicholson and his colleagues acknowledged. Perhaps a stroke of bad cosmic luck led to Earth being pummeled in relatively close succession.

    Going Deep

    It’s clearly key to more precisely constrain the age of Nadir crater, Nicholson and his collaborators maintain. Right now, the uncertainty in the structure’s age is about a million years, and that’s too large to discriminate between the breakup of a common parent asteroid and impact cluster scenarios. Drilling sediment cores from the crater would allow scientists to look for stratigraphic signatures like the iridium layer from Chicxulub that could yield a much more precise date. Nicholson and his colleagues recently submitted a drilling proposal to the International Ocean Discovery Program to do just that.

    Science paper:
    Science Advances

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    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 9:23 am on September 20, 2022 Permalink | Reply
    Tags: "Capturing Ocean Turbulence at the Underbelly of Sea Ice", , , Eos, , Precise measurements of turbulence are important for understanding ocean dynamics., The Arctic Ocean features a distinct cause of turbulence: drifting sea ice., Turbulence in the sea plays a key role in mixing ocean waters and transporting nutrients heat and dissolved gases.   

    From “Eos” : “Capturing Ocean Turbulence at the Underbelly of Sea Ice” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    9.20.22
    Sarah Stanley

    1
    For 1 year, R/V Polarstern sailed amid sea ice in the Arctic Ocean, enabling scientists from around the world to conduct polar research. New data they collected on ocean turbulence immediately beneath sea ice could deepen understanding of ocean dynamics. Credit: National Snow and Ice Data Center, CC BY 2.0

    Turbulence in the sea plays a key role in mixing ocean waters and transporting nutrients heat and dissolved gases. Sources of ocean turbulence are highly varied and include wind, currents, heating and cooling cycles, and more. The Arctic Ocean features a distinct cause of turbulence: drifting sea ice.

    Precise measurements of turbulence are important for understanding ocean dynamics. However, capturing turbulence immediately beneath moving ice poses practical challenges, and conventional methods cannot reliably measure turbulence any closer than a few meters beneath the ice.

    Now, Fer et al. demonstrate that a specially designed instrument can quantify turbulence within 1 meter of the ice-ocean interface. An ascending vertical microstructure profiler is dropped through a hole in the ice and lowered to a depth of up to 80 meters. The buoyant instrument then ascends until it reaches the underside of the ice, measuring turbulence along its vertical path.

    The researchers used this new instrument as part of the large, international Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, in which the icebreaker R/V Polarstern spent a full year drifting in the Arctic Ocean while participants carried out numerous research projects.

    During the expedition, the researchers captured a total of 167 turbulence measurements from February to September 2020, covering seasonal changes in sea ice cover and drift, as well as varying wind speeds. They found that turbulence varied significantly depending on ice and weather conditions, depth, and location within the Arctic Ocean.

    In general, under thicker “pack ice,” turbulence was greater toward the interface of ice and seawater and decreased with depth. However, after strong winds, significant turbulence extended as deep as 20 meters beneath the ice. Some measurements were taken in open waters in the central Arctic; these revealed greater levels of turbulence than seen beneath thin ice cover.

    These findings—and future research using similar methods—could help deepen understanding of Arctic ice-ocean dynamics. For instance, they could be applied to refine computational models of ocean mixing beneath ice.

    Science paper:
    Journal of Geophysical Research: Oceans

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    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 8:12 pm on September 15, 2022 Permalink | Reply
    Tags: "ECS": equilibrium carbon sensitivity, "Simpler Presentations of Climate Change", , , Eos, The basics of climate change science have been known for a long time and the predicted impact of a doubling of atmospheric carbon dioxide on global temperature hasn’t changed much in 100 years.   

    From “Eos” : “Simpler Presentations of Climate Change” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    9.13.22
    John Aber
    Scott V. Ollinger

    The basics of climate change science have been known for a long time, and the predicted impact of a doubling of atmospheric carbon dioxide on global temperature hasn’t changed much in 100 years.

    1
    Atmospheric carbon dioxide concentrations in April 2006, with warmer colors representing higher concentrations, are depicted in this snapshot from a simulation of the gas’s movement through the atmosphere performed using NASA’s Goddard Earth Observing System model, version 5. Credit: William Putman/NASA Goddard Space Flight Center.

    Has this happened to you? You are presenting the latest research about climate change to a general audience, maybe at the town library, to a local journalist, or even in an introductory science class. After presenting the solid science about greenhouse gases, how they work, and how we are changing them, you conclude with “and this is what the models predict about our climate future…”

    At that point, your audience may feel they are being asked to make a leap of faith. Having no idea how the models work or what they contain and leave out, this final and crucial step becomes to them a “trust me” moment. Trust me moments can be easy to deny.

    This problem has not been made easier by a recent expansion in the number of models and the range of predictions presented in the literature. One recent study making this point is that of Hausfather et al. [2022]*, which presents the “hot model” problem: the fact that some of the newer models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model comparison yield predictions of global temperatures that are above the range presented in the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6). The authors present a number of reasons for, and solutions to, the hot model problem.

    • See References below.

    Models are crucial in advancing any field of science. They represent a state-of-the-art summary of what the community understands about its subject. Differences among models highlight unknowns on which new research can be focused.

    But Hausfather and colleagues make another point: As questions are answered and models evolve, they should also converge. That is, they should not only reproduce past measurements, but they should also begin to produce similar projections into the future. When that does not happen, it can make trust me moments even less convincing.

    Are there simpler ways to make the major points about climate change, especially to general audiences, without relying on complex models?

    We think there are.

    Old Predictions That Still Hold True

    In a recent article in Eos, Andrei Lapenis retells the story of Mikhail Budyko’s 1972 predictions about global temperature and sea ice extent [Budyko, 1972]. Lapenis notes that those predictions have proven to be remarkably accurate. This is a good example of effective, long-term predictions of climate change that are based on simple physical mechanisms that are relatively easy to explain.

    There are many other examples that go back more than a century. These simpler formulations don’t attempt to capture the spatial or temporal detail of the full models, but their success at predicting the overall influence of rising carbon dioxide (CO2) on global temperatures makes them a still-relevant, albeit mostly overlooked, resource in climate communication and even climate prediction.

    One way to make use of this historical record is to present the relative consistency over time in estimates of equilibrium carbon sensitivity (ECS), the predicted change in mean global temperature expected from a doubling of atmospheric CO2. ECS can be presented in straightforward language, maybe even without the name and acronym, and is an understandable concept.

    Estimates of ECS can be traced back for more than a century (Table 1 [see the full article]), showing that the relationship between CO2 in the atmosphere and Earth’s radiation and heat balance, as an expression of a simple and straightforward physical process, has been understood for a very long time. We can now measure that balance with precision [e.g., Loeb et al., 2021], and measurements and modeling using improved technological expertise have all affirmed this scientific consistency.

    Settled Science

    Another approach for communicating with general audiences is to present an abbreviated history demonstrating that we have known the essentials of climate change for a very long time—that the basics are settled science.

    The following list is a vastly oversimplified set of four milestones in the history of climate science that we have found to be effective. In a presentation setting, this four-step outline also provides a platform for a more detailed discussion if an audience wants to go there.

    ~1860: John Tyndall develops a method for measuring the absorbance of infrared radiation and demonstrates that CO2 is an effective absorber (acts as a greenhouse gas).
    1908: Svante Arrhenius describes a nonlinear response to increased CO2 based on a year of excruciating hand calculations actually performed in 1896 [Arrhenius, 1896]. His value for ECS is 4°C (Table 1), and the nonlinear response has been summarized in a simple one-parameter model.
    1958: Charles Keeling establishes an observatory on Mauna Loa in Hawaii. He begins to construct the “Keeling curve” based on measurements of atmospheric CO2 concentration over time. It is amazing how few people in any audience will have seen this curve.
    Current: The GISS data set of global mean temperature from NASA’s Goddard Institute for Space Studies records the trajectory of change going back decades to centuries using both direct measurements and environmental proxies.

    The last three of these steps can be combined graphically to show how well the simple relationship derived from Arrhenius’s [1908] projections, driven by CO2 data from the Keeling curve, predicts the modern trend in global average temperature (Figure 1). The average error in this prediction is only 0.081°C, or 8.1 hundredths of a degree.

    2
    Fig. 1. Measured changes in global mean temperature (Delta T) from GISS data (open circles) are compared here with predictions (solid circles) from a one-parameter model derived from calculations performed by Svante Arrhenius in 1896 and driven by Keeling curve CO2 data. Temperature changes are relative to the baseline average temperature for the period 1951–1980.

    A surprise to us was that this relationship can be made even more precise by adding the El Niño index (November–January (NDJ) from the previous year) as a second predictor. The status of the El Niño–Southern Oscillation (ENSO) system has been known to affect global mean temperature as well as regional weather patterns. With this second term added, the average error in the prediction drops to just over 0.06°C, or 6 one hundredths of a degree.

    It is also possible to extend this simple analysis into the future using the same relationship and IPCC AR6 projections for CO2 and “assessed warming” (results from four scenarios combined; Figure 2).

    Although CO2 is certainly not the only cause of increased warming, it provides a powerful index of the cumulative changes we are making to Earth’s climate system.

    In this regard, it is interesting that the “Summary for Policy Makers” [Intergovernmental Panel on Climate Change, 2021] from the most recent IPCC science report also includes a figure (Figure SPM.10, p. 28) that captures both measured past and predicted future global temperature change as a function of cumulative CO2 emissions alone. Given that the fraction of emissions remaining in the atmosphere over time has been relatively constant, this is equivalent to the relationship with concentration presented here. That figure also presents the variation among the models in predicted future temperatures, which is much greater than the measurement errors in the GISS and Keeling data sets that underlie the relationship in Figure 1.

    A presentation built around the consistency of ECS estimates and the four steps clearly does not deliver a complete understanding of the changes we are causing in the climate system, but the relatively simple, long-term historical perspective can be an effective way to tell the story of those changes.

    Past Performance and Future Results

    3
    Fig. 2. Values of assessed global mean warming through the year 2100 from four frequently cited scenarios included in IPCC AR6 are compared here with predictions from the simple model used in Figure 1 driven by the projected CO2 concentrations from the same four scenarios. The dashed line indicates a 1:1 relationship, indicating close agreement between the two estimates.

    Projecting the simple model used in Figure 1 into the future (Figure 2) assumes that the same factors that have made CO2 alone such a good index to climate change to date will remain in place. But we know there are processes at work in the world that could break this relationship.

    For example, some sources now see the electrification of the economic system, including transportation, production, and space heating and cooling, as part of the path to a zero-carbon economy [e.g., Gates, 2021]. But there is one major economic sector in which energy production is not the dominant process for greenhouse gas emissions and carbon dioxide is not the major greenhouse gas. That sector is agriculture.

    The U.S. Department of Agriculture has estimated that agriculture currently accounts for about 10% of total U.S. greenhouse gas emissions, with nitrous oxide (N2O) and methane (CH4) being major contributors to that total. According to the EPA (Figure 3), agriculture contributes 79% of N2O emissions in the United States, largely from the production and application of fertilizers (agricultural soil management) as well as from manure management, and 36% of CH4 emissions (enteric fermentation and manure management—one might add some of the landfill emissions to that total as well).

    If we succeed in moving nonagricultural sectors of the economy toward a zero-carbon state, the relationship in Figures 1 and 2 will be broken. The rate of overall climate warming would be reduced significantly, but N2O and CH4 would begin to play a more dominant role in driving continued greenhouse gas warming of the planet, and we will then need more complex models than the one used for Figures 1 and 2. But just how complex?

    4
    Fig. 3. EPA-reported total U.S. greenhouse gas emissions in 2020 (left) amounted to 5,981 million metric tons of CO2 equivalent, led by emissions of CO2, CH4, and N2O. Major sources of N2O (center) and CH4 (right) emissions are also shown. Credit: EPA.

    In his recent book Life Is Simple, biologist Johnjoe McFadden traces the influence across the centuries of William of Occam (~1287–1347) and Occam’s razor as a concept in the development of our physical understanding of everything from the cosmos to the subatomic structure of matter [McFadden, 2021]. One simple statement of Occam’s razor is, Entities should not be multiplied without necessity.

    This is a simple and powerful statement: Explain a set of measurements with as few parameters, or entities, as possible. But the definition of necessity can change when the goals of a model or presentation change. The simple model used in Figures 1 and 2 tells us nothing about tomorrow’s weather or the rate of sea level rise or the rate of glacial melt. But for as long as the relationship serves to capture the role of CO2 as an accurate index of changes in mean global temperature, it can serve the goal of making plain to general audiences that there are solid, undeniable scientific reasons why climate change is happening.

    Getting the Message Across

    If we move toward an electrified economy and toward zero-carbon sources of electricity, the simple relationship derived from Arrhenius’s calculations will no longer serve that function. But when and if it does fail, it will still provide a useful platform for explaining what has happened and why. Perhaps there will be another, slightly more complex model for predicting and explaining climate change that involves three gases.

    No matter how our climate future evolves, simpler and more accessible presentations of climate change science will always rely on and begin with our current understanding of the climate system. Complex, detailed models will be central to predicting our climate future (Figure 2 here would not be possible without them), but we will be more effective communicators if we can discern how best to simplify that complexity when presenting the essentials of climate science to general audiences.

    References

    Arrhenius, S. (1896), On the influence of carbonic acid in the air upon temperature of the ground, Philos. Mag. J. Sci., Ser. 5, 41, 237–276, https://doi.org/10.1080/14786449608620846.

    Arrhenius, S. (1908), Worlds in the Making: The Evolution of the Universe, translated by H. Borns, 228 pp., Harper, New York.

    Budyko, M. I. (1972), Man’s Impact on Climate [in Russian], Gidrometeoizdat, St. Petersburg, Russia.

    Gates, B. (2021), How to Avoid a Climate Disaster, 257 pp., Alfred A. Knopf, New York.

    Hausfather, Z., et al. (2022), Climate simulations: Recognize the ‘hot model’ problem, Nature, 605, 26–29, https://doi.org/10.1038/d41586-022-01192-2.

    Intergovernmental Panel on Climate Change (2021), Summary for policymakers, in Climate Change 2021: The Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, edited by V. Masson-Delmotte et al., pp. 3–32, Cambridge Univ. Press, Cambridge, U.K., and New York, https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf.

    Loeb, N. G., et al. (2021), Satellite and ocean data reveal marked increase in Earth’s heating rate, Geophys. Res. Lett., 48(13), e2021GL093047, https://doi.org/10.1029/2021GL093047.

    McFadden, J. (2021), Life Is Simple: How Occam’s Razor Set Science Free and Shapes the Universe, 376 pp., Basic Books, New York.

    See the full article here .

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  • richardmitnick 11:20 am on September 9, 2022 Permalink | Reply
    Tags: "Global Seismic Networks:: Recording the Heartbeat of the Earth", "GSN": Global Seismographic Network, "WWSSN": Worldwide Standardized Seismographic Network, , , , , Eos, , GEOSCOPE network, Global broadband seismographic networks have provided the science community with 30 years of data which is being used to understand the Earth., The first global seismographic networks were relatively small (20 to 30 stations) and recorded data on paper records.   

    From “Eos” : “Global Seismic Networks:: Recording the Heartbeat of the Earth” 

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    From “Eos”

    AT

    AGU

    9.9.22
    Adam T. Ringler
    aringler@usgs.gov

    Global broadband seismographic networks have provided the science community with 30 years of data which is being used to understand the Earth.

    1
    The underground vault and nearby weather station at Global Seismographic Network station TRIS on Tristan da Cunha, in the South Atlantic. Credit: IRIS

    For the past 30 years, Earth scientists have been monitoring the entire planet from surface processes down to the innermost core using near real-time digital seismic data. These data are collected by global seismographic networks and are free and openly available to anyone. These networks operate seismic stations that are often in very remote locations such as Pitcairn Island in the middle of the south Pacific Ocean or the South Pole in Antarctica.

    A recent article published in Reviews of Geophysics [below] explores the history and resulting scientific achievements of Global Seismographic Networks. We asked the lead author to give an overview of how Global Seismographic Networks evolved, what they’ve uncovered, and what challenges remain.

    What are “Global Seismographic Networks” and how are they used?

    Global seismographic networks are collections of seismic stations that measure near real-time ground motion (movement of the earth from earthquake shaking or other sources) and send those data to scientists. The instruments at these stations are so sensitive that they can record earthquakes from all over the world. This information is then used to locate the earthquake, determine its size, and resolve how the fault that generated the earthquake moved.

    2
    a) Global Seismographic Network as of 2021 with stations colored by primary sensor type. (b) Nanometrics T-360 GSN sensor (red); (c) Nanometrics T-120 borehole sensor (purple); (d) Streckeisen STS-6 sensor (orange); (e) Streckeisen STS-1sensor (blue); (f) GeoTech KS-54000 sensor (green). These instruments range from a height of approximately 16 centimeters (e) to 2 meters (f) and are not shown to scale. Credit: Ringler et al. [2022], Figure 7.

    These data are also used to study the interior of the Earth. By using earthquakes as sources of seismic energy, scientists estimate the internal properties of the Earth via tomography, which is similar to how a computed tomography (CT) scan works using seismic waves instead of X-rays.

    In the absence of ground motions generated by earthquakes, some of the next largest signals that these instruments detect are smaller seismic waves generated through the interaction of ocean waves with Earth’s crust. Therefore, the long-running history of global seismographic network stations can also be used to track changes in global ocean wave activity, which can be important for climate science.

    What did the first seismic network look like and how has it evolved?

    The first global seismographic networks were relatively small (20 to 30 stations) and recorded data on paper records. In the 1960s, these networks evolved to be much larger (100 to 150 stations), such as the Worldwide Standardized Seismographic Network (WWSSN) to monitor for nuclear testing.

    Although the WWSSN was state of the art at the time, it was later realized that the network was unable to record the slowest oscillations of great earthquakes, also known as normal modes. Large earthquakes cause normal modes to oscillate through the Earth much like ringing a bell. The Earth rings for many days, and each oscillation takes several minutes to complete. These normal modes provide unique information about the properties of Earth’s core and lower mantle that seismic waves from smaller earthquakes can’t tell us.

    Advances in technology and a continued interest to study normal modes by scientists led to the development of the GEOSCOPE network and the Global Seismographic Network (GSN) starting in the 1980s, with continual improvements to today. These state-of-the-art networks digitally record an exceptional range of ground motion amplitudes, from movements as small as the size of an atom to accelerations capable of collapsing buildings.

    3
    Current instrumentation used in the Global Seismographic Network (GSN). Credit: Ringler et al. [2022], Figure 8.

    How have GSNs advanced our understanding of the Earth?

    Global seismographic networks have provided a wealth of information about earthquakes, properties of the Earth’s interior, and surface processes such as ocean storms, large volcanic eruptions, and glacial calving events.

    Through locating and determining slip mechanisms of earthquakes, the long-running history of these networks has also helped quantify plate tectonics through the characterization of earthquakes along tectonic plate boundaries.

    Global seismographic networks have contributed to several foundational observations of Earth’s interior, including providing the first unambiguous evidence that the inner core is solid.

    Although not an original goal of these networks, they have recently been used to provide insight into environmental changes in the oceans and polar regions, as well as unique observations about how large volcanic eruptions oscillate the Earth’s atmosphere.

    In addition, these networks have helped scientists and engineers understand regions of potential hazard and develop building codes that mitigate loss of life and property after large earthquakes.

    How can scientists continue to advance the quality of seismic data and networks?

    Continued long-term and freely accessible monitoring data provided by global seismographic networks and support from the international scientific community to ensure high-quality data would be beneficial to advancement. Many scientific discoveries made using global seismic data were only possible after decades of data collection. For example, monitoring the rotation of the inner core, which is responsible for Earth’s magnetic field, required long running high-quality data records from globally distributed stations.

    Are there any additional interdisciplinary uses for GSNs?

    The very broadband nature and multidisciplinary development of global seismic networks makes them very well-suited to be used for interdisciplinary studies. Earth scientists have been able to use global seismic network data to study changes in climate and oceans. Many global seismic stations not only record seismic data, but also atmospheric data (such as pressure) and the Earth’s magnetic field. These additional data streams can be used to study things like how large volcanoes, such as the volcanic eruption near Tonga on 15 January 2022, erupted and how the energy seismically coupled into the Earth.

    What are some remaining challenges where additional research, data or modeling efforts are needed?

    Global seismographic networks have been widely successful at imaging the interior of the Earth, quantifying where tectonic plate boundaries are, and reducing geological hazards. Similar to how new and improved telescopes produce higher resolution images of distant galaxies, continued improvements to the infrastructure and instrumentation of global seismic networks will lead to new discoveries and an improved understanding of the Earth’s structure, atmospheric interactions, and geologic hazards.

    Many questions about the Earth still remain for which seismic data may help provide answers. For example, seismic data could provide one of the key tools for better understanding the evolution of the interior of the Earth and how it interacts with Earth surface processes. The long-running data streams from global seismographic networks could also help us understand increasing extreme climate activity that interacts with the Earth’s surface. Additionally, global station coverage is sparse in some regions, including in ocean basins and in central Africa, which limits our ability to detect earthquakes and thus obtain clear images on Earth structure in these regions.

    Science paper:
    Reviews of Geophysics

    See the full article here .

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    Please help promote STEM in your local schools.

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  • richardmitnick 9:29 am on September 9, 2022 Permalink | Reply
    Tags: "Months of Gravity Changes Preceded the Tōhoku Earthquake", , , , Eos, Using GRACE satellite data researchers discovered anomalous gravimetric signals that occurred before a seismic event that started deep within Earth.,   

    From “Eos” : “Months of Gravity Changes Preceded the Tōhoku Earthquake” 

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    From “Eos”

    AT

    AGU

    9.1.22
    Sarah Derouin

    Using GRACE satellite data researchers discovered anomalous gravimetric signals that occurred before a seismic event that started deep within Earth.

    1
    An aerial view of Wakuya, Japan, after the 2011 Tōhoku earthquake shows how devastating subducting earthquakes can be. Continuous gravity monitoring of the Pacific subduction belt might highlight where big earthquakes may occur. Credit: Mass Communication Specialist 3rd Class Alexander Tidd/U.S. Navy CC BY-NC 2.0.

    Earthquakes caused by subducting tectonic plates can be highly destructive events. The 2011 Tōhoku earthquake caused immense damage to population centers in eastern Japan.

    Constant monitoring of faulted regions with seismographs and space geodesy measurements can indicate when land deformations are occurring in shallow or surficial systems, giving researchers a hand with hazard mitigation work. But for subduction zones, much of the deformation occurs deep within Earth, making it difficult to detect on the surface.

    In a new paper, Panet et al.[Journal of Geophysical Research: Solid Earth (below)] investigate whether they could detect pulling slab deformations using gravity measurements from satellites. The team used Gravity Recovery and Climate Experiment (GRACE) satellite data—GRACE is a twin satellite system launched by NASA and DLR (German Aerospace Center) that makes detailed measurements of Earth’s gravity field.

    These measurements can reveal information about changes in Earth systems, including amounts and locations of water and ice as well as crustal deformations.

    In previous studies, the team identified and mapped anomalous variations in Earth’s gravity in the months preceding the Tōhoku earthquake in both space and time. In this paper, the researchers developed a new way to detect signals along plate boundaries.

    The researchers used GRACE to study subtle changes in gravity in subduction systems. Using overlapping passes of GRACE gravity gradients from 2004 to 2011, the team tested whether they could see deep signals in the solid Earth before Tōhoku occurred, and created a method to identify solid mass redistributions based on the variation of gravity gradients.

    They found there were unique gravity signatures preceding the Tōhoku earthquake, likely associated with deep deformations in the subduction system. The team says their work presents a way to continuously monitor Pacific plate subduction movements at depth using real-time gravity-measuring satellites.

    Science paper:
    Journal of Geophysical Research: Solid Earth

    See the full article here .

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    Please help promote STEM in your local schools.

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  • richardmitnick 8:24 am on September 7, 2022 Permalink | Reply
    Tags: "Glacier Advance and Retreat:: Insights From the Top of the World", , , , Eos, , Glacial moraines Khumbu Valley in Nepal   

    From “Eos” : “Glacier Advance and Retreat:: Insights From the Top of the World” 

    Eos news bloc

    From “Eos”

    AT

    AGU

    9.7.22
    Mikaël Attal

    1
    Moraines are heaps of debris accumulated by glaciers around their edges, which are left behind when glaciers retreat. This figure shows perspective photographs of the moraines from which samples were collected for Be-10 exposure-age dating, and the ages produced (in ka) for each sample for (top) Khumbu Glacier and (bottom) Lobuche Glacier. Colored lines show various generations of moraine crests identified in this study. Credit: Hornsey et al. [2022], Figure 4(a,c)

    2
    Figure 1.Map of moraine crests and the previous geochronology for the upper Khumbu Valley (Finkel et al., 2003; Richards, 2000; Richards et al., 2000; Rowan, 2017). Ages (in ka) were measured using Be-10 exposure-age dating of moraine crests (Finkel et al., 2003; Owen et al., 2009) and OSL dating of glacial sediments (Richards, 2000; Richards et al., 2000). The ages for each glacial stage are the error-weighted mean of sample ages with the standard deviation of the internal uncertainties from Finkel et al. (2003) after recalibration and a chi-squared test to identify outliers. The two Be-10 ages identified with an asterisk are presented in Finkel et al. (2003) but produced by Aoki and Imamura (1999). The underlying image is the 8-m High Mountain Asia Digital Elevation Model (Shean, 2017) and glacier outlines from the Randolph Glacier Inventory (RGI Consortium, 2017).

    3
    Figure 2. Photographs of moraines in the upper Khumbu Valley and boulders sampled for Be-10 exposure-age dating. (a) The Last Glacial Maximum Pheriche moraines formed by Khumbu Glacier looking downvalley, (b) the Last Glacial and Holocene terminal moraines of Khumbu Glacier looking upvalley, (c) the Holocene Khumbu and Lobuche Glacier moraines looking downvalley, (d) the Changi Shar Glacier moraines looking across the former glacier ablation area to the south, (e) the Changi Nup Glacier moraines looking across the former glacier ablation area to the north, (f) the Khumbu 1 moraine looking upglacier, (g) the Khumbu 2 moraine looking upglacier, (h) the boulder from which sample KH1 was collected, (i) the Lobuche 3 moraine looking upglacier, (j) the boulder from which sample LB11 was collected. Note people and tents circled in red for scale.

    4
    Figure 3. Map of moraine crests in the Khumbu Valley and sample locations where (a) shows an overview of the Khumbu Valley and locations of the sampled boulders for each glacier, with detail of sample locations for (b and c) Changri Nup Glacier moraines, (d) Khumbu Glacier moraines, and (e) Lobuche Glacier moraines. Glacier outlines are from the Randolph Glacier Inventory (RGI Consortium, 2017). Topographic imagery is from Landsat bands 7, 5, and 4 in 2015. Moraine crests are indicated by colored lines, where those that are not differentiated into detailed glacial stages are assigned as either Holocene or Late Glacial.

    Glaciers are vulnerable to climate change, in particular in the monsoon-influenced Himalaya. Retreating glaciers pose serious risks, including risks associated with glacial lake outburst floods (GLOFs) and challenges in managing water resources for the communities living downstream. To understand how glaciers will respond to climate change in the future, we need to understand how they responded to climate change in the past.

    In the Holocene (past 12,000 years of the Earth’s history), the Earth experienced a succession of warming and cooling periods. When glaciers retreat, usually during warming periods, they leave behind heaps of debris that had accumulated around their edges, named moraines. Glaciers in the Khumbu area have left a series of well-developed moraines, which Hornsey et al. [2022] dated using Beryllium-10 (Be-10).

    Be-10 is a cosmogenic nuclide, that is, a type of atom that does not exist in rocks at depth. As soon as a rocks is brought to the surface, cosmogenic nuclides start accumulating in the rock due to interactions between the atoms that make up the rocks and the cosmic rays that continually bombard the surface of the Earth. The longer a rock is exposed at the surface of the Earth, the greater its concentration in cosmogenic nuclides. Cosmogenic nuclides can therefore be used like a stopwatch to date when a rock fragment has been brought to the surface of the Earth, for example, when a glacier retreated and left previously buried fragments behind.

    The new ages show that glaciers in the Khumbu area have responded to past climate warming in a rapid and predictable way, leaving behind complete evidence of their advance and retreat at known times of cooling and warming, respectively. This work will help scientists better understand and predict the response of Himalayan glaciers to ongoing unprecedented climate change.

    Science paper:
    Journal of Geophysical Research: Earth Surface

    See the full article here .

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  • richardmitnick 8:46 pm on August 22, 2022 Permalink | Reply
    Tags: "Swinging Strength of Earth’s Magnetic Field Could Signal Inner Core Formation", , Earth’s atmosphere owes its persistence to the geomagnetic field which thwarts the Sun’s rays from dispelling this gaseous veneer., Eos, , , , Rock transcribes where North was (direction) and how strong the field was (intensity) at the time of formation., The (failed) North American Midcontinent Rift—a region where 1.1 billion years ago there was voluminous volcanism., The combination of molten iron alloys and Earth’s rotation results in a self-sustaining magnetic field called the geodynamo., The magnetic record stored in rocks documents the liquid core’s behavior and possibly when the inner core formed., This protective geomagnetic field owes its existence to Earth’s core., Understanding how paleomagnetic intensity has changed helps scientists address when the core transitioned to a solid inner core wrapped in a liquid outer core., Unique aggregations of crystals-anorthosite xenoliths-that formed deep in Earth’s crust brought close to the surface with magma that fed lava eruptions into the rift.   

    From “Eos” : “Swinging Strength of Earth’s Magnetic Field Could Signal Inner Core Formation” 

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    From “Eos”

    AT

    AGU

    8.22.22
    Alka Tripathy-Lang

    The magnetic record stored in rocks documents the liquid core’s behavior and possibly when the inner core formed. Whether it formed half a billion or more than a billion years ago, however, is up for debate.

    1
    Researchers (including coauthor Nick Swanson-Hysell) found surprisingly high paleointensity values in rocks collected from the (failed) North American Midcontinent Rift. Credit: Yiming Zhang

    Life as we know it requires an atmosphere. It is the air we breathe, our shield from harmful ultraviolet rays, and our defense against extreme temperature swings, like those on Mars. But Earth’s atmosphere owes its persistence to the geomagnetic field which thwarts the Sun’s rays from dispelling this gaseous veneer. And this protective geomagnetic field owes its existence to Earth’s core.

    As the liquid part of the core (the outer core) swirls, the combination of molten iron alloys and Earth’s rotation results in a self-sustaining magnetic field called the geodynamo. As they archive evidence of the geodynamo’s billions-of-years-long existence, rocks can transcribe where north was (direction) and how strong the field was (intensity) at the time of formation. That transcription is possible as long as the rocks remain relatively untouched by high temperatures, fluids, or other traumas of tectonics.

    Because the strength of Earth’s magnetic field relies on the vigor with which the liquid core churns, understanding how paleomagnetic intensity has changed at the surface can help scientists address when the core transitioned from a single ball of sloshing melt to a solid inner core wrapped in a liquid outer core. In other words, paleomagnetic intensity might tell scientists when the inner core began to form, with some suggesting that the answer is a little more than half a billion years ago (i.e., only in the last ~10% of Earth’s history).

    In a new study published in the PNAS [below], paleomagnetist and University of California- Berkeley doctoral student Yiming Zhang and his coauthors collected and studied rocks from the (failed) North American Midcontinent Rift—a region where 1.1 billion years ago there was voluminous volcanism. The rocks that Zhang targeted are unique aggregations of crystals known as anorthosite xenoliths that formed deep in Earth’s crust but were brought close to the surface with magma that fed lava eruptions into the rift. The team found surprisingly high paleointensity values that signal a turbulent core—more spirited than might be expected for a liquid core lacking a solid center and stronger than Earth’s magnetic field today.

    2
    Plot of age versus paleointensity, with a dashed line showing a previously interpreted decreasing trend until about 500–600 million years ago, after which paleointensity values begin to climb, showing distinct lows but also higher highs than most data from the Precambrian, barring the data presented in the new study. A light blue band from 1,110 to 1,085 million years ago shows the age of the rocks in this study as well as their curiously high paleointensity values. Credit: Zhang et al., 2022.

    The Age of Earth’s Heart

    The energy that causes the liquid core to move, or convect, comes from two different mechanisms. Thermally convecting liquid is driven by heat that’s wanting to rise and escape, said Courtney Sprain, a paleomagnetist at the University of Florida who was not involved in this study. In the other mechanism, compositional convection stirs the cauldron because of light elements. As the mostly iron inner core solidifies, it excludes lighter, more buoyant elements that proceed to rise through the liquid outer core. “We believe [that] today, that’s one of our main sources of energy driving the geodynamo,” Sprain explained.

    Because Earth was much warmer billions of years ago and the inner core was not initially present, thermal convection may have been the primary driver generating the early magnetic field, said Richard Bono, a paleomagnetist at Florida State University who also was not involved in Zhang’s work. As Earth cooled, thermal convection—and the intensity of the magnetic field—should have tapered. But continued cooling eventually led to the beginnings of Earth’s solid metal heart, which should have boosted the waning magnetic field as compositional convection overtook its thermal counterpart.

    This transition might have begun less than 700 million years ago (much younger than canonical estimates), according to experiments designed to determine how fast iron conducts heat at extremely high pressures and temperatures, said Zhang. However, such experiments have led to different results through different approaches, enough so that various scenarios of the age of the inner core are possible.

    A 2019 paper [Nature Geoscience (below)] led by Bono, in which scientists collated high-quality paleointensity data, supported this young inner core formation timeline, with the paleomagnetic field interpreted to decrease in intensity until a 565-million-year-old low, followed by a rise toward much higher values, signifying more mixing. This timeline led to the intriguing hypothesis that the inner core began to form sometime after 565 million years ago—remarkably young.

    However, because older (1.14-billion-year-old) rocks have low paleointensity values, Zhang’s curiously high paleointensity data in 1.09-billion-year-old rocks could be interpreted as inner core nucleation similar to some previous estimates [Nature (below)]. “You need some really strong forces in the interior of the Earth to generate such strong [paleointensity] values,” said Zhang. If true, new explanations for later ebbs and flows of paleointensity are needed for around 565 million years ago, as well as at younger times [PNAS] of low to high field strength transitions. Nevertheless, these data don’t negate inner core nucleation 565 million years ago either, he said.

    “If anything, this is telling us we need to start trying to understand some of the other added complexities” like plate tectonics, said Sprain. Subducting plates move through the mantle, sometimes settling into cold piles at the core-mantle boundary. Elsewhere along this boundary, buoyant plumes of hot material rise upward. These sunken slabs and upwelling plumes affect how heat escapes from the core, a process that itself affects how quickly the outer core can convect. The core’s pattern of exhaling heat changes as this geometry shifts, which could affect how the magnetic field is generated, she explained.

    Snapshot or Long-Term Average?

    “Our magnetic field is really crazy,” said Sprain. “It can change on timescales of seconds to millions of years.”

    “When we’re trying to understand what the strength of the field is, we have to ask—how much time are we looking at, [and] how much time do we average?” said Bono. A rock that cools quickly, on the order of hundreds or thousands of years, will record a snapshot of the magnetic field. A rock that takes many tens or hundreds of thousands of years to cool smooths out the magnetic field’s short-term variation. “You really need to be looking at the time-averaged field strength” to understand what was happening in the core, he said.

    In the new study, said Sprain, Zhang has data from seven sites but only one date for these rocks. Because these are very old rocks, each date’s margin of error would be on the order of 100,000 years to greater than 1 million; collecting more dates wouldn’t necessarily help resolve the relative timing between sites. “Even if there was more than 10,000 years between [multiple samples’] cooling times, we wouldn’t be able to resolve it [because] the ages would overlap,” Sprain said.

    Nevertheless, the data are of high quality, and even averaging all the information together results in a higher-than-expected magnetic field for 1.1 billion years ago, confirming the findings of prior work [Geophysical Journal International (below)], said Bono. In this prior work, Sprain found that slightly older volcanic rocks from the Midcontinent Rift also record a strong magnetic field similar to that on Earth today.

    “What we need,” said Sprain, “is more high-quality data.” This is especially true for the Precambrian, whose rocks have had more time to endure upheavals that can erase their experiences.

    Science papers:
    PNAS 2022
    Nature Geoscience 2019
    Nature 2015
    PNAS 2021
    Geophysical Journal International 2018

    See the full article here .

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    Please help promote STEM in your local schools.

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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 2:03 pm on August 18, 2022 Permalink | Reply
    Tags: "A New Look at Preindustrial Carbon Release from the Deep Ocean", , , , Eos   

    From “Eos” : “A New Look at Preindustrial Carbon Release from the Deep Ocean” 

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    From “Eos”

    AT

    AGU

    8.18.22
    Sarah Stanley

    New research could help inform future studies of how the release of carbon dioxide from the Southern Ocean might affect global climate change.

    1
    A new study identifies where in the Southern Ocean carbon dioxide is released into the atmosphere and where the carbon comes from. Credit: F. Alexander Haumann.

    In the Southern Ocean near Antarctica, deep-ocean water upwells to the surface, where it releases carbon dioxide that entered the ocean prior to the Industrial Revolution. This process is a key component of the global carbon cycle, and recent research [Geophysical Research Letters (below)] has suggested that it returns more carbon dioxide to the atmosphere than previously thought.

    Now, a new study by Chen et al. [Global Biogeochemical Cycles (below)] explores exactly where in the Southern Ocean carbon dioxide is released into the atmosphere, where this carbon comes from, and how various factors combine to drive these processes.

    Building on earlier research, the scientists analyzed measurements that had been collected with ships and by Argo floats, free-drifting instruments that take measurements in the subsurface ocean. These data included observations of temperature, salinity, dissolved oxygen, pH, alkalinity, nutrient levels, and levels of dissolved inorganic carbon.

    Using those data, the researchers made detailed calculations related to the partial pressure of carbon dioxide in upwelling water, which helps to determine its ability to release carbon dioxide into the atmosphere. They also explored how additional factors, such as biological processes and ocean circulation, drive spatial patterns of varying partial pressure of carbon dioxide in different ocean regions and at different depths—thereby influencing its release at the surface.

    The analysis showed that preindustrial carbon dioxide is released primarily from a specific band of upwelling water that encircles Antarctica and lies between a region known as the Subantarctic Front and an inner boundary formed by the edge of the extent of sea ice during wintertime. This deep water comes from the northern Pacific and Indian Ocean basins, and it is rich in old carbon produced from organic carbon through a process known as remineralization.

    The study also showed that low temperature and high alkalinity hinder carbon dioxide release from other locations of upwelling deep waters, despite their higher carbon content.

    These findings could help inform future investigations of how carbon dioxide release from the Southern Ocean might affect—and be affected by—global climate change.

    Science papers:
    Geophysical Research Letters
    Global Biogeochemical Cycles

    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.

     
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