• Longest known continuous record of the P

    From ScienceDaily@1:317/3 to All on Thu Jul 8 21:30:32 2021
    Longest known continuous record of the Paleozoic discovered in Yukon wilderness

    Date:
    July 8, 2021
    Source:
    Stanford University
    Summary:
    Expeditions to a remote area of Yukon, Canada, have uncovered
    a 120- million-year-long geological record of a time when land
    plants and complex animals first evolved and ocean oxygen levels
    began to approach those in the modern world.



    FULL STORY ========================================================================== Hundreds of millions of years ago, in the middle of what would eventually become Canada's Yukon Territory, an ocean swirled with armored trilobites, clam-like brachiopods and soft, squishy creatures akin to slugs and squid.


    ==========================================================================
    A trove of fossils and rock layers formed on that ancient ocean floor
    have now been unearthed by an international team of scientists along the
    banks of the Peel River a few hundred miles south of the Arctic's Beaufort
    Sea. The discovery reveals oxygen changes at the seafloor across nearly
    120 million years of the early Paleozoic era, a time that fostered the
    most rapid development and diversification of complex, multi-cellular
    life in Earth's history.

    "It's unheard of to have that much of Earth's history in one place," said Stanford University geological scientist Erik Sperling, lead author of
    a July 7 study detailing the team's findings in Science Advances. Most
    rock formations from the Paleozoic Era have been broken up by tectonic
    forces or eroded over time. "There's nowhere else in the world that I
    know of where you can study that long a record of Earth history, where
    there's basically no change in things like water depth or basin type."
    Oxygen was scarce in the deep water of this and other oceans at the
    dawn of the Paleozoic, roughly 541 million years ago. It stayed scarce
    until the Devonian, roughly 405 million years ago, when, in a geological
    blink -- no more than a few million years -- oxygen likely rocketed
    to levels close to those in modern oceans and the diversity of life
    on Earth exploded. Big, predatory fish appeared. Primitive ferns and
    conifers marched across continents previously ruled by bacteria and
    algae. Dragonflies took flight. And all of this after nearly four billion
    years of Earth's landscapes being virtually barren.

    Scientists have long debated what might have caused the dramatic shift
    from a low oxygen world to a more oxygenated one that could support
    a diverse web of animal life. But until now, it has been difficult to
    pin down the timing of global oxygenation or the long-term, background
    state of the world's oceans and atmosphere during the era that witnessed
    both the so-called Cambrian explosion of life and the first of Earth's
    "Big Five" mass extinctions, about 445 million years ago at the end of
    the Ordovician.

    "In order to make comparisons throughout these huge swaths of our history
    and understand long-term trends, you need a continuous record," said
    Sperling, an assistant professor of geological sciences at Stanford's
    School of Earth, Energy & Environmental Sciences (Stanford Earth).



    ========================================================================== Context for past life With permission from the Na Cho Nyak Dun and
    Tetlit Gwitch'in communities in Yukon, Sperling's team, which included researchers from Dartmouth College and the Yukon Geological Survey,
    spent three summers at the Peel River site.

    Arriving by helicopter, the research team hacked through brush with
    machetes beside Class VI rapids to collect hundreds of fist-sized samples
    of rock from more than a mile of interbedded layers of shale, chert and
    lime mudstone.

    Back at Sperling's lab at Stanford, a small army of summer undergraduates
    and graduate students worked over five summers to help analyze the
    fossils and chemicals entombed in the rocks. "We spent a lot of time
    splitting open rocks and looking at graptolite fossils," Sperling
    said. Because graptolites evolved a vast array of recognizable body
    shapes relatively quickly, the pencil-like markings left by the fossils
    of these colony-dwelling sea creatures give geologists a way to date
    the rocks in which they're found.

    Once the researchers had finished identifying and dating graptolite
    fossils, they ground the rocks in a mill, then measured iron, carbon, phosphorus and other elements in the resulting powder to assess the
    ocean conditions at the time and place where the layers formed. They
    analyzed 837 new samples from the Peel River site, as well as 106 new
    samples from other parts of Canada and 178 samples from around the world
    for comparison.

    Winners and losers The data show low oxygen levels, or anoxia, likely
    persisted in the world's oceans for millions of years longer than
    previously thought -- well into the Phanerozoic, when land plants and
    early animals began to diversify. "The early animals were still living in
    a low oxygen world," Sperling said. Contrary to long-held assumptions,
    the scientists found Paleozoic oceans were also surprisingly free of
    hydrogen sulfide, a respiratory toxin often found in the anoxic regions
    of modern oceans.

    When oxygen eventually did tick upward in marine environments, it came
    about just as larger, more complex plant life took off. "There's a
    ton of debate about how plants impacted the Earth system," Sperling
    said. "Our results are consistent with a hypothesis that as plants
    evolved and covered the Earth, they increased nutrients to the ocean,
    driving oxygenation." In this hypothesis, the influx of nutrients to
    the sea would have given a boost to primary productivity, a measure of
    how quickly plants and algae take carbon dioxide and sunlight, turn them
    into new biomass -- and release oxygen in the process.

    The change probably killed off graptolites. "Although more
    oxygen is really good for a lot of organisms, graptolites
    lost the low oxygen habitat that was their refuge,"
    Sperling said. "Any environmental change is going to have
    winners and losers. Graptolites might have been the losers." ========================================================================== Story Source: Materials provided by Stanford_University. Original written
    by Josie Garthwaite. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Erik A. Sperling, Michael J. Melchin, Tiffani Fraser, Richard
    G. Stockey,
    Una C. Farrell, Liam Bhajan, Tessa N. Brunoir, Devon B. Cole,
    Benjamin C.

    Gill, Alfred Lenz, David K. Loydell, Joseph Malinowski, Austin
    J. Miller, Stephanie Plaza-Torres, Beatrice Bock, Alan D. Rooney,
    Sabrina A.

    Tecklenburg, Jacqueline M. Vogel, Noah J. Planavsky, Justin
    V. Strauss. A long-term record of early to mid-Paleozoic marine
    redox change. Science Advances, 2021; 7 (28): eabf4382 DOI:
    10.1126/sciadv.abf4382 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2021/07/210708185945.htm

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