• Traces of life in the Earth's deep mantl

    From ScienceDaily@1:317/3 to All on Tue Mar 8 21:30:38 2022
    Traces of life in the Earth's deep mantle

    Date:
    March 8, 2022
    Source:
    ETH Zurich
    Summary:
    The rapid development of fauna 540 million years ago has permanently
    changed the Earth - deep into its lower mantle. A team has now
    found traces of this development in rocks from this zone.



    FULL STORY ==========================================================================
    The rapid development of fauna 540 million years ago has permanently
    changed the Earth -- deep into its lower mantle. A team led by ETH
    researcher Andrea Giuliani found traces of this development in rocks
    from this zone.


    ==========================================================================
    It is easy to see that the processes in the Earth's interior influence
    what happens on the surface. For example, volcanoes unearth magmatic rocks
    and emit gases into the atmosphere, and thus influence the biogeochemical cycles on our planet.

    What is less obvious, however, is that the reverse is also true: what
    happens on the Earth's surface effect the Earth's interior -- even down to great depths. This is the conclusion reached by an international group
    of researchers led by Andrea Giuliani, SNSF Ambizione Fellow in the
    Department of Earth Sciences at ETH Zurich, in a new study published in
    the journal Science Advances. According to this study, the development
    of life on our planet affects parts of Earth's lower mantle.

    Carbon as a messenger In their study, the researchers examined rare diamond-​bearing volcanic rocks called kimberlites from different
    epochs of the Earth's history. These special rocks are messengers from
    the lowest regions of the Earth's mantle.

    Scientists measured the isotopic composition of carbon in about 150
    samples of these special rocks. They found that the composition of
    younger kimberlites, which are less than 250 million years old, varies considerably from that of older rocks. In many of the younger samples,
    the composition of the carbon isotopes is outside the range that would
    be expected for rocks from the mantle.

    The researchers see a decisive trigger for this change in composition
    of younger kimberlites in the Cambrian Explosion. This relatively short
    phase - - geologically speaking -- took place over a period of few tens of million years at the beginning of the Cambrian Epoch, about 540 million
    years ago.

    During this drastic transition, almost all of today's existing animal
    tribes appeared on Earth for the first time. "The enormous increase in
    life forms in the oceans decisively changed what was happening on the
    Earth's surface," Giuliani explains. "And this in turn affected the
    composition of sediments at the bottom of the ocean." From the oceans
    to the mantle and back For the Earth's lower mantle, this changeover is relevant because some of the sediments on the seafloor, in which material
    from dead living creatures is deposited, enter the mantle through plate tectonics. Along the subduction zones, these sediments -- along with the underlying oceanic crust -- are transported to great depths. In this way,
    the carbon that was stored as organic material in the sediments also
    reaches the Earth's mantle. There the sediments mix with other rock
    material from the Earth's mantle and after a certain time, estimated
    to at least 200-​300 million years, rise to the Earth's surface
    again in other places -- for example in the form of kimberlite magmas.

    It is remarkable that changes in marine sediments leave such profound
    traces, because overall, only small amounts of sediment are transported
    into the depths of the mantle along a subduction zone. "This confirms
    that the subducted rock material in the Earth's mantle is not distributed homogeneously, but moves along specific trajectories," Giuliani explains.

    The Earth as a total system In addition to carbon, the researchers
    also examined the isotopic composition of other chemical elements. For
    example, the two elements strontium and hafnium showed a similar pattern
    to carbon. "This means that the signature for carbon cannot be explained
    by other processes such as degassing, because otherwise the isotopes
    of strontium and hafnium would not be correlated with those of carbon," Giuliani notes.

    The new findings open the door for further studies. For example, elements
    such as phosphorus or zinc, which were significantly affected by the
    emergence of life, could also provide clues as to how processes at the
    Earth's surface influence the Earth's interior. "The Earth is really a
    complex overall system," Giuliani says. "And we now want to understand
    this system in more detail."

    ========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
    Felix Wu"rsten. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Andrea Giuliani, Russell N. Drysdale, Jon D. Woodhead, Noah
    J. Planavsky,
    David Phillips, Janet Hergt, William L. Griffin, Senan Oesch, Hayden
    Dalton, Gareth R. Davies. Perturbation of the deep-Earth carbon
    cycle in response to the Cambrian Explosion. Science Advances,
    2022; 8 (9) DOI: 10.1126/sciadv.abj1325 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220308102802.htm

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