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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