Understanding Earth's 'deep-carbon cycle'

Date:
September 10, 2020
Source:
Case Western Reserve University
Summary:
New geologic findings about the makeup of the Earth's mantle are
helping scientists better understand long-term climate stability
and even how seismic waves move through the planet's layers.



FULL STORY ==========================================================================
New geologic findings about the makeup of the Earth's mantle are helping scientists better understand long-term climate stability and even how
seismic waves move through the planet's layers.


==========================================================================
The research by a team including Case Western Reserve University
scientists focused on the "deep carbon cycle," part of the overall cycle
by which carbon moves through the Earth's various systems.

In simplest terms, the deep carbon cycle involves two steps:
* Surface carbon, mostly in the form of carbonates, is brought
into the
deep mantle by subducting oceanic plates at ocean trenches.

* That carbon is then returned to the atmosphere as carbon dioxide
(CO2)
through mantle melting and magma degassing processes at volcanoes Scientists have long suspected that partially melted chunks of this
carbon are broadly distributed throughout the Earth's solid mantle.

What they haven't fully understood is how far down into the mantle they
might be found, or how the geologically slow movement of the material contributes to the carbon cycle at the surface, which is necessary for
life itself.

Deep carbon and climate change connection "Cycling of carbon between the surface and deep interior is critical to maintaining Earth's climate in
the habitable zone over the long term -- meaning hundreds of millions
of years," said James Van Orman, a professor of geochemistry and mineral physics in the College of Arts and Sciences at Case Western Reserve and
an author on the study, recently published in the Proceedings of the
National Academy of Sciences.



========================================================================== "Right now, we have a good understanding of the surface reservoirs of
carbon, but know much less about carbon storage in the deep interior,
which is also critical to its cycling." Van Orman said this new research showed -- based on experimental measurements of the acoustic properties of carbonate melts, and comparison of these results to seismological data --
that a small fraction (less than one-tenth of 1%) of carbonate melt is
likely to be present throughout the mantle at depths of about 180-330 km.

"Based on this inference, we can now estimate the carbon concentration
in the deep upper mantle and infer that this reservoir holds a large
mass of carbon, more than 10,000 times the mass of carbon in Earth's atmosphere," Van Orman said.

That's important, Van Orman said, because gradual changes in the amount
of carbon stored in this large reservoir, due to exchange with the
atmosphere, could have a corresponding effect on CO2 in the atmosphere --
and therefore, on long-term climate change.

The first author of the article is Man Xu, who did much of the work as
a PhD student at Case Western Reserve and is now a postdoctoral scholar
at the University of Chicago.



========================================================================== Others on the project were from Florida State University, the University
of Chicago and Southern University of Science and Technology (SUSTech)
in Shenzhen, China.

Explaining seismic wave speed differences The research also sheds light
on seismology, especially deep earth research.

One way geologists better understand the deep interior is by measuring
how seismic waves generated by earthquakes -- fast-moving compressional
waves and slower shear waves -- move through the Earth's layers.

Scientists have long wondered why the speed difference between the two
types of seismic waves -- P-waves and S-waves -- peaked at depths of
around 180 to 330 kilometers into the Earth.

Carbon-rich melts seem to answer that question: small quantities of
these melts could be dispersed throughout the deep upper mantle and would explain the speed change, as the waves move differently through the melts.


========================================================================== Story Source: Materials provided by Case_Western_Reserve_University. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Man Xu, Zhicheng Jing, Suraj K. Bajgain, Mainak Mookherjee, James
A. Van
Orman, Tony Yu, Yanbin Wang. High-pressure elastic properties
of dolomite melt supporting carbonate-induced melting in deep
upper mantle.

Proceedings of the National Academy of Sciences, 2020; 117 (31):
18285 DOI: 10.1073/pnas.2004347117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910120112.htm

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