Geologists at MIT have discovered that smectite, a seabed clay mineral, has
the unexpected capacity to trap carbon over millions of years.
A single clay grain looks like the folds of an accordion under a
microscope. It is well known that these folds serve as efficient organic
carbon traps.
The MIT group has now demonstrated that plate tectonics is the source of
the carbon-trapping clays: Rocks that eventually weather into minerals like
smectite can be brought to the surface when oceanic crust collides with a
continental plate. The deceased creature fragments are eventually trapped in
the minute folds of the minerals in the clay silt when it returns to the
ocean. By doing this, the organic carbon is prevented from being ingested by
microorganisms and released as carbon dioxide back into the
atmosphere.
Smectite can have a worldwide influence over millions of years,
contributing to global cooling. The scientists demonstrated through a number
of investigations that smectite was most likely formed following a number of
significant tectonic events throughout the previous 500 million years. The
clays stored enough carbon during each tectonic event to cause Earth to cool
and trigger the ice age that followed.
These results are the first to demonstrate how the formation of
carbon-trapping smectite can initiate ice ages due to plate tectonics.
These clays are still present in some tectonically active areas, and
scientists think that smectite is still storing carbon, functioning as a
slow-acting natural barrier against human activity that warms the
planet.
Joshua Murray, a PhD student in the Department of Earth, Atmospheric, and
Planetary Sciences at MIT, states that "the influence of these unassuming
clay minerals has wide-ranging implications for the habitability of
planets." "There may even be a modern application for these clays in
offsetting some of the carbon that humanity has placed into the
atmosphere."
Murray and MIT geology professor Oliver Jagoutz published their research
results in Nature Geoscience.
A visible and distinct clay
The team's earlier research, which demonstrated that every major ice age on
Earth was probably brought on by a tectonic catastrophe in the tropics, is
being followed up on by this current study. The scientists discovered that
the atmosphere was exposed to ocean rocks known as ophiolites during each of
these tectonic episodes.
They proposed that specific weathering processes, such exposure to wind,
rain, and chemical interactions, might cause ophiolites to undergo
weathering in a tropical location following a tectonic collision, converting
the rocks into a variety of minerals, including clays.
"Those clay minerals, depending on the kinds you create, influence the
climate in different ways," says Murray.
The minerals that may emerge from this weathering action, as well as
whether and how these minerals could directly contribute to cooling the
planet, were unknown at the time. Therefore, even though it seemed that
plate tectonics and ice ages were related, it was still unclear how
precisely one might cause the other.
The goal of the new study was to determine if the carbon-trapping minerals
that the team's projected tectonic tropical weathering mechanism would
create in sufficient numbers to start a global ice age.
The group initially perused the geologic literature to gather information
on the many ways that main magmatic minerals weather over time, as well as
the kinds of clay minerals that can result from this weathering. A
weathering simulation of many rock types that are known to be exposed in
tectonic collisions was then created using these observations.
"Then we look at what happens to these rock types when they break down due
to weathering and the influence of a tropical environment, and what minerals
form as a result," Jagoutz explains.
Subsequently, they inserted every worn-out "end-product" mineral into a
model of the Earth's carbon cycle to see the potential impact of each
mineral on the interaction of inorganic carbon, which is found in the
atmosphere as carbon dioxide, or organic carbon, which includes remnants of
extinct creatures.
Smectite was the mineral whose existence and impact were most evident based
on these investigations. The clay was an exceptionally good organic carbon
trap in addition to being a naturally weathered result of tropical
tectonics. Smectite seems to provide a strong theoretical link between
tectonics and ice periods.
However, were there genuinely enough of the clays to start the preceding
four ice ages? The best way for scientists to verify this would be to
discover smectite in prehistoric rock strata that go back to each global
cooling epoch.
"Unfortunately, as clays are buried by other sediments, they get cooked a
bit, so we can't measure them directly," Murray explains. "But we can look
for their fingerprints."
a gradual increase
The scientists reasoned that because smectites are a byproduct of
ophiolites, distinctive elements like nickel and chromium, which would be
retained in ancient sediments, are also present in these ocean rocks. In the
event that smectites were formerly present, nickel and chromium ought to be
as well.
The group examined hundreds of oceanic sedimentary rocks that have been
deposited over the last 500 million years in a database to evaluate this
theory. Four distinct ice ages occurred on Earth during this time. The
presence of significant nickel and chromium spikes in the rocks at each of
these eras led the researchers to conclude that smectite must have also been
present.
Their calculations showed that the clay mineral might have contributed less
than a tenth of a percent to the preservation of organic carbon. This is a
very little sum in absolute terms. However, scientists determined that the
accumulated, trapped carbon in the clay over millions of years was
sufficient to start each of the four major ice eras.
"We found that you really don't need much of this material to have a huge
effect on the climate," Jagoutz explains.
"These clays also have probably contributed some of the Earth's cooling in
the last 3 to 5 million years, before humans got involved," says Murray.
"These clays are probably influencing the climate even in the absence of
people. It simply moves so slowly."
"Jagoutz and Murray's work is a nice demonstration of how important it is
to consider all biotic and physical components of the global carbon cycle,"
says Penn State University geosciences professor Lee Kump, who was not
involved in the research. "Feedbacks among all these components control
atmospheric greenhouse gas concentrations on all time scales, from the
annual rise and fall of atmospheric carbon dioxide levels to the swings from
icehouse to greenhouse over millions of years."
Is it possible to purposely use smectites to reduce global carbon emissions
even more? Murray sees some opportunity, such protecting carbon stores like
permafrost zones. Permafrost is expected to melt as a result of global
warming, exposing long-buried organic carbon. The clays in these areas may
stop this exposed carbon from escaping and warming the atmosphere even more
if smectites were put there.
"If you want to understand how nature works, you have to understand it on
the mineral and grain scale," explains Jagoutz. Additionally, this is how we
will go ahead in order to address this climate disaster. There's a
considerable possibility you may discover something truly helpful if you
examine these natural processes."
Provided by
Massachusetts Institute of Technology
