YSM Issue 90.1

environmental science
FOCUS
Surging carbon dioxide levels,
combined with increased radiation
from the Sun, dramatically increased
global temperatures. Ocean surface
temperatures reached upwards of 104
degrees Fahrenheit. Consequently, mass
extinctions occured, with marine and
terrestrial life suffering huge losses in
biodiversity.
Until now, the scientific community
has struggled to determine the relative
importance of the two forces that drove
the Permian-Triassic mass extinction
event: volcanic activity and erosion. A
study led by Ryan McKenzie, a postdoctoral
associate at the Yale Department
of Geology and Geophysics, has now
proposed a solution. The study argues
that on the time scale of the past several
hundred million years, volcanoes have
been the principal driver of climate
change. These revolutionary findings are
key to understanding long-term climate
change, and thus, may prove informative
in our present-day combat against global
climate change.
The greenhouse effect
Carbon dioxide (CO 2
) is a double-edged
sword—both vital to life yet potentially
harmful. On one hand, much of Earth’s
plant life depends on CO 2
to produce
food for itself and consumers, like us. At
the same time, increasing CO 2
levels since
the industrial revolution have led to rising
global temperatures, which pose a risk
to the global ecosystem. How does this
happen?
The Earth’s atmosphere normally
reflects much of the Sun’s invisible
infrared radiation back into space,
thereby preventing surface temperatures
from becoming too high. However,
when sufficiently concentrated in the
atmosphere, CO 2
can form a blanket of
sorts, which traps some of this radiation
and prevents it from leaking back to space.
Thus, CO 2
is aptly termed a greenhouse
gas.
Various processes regulate the levels
of CO 2
in the atmosphere. Volcanic
eruptions, which release gases from the
Earth’s interior, contribute to atmospheric
greenhouse gases and raise global
temperature levels. Chemical weathering,
on the other hand, has the opposite
effect. When CO 2
reacts with water vapor,
carbonic acid is formed. This weak acid
then eats away at rocks and other surfaces.
Other forms of chemical weathering
include burial of carbonate minerals,
along with burial of organic carbon. Thus,
chemical weathering is a CO 2
sink and has
the ultimate impact of decreasing global
temperatures.
The scientific community recognizes
these two forces—volcanism and
weathering—as the principal drivers of
long-term climate change. Due to these
two processes oscillating and changing
pace over time, the content of CO 2
in the
atmosphere is in constant flux. Thus, the
Earth’s temperature has risen and fallen
multiple times within its history, creating
various periods of global warming
followed by global cooling in the form of
ice ages.
The unearthing begins
The study began with McKenzie’s
fascination with the links between climate
change and biodiversity. “I became
interested in the anomalies characteristic
of the Cambrian period,” McKenzie said.
“A lot of species extinction occurred,
which many people attribute to the
Cambrian having one of the highest
atmospheric carbon dioxide levels of the
past six hundred million years.” McKenzie
set out to discover the root cause of this
carbon dioxide flux.
First, McKenzie and team needed to
obtain a record of Earth’s volcanic history.
The Earth is made up of multiple tectonic
plates, which are large pieces of the Earth’s
crust. When an oceanic plate collides
with a continental plate, a subduction
zone is formed. The oceanic plate sinks
deeper into the earth, liberating water in
the process. This water gradually seeps
upward, melting the hot mantle rocks
and forming magma in the process.
This magma finally rises to the surface
and forms a chain of active volcanoes.
Unfortunately, however, it is often difficult
to track the formation of these volcanic
emissions through Earth’s history because
erosion and destruction of volcanoes
obscures the important data. In addition,
the commonly used sea-level approach to
track volcanic rates through time relies
on too many vague assumptions. This is
where zircons come in.
Zircons, otherwise known as zirconium
silicate, are grains of sedimentary rocks
that crystallize from magma. Young zircon
is especially prevalent in the subduction
zones of continental volcanoes, such as the
Andes and the Cascade volcanoes. Zircon
grains are able to withstand high degrees
of erosion, so they represent untampered
records of volcanic activity. Fortunately,
due to zircon’s uranium impurities, the
age of zircon samples can be determined
very precisely through radioactive
isotope analysis. Thus, if one can trace an
abundance of young zircon to a specific
period, this period likely experienced
massive continental volcanic activity.
McKenzie and his team used this property
of zircon to contribute to a precise record
of continental volcanic activity throughout
the geologic timeline. The team could now
accurately map the relationships between
carbon dioxide levels and volcanic activity.
IMAGE COURTESY OF RYAN MCKENZIE
►Dr. Ryan McKenzie stands with a fuming
Mt. Bromo in Indonesia. McKenzie analyzed
sedimentary rock to more closely link volcanic
emissions to long-term climate change driven
by carbon dioxide concentration.
www.yalescientific.org
December 2016
Yale Scientific Magazine
13