WHOI Associate
Scientist Valier Galy lowers a sampling device into the Mackenzie River.
Chemical analysis of the collected samples revealed that the river’s transport
of organic carbon creates
a carbon sink, burying CO2 in Arctic Ocean sediments
without releasing it into the atmosphere.
(Photo by Robert
Hilton, Durham University)
New study traces the fate of carbon stored in thawing Arctic
soils
(August 6, 2015) As
temperatures rise, some of the organic carbon stored in Arctic permafrost meets
an unexpected fate—burial at sea. As many as 2.2 million metric tons of organic
carbon per year are swept along by a single river system into Arctic Ocean
sediment, according to a new study an international team of researchers
published today in Nature. This process locks away carbon dioxide (CO2) - a
greenhouse gas - and helps stabilize the earth’s CO2 levels over time, and it
may help scientists better predict how the natural carbon cycle will interplay
with the surge of CO2 emissions due to human activities.
“The erosion of permafrost carbon is very significant,” says
Woods Hole Oceanographic Institution (WHOI) Associate Scientist Valier Galy, a
co-author of the study. “Over thousands of years, this process is locking CO2
away from the atmosphere in a way that amounts to fairly large carbon stocks.
If we can understand how this process works, we can predict how it will respond
as the climate changes.”
Northern Canada’s
Mackenzie River is the largest river flowing into the Arctic Ocean from North
America—and the dominant source of biosphere-derived organic carbon in Arctic Ocean
sediments, according to a new study. (Photo by Robert
Hilton, Durham University)
Permafrost—frozen ground found in the Arctic and in some
alpine regions—is known to hold billions of tons of organic material. Amid
concerns about rising Arctic temperatures and their impact on permafrost, many
researchers have directed their efforts to studying the permafrost carbon
cycle—the processes through which carbon circulates between the atmosphere, the
soil and plants (the biosphere), and the sea. Yet how this cycle works and how
it responds to the warming, changing climate remains poorly understood.
Galy and his colleagues from Durham University, the Institut
de Physique du Globe de Paris, the NERC Radiocarbon Facility, Stockholm
University, and the Universite Paris-Sud set out to characterize the carbon
cycle in one particular piece of the Arctic landscape—northern Canada’s
Mackenzie River, the largest river flowing into the Arctic Ocean from North
America and that ocean’s greatest source of sediment. The researchers
hypothesized that the Mackenzie’s muddy water might erode soils along its path,
some from places where permafrost is melting, and wash that biosphere-derived
material and the organic carbon within it into the ocean, preventing the degradation
of organic carbon and associated release of CO2 into the atmosphere.
The researchers
collected samples (denoted by circles) at three locations along the Mackenzie
River—the river delta (black), Tsiigehtchic (grey), and Norman Wells
(white)—along with its major tributaries, the Liard River (red diamond), the
Arctic Red River (light blue square), and the Peel River (dark blue square).
They compared the chemical composition of these samples with those obtained
from the sediment core MTW01 in the Arctic Ocean’s Mackenzie trough
(triangle). a, The river’s major channels
(black lines) are overlain on a digital elevation model that shows sediment
catchment areas and flow accumulation and flow direction with dotted lines. b,
A map shows the status of permafrost in the upstream areas of the Mackenzie
River basin. c, White rectangles show the sample locations near the Mackenzie
River delta overlain on satellite imagery of the basin.
(Illustration
courtesy of Hilton, et al)
The researchers collected samples at various depths and
locations along the river system, lowering a specially-designed device to take
samples of the water and suspended sediments carried by the river. To take into
consideration the river’s seasonal variation—its flow increases sharply during
the spring, when warm temperatures melt the snowpack and raise water levels,
and drops during the frozen winter months—they sampled it during different
seasons across three years starting in 2009.
Then the researchers sifted through the samples to isolate
the carbon they contained. They used the presence of one specific isotope of
carbon that decays over time, carbon-14, to determine how old the carbon was.
This was important because it revealed the carbon’s origin– rock or biosphere.