A carbon sink shrinks in the Arctic
Canada Basin's diminished capacity to absorb carbon dioxide

Date:
June 15, 2020
Source:
University of Delaware
Summary:
Ice melts in the Arctic Ocean were thought to be drawing large
amounts of carbon dioxide out of the atmosphere, acting as a carbon
sink and helping to mitigate greenhouse gases. But new research
shows that may not be the case in all areas, particularly in the
Canada Basin, where the carbon sink is shrinking, inhibiting the
ocean's ability to absorb carbon dioxide from the atmosphere into
the deep ocean and store it there.



FULL STORY ==========================================================================
New research by University of Delaware doctoral student Zhangxian Ouyang
and oceanographer Wei-Jun Cai, and an international team of researchers, demonstrates that rapid warming and sea-ice loss have induced major
changes in the western Arctic Ocean.


==========================================================================
The research team's findings -- published Monday, June 15 in Nature
Climate Change -- show that the Arctic Ocean's ability to remove carbon
dioxide from the atmosphere can vary greatly depending on location.

Arctic Ocean sea-ice loss is a critical consequence of climate change. As
sea ice continues to melt in the western Arctic Ocean, more fresh water
is entering the upper portion of the water in the Canada Basin, which
sits off the coast of Alaska and Canada, south of the Chukchi Shelf.

This summertime melt cycle is exacerbating seasonal changes and increasing
the amount of carbon dioxide present in the water's topmost layer, which comprises the upper 70 feet of the water column. This is reducing the
basin's capacity to remove carbon dioxide from the atmosphere.

Prevailing thought, based on data measurements from under the ice and
in newly melted ocean margin areas in the 1990s and early 2000s, had
suggested that when the ice melted it would allow the Arctic Ocean to
draw large amounts of carbon dioxide out of the atmosphere, acting as a
carbon sink and helping to mitigate greenhouse gases. However, this may
not be the case in all places, particularly in the Canada Basin where
summer ice retreat has advanced into the deep basin since 2007.

The research team's latest findings are based on an analysis of over
20 years of global data sets collected between 1994-2017 by researchers
across the United States, China, Japan and Canada. They provide a more
accurate depiction of what is happening in this region and build on Cai's previous work from 2010, which indicated that carbon dioxide levels at
the sea surface increase rapidly and unexpectedly toward levels found
in the atmosphere in newly ice-free Arctic Ocean basins.



==========================================================================
For example, the research team's work showed that as the ice breaks up and melts in the Canada Basin, this meltwater lays on top of the sea surface, creating a "blanket" of sorts that inhibits the ocean's ability to absorb carbon dioxide from the atmosphere into the deep ocean and store it there.

Cai's team refers to this phenomenon as a "new normal" that is created
by extreme seasonal warming and meltwater in the region.

"As carbon dioxide accumulates in the surface layer of the water from
melting ice, the amount of carbon dioxide this area of the Arctic
Ocean can take from the atmosphere will continue to shrink," said Cai,
the Mary A.S. Lighthipe Professor in the College of Earth, Ocean and Environment. "We predict by 2030, the Canada Basin's ability to serve as
a carbon sink will be really minimal." Additionally, this rapid increase
of carbon dioxide content in the basin may have rapidly acidified the
surface water, a process that can endanger marine calcifying organisms
and disrupt ecosystem functioning there.

In stark contrast, farther south in the shallow Chukchi Sea, the amount
of carbon dioxide in the water's topmost layer remains very low, much
lower than what is present in the atmosphere. This means that as air
passes over the water's surface, the sea can more quickly absorb carbon
dioxide from the air.

The researchers suggest that this difference is the result of high
biological production in the Chukchi Sea due to rich nutrients being transported there on currents coming from the Pacific Ocean since the
Bering Strait has opened up due to earlier ice loss. These nutrients
enable abundant growth of phytoplankton and other marine organisms
that form the base of the marine food web and feed the broader
ecosystem. Phytoplankton also consume carbon dioxide dissolved in the
water during photosynthesis, allowing more carbon dioxide to be taken
from the surrounding atmosphere.



==========================================================================
The research team suspects that the Chukchi Sea will become a larger
carbon sink in the future and impact the deep ocean carbon cycle and
ecosystem, while the Canada Basin likely will remain less so as sea ice
in the region continues to melt and change the water chemistry.

According to Lisa Robbins, a retired senior scientist with the United
States Geological Survey (USGS) and a co-author on the paper, these
changes could have important implications for organisms in the Arctic. For instance, Arctic cod is an important fishery in the western Arctic that contributes to the region's overall economy and serves an important role
in the marine food web as a food source for other organisms, such as
Beluga whales and ringed seals. Biologists have noted that as temperature
and sea ice melt have increased, Atlantic cod are responding by moving
farther north. Changing water chemistry also may be playing a role,
said Robbins, who led three expeditions to study the region's water
chemistry in the Arctic aboard the United States Icebreaker R/V Healy
while with the USGS.

Long-term data sets, such as those used in this study, are key to
understanding and predicting future changes in the Arctic.

"The amount of insight we get from these data sets into how our
earth-ocean works is tremendous. If scientists hadn't collected data in
1994, we wouldn't have a place to start and compare with," said Robbins,
now a courtesy professor in the College of Marine Science at University
of South Florida.

A 2019 article in Wired magazine found that in northern Canada near
Greenland, glacial meltwater seems to be aiding watersheds in absorbing
carbon dioxide from the atmosphere. While alone it cannot counterbalance
the amount of carbon dioxide in the atmosphere due to carbon emissions,
it is an important illustration that the changes aren't uniform and
the subsequent effects - - positive and negative -- are the result of a
complex combination of multiple different drivers. Further research and
more international collaborative efforts can help to answer challenging unanswered questions.

As sea-ice loss accelerates, the researchers expect these seasonal
variations will cause the ocean water in the Canada Basin to have high
levels of carbon dioxide and become increasingly acidic. This will
further reduce the basin's capacity to take up carbon dioxide from the atmosphere and potentially reduce its capacity to mitigate climate change.

While this problem might seem very far away from Delaware, it's important
to remember that the ocean is one global system with circulation currents
that transport water around the world, even to the Atlantic Ocean on
the East Coast.

And greenhouse gases are a global issue.

Understanding how fundamentally important ice melt is to driving
carbonate chemistry and seasonal changes in carbon dioxide in this
region of the Arctic Ocean will help advance the science in this area,
maybe not immediately but over the long-run, said Cai.

"We are trying to understand the processes at work and if the Arctic
Ocean will continue to be a large carbon sink, while providing data that
can help Earth systems modelers to predict global changes to the carbon
cycle, and the ocean's biology and water chemistry," Cai said.

This work is funded by multiple nations, including Cai's work which
is supported through the National Science Foundation's Arctic Natural
Science Program.

Co-authors on the paper include researchers at The Third Institution of Oceanography (China), Columbia University, University of Montana, Ocean University of China, Japan Agency for Marine-Earth Science and Technology, University of South Florida and the International Arctic Research Center.


========================================================================== Story Source: Materials provided by University_of_Delaware. Original
written by Karen B.

Roberts. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zhangxian Ouyang, Di Qi, Liqi Chen, Taro Takahashi, Wenli Zhong,
Michael
D. DeGrandpre, Baoshan Chen, Zhongyong Gao, Shigeto Nishino,
Akihiko Murata, Heng Sun, Lisa L. Robbins, Meibing Jin, Wei-Jun
Cai. Sea-ice loss amplifies summertime decadal CO2 increase in
the western Arctic Ocean.

Nature Climate Change, 2020; DOI: 10.1038/s41558-020-0784-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615152106.htm

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