The Movement of CO2 Through the Frozen World of Sea Ice

Every winter, a frozen blanket known as sea ice completely covers the Arctic Ocean. For centuries, sea ice has been viewed as a solid lid on the ocean that acts as a boundary to block gases traveling between the ocean and the atmosphere. However, scientific discoveries over recent years have shown that sea ice is more like a sponge, a porous substance that is also home to microscopic life forms. The pores in sea ice are filled with very salty liquid called brine that is rich in carbon dioxide (CO2). These liquid pockets create a network of tubes or channels that move gases like CO2, similar to the way veins and arteries move blood in our bodies. In this article, you will discover how CO2 enters, exits, and is transformed in one of the harshest environments on Earth.

The seasonal sea-ice cycle in the Arctic. The orange line shows the monthly historical average from to . The red line shows , when the ice reached a record minimum.
is shown in blue. The left insert shows the sea ice cover at the end of the winter and the right insert shows it at the end of the summer. Since , the summer sea ice extent has drastically decreased. Maps and date are from the National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder, CO.

SEA ICE: A ZOOMED-OUT VIEW
On average, sea ice covers about million square kilometers of the

SEA ICE
Is frozen seawater that floats on the ocean surface. It is composed of ice crystal and salty liquid pocket called brine.
Earth's oceans, or about two-and-a-half times the area of Canada [ ].
Due to its size, sea ice is visible from space as a large white blanket on the ocean. By observing sea ice at these vast scales, we can see dramatic changes to the ice extent throughout the year and over decades-since , when the first satellite observations of sea ice were made.
Each year, as the sun sets and winter begins in the Arctic Ocean (in the far North) or the Southern Ocean (in the far South of Antarctica), sea ice forms when air temperatures decrease, and the ocean begins to freeze. As winter continues, sea ice thickens and grows outwards to cover vast areas of the ocean. In some places in the Arctic, sea ice even grows to be many meters thick! As the sun rises and the air warms in the spring, the sea ice begins to melt and break up, exposing the liquid ocean below. We call this expansion and contraction of sea ice a seasonal cycle (Figure ).
Comparing sea ice from year to year, we find that the amount of sea ice covering the ocean is changing. This long-term change is happening as the sea ice continues to grow and melt as part of its yearly seasonal cycle (Figure , blue and orange lines). In the Arctic, sea ice is melting more in summer than it used to, and we have already lost % of the summer sea ice since (Figure , yellow line). Scientists predict that, by , all the Arctic sea ice will completely melt during summer for the first time in history. This means that, although explorers can walk to the North Pole today, in the future they will have to sail to it. One of the great research questions of our time is how these changes are a ecting ocean life and our warming climate.

SEA ICE: A CLOSER LOOK
Zooming in on sea ice, to the scale of only a few centimeters, shows that it is complex. Pockets of salty liquid, known as brine, exist in the BRINE Is water with a high concentration of dissolved salt. sea ice (Figure ). Brine pockets are liquid at temperatures below zero because the salt prevents the liquid from freezing, and there is always some liquid in sea ice [ ]. Zooming in still further we find gas bubbles, salt crystals, and life within these brine pockets ( Figure , bottom panel). These brine pockets are a unique habitat for microscopic organisms and a place where chemical reactions happen. Scientists have been working to understand sea ice at these very small scales and see how sea ice a ects the chemical nature of the oceans and even life beyond the oceans.

CO IN THE ATMOSPHERE, OCEANS, AND LIVING ORGANISMS
Carbon is one of the most abundant elements on Earth, along with oxygen, nitrogen, and hydrogen. Carbon is found in the atmosphere as carbon dioxide (CO ) gas, in the ocean as dissolved CO , in some kinds of rock, and in all living organisms. Carbon is essential to life and you are made of about % carbon.
In the atmosphere, CO is a major gas that contributes to global warming [ ]. CO emitted by human activities (cars, the oil/gas industry, etc.) can move between the atmosphere, the oceans, and living organisms, and it changes forms as it moves. If CO is pumped into the deep ocean, it can be locked up there for hundreds of years, reducing global warming. The processes that move CO from the atmosphere into the ocean are called pumps. There are two main CO pumps in the ocean: the solubility pump and the biological SOLUBILITY PUMP Is a process that takes up CO from the atmosphere to the ocean's surface as dissolved CO and transports it to the bottom of the ocean.

BIOLOGICAL PUMP
Contributes to the ocean's role in taking up and storing CO from the atmosphere. The CO is transformed and stored by micro-organism as algae that use photosynthesis to grow. by human activities thanks to the solubility and biological pump.
The solubility pump (Figure , blue arrows) refers to the process by which atmospheric CO is absorbed by the ocean surface and become kids.frontiersin.org January | Volume | Article | Figure   Figure Sea ice a closer look. more CO than warm, salty water. Therefore, the cold polar oceans, like the Arctic Ocean, are great at taking up CO from the atmosphere. If the CO goes to the bottom of the ocean, it can stay there for years or more ( Figure ).
The biological pump refers to the use of CO by algae called phytoplankton. Algae are microscopic, single-celled organisms that Ice algae grow in brine pockets within the ice, in meltwater ponds at the surface, and most importantly at the base of sea ice, where the ice is touching the ocean below. Ice algae grow quickly, or kids.frontiersin.org January | Volume | Article | bloom, when light becomes available for photosynthesis in the spring PHOTOSYNTHESIS Is the process by which plants and algae make food. This chemical process uses sunlight, CO , and nutrients to produce sugars that the cell can use as energy to grow.

BLOOM
Is a rapid increase in the population of algae. An algal bloom is often recognized by the green or brown coloration of the water.
( Figure , green arrows). Ice algae known as diatoms are very good at growing quickly and largely make-up the ice-algae bloom. Although the diatoms are too small to count individually, scientists can see the bloom as a browning of the ice (Figure , bottom panel). The ice-algae bloom lasts until late summer when the cells have used up the nutrients needed to grow and when the ice around them begins to melt. Photosynthesis performed by ice algae can have a large impact on how much CO sea ice takes up in the spring (Figure , green  arrows). Generally, when the ice is brown with algae, it is expected that lots of CO is being taken up into the ice (Figure , bottom panel). Ice algae play an important role in using atmospheric CO , but they are also important for the animals living in the Arctic Ocean. The growth of ice algae supplies other organisms with a lot of food. The size of the algae bloom means that organisms can get lots of food very easily, like going to a vegetarian bu et for dinner. Nutrition gained from ice algae is transferred up the food web as one organism eats another, all the way to the polar bear.

WHAT HAPPENS TO CO TRAPPED IN SEA ICE?
As sea ice forms in winter, it traps salts and CO from the ocean in brine (Figure , bottom panel). In fact, so much CO gets trapped with salt that it is transformed into solid rock by chemical reactions. One of the most common rocks that forms from CO inside sea ice is made of calcium carbonate, also called limestone, which is the same

CALCIUM CARBONATE
Is a sedimentary rock like limestone. Calcium carbonate is produced by the precipitation (solidification) of dissolved calcium and CO in water.
substance that makes up the skeletons of corals and many seashells you might find on the beach. Researchers can see the tiny pieces of calcium carbonate when they melt a core of sea ice and look at it under the microscope (Figure , bottom panel). The CO in sea ice is also trapped in bubbles ( Figure , bottom panel). The bubbles can rise from the bottom of the sea ice to the surface through the brine channels. Once at the surface, the gases can escape into the air ( Figure  , white arrows). Sea ice also sends some CO to the bottom of the ocean. This process takes place during winter, when the salty brine from the sea ice sinks to the deep ocean, bringing CO along ( Figure  , white arrows). This is often referred to as the sea-ice pump, similar to the solubility and biological pumps described above. Through the rising bubbles and sinking brine, the sea ice loses a lot of CO that was trapped inside it. As a result, when the sun comes back in the spring, the sea ice no longer holds as much CO . Researchers have observed that, in the spring, sea ice can again absorb lots of CO from the atmosphere. Overall, researchers think that sea ice helps the ocean to absorb CO . So, sea ice helps us fight climate change.

KEY MESSAGES
Sea ice cover grows in winter and melts in summer. Thanks to the cold water and the presence of algae and sea ice, the Arctic Ocean is a carbon sink; it helps to decrease the amount of CO in the atmosphere. Firstly, sea ice algae use CO to grow and create food for larger organisms. Secondly, sea ice can trap CO in its brine and favor its transport to the bottom of the ocean. In the Arctic, the summer sea ice cover is strongly decreasing due to global warming. Global warming threatens the house of ice algae and the ability of the Arctic Ocean to exchange CO with the atmosphere.

ACKNOWLEDGMENTS
This work was a contribution to the Diatom-ARCTIC (Diatom Autecological Responses with Changes to Ice Cover) and EISPAC (E ects of ice stressors and pollutants on the Arctic marine cryosphere) project. This work was also supported by the international working group BEPSII: Biochemical exchange processes at Sea Ice Interfaces.

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

ODILE CRABECK
Odile Crabeck is a research fellow at the Fund for Scientific Research of Belgium, I study the sea ice the physical and biogeochemical state of the sea ice brine and bubbles, using fieldworks data and laboratory studies. *ocrabeck@uliege.be

KARLEY CAMPBELL
Karley Campbell is an Associate Professor of Marine Botany at The Arctic University of Norway, and an a liated researcher at the University of Bristol, UK. Her research brings together lab and field-based studies to determine how environmental change will a ect on sea ice microorganism activity, species composition and physiology.

SEBASTIEN MOREAU
Sebastien Moreau is a sea ice and ocean biogeochemist working at the Norwegian Polar Institute, Tromsø, Norway. His research focuses on phytoplankton and the biogeochemical cycle of carbon of polar oceans. He investigates these questions by using field observations, satellite data as well as D and D models.

MAX THOMAS
Max Thomas is a research fellow at the University of Otago, New Zealand. He uses laboratory observations and a range of modeling tools to study sea-ice physics.