Dispatch 25: Measuring CO₂ in the Arctic Ocean

Ashley Arroyo

October 10, 2022

Carbon dioxide (CO₂) is an important greenhouse gas, which traps and re-emits heat in the Earth’s atmosphere. The oceans are a significant carbon sink; the amount of CO₂ in the Earth’s atmosphere is of comparable magnitude to that absorbed by the ocean. Since colder surface waters can absorb more CO₂ than warmer waters, the Arctic Ocean plays an important role in global CO₂ uptake. Understanding how CO₂ is evolving in the Arctic Ocean is particularly relevant because of sea-ice decline; decreasing sea ice concentrations lead to the potential for increased air-sea gas exchange, which could alter carbon cycling and storage on local and global scales. For these reasons, it is very important to monitor CO₂ storage and air-sea fluxes in the Arctic Ocean.

Mike DeGrandpre is a professor in the chemistry department at the University of Montana, who is aboard the Louis taking measurements of pCO₂ (the partial pressure of CO₂ in a liquid or a gas) in the surface waters of the Canada Basin. On a day-to-day basis, Mike has been using an “underway system” which takes near-continuous measurements of pCO₂ of seawater from underneath the ship at a depth of 9 meters. Seawater is brought through stainless steel tubing into the ship’s main lab where it runs through an equilibrator. Within the equilibrator, a small volume of air from the lab interacts with the seawater, causing the pCO₂ of the air to “equilibrate,”5:  or become the same as the pCO₂ in the seawater. The equilibrated air is then sent into an infrared gas analyzer, which determines the pCO₂ of the air (which is the same as the pCO₂ of the sea water). The underway system produces a continuous record of pCO₂ throughout the entire expedition. Combining this data set with other measured parameters (i.e., atmospheric pCO₂, water temperature, sea ice concentration, wind speed, etc.), the air-sea flux of CO₂ into the Arctic Ocean can be computed.

In addition to the data that are recorded through the ships underway pCO₂ system, Mike along with the science crew from Woods Hole Oceanographic Institution (WHOI) have installed Submersible Autonomous Moored Instruments (SAMI-CO₂) on the Ice Tethered Profilers and moorings that have been deployed as a part of this year’s expedition. The SAMI-CO₂ sensors take measurements of pCO₂ at fixed depths on the instruments: the sensors are located at depths of 4 meters on the ITPs and 40 meters on the moorings. The data that will be produced from the SAMI-CO₂ sensors will be valuable in quantifying pCO₂ in Arctic Ocean surface waters to help us gain insight on carbon cycling and air-sea fluxes of CO₂ in the Arctic Ocean under a warming climate.