This project will investigate the use of naturally-occurring radon as a tracer for gas exchange in seasonally ice-covered waters in the Arctic Ocean, using ships of opportunity for sampling. At its maximum extent, sea ice covers nearly 10% of the ocean surface, which creates an important control on the exchange of heat and biogenic gases (CO2, O2, DMS) between the ocean and atmosphere. In the Arctic, the summer minimum in sea ice cover is rapidly decreasing and this implies a greater oscillation between the minimum and maximum sea ice extents. The effect of this increased oscillation on the net air-sea exchange of gases is not certain because sea ice is interconnected with upper ocean physics and biology. What appears certain, is that the Arctic surface ocean will experience greater variability in ice cover and this increase in variability emphasizes the need for a predictive map of air-sea gas exchange versus the forcing conditions in the sea ice zone. Sea surface convection during freezing, stratification by meltwater, and the presence of interspersed ice floes may exert important controls on the gas transfer velocity and flux of biogenic gases. A well-constrained determination of the flux of CO2 in sea ice, and in turn a well-constrained budget of polar ocean carbon, will require more detailed knowledge of gas transfer velocity in the presence of sea ice. This study proposes to make an exploratory map of the scaling relationship between the gas transfer velocity and the forcing conditions in the seasonal sea ice zone, by combining estimates of the gas transfer velocity from radon measurements with forcing conditions (such as wind and upper ocean turbulence) from Arctic Observing Network measurements and from model results. In addition to its impact on understanding carbon cycling, the project will support an early career investigator, and a graduate student.
Radon is a naturally occurring radioactive gas that forms from the radioactive decay of radium. It has a half-life of 3.8 days compared to a half-life of 1620 years for radium. In the ocean radon forms from the radioactive decay of radium dissolved in the ocean water. For most of the ocean radon is in equilibrium with radium and the production of radon from radium radioactive decay equals the destruction of radon from its radioactive decay. However, in the surface water, radon escapes to the overlying atmosphere by gas exchange resulting in a deficit of radon with respect to its production from dissolved radium. The gas exchange rate can be determined from the size of the radon deficit and the known half-life of radon and can be used to calculate the uptake or evasion of other gases, such as oxygen and carbon dioxide, from the ocean. In this study, radon and radium measurements were made at various locations in the Arctic Ocean that were totally or partially ice covered and the gas exchange rate was determined from these measurements to investigate the impact of ice cover on gas exchange. This information will lead to a better understanding of the exchange of oxygen and carbon dioxide between the Arctic Ocean and the atmosphere and thus to a better understanding of uptake of carbon dioxide by phytoplankton during photosynthesis.
