16.03.2015 | project secretary Mai korsbæk

Sea Ice: Source or sink for atmospheric CO₂?

Presentation by Nicolas-Xavier Geilfus, Post Doc, Arctic Research Centre

Arctic Ocean contributes to the global oceanic uptake of CO₂ by about 5% to 14% in taking up from 66 to 199 TgC yr^-1. However, the role of the marine cryosphere was ignored because sea ice is considered as an impermeable barrier, impeding the gas exchanges between the ocean and the atmosphere [Bates and Mathis, 2009]. However, a growing body of evidence suggests that gases exchange could occur between sea ice and the atmosphere. In this context, two arctic surveys were carried out in the framework of the International Polar Year (IPY). From there, we present a snapshot of the partial pressure of CO₂ (pCO₂) dynamics firstly during the initial sea ice growth and secondly from early spring to the beginning of the summer.

We confirmed previous laboratory measurement findings that growing young sea ice acts as a source of CO₂ to the atmosphere by measuring CO₂ efflux from the ice (4 to 10 mmol m^-2 d^-1). Further, we
show that the precipitation of calcium carbonate as ikaite in the frost flowers and throughout the ice plays a negligible role in the effluxes of CO₂. In early spring, supersaturations in CO₂ (up to 1834 μatm) were observed in sea ice as consequence of concentration of solutes in brines, CaCO₃ precipitation and microbial respiration. As the summer draw near, brine shifts to a marked undersaturation (down to almost 0 μatm) because of the brine dilution by ice meltwater, dissolution of CaCO₃ and photosynthesis during the sympagic algal bloom. During the end of winter, soon as the ice becomes permeable, CO₂ fluxes were observed: (i) from the ice to the atmosphere, as the brine were supersaturated, (ii) from the atmosphere to the ice, as brine shift to an undersaturation. Temperature appears to be the main driver of the pCO₂ dynamics within sea ice. It mainly controls the saturation state of the brine (where others processes may be added, e.g., CaCO₃ precipitation, primary production) and thus, the concentration gradient of CO₂ between sea ice and the atmosphere. It also controls the brine volume and so the brine connectivity, allowing the gas exchanges between sea ice and the atmosphere.

The Arctic Seminars is a forum for interdisciplinary discussions on research perspectives in the Arctic region. The presenters in the Seminar series come from a variety of disciplines across the four faculties involved in ARC.  All interested are welcome to participate.