Imaging air volume fraction in sea ice using non-destructive X-ray tomography

Abstract

Abstract. Although the presence of a gas phase in sea ice creates the potential for gas exchange with the atmosphere, the distribution of gas bubbles and transport of gases within the sea ice are still poorly understood. Currently no straightforward technique exists to measure the vertical distribution of air volume fraction in sea ice. Here, we present a new fast and non-destructive X-ray computed tomography technique to quantify the air volume fraction and produce separate 3-D images of air-volume inclusions in sea ice. The technique was performed on relatively thin (4–22 cm) sea ice collected from an experimental ice tank. While most of the internal layers showed air-volume fractions < 2 %, the ice–air interface (top 2 cm) systematically showed values up to 5 %. We suggest that the air volume fraction is a function of both the bulk ice gas saturation factor and the size of the brine channel. We differentiate micro bubbles (∅ < 1 mm), large bubbles (1 < ∅ < 5 mm) and macro bubbles (∅ > 5 mm). While micro bubbles were the most abundant type of air inclusions, most of the air porosity observed resulted from the presence of large and macro bubbles. The ice microstructure (granular and columnar) as well as the permeability state of ice are important factors controlling the air volume fraction. The technique developed is suited for studies related to gas transport and bubble migration and can help considerably improving parameterization of these processes in sea ice biogeochemical models.

Sea Ice Formation, Glacial Melt and the Solubility Pump Boundary Conditions in the Ross Sea

“…Sea ice formation results in the partitioning of dissolved solutes between brine pockets that are included within the ice, and dissolved solutes that are immediately or eventually deposited in the water parcel beneath sea ice (Killawee et al, 1998). This process of solute exclusion is complex and evolves through several phases that include near instantaneous segregation during formation of individual ice crystals as well as the nucleation of bubbles at the freezing interface (Crabeck et al, 2015), and brine drainage, which happens over time as the individual brine pockets coalesce into brine channels (Feltham et al, 2006;Wettlaufer, 1992). Here, the treatment of brine drainage and solute exclusion is somewhat coarse, but is an adequate approximation given what residual solutes can be measured in the water column.…”

US SOLAS Science Report

“…There are also large uncertainties in air-sea exchange in partially ice-covered environments, since ice dampens air-sea gas exchange, whereas ice leads may experience increased turbulence relative to the open ocean. Traditionally, gas exchange in ice-covered environments has been treated similarly to open ocean settings and simplistically scaled by the fraction of the ice cover, which fails to capture key underlying physical, chemical, and biological processes (Crabeck et al, 2014;Else et al, 2013, Fransson et al, 2017Gourdal et al, 2019). Coastal and estuarine systems are responsible for a large drawdown of CO2, and yet gas transfer in these systems is still highly uncertain due to limited fetch, coastal currents, shallow waters, and increased surfactants.…”