Abstract. Field investigations of the properties of heavily melted “rotten” Arctic
sea ice were carried out on shorefast and drifting ice off the coast of
Utqiaġvik (formerly Barrow), Alaska, during the melt season. While no
formal criteria exist to qualify when ice becomes rotten, the objective
of this study was to sample melting ice at the point at which its structural and
optical properties are sufficiently advanced beyond the peak of the summer
season. Baseline data on the physical (temperature, salinity, density,
microstructure) and optical (light scattering) properties of shorefast ice
were recorded in May and June 2015. In July of both 2015 and 2017, small
boats were used to access drifting rotten ice within ∼32 km of Utqiaġvik. Measurements showed that pore space increased as ice
temperature increased (−8 to 0 ∘C), ice salinity
decreased (10 to 0 ppt), and bulk density decreased (0.9 to
0.6 g cm−3). Changes in pore space were characterized with thin-section
microphotography and X-ray micro-computed tomography in the laboratory. These
analyses yielded changes in average brine inclusion number density (which
decreased from 32 to 0.01 mm−3), mean pore size (which
increased from 80 µm to 3 mm), and total porosity (increased from
0 % to > 45 %) and structural anisotropy (variable, with
values of generally less than 0.7). Additionally, light-scattering coefficients
of the ice increased from approximately 0.06 to > 0.35 cm−1 as the ice melt progressed. Together, these findings indicate that
the properties of Arctic sea ice at the end of melt season are significantly
distinct from those of often-studied summertime ice. If such rotten ice were
to become more prevalent in a warmer Arctic with longer melt seasons, this
could have implications for the exchange of fluid and heat at the ocean
surface.
Monitoring abiotic degradation in sinking versus suspended Arctic sea ice algae during a spring ice melt using specific lipid oxidation tracers