Relationship of physical and mechanical properties of sea ice during the freeze-up season in Nansen Basin

Sea-ice properties, such as porosity and strength, can have significant spatial variability at small scales. Quantifying this variability may give better estimates of the ice properties and their interrelation. Additionally, correlating in situ ice strength measurements and ice properties, including porosity, may improve understanding of the factors influencing ice strength. This paper presents measurements of sea-ice properties and strength on first- and second-year sea ice during the GoNorth expedition to the Arctic Ocean in October 2022. In situ borehole indentation measurements were co-located with measurements of physical properties, and the meter-scale variability of the physical properties and strength was investigated. Bulk density values found from hydrostatic weighing were 911 ± 5 kg m⁻³ for first-year and 904 ± 5 kg m⁻³ for second-year ice, with significantly less uncertainty than density values from the mass/volume method or from freeboard/draft measurements. The second-year ice was relatively saline, with a mean bulk salinity of 3.1 ± 0.5, and the ice was desalinated in the upper and lower 0.2 m. The bulk gas fraction in the second-year ice was 2.5 ± 0.5 %, similar to the first-year ice gas fraction of 2.8 ± 0.5 %. Gas fractions up to 6.5 % were observed in the second-year ice without any obvious correlation with the brine fraction. The second-year ice had larger spatial variability in thickness, porosity, grain structure, and ice strength compared to the first-year ice. Variability in bulk density and gas fraction were similar for first- and second-year ice, as the larger variability was mostly seen below the upper 0.4 m of the second-year ice. The borehole strength was 26.0 ± 4.4 MPa for first-year and 41.0 ± 12.1 MPa for second-year ice. There were indications that the total microporosity at indentation depth was related to in situ borehole strength ($R^2$ = 0.82), and that brine volume was the most influential parameter. The relative variability in the local microporosity in the second-year ice (0.43) was greater than the relative variability in borehole strength (0.27), while the opposite was true for the first-year ice (0.09 versus 0.17).