Updated Temperature, Salinity and Strength Distributions for Old Ice Available to Purchase

This paper presents updated distributions of the temperatures, salinities and borehole strengths of old ice, as a function of time of year. Thousands of measurements are compiled from old ice floes across the Arctic. Probability of exceedance curves are used to show the more ‘extreme’ values that can occur at various depths in the ice, whereas properties over the full ice thickness are obtained from (i) the means of populations and (ii) by depth-averaging measurements from the same borehole. Ice temperature and borehole strength show strong seasonal variability (temperature and borehole strength being inversely proportional), whereas the mean salinity of old ice varies little over the course of a year (on average, 1.8‰ for all months). The compilation showed that temperatures colder than approximately -34°C have not been measured in old ice in the marine environment during the three coldest months of the year. Borehole strengths upwards of 40MPa have been measured at various depths in old ice. Since such cold temperatures (-34°C) and high strengths (40MPa) are not representative of the full thickness of old ice, particular attention is paid to the range of depth-averaged ice temperatures spanning all months (0 to -20°C). Flexural strengths calculated from the Timco and O’Brien equation (for old ice having a mean salinity of 1.8‰ – which incidentally is also the mean bulk salinity of old ice from the data compilation – are comparable to the limited number of flexural strengths measurements from multi-year ice specimens. Normalized values of mean borehole strength and calculated flexural strength (for 1.8‰ ice) exhibit similar trends of decreasing strength with increasing ice temperature however, the Timco and O’Brien equation tends to underestimate the flexural strength of old ice near the melting point (ice temperatures warmer than -3°C). That, coupled with the relatively constant bulk salinity of old ice throughout the year, suggests that brine volume plays a lesser role in the deterioration of old ice than first-year ice. As a rough rule of thumb, the mean depth-averaged strength of old ice with a bulk ice temperature of -10°C, -5°C and 0°C decreases to respectively ~85%, 70% and 30%, relative to its maximum winter strength (-20°C ice temperature). On average, the mean, depth-averaged borehole strength of old ice is 30 times higher than the calculated flexural strength of 1.8‰ old ice, over the ice temperature range -1°C to -20°C. Caution should be exercised when applying results in this paper, given the considerable variability in old ice strength with temperature.