Spatiotemporal distributions of icebergs in a temperate fjord: Columbia Fjord, Alaska

Abstract

Much of the world's ice enters the ocean via outlet glaciers terminating in fjords. Inside fjords, icebergs may affect glacier-ocean interactions by cooling incoming ocean waters, enhancing vertical mixing, or providing back stress on the terminus. However, relatively few studies have been performed on iceberg dynamics inside fjords, particularly outside of Greenland. We examine icebergs calved from Columbia Glacier, Alaska, over 8 months spanning late winter to mid-fall using 0.5m resolution satellite imagery, identifying icebergs based on pixel brightness. Iceberg sizes fit a power-law distribution with an overall power-law exponent, m, of -1.26±0.05. Seasonal variations in the power-law exponent indicate that brittle fracture of icebergs is more prevalent in the summer months. Combining our results with those from previous studies of iceberg distributions, we find that iceberg calving rate, rather than water temperature, appears to be the major control on the exponent value. We also analyze icebergs' spatial distribution inside the fjord and find that large icebergs (10 000-100 000 m2 cross-sectional area) have low spatial correlation with icebergs of smaller sizes due to their tendency to ground on shallow regions. We estimate the surface area of icebergs in contact with incoming seawater to be 3.0±0.63 104 m2. Given the much larger surface area of the terminus, 9:7±3:7×105 m2, ocean interactions with the terminus may have a larger impact on ocean heat content than interactions with icebergs.