Drift-dependent changes in iceberg size-frequency distributions

Abstract

Although the size-frequency distributions of icebergs can provide insight into how they disintegrate, our understanding of this process is incomplete. Fundamentally, there is a discrepancy between iceberg power-law size-frequency distributions observed at glacial calving fronts and lognormal size-frequency distributions observed globally within open waters that remains unexplained. Here we use passive seismic monitoring to examine mechanisms of iceberg disintegration as a function of drift. Our results indicate that the shift in the size-frequency distribution of iceberg sizes observed is a product of fracture-driven iceberg disintegration and dimensional reductions through melting. We suggest that changes in the characteristic size-frequency scaling of icebergs can be explained by the emergence of a dominant set of driving processes of iceberg degradation towards the open ocean. Consequently, the size-frequency distribution required to model iceberg distributions accurately must vary according to distance from the calving front.

Figure 1

The lognormal shift in iceberg size magnitude-frequency distributions with distance away from the calving front at Jakobshavn Isbræ. (a) Location of the study site in West Greenland and (b) the area surrounding Vaigat displaying regional bathymetry and the locations referred to in the text. Seismometers are displayed as red dots with their 15 km effective detection radiuses shown as blue circles. (c) Idealised comparison between the probability distributions, p(x), of an inverse power-law (green line 1) and two lognormal distributions (curves) of decreasing complexity, labelled 2. and 3. As the hierarchy of processes responsible for the lognormal distribution becomes more complex (curve 3. to 2), the distribution becomes broader, providing a greater degree of overlap with the inverse power-law distribution (line 1). The complexity of the breakage process responsible for generating the magnitude-frequency distribution of iceberg sizes decreases with distance away from the calving front of Jakobshavn Isbræ owing to the emergence of a dominant set of decay mechanisms. As a result, the power-law distribution of iceberg sizes initially present proximal to Jakobshavn Isbræ (1) evolves towards lognormal scaling, and becomes more characteristically lognormal, as icebergs transit from Jakobshavn Isbræ to Vaigat (2) and towards the open ocean (3). Bathymetric data is obtained from the IBCAO V. 3.0 dataset and drawn using ArcMap 10.3.

Figure 2

Cumulative size-frequency distributions, Pr(E ≥ e), for the energy released by iceberg fragmentation processes: (a) cracking, (b) microfracturing, (c) calving and rolling, (d) all detected events. Optimal lognormal and power-law approximations of the data are displayed as dashed red and solid blue lines, respectively.

Figure 3

Optimal lognormal (red line) and power-law (blue line) approximations of the distribution of planform iceberg areas located: (a) within Vaigat and (b) proximal to Jakobshavn Isbræ. The slope of the fitted power law in (b) is 2.4.