Why can air and water pass through the upper layers of a newly calved iceberg?

Air and water can pass through the upper layers of a newly calved iceberg because this region is composed of firn rather than solid ice.1

In these upper layers, the material has not yet reached the density required to become impermeable. Specifically, the density of the surface material may be as low as 400 kg per cubic metre, leaving open spaces between the crystal grains that allow fluids to percolate. It is only when the material reaches a density of approximately 800 to 840 kg per cubic metre—typically found at depths of 40 to 60 metres—that the air channels collapse and seal off, rendering the deeper ice impermeable to fluids.12