An upside-down glacier of blue ice
Beneath the vast, white expanse of the Amery Ice Shelf in East Antarctica lies a hidden world of brilliant blue ice. This is not ordinary glacial ice. It is marine ice, formed from frozen ocean water that accretes to the bottom of the shelf. This "upside-down" glacier is created through a complex process of melting and refreezing driven by ocean circulation deep beneath the floating ice, which can be over 650 meters thick.
As the immense ice shelf, fed by the Lambert Glacier, pushes out into Prydz Bay, warmer, saltier ocean water circulates deep underneath. This causes melting at the base of the ice shelf in its southern regions. The resulting meltwater mixes with the seawater, making it colder and fresher. As this supercooled water circulates northward, it refreezes onto the ice shelf's base as frazil ice, a collection of small, needle-like ice platelets. Over time, these platelets consolidate into a solid layer of marine ice. This layer can become several hundred meters thick in places.
Unlike the meteoric ice of the main glacier, which is white from trapped air bubbles, this marine ice is exceptionally clear and dense. Its crystalline structure scatters blue light, giving it a spectacular color. When parts of the ice shelf break away, this material sometimes flips over, revealing stunningly blue or jade-colored icebergs.
A conveyor belt for ocean nutrients
The formation of this marine ice concentrates for oceanic materials. As frazil ice crystals form, they scavenge and incorporate sediments, dissolved organic matter, and micronutrients from the seawater. The porous nature of the newly formed ice traps this material, which becomes locked in as the ice consolidates.
Iron is a micronutrient, a micronutrient that is scarce in the Southern Ocean and limits the growth of phytoplankton, the base of the marine food web. The marine ice of the Amery Ice Shelf becomes a reservoir of this bioavailable iron. As the ice shelf flows towards the sea and its front edge calves off icebergs or melts, this iron-rich blue ice is released into the sunlit surface waters of Prydz Bay.
The sudden injection of this potent natural fertilizer triggers massive phytoplankton blooms. Satellite imagery often shows these blooms downstream from the Amery Ice Front, evidence of the powerful connection between the sub-glacial freezing processes and the surface ecosystem. The soluble iron supplied by the melting marine ice can be an order of magnitude larger than what is needed to sustain the bay's large annual phytoplankton bloom. This process supports a highly productive marine environment, feeding everything from tiny krill to the region's penguins, seals, and whales.