Icebergs Can Be Green

While most icebergs appear white or a brilliant blue, mariners and scientists have long been puzzled by rare, jade-colored behemoths drifting in the Southern Ocean. The typical white color of an iceberg is caused by tiny air bubbles in the ice scattering light, while dense, bubble-free glacial ice appears blue for the same reason our sky does. Green icebergs, however, are fundamentally different. They are formed not from compressed snow but from frozen (Review) ocean water, known as marine ice, that has collected on the underside of an ice shelf. This ice is crystal clear and lacks air bubbles, setting the stage for its unusual coloration.

The secret to their green hue is a story of geology and light. As Antarctica's massive glaciers flow over the continent, they grind (Review) the underlying bedrock into a fine powder called glacial flour. This powder, rich in iron oxides, is washed out into the sea and gets trapped as the seawater freezes to the bottom of the ice shelf. When a piece of this iron-laden marine ice breaks away, it carries its mineral cargo with it. The yellowish-red iron particles act as a color filter. When blue light from the sun passes through the dense, pure ice, it interacts with the suspended iron particles, resulting in the spectacular green light that we see.

These floating emeralds are more than just a geological curiosity; they are vital to the Antarctic ecosystem. As they drift and melt, they slowly release their iron into the ocean. Iron is a crucial nutrient for phytoplankton, the microscopic plants that form the base of the marine food web. In the often iron-poor Southern Ocean, these green icebergs act as massive, mobile fertilization packets, seeding the water (Review) with life-giving nutrients and creating temporary oases for marine life.