LAVISH GIANT ICEBERGS ON THE SOUTHERN OCEAN

Carbon dioxide you breathe every day is stacked in the atmosphere, but it is constantly balanced by carbon fixation, i.e. building organic materials using atmospheric carbon. Although plant is a typical example of carbon fixation, about a half of carbon fixation is conducted through marine organisms. Among several vast oceans in the Earth, Southern Ocean takes a significant role on the global carbon sink, partly because this ocean is the starting point of cold bottom current.

At the carbon-rich ocean, a high concentration of dissolved iron means a bloom of lives. In marine environment, sources or nutritious sediments varies: islands, continental shelves, and icebergs. Although atmospheric iron input to ocean is the main source, in local areas, effects of terrestrial origin iron fluxes can be predominant.

As mentioned before, southern ocean has potential to consume a lot of carbon dioxide, but there are not enough iron to increase the number of lives. So, it is a powder keg for explosion of lives. Under this circumstance, if sediment-rich icebergs bombard irons, organisms will flourish in the seawater around the iceberg.

Despite of the large size, the giant icebergs have not been considered as a major factor of increased iron concentration and consequent blooming. Researchers already recognized the life-blooming near a giant iceberg, nonetheless they presumed that its effect is minor, due to unusual calving of giant icebergs.

Duprat et al. showed that the effect of giant icebergs, discovered by using remote sensing, is much greater than the previous expectations. First, a giant iceberg makes high chlorophyll concentration halo, shield-shaped at the front and tail-shaped along its rear. Secondly, the concentration of chlorophyll decays as the giant iceberg moves away from a certain location. Furthermore, this surge of chlorophyll became stronger when the iceberg floats on the Atlantic and Indian Ocean sector of the Southern Ocean. Authors explained that different arrangement of basement rock types, hard or soft, in Antarctica accounts for this location-based difference in chlorophyll levels.

A rough estimation of this research suggests that an episodic giant iceberg may contribute about 10~20 % of total carbon export of the Southern Ocean. Despite the large error range, this estimation is far beyond of the conventional thoughts. Considering recently occurring abrupt disintegrations of large ice bodies in Antarctica (e.g. Larsen Ice Shelf), which is supposed to be a respond to the climate change, giant icebergs and their strong impact on biological activity in the Southern Ocean need to be re-evaluated in terms of ecology and climatology.