Iceberg A-68: The Story of How a Megaberg Changed the Ocean

Icebergs are created unique and unusual temporary ecosystems around you – A-68 did this on a huge scale. In the surface waters around the iceberg, Tarling and his colleagues discovered increased nutrient levelsincluding nitrates and phosphates. Concentrations were more typical of those found in deeper waters. What appears to have happened is that less dense fresh water melting on the underside of the iceberg, which extended up to 463 feet (141 m) below the surface, picked up these nutrients from the deeper seawater and carried them upward with it.

These nutrient-rich meltwaters were dominated by ice-associated algae: species that prefer to live in or near ice. “They can handle large differences in salinity,” Tarling says, which allows them to survive by moving between the fresh water of the iceberg and the salty seawater.

“You get a sort of halo effect,” Tarling says. Algae associated with ice bloomed around A-68a and its “children” icebergs. This attracted tiny animals called zooplankton to feed on them. If the research ship had stayed longer, Tarling says, they likely would have seen larger animals arrive to feed on the zooplankton.

This probably included baleen whales, the largest animals on Earth. “The whales will definitely be there and thriving on this kind of productivity, given a few more weeks,” Tarling says. “They are great at finding areas of productivity. That’s why they thrive.” He suggests they might even know that icebergs tend to leave flowers behind. “They’re really smart animals.”

The giant iceberg also affected the overall structure of the waters around it, but not in the same way as most icebergs. Typically, freshwater discharge creates stratification, in which layers of water stacked on top of each other are less mixed than normal. “If you have fresh water at the surface, you essentially increase stratification,” Senedez says. “You make it more stable, so it’s harder to mix.”

But not around A-68a. “Everything happened so quickly that it completely changed the whole dynamic of what was happening,” Tarling says. “What was happening was these huge piles of water were coming in.” The weight of fresh water sank the lower layers so that conditions normally found at a depth of 164 feet (50 m) were instead found at a depth of 328 feet (100 m).

Any food particles drifting in the water were also thrown down. “This deepening of the water masses creates an effect that we’ve never seen before, taking all of these solids with it,” Tarling says.

This may actually have increased the amount of carbon buried at the bottom of the Southern Ocean. Typically, organic material slowly floats down the water and some of it is eaten, so only some reaches the seabed and lingers there. But the crushing weight of fresh water from A-68a as it melted may have helped push the carbonaceous material faster to depths where it was less likely to be consumed.

“Nobody ever reported it,” Tarling says. The implication is that massive icebergs like A-68 could help transport carbon into the deep sea, slightly reducing the concentration of greenhouse gases in the atmosphere.