The iron fertilization hypothesis offered a silver lining: as temperatures rise and Antarctic glaciers melt, ice-trapped iron would be released into the Southern Ocean, feeding blooms of microscopic algae that pull carbon dioxide from the atmosphere. More melting, more iron, more algae, more carbon drawdown. A negative feedback loop — warming partially correcting itself.
Published in Communications Earth & Environment, Sherrell and colleagues at Rutgers traveled aboard the icebreaker Nathaniel B. Palmer to the Dotson Ice Shelf in the Amundsen Sea, collecting meltwater samples at the source. They found that the meltwater itself carries very little iron. Most of the iron near the ice shelf comes from the grinding and dissolving of bedrock into the liquid layer between the bedrock and the ice sheet — not from the ice that is driving sea level rise.
The structural insight is about misidentified sources. The iron is real. It is present in the waters near the ice shelf. But it does not come from where the hypothesis assumed. The ice contributes meltwater; the bedrock contributes iron. As the ice sheet retreats and thins, the meltwater increases but the iron source — bedrock grinding at the base — may not increase proportionally, and could even decrease as the ice lifts off the rock. The negative feedback loop assumed that melting and iron release were the same process. They are not. They are adjacent processes that happen to occur in the same location but respond to different physical drivers. The silver lining was real iron from the wrong source, and the source does not scale with the problem it was supposed to mitigate.