New research from Rutgers University reveals that meltwater from Antarctic ice shelves contributes far less iron to surrounding ocean waters than scientists had assumed. Instead, most iron originates from deep ocean water and continental sediments. The findings challenge expectations about iron fertilization and its role in carbon absorption.
For years, scientists have considered iron released from melting Antarctic glaciers as a potential natural mechanism to combat climate change. The concept of iron fertilization suggested that this iron would stimulate algal blooms in the Southern Ocean, which in turn absorb carbon dioxide through photosynthesis. However, a recent study published in Communications Earth and Environment disputes this idea based on direct field measurements.
In 2022, researchers led by Rob Sherrell, a professor in the Department of Marine and Coastal Sciences at Rutgers University-New Brunswick, conducted expeditions aboard the U.S. icebreaker Nathaniel B. Palmer to the Dotson Ice Shelf in the Amundsen Sea of West Antarctica. This region is significant as it drives much of the sea level rise from Antarctic ice melt. The team sampled water at entry and exit points of cavities beneath the ice shelf, where warm deep ocean water mixes with glacial meltwater.
Lead author Venkatesh Chinni, a postdoctoral scholar at Rutgers, analyzed iron concentrations, including dissolved and particulate forms. Collaborators Jessica Fitzsimmons and Janelle Steffen from Texas A&M University, along with Tim Conway from the University of South Florida, examined isotopic ratios to trace iron origins. The results showed that meltwater accounts for only about 10% of the dissolved iron exiting the cavity. In contrast, 62% comes from deep ocean water, and 28% from sediments on the continental shelf.
"Roughly 90% of the dissolved iron coming out of the ice shelf cavity comes from deep waters and sediments outside the cavity, not from meltwater," Chinni stated. Sherrell noted that earlier estimates of iron from meltwater were several times higher. Isotope data also indicated a subglacial liquid layer low in dissolved oxygen, which may dissolve iron from bedrock more effectively than ice melt itself.
"Our claim in this paper is that the meltwater itself carries very little iron, and that most of the iron that it does carry comes from the grinding up 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," Sherrell explained. These insights suggest a need to revise climate models regarding iron sources in the Southern Ocean, which plays a crucial role as the world's largest oceanic sink for CO2. The researchers call for further studies on subglacial processes to better understand their impact on ocean chemistry and global climate dynamics.