This article was adapted from .
For scientists who study the Southern Ocean, the theory of iron fertilization has offered a bright spot amid rising temperatures and melting glaciers in Antarctica. The theory posits that iron 鈥 once trapped in ice 鈥 will be released into the ocean as glaciers melt, feeding blooms of microscopic algae that would pull carbon dioxide from the atmosphere as they grow.
But that theory faces new challenges following what researchers describe as the most accurate measurement of iron inputs from a glacier in Antarctica. Led by marine scientists at Rutgers University-New Brunswick and including oceanographers from the University of South Florida, a new study suggests that meltwater from an Antarctic ice shelf supplies far less iron to surrounding waters than once thought.
The , published in the journal Communications Earth and Environment, raise questions about the sources of iron in the Southern Ocean near Antarctica, and could significantly alter how climate predictions are forecasted and modeled, the researchers said.
鈥淭his paper shows that melting ice shelves are not supplying much iron to the waters in the Amundsen Sea in West Antarctica, but their meltwater pump systems do bring iron-rich deep waters to the surface where they can be a driver of biological blooms and carbon uptake,鈥 said Tim Conway, associate professor at the USF College of Marine Science and a co-author of the study.
The research builds on previous work of Conway鈥檚 doctoral students Hung-an Tian and Zach Bunnell with similar findings in 2023.
Despite being shrouded in darkness for several months a year, the Antarctic waters of the Southern Ocean are a highly productive region for growth of phytoplankton, the vital food source for krill, which in turn feed penguins, seals, and whales. As phytoplankton grow, they absorb vast amounts of carbon dioxide through photosynthesis, making the region the for the climate-warming gas.
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While previous studies into iron sources in the Southern Ocean have primarily been through , researchers behind the new study took a different approach. In 2022, they traveled aboard a now-decommissioned U.S. icebreaker, the , to the Dotson Ice Shelf, located in the Amundsen Sea, to collect melting glacial water at the source. The Amundsen Sea accounts for most of the sea level rise .
鈥淚t has been widely assumed that glacial melting underneath ice shelves contributes considerable bioavailable iron to these shelf waters, in a process of natural glacier-driven iron fertilization,鈥 said , a professor in the Department of Marine and Coastal Sciences at the Rutgers School of Environmental and Biological Sciences and the study鈥檚 principal investigator.
Sherrell said the study modifies those assumptions by determining that the amount of iron in meltwater is several times lower than previously thought and that most of that iron comes from a different type of meltwater than is produced by ice shelves melting.
In the Amundsen Sea, glacial meltwater comes from beneath floating ice shelves 鈥 the seaward extensions of glaciers from the continent 鈥 and the melting is caused primarily by warm water that flows from the deep ocean into the cavities under the ice.
At the Dotson Ice Shelf, the team identified where seawater enters one such cavity and where it exits after meltwater is added. They collected water samples from entry and exit points.
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Back in New Jersey, Sherrell鈥檚 colleague , a postdoctoral scholar and lead author of the study, analyzed the samples for iron content in both its dissolved state and in suspended particles. Collaborators , a professor and chemical oceanographer, and , an assistant research scientist, both at Texas A&M University, measured the isotopic ratios to 鈥渇ingerprint鈥 and distinguish the sources.
Steffen carried out initial isotopic measurements in at the USF College of Marine Science.
Chinni and the team then calculated how much more iron was coming out of the cavity than went in and deduced from the isotopic data the type of melting that was responsible.
The results showed that total meltwater contributed about 10 percent of the outflowing dissolved iron, with the majority (62 percent) contributed by inflowing deep water, and another 28 percent as inputs from shelf sediments.
鈥淩oughly 90 percent of the dissolved iron coming out of the ice shelf cavity comes from deep waters and sediments outside the cavity, not from meltwater,鈥 Chinni said.
Additionally, iron isotope ratios from the samples suggest that somewhere beneath the glacier is a liquid meltwater layer that lacks dissolved oxygen, a condition that promotes the dissolution of solid iron oxides in the bedrock, seemingly a larger source of iron than ice shelf melting, Chinni said.
Taken together, the findings challenge prevailing assumptions about iron sources in the Southern Ocean in a warming world, though additional research is needed to better understand how the subglacial processes are involved, the team said.
鈥淥ur 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,鈥 Sherrell said.
For some colleagues, he added, this will be a very surprising realization.