The findings, published in the journal Nature Communications Earth & Environment, show that meltwater from the Antarctic ice shelf pumps much less iron into the surrounding waters than scientists had expected. The authors argue that this undermines the iron fertilization theory, which posits that iron-rich glacial meltwater will feed algae blooms that pull planet-warming carbon dioxide out of the atmosphere as they grow.
“Our claim in this paper is that meltwater contains very little iron, and most of the iron it carries comes from the grinding and dissolving of rock in the liquid layer between the rock and the ice sheet, rather than from the ice itself that is causing sea level rise,” lead investigator Rob Sherrell, a professor of biogeochemistry at Rutgers University, said in a statement.
Theory of Field Data Challenges
Evidence supporting the iron fertilization theory comes primarily from simulations and computer modeling. Sherrell and his colleagues wanted to test its validity using real-world data, so they launched an expedition to the Amundsen Sea in West Antarctica.
The Amundsen ice shelf has the highest rate of thinning in Antarctica and therefore Antarctic melting accounts for most of the sea level rise. When warm seawater rises from the deep ocean and enters cavities beneath an ice shelf – a marine extension of a glacier away from the continent – it melts the shelf from below, releasing fresh water into the ocean.
On the Dotson Ice Shelf, researchers collected water samples at the point where seawater enters such a cavity and the point where it exits after meltwater flow. The study’s lead author Venkatesh Chinni, a postdoctoral researcher at Rutgers, then analyzed the samples in the lab to assess the iron content in them, while colleagues at Texas A&M University measured isotopic ratios to figure out where the iron was coming from.
The analysis revealed that total meltwater contributes only 10% of the dissolved iron flowing out, while 62% comes from deep water flow. The researchers determined the remaining 28% from inputs derived from shelf sediments. This refutes the idea that meltwater could stimulate algal blooms large enough to offset global warming.
Complications of glacial melting and warming
While the study makes a compelling case against the iron fertilization theory, the authors note that a true understanding of the subglacial processes involved in iron flux requires additional research.
Furthermore, they only examined one Antarctic ice shelf, and the way water moves and mixes within subglacial cavities can vary depending on the size of the shelf, surrounding ocean conditions, and the properties of meltwater outflow. The researchers believe that the fundamental balance of dissolved iron sources they observed at Dotson may generally apply to other ice shelves, but more research will also be needed to verify this.
This is not the first study to question the iron fertilization theory. For example, previous research found that past spikes in iron concentrations in the equatorial Pacific Ocean had little or no effect on carbon-capturing algae. Meanwhile, there is enough evidence to suggest that glacial melt may increase regional temperatures – and hence melting – by darkening the glacier surface, causing it to absorb more solar radiation.
The interplay between global climate and the ocean processes that melt glaciers is incredibly subtle, which is why it is so important for regional studies to validate findings and theories based on modeling. Such work may not offer many rays of hope, but it will help scientists understand the future of a warming Antarctica.
<a href