Study Highlights Importance of Mineral Iron in the Ocean

New research published today in Nature by an international team of researchers, including one from Bigelow Laboratory for Ocean Sciences, has revealed the importance of mineral forms of iron in regulating the cycling of this bio-essential nutrient in the ocean. The findings pave the way for new work on the relationship between the iron and carbon cycles and how they may interact with changing ocean oxygen levels in the future.

"To date, we have not fully appreciated the role that mineral forms of iron have played in driving the distributions and temporal dynamics of iron in the ocean,” said Alessandro Tagliabue, a professor at the University of Liverpool and the paper’s lead author.

The study involved collaborators in the U.K., U.S., Australia, and France and addresses a significant knowledge gap in ocean research.

"This study highlights our still evolving understanding of what happens to iron in the ocean and what controls the availability of iron to phytoplankton," said Ben Twining, a Senior Research Scientist at Bigelow Laboratory and one of the study's co-authors. "We need to understand that process to understand how increasing wildfires, dust plumes, and human activity supplying iron to the ocean might alter ocean productivity in the future.”

The ocean of the early Earth was low in oxygen and rich in iron, the latter of which was incorporated as a catalyst in many biological reactions. This included photosynthesis, which, in turn, helped oxygenate the Earth system. As iron is less soluble in well-oxygenated seawater, precipitation and the sinking of iron oxides led to declining iron levels. Consequently, iron now plays a critical role in regulating productivity across the contemporary ocean as an important limiting nutrient.

Traditionally, it was thought that iron levels were largely regulated above their soluble thresholds by organic molecules called ligands, which bind iron. This view has underpinned the representation of the marine iron cycle in global models used to explore how changes in climate may affect levels of biological productivity in the future.

However, oceanographers have been puzzled as to why there seemed to be a much larger loss of iron from the ocean due to insolubility than expected given the measured high levels of ligands. The ocean models built according to the expected pattern have generally performed poorly in reproducing real-world observations.

To address that gap, the team of researchers examined the processes driving the cycling of iron over an annual cycle for the first time. They found that iron was largely cycling independently of ligands in the upper ocean; instead, the process was controlled by the clustering of iron oxide colloids to form so-called ‘authigenic’ particles that are lost from the upper ocean.

The authors developed a new numerical model to both explain their results and extrapolate their findings across the ocean. The new model performed markedly better in reproducing independent observations and highlighted that this new process was important in around 40% of upper ocean waters.

By illuminating how iron cycling occurs via the co-aggregation of iron oxide and carbon, their findings have implications for the global carbon cycle and suggest that iron cycling may be sensitive to trends of ocean oxygen loss in the future.

"These findings will cause us to reassess our understanding of the iron cycle and its sensitivity to changing environmental conditions," Tagliabue said.

This project was a process study contribution to the international GEOTRACES effort jointly funded by the National Science Foundation and the UK Natural Environment Research Council. In addition to the University of Liverpool and Bigelow Laboratory, the research team involved scientists from the University of South Florida, Oregon State University, Sorbonne Université, University of Tasmania, University of Leeds, the Bermuda Institute of Ocean Sciences, University of Georgia, and Old Dominion University. The work was made possible by measuring multiple different forms of iron over the annual cycle at the Bermuda Atlantic Time Series site.

"By piggybacking on the Bermuda Atlantic Time Series, and getting iron measurements over the course of a year, we have a better understanding of how iron cycles in the ocean," Twining said. "Going forward, we hope to replicate this approach in other places to complete that picture of what happens to iron in the ocean and how that might change in the future."

Photo: Peter Sedwick, a professor at Old Dominion University and a co-author on the study, deploys the clean rosette system the team used to collect water samples during project fieldwork. Alessandro Tagliabue.