According to new research published in Nature, mineral forms of iron play an important role in regulating the cycling of this bio-essential nutrient in the ocean.
The discoveries pave the way for future research into the relationship between the iron and carbon cycles, as well as how changing ocean oxygen levels may interact.
The research, led by the University of Liverpool and involving collaborators from the United States, Australia, and France, aims to fill a knowledge gap in ocean science.
Principal Investigator Professor Alessandro Tagliabue said, “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.”
The early Earth’s ocean was low in oxygen and high in iron, which served as a catalyst in many biological reactions. These include photosynthesis, which oxygenated the earth’s system through its proliferation.
Because iron is less soluble in well-oxygenated seawater, precipitation and sinking of iron oxides resulted in a decrease in iron levels. As a result, iron now plays a critical role in regulating ocean productivity and thus ecosystems throughout the modern ocean.
It is thought that organic molecules called ligands, which bind iron, regulate iron levels above their soluble thresholds. This viewpoint has underpinned the representation of the marine iron cycle in global models used to investigate how future climate changes will affect levels of biological productivity.
However, oceanographers have been perplexed as to why there appeared to be a much larger loss of iron due to insolubility in the ocean than would be expected based on the measured high levels of ligands. In general, ocean models built in accordance with the expected pattern have performed poorly in reproducing observations.
It was discovered that iron in the upper ocean was largely cycling independently of ligands and was instead controlled by the clustering of iron oxide colloids to form so-called ‘authigenic’ particles that are lost from the upper ocean.
The authors created a new numerical model to explain their findings and extrapolate them across the ocean. The new model reproduced other independent observations significantly better, indicating that this new process was important in approximately 40 per cent of upper ocean waters.
The co-aggregation of iron oxides and carbon is a key implication of this process, which has implications for the global carbon cycle and may be sensitive to future trends in ocean oxygen loss.
“These findings will cause us to reassess our understanding of the iron cycle and its sensitivity to changing environmental conditions,” said Professor Tagliabue.
Researchers from the University of South Florida, Oregon State University, Bigelow Laboratory for Ocean Sciences, Sorbonne Université, University of Tasmania, University of Leeds, Bermuda Institute of Ocean Sciences, University of Georgia, and Old Dominion University participated in the study led by the University of Liverpool.
“Our work was only possible because of the efforts to measure multiple different forms of iron in seawater over the annual cycle at the Bermuda Atlantic Time Series site,” said Professor Tagliabue.