Deep within layers of mud from the deep-sea floor of the South Pacific is a history of iron

In a surprising discovery, researchers at the University of Hawaiʻi at Mānoa found that the South Pacific Ocean—a region known for being iron-poor—is currently receiving more iron-rich dust than at any point in the last 93 million years.

This new study, based on the analysis of deep-sea sediment cores, sheds light on the critical role of iron in Earth’s climate history by revealing how its sources have shifted over time. The research provides crucial insights into the long-term interplay between iron, marine life, and atmospheric carbon dioxide levels.

Iron is a vital nutrient for marine life and plays a significant role in regulating atmospheric carbon dioxide by influencing the growth of phytoplankton, which absorb carbon dioxide. Although the importance of iron today is well-established, researchers have a limited understanding of how past iron availability may have shaped the marine ecosystem.

To learn about the ocean’s history of iron, researchers studied layers of mud from the deep-sea floor in the South Pacific.

“Over the past 93 million years, we found that five primary sources of iron have influenced the South Pacific Ocean: dust, iron from far-off ocean sources, two distinct hydrothermal sources, and volcanic ash,” said Logan Tegler, lead author and oceanography postdoctoral researcher at the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology. “These sources shifted over time as the sites gradually migrated away from mid-ocean ridges.”

The study showed that the ocean’s main source of iron switched from hot vents on the seafloor to dust about 30 million years ago.

“Understanding this historical context helps us comprehend how iron has shaped ecosystems,” Tegler said. “It also raises questions about how the iron cycle might have favored certain microbes over others—an ecosystem with persistently low iron could favor microbes adapted to survive under iron-limited conditions, such as diatoms.”

In many regions of the Pacific Ocean, iron availability limits the growth of phytoplankton, thereby limiting the amount of carbon dioxide removed from the atmosphere.

“Modern dust deposition in the South Pacific is extremely low,” Tegler said. “However, our findings surprisingly suggest that the South Pacific is currently receiving more dust than it has at any point in the last 90 million years, which is remarkable given its current reputation as an iron-poor region!”

This study sheds light on iron cycling across the broader Pacific basin and enhances understanding of how essential nutrients like iron shape ocean ecosystems and climate over millions of years.

“As human activities increase iron input to the oceans through industrial emissions and biomass burning, understanding past perturbations of the iron cycle is crucial for predicting and mitigating adverse effects,” added Tegler.