How ocean iron has shaped Earth's climate history

When people think about climate change, carbon dioxide and melting ice usually come to mind. But there’s another, less obvious factor at play: iron in the ocean.

A new study from the University of Hawai‘i at Mānoa shows how the supply of iron in the South Pacific shifted over the last 93 million years.

The shift in iron changed how marine life grew and how carbon dioxide moved in and out of the atmosphere.

Why ocean iron matters

Iron feeds phytoplankton, the tiny plants floating in seawater. When they grow, they pull carbon dioxide out of the air. Without enough iron, phytoplankton struggle, and less carbon dioxide gets absorbed.

Today we know this link well, but the UH Mānoa team wanted to know how iron shaped oceans in the distant past. To investigate, the researchers studied deep-sea sediment cores collected far from any continent.

Changing iron in ocean

Study lead author Logan Tegler is a postdoctoral researcher in the UH Mānoa School of Ocean and Earth Science and Technology.

“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 a volcanic ash,” explained Tegler.

“These sources shifted over time as the sites gradually migrated away from mid-ocean ridges.”

At first, most iron came from underwater hydrothermal vents. Later, dust carried by wind slowly became more important. Around 30 million years ago, it took over as the main source.

Climate shifts and dust

The study connects shifts in iron supply directly to major climate events in Earth’s history.

During warmer greenhouse periods, when the planet lacked polar ice, volcanic ash and seafloor hydrothermal vents acted as the main suppliers of iron to the ocean. These sources provided a steady flow of nutrients, helping sustain marine productivity.

Later, as Earth cooled and Antarctica developed permanent ice sheets, the dynamics changed. Stronger winds began carrying larger amounts of continental dust into the South Pacific.

The dust became a dominant source of iron, replacing volcanic and hydrothermal inputs. The change mattered because not all iron sources are equal. Some forms are more accessible to microbes, while others are harder to use.

These differences influenced which organisms could thrive, altering marine food webs and carbon uptake. By linking climate transitions with nutrient supply, the study shows how even small elements can reshape the global carbon cycle.

Ocean life under low iron

“Understanding this historical context helps us comprehend how iron has shaped ecosystems,” said Tegler.

“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 large parts of the Pacific, the amount of available iron still sets the limit on how much phytoplankton can grow. Fewer phytoplankton mean less carbon dioxide leaves the air.

Today’s unusual pattern

“Modern dust deposition in the South Pacific is extremely low,” said Tegler. “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 isn’t just random. Rising dust relates to shifts in climate and geology. Drying in Australia, uplift of landmasses, altered wind paths, human-driven changes, and broader atmospheric circulation patterns all play a part. Compared to the deep past, today’s South Pacific is in an unusual state.

Ocean changes due to iron

“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,” noted Tegler.

Iron’s history shows how one nutrient can shift ecosystems and climate. By studying how iron availability changed over millions of years, scientists gain insight into today’s environmental challenges.

Ultimately, the research helps predict future impacts on Earth’s carbon balance, marine life, and overall climate resilience.

The study is published in the journal Paleoceanography and Paleoclimatology.

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