Iron helped scientists trace Earth's oxygen. Now it's telling a bigger story

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, scientists have studied iron preserved in ancient rocks to reconstruct how Earth's oceans and atmosphere became rich in oxygen. A new study suggests iron has recorded other major transformations too.

Michigan State University researcher Dalton Hardisty and colleagues found that changes in Earth's iron record also reflect the building of mountains and the rise of land plants. This discovery offers scientists a more nuanced way to interpret more than a billion years of Earth's history—and could offer clues to how a warming climate will reshape the planet. The results are published in the Proceedings of the National Academy of Sciences.

"The iron cycle has new things to tell us," said Hardisty, the MSU Endowed Assistant Professor of Global Change Processes and co-author of the study. "The record may reflect the delivery of iron from land, telling us more about changes to Earth's surface, weathering processes and land plants." Throughout most of Earth's history, the ocean was enriched in dissolved iron. In the absence of oxygen, the dissolved iron combined with sulfur to form pyrite, commonly known as "fool's gold." Early marine microbes released oxygen as a byproduct of photosynthesis.

This oxygen also combined with dissolved iron to form hematite. A prominent example includes the banded iron formations found across the Lake Superior region of Michigan and Minnesota. Geologists look to the rock record and use the transition from pyrite to hematite to mark when the ocean, and subsequently the atmosphere, accumulated oxygen.

Hardisty joined colleagues from the University of Hamburg, Germany, and ETH Zurich to focus on iron cycling at Earth's surface. The team augmented data gathered from the Sedimentary Geochemistry and Paleoenvironments Project database to reconstruct the iron record for a vast segment of recent geologic history, spanning the last 1.2 billion years. Their analysis yielded two remarkable insights.

First, continents played a critical and underappreciated role in supplying iron to the ocean. Second, pyrite formed from the continental supply of iron was important in Earth's oxygenation. "The iron that we were tracing was tracking more than changes in oxygen in the ocean, which is how the records were interpreted in the past," Hardisty said.