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 research 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 were 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 on the study. "The record may reflect the delivery of iron from land, telling us more about changes of Earth's surface, weathering processes and land plants."
From iron to oxygen
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 the 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 the transition when the ocean, and subsequently the atmosphere, accumulated oxygen.
Mountains, plants and iron
Hardisty joined colleagues from the University of Hamburgh, Germany and ETH in 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 made 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. "We didn't tear down the tool — we added another component to it to broaden the application and added new insight."
For the oldest portion of the study, the researchers found the long-standing theory holds. The iron record in ocean sediment was controlled by low oxygen conditions, while other influences became important later.
At this point, the delivery method of iron to the ocean took center stage. The team found that peaks in the iron record coincide with major mountain building events during the past 500 million years. They showed that prominent events that altered Earth's surface—including the Variscan mountain building event that led to the formation of the supercontinent Pangea—correlate to prominent peaks in the iron cycle. According to Hardisty, the change in elevation combined with atmospheric oxygen intensified weathering of the iron-rich rocks at Earth's surface.
Plants on land accelerated the delivery of iron in several important ways. Hardisty explained that plant roots release chemicals that help break down rocks and sediments, freeing the iron they contain. They also hold streambed sediments in place so that there's more time for the iron to react with oxygen and form iron-oxide minerals that can eventually be carried to the ocean.
Understanding what shaped the iron cycle in the past could also help scientists better interpret how Earth responds to future environmental change.
"Iron remains an important way to understand the past," Hardisty said. "Our findings will help researchers by expanding their tools to continue to study how climate change will affect the planet."
The study was supported by the German Research Foundation.
By Stacy Kish
