A new study suggests that iron preserved in ancient rocks has recorded major transformations in Earth's history beyond the rise of atmospheric oxygen, including the building of mountains and the rise of land plants. Researchers from Michigan State University, including Dalton Hardisty, and colleagues from the University of Hamburg, Germany, and ETH Zurich, analyzed the iron record over the last 1.2 billion years. Their findings, published in the Proceedings of the National Academy of Sciences, indicate that changes in the iron record reflect not only oxygen levels but also surface processes such as weathering and the influence of land plants. Throughout much of Earth's history, the ocean was enriched in dissolved iron, which combined with sulfur to form pyrite in the absence of oxygen. Early marine microbes released oxygen as a byproduct of photosynthesis, which also combined with dissolved iron to form hematite. This transition from pyrite to hematite is a key indicator of oxygen accumulation in the ocean and atmosphere. However, the study suggests that the iron record also reflects the delivery of iron from land, providing insights into changes to Earth's surface, weathering processes, and the emergence of land plants. The researchers found that peaks in the iron record coincide with major mountain-building events over the past 500 million years, including the Variscan event that led to the formation of the supercontinent Pangea. These events altered Earth's surface, intensifying the weathering of iron-rich rocks. Land plants contributed to the delivery of iron by releasing chemicals that break down rocks and sediments, and by stabilizing streambeds, allowing more time for iron to react with oxygen and form minerals that can be transported to the ocean. The study expands the understanding of the iron cycle's role in Earth's history and could provide clues about how a warming climate will reshape the planet. The findings offer scientists a more nuanced way to interpret Earth's history and enhance their tools for studying future environmental changes.