What is it about?
When and how Earth’s oceans first became oxygenated, long before the Great Oxidation Event began approximately 2.4 billion years ago, remains a major question in understanding the evolution of early Earth. Heavy molybdenum isotope signatures preserved in ancient marine sediments have traditionally been interpreted as evidence for the accumulation of oxygen in surface seawater. By analysing molybdenum isotopes in 2.57–2.51-billion-year-old banded iron formations from China, we found evidence of a relatively stable, isotopically heavy molybdenum reservoir in the Neoarchean ocean. Combining these findings with a new oceanic isotope mass-balance model, we propose an alternative explanation: the heavy molybdenum isotope signatures resulted primarily from the preferential removal of lighter molybdenum isotopes during iron oxidation and banded iron formation deposition, under largely oxygen-free conditions. In other words, extensive iron oxidation did not necessarily require widespread oxygenation of the oceans.
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Why is it important?
Our findings challenge the conventional interpretation of heavy molybdenum isotope signatures as direct evidence for widespread ocean oxygenation before the Great Oxidation Event. Instead, they highlight the previously underappreciated role of iron cycling in regulating the chemistry of Earth’s early oceans. We propose that photoferrotrophic bacteria, which use sunlight to oxidize dissolved iron without producing oxygen, were likely major contributors to iron oxyhydroxide precipitation and the widespread deposition of banded iron formations during this period. This provides a new explanation for how extensive iron oxidation and heavy molybdenum isotope signatures could have developed while the oceans remained largely anoxic. More broadly, our work highlights the importance of considering the interactions between microbial activity and ocean chemistry when interpreting the geochemical record of Earth’s early oxygenation.
Perspectives
What I find particularly interesting about this work is that iron oxidation and ocean oxygenation are not necessarily the same thing. Microorganisms could have driven extensive oxidation of dissolved iron billions of years before oxygen became widespread in the oceans. This distinction provides a new perspective on how we interpret the early oxygenation of Earth and the role microbial life played in transforming our planet’s surface environments. This study is also particularly meaningful to me because it builds on several decades of research by our team on Chinese banded iron formations. I am very grateful to all our collaborators for their expertise, insights, and contributions. This work would not have been possible without their collective efforts.
Professor Changle Wang
Institute of Geology and Geophysics, Chinese Academy of Sciences
Read the Original
This page is a summary of: A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation, Proceedings of the National Academy of Sciences, November 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2608195123.
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