Geologists have discovered a “proxy” chemical that allows them to measure and study oxygen levels in the Earth’s oldest oceans.
“More than 2.5 billion years ago, there was little to no oxygen in the oceans, as methane shrouded the Earth in a haze,” says Zunli Lu, an assistant professor in the Earth science department at Syracuse University.
“Organisms practicing photosynthesis eventually started to overpower reducing chemical compounds (i.e., electron donors), and oxygen began building up in the atmosphere. This period has been called the Great Oxidation Event,” explains Lu.
Using a novel approach called iodine geochemistry, Lu and colleagues have confirmed the earliest appearance of dissolved oxygen in the ocean’s surface waters. The study appears in the journal Geology.
Central to their approach is iodate, a form of iodine that exists only in oxygenated waters. When iodate is detected in carbonate rocks in a marine setting, the researchers can measure the elemental ratio of iodine to calcium.
This measurement, known as a proxy for ocean chemistry, helps them figure out how much oxygen has dissolved in the water.
“Iodine geochemistry enables us to constrain oxygen levels in oceans that have produced calcium carbonate minerals and fossils,” says Lu, who developed the proxy.
“What we’ve found in ancient rock reinforces the proxy’s reliability. Already, we’re using the proxy to better understand the consequences of ocean deoxygenation due to rapid global warming.”
Researchers from the University of California, Riverside, Yale University, University of Manitoba, and the University of Denis Diderot in Paris coauthored the study.
Source: Syracuse University