Geologists Confirm Oxygen Levels in Earth's Oldest Oceans

In a new study, geologists used a novel approach to study the level of oxygen in Earth's oldest oceans.

The new research, led by researchers at Syracuse University, offers crucial implications for the study of marine ecology as well as warming of the planet.  Led by Zunli Lu and Xiaoli Zhou, an assistant professor and Ph.D. student respectively, in the Department of Earth Sciences, the research used a new technique called iodine geochemistry to confirm the initial appearance of dissolved oxygen in the surface waters of the ocean.

"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 Lu, a member of Syracuse University's Low-Temperature Geochemistry Research Group, in a statement. "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."

The researchers focused on iodate, a form of iodine present only in oxygenated waters.  The researchers can measure the elemental ratio of iodine to calcium only when iodate is detected in the marine carbonate rocks. Known as proxy for ocean chemistry, the researchers can use this measurement to trace the amount of oxygen that 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."

The finding is documented in the journal Geology.