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In the effort to understand the history of Earth’s oxygenation, scientists have sought geochemical proxies, but studies don’t always fully consider how samples get preserved within the ancient environment and how that may affect results, especially when it comes to marine carbonates which can be altered during rock transitions and formations.

NASA Astrobiology Postdoctoral Program fellow Ashleigh Hood and the Alternative Earths team looked at uranium isotopes in samples from a reef complex in South Australia dating back over 635 millions ago. The research paper, “Integrated geochemical-petrographic insights from component-selective δ238U of Cryogenian marine carbonates” was featured on the cover of Geology.

The team’s results showed high variability in uranium isotopes between different carbonates within a single sample and also from less well-preserved samples. The best-preserved samples found in marine cements had isotope values indicating past anoxic and iron-rich marine settings, which are thought to be the ocean conditions that led up to the rise of animals.

Because of the variability, the scientists call out the need for a fundamental change in carbonate trace metal work methodology that includes careful screening when collecting marine samples for chemical analysis.

The research was supported by the Australian Research Council Discovery Grant, the U.S. National Science Foundation Earth-Life Transitions Program, the NASA Astrobiology Institute (NAI), the NASA Postdoctoral Program, and the AAA Chevron Fellowship.