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An analysis of sulfide ore deposits from one of the world鈥檚 richest base-metal mines confirms that oxygen levels were extremely low on Earth 2.7 billion years ago, but also shows that microbes were actively feeding on sulfate in the ocean and influencing seawater chemistry during that geological time period.

The research, reported by a team of Canadian and U.S. scientists in聽Nature Geoscience,聽provides new insight into how ancient metal-ore deposits can be used to better understand the chemistry of the ancient oceans 鈥� and the early evolution of life.

Sulfate is the second most abundant dissolved ion in the oceans today. It comes from the 鈥渞usting鈥� of rocks by atmospheric oxygen, which creates sulfate through chemical reactions with pyrite, the iron sulfide material known as 鈥渇ool鈥檚 gold.鈥�

The researchers, led by PhD student John Jamieson of the University of Ottawa and Prof. Boswell Wing of 91社区, measured the 鈥渨eight鈥� of sulfur in samples of massive sulfide ore from the Kidd Creek copper-zinc mine in Timmins, Ontario, using a highly sensitive instrument known as a mass spectrometer. The weight is determined by the different amounts of isotopes of sulfur in a sample, and the abundance of different isotopes indicates how much seawater sulfate was incorporated into the massive sulfide ore that formed at the bottom of ancient oceans. That ancient ore is now found on the Earth鈥檚 surface, and is particularly common in the Canadian shield.

The scientists found that much less sulfate was incorporated into the 2.7 billion-year-old ore at Kidd Creek than is incorporated into similar ore forming at the bottom of oceans today. From these measurements, the researchers were able to model how much sulfate must have been present in the ancient seawater. Their conclusion: sulfate levels were about 350 times lower than in today鈥檚 ocean. Though they were extremely low, sulfate levels in the ancient ocean still supported an active global population of microbes that use sulfate to gain energy from organic carbon.

鈥淭he sulfide ore deposits that we looked at are widespread on Earth, with Canada and Quebec holding the majority of them,鈥� says Wing, an associate professor in 91社区鈥檚 Department of Earth and Planetary Science. 鈥淲e now have a tool for probing when and where these microbes actually came into global prominence.鈥�

鈥淒eep within a copper-zinc mine in northern Ontario that was once a volcanically active ancient seafloor may not be the most intuitive place one would think to look for clues into the conditions in which the earliest microbes thrived over 2.7 billion years ago,鈥� Jamieson adds.聽 鈥淗owever, our increasing understanding of these ancient environments and our abilities to analyze samples to a very high precision has opened the door to further our understanding of the conditions under which life evolved.鈥�

The other members of the research team were Prof. James Farquhar of the University of Maryland and Prof. Mark D. Hannington of the University of Ottawa.

The Natural Sciences and Engineering Research Council of Canada made this study possible through fellowships to Jamieson and a Discovery grant to Wing.

To access the study鈥檚 abstract:聽