An analysis of sulphide ore deposits from one of the world's richest base-metal mines, located in Canada, confirms that oxygen levels were extremely low on Earth 2.7 billion years ago but also shows that microbes were actively feeding on sulphate in the ocean and influencing sea water chemistry during that geological time period.
The research, reported by a team of Canadian and U.S. scientists in Nature Geoscience, provides new insight into how ancient metal-ore deposits can be used to better understand the chemistry of the ancient oceansand the early evolution of life.
Sulphate is the second most abundant dissolved ion in the oceans today. It comes from the rusting of rocks by atmospheric oxygen, which creates sulphate through chemical reactions with pyritethe iron sulphide material known as fool's gold.
The research team, led by PhD student John Jamieson of the University of Ottawa and professor Boswell Wing of McGill, measured the weight of sulphur in samples of massive sulphide 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 sulphur isotopes in a sample; the abundance of different isotopes indicates how much sea water sulphate was incorporated into the massive sulphide ore that formed at the bottom of ancient oceans. That ancient ore is now found on the Earth's surface and is particularly common in the Canadian Shield.
The scientists found that much less sulphate 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 sulphate must have been present in the ancient sea water. They concluded that sulphate levels were about 350 times lower than in today's oceans. Though the sulphate levels were extremely low, the levels in the ancient ocean still supported an active global population of microbes that use sulphate to gain energy from organic carbon.
The sulphide ore deposits that we looked at are widespread on Earth, with Canada and Quebec holding the majority of them, says Wing, an associate professor in McGill's Department of Earth and Planetary Science. We now have a tool for probing when and where these microbes actually came into global prominence.
Deep 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, Jamieson adds. However, 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.
The other members of the research team were professor James Farquhar of the University of Maryland and professor Mark D. Hannington of the University of Ottawa.
The National Science and Engineering Research Council of Canada made this study possible through fellowships to John Jamieson and a Discovery grant to Boswell Wing.
The abstract of the study can be accessed at: www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo1647.html
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