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Long-standing recycling of ancient sulfur in billion-year-old rocks supports unique ecosystems in terrestrial deep subsurface and sheds insights into search for life on Mars

To the naked eye, ancient rocks may look completely inhospitable, but in reality, they can sustain an entire ecosystem of microbial communities in their fracture waters isolated from sunlight for millions, if not billions, of years. New scientific findings discovered the essential energy source to sustain the life kilometres below Earth’s surface with implications for life not only on our planet but also on Mars. 

“Most life lives on sunlight, but these deep subsurface microbes seem to live on the limited energy they can only get inside the water trapped in these ancient rocks, ” says Long Li, assistant professor in the University of Alberta’s Department of Earth and Atmospheric Sciences. The two essential substances in the fracture water by the deep subsurface microbes are hydrogen (as electron donor) and sulfate (as electron acceptor). There is a basic understanding that reactions between the water and minerals in the rock produce hydrogen, but what about sulfate? 

Li and colleagues at University of Toronto and McGill University examined the distribution patterns of multiple isotopes--the sulfur atoms with different number of neutrons--in the dissolved sulfate in the “billion-year-old” water collected from 2.4 kilometers below the surface in northern Ontario, part of the Precambrian Canadian Shield. They observed a unique distribution pattern called sulfur isotope mass-independent fractionation. 

Such a signature is thought to be produced by photochemical reactions in early Earth’s atmosphere prior to the Great Oxidation Event 2.4 billion years ago. To date, this signature of ancient Earth sulfur has only been found in rocks and minerals. Based on the match in the isotopic signature between the dissolved sulfate and the pyrite minerals in the 2.7 billion-year-old host rocks, the authors demonstrated that the sulfate was produced by oxidation of sulfide minerals in the host rocks by oxidants generated by radiolysis of water. 

“The sulfate in this ancient water is not modern sulfate from surface water flowing down. What we’ve found is that the sulfate, like the hydrogen, is actually produced in place by reaction between the water and rock. What this means is that the reaction will occur naturally and can persist for as long as the water and rock are in contact, potentially billions of years.

“The wow factor is high,” says Li. Billion-year-old rocks, exposed or unexposed, compose nearly 70 percent of Earth’s continental crust. “If geological process can naturally supply a steady energy source in these rocks, the modern terrestrial subsurface biosphere may expand significantly both in breadth and in depth.”

Billion-year-old rocks similarly dominate the surface of the mysterious red planet. Some locations on Mars have similar mineral assemblage to the studied area in northern Ontario. This allows the scientists to speculate that microbial life can indeed be supported on Mars. 

“Because this is a fairly common geological setting in early Earth as well as modern Mars, we think that as long as the right minerals and water are present, likely kilometers below the surface, they can produce the necessary energy source to support the microbes. I’m not saying that these microbes definitively exist, but the conditions are right to support microbial life on Mars.”

Li concludes that if there is any life on Mars right now--a question that has long piqued people’s curiosity--the best bet is to look below the surface.  

This conclusion is seconded by McGill’s Lyle Whyte, who was Principal Investigator of the NSERC-sponsored Collaborative Research and Training Program (CREATE) Canadian Astrobiology Training Program (CATP) that facilitated this pan-Canadian research project. “This is an extraordinarily exciting result in that it shows how microbial life, using substrates produced from radiation, could exist in the deep subsurface of Earth but also within other solar system bodies such as Mars, Europa and Enceladus.”

Whyte adds, “New discoveries like these have been a hallmark of the CREATE program in general, and the McGill-led CATP in particular. They are a real testament to the collaborative nature that has long characterized Canadian science.”

“Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks” appeared in the October 27 issue of Nature Communications, an open access journal part of the Nature group of publications.

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