A lack of atmospheric oxygen helps explain why some of the world’s largest mineral deposits came to rest where they did some 2.7 billion years ago, geological scientist Andrey Bekker and his colleagues report in the November 19th issue of the journal Science.

The findings apply to iron-nickel sulfide deposits and it was the sulfur in this equation that Bekker focused on.

Some of the largest known iron-nickel sulfide deposits are associated with volcanic rocks rich in iron and magnesium, two elements forged in the Earth’s mantle at very high temperatures before they arrive to the Earth’s surface. But considering all we know about the mantle and how things move from it to the surface, the levels of sulfur needed to create these deposits is unaccounted for. So where is this sulfur coming from? It has been debated since the 60s when these deposits were first discovered in Western Australia.

Bekker has long been interested in how early Earth processes affected the content of oxygen in the ocean and atmosphere, and he applied sulfur isotopes to answer this question. Isotopes are atoms of the same element that have a different mass and sulfur isotopes come in four varieties called 32, 34, 33, and 36. The first two are the most abundant and for years it was well established that scientists would only measure the 34 to 32 ratio. But some were unsatisfied with this arrangement and developed ways to also analyze the 32 to 33 and 36 to 32 ratios.

This new tool cleaned the window to the ancient past and allowed scientists, including Bekker, to see an anomaly in the sedimentary rock record. Sulfur isotopes appear in the sedimentary rock record in predictable ways until you go back to about 2.4 billion years ago. At this point, things change because the atmosphere had no oxygen and therefore there was no ozone to block ultraviolet radiation from the sun. This allowed the sulfur coughed from volcanoes to drift high into the air where the harsh, unhindered rays of the sun “produced these crazy photochemical fractionations that we don’t really understand but can nevertheless read their signature locked in sedimentary rocks,” Bekker said. These sulfur atoms eventually came to rest on the ocean floor where they incorporated into sedimentary rock. And like a parrot amongst crows, these anomalous sulfur atoms stand out.

Bekker’s hypothesis held that if lava assimilated sulfur by flowing over sedimentary rocks on the ocean floor that were old enough to have these anomalous sulfur isotope ratios in them, the signature would be preserved and observable. And after collecting samples from Ontario and Australia with his colleagues, he found just that. So, combined with other data, this finding constrains the models used to predict where the ore can be found.

“It was great to discover this. It sort of makes you feel that suddenly you connect your idea to something real,” Bekker said.

“As a geologist you try to characterize how the Earth works and it’s like driving through a cloud into a city. You drive and see nothing but in the mist you suddenly see some skyscrapers but you’re not sure. Maybe it’s an illusion. But then things become clear and you see buildings and houses and you see the city. And you say ‘wow’. You didn’t miss you. You came to the right place.”

Bekker’s project received funding from the Natural Sciences and Engineering Research Council of Canada (NSERC).

For more information contact Sean Moore, public affairs, University of Manitoba, 204-474-7963 (sean_moore@umanitoba.ca).