(CN) — Traces of the world's oldest glaciers have been found under its largest gold deposits, according to a study published Wednesday in the journal Geochemical Perspectives Letters.
Ilya Bindeman, a professor at the University of Oregon, and Axel Hofmann, a professor at the University of Johannesburg, studied glacial deposits in the rocks under the gold fields of South Africa. Bindeman described those fossilized glacial moraines as the debris that a glacier leaves behind as it gradually melts and contracts.
Through their work, the researchers learned that the glacier traces dated back to 2.9 billion years ago, making them the oldest on record. To determine their age, the study says that the researchers needed to examine the moraines’ oxygen isotopes to see what the temperature was like when the glaciers deposited their rocks.
“We looked at the relative amounts of three oxygen isotopes, 16O, 17O, and 18O. These are all types of oxygen but have very slightly different weights. We found that these rocks had very low amounts of 18O, and very high amounts of 17O, indicating that they were formed at icy temperatures. This means ice. Couple that geochemical evidence with the moraine evidence, and it means glaciers, the oldest glaciers yet found on Earth,” Bindeman said.
Hofmann offered two explanations as to how this happened.
The first explanation suggests that the gold fields of South Africa were closer to the poles than previously thought, which also suggests that continental ice caps were present at the time.
As for the second explanation, Hofmann theorized that the glaciers’ age indicates a very cold period in Earth’s history.
“Another possibility is that the whole Earth was in a ‘snowball Earth’ period, when low atmospheric concentrations of CO2 and CH4 led to a ‘reverse greenhouse effect,’ causing much of the planet to freeze. Scientists believe that this may have happened on a couple of occasions in the more recent past. If so, this would be the earliest such global cooling period recorded. Either possibility is scientifically interesting,” said Hofmann.
Regarding the moraines, Bindeman commented on their level of preservation, giving credit to the area where they uncovered the glaciers.
“We found extremely well-preserved glacial deposits close to the gold fields of South Africa," said Bindeman. "This is one of the few areas which remain fairly intact and unchanged from the early Earth.”
Aside from this, Hofmann added that the location of some of Earth’s gold deposits raises questions about how the planet’s past climate variations influenced the formation of the gold deposits, which could prompt further study on the topic.
“The largest sedimentary gold deposits in the world are found in slightly younger rocks sitting above the rocks we studied. It’s possible that a change from icehouse to greenhouse conditions may have aided in the formation of those gold deposits, but this needs to be confirmed and requires further work,” said Hofmann.
This work will be presented at the Goldschmidt Conference on Wednesday.
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