Scientists have discovered the traces of the world’s oldest known glaciers, dating from 2.9 billion years ago, in rocks sitting under the planet’s largest gold deposits in South Africa.
The finding suggests the presence of continental ice caps in the past and that either the area was closer to the poles, or that parts of the Earth may have been frozen in a previously unknown “snowball Earth” period of extreme cold weather.
The study, published in the journal Geochemical Perspectives Letters, found evidence from relative oxygen isotope concentrations in ancient rocks, as well as physical proof, showing firm evidence of glaciers, 2.9 billion years ago.
“We found extremely well-preserved glacial deposits close to the gold fields of South Africa. This is one of the few areas which remain fairly intact and unchanged from the early Earth,” said Professor Ilya Bindeman from the University of Oregon, US.
“These deposits are fossilised glacial moraines, which are basically the debris left by a glacier as it gradually melts and contracts. These are the oldest moraine deposits ever found,” Bindeman said.
The researchers were also able to correlate this with analysis of oxygen isotopes from these rocks, which showed that the climate must have been cold when the rocks were deposited.
They looked at 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,” Bindeman said. “This means ice. Couple that geochemical evidence with the moraine evidence, and it means glaciers, the oldest glaciers yet found on Earth,” he added.

The researchers put forward a couple of possible explanations: It may be that this area was close to the poles. Another possibility is that the whole Earth was in a “snowball Earth” period, when low atmospheric concentrations of carbon dioxide and methane 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 Professor Axel Hofmann, from the University of Johannesburg, South Africa.
“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,” Hofmann added. Source: PTI