The northeast corner of South Africa is a patchwork of landscapes, from open plains dotted with hoodoos in the interior and wetlands along the coast to startlingly dense coniferous forests – a relic of the colonial period, when exotic species were imported from Europe to create huge pine plantations. Beneath this diversity lies one of the largest deposits of gold in the world, and below, more than five kilometers underground, lies a geologic treasure trove of deep times: layers of rock that bear witness to the first deep freeze of Earth.
According to a recent article published in Geochemical Outlook Letters, these rocks contain multiple clues to the oldest glaciers in the world, patches of ice that covered the region 2.9 billion years ago. Now the big question is: what was the ice doing there?
THE oldest glaciers still hanging today are in Antarctica dry valleys, and are around a million years old, although some researchers believe that at least one, buried under thick sediment, could be up to eight million years old. However, this ancient frozen water is only a drop in the ice bucket compared to the longevity of glaciers during the Huronian, a period also known as “Snowball Earth”.
“Snowball Earth” has been applied to a few icy chapters in the planet’s ancient history (geologists love a catchy term as much as anyone else). But the Huronian glaciation was, until the recent South African discovery, Earth’s oldest known ice age – and it was a doozy. Overall, the period spanned over 200 million years, beginning around 2.45 billion years ago. Although there were some climatic fluctuations during the Huronian, including relatively warm and humid periods, there were several major global coolings that lasted 10 million years or more.
The discovery in South Africa, at sites near the border of Eswatini (formerly Swaziland), reveals Earth’s first glaciers formed about half a century ago billion years before the Huronian. We don’t know much about this period of Earth’s history; when the South African glaciers existed, the planet was about 1.6 billion years old and already home to microbial lifeforms, including some of the earliest multicellular organisms. But more complex life will not evolve until more than a billion years later.
To find the oldest glaciers, the research team analyzed levels of oxygen isotopes trapped in rock strata and identified the specific geochemical signature of an icy climate. Within the layers, they also documented the oldest known moraine on the planet, the distinctive dumping ground of debris left by the melting and contraction of a glacier.
The moraine and other evidence has been preserved for almost 3 billion years because it rests on the Kaapvaal Craton, one of the oldest pieces of continental crust. This ancient landmass, found beneath much of South Africa, began forming around 3.7 billion years ago and may have been part of Earth’s first continent, along with Australia Pilbara Craton.
Geologists like to debate exactly when and where this first continent was formed, and the further back in time we go, the more obscure the data becomes. The same goes for our understanding of plate tectonics, the slow dance of continents across the surface of the planet. The co-authors of the new South African glacier paper believe the area where they found the ancient moraine may have been tectonically close to a pole and part of an ice sheet before it does not slide back to its current location for millions of years.
However, there is another possible explanation for ultra-old glaciers. They may be the first solid proof of a long and hotly debated lost ice age: the very first Snowball Earth, a deep planetary freeze that would have covered almost all of the Earth in a million winters or more.