Huronian Glaciation: The Long Ice Age 2.4 Billion Years Ago

The Huronian glaciation was a series of ice ages that gripped parts of the Earth roughly 2.45 to 2.22 billion years ago, during the Paleoproterozoic Era. It takes its name from the Huronian Supergroup, a thick sequence of ancient sedimentary rocks on the north shore of Lake Huron in Ontario, Canada, which preserves three separate glacial deposits. The glaciation is among the oldest well-documented ice ages on Earth and is closely tied to the first lasting rise of oxygen in the atmosphere.

The Huronian Supergroup

The Huronian Supergroup is a pile of sedimentary and volcanic rocks up to about 12 kilometres thick, laid down along the southern edge of the ancient Superior craton, the stable core of what is now central Canada. It stretches for more than 300 kilometres between Sault Ste. Marie and the Sudbury and Temagami areas.

Within this sequence, geologists recognise three formations containing glacial deposits. From oldest to youngest they are the Ramsay Lake, Bruce and Gowganda formations. Each is overlain by finer sediments and limestones or sandstones, suggesting that ice advanced and retreated at least three times.

The upper age limit is fixed by the Nipissing diabase, sheets of volcanic rock that cut through the Huronian rocks and are dated to about 2.22 billion years ago. Volcanic rocks near the base give ages of around 2.45 billion years. The glaciations therefore occurred somewhere within that window, though the precise date of each one is uncertain.

How geologists recognise an ancient ice age

The key evidence is a rock called tillite or diamictite: a jumble of stones of many sizes, from pebbles to boulders, set in a fine-grained matrix. Unlike a river or beach, which sorts sediment by size, a glacier carries and dumps everything together. The Gowganda Formation contains some of the most famous Precambrian examples.

Other clues include dropstones, single stones that fell into fine, layered lake or seabed mud, probably after melting out of floating ice. Thinly layered sediments resembling the seasonal layers of modern glacial lakes, and grooved or striated rock surfaces scratched by moving ice, add to the case.

The Canadian geologist A. P. Coleman described the Gowganda tillite in the early 1900s and argued that it recorded a “Lower Huronian” ice age. At the time, the idea of glaciation so deep in the Precambrian was surprising, and it took decades for the interpretation to be widely accepted.

A worldwide event?

Glacial rocks of broadly similar age are known from several continents. They include deposits in Michigan and Wyoming in the United States, Finland and Russian Karelia, the Transvaal Supergroup of South Africa and the Turee Creek Group of Western Australia.

The South African Makganyene Formation is especially important. In 1997 David Evans, Nicolas Beukes and Joseph Kirschvink reported magnetic evidence that it formed close to the equator. If ice reached sea level in the tropics, the planet may have been almost entirely frozen, an early version of the later Snowball Earth glaciations of the Cryogenian Period.

Matching deposits across continents is difficult because dates are imprecise. Some researchers think there were several regional glaciations at different times; others see evidence for one or more global events. Some reconstructions place many of these regions on the margins of the ancient Kenorland supercontinent, which was breaking up during this period.

The link to rising oxygen

The Huronian glaciation overlaps with the Great Oxidation Event, when free oxygen first built up in the atmosphere. Many scientists think the two are connected.

The young Sun was roughly 15 to 20 percent fainter than today, yet the early Earth was not permanently frozen. One explanation is that methane, a powerful greenhouse gas produced by microbes, helped keep the planet warm. When cyanobacteria began releasing oxygen in large amounts, the oxygen would have reacted with methane and removed much of it from the air, cooling the climate. In 2005 Robert Kopp and colleagues argued that this chain of events could have triggered a global freeze.

The Huronian rocks themselves record the chemical change. The older units contain grains of pyrite and uraninite carried by rivers, minerals that break down in the presence of oxygen. Above the Gowganda Formation, the Lorrain Formation includes red, iron-oxide-stained sediments, a sign that oxygen was then present. Sulphur isotope measurements in the sequence also suggest that the atmosphere changed during the glacial interval.

Other factors may have contributed, including increased weathering of newly exposed land, which draws carbon dioxide out of the air, and the changing positions of continents. Whether oxygen rose before, during or after each glaciation is still under study.

By the numbers

Myths and facts

Myth: it was one ice age lasting 300 million years. The often-quoted figure of about 300 million years describes the span within which the glaciations happened, not the length of a single continuous ice age. The rocks show at least three separate glacial episodes, with warmer intervals between them.

Myth: the whole planet was certainly frozen. A global “Snowball Earth” at this time is a serious hypothesis, supported mainly by the low-latitude Makganyene deposit, but it is not established. The extent of the Huronian ice in particular remains uncertain.

Legacy for science and for Ontario

The Huronian rocks have shaped how geologists think about the early Earth. They provided some of the first evidence that ice ages were not a feature only of recent geological time, and they remain a reference section for studying the transition from an oxygen-free to an oxygenated world. Researchers return to the same road cuts and lakeshore outcrops to collect samples for new isotope and dating methods.

The rocks also have economic importance. Ancient river gravels near the base of the sequence host the uranium deposits of the Elliot Lake district, which were mined from the 1950s to the 1990s. The uranium was concentrated as grains of uraninite that could only survive transport because the air then contained so little oxygen, a direct link between the region’s mining history and the chemistry of the atmosphere more than two billion years ago.

Timeline

• c. 2.45 billion years ago: Volcanic rocks at the base of the Huronian Supergroup are erupted as rifting begins.
• c. 2.45–2.3 billion years ago: The Ramsay Lake and Bruce glaciations leave glacial deposits in Ontario.
• c. 2.3 billion years ago: The Gowganda glaciation, the most extensive of the three, deposits widespread tillites.
• Before c. 2.22 billion years ago: After the Gowganda glaciation, the red sediments of the Lorrain Formation record an oxygenated environment.
• c. 2.22 billion years ago: The Nipissing diabase intrudes the Huronian rocks, marking a minimum age for the glaciations.
• Early 1900s: A. P. Coleman describes the Gowganda tillite as evidence of a Precambrian ice age.
• 1997: Evans, Beukes and Kirschvink report evidence that the Makganyene glaciation in South Africa occurred near the equator.
• 2005: Kopp and colleagues propose that oxygenic photosynthesis triggered a Paleoproterozoic Snowball Earth.

Why it matters

The Huronian glaciation shows that Earth’s climate has swung to extremes long before humans existed. It is also one of the clearest examples of life altering the climate: if the methane hypothesis is correct, microbes that learned to produce oxygen pushed the planet into deep cold. Studying these rocks helps scientists understand how the atmosphere, oceans and life interact over vast stretches of time.