Lauren Fuge
Australian researchers have found evidence that supermountains – as tall as the Himalayas and as wide as supercontinents – formed at two critical moments in the evolution of life.
“There’s nothing like these two supermountains today,” says Ziyi Zhu, a PhD candidate at the Australian National University (ANU). “It’s not just their height – if you can imagine the 2,400 km long Himalayas repeated three or four times you get an idea of the scale.”
Zhu is the lead author of a new study published in the journal Earth and Planetary Science Letters, in which she and colleagues used zircons to track when these massive mountains formed.
Their findings fit in with what we know about the supercontinent cycle: the idea that the most fundamental pulse of the planet is the formation and breaking-up of the continents into supercontinents. This cycle appears to operate over 700–800 million years, and the dates these supermountains formed line up with when continents were smashing together into supercontinents.
The first example, called the Nuna Supermountain after the supercontinent that was forming at the time, dates back to between 2,000 and 1,800 million years ago.
“It coincides with the likely appearance of eukaryotes, organisms that later gave rise to plants and animals,” Zhu says.
“The second, known as the Transgondwanan Supermountain, coincides with the appearance of the first large animals 575 million years ago and the Cambrian explosion 45 million years later, when most animal groups appeared in the fossil record.”
According to co-author Professor Jochen Brocks, also from ANU: “What’s stunning is the entire record of mountain building through time is so clear. It shows these two huge spikes: one is linked to the emergence of animals and the other to the emergence of complex big cells.”
This study is an addition to the growing body of evidence that mountains played a crucial role in the rise of life on Earth, an idea first proposed in a 2006 paper also published in Earth and Planetary Science Letters.
Wait, how can mountains influence evolution?
It’s all in the erosion.
When mountains form, they bring elements from deep in Earth’s interior up to the surface. Then, as rain and wind and glaciers wear away at the peaks over millennia, these elements – such as iron and phosphorous – are released and make their way through rivers to the ocean.
This helps drive systems like the climate and carbon cycle, as well as supply nutrients key for the development of life.
It’s thought that for a long time in the planet’s history, life was being “held back” because the nutrients it needed to develop were not in abundance. For example, eukaryotes appeared on Earth about 1.7 billion years ago but didn’t rise to dominance until some 800 million years ago. This time interval is known as the ‘Boring Billion’, because there was almost no advance in evolution.
“The slowing of evolution is attributed to the absence of supermountains during that period, reducing the supply of nutrients to the oceans,” explains co-author Professor Ian Campbell from ANU.
This idea is supported by previous research, which has also suggested that the reason for this hiatus is a lack of mountain formation.
Then, when plate tectonics smashed plates into supercontinents and thrust massive mountain chains up above the surface, a new round of erosion supplied the key ingredients that life needed to take off.
These nutrients may have increased oxygen levels in the atmosphere.
“The early Earth’s atmosphere contained almost no oxygen,” Zhu says. “Atmospheric oxygen levels are thought to have increased in a series of steps, two of which coincide with the supermountains.
“The increase in atmospheric oxygen associated with the erosion of the Transgondwanan Supermountain is the largest in Earth’s history and was an essential prerequisite for the appearance of animals.”
“This study gives us markers,” Campbell says, “so we can better understand the evolution of early, complex life.”
Seems like a grand claim – how was the study conducted?
Zhu and colleagues looked at zircons: hardy minerals that are like crystalline time capsules. Importantly, when they form they act like “sponges” for many different elements.
“They’re gorgeous,” says Professor Alan Collins, a geologist at the University of Adelaide who was not involved in this study. “They have a lot of small trace amounts of lots of different elements – like rare earth elements, uranium and all sorts of stuff in them.”
If you find different concentrations of elements within zircons, he explains, this can tell you something about how it formed.
Zhu and team looked at zircons with a low content of lutetium, a heavy rare earth element. They argue that the zircons are depleted in lutetium because of the environment they were growing in – in a high-pressure “soup” of magma beneath a volcano in a mountain range.
Here, lutetium and other elements would have been floating around along with other elements. A variety of minerals would have been growing, each taking in different concentrations of the available elements. The study argues that zircon was growing in competition with garnet, which would have taken in a lot of lutetium. Garnet can only grow in significant quantities in intensely high-pressure environments – like those beneath the weight of massive mountains.
“It’s quite a long logic train, and it is controversial,” Collins says. “There are other reasons why you might have those sorts of depletions, but… we’re getting into a world where we’re getting an awful lot of data. We’re starting to be able to really treat these things in quite rigorous statistical ways to look for these trends.”
That’s what this study is doing: although the team are making some big leaps, Collins believes they are doing so in a careful and logical way, comparing their data with other datasets about the rise of oxygen and life.
How can their argument be backed up?
According to Collins, there are a number of different chemical proxies pointing to massive mountain chains existing at this time, but there are ways to verify and improve upon this new data.
The study looked at zircon grains from a global dataset, which were not attached to a certain place. One way to verify their findings is to look for these zircons in situ, at the suspected location of an ancient mountain range.
Collins is currently involved in a related study doing just this, looking at mountain ranges from 1 billion to 500,000 million years ago.
“We’re looking at the remains of those actual mountain ranges themselves, which exist, though they’re all flat now,” he says.
“Now, you see the rocks that had formed at the bottom of those mountains exposed, and then from those we’re trying to work out the pressures that they were formed and/or they experienced – and then from that, work out how much rock must have been above them, so how high the mountains must have been at that time.”
This method, he says, could be an independent way to verify the existence of such supermountains, and would make it possible to map where those mountains actually were on the surface of the Earth.
“Once you can start to map out where these mountains are, you can start to actually put them into the global climate models we use today,” Collins says, “and try and build that back in time.”
A number of different research teams are working on this at the moment, trying to model what the world looked like back then, and thus see how geological processes influenced surface processes like biology and chemistry.
This new study is just another thread in the complex story of our planet, and in the quest to understand the relationships between the Earth’s systems through deep time.