Ziyi Zhu wanted to know when these ranges formed, but then she also stumbled upon their influence on life on Earth
Nestled in a zircon grain, uranium marks the march of time, as its atoms inexorably turn to lead.
Using this property of zircon, researchers have found times that supermountains existed, and, perhaps more importantly, how they impacted evolution.
Ziyi Zhu, a PhD candidate from the Australian National University and first author of an Earth and Planetary Science Letters paper on the work, described her reaction when she made the connection.
“I was thrilled about the finding. I am always amazed by the large amount of information that tiny zircons contain,” she told Truly Curious.
Long before humans turned up, there were mountains that rivaled the Himalayas in height and stretched distances as long as a return trip from New York to Los Angeles. But little was known about when these “supermountains” existed. Until now.
To understand the history of supermountains, scientists look not in the heights of the Himalayas or the Alps, but down in the rivers.
Zhu says that river sediments are “a representative sample of the whole drainage basin.” She and her colleagues used the worldwide river database to study sediments from 52 of the world’s major rivers, to allow for a fuller representation of Earth’s crust.
A right turn
Though now well-known for her research, Zhu’s career could have been quite different, had it not been for a critical decision made in her undergraduate years.
Zhu had developed a passion for the outdoors and nature ever since her mother, Li, took her to the countryside, venturing onto a new path every time. But when choosing her undergraduate degree, she bowed to her family’s suggestion to pick a field that promised a good salary and job security: computer science. But sitting before a computer lacked the allure the natural sciences provided.
“Every time I am surrounded by mountains, rivers, trees, rocks…, I feel a deep sense of peace and joy,” Zhu said. After six months of creating and correcting code, she threw up computer science to enroll for an earth science major at China’s University of Geosciences in Wuhan.
When she went on to do a PhD in geochemistry in Canberra, a unique dataset was awaiting her: a record of lutetium, the heaviest rare earth element, one found in the mineral zircon. As mentioned, zircon, through its uranium content, provided a radiometric clock for the age at which the mineral with the lutetium formed.
Pressure makes the difference
Lutetium content tells the story of the torrid relationships seen at high/ultrahigh pressures between the numerous garnet minerals and the less abundant zircons. The crux is at around 1.2 Gpa (that’s over 10 times the pressure of the deepest part of the ocean, the Mariana Trench). At roughly 45 km within the Earth’s crust, the garnets steal most of the lutetium from the zircons. But the zircons have the last laugh: they are still found millions of years later in the sediments of rivers such as the Seine and Loire in France. That is where Zhu and her colleagues collect and study them.
“I really enjoy playing around with the zircon data, understanding the processes that are responsible for the chemical variations, and piecing together the stories hidden behind the data,” Zhu said.
The story she came up with was that garnets took the lutetium away and left the zircons low on lutetium in areas of high pressure. This provided essential information about when supermountains formed. Based on this idea, Ziyi and her team identified two main periods of supermountain formation: 2,000-1,800 million years ago, and 650-500 million years ago. These coincide with the formation of two of Earth’s supercontinents, Nuna and Gondwana.
Alone, this would have been a remarkable find. However, the pandemic gave the team time and space to reflect and expand on the idea.
From mountains to metazoa
“At the time, I had more energy and time to read papers from other researchers,” Zhu recalled. One day she happened to read a paper from Payne et al. 2009 about biovolume (the volume of the largest preserved organisms), and she noticed an uncanny similarity in the timing.
The Nuna supermountain period aligned with the advent of the first macroscopic fossils, some 2,000-1,800 million years ago. The Gondwana period occurred approximately 575 million years ago, when large animals appeared, followed by another explosion of even larger animal-like organisms at around 540 million years ago.
Zhu remembers that she “spent the whole evening putting the two images together,” and, sure enough, they aligned.
This was not surprising to her. Supermountain-formation goes hand in hand with erosion. The result is a flux of nutrients entering the oceans, promoting the growth of primitive organisms, such as algae and cyanobacteria. The oxygen the algae and cyanobacteria excreted, and the rapid burial of organic carbon that could have taken up the gas, led to an increase in atmospheric oxygen, the basis of most modern respiration in larger and more complex life. Geologists describe these sharp increases as Great Oxygenation Events.
For a geologist like Zhu, this was new territory. She was out of her depth, but was convinced that going down this rabbit hole would provide a new dimension to her work.
She read paper after paper to inform her writing, but her submission was initially rejected.
Zhu blamed her shortcomings in biology for that.
“At the beginning, I was too assertive,” she said. “I wasn’t experienced enough to be more open, to leave space for more work… I didn’t realize that the arguments I provided were just one hypothesis that fits the observation.” However, with the support of co-author Jochen J. Brocks, a biogeochemist from the same institution, Zhu got the help she needed in biology to buttress her story about how the geology matched the evolutionary biology.
Funnily enough, when the paper was accepted, there was no big celebration.
“I felt a sense of relief. Finally, after almost a year of submissions, it got through,” was how Zhu put it. This paper is one of three she will submit to gain her PhD qualification later this year. What happens next is still unclear.
“Maybe what I want to pursue will change in the future… but understanding Earth’s geological evolution through time is what I think fascinating at the moment, so I will just keep on going down this path,” she said.
Zhu described her paper as a testament to collaboration. That went from using data sets collected by several researchers over 20 years to bringing together the fields of geology and biology.
“It is built on lots of people’s efforts,” she said.
Thanks to them, we now have a clearer idea of how mountains may have influenced our evolution.
Hannah Brightley is a science communicator based in New Zealand. She has an M.Sc. in geology and is passionate about working with scientists to showcase their research to the general public.
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Life on Earth may be older than believed