Deep in the wilderness of the Peel River watershed in the northeast corner of the Yukon sits a peerless historical record written in layers of rock formed from the bed of an ancient ocean.
Researchers have recently unearthed fossils and rock layers, which tell a tale spanning nearly 120 million years of the early Paleozoic era. According to a report on the research team’s findings published by Stanford University, the time period frozen in the sedimentary rock was marked by oxygen changes at the sea floor that kicked off the fastest development and diversification of complex multi-cellular life in Earth’s history.
The research team, led by Erik Sperling, an assistant professor of geological sciences at Stanford, unearthed the fossils of armoured trilobites and life forms similar to today’s clams, slugs and snails.
There are other places on earth where researchers can scan layers of rock in search of information about the Paleozoic era, but Sperling said he has no knowledge of a record as complete as the one along the Peel that he and his collaborators studied.
Sperling said that in most places, the ancient rock formations are eroded or broken up by tectonic forces.
At the area researchers studied, south of the Beaufort Sea, Sperling said there is a long record that shows basically no change in water depth or basin type.
“It’s unheard of to have that much of the Earth’s history in one place,” he said.
Sperling said the long record allows for important comparisons across huge swathes of the Earth’s history.
Drawing those comparisons took years of work both in the Yukon wilderness and back at Sperling’s Stanford lab.
According to Stanford, the work at the Peel River site was done with permission from the Na Cho Nyak Dun and Tetlit Gwitch’in communities. Sperling’s team also included researchers from Dartmouth College and the Yukon Geological Survey. Sperling said they made trips to the site three summers in a row.
Sperling had worked on other geological studies in the Yukon since 2011, but he described the first excursion to the Peel as a bit of a shot in the dark. He said the area had received some attention from researchers during Geological Survey of Canada field campaigns in the 1960s and 70s but there hadn’t been a study of it done in decades. As Sperling and his team boarded the helicopter to fly into the unknown, a publication from 1972 was their best guide to the area’s geological make up.
A stretch of river that Sperling said spanned between eight and 10 kilometres was chosen as the researchers’ focus. He said exposed rock along the river was unbroken by faults or other forces. Where other sites might offer a good record of a five-million-year period followed by a break and then another five million years, the rocks along the Peel offered an unfragmented look at over 100 million years of the planet’s history.
The section of river chosen for study proved to be well worth the long journey and the concerns about bears near the team’s wilderness campsites. Living in tents on a diet of freeze-dried meals, the researchers began the work of collecting hundreds of fist-sized rock samples from the layers of shale, chert and lime mudstone.
Sperling said flat ground along the river made for relatively easy travel as they collected samples but when the team had to skirt around cliffs and rapids they were faced with steep terrain and hordes of mosquitoes in the dense and brushy boreal forest.
Along with the three July journeys to the Peel watershed, the study consisted of thorough lab analysis by what Josie Garthwaite of Stanford’s School of Earth, Energy and Environmental Sciences called “a small army of summer undergraduates and graduate students.”
Over the course of five summers, the fossils and chemicals preserved in the rocks were studied.
Sperling said a lot of this lab time was spent splitting open rocks to look at graptolite fossils.
According to information from the British Geological Survey, graptolites were tiny animals which lived in colonies encased in interconnecting tubes between 520 and 350 million years ago during the Cambrian and Carboniferous periods of Earth’s history. The seabed-dwelling creatures lived attached to boulders or rooted in soft mud at first but later, they became free floating to eat single-celled organisms more effectively in shallower water.
When fossilized, the graptolites leave pencil-like markings in the rocks. Because they evolved an array of recognizable body shapes over a relatively short time, the graptolites offer researchers like Sperling a way to date the rocks they found.
Once the fossils were identified, researchers ground up the rock samples and measured levels of iron, carbon, phosphorus and other elements in the resulting powder. This allowed them to draw conclusions about conditions in the ancient ocean at the time and place the rock layer was formed.
“The most intriguing part for me was learning about geochemistry and how it is so useful for figuring out the environmental backdrop of ancient times. It really gives you the whole context for past life,” said Stephanie Plaza-Torres, a co-author of Sperling’s study while she was an undergrad.
Sperling and his co-authors published their findings in the Science Advances journal on July 7. The 120-million year record they analyzed shows a rapid oxygenation of the ocean coming at about the same time as larger, more complex plant life on earth.
“There’s a ton of debate about how plants impacted the Earth system,” Sperling said.
The results of the research on the Peel support a hypothesis that as plants evolved across the world they increased nutrients in the ocean, driving oxygenation.
The findings Sperling and his colleagues published are only a part of the story the Peel’s rocks can tell.
He said the study his team worked on looked at the 120-million-year record in its entirety but added there are at least 15 interesting events spread across those millions of years that could benefit from closer study — he hopes to do some of it himself one day.
Sperling said the Yukon is a very special area of study for him, both because of the rich geological history underfoot and the place it is today.
Contact Jim Elliot at email@example.com