Yukon wilderness yields longest known continuous record from Paleozoic Era

(A) Paleogeographic map of Laurentia (ca. 480 Ma) with the study area depicted in the box [from (30)]. (B) Early Paleozoic paleoenvironmental reconstruction of Yukon, Northwest Territories, and British Columbia [modified after (30)]. The northwestern Laurentian margin was divided into a series of shallow-water carbonate platforms (blue) and deep-water shale basins (gray). The Road River Group was sampled at the Peel River locality in the Richardson trough. (C) The majority of the Road River Group consists of unbioturbated interbedded organic-rich mudstone/shale and lime mudstone with local diagenetic chert replacement. Shown here are Tremadocian dark gray to black shale and lime mudstone of the Mount Hare Formation. Seated geologist is at ~38 m in section J1727 [see (30) for detailed sedimentological discussion of the Peel River section]. (D) Trace fossils occur sporadically throughout the Road River Group, but the primary interval of extensive, continuous bioturbation occurs in the lower to middle Katian (Upper Ordovician). Bedding plane view of heavily bioturbated Katian strata from 81 to 93 m in J1518; 30-cm-long hammer for scale. MCE, Misty Creek embayment; MRE, Meilleur River embayment. Photo credit: Erik Sperling, Stanford University.

TORONTO — Millions of years ago, the middle of Yukon was covered by an ocean, and over years of excavation, researchers have uncovered a treasure trove of Earth’s history in the territory.

Researchers from Stanford University and Dartmouth College, with the permission of Na Cho Nyak Dun and Tetlit Gwitch'in communities, helicoptered into Yukon wilderness and worked their way through brush and forest to arrive at the Peel River where for three summers they collected hundreds of samples alongside river rapids.

Their findings were published on July 7 in Science Advances and they detail the longest known continuous record of the Paleozoic Era, shining a light on 120 million years of ancient Earth.

"It's unheard of to have that much of Earth's history in one place," Stanford University geological scientist Erik Sperling and lead author of the study said in a press release.

Most fossils and rock formations from this time period on Earth have broken up due to tectonic forces or erosion over millions of years.

“There's nowhere else in the world that I know of where you can study that long a record of Earth history, where there's basically no change in things like water depth or basin type,” he added.

The fossils that the researchers found uncover the life that would have been found in Yukon’s ancient ocean, including trilobites, brachiopods and squishy slug and squid-like creatures.

Early on in the Paleozoic Era, 541 million years ago, oceans had very little oxygen and remained that way for millions of years until in a “geological blink,” or a few million years, oceans swelled with oxygen, large fish appeared, and the Earth’s barren land became covered in conifers and ferns. The why’s and how’s of the sudden appearance of an abundant land have been long debated by scientists and the Yukon discovery could help provide answers.

"In order to make comparisons throughout these huge swaths of our history and understand long-term trends, you need a continuous record," said Sperling.

The researchers spent time in the lab breaking open rocks looking for graptolite fossils because, according to the press release, they “evolved a vast array of recognizable body shapes.” It helped provide the geologists with a date for the fossils.

After identifying and dating the graptolite fossils, they ground up the rocks and measured the resulting powder for elements such as phosphorus, iron and carbon to assess the conditions of the ocean at the time and place the layers formed.

The researchers found that the ocean had low oxygen for millions of years longer than they thought and that this likely applied to all of the world’s oceans, according to the researchers.

"The early animals were still living in a low oxygen world," Sperling said.

The researchers found that just as marine oxygen levels began to increase, larger, complex plant life also increased.

"There's a ton of debate about how plants impacted the Earth system," Sperling said. "Our results are consistent with a hypothesis that as plants evolved and covered the Earth, they increased nutrients to the ocean, driving oxygenation."