Hundreds of millions of years ago in the middle of what would eventually become Canada’s Yukon Territory, a sea of armored trilobites, mussel-like brachiopods and soft, squishy creatures resembling snails and squid swirled.
A trove of fossils and rock layers formed on the ancient seabed has now been uncovered by an international research team along the banks of the Peel River a few hundred miles south of the Arctic Beaufort Sea. The discovery reveals oxygen changes on the ocean floor for nearly 120 million years from the early Paleozoic era, a time that promotes the fastest development and diversification of complex, multicellular life in Earth’s history.
“It’s unheard of to have so much of Earth’s history in one place,” said Stanford University geological scientist Erik Sperling, lead author of a July 7 study describing the team’s findings in Scientific progress. Most rock formations from the Paleozoic era have been broken up by tectonic forces or eroded over time. “There is no other place in the world that I know of where you can study as long as an overview of the history of the Earth, where there is virtually no change in things like water depth or basin type.”
Oxygen was scarce in the deep waters of this and other oceans at the beginning of the Paleozoic about 541 million years ago. It remained scarce until Devonia for approx. 405 million years ago, when oxygen probably rocketed to levels close to those in modern oceans, and the diversity of life on Earth exploded in a geological flash – no more than a few million years. Large predatory fish appeared. Primitive ferns and conifers marched across continents previously ruled by bacteria and algae. Dragonflies took flight. And all this after almost four billion years, when the landscapes of the Earth were almost barren.
Researchers have long debated what could have caused the dramatic shift from a low oxygen world to a more oxygenated one that could support a diverse orbit of wildlife. But until now, it has been difficult to determine the time of global oxidation or the long-term background status of the world’s oceans and atmosphere at the time, which witnessed both the so-called Cambrian explosion of life and the first of the Earth’s “Big Five” mass extinctions for approx. 445 million years ago at the end of the Ordovician.
“To be able to compare through these vast sections of our history and understand long-term trends, you need a continuous record,” said Sperling, an assistant professor of geological sciences at the Stanford School of Earth, Energy & Environmental Sciences (Stanford Earth).
Context to past lives
With the permission of the Na Cho Nyak Dun and Tetlit Gwitch’in communities in the Yukon, Sperling’s team, which included researchers from Dartmouth College and the Yukon Geological Survey, spent three summers at the Peel River site. When we arrived by helicopter, the research team hacked a brush with machetes next to Class VI currents to collect hundreds of fist-shaped rocks from more than a mile of embedded layers of slate, cherry and lime mud.
Back at Sperling’s laboratory in Stanford, a small army of summer and graduate students worked over five summers to help analyze the fossils and chemicals contained in the rocks. “We spent a lot of time splitting open rocks and looking at graptolite fossils,” Sperling said. Because graptolites evolved a wide range of recognizable body shapes relatively quickly, the pencil-like markings left by fossils from these colonial sea creatures give geologists a way to date the rocks where they are found.
When the scientists finished identifying and dating graptolite fossils, they ground the rocks in a mill and then measured iron, carbon, phosphorus and other elements in the resulting powder to assess sea conditions at the time and place where the layers formed. They analyzed 837 new samples from the Peel River site as well as 106 new samples from other parts of Canada and 178 samples from around the world for comparison.
Stephanie Plaza-Torres, a co-author of the study, who analyzed dozens of samples for the project in 2016 as a student in Stanford’s Summer Undergraduate Research in Geoscience and Engineering Program (SURGE), has continued to pursue a PhD student in geological sciences. at the University of Colorado, Boulder. “The most exciting part for me was learning about geochemistry and how it is so useful to find out the environmental background from ancient times. It really gives you the whole context of past lives, ”Plaza-Torres said. “After SURGE, I was hooked on geochemistry.”
Winners and losers
The data show low oxygen levels or anoxia, which probably persisted in the world’s oceans for millions of years longer than previously thought – far into the Phanerozoic, when land plants and early animals began to diversify. “The early animals still lived in a low-oxygen world,” Sperling said. Contrary to long-standing assumptions, the researchers found that Paleozoic oceans were also surprisingly free of hydrogen sulfide, a respiratory toxin often found in the anoxic regions of modern oceans.
When oxygen eventually crossed upward into marine environments, it happened just as larger and more complex plant life took off. “There’s a lot of debate about how plants affected the earth’s system,” Sperling said. “Our results are consistent with a hypothesis that as plants evolved and covered the earth, they increased nutrients to the ocean and carried oxygenation.” In this hypothesis, the influx of nutrients into the sea would have given a boost to primary productivity, a measure of how quickly plants and algae take carbon dioxide and sunlight, turn them into new biomass – and release oxygen in the process.
The change probably killed graptolites. “While more oxygen is really good for many organisms, graptolites lost the low oxygen habitat that was their refuge,” Sperling said. “Every environmental change will have winners and losers. The Graptolites could have been the losers. ”
Co-author Richard Stockey is a PhD student, Tessa Brunoir is a research assistant and Una Farrell is affiliated with the Department of Geological Sciences at Stanford. Liam Bhajan, ’17, studied geological science as a bachelor. Additional co-authors are affiliated with St. Francis Xavier University; Yukon Geological Survey; Trinity College, Dublin; Georgia Institute of Technology; Virginia Polytechnic University and State University; Western University Canada; University of Portsmouth; Dartmouth College; University of Puerto Rico, Mayagüez; Vanderbilt University; and Yale University.
This work was supported by an Ocean Sciences Research Fellowship of the Alfred P. Sloan Foundation, the subsidiaries of the Stanford Program on Deepwater Deposition Systems, the McGee-Levorsen Foundation at Stanford, the National Science Foundation, the Agouron Institute, and the Natural Sciences and Engineering Research Council (Canada).