120 million years of intact geological record discovered | Polarjournal
Ordovician black shales of the Mount Hare Formation, Road River Group (c. 465 million years old) rise above conglomerates of the Aberdeen Member. The dangerous rapids of Aberdeen Canyon (Nan Zhak Nadhàdlaii), created by the Peel River cutting through the resistant conglomerates, are shown at lower left. Photo: Erik Sperling

Hundreds of millions of years ago, in the middle of what would eventually become Canada’s Yukon Territory, an ocean swirled with armored trilobites, clam-like brachiopods and soft, squishy creatures akin to slugs and squid. A trove of fossils and rock layers formed on that ancient ocean floor have now been unearthed by an international team of scientists along the banks of the Peel River a few hundred miles south of the Arctic’s Beaufort Sea. The discovery reveals oxygen changes at the seafloor across nearly 120 million years of the early Paleozoic era, a time that fostered the most rapid development and diversification of complex, multi-cellular life in Earth’s history.

“It’s unheard of to have that much of Earth’s history in one place,” said Stanford University geologist Erik Sperling, lead author of the study published in early July in the journal Science Advances. Most rock formations from the Paleozoic Era have been broken up by tectonic forces or eroded over time.

Oxygen was scarce in the deep water of this and other oceans at the dawn of the Paleozoic, roughly 541 million years ago. It stayed scarce until the Devonian, roughly 405 million years ago, when, in a geological blink – no more than a few million years – oxygen likely rocketed to levels close to those in modern oceans and the diversity of life on Earth exploded. Big, predatory fish appeared. Primitive ferns and conifers marched across continents previously ruled by bacteria and algae. Dragonflies took flight. And all of this after nearly four billion years of Earth’s landscapes being virtually barren.

“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.”

Erik Sperling, geologist at Stanford University

Scientists have long debated what might have caused the dramatic shift from a low oxygen world to a more oxygenated one that could support a diverse web of animal life. But until now, it has been difficult to pin down the timing of global oxygenation or the long-term, background state of the world’s oceans and atmosphere during the era that witnessed both the so-called Cambrian explosion of life and the first of Earth’s “Big Five” mass extinctions, about 445 million years ago at the end of the Ordovician.

According to Sperling, continuous recording is necessary to make comparisons over these vast periods of time and to understand long-term trends.

A – Paleogeographic map of the Laurentia continental block (ca. 480 Ma) showing the study area (rectangle outlined in black). B – Early Paleozoic reconstruction of the paleoenvironment of Yukon, Northwest Territories, and British Columbia. 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 site in the Richardson Trough. C – The majority of the Road River group consists of organic-rich mudstone/shale and calcareous mudstone that have not been scoured and mixed by living organisms (bioturbation). Shown here are dark gray to black shales and limestone mudstones of the Mount Hare Formation from the Tremadocian. D – Trace fossils occur sporadically throughout the Road River Group, but the primary interval of extensive, continuous bioturbation occurs in the Upper Ordovician. Graphic: Science Advances/ Sperling et al. 2021

Context for past life
With permission from the Na Cho Nyak Dun and Tetlit Gwitch’in communities in Yukon, Sperling’s team, which included researchers from Dartmouth College and the Yukon Geological Survey, spent three summers at the Peel River site. Arriving by helicopter, the research team hacked through brush with machetes beside Class VI rapids to collect hundreds of fist-sized samples of rock from more than a mile of interbedded layers of shale, chert and lime mudstone.

Back at Sperling’s lab at Stanford, a small army of summer undergraduates and graduate students worked over five summers to help analyze the fossils and chemicals entombed in the rocks. “We spent a lot of time splitting open rocks and looking at graptolite fossils,” Sperling said. Because graptolites evolved a vast array of recognizable body shapes relatively quickly, the pencil-like markings left by the fossils of these colony-dwelling sea creatures give geologists a way to date the rocks in which they’re found.

Once the researchers had finished identifying and dating graptolite fossils, they ground the rocks in a mill, then measured iron, carbon, phosphorous and other elements in the resulting powder to assess the ocean 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.

Graptolites were polyp-like, colony-forming marine animals that needed a low-oxygen environment to survive. They possibly became extinct as oxygen levels on Earth increased. Photo: Wikipedia/KevinWalsh

Winners and losers
The data show low oxygen levels, or anoxia, likely persisted in the world’s oceans for millions of years longer than previously thought – well into the Phanerozoic, when land plants and early animals began to diversify. “The early animals were still living in a low oxygen world,” Sperling said. Contrary to long-held assumptions, the scientists found Paleozoic oceans were also surprisingly free of hydrogen sulfide, a respiratory toxin often found in the anoxic regions of modern oceans.

When oxygen eventually did tick upward in marine environments, it came about just as larger, more complex plant life took off. “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.” In this hypothesis, the influx of nutrients to 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 off graptolites. “Although more oxygen is really good for a lot of organisms, graptolites lost the low oxygen habitat that was their refuge,” Sperling said. “Any environmental change is going to have winners and losers. Graptolites might have been the losers.”

Source: Stanford University press release

Link to the study: Erik A. Sperling, Michael J. Melchin, Tiffani Fraser, Richard G. Stockey, Una C. Farrell, Liam Bhajan, Tessa N. Brunoir, Devon B. Cole, Benjamin C. Gill, Alfred Lenz, David K. Loydell, Joseph Malinowski, Austin J. Miller, Stephanie Plaza-Torres, Beatrice Bock, Alan D. Rooney, Sabrina A. Tecklenburg, Jacqueline M. Vogel, Noah J. Planavsky, Justin V. Strauss. A long-term record of early to mid-Paleozoic marine redox change. Science Advances, 2021; 7 (28): eabf4382 DOI: 10.1126/sciadv.abf4382

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