“The early animals were still living in a low oxygen world,” Sperling said. 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. ![]() 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. 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. 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. “We spent a lot of time splitting open rocks and looking at graptolite fossils,” Sperling said. 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.īack 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. 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. “In order to make comparisons throughout these huge swaths of our history and understand long-term trends, you need a continuous record,” said Sperling, an assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). ![]() 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. 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. And all of this after nearly four billion years of Earth’s landscapes being virtually barren. ![]() Primitive ferns and conifers marched across continents previously ruled by bacteria and algae. 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. Oxygen was scarce in the deep water of this and other oceans at the dawn of the Paleozoic, roughly 541 million years ago.
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