UMD Geologists Studying Shale Formation Discovered How Rising Seas Created a ‘Kill Mechanism’ 350 Million Years Ago
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The Bakken Shale Formation—a 200,000-square-mile shale deposit below parts of Canada and North Dakota—has supplied billions of barrels of oil and natural gas to North America for 70 years. A new discovery there opens a window on Earth’s complicated geological history and explains a mass die-off of marine life long before the rise of the dinosaurs.
A research team including geologists from the University of Maryland, George Mason University and the Norwegian oil and gas company Equinor developed a new framework for analyzing paleontological and biogeochemical data extracted from the formation’s rock. In a paper published today in the journal Nature, the team pinpoints a major trigger of several closely spaced biotic crises, or mass extinctions, during the late Devonian Period almost 350 million years ago: euxinia, or the depletion of oxygen and expansion of hydrogen sulfide in large bodies of water. The findings may carry lessons applicable to the modern climate crisis, the researchers said, demonstrating links between sea level, climate, ocean chemistry and biotic disruption.
“There have been other mass extinctions presumably caused by expansions of hydrogen sulfide before, but no one has ever studied the effects of this kill mechanism so thoroughly during such a critical period of Earth’s history,” said UMD Geology Professor Alan Jay Kaufman, a senior author of the paper.
The late Devonian Period was a “perfect storm” of factors that played a large role in forming the Earth we know today, Kaufman said. Vascular plants and trees were especially crucial to the process; as they expanded on land, plants stabilized soil structure, helped spread nutrients to the ocean, and added oxygen and water vapor to the atmosphere while pulling carbon dioxide out of it.
The Devonian Period ended around the same time the Bakken sediments accumulated, allowing the layers of organic-rich shale to ‘record’ the environmental conditions that occurred there. Because the Earth’s continents were flooded during that time, various sediments including black shale gradually accumulated in inland seas that formed within geological depressions like the massive Williston Basin, which preserved the Bakken formation that lies under parts of North Dakota and Montana and stretching north to Manitoba and Saskatchewan.
Undergraduate laboratory assistant Tytrice Faison ’22—a geology major who joined Kaufman’s lab after taking a course with him through the Carillon Communities living-learning program—prepared more than 100 shale and carbonate samples taken from the formation. After analyzing the samples, Kaufman, Faison and the rest of the Bakken team deciphered clear layers of sediment representing three key biotic crises known as the Annulata, Dasberg and Hangenberg events—the last of which is associated with one of the greatest mass extinctions in Earth history.
“We could see anoxic events (where large expanses of water were oxygen-deprived) distinctly marked by black shale and other geochemical deposits, which are likely linked to a series of rapid rises in sea level” linked to melting South Pole ice sheets, Kaufman explained
Higher sea levels would have flooded areas known as interior continental margins, causing high levels of nutrients such as phosphorus and nitrogen to trigger algal blooms and sap the water of oxygen. The resulting dead zones in turn would have increased toxic hydrogen sulfide right where most marine animals lived, killing both ocean-dwelling creatures and some living on land around the shoreline.
The team’s research may also apply to the oceans of today affected by global warming, Kaufman said. He compared the ocean’s circulatory system to a “conveyor belt” carrying nutrients, oxygen and microorganisms from place to place.
“This oceanic jet stream helps to spread life-sustaining oxygen through the oceans,” Kaufman explained. “If that conveyor belt were to be slowed down due to global warming, parts of the ocean might be deprived of oxygen and potentially become euxinic.”
The collateral damage caused by global warming might then promote animal migration out of dead zones or put Earth on a path to decreased diversity and increased rates of extinction, he added.
“Our study helps us to understand several things about Earth’s growing pains across a critical transition from a world we would not recognize today to one we would find more familiar,” Kaufman said. “It provides evidence for a kill mechanism that may be general to many of the many mass extinctions that occurred in the past, but also explains the origin of a major source of oil and gas to the United States.”
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