The Bakken shale formation — a 200,000-square-mile deposit of shale under 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 reveals that the rocks also open a uniquely informative window into Earth’s complex geological history.
A research team, which included geologists from the University of Maryland, George Mason University and Norwegian oil and gas company Equinor, developed a new framework for analyzing paleontological and biogeochemical data extracted from the formation’s rock. Using this technique, the team identified a major trigger of several nearby biotic crises during the late Devonian period nearly 350 million years ago: the rapidity, or depletion, of oxygen and the expansion of hydrogen sulfide in large bodies of water. Published in the journal Nature on March 8, 2023, the team’s findings demonstrate relationships between sea level, climate, ocean chemistry and biotic disturbance.
“For the first time, we can pinpoint a specific kill mechanism responsible for a series of major biotic disturbances during the late Devonian period,” said UMD Geology Professor Alan Jay Kaufman, senior author of the paper. “There have been other mass extinctions possibly caused by hydrogen sulfide expansions in the past, but no one has ever studied the effects of this killing mechanism so thoroughly during such a critical period in Earth’s history.”
According to Kaufman, the late Devonian period was a “perfect storm” of factors that played a big role in how Earth is today. Vascular plants and trees were especially vital to the process. As they spread over land, plants stabilized soil structure, helped spread nutrients to the ocean, and added oxygen and water vapor to the atmosphere while pulling carbon dioxide from it.
“The introduction of land plants capable of photosynthesis and respiration stimulated the hydrologic cycle, which initiated Earth’s capacity for more complex life as we know it today,” Kaufman said.
The Devonian Period ended around the same time as the Bakken sediments accumulated, allowing layers of organic-rich shale to “record” the environmental conditions that occurred there. Because Earth’s continents were flooded at that time, various sediments, including black shale, gradually accumulated in the inland seas that formed within geological depressions such as the Williston Basin, the preserved Bakken Formation.
Undergraduate lab assistant Tytrice Faison (BS ’22, geology) — who joined Kaufman’s lab after taking a class with him through the Carillon Communities living learning program — prepared and analyzed 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 major biotic crises known as the Annulata, Dasberg and Hangenberg events, with the latter crisis associated with one of the largest mass extinctions in history land. .
“We could see anoxic events distinctly marked by black shale and other geochemical deposits, which are likely associated with a series of rapid sea-level rises,” Kaufman explained. “We suspect that sea level may have risen during the pulse events due to the ice sheets currently melting around the South Pole.”
The higher sea level would result in the flooding of the inner continental margins or the transition zone between oceanic and continental crust. In these conditions, high levels of nutrients such as phosphorus and nitrogen could have triggered algal blooms that create low-oxygen zones in large bodies of water. These zones in turn would have increased toxic hydrogen sulfide right where most marine animals would have lived. Under these conditions, animals in the oceans and on land around the coastline would have died out during these late Devonian events.
The team’s research is not exclusive to global biotic disturbances from hundreds of millions of years ago. Kaufman suggests that their findings apply not only to the shallow inland seas of the Devonian Period, but perhaps also to today’s oceans affected by global warming. He compared the ocean’s circulatory system to a “conveyor belt” that transports nutrients, oxygen and microorganisms from place to place.
“Cold, salty water develops in the North Atlantic region before it sinks and eventually makes its way to the Indian and Pacific Oceans, cycling around the globe. This ocean jet stream helps spread life-sustaining oxygen through of the oceans,” Kaufman explained. “If this conveyor belt slows down due to global warming, parts of the ocean may become oxygen-deprived and potentially euxic.”
Collateral damage caused by global warming may then promote the migration of animals out of dead zones or put Earth on a path to reduced diversity and increased extinction rates, he added.
“Our study helps us understand many things about Earth’s growing pains in 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 killing mechanism that may be general to many of the many mass extinctions that have occurred in the past, but also explains the origin of an important source of oil and natural gas in the United States.”