La formación de Bakken Shale ha proporcionado nuevos conocimientos sobre la historia geológica de la Tierra, revelando un desencadenante clave de varias crisis bióticas durante el Devónico tardío: euxinia, o agotamiento de oxígeno y expansión de sulfuro de hidrógeno en grandes masas de agua. Esta investigación no solo ayuda a comprender el pasado de la Tierra, sino que también advierte sobre las posibles consecuencias del calentamiento global, como la disminución de la diversidad y el aumento de las tasas de extinción.

Una importante fuente de petróleo de América del Norte está revelando información sobre una de las extinciones masivas más devastadoras del planeta.

La Formación Bakken Shale, que se extiende por 200 000 millas cuadradas debajo de partes de Canadá y Dakota del Norte, ha sido una fuente prolífica de petróleo y gas natural para América del Norte durante los últimos 70 años. Hallazgos recientes ahora han revelado que estas rocas ofrecen una visión única de la compleja historia geológica de nuestro planeta.

Un equipo de investigadores, formado por geólogos de la Universidad de Maryland, la Universidad George Mason y la empresa noruega de petróleo y gas Equinor, ha creado un nuevo enfoque para estudiar la información biogeoquímica y fósil obtenida de las rocas de formación.

Usando esta técnica, el equipo identificó una causa importante de varias crisis bióticas convergentes durante el período Devónico tardío hace aproximadamente 350 millones de años: euxinia, o agotamiento de oxígeno y expansión de sulfuro de hidrógeno en grandes masas de agua. Publicado recientemente en la revista naturalezaLos hallazgos del equipo ilustran los vínculos entre el nivel del mar, el clima, la química oceánica y la bioturbación.

Los investigadores observan y discuten muestras de rocas de la Formación Bakken Shale. Crédito: Alan Jay Kaufman

«Por primera vez, podemos señalar un mecanismo de muerte específico responsable de una serie de perturbaciones bióticas importantes durante el Devónico tardío», dijo el profesor Alan Jay Kaufman, profesor de geología de la UMD. «Ha habido otras extinciones masivas supuestamente causadas por la expansión del sulfuro de hidrógeno antes, pero nadie ha estudiado exhaustivamente los efectos de este mecanismo de muerte durante este período crítico en la historia de la Tierra».

Según Kaufman, el período Devónico tardío fue una «tormenta perfecta» de factores que jugaron un papel muy importante en cómo es la Tierra hoy. Las plantas vasculares y los árboles fueron de particular interés en este proceso; A medida que se expandieron en la tierra, las plantas estabilizaron la estructura del suelo, ayudaron a difundir los nutrientes al océano y agregaron oxígeno y vapor de agua a la atmósfera mientras extraían dióxido de carbono de ella.

Introducción de plantas terrestres capaces de[{» attribute=»»>photosynthesis and transpiration stimulated the hydrological cycle, which kick-started the Earth’s capacity for more complex life as we know it today,” Kaufman said.

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 Williston Basin, the preserved the Bakken formation.

Undergraduate laboratory assistant Tytrice Faison (B.S. ’22, geology)—who joined Kaufman’s lab after taking a course 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 key biotic crises known as the Annulata, Dasberg, and Hangenberg events, with the last crisis associated with one of the greatest mass extinctions in Earth history.

“We could see anoxic events distinctly marked by black shale and other geochemical deposits, which are likely linked to a series of rapid rises in sea level,” Kaufman explained. “We suspect that sea levels may have risen during the pulsed events due to the melting ice sheets around the South Pole at this time.”

Higher sea levels would have resulted in the flooding of interior continental margins, or the transitional region between oceanic and continental crusts. In these settings, high levels of nutrients, such as phosphorous and nitrogen, could have triggered algal blooms which 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 those conditions, animals in the oceans and on land around the shoreline would have died during these late Devonian events.

The team’s research is not exclusive to global biotic disruptions from hundreds of millions of years ago. Kaufman suggests that their findings are not just applicable to the shallow inland seas of the Devonian Period, but perhaps also to the oceans of today affected by global warming. He compared the ocean’s circulatory system to a “conveyor belt” carrying 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 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.”

Reference: “Basin-scale reconstruction of euxinia and Late Devonian mass extinctions” by Swapan K. Sahoo, Geoffrey J. Gilleaudeau, Kathleen Wilson, Bruce Hart, Ben D. Barnes, Tytrice Faison, Andrew R. Bowman, Toti E. Larson and Alan J. Kaufman, 8 March 2023, Nature.
DOI: 10.1038/s41586-023-05716-2

The study was funded by Equinor.