Piecing together the timeline for an ancient shock to the carbon cycle

The view looking northeast of the Ediacaran Rackla Group of the Wernecke Mountai

The view looking northeast of the Ediacaran Rackla Group of the Wernecke Mountains, in Yukon, Canada. The bright maroon- and tan-colored ridge in the foreground comprises a full exposure of the Gametrail Formation, which hosts the Shuram negative carbon isotope excursion, Earth’s largest known carbon cycle perturbation. (Photo credit: Justin V. Strauss)

The story of life on Earth is a tale of beginnings and endings - and of the interplay of geochemical forces, plate tectonics, and climate cycles over millions of years.

Now, a team of researchers led by Yale and the Massachusetts Institute of Technology has sorted out the timing of one part of the story, ruling it out as a cause of a mass extinction of animal species.

In a new study in Proceedings of the National Academy of Sciences, Yale geologist Alan Rooney and colleagues determined the beginning and end dates of the Shuram carbon excursion, one of the largest disturbances to the carbon cycle in Earth’s history. Knowing the timing of the Shuram event clarifies its role in the rise and fall of animal life on Earth.

The researchers analyzed rhenium-osmium (Re-Os) isotopes (an isotope is an atom with a higher or lower number of neutrons than normal) in ancient rocks to determine that the Shuram event began 574 million years ago and ended 567 million years ago. The analysis showed the same timing for rocks in both Oman and Northwest Canada, indicating that Shuram was a synchronous, global event.

This timing suggests the Shuram excursion was not responsible for a global mass extinction that almost stopped the evolution of animal life during the Ediacaran period, which occurred from 635 to 541 million years ago.

The Ediacaran period on Earth was characterized by tube-shaped and frond-shaped marine animals. After their extinction came the Cambrian period, during which a vast array of complex animal life radiated across the planet.

" For decades, the paleontological community believed that the Shuram event played a role in the appearance or extinction of some of the earliest animal life," said Rooney, an assistant professor in the Department of Earth & Planetary Sciences and co-lead author of the study. "Our new data refute that hypothesis. The Shuram event was not connected to the Ediacaran extinction - or the explosion of species that came later."

Marjorie Cantine of MIT is the study’s other co-lead author. The study included co-authors from the Sultanate of Oman, Stanford University, and Dartmouth College.

Jim Shelton

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