John Marshall on the Riddle of the Mass Extinction 360 Million Years Ago
John Marshall is a Professor at the School of Ocean & Earth Science at the University of Southampton. He is a fossil expert specializing in mass extinction events. He explains how his recent research has uncovered new evidence that may finally explain what caused the mass extinction at the end of the Devonian period.
Here he is on his way to a field location in East Greenland.
Photo courtesy of Jon Lakin
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Podcast Illustrations
All images courtesy of John Marshall unless otherwise noted.
Dropping off equipment and supplies for John Marhsall’s team on Rebild Bakker, the East Greenland island on which the Devonian lake-bed sediments were discovered.
The ravine on Rebild Bakker in East Greenland where the malformed spores were discovered.
The end-Devonian period (360 million years ago) lake-bed mudstone in East Greenland in which well-preserved malformed spores were found. The huge ancient lake bed was in the arid interior of the Old Red Sandstone Continent, made up of Europe and North America. This lake was situated in the Earth’s southern hemisphere and would have been similar in nature to modern-day Lake Chad on the edge of the Sahara Desert. Though the surface is shattered by freeze-thaw cracking, there are good unweathered samples just below the surface. The boundary between the Devonian and Carboniferous Periods lies at the base of the yellow notebook.
Disposition of the continents in the mid-Devonian period, about 30 million years before the end-Devonian extinction. The present-day coastlines are shown, with Greenland being a part of a large continent, called the Old Red Sandstone continent, located just south of the equator.
Courtesy of C.R. Scotese
More detailed reconstructed map of the Old Red Sandstone continent of the late Devonian period showing the location of the crust that make up present-day Greenland.
Courtesy of Trond Torsvik and Robin Cocks
Photomicrograph of a normal Grandispora Cornuta spore, which is the Latin for a big spore with horns, i.e., spines. Though the spore is somewhat squashed and folded, the spines are all the same, regularly distributed, and end in identical points. The diameter is 75 microns, i.e., less than a thousandth of a millimeter.
The same species of spore, Grandispora Cornuta, but with significant malformation. The evenly spaced spines are reduced to irregular blobs with an irregular overgrowth at the top of the specimen. The spore has also turned a darker color, which may be a response to increased radiation with the growing plant able to increase the amount of UV-absorbing chemicals in the spore wall, akin to a sun-tan in humans. Same scale as above.
Cartoon illustrating how an initial warming event causing an initial reduction in the ozone layer is in turn amplified by the resultant collapse of the forest ecosystem.
Changes in the eccentricity, i.e., in how much the orbit departs from a perfect circle, affects the strength of the incident solar radiation at any given latitude.
Changes in the tilt (obliquity) of the Earth’s axis of rotation with respect to the plane of the Earth’s orbit around the sun (the ecliptic) vary the severity of the seasons.
Precession changes the timing of the seasons with respect to the orbit of the Earth around the sun. For example, if at the start of a precessional cycle, summer in the northern hemisphere occurs when the Earth’s orbit is at its closest point to the sun, then in the middle of the cycle, 13,000 years later, summer in the northern hemisphere will occur when the Earth is at its furthest point in its orbit.
About every 41,000 years, the effects of the changes in orbital eccentricity, axial tilt (obliquity) and precession align with each other, causing climate changes, such as ice ages or periods of global warming.
John Marshall suggests that changes in incident solar radiation caused by these orbital effects, together with a planet that was unusually susceptible to being tipped into an extreme warm state, undermined the forest ecosystem which in turn released methyl halogens that damaged the ozone layer, thus letting more ultraviolet from the sun reach the Earth’s surface and cause the end-Devonian mass extinction.