Fast the oceans have become hostile to life

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A team of German, Belgian, American and Chinese geologists examined climatic cycles in different rock formations to determine the precise timing of the Devonian extinction. Their results, published this week , reveal how quickly the oceans became hostile to life during the Devonian period.

In whole Earth’s history, geologists distinguish five mass extinction events that were particularly severe. The Devonian mass extinction event is one of those five and occurred 374 million years ago, more than 300 million years before the meteorite impact that wiped out the dinosaurs. The Devonian was a very peculiar period: large fish reigned the oceans and coral reefs were flourishing, but there were no large land animals yet. "The Devonian climate can be best described as an extreme greenhouse climate, with much more CO2 in the atmosphere compared to today," says David De Vleeschouwer, geologist at the MARUM - Center for Marine Environmental Sciences at the University of Bremen. Despite the intriguing aspects of the Devonian world, scientists did not know exactly how fast the Earth became inhospitable for life across Devonian mass extinction event. A team of geologists from Germany, Belgium, the USA and China looked at rhythmical climate cycles in the rock record to figure out the precise timing of the Late Devonian extinction. "We now know how fast the Devonian oceans turned hostile to life, and we published these findings ".

The geological expression of the Late Devonian mass extinction event in Germany. The mass extinction event is displayed by a level of black shales (indicated by the geological hammer) interrupting the succession of limestone layers. Credit :Steinbruch Schmidt

For reasons still debated by scientists, the ocean water turned low in oxygen during the Devonian mass extinction event. "This event suffocated the majority of life in the ocean, and the extremely diverse Devonian reef-builders were among the major victims," explains Anne-Christine Da Silva from University of Liège (Geology Researh Unit) in Belgium. The main cause of the extinction is a hot topic in geology, as there is no smoking gun that can be identified as the trigger of this underwater-massacre. In fact, oxygen levels in the ocean water dropped twice across the course of the extinction. Geologists know that by virtue of two levels of black shale in the worldwide rock record, marking the extinction interval. These two levels of black shale are rich in organic components because there was no sufficient oxygen for the organic material to rot, nor for under-water organisms to breath.

As a geologic clockwork, David De Vleeschouwer and his team used a technique called cyclostratigraphy. "We combined information from geologic sections spanning the Devonian extinction event, from Belgium, Poland, China, Canada and the USA. In all sections, we found the effects of cyclical changes in the eccentricity of the Earth’s orbit around the Sun." When eccentricity is low, the Earth rotates around the Sun on an orbit that is very close to a perfect circle; but, when eccentricity is high, the Earth’s orbit around the Sun is much more elliptical, allowing for very strong differences in the amount of solar energy the Earth receives during winter and summer. Changes in eccentricity are very rhythmical, with fixed periods of 100 and 405 thousand years. The authors used these cycles to constrain the duration between the two black shales, and found that the second episode of low oxygen levels commenced 600,000 years after the first. This result is the first precise time measurement of this essential episode in the evolution of life on Earth, and turned out to be much shorter than expected. What is more, is that the team noticed that the main pulse of extinction coincided with a prolonged period of low eccentricity. This means the Earth’s orbit was close to circular for several tento hundred thousands of years, giving way to quite steady climates. These invariable climates gave way to sluggish ocean circulation and stratification of the water column, both favouring low oxygen levels in the world’s oceans.

The eccentricity of the Earth’s orbit changes rhythmically, from a perfect circle to a more elliptical shape. Note that this figure is not to scale.

Yet, the authors do not claim that they unmasked the so-called "smoking gun". The authors believe land plants are the most likely culprits of the devastating Devonian die-off. During the Devonian, land plants developed deep root systems and woody tissues, giving them the evolutionary advantage to colonize different environments. The success of land plants came at a cost though: when a plant dies, its biomass get washed into waterways and into the ocean. "In other words, the Devonian seas got smothered with nutrients from rotting plants, a process during which oxygen is absorbed and other life is starved out," explains David De Vleeschouwer. "However, the evolution of land plants is a slow and gradual process. Only when the eccentricity configuration of the Earth’s orbit favoured sluggish ocean circulation, all factors aligned to push the Earth’s system beyond its tipping point, causing the Devonian mass extinction."

Scientific reference

Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity ’, Nature Communications
Auteurs : David De Vleeschouwer 1,2, Anne-Christine Da Silva 3,4, Matthias Sinnesael2, Daizhao Chen5, James E. Day6, Michael T. Whalen7, Zenghui Guo5 & Philippe Claeys2

1MARUM--Center for Marine Environmental Sciences, University of Bremen, Germany
2 Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Belgium.
3 Sedimentary Petrology Laboratory , University of Liège, Belgium
4 Paleomagnetic Laboratory, Utrecht University, Budapestlaan 17, The Netherlands
5 Institute of Geology and Geophysics, Chinese Academy of Science, Beijing, China
6 Department of Geography-Geology, Illinois State University, USA
7 Department of Geosciences, University of Alaska, Fairbanks, USA.

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