A new study argues that oxygen depletion and hydrogen sulfide expansion in the oceans could have caused one of the most significant mass extinction events on Earth more than 350 million years ago.
Live Science reports that researchers think the changes may have been due to the rising sea levels, which have some eerie similarities to the current climate condition of Earth.
Devonian Period
Sea life proliferated throughout the Devonian epoch from placoderms, or jawless fishes, that evolved widely in the waters that ringed the supercontinents Gondwana and Euramerica, to trilobites and early ammonites in the oceans, and vast reefs in continents.
Moreover, Earth's earliest forests of ferns and trees appeared on land during that era. It was also when Tiktaalik roseae, Earth's first known tetrapod, crawled out of the sea during the mid-Devonian.
However, that period of Earth's history also witnessed some of the most mass extinction events on the planet. One of the five infamous mass extinction events even resulted in the creation of the flora and fauna of the modern world. Placoderms, trilobites, and early ammonites became extinct while cartilaginous fish-like sharks and rays thrived.
Devonian marine life couldn't survive in the oxygen-depleted waters. Researchers said that by the end of the Devonian, 75% of all life had gone extinct. Kaufman said their findings could be applied to today's oceans.
How Oxygen Depletion and Hydrogen Sulfide Expansion Caused a Mass Extinction Event
In the new study, titled "Basin-scale Reconstruction of Euxinia and Late Devonian Mass Extinctions" published in the journal Nature, researchers examined black shale samples from the Bakken Formation, a 200,000-square-mile (518,000-square-kilometer) region partly laid down during the late Devonian that includes parts of North Dakota and Canada.
The team discovered evidence that Earth underwent periods of oxygen depletion and hydrogen sulfide expansion, which likely contributed to the massive extinction events that plagued Earth between 419.2 and 358.9 million years ago, known as the "Age of Fishes."
When algae decompose on the ocean floor, hydrogen sulfide is formed. The breakdown process also depletes the oxygen region. Study co-author Alan Jay Kaufman, a geologist at the University of Maryland, said in a press release that no one has ever studied the effects of this killing mechanism although it has been theorized that other mass extinctions were caused by hydrogen sulfide expansion.
Kaufman explained that the late Devonian Period was a "perfect storm" of forces that had a significant part in the formation of modern Earth. Vascular plants and trees were especially important because they stabilized soil structure, helped transport nutrients to the ocean, and contributed oxygen and water vapor to the atmosphere while taking carbon dioxide out of it.
Devonian Period ended as the Bakken sediments accumulated, which allowed layers of organic-rich shale to record environmental conditions at that time. Various sediments gradually formed in inland seas that formed within geological depressions and preserved the Bakken formation that lies under parts of North Dakota and Montana, and in Manitoba and Saskatchewan.
Undergraduate laboratory assistant Tytrice Faison said that they could see anoxic events distinctly marked by black shale and other geochemical deposits that were likely linked to a series of rising sea levels.
Higher sea levels flooded some areas which led to high levels of phosphorous, nitrogen, and other nutrients to trigger algal blooms and sap the water of oxygen, resulting in dead zones that increased toxic hydrogen sulfide that killed ocean-dwelling creatures and some on land.
RELATED ARTICLE: How Did Mass Extinctions Like Dinosaur's Death Shape Earth's History? Here Are 5 Examples
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