A lack of oxygen and rising levels of hydrogen sulfide in the oceans more than 350 million years ago could have been responsible for one of the most significant mass extinctions on Earth, a new study suggests. The changes were likely caused by sea level rise and bear some chilling parallels to today’s conditions.
Researchers examined samples of black shale from the Bakken Formation, a 200,000 square-mile (518,000-square-kilometer) region partially laid down during the Late Devonian and encompassing portions of North Dakota and Canada and one of the largest contiguous deposits of natural gas and oil (opens in new tab) in the United States. The team found evidence that Earth experienced periods of oxygen starvation and hydrogen sulfide expansion that likely contributed to the large-scale extinction events that killed Earth during the Devonian Period (419.2 and 358.9 million years ago), or the “Age of Fish” devastated.
Hydrogen sulfide is formed when algae on the sea floor decompose. The decomposition process also depletes the area of oxygen.
“There have been other mass extinctions before that are believed to have been caused by the spread of hydrogen sulfide, but no one has ever examined the effects of this killing mechanism so thoroughly during such a critical period in Earth’s history,” said the study’s co-author Alan Jay Kaufman (opens in new tab)a geologist at the University of Maryland said in a opinion (opens in new tab).
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During the Devonian period, sea life was profiled. Jawless fish known as placoderms became widespread in oceans surrounding the Gondwana and Euramerica supercontinents. The oceans were also teeming with trilobites and early ammonites, and extensive reefs lined the continents. On land, the earth saw its first forests of ferns and early trees. Earliest known tetrapod on Earth until the Middle Devonian Tiktalik roseae had crawled out of the sea.
However, the Devonian period also saw some of the most significant extinction events in Earth’s history, including one of the five infamous “mass extinctions” Events that led to the development of the flora and fauna that we know today. Placoderms, trilobites, and early ammonites disappeared while cartilaginous, fishlike sharks and rays proliferated.
To improve on the Devonian extinction, the research team analyzed more than 100 core samples drilled from black shale deposits in the Bakken Formation. This organic matter-rich sediment accumulated towards the end of the Devonian period and chronicled the chemical composition of the environment.
The team found evidence of “anoxic events,” where the water was completely depleted of oxygen, they reported in the study, published March 8 in the journal Nature (opens in new tab).
These sharp declines “are likely associated with a series of rapid sea level rises” attributed to the melting of the South Pole ice sheets during the preceding Silurian period (443.8 million to 419 million years ago), Kaufman said in the statement .
At the same time, plants turned rocky land into soil that would have released nutrients to flow into these rising oceans. The release of nutrients into the oceans would have triggered massive algal blooms that died, decomposed and took up oxygen. As they decomposed, the dead algae released hydrogen sulfide, increasing levels of the toxic chemical.
The oxygen-poor seas were too much for Devonian marine life. Researchers estimate that by the end of the Devonian, 75% of all living things would be extinct.
The Devonian mass extinction is a warning for today, the study authors wrote. poor in oxygen Dead zones are created in the oceans every year in places like the Gulf of Mexico and the Baltic Sea (opens in new tab). The intensive use of fertilizers as well as the runoff of sewage increase the nutrient content of the ocean and promote massive algal blooms. And as the globe warms and sea level risethe oceans will also not circulate oxygen, Kaufman said in the statement.
Past mass extinctions can help scientists understand the consequences of our actions today. Although the reasons for sea-level rise and nutrient input into the Devonian are different than they are today, they could lead to the same result — a massive loss of life in our planet’s oceans, the researchers argue.