in paper Published in the journal Nature ChemistryScientists have proposed a new explanation for a series of severe environmental crises known as marine anoxia, which occurred between 185 and 85 million years ago (during the Mesozoic era) when the amount of dissolved oxygen in the oceans became extremely low.
Oceanic anoxia was a geologically abrupt phase of extreme oxygen depletion in the oceans that disrupted marine ecosystems and led to evolutionary shifts. These events, which usually lasted about 1.5 million years, occurred frequently during the Mesozoic Era, between about 183 and 85 million years ago. One hypothesis suggests that anoxia resulted from increased chemical weathering of the Earth's surface on a greenhouse world with high volcanic carbon emissions. Gernon othersA combination of plate reconstructions, tectonic geochemical analyses and global biogeochemical modelling tested this hypothesis. Image courtesy of the University of Southampton.
“Ocean anoxia is like hitting the reset button on the Earth's ecosystems,” said Prof Tom Gernon, from the University of Southampton.
“The challenge was to understand what geological forces pushed the button.”
The researchers investigated the influence of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic Era.
“This period in Earth's history is also known as the Age of the Dinosaurs and is well exposed along the cliffs of the Jurassic Coast on the south coast of England, Whitby in Yorkshire and Eastbourne in East Sussex,” Prof Gernon said.
Scientists have combined statistical analysis and advanced computer modelling to investigate how the ocean's chemical cycle may have responded to the breakup of the supercontinent Gondwana, the giant landmass once home to dinosaurs.
“During the Mesozoic Era, the continents broke apart and intense volcanic activity occurred around the world,” Prof Gernon said.
“As the plates shifted and new ocean floor was formed, phosphorus, a nutrient essential to life, was released in large quantities into the ocean from weathered volcanic rocks.”
“Importantly, we found evidence of multiple instances of chemical weathering on both the ocean floor and the continents, alternating between destroying the oceans — like a geological tag team.”
The authors find that the timing of these weathering waves coincides with most of the marine anoxic events in the rock record.
They argue that phosphorus that entered the ocean through weathering acted as a natural fertilizer, promoting the growth of marine life.
However, this fertilization phenomenon came at a great cost to marine ecosystems.
“Increased biological activity caused huge amounts of organic matter to sink to the seafloor, consuming huge amounts of oxygen,” said Prof Benjamin Mills, from the University of Leeds.
“This process ultimately left large swaths of the ocean anoxic, or 'dead zones' where oxygen was depleted and most marine life died.”
“The anoxic conditions typically lasted for one to two million years and had profound effects on marine ecosystems, the effects of which are still felt today.”
“The organic-rich rocks that accumulated during these events are the source of the world's largest commercial oil and gas reserves to date.”
The findings explain the causes of extreme biological chaos during the Mesozoic Era and highlight the devastating effects of nutrient overload on marine environments today.
“Studying geological events provides valuable insights that help us understand how Earth will respond to future climatic and environmental stresses,” Professor Gernon said.
Overall, the results reveal stronger-than-expected connections between Earth's solid interior and its surface environment and biosphere, especially during periods of tectonic and climatic upheaval.
“It is remarkable how a series of events occurring inside the Earth can have such profound, often devastating, effects on the surface,” Prof Gernon said.
“Continental breakup could have profound effects on evolutionary processes.”
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TM Gernon othersSolid Earth forcing of Mesozoic oceanic anoxia. National GeographyPublished online August 29, 2024, doi: 10.1038/s41561-024-01496-0
This article has been adapted from an original release from the University of Southampton.
Source: www.sci.news
