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In a paper published in the journal Nature Geosciencescientists offer a new explanation for a string of severe environmental crises, called oceanic anoxic events, which happened between 185 and 85 million years ago (Mesozoic era); these occurred when the seas became critically depleted of dissolved oxygen.
Oceanic anoxic events are geologically abrupt phases of extreme oxygen depletion in the oceans that disrupted marine ecosystems and brought about evolutionary turnover. Typically lasting approximately 1.5 million years, these events occurred frequently during the Mesozoic era, from about 183 to 85 million years ago. One hypothesis suggests that anoxic events resulted from enhanced chemical weathering of Earth’s surface in a greenhouse world shaped by high volcanic carbon outgassing. Gernon et al. tested this hypothesis using a combination of plate reconstructions, tectonic-geochemical analysis and global biogeochemical modeling. Image credit: University of Southampton.
“Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems,” said University of Southampton’s Professor Tom Gernon.
“The challenge was understanding which geological forces hit the button.”
The researchers examined the impact of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic era.
“This chapter of Earth’s history is often dubbed the age of the dinosaurs and is famously exposed along the Jurassic Coast on the UK’s south coast as well as along the cliffs of Whitby in Yorkshire and Eastbourne in East Sussex,” Professor Gernon said.
The scientists combined statistical analyses and sophisticated computer models to explore how chemical cycles in the ocean could have feasibly responded to the breakup of the supercontinent Gondwana, the great landmass once roamed by the dinosaurs.
“The Mesozoic era witnessed the breakup of this landmass, in turn bringing intense volcanic activity worldwide,” Professor Gernon said.
“As tectonic plates shifted and new seafloors formed, large amounts of phosphorus, a nutrient essential for life, were released from weathering volcanic rocks into the oceans.”
“Crucially, we found evidence of multiple pulses of chemical weathering on both the seafloor and continents, which alternately disrupted the oceans. It’s like a geological tag-team.”
The authors found the timing of these weathering pulses matched up with most oceanic anoxic events in the rock record.
They propose that the weathering-related influx of phosphorus to the ocean acted like a natural fertilizer, boosting the growth of marine organisms.
However, these fertilization episodes came at a major cost for marine ecosystems.
“The increase in biological activity led to huge amounts of organic matter sinking to the ocean floor, where it consumed large quantities of oxygen,” said University of Leeds Professor Benjamin Mills.
“This process eventually caused swathes of the oceans to become anoxic, or oxygen-depleted, creating ‘dead zones’ where most marine life perished.”
“The anoxic events typically lasted around one to two million years and had profound impacts on marine ecosystems, the legacy of which are even felt today.”
“The rocks rich in organic matter that accumulated during these events are by far the largest source of commercial oil and gas reserves globally.”
As well as explaining the cause of extreme biological turmoil in the Mesozoic era, the findings highlight the devastating effects that nutrient overloading can have on marine environments today.
“Studying geological events offers valuable insights that can help us grasp how the Earth may respond to future climatic and environmental stresses,” Professor Gernon said.
Overall, the results reveal a stronger-than-expected connection between the Earth’s solid interior and its surface environment and biosphere, especially during periods of tectonic and climatic upheaval.
“It’s remarkable how a chain of events within the Earth can impact the surface, often with devastating effects,” Professor Gernon said.
“Tearing continents apart can have profound repercussions for the course of evolution.”
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T.M. Gernon et al. Solid Earth forcing of Mesozoic oceanic anoxic events. Nat. Geoscipublished online August 29, 2024; doi: 10.1038/s41561-024-01496-0
This article was adapted from an original release by the University of Southampton.
