The course of evolution on Earth was altered by a series of severe environmental crises caused between 185 and 85 million years ago in the oceans, according to scientists.
The phenomenon, described as a ‘tag-team’ between the oceans and continents, severely harmed the marine life which existed during that phase and also changed the evolution course on our planet.
The oceanic anoxic events, as per the term given by the researchers, occurred when the dissolved oxygen in the water depleted to a critically low level.
The oxygen depletion led to major biological changes – including mass extinction of marine species – according to the researchers from the University of Southampton.
The study was led by researchers from University of Southampton in collaboration with academics from the universities of Leeds, Bristol in the United Kingdom, Adelaide in Australia, Utrecht in the Netherlands, Waterloo in Canada, and Yale in the United States of America.
“Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems. The challenge was understanding which geological forces hit the button,” Tom Gernon, a Professor of Earth Science at Southampton and also the lead author of the paper said.
Chemical cycles in ocean reacted positively to supercontinent breakup
The researchers studied the impact of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous Periods, collectively known as the Mesozoic era.
Commonly known as the age of the dinosaurs on Earth, the remnants from this period can be found 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.
The researchers combined statistical analysis and used computer models to understand how the chemical cycles in oceans would have responded positively to the breakup of the supercontinent Gondwana .
Gondwana, the supercontinent landmass on which dinosaurs once roamed freely, broke up sometime during the Mesozoic era . This also resulted in intense volcanic activity around the world.
According to the researchers, this volcanic activity played a key role in shaping the course of the planet.
“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,” Gernon said.
Terming it a “geological tag-team,” Gernon and the team of researchers state that they “found the timing of these weathering pulses matched up with most oceanic anoxic events in the rock record.”
Phosphorus boosted growth of marine organisms
According to the study, the researchers propose that the phosphorus in the ocean acted as a natural fertilizer and boosted the growth of marine organisms.
However, this increased biological activity also caused huge organic matter sinking into the ocean floor, and consumption of large amounts of oxygen, according to co-author Benjamin Mills, a Professor of Earth System Evolution at the University of Leeds.
“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,” Mills said.
He also stated that the rocks which accumulated during these events were rich in organic matter and has today emerged as the biggest source of commercial oil and gas reserves globally.
Importance of understanding the oceanic oxygen levels
The researchers state that study can also show the negative effects of nutrient overloading on ocean ecosystems, which should serve as a warning sign for the modern world.
According to them, human activities have “reduced mean oceanic oxygen levels by about two per cent – leading to a significant expanse in anoxic water masses.”
Their findings reveal a strong 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,” added Prof Gernon.
“Tearing continents apart can have profound repercussions for the course of evolution.”
The findings were published in the journal Nature Geoscience.
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