Rising tropical ocean oxygen levels during dramatic global warming in geological history Oxygen is essential to sustain life on Earth. The ocean receives oxygen from the upper layers in contact with the atmosphere. However, as the Earth warms, the ocean’s capacity to hold oxygen is slowly decreasing, leading to significant impacts on marine ecosystems and the human activities that depend on them. Although these trends are expected to continue, the future distribution of oxygen in the deep ocean remains uncertain. This is because ocean currents and biodegradation of biomass have a greater impact than atmospheric dispersion in these regions.

“Marine sediments are the history book of the oceans. Simone Moretti, lead author of the article published in the journal Research, said: “By examining past periods of rapid temperature increase, we can obtain valuable information about how ocean oxygen and biology respond to climate change.” Science.

A team of researchers led by scientists from the Max Planck Institute for Chemistry in collaboration with Princeton University reconstructed the oxygenation response using a combination of chemical and morphological measurements of foraminifera, microscopic fossils preserved in marine sediments for millions of years. Tropical ocean during PETM.

Nitrogen isotopes and the size of fossils determine the oxygen content of seawater

Nitrogen isotopes preserved in fossil foraminifera allowed scientists to track past changes in the aquatic denitrification column. This process in which nitrates are converted to molecular nitrogen (N) ₂ ) is caused by bacteria, it occurs only in ocean waters where oxygen is most depleted, that is, in oxygen-deficient areas. “Our measurements showed that, contrary to most expectations, denitrification decreased during the PETM, meaning that oxygen-deficient areas in the ocean decreased during this period of sharp global warming,” said Alfredo Martínez-Garcia, head of the MPIC laboratory. research was conducted.

It also turns out that the size of foraminiferal fossils is a key piece of the puzzle. Models describing the metabolism of marine organisms allow us to relate their body size to the temperature of the environment and the oxygen content of the water in which they live. Reduction in body size is an effective adaptation to a warming climate because it allows organisms to reduce their metabolism in times of stress.

“Remarkably and unexpectedly, the evidence shows that planktonic foraminifera in the Central Tropical Pacific grew during PETM warming, indicating increased tropical oxygen in the upper ocean,” commented Curtis Deutsch, professor of Earth Sciences at Princeton University. Who is the co-author of this study? Planktonic foraminifera live in the upper layers of the ocean, unlike the lower layers.

Increase in oxygen may have mitigated upper ocean mass extinction

The finding that oxygen levels in the tropical ocean increased rather than decreased during the warming of the PETM also gives researchers a clue to another puzzle about changes in marine biodiversity. The PETM was the largest deep-sea ocean extinction during the Cenozoic Era spanning the last 66 million years. One of the many mysteries surrounding the PETM is that although this mass extinction occurred at great depths, organisms living in the upper ocean were less affected.

“The transient tropical oxygenation revealed by our study may have contributed to the preservation of viability despite significant heat stress,” said Simone Moretti. “But the fauna in the surface ocean was still severely impacted during the PETM, and it took more than a hundred thousand years for these ecosystems to return to their original state, which is an eternity on the time scale of human civilization.”