Study finds increased oxygen in upper layers of tropical ocean during Paleocene-Eocene thermal maximum

Oxygen is essential for sustaining life on Earth. The ocean gets its oxygen from its upper layers in contact with the atmosphere. As our planet continues to warm, the ocean is gradually losing its ability to absorb oxygen, with serious consequences for marine ecosystems and the human activities that depend on them. Although these trends are likely to continue in the future, it remains unclear how ocean oxygen will redistribute within the oceans, where ocean currents and biological degradation of biomass dominate atmospheric diffusion.

“Marine sediments are the history book of the ocean. By studying past time intervals during which temperatures have increased rapidly, we can gain valuable information about how oxygen and ocean biology have responded to climate change,” said Simone Moretti, lead author. from a study published in Science.

Using a combination of chemical and morphological measurements on foraminifera, microscopic fossils preserved in marine sediments for millions of years, a team of researchers led by scientists from the Max Planck Institute for Chemistry in collaboration with the University of Princeton reconstructed the tropical ocean oxygenation response during the Paleocene-Eocene Thermal Maximum (PETM).

Nitrogen isotopes and fossil sizes reveal the oxygen content of seawater

Nitrogen isotopes preserved in fossil foraminifera have allowed scientists to track past changes in water column denitrification. This process, in which nitrate is converted to molecular nitrogen (N₂) by bacteria, only occurs in the ocean waters poorest in oxygen: oxygen-deficient zones.

“Our measurements showed that contrary to most expectations, denitrification declined during the PETM, implying that oxygen-deficient ocean areas contracted during this interval of abrupt global warming,” said Alfredo Martínez-García, head of the MPIC laboratory where the study was carried out.

Additionally, the size of foraminiferal fossils turned out to be a fundamental piece of the puzzle. Models that describe the metabolism of marine organisms make it possible to relate their body size to the ambient temperature and the oxygen content of the water in which they live. A reduction in body size is an effective adaptation to global warming because it allows organisms to reduce their metabolism. in times of stress.

“Remarkably and unexpectedly, evidence shows that planktonic foraminifera in the central tropical Pacific grew larger during PETM warming, implying an increase in tropical oxygen in the upper layers of the ocean,” commented Curtis Deutsch , professor of geosciences at Princeton University. who co-authored this study. Planktonic foraminifera live in the upper layers of the ocean, unlike those found at the bottom.

Increased oxygen could have mitigated mass extinction in the upper ocean

The finding that oxygen levels in the tropical ocean increased rather than decreased during PETM warming also provides researchers with a clue to another puzzle, that of changes in marine biodiversity. The PETM was the largest extinction event among deep ocean organisms during the Cenozoic Era, spanning the past 66 million years. One of the many mysteries related to the PETM is that although this large-scale extinction event took place at greater depths, organisms living in the higher part of the ocean were less affected.

“The transient tropical oxygenation revealed by our study may have helped preserve habitability despite significant thermal stress,” Moretti said. “However, during the PETM, ocean surface fauna were nonetheless greatly impacted, and it took over a hundred thousand years for these ecosystems to return to their original state, an eternity on a global scale. human civilization.”