Oxygen is essential to sustain life on Earth. The ocean gets its oxygen from the top layer, which is in contact with the atmosphere. However, as the planet warms, the ocean's ability to hold oxygen is slowly diminishing, with significant consequences for marine ecosystems and the human activities that depend on these ecosystems. While these trends are expected to continue, the future distribution of oxygen within the ocean remains uncertain. This is because ocean currents and biological decomposition of biomass have a greater impact than atmospheric dispersion in these regions.
"Ocean sediments are the history books of the ocean. By studying past periods of rapidly rising temperatures, we can gain valuable insights into how ocean oxygen and life respond to climate change," said Simone Moretti, first author of a paper published in the research journal Science.
Foraminifera fossils are tiny fossils preserved in ocean sediments for millions of years. A research team led by scientists at the Max Planck Institute for Chemistry, in collaboration with Princeton University, used a combination of chemical and morphological measurements of foraminifera to reconstruct the response of tropical ocean oxygen levels during the PETM.
Nitrogen isotopes preserved in foraminiferal fossils allow scientists to track past changes in columnar denitrification in water bodies. Bacteria convert nitrate into molecular nitrogen (N2) This process occurs only in the most oxygen-deficient waters of the ocean: the anoxic zone.
"Our measurements show that, contrary to most expectations, denitrification decreased during the PETM, which means that the anoxic zone of the ocean shrank during this period of sudden global warming," said Alfredo Martínez-García, head of the MPIC laboratory that conducted the study.
In addition, the size of the foraminiferal fossils also proved to be a fundamental part of the puzzle. Through models describing the metabolism of marine organisms, it is possible to relate the body size of marine organisms to the temperature and oxygen content of the environment in which they live. Shrinking body size is an effective adaptation to a warming climate because it allows organisms to reduce their metabolism when stressed.
"There is evidence that planktonic foraminifera in the central tropical Pacific grew larger during the PETM warming, which means that tropical oxygen in the upper ocean rose, which is surprising and unexpected," commented Curtis Deutsch, co-author of the study and professor of earth sciences at Princeton University. "Planktonic foraminifera live in the upper layers of the ocean and are different from foraminifera on the seafloor."
During the PETM warming period, oxygen levels in tropical oceans rose instead of falling, a finding that also provides researchers with a clue to another piece of the puzzle: changes in marine biodiversity. PETM was the largest extinction event of deep-sea organisms in the Cenozoic over the past 66 million years. One of the many mysteries associated with PETM is that although this mass extinction event occurred deeper on the ocean floor, organisms living in the uppermost layers of the ocean were less affected.
Simone Moretti said: "The transient tropical oxygen enrichment revealed by our study may have helped maintain habitability under huge temperature pressures. However, during the PETM, animals in the surface ocean were still severely affected, and it took more than 100,000 years for these ecosystems to return to their original state, which is an eternity on the time scale of human civilization."
Compiled source: ScitechDaily