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'History book' of the ocean reveals future impacts of warming 

About 56 million years ago, Earth went through a super-fast global warming spell called the Paleocene-Eocene Thermal Maximum (PETM). It was one of the most intense warmings ever. The temperatures jumped by 5-8 degrees Celsius (9-14 degrees Fahrenheit) in just a few thousand years.

Scientists from the Max Planck Institute for Chemistry and Princeton University studied how the oceans responded to PETM. 

Surprisingly, this period is similar to our current climate challenges. So, the study helps us understand how oceans handled oxygen levels during such dramatic warming.

Surprising secrets of ocean warming

To uncover secrets from the past, the researchers needed to go back in time.

“Marine sediments are the history book of the ocean. By studying past intervals of time in which temperatures increased rapidly, we can gain precious insights on how ocean oxygen and biology responded to changes in climate,” said study lead author Simone Moretti.

Therefore, the team dug into the ocean floor, looking at clues in the sediment to understand old nutrient cycles and oxygen levels. 

Usually, when water gets warmer, it cannot hold as much air. So, during the PETM, the ocean’s top layers should have been warm and contain less oxygen.

Surprisingly, the study found increased oxygen levels in the upper ocean during the PETM. This challenges our current understanding of how oceans react to warming. 

Ocean fossils

Other areas with very low oxygen, called “dead zones,” actually got smaller during the PETM. The shrinking dead zones also indicate increased oxygen in the upper ocean.

The study suggests that the ocean has a way of balancing itself when things change rapidly. This discovery is important because it shows how the ocean can adapt to big environmental changes.

The scientists also examined any fossils they came across. They found fossils of microscopic creatures called foraminifera, which lived in the ocean and built miniature shells from minerals. 

These tiny fossils held a big secret. During PETM, the foraminifera had larger shells than usual. This suggested they responded to their environment, hinting at warmer waters with more oxygen.

Foraminifera are sensitive to changes in temperature, acidity, and oxygen levels. So, their bigger shells during the PETM suggest they flourished in the warmer, more oxygen-rich conditions of that time.

“Remarkably, and unexpectedly, evidence shows that planktonic foraminifera from the central tropical Pacific got bigger during the PETM warming, implying a tropical oxygen rise in the upper layers of the ocean,” said study co-author Professor Curtis Deutsch.

Animal adaptation

The deep ocean, sensitive to temperature and oxygen shifts, likely suffered during the PETM’s rapid warming. Many deep-sea creatures, pushed beyond their limits, couldn’t adapt, leading to a sharp drop in the variety of life there.

On the other hand, surface dwellers in the ocean’s top layers may have fared better. A temporary boost in oxygen during this period gave them a breathing room, allowing them to keep their living conditions somewhat stable despite the chaos around them. 

This extra oxygen might have even lessened the warming’s impact, making the upper ocean a more welcoming place for these surface-dwelling marine animals than their deep-sea neighbors.

Ocean warming recovery period

Marine ecosystems took a long and arduous journey to recover even with a temporary oxygen boost. Over 100,000 years passed before they fully bounced back, highlighting the lasting effects of rapid climate change on ocean life.

The reason for this slow recovery lies in the complex interplay of factors triggered by the PETM. It wasn’t just warming temperatures but also changes in ocean chemistry, like acidification and disruptions to nutrient cycles. These factors continued to stress marine life long after the initial warming event.

Organisms had to adjust to new conditions, repopulate lost habitats, and rebuild the life that the PETM had disrupted. This process unfolded gradually over millennia, a testament to the resilience of marine ecosystems.

But their resilience has limits. The extended recovery also underscores the long-term consequences of rapid climate change on biodiversity and ecosystem stability. Studying the PETM’s recovery helps us understand how modern marine ecosystems might respond to current climate challenges.

The study is published in the journal Science.


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