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Good news: Ocean oxygen loss may ultimately reverse, saving marine life species

In a time where news about climate change is often gloom-and-doom, an international study led by Rutgers University provides an unexpected silver lining. The scientists have found evidence that ongoing oxygen depletion in the world’s oceans could potentially reverse in the future.

A disturbing decline in ocean oxygen levels has been witnessed in recent decades. This phenomenon threatens to create vast, oxygen-deficient zones in key parts of our oceans.

Scientists have linked this alarming trend to rising global temperatures. This trend has a direct impact on the oceans’ capacity to absorb oxygen from the atmosphere.

Looking to the past to learn about current ocean oxygen

The current research, published in the journal Nature, delves into Earth’s past climates to offer a glimmer of hope for the future.

Using ocean sediment data, the scientists found that during the Miocene epoch approximately 16 million years ago – a period when the Earth’s temperature was significantly higher than present day – oxygen levels in the ocean were higher in a key area.

“Our study shows that the eastern equatorial Pacific, which today is home to the largest oxygen-deficient zone in the oceans, was well oxygenated during the Miocene warm period, despite the fact that global temperatures at that time were higher than at present,” said study lead author Anya Hess, a doctoral student at Rutgers working with Professor Yair Rosenthal.

The research introduces an optimistic perspective on the current situation. This suggests that current oxygen loss may ultimately reverse.

The potential recovery of ocean oxygen levels could help alleviate the burden faced by marine life. Many species of marine life are currently threatened by the expanding and shallowing oxygen-deficient zones that constrict their habitat.

How the ocean oxygen study was conducted 

With current climate models diverging on their predictions regarding the future response of these oxygen-deficient zones post-2100, the researchers decided to study Earth’s past to better understand the potential outcomes. They focused on the mid-Miocene period. In terms of climatic conditions, this period bore similarities to the predictions for the upcoming centuries in our ongoing era of climate change.

The team examined ocean sediments from the mid-Miocene period. They were collected from the eastern equatorial Pacific by scientists aboard the JOIDES Resolution. This is a research vessel funded by the National Science Foundation. The vessel was part of the International Ocean Discovery Program (IODP). The IODP is an international marine research collaboration that explores Earth’s history and dynamics.

From these sediments, the researchers were able to extract and analyze the fossilized remains of minute water-dwelling microorganisms known as foraminifera. Their chemical composition mirrors that of the ancient ocean. This fact allows the scientists to discern oxygen levels of the seas millions of years ago.

The team used innovative methodologies, including nitrogen isotopes as detectors sensitive to denitrification. This is a process that occurs only at very low oxygen levels. They also used a comparative analysis of iodine and calcium levels. 

What the researchers learned 

The study revealed that during the peak of Miocene warmth, the analyzed area of the ocean was well oxygenated. The oxygen levels approaching those found in today’s open-ocean South Pacific.

“These results were unexpected and suggest that the solubility-driven loss of oxygen that has occurred in recent decades is not the end of the story for oxygen’s response to climate change,” said Rosenthal.

More about ocean oxygen loss

Ocean deoxygenation is the decrease in the level of oxygen in the oceans, which is primarily caused by human activities. The main drivers are global warming and nutrient pollution.

Global warming

Warmer waters hold less oxygen. Additionally, increased ocean temperatures stratify the water column, with warm water on top and cooler water below. This decreases the amount of vertical mixing and inhibits the transfer of oxygen from the atmosphere to deep waters.

Nutrient pollution

Nutrient runoff from agricultural fertilizers, sewage, and industrial pollutants can lead to the excessive growth of marine plants and algae. This process is known as eutrophication. When these plants and algae die and decompose, eutrophication consumes oxygen. This leads to areas of low oxygen known as “dead zones.”

Effects of ocean deoxygenation

The effects of ocean deoxygenation are severe. It can lead to the death of marine animals that can’t escape these low oxygen zones, including fish, crabs, and other important species. Many of these are economically important and their loss could impact people who rely on them for food and jobs.

There’s also a negative feedback loop involved: as the ocean loses oxygen, its ability to absorb CO2 decreases. This can lead to a further acceleration of global warming.

To mitigate the effects of ocean deoxygenation, it’s necessary to reduce global warming by decreasing greenhouse gas emissions. We must also limit nutrient pollution through better agricultural practices, waste management, and industrial controls.


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