Sea ice has prevented oxygen from reaching deep oceans during the last ice age

Extensive sea ice covered the world’s oceans during the last ice age, which could not penetrate oxygen into the deep ocean water.

“Sea ice is as effective as a closed window to the ocean,” said Andreas Schmittner, a climate scientist at Oregon State University and co-author of the article. “The closed window keeps fresh air out; the sea ice has acted as a barrier to prevent oxygen from entering the ocean, like stale air in a room full of people. When you open the window, oxygen can enter from outside. and the air is not so old. ‘

The findings, recently published in Natural Sciences, challenges the assumptions about the relationship between oxygen and carbon dioxide in deep ocean waters. Understanding this relationship gives researchers important insights into how the world’s oceans can respond to climate change, said Schmittner, a professor at OSU’s College of Earth, Ocean, and Atmospheric Sciences.

The ocean plays an important role in the carbon cycle; carbon dioxide from the atmosphere dissolves in surface water, where algae turn the carbon into organic matter. Breathing of the organic material removes oxygen when carbon sinks in the deep ocean. The process of transferring carbon from the ocean to the depths is known as the biological pump.

Currently the ocean is losing oxygen and the trend is expected to continue as the solubility of oxygen decreases as the temperature warms. This would lead scientists to expect higher oxygen concentrations during the last ice age, when oceans were colder, Schmittner said, but sediment previously collected from below the seabed shows lower oxygen levels in the deep ocean during the period.

Researchers have previously theorized that the biological pump was stronger during the last ice age, which increased carbon respiration. But it assumes that oxygen on the surface of the ocean is equal to the atmosphere, Schmittner said.

In their new work, Schmittner and his colleagues, Ellen Cliff and Samar Khatiwala of the University of Oxford, used modeling to investigate the lower oxygen levels in the deep ocean.

They found that imbalance plays an important role in the carbon cycle. Deep ocean oxygen concentrations were lowered because surface water was less in equilibrium with the atmosphere. The imbalance was due to the vast sea ice, mainly across the Southern Ocean, as well as higher iron fertilizer from the ice age atmosphere, which was dusty, Schmittner said.

This means that the oxygen levels in the deep ocean are affected not only by the biological pumping process, but also by the sea ice or the lack thereof, just as the air quality of a room can change through the opening or closing of a window. to notify, he said.

The researchers’ method of understanding the role of sea ice and other processes in the ocean-carbon and oxygen cycles can also be applied to future climate modeling, Schmittner said.

“Current models cannot separate the effects of the biological pump on ocean oxygen from sea ice or other influences,” he said. “It changes our understanding of the process and the reasons for it.”