Deep ocean oxygen levels are affected by sea ice

During the last ice age, extensive sea ice prevented oxygen from reaching waters in the deep ocean. This interference complicated the relationship between oxygen and carbon, according to a new study from Oregon State University.

“The sea ice is effectively like a closed window for the ocean,” said study co-author Andreas Schmittner. “The closed window keeps fresh air out; the sea ice acted as a barrier to keep oxygen from entering the ocean, like stale air in a room full of people. If you open the window, oxygen from outside can come in and the air is not as stale.”

The results of the research contradict previous ideas about the relationship between oxygen and carbon dioxide in deep ocean waters. A better understanding of this relationship will provide valuable insights into how the world’s oceans may respond to climate change, explained Schmittner.

The ocean plays an important role in the carbon cycle, in which carbon dioxide from the atmosphere dissolves in surface waters. Here, algae convert the carbon into organic matter, and respiration of that organic matter removes oxygen as carbon sinks to the deep ocean. 

The process of transferring carbon from the surface waters to the deep ocean is called the biological pump.

As temperatures rise, the ocean is losing oxygen – a trend that is expected to persist. This would lead scientists to expect higher oxygen concentrations during the last ice age when oceans were colder, said Schmittner, but sediment collected previously from below the sea floor shows lower oxygen levels in the deep ocean during that period.

To investigate, the researchers used modeling to calculate the lower oxygen levels in the deep ocean. The study revealed that disequilibrium played an important role in the carbon cycle. Deep ocean oxygen concentrations were reduced because surface waters were less equilibrated with the atmosphere. 

According to Schmittner, the disequilibrium was a result of the vast sea ice mainly over the Southern Ocean, as well as higher iron fertilization from the ice age atmosphere, which was dustier.

This means that deep ocean oxygen levels are not just influenced by the biological pump process, but also by sea ice extent, just as a room’s air quality may change by the opening or closing of a window, said Schmittner. He noted that the method used for this study could also be applied to future climate modeling.

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

The study is published in the journal Nature Geoscience.

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By Chrissy Sexton, Earth.com Staff Writer