Sea ice prevented oxygen from reaching the deep ocean during the last ice age

Vast sea ice covered the world’s oceans during the last ice age, preventing oxygen from entering the deep ocean waters, complicating the relationship between oxygen and carbon, according to one new study.

“Sea ice is actually like a closed window to the ocean,” said Andreas Schmittner, climatologist at Oregon State University and co-author of the article. “The closed window keeps cool air out; sea ice has 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 the outside can enter and the air is not as stale. “

The results, recently published in Geoscience of nature, challenge previous assumptions about the relationship between oxygen and carbon dioxide in deep ocean waters. Understanding this relationship gives researchers important information about how the world’s oceans might respond to climate change, said Schmittner, a professor in the College of Earth, Ocean, and Atmospheric Sciences at OSU.

The ocean plays an important role in the carbon cycle; carbon dioxide from the atmosphere dissolves in surface waters, where algae convert carbon into organic matter. The respiration of this organic matter removes oxygen as the carbon descends into the deep ocean. The process of transferring carbon from the ocean surface to the depths is known as the biological pump.

Currently, the ocean is losing oxygen and this trend is expected to continue as the solubility of oxygen decreases as temperatures warm. This would lead scientists to expect higher oxygen concentrations during the last ice age, when the oceans were colder, Schmittner said, but sediments previously collected under the sea floor show levels of lower oxygen in the ocean depths during this time.

Researchers have already hypothesized that the biological pump was more powerful during the last ice age, increasing the respiration of carbon. But that assumes that the ocean’s surface oxygen is balanced with the atmosphere, Schmittner said.

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

They found that imbalance played an important role in the carbon cycle. Oxygen concentrations in the high seas were reduced because surface waters were less balanced with the atmosphere. The imbalance was the result of the vast sea ice mostly over the Southern Ocean, as well as higher iron fertilization of the Ice Age atmosphere, which was dustier, Schmittner said.

This means that the oxygen levels in the ocean depths are informed not only by the biological pumping process, but also by sea ice, or its lack, just as the air quality in a room can change by opening or closing a window, he said.

The researchers’ method for understanding the role of sea ice and other processes in ocean carbon and oxygen cycles could 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 these changes.”