The eastern tropical Pacific Ocean is known for its large low-oxygen zones that are increasing in size, putting marine life at risk. New research shows that 15 million years ago, the opposite was true.
A Michigan State University study found that oxygen-deficient waters were distributed very differently during the mid-Miocene Epoch than they are today. The Pacific Ocean's oxygen-deficient zones were much smaller, while the Atlantic's were much larger. Scientists had never documented this reversal before.
A computer model helped explain why. By recreating ancient ocean conditions, Associate Professor Dalton Hardisty's team learned that a channel between North and South America allowed water to move freely between the Pacific and Atlantic oceans, reshaping ocean circulation and changing where low-oxygen waters formed.
The findings suggest that warming alone cannot explain how ocean oxygen levels change. Ocean circulation and the position of the continents may be just as important in determining where oxygen-rich and poor waters form.
"When we talk about climate change and global impacts, we say oxygen in the ocean is going to up or down, but this study has brought to light that that's not necessarily true everywhere," said Janet Burke , a fixed-term assistant professor and first author of the paper. "It's going to change the climate system, ocean circulation and atmospheric circulation in ways that might increase oxygen levels in some places and decrease them in other places. The impacts are regionally variable."
About 15 million years ago, Earth experienced a warmer climate with higher carbon dioxide levels, making it a useful window into how oceans may respond to future warming. Scientists are interested in how warming shifts oxygen levels because less of the vital gas makes survival difficult for marine life.
To study it, they turn to a type of plankton called Foraminifera or "forams". These microscopic organisms float through the ocean and sit at the base of the marine food web. They also absorb the chemical composition of the surrounding water and incorporate it into their shells.
"Their little shells are like a snapshot of the ocean's chemical composition where they lived at that time," Burke said. "If you know how they incorporate different elements into their shells, you can use them as a proxy measurement for the chemistry of the ocean water."
Planktonic forams look like grains of sand to the naked eye. Under a microscope, those grains transform into tiny seashells that once housed amoeba-like creatures. When they died, the seashells drifted down to the ocean floor and made up the sand.
Those layers collected over time, writing the record books of the ocean's history. Scientists study that history by drilling into the ocean floor and pulling up a sediment core that tells the story of ancient waters.
"Each layer represents a different time period," Burke said. "We study them and learn what species were living there, how big their shells were, were and how healthy and happy their communities seemed. Then we use their chemistry to learn what the ocean was like millions of years ago."
Samples are available to scientists thanks to a program called the International Ocean Discovery Program, or IODP. Burke requested a sample from the Miocene Epoch and spent hours picking plankton from the sediment. The selected shells were washed, crushed and dissolved to analyze the chemical contents.
Burke planned to gather more evidence of the Pacific's low oxygen zones from that era, and her Atlantic samples were intended as a control. She was shocked when her results flipped her expectations upside down. Her Pacific samples near the coast of Peru showed signs of elevated oxygen, while the Atlantic samples were consistently oxygen poor.
"My first thought was that something was wrong," Burke said. "I thought I might have to disregard the samples."
Instead of scrapping the experiment, Hardisty pulled in Keyi Cheng, a graduate student in his lab. Cheng had worked separately on a computer model that incorporates everything from algae growth and bacterial processes to ocean circulation and the iodine cycle.
Using the model, Hardisty and Cheng projected the global conditions, including the position of the continents, as they would have been in the mid-Miocene Epoch when Burke's samples were formed. The model confirmed what Burke's data had shown – the paleogeography and resulting changes in ocean circulation caused the Atlantic Ocean to have a large oxygen-deficient zone, while the Pacific's oxygen-deficient zone shrank significantly.
"The implication was that this actually has little to do with it being warm," Hardisty said. "We have to consider ocean circulation as well."
In today's climate, the study suggests that changing ocean oxygen levels could mean something different for regions around the world. Oxygen-deficient zones might increase in some areas, but others might gain oxygen because of changes in overall ocean circulation.
Hardisty hopes to continue studying the Atlantic Ocean's oxygen-deficient zones. He wants to pull samples from other regions of the ocean to learn more details about how large the zone was, how long it lasted and whether the changing levels coincided with major changes in ocean circulation and seaway connections.
More mysteries lie hidden in ancient plankton shells. Hardisty's team hopes to uncover what they reveal about the future of Earth's oceans.
By Bethany Mauger
