Earth’s atmosphere: Why do levels of carbon dioxide vary over millennia?

Why do carbon dioxide levels in the atmosphere wax and wane in conjunction with the warm and cold periods of Earth’s past? Scientists have been trying to answer this question for many years, and thanks to chemical clues left in sediment cores extracted from deep in the ocean floor, they are starting to put together pieces of that puzzle. Recent research suggests that there was enhanced storage of respired carbon in the deep ocean when levels of atmospheric carbon dioxide concentrations were lower than today’s levels.

But now, new research led by a Texas A&M scientist has reached back even further in time, for the first time revealing insights into atmospheric carbon dioxide levels in the 50,000 years before the last ice age.

“One of the biggest unknowns about past climate is the cause of atmospheric carbon dioxide variability over global warm-cold cycles,” said Franco Marcantonio, lead author of the study and professor and Jane and Ken R. Williams ’45 Chair in the Department of Geology and Geophysics at Texas A&M.

“Here we investigated the ‘how’ of varying carbon dioxide with the ‘where’ — namely, the Eastern Equatorial Pacific Ocean, which is an important region of the world ocean where, today significant carbon dioxide is exhaled into the atmosphere and the greatest rates phytoplankton growth are found.” 

The National Science Foundation-funded research was recently published in Scientific Reports, a Nature Research journal. 

To examine ancient carbon dioxide levels, Marcantonio and a team of researchers analyzed an ocean floor sediment core extracted from the deep Eastern Equatorial Pacific Ocean. The ten-meter long core spans about 180,000 years, and the chemistry of the layers of sediment provide scientists with a window into past climates. The chemical measurements they make serve as a proxy for oxygen levels of the deep sea.

Measuring minute traces of uranium and thorium isotopes, the team was able to associate periods of increased storage of respired carbon (and low deep-sea oxygen levels) with periods of decreased global atmospheric carbon dioxide levels during the past 70,000 years.

Co-authors of the study were Ryan Hostak, a former Texas A&M graduate student who earned his master’s degree in geology in 2019; Jennifer E. Hertzberg, who received her Ph.D. in oceanography from Texas A&M in 2015 and is now a postdoctoral researcher in the Department of Earth, Ocean and Atmospheric Sciences at Old Dominion University; and Matthew W. Schmidt, associate professor of Ocean, Earth and Atmospheric Sciences at Old Dominion.