Study reveals carbon cycle trends of the past 50 million years

To make predictions about how Earth’s climate will change in the future, scientists must understand environmental changes of the past. In a new study from the University of Hawaii at Mānoa, oceanographers developed a model to reconstruct carbon cycle trends over the past 50 million years.

The study reveals dramatic shifts in both the climate and carbon cycle, and some of these changes contradict what was expected about carbon cycle dynamics.

Less carbon dioxide in the atmosphere should result in a shallower calcite compensation depth (CCD), which is the ocean depth where the rate of carbonate material raining down equals the rate of carbonate that is dissolved. The CCD can be examined over the geological past by measuring the calcium carbonate content in seafloor sediment cores.

The current study, led by former PhD student Nemanja Komar, is the first to quantitatively account for all of the important components of the carbon cycle over the past 66 million years.

Contrary to what was expected based on previous assumptions about the carbon cycle, the carbonate records revealed that as atmospheric CO2 decreased over the past 50 million years, the global CCD became deeper instead of shallower.

“The variable position of the paleo-CCD over time carries a signal of the combined carbon cycle dynamics of the past,” said Komar. 

“Tracing the CCD evolution across the Cenozoic and identifying mechanisms responsible for its fluctuations are therefore important in deconvolving past changes in atmospheric CO2, weathering, and deep-sea carbonate burial. As CO2 and temperature dropped over the Cenozoic, the CCD should have shoaled but the records show that it actually deepened.”

The computer model developed for the study allowed the researchers to investigate the potential mechanisms responsible for the long-term trends they identified.

“Surprisingly, we showed that the CCD response was decoupled from changes in silicate and carbonate weathering rates, challenging the long-standing uplift hypothesis, which attributes the CCD response to an increase in weathering rates due to the formation of the Himalayas and is contrary to our findings,” explained Komar.

The research indicates that the disconnect was partly caused by an increasing proportion of carbonate buried in the open ocean compared to the carbonate present on the continental shelf.

“Our work provides new insight into the fundamental processes and feedbacks of the Earth system, which is critical for informing future predictions of changes in climate and carbon cycling,” said Komar.

The study is published in the journal Science Advances. 

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By Chrissy Sexton, Earth.com Staff Writer