How seafloor weathering drives the slow carbon cycle

February 14, 2018, University of Sydney
The oceanic slow carbon cycle. Credit: Adriana Dutkiewicz

A previously unknown connection between geological atmospheric carbon dioxide cycles and the fluctuating capacity of the ocean crust to store carbon dioxide has been uncovered by two geoscientists from the University of Sydney.

Prof Dietmar Müller and Dr Adriana Dutkiewicz from the Sydney Informatics Hub and the School of Geosciences report their discovery in the journal Science Advances.

Many of us are familiar with the Slow Movement philosophy, which includes slow living, slow cooking, slow fashion, and even slow TV. But most of us would not have heard of the slow , which is about the slow movement of carbon between the solid Earth and the atmosphere.

The slow carbon cycle predates humans and takes place over tens of millions of years, driven by a series of chemical reactions and tectonic activity. The slow carbon cycle is part of Earth's life insurance, as it has maintained the planet's habitability throughout a series of hothouse climates punctuated by ice ages.

One idea is that when rises, the weathering of continental rock exposed to the atmosphere increases, eventually drawing down carbon dioxide and cooling the Earth again.

Less well-known is that weathering exists in the deep oceans too. Young, hot, volcanic crust is subject to weathering from the circulation of seawater through cracks and open spaces in the crust. Minerals such as calcite, which capture carbon in their structure, gradually form within the crust from the seawater.

Recent work has shown that the efficiency of this seafloor weathering process depends on the temperature of the water at the bottom of the ocean—the hotter it is, the more carbon dioxide gets stored in the ocean crust.

Prof Müller explains: "To find out how this process contributes to the slow carbon cycle, we reconstructed the average bottom water temperature of the oceans through time, and plugged it into a global computer model for the evolution of the ocean crust over the past 230 million years. This allowed us to compute how much carbon dioxide is stored in any new chunk of crust created by seafloor spreading."

Animation illustrating the changing CO2 storage in ocean crust over the past 200 million years. Credit: Dietmar Müller

Dr Dutkiewicz adds: "Our plate tectonic model also allows us to track each package of ocean floor until it eventually reaches its final destination—a . At the subduction zone, the crust and its calcite are recycled back into the Earth's mantle, releasing a portion of the carbon dioxide into the atmosphere through volcanoes."

The computer model reveals that the capacity of the ocean crust to store carbon dioxide changes through time with a regular periodicity of about 26 million years.

Several geological phenomena including extinctions, volcanism, salt deposits and atmospheric carbon dioxide fluctuations reconstructed independently from the geological record all display 26 million-year cycles.

A previous hypothesis had attributed these fluctuations to cycles of cosmic showers, thought to reflect the Solar System's oscillation about the plane of the Milky Way Galaxy.

Prof Müller says: "Our model suggests that characteristic 26 million-year periodicity in the slow carbon cycle is instead driven by fluctuations in seafloor spreading rates that in turn alter the capacity of the ocean to store carbon dioxide. This raises the next question: what ultimately drives these fluctuations in crustal production?"

Subduction, the sinking of tectonic plates deep into the convecting mantle, is regarded as the dominant plate driving force of plate tectonics. It follows that cyclicities in seafloor spreading rates should be driven by equivalent cycles in subduction.

An analysis of subduction zone behaviour suggests that the driving force in the 26 million-year periodicity originates from an episodicity in subduction zone migration. This component of the slow carbon cycle needs to be built into global carbon cycle models.

Better understanding of the slow carbon cycle will help us predict how the Earth will react to the human-induced rise in atmospheric carbon dioxide. It will help us answer the question: To what extent will the continents, oceans and the take up the extra in the long run?

Explore further: Findings may help scientists understand how much carbon dioxide can be released while still limiting global warming

More information: R.D. Müller el al., "Oceanic crustal carbon cycle drives 26-million-year atmospheric carbon dioxide periodicities," Science Advances (2018). DOI: 10.1126/sciadv.aaq0500 , http://advances.sciencemag.org/content/4/2/eaaq0500

Related Stories

When continents break it gets warm on Earth 

November 13, 2017

The concentration of carbon dioxide (CO2) in the atmosphere determines whether the Earth is in greenhouse or ice age state. Before humans began to have an impact on the amount of CO2 in the air, it depended solely on the ...

New timeline proposed for plate tectonics

May 11, 2017

Earth's history should include 'pre-plate tectonic' and 'plate tectonic' phases beginning less than a billion years ago, according to a team of geoscientists in the journal Geology.

Scientists make new estimates of the deep carbon cycle

June 19, 2015

Over billions of years, the total carbon content of the outer part of the Earth—in its upper mantle, crust, oceans, and atmospheres—has gradually increased, scientists reported this month in the journal Proceedings ...

Falling sea level caused volcanos to overflow

July 7, 2017

Throughout the last 800,000 years, Antarctic temperatures and atmospheric carbon dioxide concentrations have showed a similar evolution. However, they were different during the transition to the last ice age—approximately ...

Recommended for you

Ocean acidification may reduce sea scallop fisheries

September 21, 2018

Each year, fishermen harvest more than $500 million worth of Atlantic sea scallops from the waters off the east coast of the United States. A new model created by scientists at the Woods Hole Oceanographic Institution (WHOI), ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

nrauhauser
not rated yet Feb 14, 2018
So our 26my cycle isn't anything cosmological, it's something slow moving here on Earth? This is really good news, it means further study is possible, as opposed to the waiting/speculating we've seen with other theories.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.