Warmer seas may hinder organic carbon burial in ocean sediments

Ocean warming may decrease the rate at which carbon from dead and decaying marine organisms is buried and sequestrated on the sea floor.

The storage of this ‘organic’ carbon in ocean sediments prevents it from being converted into carbon dioxide and reaching the atmosphere, where it would increase greenhouse levels and contribute to global warming.

A group of scientists from the University of Leeds, Texas A&M University and Rice University found evidence that a rise in ocean temperatures during a famous ancient period of global warming 15 million years ago increased the metabolic activity of bacteria in the sea, which breaks down dead marine organisms before they can sink to the ocean bed.

This breakdown of organic carbon releases carbon dioxide.

Dr Benjamin Mills, a biogeochemist in the School of the Earth and Environment at Leeds, says the new research is challenging conventional thinking about the rate at which organic carbon is stored in ocean sediments.

He said: “We have built models of these ancient global warming episodes, but we have always assumed that during periods of global warming, we would see greater uptake of carbon dioxide by algae and plankton in the ocean. So, we would expect to see more organic carbon buried in the sediments.”

the researchers say their findings came as a surprise as they developed an alternative and more accurate approach to measuring the rate at which organic carbon is buried.

In a paper published in the scientific journal Nature, ‘Neogene burial of organic carbon in the global ocean,’ the researchers say their findings came as a surprise as they developed an alternative and more accurate approach to measuring the rate at which organic carbon is buried.

Professor Yige Zhang, an oceanographer at Texas A&M University, who led the research effort, said “Experts have traditionally used a system of isotopic calculations to estimate organic carbon burial. Photosynthesis prefers the lighter isotope of carbon, so isotope measurements can be used to differentiate between organic or inorganic carbon burial in the global ocean, but this method is known to be very uncertain.”

Instead, the researchers looked at data records from more than 1,500 scientific drilling missions into the sea floor conducted - over 50 years - as part of the Ocean Discovery Program.

Warmer ocean temperatures allow bacteria’s metabolic rates to increase, resulting in the degradation of organic matter, returning it to carbon dioxide

Professor Zhang said: “We had this idea of compiling a huge global database and calculating the organic carbon burial rate in each location to come-up with the global carbon burial rate. We wanted to see if this ‘bottom-up’ method agreed with the traditional method of isotopic calculations, which is more ‘top down.”

But he says they were shocked when they compared the results from the new and old approaches.

For example, during a period called the mid-Miocene, about 15 million years ago, popular belief is that a large amount of organic carbon was buried around this interval, exemplified by the organic-rich “Monterey Formation” found in California.

The team’s new result, however, suggests instead that the smallest amount of organic carbon was buried during this interval over the last 23 million years or so.

Dr Ziye Li, who conducted the work as a visiting student in Texas, said: “Warmer ocean temperatures allow bacteria’s metabolic rates to increase, resulting in the degradation of organic matter, returning it to carbon dioxide.”

The process, adds Dr Mills, is like respiration - the opposite of the process of photosynthesis.

 Instead of marine organisms helping to remove CO2 when the planet warms, we actually have the opposite, where marine microbes release more CO2 and may drive further warming

Dr Mills added: “Our conclusion is that as we warm up the ocean, it will make it harder for organic carbon to find its way to be buried in the marine sediments. The lowest rates of carbon sequestration happen when the planet was warming. This means that instead of marine organisms helping to remove CO2 when the planet warms, we actually have the opposite, where marine microbes release more CO2 and may drive further warming.

The team’s research was funded by the American Chemical Society’s Petroleum Research Fund (Grant No. 59797-DNI2). Their paper, “Neogene burial of organic carbon in the global ocean,” can be viewed online along with related figures and captions.

Photo by Marek Okon on Unsplash