"Remineralization of sinking organic carbon in the ocean's interior is a key process that transports carbon from the surface ocean to the deep ocean," Eun Young Kwon of Princeton University, US, told environmentalresearchweb. "The depth of remineralization is determined by the balance between the decay rate of organic carbon and the sinking speed of organic carbon, which depends on climate sensitive factors such as temperature and oxygen concentrations. Thus it is likely that the remineralization depth might change with climate change."
That said, it's not clear how remineralization depth will respond to climate change. And until now, it hasn't been apparent how changes in remineralization depth could affect atmospheric carbon dioxide concentration and hence climate. With this in mind, Kwon and colleagues from Princeton and the University of California, Irvine, tackled this second question using a 3D global ocean biogeochemistry model.
The team found that if the depth at which 63% of sinking carbon is respired increases by 24 m around the world, atmospheric carbon dioxide concentrations would fall by 10–27 ppm. For comparison, there is currently around 380–ppm of carbon dioxide in the atmosphere.
"The impact of remineralization depth on air-sea carbon partitioning was poorly known partly due to the high cost associated with carrying out model sensitivity studies of remineralization depth," said Kwon. The researchers say that the computational efficiency of their model enabled them to examine the sensitivity of atmospheric carbon dioxide levels to changes in the remineralization depth and the underlying mechanisms of the sensitivity.
"We showed that changes in the remineralization depth result in the redistribution of respired carbon between intermediate waters and bottom waters," said Kwon. "Because the ocean's turnover timescales increase with depth – ranging from seasons at the surface to millennia at the bottom of the ocean – the vertical redistribution of respired carbon could result in a substantial change in the timescales at which respired carbon stays in the deep ocean before being exposed to the atmosphere. At steady state, an increase in the residence time of respired carbon in the ocean's interior results in a decrease in atmospheric carbon dioxide concentration."
During the Last Glacial Maximum climate was probably colder and dustier than today. The researchers believe that the colder seawater may have slowed the decay rates of sinking organic carbon and that dustier conditions increased the amount of ballast minerals and raised the organic carbon's sinking speed. Both these factors would have caused an increase in the remineralization depth of organic carbon and so reduced carbon dioxide in the atmosphere. "We suggest that the carbon dioxide drawdown during glacial times might be due in part to a change in the remineralization depth," said Kwon.
According to Kwon, the warmer temperatures of the future might increase the rate at which bacteria metabolize sinking organic material, resulting in a decrease in remineralization depth. On the other hand, enhanced stratification of the global ocean could decrease oxygen concentrations, slowing down bacterial metabolic rates, she said.
"For now, we don't have a full understanding of what controls the depth of remineralization, and we don't know how remineralization depth might change in response to climate change," explained Kwon. "This is the major gap to be filled by our research community in the future. Nonetheless, our study suggests that a possible response of remineralization depth to climate change could result in a significant feedback on changing climate."
So what's next? Kwon says that an important step is to develop a prognostic model of remineralization processes and incorporate the model in climate models to project future climate change and the carbon cycle.
The researchers reported their work in Nature Geoscience.