• Austin Pugh

New paper: Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2

Sediment on the sea floor is often created from the skeletons of microscopic shell forming animals that build their shells out of CaCO3 (calcium carbonate). Because seawater tends to become naturally more acidic as depth increases, when these animals die and sink to the sea floor, they are vulnerable to dissolution. The risk of dissolution of the sea floor is elevated the more acidic the ocean becomes, however the degree to which they are vulnerable is still unknown. We interviewed Dr. Olivier Sulpis whose recent study examines how anthropogenic CO2 affects these sediments on the sea floor.


Dr. Sulpis is currently a Postdoctoral Researcher based in Utrecht, Netherlands working in the Geochemistry research group of the Department of Earth Sciences at Utrecht University.


What is your background?


I have a bachelor degree of Earth Sciences from the Université de Lyon in France. There, I learned about the climate crises that have occurred along the long history of Earth, in particular that of the Anthropocene, characterized by CO2 emissions, ocean acidification and mass extinctions. I also enjoyed chemistry and working in a lab. So I merged these two things together for my PhD at McGill University, Montreal, and wrote my thesis about the dissolution of sediments at the bottom of the oceans driven by human activities.


Two men in red hardhats kneel on the deck of a ship holding sampling tubes
Professor Alfonso Mucci (left) and then PhD student Olivier Sulpis (right) aboard the Coriolis II in 2017, sampling waters in the St. Lawrence Estuary for dissolved oxygen analyses. Credits: Louise Delaigue.


For those of us not familiar with your area of research, could you give us a little bit of background on your research project?


Calcium carbonates are a family of natural minerals, produced by marine organisms to constitute their shells and skeletons and covering the seafloor. These minerals dissolve when they are immersed in water above a certain acidity threshold, and when they do so it neutralizes some of the acid, acting like an anti acid tablet. Today CO2 emissions from human activities acidify the ocean, and it causes the calcium carbonate minerals at the seafloor to dissolve.


"...If we want to know how fast the seafloor is dissolving, we need to have a good idea of what controls dissolution..."

A gray scale microscope image of spherical marine organisms that are made of circular plates and fragments of these plates
Coccoliths under the microscope, which are plates of calcium carbonate secreted by coccolithophores, abundant unicellular algae living at the surface of the oceans and covering large areas of the seafloor. Credits : Nicole Marshall


What was the motivation or inspiration for this research?


What causes the dissolution, other than the level of acidity in the water, was not well known. The goal of this research was to build up on studies documenting dissolution of calcium carbonates in well-mixed seawater, and to look at what happens when these calcium carbonates are packed into a sediment. If we want to know how fast the seafloor is dissolving, we need to have a good idea of what controls dissolution and how fast chemical reactions involved are happening


"...Ocean acidification has now reached the bottom of the ocean..."

What was the main question of this research?


How fast are calcium carbonate minerals at the seafloor dissolving?


How did you conduct this research or how did you go about answering your question?


First, I did a series of laboratory experiments in which I reproduced tiny seafloor environments in laboratory reactors (see fig. 2), placed sediments of natural calcium carbonate grains in them (see fig. 1), and measured how fast they were dissolving. Then, I took results from these laboratory experiments and applied them at the real seafloor, where we know pretty well now what is the chemical composition of the seawater since it has been sampled by oceanographic cruises worldwide since the 70s.



 disk of synthetic white sediment containing calcium carbonates, placed in a reactor that is filled with natural seawater
A disk of synthetic sediment containing calcium carbonates, placed in a reactor that is filled with natural seawater in order to study its dissolution rate. Credits: Olivier Sulpis
"...This dissolution is more of a warning signal..."


What were the main findings of your work?


With my colleagues, we found that calcium carbonates at the seafloor were dissolving faster at certain areas of the seafloor, where seawater is rich in anthropogenic substances, including CO2, and thus more acidic than elsewhere. That means that ocean acidification has now reached the bottom of the ocean, and is sufficiently advanced to alter its composition.


"What was surprising though is that this dissolution is already happening today, when we thought it would be something that would take place in the future."


A heat map of the globe with representing carbonate dissolution showing high levels of dissolution in the western North Atlantic and various hotspots in the southern hemisphere's oceans
Extent of calcium carbonate dissolution driven by human-made CO2 at the seafloor. Credits: Olivier Sulpis

Did you find anything unexpected?


It was not really a surprise, we know that the ocean are acidifying today, we can measure that, and we know that calcium carbonates dissolve upon acidification. We see that in the sediment record, where sediment layers that correspond to episodes of Earth history associated with elevated atmospheric CO2 concentrations are often of a brown color, because the white calcium carbonates dissolved. What was surprising though is that this dissolution is already happening today, when we thought it would be something that would take place in the future.



What is the one take-home of this work that you want everyone to know or remember?


Calcium carbonates dissolving at the seafloor sound scary, but we should not worry too much about that. If anything, this dissolution is helping us because it is neutralizing the excess acid. What we should worry about though is the many other negative effects of today’s rapid CO2 emissions. This dissolution is more of a warning signal, reminding us that we, humans, are now altering all kinds of environments on Earth, even places so remote that we have never set foot on, like the deep seafloor.


Read Sulpis et al., 2018 here (open access):

Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2


Citation: Sulpis, O., Boudreau, B.P., Mucci, A., Jenkins, C., Trossman, D.S., Arbic, B.K., and Key., R.Mm. 2018. Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2. Proceedings of the National Academy of Science, 115:46, 11700-11705.

https://doi.org/10.1073/pnas.1804250115


To learn more about Dr. Sulpis and his research, please follow him on Twitter (@OlivierSulpis) and view his website (https://deep-c.xyz/).



Acknowledgements:


Thanks to Dr. Olivier Sulpis for taking the time to answer our questions and provide insight on this important new research. All photo/figure captions were provided by Dr. Sulpis.

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