As oceans become more acidic, they gnaw their protective soils


Much of the deep sea has never been explored closely by man. Some submarines have furrowed its depths, but reaching the bottom of the ocean is a complicated and expensive journey, as the seabed is under three miles of water, which puts tremendous pressure. "We know more about space than about the bottom of our own planet's oceans, even though more than two-thirds of the Earth's surface is covered with marine sediments," said Olivier Sulpis, researcher and PhD student at McGill University. Department of Earth Sciences and Planets.

Thus, "we hear less about the effects of human activity on the seabed than on corals, for example, simply because coral bleaching seems more attractive than mud dissolving at the bottom of the sea. sea, "he added. "When you think that before the arrival of Google Maps, it took several centuries for humans to map the continents, it is easy to understand why the deep sea exploration is so hard."

Nevertheless, he and his colleagues found a way to study it without actually going there. They recreated his environment in the laboratory, building small boxes filled with seawater-covered sediments, keeping them in the dark. They duplicated the temperature and chemistry of the seawater, as well as the composition of the sediment. By mimicking seabed conditions, "we do not have to go to the bottom of the sea to do measurements, we save time and energy," said Sulpis.

What they found was disturbing. It has already been established that climate change – particularly atmospheric carbon dioxide emitted by burning fossil fuels – has acidified the oceans, damaged fragile coral reefs and disrupted vulnerable marine ecosystems. But McGill scientists have discovered that carbon dioxide has also begun to drift to the bottom of the ocean, thus dissolving materials that help curb acidification.

"Humans have become a geological force, and there is not a single part of the surface of our planet where we can not find any trace of human activity," Sulpis said. "Paleoclimate scientists and Earth science students have heard and described several times the sudden changes in climate and episodes of ocean acidification millions of years ago that caused mass extinctions around the world. whole. These events were caused by meteorite impacts, forest fires, volcanic eruptions, etc. Today, it seems like we are at the dawn of one of these catastrophic events and we do not need to look far to find the cause. Each of us is the cause. "

Normally, the seabed is white limestone, consisting largely of calcite formed from skeletons and shells of planktonic organisms and corals. Calcite neutralizes the acidity of carbon dioxide, preventing the seawater from becoming too acidic. But these days, at least in some hot spots such as the North Atlantic and the Southern Oceans, the limestone bed of the ocean becomes dark brown, result of human activities that make the levels of carbon dioxide in the water become too high and the water too acidic, according to new research published in Proceedings of the National Academy of Sciences. The researchers finally predicted that calcite would not be able to keep pace with acidification, dissolve before it can do its job.

"Calcite at the bottom of the ocean is like a big anti-acid pill," said Sulpis. "It dissolves when there is too much CO2, which neutralizes excess CO2 in the process. If the bottom of the sea lacks calcite, the ocean loses its anti-acid pill and we risk sinking into the disturbing acidification of the ocean. "

The researchers measured the speed with which the sediments dissolve when they are placed in boxes covered with seawater. "We did this for a few years and we finally realized that we understood this well enough. dissolution reaction to describe it with the help of simple mathematical equations, "explained Sulpis. "If we know the chemical conditions and the properties of sediments, we can calculate the rate of dissolution of calcite in these sediments. "

They used state-of-the-art ocean models to calculate the dissolution rates of calcite on the seabed. The experiments provided a better understanding of what controls the dissolution of calcite in marine sediments. By comparing dissolution rates of pre-industrial and modern seabed, they were able to determine how much of the total dissolution was caused by man.

Scientists know that calcite has always helped to deacidify the ocean. "What is surprising and disturbing, however, is that it is happening now," Sulpis said. "Scientists thought that it would take a lot longer before we see a dissolution of the calcite at the bottom of the sea caused by our CO2. We know how our climate works. We know how our oceans work, but what we are unable to predict is our society and how we will adapt our behavior to this changing world. "

The results have far-reaching implications. "Just as climate change is not just about polar bears, ocean acidification is not just about coral reefs," said David Trossman, currently a research associate at the University of Texas-Austin and co-author of the study. "Our study shows that the effects of human activities have become evident to the bottom of the sea in many areas and that the resulting increased acidification in these areas could affect our ability to understand climate history. of the earth."

In future work, researchers plan to study the likely evolution of dissolution over the next centuries. Since much of the carbon dioxide from burning fossil fuels still remains on the surface of the ocean, it may take decades, if not centuries, for CO2 to fall back to the ocean. bottom of the ocean, scientists said. So all is not lost – at least not yet, he says.

"Fortunately for us, there is a lot of calcite and only a quantity of fossil fuels that we can burn," said Sulpis. "It does not mean we have the green light to pollute more – it just means the oceans will be there to clean up our mess, if it takes thousands of years …" I hope, a- he added, burning fossil fuels, either because we ran out of stock, or – a smarter option – because we would have developed alternative, renewable energies. "

Marlene Cimons writes for Nexus Media, an outsourced newswire covering climate, energy, politics, art, and culture.