Earth's oceans have become approximately 30% more acidic since the industrial revolution began more than 200 years ago. Acidification changes marine chemistry and depletes key minerals that calcifying organisms, such as corals and clams, need to build their skeletons and shells. The Northeastern Pacific is naturally more acidic than other oceans, fueling debate about how much its chemistry will change in the coming decades.
The study, published Nov. 13 in Nature Communications, shows that high baseline acidity makes the water more sensitive to additional carbon dioxide from human activities. Analyses of coral skeletons from the past century revealed that CO2 has been accumulating in North American waters faster than in the atmosphere, driving rapid acidification.
"This fits into a class of really important records that show how the world has changed over the human era," said senior author Alex Gagnon, a University of Washington associate professor of oceanography.
"The findings implicate not only marine ecosystems, but all of the people who depend on them as well," added lead author Mary Margaret Stoll, a UW doctoral student of oceanography.
The ocean becomes more acidified when carbon dioxide dissolves to form an acid that releases hydrogen and bicarbonate ions, lowering the water's pH level. In North America, a powerful current system - the California Current - transports cool water south along the coast. The combination of current flow and wind creates optimal conditions for upwelling, a process that cycles deep water to the surface.
Organic matter - dead plants and animals - sinks to the bottom of the ocean, where it decomposes and releases carbon dioxide back into the water. Upwelling surfaces this CO2-rich water, increasing the acidity of subsurface and surface zones. These natural fluctuations complicate researchers' efforts to predict how much acidification will occur from human activities.
This study helps resolve these questions with records kept by centuries-old corals.
Coral incorporates elements and minerals from seawater as it grows, leaving behind a valuable record of environmental conditions in its skeleton. The Pacific Ocean is home to a small vibrant species called orange cup corals. Gagnon's lab was already studying orange cup corals when the researchers became interested in historic samples.
In 2020, the researchers began collecting samples - first from the Smithsonian Museum, and then from labs and museums all over the U.S. and Canada. They procured a total of 54 samples collected between 1888 and 1932 from the Salish Sea, the body of water connecting Washington state and Canada, and North American coastal waters.
Using handwritten records in logbooks, the researchers then navigated back to the original collection sites. They took orange cup corals from the same spots, sometimes more than a century later.
To plot CO2 and acidity over time, the researchers analyzed boron levels in the coral skeletons. In seawater, boron exists in several chemical forms that vary with acidity. Corals incorporate one of these forms into their skeletons as they grow, so the boron ratio in coral skeletons reflects the acidity of the seawater in which they formed.
Between 1888 and 2020, coral skeletons indicate that CO2 in seawater increased at a rate that outpaced the addition of greenhouse gases to the atmosphere. The magnitude of acidification was also higher 100 to 200 meters below the surface, even though ocean acidification is typically characterized as a surface process.
"No one has acidity measurements older than a few decades," Gagnon said. "We had to go back in time and do some detective work to pull some kind of chemical signal out of the world and show this unfortunate amplification effect."
The amplification effect will likely strengthen as atmospheric CO2 levels continue to climb. In the study, the researchers modeled worst case scenarios to see what could happen to species if acidification continues unchecked.
"The changes in ocean chemistry were really dramatic," Stoll said. "The Salish Sea is a region with a lot of cultural, commercial and recreational ties to marine organisms that are all rooted in the health of these ecosystems."
Despite the tenor of their results, the researchers say there is still time to course correct.
"This is no time for nihilism. The ocean is not destroyed," Gagnon said. "As very large emitters per capita, we have the power to change our emissions and influence outcomes for the oceans."
Studying regions where ocean acidification is happening faster than elsewhere can also provide key insights and warning signs.
"This is a uniquely important area to study," Stoll said. "It is at the leading edge of ocean acidification impacts and provides a window into conditions predicted for the rest of the ocean in the coming decades."
Research Report:A century of change in the California Current: upwelling system amplifies acidification
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