By Anne Snabes
Medill Reports

Calcium carbonate, a primary ingredient in the shells of tiny marine organisms, reduces the acidification of our world’s oceans.

The ocean is approximately 30% more acidic than when the Industrial Revolution began, and carbon dioxide emissions from human use of fossil fuels have greatly contributed to this increase.

When microscopic organisms called zooplankton and phytoplankton die, they sink to the bottom of the ocean, and their calcium carbonate shells dissolve. This process makes the ocean less acidic. But new research suggests that scientists don’t fully understand how calcium carbonate dissolves in the ocean.

Kassandra Costa, a postdoctoral scholar at Woods Hole Oceanographic Institution in Massachusetts, found that calcium carbonate dissolves at a shallower sea floor depth in the North Pacific Ocean than scientists predicted. She presented her research this October at the Comer Climate Conference, an annual meeting of climate scientists in Southwest Wisconsin. Costa told the Medill News Service that there could be a problem with how scientists predict the depth at which calcium carbonate dissolves.

Costa said that when carbon dioxide enters the ocean, it reacts with water and carbonate ions to produce bicarbonate. When calcium carbonate dissolves in water, carbonate ions are produced. This process replenishes the carbonate ions that were used up by carbon dioxide when it entered the ocean.

Costa said carbonate ions pick up some of the protons (positive particles in atoms) in the water, which makes the water less acidic. Because calcium carbonate is a source of the carbonate ion, it reduces ocean acidification.

“When calcium carbonate dissolves in the ocean, as she mentioned, that essentially buffers the acidity that is added by CO²,” said Jerry McManus, a geochemistry professor at Columbia University.

Costa said scientists predicted that calcium carbonate can be found at the bottom of the North Pacific up to depths of 4,400 meters (14,432 feet). This prediction was made using chemistry measurements and theory. Through research, Costa and her colleagues discovered that calcium carbonate can in fact only be found up to depths of about 3,000 meters (9,840 feet), some two-thirds of the predicted value.

Costa sailed in 2014 with other researchers 500 km (about 311 miles) off the coast of Oregon to the Juan de Fuca Ridge, where she collected sediment from the ocean floor by using a device called a multi-core. She said the researchers gently lower the device to the bottom of the ocean. It punches a hole in the sediment to extract a core of it. Then the researchers bring the instrument and the core back to the boat. The sediment in the multi-core contains clay as well as calcium carbonate from dead zooplankton and phytoplankton.

These tiny organisms are at the bottom of the ocean food chain, feeding other marine species.

Calcium carbonate dissolves in a gradient in the North Pacific. At a depth of 2,300 m (7,544 feet) the sediment at the ocean floor is 60% calcium carbonate and 40% clays. At a 3,000 m sea floor depth, the sediment is only 10% calcium carbonate, as most of the material has dissolved. This means that there is little calcium carbonate in the North Pacific at depths below 3,000 m.

McManus, who also went on the research trip, said Costa’s observation points to the limitation of “broad-brush theoretical constructions.” Costa made actual measurements, instead of solely relying on theory.

Costa said the mismatch between the expected and observed depths suggests that something is missing in our understanding of calcium carbonate dissolution in the sea.

“There’s something extra about the calcium carbonate in the North Pacific that makes it, in actuality, dissolve at much shallower depths than we expected,” she said.

Costa told her audience at the Comer Conference that scientists have already observed calcium carbonate dissolving on the sea floor due to human activity that produces CO₂.

Because calcium carbonate is dissolving at shallower depths than predicted, there may be a slightly lower amount of calcium carbonate in the North Pacific than expected. This would mean that calcium carbonate could not compensate for as much carbon dioxide as scientists expected, but more research needs to be done.

Costa said it will take thousands of years for calcium carbonate in the ocean to run out.

“Once that buffer does run out, the acidification of the ocean will proceed much more rapidly,” she explained.