Could the oceans save the Earth from the next climate warming?

Humans get carbon out of the earth by burning fossil fuels deposited millions of years ago, and bringing it up to the atmosphere as carbon dioxide. In contrast, the current rate of formation of new fossil fuels is very low. An alternative, long-term geological mechanism for carbon storage today is the formation of seashells, which are preserved as sediments on the ocean floor.

It is true that we think of trees and soil as natural carbon sinks, but the oceans have much larger stores of carbon and are much more efficient at storing carbon permanently.

In new research published Jan. 26 in Paleoceanography and Paleoclimatology, researchers investigate the long-term rate of permanent carbon removal by the formation of "shells" of plankton in the ocean near New Zealand.

The research shows that seashells have withdrawn the amount of carbon that is approximately equivalent to the regional emissions of carbon dioxide, and the research also found that the process of carbon withdrawal through seashell formation was higher during the warming of the climate, during the past periods of the Earth's history.

Zealandia submerged in the ocean

This project is part of the International Ocean Discovery Program/IODP, where Expedition 371 drilled the sea floor of Zealandia to investigate how the continent formed and to analyze ancient environmental changes recorded in its sediments.

The researchers found that carbon dioxide emissions from burning fossil fuels add about 45 million tons annually to the submerged continent in the Pacific Ocean, which represents 0.12% of the global total.

The continent of Zealandia or New Zealand is the largest continent currently submerged in the Pacific Ocean after its separation from Australia 60 million years ago, and its separation from Antarctica 130 million years ago. Its area is twice the size of India, and most of it is located at a depth of more than 1,000 meters in the southwest Pacific Ocean. It includes the islands of New Zealand and New Caledonia.

How is carbon pulled to the ocean floor?

In all oceans, organic carbon in the form of dead plants, algae and animals is usually taken up by other organisms, especially bacteria. Most of these organisms are very small, less than 1 millimeter in size, and remain invisible, but when they die and sink, they transport carbon to the depths of the ocean. Their shells can accumulate on the sea floor to form huge deposits of chalk and limestone.

As for the fish deposits that were excavated in this study, they reached hundreds of meters. These deposits were formed during periods when the climate was warmer, and this is somewhat similar to the coming decades and centuries in which scientists expect the climate to rise.

Over the past million years, the average rate of accumulation of shells, which are calcium carbonate and sequester large amounts of carbon, has averaged about 20 tons per square kilometer per year.

The total area of ​​Zealandia is about 6 million square kilometers, so the average storage rate of calcium carbonate was about 120 million tons per year, which is equivalent to 53 million tons of carbon dioxide per year. This is roughly the same as emissions from burning fossil fuels on the continent today.

Warmer climate

About 4-8 million years ago, the climate was much warmer, carbon dioxide levels were similar or even higher than they are today, and the ocean was more acidic. However, the scientists found that the average rate of seashell accumulation in Zealandia was more than twice the rate of accumulation in the last million years.

Warmer climates during this period had oceans that produced more seashells, but these data reflect average rates of accumulation over time scales of one million years. Thus, the mechanism by which these oceans produce more shells as the climate warms remains an ongoing topic of research.

According to the report published in The Conversation, this project highlights the important role that the ocean, and in particular the microscopic life within it, plays in the eventual restoration of balance to our planet.

This is because the rate at which dead plankton draw carbon to the deep ocean and the tiny shells permanently store it on the sea floor accounts for a large proportion of human carbon dioxide emissions.

This work also reveals that an ocean that will inevitably warm may produce more calcium carbonate shells than the oceans currently do, although ocean acidification will almost certainly occur as well.

According to the researchers, how quickly the ocean's natural carbon sequestration may change remains highly uncertain. To know this, we need several more centuries before we reach an oceanic state similar to that which existed 4-8 million years ago.