A vast ocean conveyor carries water from Greenland through the Atlantic for 350 years and shapes rainfall in Brazil; 15,400 years ago, it suddenly surged beyond today's strength

A powerful system of Atlantic Ocean currents can change far more abruptly than scientists once thought. New evidence from sediments collected off Brazil’s northeastern coast shows that the Atlantic Meridional Overturning Circulation, or AMOC, experienced sudden bursts of strengthening thousands of years ago, even while the system was otherwise much weaker.

The finding comes from researchers in Brazil and Germany, who reconstructed past changes in the AMOC using tiny fossil shells preserved in a marine sediment core. Their results suggest that the Atlantic circulation may be capable of a series of abrupt changes rather than simply moving steadily toward a weaker state.

The study,“Centennial-scale intensifications of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 1,” was published in Nature Communications by Partha Sarathi Jena, Cristiano M. Chiessi, Ines Beese and colleagues. The research was conducted under the auspices of the Climate Crisis and Disasters Resilience Research Center (CLIMARES), a FAPESP Research, Innovation, and Dissemination Center.

What happened to the AMOC 15,400 years ago?

The AMOC is often described as a giant conveyor belt that moves heat through the Atlantic Ocean. Warm water travels from tropical and equatorial regions toward the North Atlantic, while colder, denser water sinks near Greenland and flows southward through the deep ocean before eventually returning toward the surface.

That circulation helps regulate temperatures in Europe and parts of North America and influences rainfall across tropical regions of Africa and South America.

The new study, led by Cristiano Mazur Chiessi of the School of Arts, Sciences, and Humanities at the University of São Paulo and Stefan Mulitza of the University of Bremen, examined a period known as Heinrich Stadial 1, or HS1, between about 17,800 and 14,800 years ago.

For much of that period, the AMOC was considerably weaker than it is today. Yet the researchers found evidence of two sudden periods when the circulation intensified.

The first lasted from roughly 16,500 to 15,800 years ago. The second was much shorter, lasting about 100 years around 15,400 years ago. During that later episode, the AMOC became even stronger than its present-day level.

“This is the first time it’s been shown that the AMOC can experience bursts of strengthening during periods when it’s weakened,” Chiessi says. “As one of the article’s reviewers wrote, this completely changes the way we understand the Atlantic Meridional Overturning Circulation.”

How did sediments reveal the AMOC’s past?

The evidence came from a sediment core collected during a 2012 expedition by the German research vessel RV Maria S. Merian.

The sample was taken about 189 kilometers off the coast of Maranhão in northeastern Brazil, at a depth of 1,367 meters. The location proved especially useful because heavy rainfall in the region had produced a high rate of sedimentation, preserving numerous layers for researchers to examine.

“It’s like having a movie with many frames per second in extremely high resolution,” Chiessi explains.

The team used radiocarbon ventilation dating to reconstruct how quickly deep ocean water had circulated in the past. Researchers Partha Sarathi Jena and Ines Beese conducted the analyses during their postdoctoral fellowships at EACH-USP and the University of Bremen, respectively.

They examined fossil shells belonging to two groups of microscopic organisms known as foraminifera. Planktonic foraminifera lived near the ocean surface, while benthic foraminifera lived on the seafloor.

Because carbon-14 is taken up at the surface and reaches the deep ocean through sinking water, the difference in apparent age between the two types of shells provides a way to estimate how long water has remained isolated from the atmosphere.

Today, that circulation takes about 350 years in the equatorial Atlantic. The researchers used this relationship to reconstruct changes in AMOC intensity.

“The age difference between a shell formed at the surface and one formed at the bottom is a direct indicator of how intense the AMOC is,” Chiessi concludes.

The results showed a striking pattern. Before HS1, the age difference was about 325 years. During most of HS1, it rose to 960 years, consistent with a much weaker AMOC.

But during the first strengthening episode, the difference dropped to 450 years. During the shorter event around 15,400 years ago, it fell even further, to about 200 years, indicating that the circulation had intensified beyond its current level.

What does Brazil’s rainfall have to do with the ocean?

The researchers also compared their findings with other records of ancient climate conditions, including precipitation estimates from stalagmites in caves in Mato Grosso do Sul and Bahia.

The two periods of AMOC strengthening matched periods when heavy rainfall in Brazil gave way to a drier climate. That reduced the amount of freshwater entering the ocean.

The researchers also found that the AMOC intensification episodes coincided with increases in atmospheric carbon dioxide recorded in Antarctic ice.

Chiessi and his colleagues suggest that stronger circulation may have transported deep, carbon dioxide-rich waters, which had remained relatively stagnant while the AMOC was weak, toward the Antarctic Circumpolar Current. There, some of that carbon dioxide was released into the atmosphere.

The discovery matters because the AMOC is already being affected by modern climate change. Greenhouse gas emissions have warmed the planet, while melting Greenland ice, a warmer Arctic Ocean and increased rainfall can reduce the salinity and density of surface water. That makes it harder for water to sink and can weaken the circulation.

Recent predictions cited in the study suggest the AMOC could weaken substantially by 2100 even if countries meet their greenhouse gas reduction commitments.

Could the AMOC change suddenly again?

The ancient record does not provide a timetable for what will happen in the future. Chiessi stresses that the climate conditions during HS1 were very different from those of today, including much lower atmospheric carbon dioxide concentrations.

“Climate conditions were entirely different,” he explains. “During that period, the concentration of carbon dioxide in the atmosphere was lower than in the pre-industrial era. We can learn from past events, but they aren’t perfect analogues.”

What the sediment record does show is that a weakened AMOC was not necessarily stable. It could undergo abrupt episodes of strengthening, adding another layer to scientists’ understanding of how the Atlantic circulation responds to major climate shifts.

Chiessi hopes this work will eventually help improve AMOC forecasts and identify climate signals that could warn of sudden changes.

“We’ll need a great deal of resilience, but we can still prevent the worst from happening if we take action to drastically reduce greenhouse gas emissions,” he notes.

FAQs

What is the AMOC?
A major Atlantic system that moves heat through the ocean.

When did the strongest ancient surge occur?
Around 15,400 years ago.