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New research from an international group looking at ancient sediment cores in the North Atlantic has for the first time shown a strong correlation between sediment changes and a marked period of global cooling that occurred in the Northern Hemisphere some 3.6 million years ago. The changes in sediments imply that profound changes in the circulation of deep water currents occurred at this time.

This crucial piece of work, which showed sediments changed in multiple sites east of the mid-Atlantic ridge but not west of that important geographical feature, opens multiple doors to future research aimed at better understanding the link between deep water currents, Atlantic Ocean heat and salt distribution and ice-sheet expansion, and climatic change.

The new work, just in Nature Communications, was led by Dr. Matthias Sinnesael from Trinity College Dublin's School of Natural Sciences and Dr. Boris Karatsolis from Vrije Universiteit Brussel.

"In recent decades, humanity has increasingly felt the impacts of global warming," said Dr. Sinnesael. "From rising sea levels that endanger coastal cities to heat waves and floods, the world is currently living within the 'storm' of extreme weather events. Collectively, the short-term daily-to-monthly changes in weather that persist over long periods, eventually become part of the climate, which constitutes the long-term average state of these weather conditions.

"At the same time, changes in climate can act on timescales larger than humanity itself, influenced by complex interactions between various processes like plate tectonics, greenhouse gases, biotic evolution, and ocean circulation patterns."

Climate researchers around the world aim to understand these long-term processes and the effect they have on climate, as well as to decouple them from the ones humans are inducing. To do that, they target major climatic systems (e.g., icesheets, river and ocean basins) and look for clues of their evolution in past periods of Earth's history.

One of the most crucial systems for Earth's climate is the ocean "conveyor belt"—a set of currents that act together to redistribute heat on the global ocean. The Gulf Stream is the upper limb of this belt, known for bringing warm waters from the tropics to higher latitudes, resulting in the relatively mild climate that western Europe is experiencing today.

The lower limb of the "conveyor belt" acts in the deeper ocean and consists of three main systems of southward flowing currents: the Iceland Scotland Overflow Water (ISOW), the Denmark Strait Overflow water (DSOW) and the Labrador Sea Water (LSW). Together, they form the return flow known as the North Atlantic deep water (NADW).

Dr. Karatsolis said, "Concerns have been growing that the conveyor belt is slowing down due to ocean warming and ice melting, with serious implications for all life on Earth and in the oceans."

"To be able to predict how—and why—these changes may occur, we first need to understand what happened when things shifted in the deep past. Our chief aim with this work was to reconstruct the past activity of the 'conveyor belt' during a period of Earth's history when temperatures and CO₂ concentrations were higher than today but similar to the ones projected for the next hundreds of years."

Drs. Sinnesael and Karatsolis led this study as part of an international, multidisciplinary project, implemented by the International Ocean Discovery Program. The project, with the code name "IODP Expedition 395/395C," involved two seagoing research expeditions in the North Atlantic (summer of 2021 and 2023) and focused on recovering and investigating deep sea sediments.

These sediments are nowadays transported in the bottom of the ocean by strong deep-sea currents related to the lower limb of the "conveyor belt" (ISOW and DSOW) and therefore hold information about the activity of the NADW in hundred-to-million-year timescales.

Although this area had been previously drilled, "IODP Expeditions 395C/395" managed to go deeper and therefore investigate the sediment deposition during a warmer-than-present period of Earth's history that occurred roughly 5 to 2.8 million years ago.

Key results

After analyzing the composition and the physical properties of the sediments, the scientists noticed a remarkable change in the type of sediments they were retrieving: a pale-looking carbonate mud very sharply transitioned into a dark gray pile of fine silt and clay particles.

Interestingly, they kept finding the same change in multiple sites located to the east of mid-Atlantic ridge, all related to the ISOW deep current system. In contrast, the sites west of the mid-Atlantic ridge do not show much change, looking rather the same throughout the studied interval.

Dr. Sinnesael added, "After detailed further investigations, it turns out that the changes seen in sediments east of the mid-Atlantic ridge all happened around the same time—about 3.6 million years ago. The timing of this change is intriguing as it coincides with a period of strong cooling and the development of large ice bodies in the Northern Hemisphere."

"We must be careful not to infer a cause-and-effect relationship before we understand the system more completely, but the most intuitive way to interpret the change in the type of sediment is to assume it reflects a fundamental shift in ocean circulation in the North Atlantic Ocean, which is likely related to strong formation of deep-water currents—much like we know them today—in the eastern part of the Atlantic Ocean."

"Further research will refine our understanding of the link between deep ocean circulation and the development of contemporaneous ice sheets, and help us predict what is likely to come in the future."