In a ground-breaking study published today in Nature Communications, an international team of researchers has established a compelling link between shifts in deep ocean currents and the cooling of the Northern Hemisphere that occurred approximately 3.6 million years ago. By analysing the composition of ancient ocean sediments, the team uncovered significant changes in deep water circulation patterns during this pivotal period in Earth's history.

Above: The scientific research vessel Joides Resolution. Image credit: T. Fulton / IODP / TAMU.

This crucial research, spearheaded by Dr. Matthias Sinnesael from Trinity College Dublin and Dr. Boris Karatsolis from Vrije Universiteit Brussel, revealed distinct alterations in sediment composition at multiple locations east of the Mid-Atlantic Ridge, while sites to the west remained largely unchanged. These findings unlock new avenues for future research aimed at unravelling the intricate relationships between deep water currents, the distribution of heat and salt in the Atlantic Ocean, the expansion of ice sheets, and broader climatic shifts.

Above: The research team discussing preliminary results on the ship at the core table. Image credit: J Field.

Dr. Sinnesael emphasised the relevance of this work in the context of current climate challenges: "Humanity is increasingly experiencing the consequences of global warming, from rising sea levels to extreme weather events. While we grapple with short-term weather variations that shape our climate over time, long-term climate changes, influenced by factors like plate tectonics and ocean circulation, operate on timescales far exceeding human lifespans."

He further explained that climate researchers worldwide are dedicated to understanding these long-term processes to differentiate them from human-induced changes. Their approach involves examining major climatic systems, such as ice sheets and ocean basins, for historical clues.

The ocean's "conveyor belt," a network of currents redistributing heat globally, plays a vital role in Earth's climate. The Gulf Stream, its upper component, is renowned for transporting warm tropical waters to higher latitudes, contributing to Western Europe's mild climate. The deeper limb of this system comprises three key southward-flowing currents: the Iceland Scotland Overflow Water (ISOW), the Denmark Strait Overflow Water (DSOW), and the Labrador Sea Water (LSW), which collectively form the North Atlantic Deep Water (NADW) return flow.

Dr. Karatsolis highlighted growing concerns about a potential slowdown of this "conveyor belt" due to ocean warming and ice melt, with potentially severe global consequences. "To predict future changes, we must first understand past shifts," he stated. "Our primary goal was to reconstruct the activity of this 'conveyor belt' during a period when Earth's temperatures and CO2 concentrations were higher than today but comparable to future projections."

The research was conducted as part of the International Ocean Discovery Program (IODP) Expedition 395/395C, involving two research expeditions in the North Atlantic in 2021 and 2023 aboard the scientific research vessel Joides Resolution. These expeditions focused on recovering and analysing deep-sea sediments transported by strong deep-sea currents linked to the lower limb of the "conveyor belt," providing a record of NADW activity over hundreds of thousands to millions of years.

While the region had been previously studied, IODP Expeditions 395/395C achieved greater depths, allowing investigation of sediment deposition during a warmer period in Earth's history, approximately 5 to 2.8 million years ago.

Key Findings:

Analysis of the sediment composition and physical properties revealed a striking transition from pale carbonate mud to dark grey fine silt and clay particles. This change was consistently observed in multiple sites east of the Mid-Atlantic Ridge, all associated with the ISOW deep current system. Conversely, sites west of the ridge showed little to no change throughout the studied time interval.

Dr. Sinnesael noted the significance of the timing of this sedimentary shift: "Our detailed investigations revealed that these changes east of the Mid-Atlantic Ridge occurred around 3.6 million years ago. This timing is particularly interesting because it coincides with a period of significant cooling and the expansion of large ice sheets in the Northern Hemisphere."

He cautioned against immediately concluding a cause-and-effect relationship but suggested that the change in sediment type likely reflects a fundamental alteration in North Atlantic Ocean circulation, potentially linked to the strong formation of deep-water currents, similar to the present-day system in the eastern Atlantic.

Further research will delve deeper into the connection between deep ocean circulation and the development of contemporaneous ice sheets, offering valuable insights into potential future climate scenarios.

The full research article is available on the Nature Communications website.