For decades, one relationship in ocean science has been treated as close to a law of nature: when warm, salty water leaking from the Indian Ocean around the tip of South Africa — the so-called “Agulhas Leakage” — slows down, the Atlantic Meridional Overturning Circulation (AMOC) should slow down with it. New evidence pulled from marine sediment 3.4 million years old shows that assumption doesn’t always hold.
A study published in Nature Geoscience, “Disconnection of the late Pliocene Agulhas Leakage from Atlantic Meridional Overturning Circulation,” led by Dr. Suning Hou of Utrecht University, reports a glacial period in the late Pliocene, around 3.4 million years ago, when the Agulhas Leakage nearly shut off — yet the AMOC intensified rather than weakened. “This was basically the first textbook concept,” Hou said of the assumed link. “It was surprising to find geological evidence showing that it isn’t universally true.”
What the sediment cores showed
The team reconstructed ocean conditions by analyzing organic molecules and other proxies preserved in layered marine sediment recovered from two sites on opposite sides of the Atlantic basin: IODP Site U1475 on the Agulhas Plateau, roughly 500 km south of South Africa, and ODP Site 625 in the northern Gulf of Mexico. Comparing the two records let the researchers track the Agulhas Leakage and the AMOC as separate, independently dated signals through the same glacial interval, rather than inferring one from the other.
That comparison is what exposed the disconnection: the Indian Ocean inflow around southern Africa dropped off sharply, but the overturning circulation in the Atlantic did not follow it down. The finding indicates the AMOC has more than one way of sustaining or even strengthening itself, independent of the Agulhas “leak” that is often described as one of its key ingredients.
Two drilling programs, one shared archive
Neither of the two sediment cores was drilled for this study. Both come from the international scientific ocean drilling programs — the International Ocean Discovery Program (IODP) and its predecessor, the Ocean Drilling Program (ODP) — whose cores are curated in shared repositories and made available for reuse by any qualified research team, long after the original expedition ends. That a Pliocene glacial event on the far side of the Atlantic from the original drill sites could be reconstructed this precisely, decades after the cores came out of the seafloor, is itself a demonstration of why long-lived, openly accessible core archives are treated as core research infrastructure rather than one-off expedition output.
An international collaboration, ERC-funded
The study was carried out by a multi-national team spanning the Netherlands, United States, China, and United Kingdom, with contributing authors including Carolien M. H. van der Weijst, Francien Peterse, Alejandra Cartagena-Sierra, Ning Tan, Malte Stockhausen, Fenghao Liu, Melissa A. Berke, Isla S. Castañeda, Aidan Starr, Francesca Sangiorgi, Anna S. von der Heydt, and Peter K. Bijl. The work was funded through the European Research Council’s OceaNice project, based at Utrecht University.
Why it matters for AMOC stability
The AMOC carries warm surface water north and returns cold, dense water south at depth, and its behavior is central to projections of future climate change, including concerns that a warming world could push the circulation toward a slowdown or collapse. Much of that concern rests on assumed feedback loops, including the Agulhas Leakage connection this study complicates. A circulation system that can apparently decouple from one of its textbook drivers during a real past climate transition is a system that is harder to predict from a single mechanism alone — a caution for climate models that lean heavily on the Agulhas-AMOC link as a stabilizing or destabilizing feedback.
The researchers frame the result as evidence that the AMOC’s past behavior was more varied, and its stability more contingent on multiple interacting factors, than a single textbook relationship can capture.
Source
Hou, S. et al. “Disconnection of the late Pliocene Agulhas Leakage from Atlantic Meridional Overturning Circulation,” Nature Geoscience (2026). Reported by ScienceDaily, “Ancient ocean ‘detour’ reveals a surprising secret about Earth’s climate system” (August 6, 2026), based on research from Utrecht University.
