A groundbreaking international study investigating ancient sediment cores from the North Atlantic has unveiled compelling evidence linking sedimentary shifts to a significant period of global cooling in the Northern Hemisphere approximately 3.6 million years ago. This discovery provides novel insights into the profound changes in deep ocean circulation during a pivotal phase of Earth’s climatic history. The research, led by Dr. Matthias Sinnesael of Trinity College Dublin and Dr. Boris Karatsolis of Vrije Universiteit Brussel, elucidates the dynamic relationship between sediment composition and the activity of deep-water currents, enhancing our understanding of long-term climate variability.
The research team embarked on an ambitious campaign under the International Ocean Discovery Program (IODP), specifically through Expedition 395 and its extension, 395C. These two research voyages, conducted in the summers of 2021 and 2023, focused on meticulously retrieving deep-sea sediment cores from strategically selected sites east and west of the mid-Atlantic Ridge. Notably, the mid-Atlantic Ridge—a significant underwater mountain range—served as a critical geographical boundary demarcating distinct oceanographic regimes characterized by contrasting sediment deposition patterns.
Analyzing these sediment cores through detailed geochemical and sedimentological methods, the researchers identified a stark and synchronous transition at sites east of the mid-Atlantic Ridge. The sediment shifted abruptly from pale carbonate-rich muds into darker, fine-grained silts and clays. This transformation, absent in western sites, implicates a dramatic reorganization of ocean currents, specifically those linked to the Iceland-Scotland Overflow Water (ISOW) — a principal component of the North Atlantic Deep Water (NADW) system responsible for conveying dense water masses to the deep ocean.
The findings suggest that around 3.6 million years ago, the activity of the ISOW intensified markedly, coinciding with global cooling and the expansion of large ice sheets across the Northern Hemisphere. This timing aligns with well-documented climatic transitions during the late Pliocene, a period characterized by decreasing atmospheric CO₂ concentrations and the onset of Northern Hemisphere glaciation. The deep-water current enhancements documented through sediment proxies reflect a reconfiguration of the ocean’s global “conveyor belt,” a thermohaline circulation system indispensable for heat and salinity redistribution.
Understanding changes in the Atlantic meridional overturning circulation (AMOC), represented here by the NADW system, remains paramount due to its vital role in moderating regional and global climates. The research underscores that deep ocean currents like ISOW, Denmark Strait Overflow Water (DSOW), and Labrador Sea Water (LSW) generate a return flow that helps regulate surface climate, especially in Europe and North America. Enhanced activity of these currents in the past likely contributed to altering heat transport dynamics, which may have cascaded to influence atmospheric patterns and ice-sheet stability.
Dr. Sinnesael remarked on the significance of these results, emphasizing the intricate feedbacks between deep ocean circulation and climate evolution. He cautioned, however, against prematurely attributing definitive cause-and-effect relationships without further study, advocating for ongoing research into complex ocean-atmosphere-ice interactions. These insights also offer crucial analogues for near-future climate scenarios, as current anthropogenic warming threatens to disrupt the “conveyor belt” through warming ocean waters and accelerated ice melt.
The sophisticated sediment analysis combined physical properties with sediment composition to reconstruct paleoceanographic conditions with unprecedented precision. This approach allowed the researchers to discern not only the timing of the shifts but also infer changes in water mass properties and flow intensities. The sediments act as time capsules, transmitting the imprints of ocean current strength, sediment transport mechanics, and chemical signatures reflective of water mass sources and pathways.
Moreover, the data suggest a geographically constrained impact of changing circulation, with eastern sites displaying definitive sediment transitions while western sites remained relatively unchanged. This spatial heterogeneity indicates that transformation in the deep water formation and flow was preferentially localized or at least most pronounced east of the mid-Atlantic Ridge. Such regional differences highlight the need to consider bathymetric and oceanographic complexities in modeling past and future changes in thermohaline circulation.
Dr. Karatsolis pointed out the broader implications of this research for understanding the Earth system’s response to elevated CO₂ levels and warmer climates. The late Pliocene period, when these changes occurred, is often cited as a valuable analogue for possible future climate trajectories given similar atmospheric greenhouse gas concentrations projected for the coming centuries. By examining when and how the ocean “conveyor belt” evolved naturally during warmer intervals, scientists can refine predictive models of ocean circulation dynamics under anthropogenic pressures.
The breakthrough also demonstrates the power of multidisciplinary collaboration, integrating marine geology, geochemistry, and climate science across international institutions. Utilizing advanced drilling technology aboard research vessels, the team could access sediment records dating back millions of years, enabling a retrospective lens on deep ocean currents and their roles in shaping Earth’s climate system. This synergy between fieldwork and laboratory analysis catalyzes new frontiers in paleoceanography and climatology.
Looking ahead, the authors advocate for further high-resolution sediment recovery campaigns complemented by coupled ocean-atmosphere modeling to unravel the mechanisms driving these ancient oceanographic changes. Such research endeavors will be instrumental in elucidating how shifts in deep water circulation interplay with ice sheet dynamics and atmospheric conditions, ultimately informing projections of future climate patterns and extremes.
This study marks a pivotal step in deepening our comprehension of fundamental Earth processes, bridging the gap between past climatic upheavals and contemporary environmental challenges. By decoding the sedimentary archives, scientists are piecing together a more detailed narrative of how the ocean’s hidden currents have historically moderated and responded to climatic shifts—knowledge that is crucial as humanity navigates an uncertain climatic future.
Subject of Research: Paleoceanography and deep water circulation changes related to the North Atlantic Deep Water system during the late Pliocene epoch (circa 3.6 million years ago).
Article Title: Onset of strong Iceland-Scotland overflow water 3.6 million years ago
Web References:
10.1038/s41467-025-59265-5
Image Credits: Credit: J Field
Keywords: North Atlantic Deep Water, Iceland-Scotland Overflow Water, sediment cores, deep ocean circulation, late Pliocene, global cooling, thermohaline circulation, ocean conveyor belt, paleoclimate, ice-sheet expansion, paleoceanography, climate change
