The Holocene epoch, the current interglacial period spanning the last ~11,500 years, is characterized by a gradual decline in Earth's axial tilt, increasing precession index, waning continental ice sheets, and fluctuating greenhouse gas concentrations. Millennial-scale variability is evident in high-latitude surface ocean conditions, including wind strengths, sea-ice extent, and freshwater flux, which can influence meridional overturning circulations in the North Atlantic and Southern Ocean. These circulations play a critical role in deep-ocean ventilation and air-sea carbon cycling. Atmospheric *p*CO₂ showed a slight decrease in the early Holocene (~11-6 ka) followed by a ~20 ppmv increase from ~6 ka to the pre-industrial period. Understanding the interplay between these climate forcings, ocean circulation, and atmospheric CO₂ during this warm period is crucial. Prior studies have attempted to reconstruct Holocene overturning using various proxies, but these reconstructions show divergent trends, partly due to the complex dynamics of the Atlantic Meridional Overturning Circulation (AMOC) and a lack of suitable archives for the Southern Ocean. The relationship between atmospheric *p*CO₂ and polar ocean overturning during the Holocene remains unclear, prompting the need for this study to address this gap in our understanding of past climate change and its carbon cycle implications.

Previous research has utilized multiple approaches to infer the history of Holocene overturning in the North Atlantic, including reconstructions of bottom water flow speed, deep-water transport flux, and mid-to-high-latitude salinity/temperature anomalies. However, these reconstructions often present contrasting trends. In the Southern Ocean, detailed overturning and ventilation reconstructions have been limited by the lack of suitable sedimentary archives, and there's ongoing debate regarding the position of the Southern Hemisphere westerlies. The existing literature highlights the need for a more comprehensive and robust dataset to resolve the relationship between atmospheric *p*CO₂ and polar ocean overturning during the Holocene. This study aims to address this critical knowledge gap by utilizing a novel approach based on deep-sea coral radiocarbon records.

This study employed radiocarbon analysis of deep-sea corals as a proxy for ocean ventilation and overturning. Precise and accurate calendar ages are crucial for reliable 14C evolution reconstruction. The aragonite skeletons of deep-sea corals record ambient seawater 14C during growth, and their ages were determined using the uranium (U)-series disequilibrium method. Samples were collected from various sites in the Drake Passage (Burdwood Bank, Cape Horn, Sars Seamount, Shackleton Fracture Zone) and the Reykjanes Ridge, south of Iceland, at depths ranging from 0.3 to 1.9 km. The U-series dating of deep-sea corals involved isotope spiking, column chemistry, and mass spectrometry to determine precise ages. Radiocarbon analysis involved sample preparation, graphitization, and measurement using accelerator mass spectrometry (AMS). To interpret radiocarbon data in terms of overturning changes, the researchers accounted for the influence of atmospheric Δ14C changes and the atmospheric *p*CO₂ effect on air-sea carbon isotope exchange efficiency. Data was projected to the Marine20 calibration curve to estimate deep-water ventilation age. This approach incorporates the effects of changes in ocean mixing and air-sea gas exchange, providing a more accurate representation of deep-water ventilation age.

Analysis of the reconstructed Δ14C values across various sites showed a gradual decrease over the past 10,000 years, generally following the trend of atmospheric radiocarbon. Distinctive Δ14C values were observed between sites, reflecting differences in ventilation. Interestingly, shallower depths at Sars Seamount exhibited enriched 14C signatures around 9.6 ka. Projecting the 14C data to the Marine20 curve revealed surprisingly stable ventilation ages across the sites studied, with only minor variations within the analytical uncertainties. For instance, the ventilation age of the deepest samples from Burdwood Bank showed less than 10% variability. This near-constant ventilation gradient between the North Atlantic and Southern Ocean, and the lack of long-term trends at each site (except Sars Seamount), strongly suggests that Southern Ocean overturning remained stable. The limited variability observed in neodymium isotope data further supports the conclusion of stable overturning. Two radiogenic isotope signatures in Sars Seamount corals may be attributable to zonal mixing of Pacific waters. The study notes that while short-term AMOC slowdowns following major meltwater pulses cannot be entirely ruled out, the findings support the concept of a stable, strong AMOC mode once atmospheric CO2 levels reach pre-industrial levels.

The remarkable stability of millennial-scale polar ocean overturning during the Holocene suggests that changes in ocean circulation did not primarily drive the long-term *p*CO₂ evolution. Specifically, during the major phase of rising atmospheric *p*CO₂, no significant ventilation changes were observed, indicating that overturning in the North Atlantic and Southern Ocean did not cause oceanic carbon release. The study points towards alternative mechanisms, such as changes in carbon and nutrient distribution within the water column and changes in land organic carbon stock. The observed decrease in deep oceanic [CO2] content during the early Holocene points to ocean alkalinity removal and carbon release to the atmosphere, while nitrogen isotope data suggests a gradually decreased nutrient utilization rate and increased nutrient supply towards the late Holocene. These changes in nutrient utilization and carbon distribution, likely driven by deglaciation and obliquity-driven changes, rather than changes in large-scale ocean circulation, might have been responsible for regulating the atmospheric CO2 budget throughout the Holocene.

This study provides strong evidence for the remarkable stability of large-scale polar ocean overturning during the Holocene, as determined by radiocarbon data from deep-sea corals. This stability suggests that the observed changes in atmospheric CO2 concentrations were not primarily driven by changes in ocean circulation, highlighting the importance of other mechanisms like nutrient redistribution and terrestrial carbon changes. Further research could investigate these alternative mechanisms in more detail and explore the potential impacts of short-term ocean circulation changes on the carbon cycle.

The study acknowledges potential limitations. Short-term, high-magnitude events like the 8.2 ka event and other meltwater pulses might have caused short-term AMOC slowdowns that are not resolved by the study's sampling resolution. The interpretation relies on the accuracy of the Marine20 surface 14C curve, which assumes no Holocene circulation changes. However, the consistency of findings across multiple sites strengthens the overall conclusion.