Radiocarbon evidence for the stability of polar ocean overturning during the Holocene

The Holocene (~11.5 ka to present) was characterized by declining axial tilt, rising precession index, retreating ice sheets, and changing greenhouse gases. High-latitude surface ocean conditions showed millennial-scale variability (winds, sea ice, freshwater fluxes), which could influence meridional overturning in the North Atlantic and Southern Ocean, thereby affecting deep-ocean ventilation and air–sea carbon cycling. Atmospheric pCO₂ decreased by ~5 ppm from ~11–6 ka and then rose ~20 ppm from ~6 ka to pre-industrial. However, reconstructions of Holocene overturning—especially in polar regions—are inconsistent or sparse, leaving unclear whether changes in polar overturning drove the Holocene CO₂ evolution. This study tests the hypothesis that polar ocean overturning remained stable during the Holocene by generating absolutely dated radiocarbon records from deep-sea corals in the Drake Passage and the North Atlantic.

Previous North Atlantic Holocene overturning reconstructions used proxies for bottom water flow speed, deep-water transport flux, and mid- to high-latitude salinity/temperature anomalies, but trends diverge, likely reflecting multiple AMOC regimes and pathways. In the Southern Ocean, detailed Holocene overturning and ventilation reconstructions have been hampered by a lack of suitable sediment archives and uncertainties in westerly wind positions, complicating assessments of overturning–CO₂ linkages. Radiocarbon is a widely used proxy for ventilation and overturning because it is set at the surface by air–sea exchange and transported to depth during deep-water formation, with decay providing a time-integrated measure of aging and ventilation. Atmospheric radiocarbon histories (IntCal20) and marine calibration curves (Marine20) provide benchmarks for interpreting ocean Δ14C and ventilation ages. Prior coral radiocarbon and neodymium isotope studies in the North Atlantic (for example, Rockall Trough) and limited Drake Passage corals informed deglacial to Holocene changes but lacked the spatial–temporal coverage to resolve long-term polar overturning stability.

- Sampling: Deep-sea coral fossils were dredged from multiple Drake Passage sites (Burdwood Bank, Cape Horn, Sars Seamount, Shackleton Fracture Zone; depths ~0.3–1.9 km) during RVIB Nathaniel B. Palmer cruises 0805 (2008) and 1103 (2011), and from the Reykjanes Ridge south of Iceland (~1.29–1.43 km) during Celtic Explorer cruise CE0806 (2008). Species include mostly solitary Desmophyllum with some Flabellum, Balanophyllia, Caryophyllia (Drake Passage), and framework corals Lophelia pertusa and Madrepora oculata (North Atlantic). Sixty-one new Drake Passage Holocene 14C data points are presented in addition to previously reported Holocene samples. - Context of water masses: Drake Passage upper waters (<2 km) are dominated by Upper Circumpolar Deep Water (UCDW; low O₂ ~<180 µmol kg⁻¹) formed from mixed Pacific/Indian/Atlantic deep waters; overlying are Antarctic Intermediate Water and mode waters. North Atlantic mid-depth sites lie within the subpolar gyre, ventilated primarily by Labrador Sea convection with potential subtropical intermediate water influences. - U-series dating: Approximately 0.2 g aragonite per coral was cleaned, spiked (236U–229Th), U and Th purified via anion-exchange after Fe(OH)₃ co-precipitation, and isotopes measured on a Neptune MC-ICP-MS using sample–standard bracketing. Decay constants followed Cheng et al. Initial 230Th/232Th was assumed (Drake Passage: 37 ± 37; North Atlantic: 14.8 ± 14.8; 2σ). Age uncertainties were estimated via Monte Carlo propagation of analytical and initial 230Th uncertainties. - Radiocarbon: ~10 mg leached carbonate was graphitized and measured on a MICADAS AMS (University of Bristol). Blank and 8¹⁴C corrections applied; fossil corals >100 ka served as blanks (yielding apparent 46–50 ka). Inter-lab consistency was checked against UC Irvine AMS repeats. - Calculations: Coral Δ14C was computed as Δ14C_coral = (F14C × e^(calendar age/8,267) − 1) × 1000. To isolate ventilation, coral records were projected to the Marine20 surface ocean 14C curve to obtain projection ages (t_proj or f_proj), accounting for surface ocean smoothing of atmospheric signals (Marine20 generated with the BICYCLE box model forced by IntCal20 and ice-core pCO₂). Errors from U-series ages, 14C, and Marine20 uncertainties were propagated (±2σ) and shown as error ellipses. Site-to-site t_proj offsets relative to Reykjanes Ridge were also evaluated to assess spatial ventilation gradients. - Data handling: Samples were not grouped by modern water mass labels to avoid misattribution to past conditions. Individual coral data are considered decadal to centennial averages given coral growth rates and sampling integration. Sample metadata and code for Δ14C and t_proj calculations are publicly available.

- Deep-ocean Δ14C from Drake Passage and Reykjanes Ridge decreases gradually over ~10 ka broadly following atmospheric IntCal20, with distinct site-dependent values reflecting water mass structure; Reykjanes Ridge samples closely track atmospheric trends. - Ventilation stability: Projection ages (f_proj/t_proj) show no significant long-term trends at individual sites through the Holocene within propagated uncertainties, except for a transient 14C-enriched anomaly at Sars Seamount around ~9.6 ka (reproduced in two samples), likely due to local/zonal mixing rather than basin-scale overturning changes. - Quantitative constraints: Mean Holocene f_proj values include (±2σ): Burdwood Bank deepest UCDW 870 ± 67 years (n = 10); and site offsets relative to Reykjanes Ridge are small and nearly constant: Cape Horn 450 m: 179 ± 98 years; Burdwood Bank 816 m: 355 ± 146 years; Cape Horn 1,012 m: 532 ± 123 years; Burdwood Bank 1,879 m: 890 ± 102 years. Variabilities are comparable to analytical uncertainties, implying stable ventilation gradients between the North Atlantic and Southern Ocean. - Consistency with other proxies: Coral Nd isotope data from Drake Passage show limited variability (aside from two mid-Holocene radiogenic anomalies at Sars Seamount), supporting stable large-scale overturning. The study cannot exclude short-lived AMOC slowdowns tied to meltwater events (e.g., 8.2 ka), but finds no evidence for prolonged or large-scale Holocene perturbations. - Implication for carbon cycle: During the main rise in atmospheric pCO₂ (7–2 ka), deep records show no ventilation changes, indicating overturning did not drive millennial-scale Holocene CO₂ variations. Other records indicate early Holocene decreases in deep [CO₂] and evolving nutrient utilization, consistent with internal nutrient/carbon redistribution rather than overturning changes.

The radiocarbon evidence from absolutely dated deep-sea corals in both hemispheric polar regions demonstrates that millennial-scale polar overturning circulation (UCDW in the Southern Ocean and NADW-influenced mid-depth North Atlantic) remained stable through the Holocene within proxy resolution. Stable f_proj at each site and nearly constant inter-basin ventilation gradients argue against a sustained, large-scale change in overturning being responsible for the ~20 ppm rise in atmospheric pCO₂ after ~6 ka. Short-lived events, such as the 8.2 ka perturbation, may have occurred but are not captured as prolonged ventilation shifts. The decoupling between overturning and biogeochemical proxies suggests that Holocene CO₂ evolution likely reflects processes such as post-deglacial redistribution of nutrients and carbon within the ocean (for example, changes in denitrification locations and oxygen minimum zones affecting nitrate supply and δ15N), continuous upwelling/advection pathways, and changes in terrestrial organic carbon stocks. The findings align with conceptual frameworks proposing that AMOC stabilizes in a strong mode once atmospheric CO₂ approaches pre-industrial levels, independent of residual ice-sheet volume changes.

Deep-sea coral radiocarbon records from the Drake Passage and Reykjanes Ridge reveal stable polar ocean ventilation throughout the Holocene, with invariant site-specific projection ages and consistent ventilation gradients between the North Atlantic and Southern Ocean. Therefore, long-term, large-scale changes in polar overturning did not drive the millennial-scale Holocene atmospheric pCO₂ evolution. Instead, internal ocean biogeochemical adjustments and terrestrial carbon stock changes likely modulated the CO₂ budget. Future work should target higher-resolution coral archives to test for short-lived overturning perturbations, expand spatial coverage (including Antarctic Bottom Water pathways), and integrate radiocarbon with nutrient and carbon system proxies to quantify the roles of nutrient redistribution, denitrification, and land-use/biomass changes.

- Temporal resolution: Deep-sea corals integrate over decades and radiocarbon responds on centennial–millennial timescales, limiting detection of brief overturning perturbations; short-term AMOC slowdowns after meltwater pulses (e.g., 8.2 ka) cannot be excluded. - Spatial/depth coverage: Coral sites sample upper to mid-depth waters; Antarctic Bottom Water was not directly recorded, potentially missing variability in the lower overturning cell. - Calibration/model dependence: Projection ages rely on Marine20, which is generated without prescribed Holocene circulation changes; although conclusions do not hinge on Marine20 accuracy (given stable inter-site offsets), model assumptions remain a caveat. - Proxy scope: The study focuses on ventilation proxies and does not directly constrain biogeochemical processes responsible for CO₂ changes; interpretations of nutrient redistribution are based on external proxies and literature. - Local anomalies: Sars Seamount mid-Holocene 14C enrichments likely reflect local/zonal mixing, highlighting potential site-specific effects.