Radiocarbon constraints on the extent and evolution of the South Pacific glacial carbon pool

Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

Five million years of Antarctic Circumpolar Current strength variability

The Antarctic Circumpolar Current (ACC) represents the world's largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability1-3. Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity4. Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial-interglacial cycles5-8, the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling9 and increasing global ice volume10. Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings11-13. We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability14. A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO2 during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming.

Southern Ocean contribution to both steps in deglacial atmospheric CO2 rise

The transfer of vast amounts of carbon from a deep oceanic reservoir to the atmosphere is considered to be a dominant driver of the deglacial rise in atmospheric CO₂. Paleoceanographic reconstructions reveal evidence for the existence of CO₂-rich waters in the mid to deep Southern Ocean. These water masses ventilate to the atmosphere south of the Polar Front, releasing CO₂ prior to the formation and subduction of intermediate-waters. Changes in the amount of CO₂ in the sea water directly affect the oceanic carbon chemistry system. Here we present B/Ca ratios, a proxy for delta carbonate ion concentrations Δ[CO₃^2-], and stable isotopes (δ¹³C) from benthic foraminifera from a sediment core bathed in Antarctic Intermediate Water (AAIW), offshore New Zealand in the Southwest Pacific. We find two transient intervals of rising [CO₃^2-] and δ¹³C that that are consistent with the release of CO₂ via the Southern Ocean. These intervals coincide with the two pulses in rising atmospheric CO₂ at ~ 17.5-14.3 ka and 12.9-11.1 ka. Our results lend support for the release of sequestered CO₂ from the deep ocean to surface and atmospheric reservoirs during the last deglaciation, although further work is required to pin down the detailed carbon transfer pathways.

Radiocarbon: A key tracer for studying Earth's dynamo, climate system, carbon cycle, and Sun

[Figure: see text].

Deglacial patterns of South Pacific overturning inferred from 231Pa and 230Th

The millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is well documented for the last glacial termination and beyond. Despite its importance for the climate system, the evolution of the South Pacific overturning circulation (SPOC) is by far less well understood. A recently published study highlights the potential applicability of the ²³¹Pa/²³⁰Th-proxy in the Pacific. Here, we present five sedimentary down-core profiles of ²³¹Pa/²³⁰Th-ratios measured on a depth transect from the Pacific sector of the Southern Ocean to test this hypothesis using downcore records. Our data are consistent with an increase in SPOC as early as 20 ka that peaked during Heinrich Stadial 1. The timing indicates that the SPOC did not simply react to AMOC changes via the bipolar seesaw but were triggered via Southern Hemisphere processes.

Equatorial Pacific seawater pCO2 variability since the last glacial period

The ocean may have played a central role in the atmospheric pCO2 rise during the last deglaciation. However, evidence on where carbon was exchanged between the ocean and the atmosphere in this period is still lacking, hampering our understanding of global carbon cycle on glacial-interglacial timescales. Here we report a new surface seawater pCO2 reconstruction for the western equatorial Pacific Ocean based on boron isotope analysis-a seawater pCO2 proxy-using two species of near-surface dwelling foraminifera from the same marine sediment core. The results indicate that the region remained a modest CO2 sink throughout the last deglaciation.

Enhanced North Pacific deep-ocean stratification by stronger intermediate water formation during Heinrich Stadial 1

The deglacial history of CO₂ release from the deep North Pacific remains unresolved. This is due to conflicting indications about subarctic Pacific ventilation changes based on various marine proxies, especially for Heinrich Stadial 1 (HS-1) when a rapid atmospheric CO₂ rise occurs. Here, we use a complex Earth System Model to investigate the deglacial North Pacific overturning and its control on ocean stratification. Our results show an enhanced intermediate-to-deep ocean stratification coeval with intensified North Pacific Intermediate Water (NPIW) formation during HS-1, compared to the Last Glacial Maximum. The stronger NPIW formation causes lower salinities and higher temperatures at intermediate depths. By lowering NPIW densities, this enlarges vertical density gradient and thus enhances intermediate-to-deep ocean stratification during HS-1. Physically, this process prevents the North Pacific deep waters from a better communication with the upper oceans, thus prolongs the existing isolation of glacial Pacific abyssal carbons during HS-1.

Neodymium isotope evidence for glacial-interglacial variability of deepwater transit time in the Pacific Ocean

There is evidence for greater carbon storage in the glacial deep Pacific, but it is uncertain whether it was caused by changes in ventilation, circulation, and biological productivity. The spatial ε_Nd evolution in the deep Pacific provides information on the deepwater transit time. Seven new foraminiferal ε_Nd records are presented to systematically constrain glacial to interglacial changes in deep Pacific overturning and two different ε_Nd evolution regimes occur spatially in the Pacific with reduced meridional ε_Nd gradients in glacials, suggesting a faster deep Pacific overturning circulation. This implies that greater glacial carbon storage due to sluggish circulation, that is believed to have occurred in the deep Atlantic, did not operate in a similar manner in the Pacific Ocean. Other mechanisms such as increased biological pump efficiency and poor high latitude air-sea exchange could be responsible for increased carbon storage in the glacial Pacific.

The response of deep Pacific overturning to glacial-interglacial climate change is still debated. Here the authors show a generally faster deep Pacific overturning operated in recent glacial periods based on a novel application of Nd isotopes recorded in foraminifera.

Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO2 rise

The early part of the last deglaciation is characterised by a ~40 ppm atmospheric CO₂ rise occurring in two abrupt phases. The underlying mechanisms driving these increases remain a subject of intense debate. Here, we successfully reproduce changes in CO₂, δ¹³C and Δ¹⁴C as recorded by paleo-records during Heinrich stadial 1 (HS1). We show that HS1 CO₂ increase can be explained by enhanced Southern Ocean upwelling of carbon-rich Pacific deep and intermediate waters, resulting from intensified Southern Ocean convection and Southern Hemisphere (SH) westerlies. While enhanced Antarctic Bottom Water formation leads to a millennial CO₂ outgassing, intensified SH westerlies induce a multi-decadal atmospheric CO₂ rise. A strengthening of SH westerlies in a global eddy-permitting ocean model further supports a multi-decadal CO₂ outgassing from the Southern Ocean. Our results highlight the crucial role of SH westerlies in the global climate and carbon cycle system with important implications for future climate projections.

Despite decades of research, the sequence of events leading to the deglacial atmospheric CO₂ rise remains unclear. Menviel et al. show that Southern Ocean convection driven by intensified Southern Hemisphere westerlies during Heinrich stadial 1 can explain the abrupt pCO₂ rise and changes in atmosphere and ocean carbon isotopes.

Breakup of last glacial deep stratification in the South Pacific

Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO₂ rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO₂ through upwelling.

Repeated storage of respired carbon in the equatorial Pacific Ocean over the last three glacial cycles

As the largest reservoir of carbon exchanging with the atmosphere on glacial–interglacial timescales, the deep ocean has been implicated as the likely location of carbon sequestration during Pleistocene glaciations. Despite strong theoretical underpinning for this expectation, radiocarbon data on watermass ventilation ages conflict, and proxy interpretations disagree about the depth, origin and even existence of the respired carbon pool. Because any change in the storage of respiratory carbon is accompanied by corresponding changes in dissolved oxygen concentrations, proxy data reflecting oxygenation are valuable in addressing these apparent inconsistencies. Here, we present a record of redox-sensitive uranium from the central equatorial Pacific Ocean to identify intervals associated with respiratory carbon storage over the past 350 kyr, providing evidence for repeated carbon storage over the last three glacial cycles. We also synthesise our data with previous work and propose an internally consistent picture of glacial carbon storage and equatorial Pacific Ocean watermass structure.

During glacial periods the oceans stored carbon removed from the atmosphere, yet identifying precisely where that storage occurred remains challenging. Here, the authors show that the deep equatorial Pacific Ocean was a reservoir for respired carbon during glacial periods for at least the last 350 kyr.

Radiocarbon constraints on the glacial ocean circulation and its impact on atmospheric CO2

While the ocean’s large-scale overturning circulation is thought to have been significantly different under the climatic conditions of the Last Glacial Maximum (LGM), the exact nature of the glacial circulation and its implications for global carbon cycling continue to be debated. Here we use a global array of ocean–atmosphere radiocarbon disequilibrium estimates to demonstrate a ∼689±53 ¹⁴C-yr increase in the average residence time of carbon in the deep ocean at the LGM. A predominantly southern-sourced abyssal overturning limb that was more isolated from its shallower northern counterparts is interpreted to have extended from the Southern Ocean, producing a widespread radiocarbon age maximum at mid-depths and depriving the deep ocean of a fast escape route for accumulating respired carbon. While the exact magnitude of the resulting carbon cycle impacts remains to be confirmed, the radiocarbon data suggest an increase in the efficiency of the biological carbon pump that could have accounted for as much as half of the glacial–interglacial CO₂ change.

Establishing the efficiency of the biological carbon pump is needed to constrain the impact of ocean circulation on the carbon cycle. Here, the authors compile a global array of ocean–atmosphere radiocarbon disequilibrium estimates and evaluate the strength of the carbon pump over the last glacial maximum.

Synchronous deglacial thermocline and deep-water ventilation in the eastern equatorial Pacific

The deep ocean is most likely the primary source of the radiocarbon-depleted CO₂ released to the atmosphere during the last deglaciation. While there are well-documented millennial scale Δ¹⁴C changes during the most recent deglaciation, most marine records lack the resolution needed to identify more rapid ventilation events. Furthermore, potential age model problems with marine Δ¹⁴C records may obscure our understanding of the phase relationship between inter-ocean ventilation changes. Here we reconstruct changes in deep water and thermocline radiocarbon content over the last deglaciation in the eastern equatorial Pacific (EEP) using benthic and planktonic foraminiferal ¹⁴C. Our records demonstrate that ventilation of EEP thermocline and deep waters occurred synchronously during the last deglaciation. In addition, both gradual and rapid deglacial radiocarbon changes in these Pacific records are coeval with changes in the Atlantic records. This in-phase behaviour suggests that the Southern Ocean overturning was the dominant driver of changes in the Atlantic and Pacific ventilation during deglaciation.

Potential age model problems with marine Δ¹⁴C records have obscured our understanding of the role of the deep-ocean in deglacial atmospheric CO₂ rise. Here, the authors show that deglacial ventilation of EEP thermocline and deep waters occurred synchronously and was coeval with changes in Atlantic records.

Massive remobilization of permafrost carbon during post-glacial warming

Recent hypotheses, based on atmospheric records and models, suggest that permafrost carbon (PF-C) accumulated during the last glaciation may have been an important source for the atmospheric CO₂ rise during post-glacial warming. However, direct physical indications for such PF-C release have so far been absent. Here we use the Laptev Sea (Arctic Ocean) as an archive to investigate PF-C destabilization during the last glacial–interglacial period. Our results show evidence for massive supply of PF-C from Siberian soils as a result of severe active layer deepening in response to the warming. Thawing of PF-C must also have brought about an enhanced organic matter respiration and, thus, these findings suggest that PF-C may indeed have been an important source of CO₂ across the extensive permafrost domain. The results challenge current paradigms on the post-glacial CO₂ rise and, at the same time, serve as a harbinger for possible consequences of the present-day warming of PF-C soils.

Atmospheric CO₂ increases during the last deglaciation have been linked to the destabilisation of permafrost carbon reservoirs. Here, using a sediment core from the Laptev Sea, Tesi et al. indicate a massive supply of permafrost carbon was released from Siberia following active layer deepening.