Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events

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.

Arctic and Antarctic forcing of ocean interior warming during the last deglaciation

Subsurface water masses formed at high latitudes impact the latitudinal distribution of heat in the ocean. Yet uncertainty surrounding the timing of low-latitude warming during the last deglaciation (18-10 ka) means that controls on sub-surface temperature rise remain unclear. Here we present seawater temperature records on a precise common age-scale from East Equatorial Pacific (EEP), Equatorial Atlantic, and Southern Ocean intermediate waters using new Li/Mg records from cold water corals. We find coeval warming in the tropical EEP and Atlantic during Heinrich Stadial 1 (+ 6 °C) that closely resemble warming recorded in Antarctic ice cores, with more modest warming of the Southern Ocean (+ 3 °C). The magnitude and depth of low-latitude ocean warming implies that downward accumulation of heat following Atlantic Meridional Overturning Circulation (AMOC) slowdown played a key role in heating the ocean interior, with heat advection from southern-sourced intermediate waters playing an additional role.

Enhanced subglacial discharge from Antarctica during meltwater pulse 1A

Subglacial discharge from the Antarctic Ice Sheet (AIS) likely played a crucial role in the loss of the ice sheet and the subsequent rise in sea level during the last deglaciation. However, no direct proxy is currently available to document subglacial discharge from the AIS, which leaves significant gaps in our understanding of the complex interactions between subglacial discharge and ice-sheet stability. Here we present deep-sea coral 234U/238U records from the Drake Passage in the Southern Ocean to track subglacial discharge from the AIS. Our findings reveal distinctively higher seawater 234U/238U values from 15,400 to 14,000 years ago, corresponding to the period of the highest iceberg-rafted debris flux and the occurrence of the meltwater pulse 1A event. This correlation suggests a causal link between enhanced subglacial discharge, synchronous retreat of the AIS, and the rapid rise in sea levels. The enhanced subglacial discharge and subsequent AIS retreat appear to have been preconditioned by a stronger and warmer Circumpolar Deep Water, thus underscoring the critical role of oceanic heat in driving major ice-sheet retreat.

Early sea ice decline off East Antarctica at the last glacial-interglacial climate transition

Antarctic climate warming and atmospheric CO2 rise during the last deglaciation may be attributed in part to sea ice reduction in the Southern Ocean. Yet, glacial-interglacial Antarctic sea ice dynamics and underlying mechanisms are poorly constrained, as robust sea ice proxy evidence is sparse. Here, we present a molecular biomarker-based sea ice record that resolves the spring/summer sea ice variability off East Antarctica during the past 40 thousand years (ka). Our results indicate that substantial sea ice reduction culminated rapidly and contemporaneously with upwelling of carbon-enriched waters in the Southern Ocean at the onset of the last deglaciation but began at least ~2 ka earlier probably driven by an increasing local integrated summer insolation. Our findings suggest that sea ice reduction and associated feedbacks facilitated stratification breakup and outgassing of CO2 in the Southern Ocean and warming in Antarctica but may also have played a leading role in initializing these deglacial processes in the Southern Hemisphere.

Pre-aged terrigenous organic carbon biases ocean ventilation-age reconstructions in the North Atlantic

Changes in ocean ventilation have been pivotal in regulating carbon sequestration and release on centennial to millennial timescales. However, paleoceanographic reconstructions documenting changes in deep-ocean ventilation using ¹⁴C dating, may bear multidimensional explanations, obfuscating the roles of ocean ventilation played on climate evolution. Here, we show that previously inferred poorly ventilated conditions in the North Atlantic were linked to enhanced pre-aged organic carbon (OC) input during Heinrich Stadial 1 (HS1). The ¹⁴C age of sedimentary OC was approximately 13,345 ± 692 years older than the coeval foraminifera in the central North Atlantic during HS1, which is coupled to a ventilation age of 5,169 ± 660 years. Old OC was mainly of terrigenous origin and exported to the North Atlantic by ice-rafting. Remineralization of old terrigenous OC in the ocean may have contributed to, at least in part, the anomalously old ventilation ages reported for the high-latitude North Atlantic during HS1.

Global reorganization of deep-sea circulation and carbon storage after the last ice age

Using new and published marine fossil radiocarbon (¹⁴C/C) measurements, a tracer uniquely sensitive to circulation and air-sea gas exchange, we establish several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. We find the most ¹⁴C-depleted water in glacial Pacific bottom depths, rather than the mid-depths as they are today, which is best explained by a slowdown in glacial deep-sea overturning in addition to a "flipped" glacial Pacific overturning configuration. These observations cannot be produced by changes in air-sea gas exchange alone, and they underscore the major role for changes in the overturning circulation for glacial deep-sea carbon storage in the vast Pacific abyss and the concomitant drawdown of atmospheric CO₂.

Evidence for late-glacial oceanic carbon redistribution and discharge from the Pacific Southern Ocean

Southern Ocean deep-water circulation plays a vital role in the global carbon cycle. On geological time scales, upwelling along the Chilean margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we develop an X-ray Micro-Computer-Tomography method to assess foraminiferal test dissolution as proxy for paleo-carbonate ion concentrations ([CO₃^2-]). Our subantarctic Southeast Pacific sediment core depth transect shows significant deep-water [CO₃^2-] variations during the Last Glacial Maximum and Deglaciation (10-22 ka BP). We provide evidence for an increase in [CO₃^2-] during the early-deglacial period (15-19 ka BP) in Lower Circumpolar Deepwater. The export of such low-carbon deep-water from the Pacific to the Atlantic contributed to significantly lowered carbon storage within the Southern Ocean, highlighting the importance of a dynamic Pacific-Southern Ocean deep-water reconfiguration for shaping late-glacial oceanic carbon storage, and subsequent deglacial oceanic-atmospheric CO₂ transfer.

Deglacial Subantarctic CO2 outgassing driven by a weakened solubility pump

The Subantarctic Southern Ocean has long been thought to be an important contributor to increases in atmospheric carbon dioxide partial pressure (pCO₂) during glacial-interglacial transitions. Extensive studies suggest that a weakened biological pump, a process associated with nutrient utilization efficiency, drove up surface-water pCO₂ in this region during deglaciations. By contrast, regional influences of the solubility pump, a process mainly linked to temperature variations, have been largely overlooked. Here, we evaluate relative roles of the biological and solubility pumps in determining surface-water pCO₂ variabilities in the Subantarctic Southern Ocean during the last deglaciation, based on paired reconstructions of surface-water pCO₂, temperature, and nutrient utilization efficiency. We show that compared to the biological pump, the solubility pump imposed a strong impact on deglacial Subantarctic surface-water pCO₂ variabilities. Our findings therefore reveal a previously underappreciated role of the solubility pump in modulating deglacial Subantarctic CO₂ release and possibly past atmospheric pCO₂ fluctuations.

Using paired reconstructions of seawater pCO₂, temperature, and nutrient utilization, Dai et al. show underappreciated influences of the solubility pump on deglacial Subantarctic surface-water pCO₂ variabilities compared to the biological pump.

A deep Tasman outflow of Pacific waters during the last glacial period

The interoceanic exchange of water masses is modulated by flow through key oceanic choke points in the Drake Passage, the Indonesian Seas, south of Africa, and south of Tasmania. Here, we use the neodymium isotope signature (ε_Nd) of cold-water coral skeletons from intermediate depths (1460‒1689 m) to trace circulation changes south of Tasmania during the last glacial period. The key feature of our dataset is a long-term trend towards radiogenic ε_Nd values of ~−4.6 during the Last Glacial Maximum and Heinrich Stadial 1, which are clearly distinct from contemporaneous Southern Ocean ε_Nd of ~−7. When combined with previously published radiocarbon data from the same corals, our results indicate that a unique radiogenic and young water mass was present during this time. This scenario can be explained by a more vigorous Pacific overturning circulation that supported a deeper outflow of Pacific waters, including North Pacific Intermediate Water, through the Tasman Sea.

Using cold-water corals, this work identifies a deep outflow of Pacific waters via the Tasman Sea during the last ice age, thus highlighting the role of this area for the interoceanic exchange of water masses on climatic time scales.