Reduced Antarctic Bottom Water overturning rate during the early last deglaciation inferred from radiocarbon records

Domain: earth_science
Published: 2025-08-20
Editorial outcome: accept

Abstract
The rapid CO2 rise during the early deglaciation is often linked to enhanced ventilation by intensified Antarctic Bottom Water (AABW) overturning. The recorded radiocarbon ventilation seesaw during the early deglaciation, which describes improved Southern Ocean and reduced North Atlantic abyssal radiocarbon ventilation, has been interpreted as intensified AABW and reduced North Atlantic Deep Water convections. However, abyssal radiocarbon records also reflect changes in surface reservoir ages and interior water mass mixing. Using isotope-enabled simulations, we show that this seesaw results from weakened AABW overturning and decreased Southern Ocean surface reservoir age. With AABW occupying the abyssal ocean, weakened AABW overturning increases transit time, with the magnitude increasing northward. This transit time increase outpaced the declining Δ14Catm Southern Ocean surface reservoir age decrease in the abyssal North Atlantic, but not in the abyssal Southern Ocean, thus producing a radiocarbon ventilation seesaw. Our results suggest sluggish deep water overturning from both poles during the early deglaciation.

Introduction
Understanding past rapid changes in atmospheric CO2 is essential for grasping the mechanisms and consequences of swift climate changes, thereby guiding projections and strategies for future climate scenarios. Notably, during the early phase of the last deglaciation (~17 ka to 15 ka), atmospheric CO2 increased by ~30 ppm over a span of 2000 years1. This increase constituted about half of the total CO2 rise during the last deglaciation, but occurred within just a quarter of the entire period. A predominant hypothesis attributes this CO2 rise to enhanced ventilation of the abyssal ocean2–6. Given the abyssal ocean is primarily filled with the Antarctic Bottom Water (AABW)7,8, strengthened AABW overturning rate would improve abyssal ventilation and thereby increase atmospheric CO2 due to enhanced outgassing and reduced carbon sequestration in the abyssal ocean on millennial timescales9–12.

A key evidence that is used to support this enhanced AABW overturning hypothesis lies in deglacial reductions in radiocarbon ventilation ages reconstructed from marine sediments. Radiocarbon enters the ocean through surface gas exchange and is transported in the ocean interior with radioactive decay along the transport pathway, thus providing a unique tracer to quantify past water transit time and, in turn, circulation rates13. The radiocarbon ventilation age is defined here as the radiocarbon age difference between the ocean and the contemporary atmosphere (“Methods”). Specifically, there was a noticeable decrease in radiocarbon ventilation ages in the abyssal Southern Ocean during Heinrich Stadial 1 (HS1, ~17.5-14.7 ka)14 when the North Atlantic Deep Water (NADW) convection nearly collapsed15. This suggests enhanced ventilation in the abyssal Southern Ocean during the early last deglaciation. Concurrently, increased radiocarbon ventilation ages during HS1 were recorded at both mid and abyssal water depths at the Iberian Margin in the North Atlantic16,17. These opposite radiocarbon ventilation age trends between the Southern Ocean and the North Atlantic are referred to as the “bipolar radiocarbon ventilation seesaw”17, traditionally interpreted as a consequence of antiphase changes in NADW and AABW productions, a phenomenon termed as the “deepwater production seesaw”18,19.

However, the use of radiocarbon ventilation age as an indicator for deepwater production changes is ambiguous. In addition to the water transit time from the surface to the interior ocean, which is the “true ventilation time”, radiocarbon in the ocean interior is influenced by surface gas exchange and the mixing of water masses originating from different sources13,20. Modeling studies suggest that deep water radiocarbon ventilation ages are only weakly affected by the true ventilation time determined by the deep water convection rate21. Instead, they strongly depend on changes in the surface reservoir age, which is defined as the radiocarbon age difference between the ocean surface and the contemporary atmosphere (“Methods”). This surface reservoir age effect is particularly critical in the Southern Ocean21,22. During the early deglaciation, opposite trends in the radiocarbon surface reservoir ages were observed in the subpolar North Atlantic and the Southern Ocean23, which urges careful consideration of potential influences of surface gas exchange in explaining the bipolar radiocarbon ventilation seesaw16. In addition, deep water radiocarbon ventilation ages are affected by water mass mixing in the ocean interior20,21,24, which is poorly constrained by proxy reconstructions during the early deglaciation and thus complicates the interpretation of the radiocarbon records in terms of past deepwater convection changes.

To interpret the radiocarbon ventilation seesaw, we utilize an isotope-enabled transient ocean simulation that effectively reproduces the overall trends in the reconstructed radiocarbon records. Our simulation reveals that the radiocarbon ventilation seesaw in reconstructions can be explained by weakened AABW production, coupled with a decrease in the Southern Ocean surface reservoir age during the early last deglaciation. In addition, we explore the associated mechanisms underlying the AABW overturning rate and Southern Ocean surface reservoir age changes.

Results
Simulated deglacial physical ocean and radiocarbon changes
C-iTRACE is a transient ocean-only simulation driven by surface forcings from a fully coupled transient simulation (TRACE21K)25 (“Methods”). C-iTRACE is capable of reproducing multiple isotopic changes as revealed by proxy records at the LGM26 and during the last deglaciation27,28, suggesting a reasonable model representation of deglacial ocean changes. With freshwater inputs in the high latitude North Atlantic during HS1, the simulated NADW nearly collapses, with the simulated 231Pa/230Th reproducing 231Pa/230Th records from the Bermuda Rise15 (Fig. 1A). In C-iTRACE, AABW transport rate weakens from 11 Sv at the LGM to 5 Sv at HS1, parallel to the NADW reduction during the deglaciation (Fig. 1A and Supplementary Fig. 1). Therefore, no deep water production seesaw is simulated in C-iTRACE. The parallel reductions in NADW and AABW overturning rates during the early deglaciation are also produced by a fully coupled transient deglacial simulation with a higher model resolution (iTRACE)29 (Supplementary Fig. 1).

C-iTRACE reasonably reproduces the first-order trends in reconstructed radiocarbon ventilation age changes during the early deglaciation, thus presenting a useful tool for understanding radiocarbon records. The simulated surface reservoir ages in the Southern Ocean decreases by ~660 years from the LGM to HS1, in agreement with the compiled reconstructions23 (Fig. 1D). The simulated radiocarbon ventilation age changes between the LGM and HS1 in the ocean interior also agree with published reconstructions in the Southern Ocean and the Atlantic within data uncertainties (Supplementary Table. 1)30 (Fig. 1B, C and Supplementary Figs. 2 and 3), although the magnitude of the radiocarbon ventilation ages seems to show a systematic bias from reconstructions especially at the Iberian Margin (Fig. 1B). The higher simulated deepwater radiocarbon ventilation ages than the reconstructions at the Iberian Margin might imply that AABW overturning rate was not strong enough during the LGM in C-iTRACE. Nevertheless, the bipolar radiocarbon ventilation seesaw trend is simulated qualitatively in C-iTRACE. From the LGM to HS1, the simulated B-A age increases by 141 years at the Iberian Margin but decreases by 329 years in the Southern Ocean below ~3 km water depth, capturing the first-order increasing and decreasing trends in reconstructions from the Southern Ocean and Iberian Margin respectively (Fig. 1B, C and Supplementary Fig. 4). The decrease of radiocarbon ventilation age record at the Iberian Margin staring at 15.5 ka is related to the transition into the Bølling–Allerød (BA), which is earlier than the transition in 231Pa/230Th records from the Bermuda Rise15 probably due to chronological uncertainties and bioturbation in the radiocarbon records. Combined with the simulated parallel reductions in AABW and NADW production rates, we find that the radiocarbon ventilation seesaw occurs without a deep-water production seesaw in C-iTRACE.

The simulated abyssal ocean is occupied mainly by AABW and experiences little change in water mass mixing ratio during the early deglaciation as indicated by the idealized dye tracer released in the surface Southern Ocean (Methods) (Fig. 1G and Supplementary Fig. 5). At the LGM, NADW shoaled to above 3 km, and the abyssal ocean is dominated by AABW below 3 km (Supplementary Fig. 5A). Under this LGM water mass geometry, C-iTRACE reproduces δ13C, εNd and 231Pa/230Th in agreement with reconstructions26. At HS1, reduced NADW overturning leads to a decrease in the NADW percentage at the mid-depth above 3 km28. However, the AABW percentage is changed by less than 10% in the abyssal ocean below 3 km (Supplementary Fig. 5C): the abyssal Southern Ocean is occupied by 100% AABW at LGM and 98% AABW at HS1, while the Iberian Margin below ~3 km is occupied by 91% AABW at the LGM and 82% AABW at HS1 (Fig. 1G). The limited change in abyssal water mass mixing might be supported by little changes in εNd records from 20 ka to 15 ka31–35, although the interpretation of εNd records might be complicated by benthic fluxes32,36,37. Under such conditions, deglacial radiocarbon ventilation changes in the abyssal ocean is mainly affected by changes in AABW, instead of NADW as previously suggested17,38.

Decomposing abyssal radiocarbon ventilation age changes
The deglacial radiocarbon ventilation seesaw resulted from a synergy of reduced Southern Ocean surface reservoir ages and weakened AABW overturning rate in the model. Since the abyssal ocean is almost entirely occupied by AABW from the LGM to HS1, radiocarbon ventilation age changes can be decomposed into (1) changes in Southern Ocean surface reservoir ages and (2) changes in water transit time from the surface to the interior ocean represented by the ideal age in the model (“Methods”). The sum of anomalies of these two components reproduce, qualitatively, the trends of radiocarbon ventilation age anomalies in both abyssal Southern Ocean and the Iberian Margin (Fig. 1F), although the sum of water transit time and Southern Ocean surface reservoir age anomalies (Fig. 1F dashed lines) is systematically higher than the deep ocean radiocarbon ventilation age anomaly (Fig. 1F solid lines) due to the decreasing trend in the atmospheric radiocarbon activities (Δ14Catm) during the deglaciation (Supplementary text). From LGM to HS1, a weaker AABW overturning rate increases the ideal age in the abyssal Southern Ocean and North Atlantic (Fig. 1E). The magnitude of the ideal age increases linearly with the distance away from the AABW source region (Supplementary text). Therefore, the ideal age increases more at the Iberian Margin than in the Southern Ocean. From 19 ka to 15 ka, the slightly increased transit time (~+557 years) is smaller than the decreased surface reservoir age (~−654 years) in the abyssal Southern Ocean, while the much larger transit time increase (~+1274 years) is larger than the decreased surface reservoir age (~−654 years) for abyssal waters at the Iberian Margin, leading to the net radiocarbon ventilation age decrease in the abyssal Southern Ocean and increase in the abyssal North Atlantic, which generates the bipolar radiocarbon ventilation seesaw.

Both weakened AABW overturning rate and reduced Southern Ocean reservoir ages are essential in producing the bipolar radiocarbon ventilation seesaw. If there is no significant decrease in Southern Ocean reservoir ages, as shown in the sensitivity experiment Fix_pCO2&14C (“Methods”, Fig. 2D red), reduced AABW overturning rate alone would increase transit time and, in turn, increase the radiocarbon ventilation ages in both the abyssal Southern Ocean and the North Atlantic (Fig. 2E, F red). If there is no change in AABW overturning rate, as illustrated in the sensitivity experiment Fix_circulation (Methods) (Supplementary Fig. 6B blue), reduced reservoir ages in the Southern Ocean (Fig. 2D blue) would be propagated into the entire abyssal ocean, leading to reduced radiocarbon ventilation ages in both the Iberian Margin and the Southern Ocean (Fig. 2E, F blue). Therefore, reduced Southern Ocean surface reservoir ages23 combined with increased radiocarbon ventilation ages at the Iberian Margin must require an increased AABW transit time, suggesting a weaker AABW overturning rate during the early deglaciation.

Mechanism for reduced Southern Ocean surface reservoir ages
The surface reservoir age is influenced by air-sea gas exchange modulated by sea ice insulation39,40, physical circulation41, atmospheric CO2 levels (pCO2)42 and Δ14Catm 41,43, all of which favor reduced Southern Ocean reservoir ages during HS1. Decreased NADW overturning during HS1 leads to surface cooling in the North Atlantic and warming in the Southern Ocean, a phenomenon referred to as the “thermal bipolar seesaw”44,45. In addition, increasing atmospheric CO2 would also warm up the Southern Ocean (Fig. 3E). This warming results in reduced sea ice coverage (Fig. 2C and Supplementary Fig. 7), dominating the change in CO2 gas exchange velocity (piston velocity) in the Southern Ocean during the early deglaciation (Supplementary Fig. 8). This enhanced air-sea gas exchange by sea ice retreat leads to a younger Southern Ocean surface reservoir age (Supplementary Fig. 9I). However, the associated sea ice insulation effect contributes only approximately 20% to the deglacial Southern Ocean reservoir age decrease in the AABW formation region, as illustrated by the difference between C-iTRACE and experiment Fix_sea_ice (Methods and Supplementary Table 2) (Supplementary Figs. 9I and 10). This suggests that sea ice coverage is not a dominant factor controlling the early deglacial Southern Ocean reservoir age changes46. Other Southern Ocean physical condition changes, including sea surface warming (Fig. 3E) and shoaling of mixed layer depth (Supplementary Fig. 11), can also contribute to younger surface reservoir ages. However, their contributions are even smaller, as shown by the difference between experiment Fix_sea_ice and Fix_circulation (Methods and Supplementary Table 2) (Supplementary Figs. 9G and 10). We find that the deglacial Southern Ocean surface reservoir age changes are reduced, mostly contributed by atmospheric CO2 changes, as manifested by the difference between C-iTRACE and experiment Fix_pCO2&14C (Methods and Supplementary Table 2) (Supplementary Figs. 9 and 10). Without changes in atmospheric CO2, Southern Ocean surface reservoir ages would only experience a small decrease, as demonstrated in experiment Fix_pCO2&14C (Fig. 2D).

The decreased surface reservoir ages in the Southern Ocean are contributed predominantly by the declining Δ14Catm. The atmospheric CO2 forcings include the increasing pCO2 level and the declining Δ14Catm, both of which might contribute to the decreasing surface reservoir ages in the Southern Ocean. Ignoring mixing with underlying waters, the surface water reservoir age has been suggested to be linearly related to 1/pCO2 42. As a result, increasing pCO2 during the early deglaciation would be accompanied by decreasing surface reservoir ages. However, mixing with deep waters cannot be neglected in the AABW formation regions due to deep convection resulting from sea ice production and brine rejection. Because deglacial Δ14Catm was declining, mixing with deep waters which exchanged 14C with the atmosphere with higher Δ14Catm at an earlier time (e.g., a millennium ago) will increase Δ14C in the surface ocean and, in turn, decrease its reservoir ages (memory effect)43. To explore the relative importance of the increasing pCO2 and declining Δ14Catm in the reduction of surface reservoir ages in the AABW formation region, a sensitivity experiment, Fix_circulation&pCO2, is carried out, with changing Δ14Catm and fixed circulation and pCO2 (“Methods”). The Southern Ocean surface reservoir ages in Fix_circulation&pCO2 are almost identical to those in experiment Fix_circulation (Fig. 2D), suggesting a limited contribution by increasing pCO2 but a dominant contribution by declining Δ14Catm on reservoir age changes in the AABW formation region during the early deglaciation.

Buoyancy-driven reduction in AABW overturning rate
Surface wind and buoyancy forcings, along with internal mixing, drive the global ocean circulation. A previous modeling study suggests that intensified Southern Hemisphere westerlies during HS1 enhance AABW overturning rate, based on artificial wind stress forcing10. However, the surface momentum forcing in C-iTRACE, taken from a fully coupled simulation that reproduces major observed climate changes25 (“Methods”), shows little change in Southern Hemisphere westerlies during the early deglaciation (Fig. 3A and Supplementary Figs. 12A, B), consistent with another fully coupled simulation with higher resolution29 (Supplementary Figs. 12D, E). The evolution of AABW overturning rate in C-iTRACE during the early deglaciation is predominantly influenced by surface buoyancy flux, with the haline component playing a key role47 (Fig. 3C, “Methods”). From the LGM to HS1, the Southern Ocean warms due to the thermal bipolar seesaw as NADW weakens (Fig. 3E), which results in sea ice retreat (Fig. 3D and Supplementary Fig. 7), consistent with sea ice records48. Less sea ice production contributes to a decrease in the haline density flux associated with brine rejection (Fig. 3C). Although stronger AABW overturning rate are simulated during Heinrich Stadials in previous studies using LOVECLIM and UVIC models10,12, these apparently opposite AABW responses during Heinrich Stadials in different numerical experiments can be reconciled, in terms of physical mechanisms, by considering opposite surface buoyancy forcings (see details in “Methods”): a negative freshwater forcing/increasing density flux over the Southern Ocean lead to the enhancement of AABW overturning rate in LOVECLIM and UVIC models10,12, while a decreasing density flux due to sea ice retreat over the Southern Ocean lead to the decrease of AABW overturing in C-iTRACE (Fig. 3). Therefore, freshening surface Southern Ocean results in weakening of AABW overturning rate and vice versa is a robust feature in different climate models. The reconstructed sea ice retreat and thus reduced density flux in the Southern Ocean during the early last deglaciation48 probably suggests a weakened AABW overturning rate, based on the above physical mechanisms.

Moreover, with a fixed surface buoyancy forcing, a 30% increase in the Southern Hemisphere westerlies strength (experiment SO_wind) causes little change in AABW overturning rate (Fig. 3A, B). This suggests that the surface buoyancy forcing is the primary factor influencing AABW overturning rate during the early deglaciation. Our results suggest parallel decreases in AABW and NADW convection during the early last deglaciation, driven by surface buoyancy forcings which are connected by the thermal bipolar seesaw.

The reduced AABW overturning rate during HS1, resulting from the surface Southern Ocean warming in C-iTRACE, is also a robust feature in other climate models49,50 and aligns with observed and projected AABW weakening in the recent and upcoming decades51,52. Sensitivity experiments with different magnitudes and locations of meltwater in the North Atlantic (“Methods”)53 also consistently show weakening of AABW parallel to NADW slowdown (Fig. 4). While sea ice retreat may not significantly contribute to the early deglacial surface reservoir age changes in the Southern Ocean as discussed above, it does play a crucial role in determining the transit time in the interior ocean by controlling the AABW production. This, in turn, influences the radiocarbon ventilation age in the ocean interior.

Southern Ocean upwelling and AABW overturning rate
The global overturning circulation comprises an upper cell associated with NADW and a lower cell associated with AABW54. Based on the increased biological productivity55, enhanced upwelling in the Southern Ocean during the early deglaciation has been invoked to strengthen the AABW overturning rate and improve the abyssal ocean ventilation5,9, because part of the wind-driven upwelled water moves southward and subsequently feeds the formation of AABW. However, while the upper cell relies on the wind-driven upwelling in the Southern Ocean to balance the NADW formation56, AABW formation by brine rejection57 is balanced by interior diapycnal mixing in the lower cell58. The intensified wind-driven Southern Ocean upwelling would increase the NADW overturning56,59, but its exact role in the AABW overturning has not been firmly determined58. Therefore, enhanced Southern Ocean upwelling does not necessarily indicate an enhanced AABW overturning rate, which is supported by experiment SO_wind. In SO_wind with fixed surface buoyancy forcing, a 30% increase in the Southern Hemisphere westerlies results in enhanced upwelling and opal productivity in the Southern Ocean (Supplementary Fig. 13) and strengthening of the NADW overturning rate although it still remains relatively weak (Supplementary Fig. 6A). By contrast, AABW overturning rate remains nearly unchanged (Supplementary Fig. 6B). Consequently, Southern Hemisphere westerlies appear to primarily influence the upper cell circulation. Therefore, Southern Ocean upwelling and AABW overturning are driven by distinct physical mechanisms, that is, enhanced Southern Ocean upwelling is not necessarily associated with a stronger AABW overturning rate or abyssal ventilation during the early deglaciation.

Methods
Isotope-enabled ocean model
The Parallel Ocean Program version 2 (iPOP2)70 implemented with several isotopes, including radiocarbon71 is used for the deglacial transient simulation and sensitivity experiments. The biotic version of radiocarbon71 is analyzed in this study. The ocean model configuration is nominal 3° horizontal resolution and 60 vertical layers, with a 10 m resolution in the upper 200 m, expanding to 250 m resolution below 3000 m.

In addition to isotopes, idealized dye tracer and ideal age are implemented to quantitatively represent the water mass mixing ratios and water transit time in the model, respectively. A dye tracer is released with a surface value of 0 except in the source region, where its surface value is specified at 128. In the interior ocean, dye tracers are transported as other conservative passive tracers. For example, a dye tracer is released over the Southern Ocean (south of 34°S) with surface values of 1 and 0 within and outside the Southern Ocean, respectively, which indicates the percentage of AABW in the abyssal ocean. The ideal age is set to 0 at the surface and increases with 1 yr/yr in the ocean interior, which therefore indicates how long since the water mass has left the surface.

Deglacial transient simulation and sensitivity experiments
A transient simulation of the last deglaciation (C-iTRACE) is simulated using iPOP227, which is forced by monthly surface forcings from a fully coupled simulation TRACE21K25. TRACE21K is a transient simulation under reconstructed insolation, greenhouse gas, and continental ice sheet using CCSM3, and the magnitude and location of the meltwater flux in TRACE21K are developed under the constraint of reconstructed sea level, Greenland temperature and AMOC72. Although the meltwater scheme in TRACE21K can be further improved to reach better agreement with the recent sea level estimate73, TRACE21K captures major climate changes during the last deglaciation, thus presenting a useful tool for paleoclimate research25,45. In the meltwater scheme developed in TRACE21K, there is no freshwater flux added to the Southern Ocean during the early deglaciation. C-iTRACE is forced by monthly surface momentum, heat and freshwater fluxes and surface sea ice fractions in TRACE21K, with surface temperature and salinity restoring to the monthly average of TRACE21K output. The restoring timescale is 30 days for temperature and 60 days for salinity. C-iTRACE reproduces the physical circulation in TRACE21K27. C-iTRACE is able to reproduce different isotope changes consistent with observations during the deglaciation27,28. The atmospheric pCO2 and Δ14Catm are prescribed by reconstructed records74,75 (Fig. 2A, B). The physical circulation and radiocarbon are initialized from the LGM state in reference No76. and further spun up for another 6788 years, therefore, radiocarbon reaches quasi equilibrium under the LGM condition.

The iPOP2 is used as a tool to explore the mechanism of the radiocarbon age and AABW changes in this study, and several sensitivity experiments are carried out to diagnose the contribution of pCO2, Δ14Catm, and circulation to radiocarbon ventilation age and buoyancy and momentum control on AABW overturning rate during the early deglaciation:

(1) Experiment Fix_pCO2&14C: the atmospheric CO2 and radiocarbon are fixed at LGM values (Fig. 2A, B), which are 188 ppmv and 393, respectively. The surface fluxes and sea ice are transient, which are the same as in C-iTRACE. The NADW and AABW overturning rates are identical to C-iTRACE (Supplementary Fig. 6). The difference between C-iTRACE and Fix_pCO2&14C shows the effect of atmospheric CO2 forcings, including pCO2 and Δ14Catm on marine radiocarbon (Supplementary Table 2).

(2) Experiment Fix_circulation: the ocean circulation is fixed at the LGM condition by looping the TRACE21K surface fluxes (heat flux, freshwater flux, momentum flux) and sea ice from 20 ka to 19 ka (Fig. 2C), while the atmospheric pCO2 and Δ14Catm are transient (Fig. 2A, B). The NADW and AABW overturning rates stay at LGM values (Supplementary Fig. 6). The difference between Fix_sea_ice and Fix_circulation shows the effect of deglacial physical condition change, including sea surface temperature, mixed layer depth and mixing with underlying deep water, on marine radiocarbon (Supplementary Table 2).

(3) Experiment Fix_sea_ice: the sea ice coverage for gas exchange is fixed at the LGM state (Fig. 2C), with transient surface fluxes and atmospheric pCO2 and Δ14Catm (Fig. 2A, B). The sea ice effect on the buoyancy forcing is included in the transient freshwater flux so that the sea ice effect on the circulation is included in the transient simulation, such as Fig. 6A. Therefore, the difference between C-iTRACE and Fix_sea_ice shows the effect of insulation of gas exchange by sea ice on marine radiocarbon (Supplementary Table 2).

(4) Experiment Fix_circulation&pCO2: the ocean circulation is fixed at LGM condition by looping the TRACE21K surface fluxes (heat flux, freshwater flux, momentum flux) and sea ice from 20 ka to 19 ka (Fig. 2C). The atmospheric pCO2 is fixed at 188 ppm (Fig. 2A) and the Δ14Catm is transient (Fig. 2B). The difference between Fix_circulation&pCO2 and Fix_circulation shows the effect of deglacial atmospheric pCO2 change on marine radiocarbon (Supplementary Table 2).

(5) Experiment SO_wind: the westerlies over the Southern Ocean (40°S-60°S) is increased by 30% from 17 ka to 15 ka (Fig. 3A and Supplementary Fig. 12C). This experiment shows the direct effect of momentum forcing on AABW overturning rate and abyssal ventilation.

AABW and NADW overturning rate in the model
The overturning streamfunction used in the analysis considers both Eulerian velocity and bolus velocity, which represent the eddy effect through eddy parameterization77. Therefore, the residual overturning circulation is used, which is more representative of tracer property transports56. NADW overturning rate is defined as the maximum of the Atlantic overturning streamfunction in the North Atlantic between 500 m and 1600 m. AABW overturning rate is defined as the minimum of the global overturning streamfunction between 33°S and 10°S below 2000 m.

AABW response during Heinrich Stadial in different climate models
In C-iTRACE and iTRACE experiments by CESM, the simulated AABW overturning rate is weakened during the Heinrich Stadial. However, other modeling studies suggest enhanced AABW overturning rate during Heinrich Stadial in LOVECLIM and UVIC models10,12. These opposite AABW responses are caused by different surface buoyancy forcings in different studies. In references10 and12, a negative freshwater forcing is applied to the Southern Ocean, which increases the surface density and, in turn, the AABW overturning rate. However, in C-iTRACE and iTRACE simulated by CESM, no additional freshwater forcing is added to the Southern Ocean, and the surface buoyancy flux is dominated by the reduced sea ice in response to the Southern Ocean warming. Therefore, in CESM, warming in the Southern Ocean leads to reduced AABW overturning rate during Heinrich Stadial. If similar warming is applied in the UVIC model, AABW production will also show a decrease49, suggesting a robust weakening AABW response to the Southern Ocean warming.

Robust AABW weakening in response to North Atlantic freshwater input
The responses of AABW and NADW overturing to the freshwater input in the North Atlantic are analyzed in idealized sensitivity experiments with varying locations and magnitudes of freshwater input under preindustrial initial conditions using fully-coupled CESM53. In experiments 0.25Sv_North_Atlantic and 0.5Sv_North_Atlantic, 0.25Sv and 0.5Sv freshwater are added to the 50°-70°N North Atlantic, respectively. In 0.5Sv_Gulf_of_Mexico, 0.5Sv freshwater is added to the Gulf of Mexico (15°–33°N, 255°–279°E). Each experiment has been integrated for 200 years. In the year 200, the NADW overturning rate has reached quasi-equilibrium, and the AABW overturning rate is still adjusting. With different magnitudes of reductions in NADW overturning rate, AABW shows a long-term decreasing trend in all three idealized experiments, suggesting a robust parallel weakening of AABW and NADW in response to meltwater input in the North Atlantic.

Buoyancy flux calculation
The density flux is calculated using the linearized equation of state of seawater: Fρ = -α*Cp*Q + ρ(0,T)*β* (E-P-R-I)*S, where α= -1/ρ ∂ρ/∂T|P,S and β= 1/ρ ∂ρ/∂S|P,T are the thermal expansion and haline contraction coefficients, respectively. Here, S is salinity and T is temperature. Q is the total heat flux. Cp is the specific heat capacity. ρ(0,T) is the density of freshwater with salinity of 0 and temperature of T. E, P, R, and I represent the freshwater fluxes related to evaporation, precipitation, river runoff, and sea-ice melting and brine rejection, respectively. The density flux can be further separated into the thermal density flux (-α*Q/Cp) and the haline density flux (ρ(0,T)*β* (E-P-R-I)*S). The contribution of the brine rejection can be estimated as ρ(0,T)*β* (-I)*S.

Radiocarbon ages calculation in the model
The radiocarbon ventilation age is calculated as the benthic-atmosphere age differences: 8033×ln(Δ14Catm+1 / Δ14COcean+1) (14C Libby half-life is 5568 years). The reservoir age in the model is defined as the difference of the radiocarbon age between the surface ocean (0–100 m average) and the atmosphere: 8033×ln(Δ14Catm+1 / Δ14COcean_surface+1).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Gu and colleagues present radiocarbon-enabled simulations performed with CESM of the early stage of the last\ndeglaciation, testing whether the production rates of the Atlantic’s two dominant water masses, NADW and AABW, changed\nin tandem or in a compensating manner. The authors find that the bipolar radiocarbon ventilation seesaw reconstructed for\nthe deglacial deep Atlantic can be attributed to changes in reservoir ages rather than alterations in transit times. Through a\nrange of well-designed sensitivity experiments the authors disentangle the different contributing effects and clearly show that\nthe reconstructed radiocarbon data is in agreement with a concurrent reduction of both NADW and AABW during Heinrich\nStadial 1.\n\nThe manuscript is well-written and clearly describes the well-designed and clean sensitivity experiments, presenting a\nconvincing argument that the model simulations can account for the reconstructions by simulating both weak NADW and\nAABW formation rates during HS1. The findings are in line with a number of recent studies that use isotope-enabled models\nto better understand the deglacial changes of the most widely used proxies, which led to revised but more robust\ninterpretations of proxy timeseries. As such, I only have minor comments on the manuscript.\n\nNevertheless, it may be worth considering study’s relatively focused scope. The broader implications on the large-scale\nclimate change and carbon cycle are not addressed in the paper. The revised deep water production rates and changes in\nglobal water mass fractions might hold clues for a better understanding of the initial CO2 rise during the last deglaciation,\nwhich still remains not fully understood. This is not to suggest that the authors should add a lengthy discussion or further\nexperiments into the current manuscript, which would be clearly beyond the scope of this study. The study, in its present\nform, is already a well-rounded piece of research. However, these broader considerations might be relevant for the choice of\njournal, and Nature Communications might have a too broad readership for the rather specialized topic at hand. Clearly, this\ndecision rests with the authors and editor, and should they deem this appropriate for Nature Communications, I would be\nhappy to see this manuscript published there.\n\nMajor point\nFigures 1-3 depict nearly the entire deglaciation until 13 ka. However, the manuscript only discusses the transition from the\nLGM to HS1. I would therefore strongly suggest to also only show the timeseries until 15 ka, i.e., the end of HS1. This is\nbecause it might generally be confusing if the BA is also shown, and more importantly, the BA was characterized by very\ndifferent ocean dynamics that are not fully captured by the model simulation, leading to a poor model-data match\n(especially in Fig. 1). Event though this is an independent issue, it currently slightly undermines how convincing the study appears to\nthe general reader, and I feel this is fixed easily by just not showing the BA and YD.\n\nMinor/technical points\nL71: Please add references to Muglia and Schmittner. (2021, doi:10.1016/j.quascirev.2021.106844) and Poeppelmeier et al.\n(2023, doi:10.1038/s41561-023-01140-3) who showed that 14C ventilation ages do not reliably indicate deep ocean\nadvection rates.\nL121-126: Maybe again highlight here that the radiocarbon ventilation seesaw occurs in the simulation without a deep water\nseesaw.\nL138: More recent studies (Poeppelmeier et al., 2022, doi:10.1016/j.quascirev.2022.107404; Du et al., 2020,\ndoi:10.1016/j.quascirev.2020.106396) suggest that Nd isotope records need to be interpreted more carefully, as changes in\nweathering also a considerable role. I therefore suggest to phrase this a bit more carefully, maybe also highlighting that Nd\nisotope evidence remains also ambiguous.\nL156: Can you please put numbers on these effects here to be a bit more quantitative.\nL162: AABW production rate.\nL177: Maybe note that the thermal bipolar seesaw effects adds to the general global warming initiated during the early\ndeglaciation, leading to further sea-ice loss.\nL270: Better highlight that this only holds true if changes in water mass mixing between NADW and AABW are very small,\nas in the early deglaciation. During later stages of the last deglaciation, when NADW formation substantially enhanced, any\nchange in the AABW advection strength may be overwhelmed by changes in water mass mixing.\nL290: But AABW has a higher DIC concentration than NADW, which would indicate more carbon storage at globally higher\nAABW fractions. How can this be reconciled?\nFig. 2: To increase clarity, please use more descriptive axes labels, i.e., ice fraction instead of IFRAC, SO Reservoir age\ninstead of SO Rage, and spell out Iberian Margin.\nFig. 3: Units are missing in a couple of panels.", "reviewer_2": "This manuscript highlights the Atlantic abyssal ventilation during the last deglaciation by analyzing model results from a\nisotope-enabled complex ocean model which providing direct comparison between the reconstruction and modeling\nresults.Numerical simulation using such model is quite limited and experiments setup in the manuscript is smart and\nrequires quite amount of work.Hence,such work will make significant contribution in studying deglaciation global climate\nchange and abrupt climate events.Manuscript conclusion is concise and could be supported by analysis. The questions that\nI would like to address are listed below:\n1. Considering Southern Ocean stratification is primarily determined by sea ice, What kind of process decelerate the AABW\nin sea_ice sensitivity experiment?\n2. In extended figure 2d, seems that increased model resolution in iPOP one degree HS1 experiment, AABW almost reverse\nits sigh compare to three degree run. What are the possible reasons would lead to such difference considering only\nresolution has changed? Will the difference affect the final conclusion?\n3. Which atmospheric forcing is included in the control experiments? The author might add a few line in method section.", "reviewer_3": "This manuscript presents modeling results from the C-iTRACE simulation (Gu et al., 2019; 2020; 2021), generated the\nisotope-enabled ocean component (iPOP2) of the NCAR Community Earth System Model (CESM1), exploring a potential\nalternative explanation for observational data (e.g., Nd isotopes, Pa/Th, 14C) that suggests Antarctic Bottom Water (AABW)\nventilation was enhanced as North Atlantic Deep Water (NADW) convection slowed early in the last glaciation. The authors\npostulate that simultaneous decreases in surface ocean reservoir age (e.g., the offset between the surface ocean and the\natmosphere) occurred in both the Antarctic and high-latitude North Atlantic during Heinrich Stadial 1 (HS1). The resulting\nreduced surface ocean reservoir age at the point of bottom water formation decreases the radiocarbon reservoir ages, and\napparent ventilation age, in both AABW and NADW – even as deep convection slowed in both hemispheres in the C-\niTRACE simulation. In the case of NADW, the authors postulate that the decrease in preformed radiocarbon reservoir age is\noverwhelmed by the increased transit time associated with the reduced convection in the North Atlantic, but dominates the\napparent ventilation age in the Southern Ocean, leading to an apparent ‘bipolar radiocarbon ventilation seesaw’ in spite of\ntrue ventilation being reduced in both hemispheres.\n\nThe authors note that the CESM1 model findings presented here, specifically regarding a weakening of AABW formation\nduring HS1, are distinct from simulations using LOVECLIM and the UVIC Earth System Climate Model (i.e., Menviel et al.,\n2015). In understanding the potential significance of the C-iTRACE results, I believe it would strengthen the paper if the\nauthors could elaborate on why these model predictions regarding the existence (or not) of a bipolar ventilation seesaw in\nearly deglaciation are so distinct? The authors offer the favorable agreement of the C-iTRACE simulation with deglacial\nNorth Atlantic proxy records as reason for confidence in these model results (e.g., lines 101-105; 216-219; 312-313). This is\nnot a criticism of the novel ideas advanced in the paper under review, but it is worth noting that iTRACE relies on significant,\nand to date unattributed, ‘pre-hosing’ to weaken AMOC prior to the Laurentide freshwater release of Heinrich Stadial 1 (HS1)\nto achieve the AMOC collapse that is in agreement with deglacial proxy records – so agreement with the proxy records is not\nan objectively independent outcome of processes captured by the CESM1, rather the simulation is forced to that agreement\nby design.\n\nAlthough the authors assert in the text that the C-iTrace results agree with available observed radiocarbon ventilation age\nchanges between the LGM and HS1 ‘within data uncertainties’ (Line 120), the simulation results presented in Figure 1 seem\nboth qualitatively and quantitatively distinct from the observational data. In the case of the Benthic – Atmosphere (B-A) 14C\nage offsets for the Iberian Margin (IM, the model results suggest little change in the deep ocean through early deglaciation\ninto HS1, contrasting with the increase observed in the data, and are overall **greater** than observations. In the case of the\nSouthern Ocean (SO) model-data comparison, the model results do show a decrease in 14C age B-A offsets through HS1 –\nalmost imperceptible in magnitude relative to the proxy record . However in the SO case the modeled offsets are **lower**\nthan observations during early deglaciation, becoming greater at the end of the simulation largely because the model results\nare invariant while the proxy offset decreases by over a thousand years.\n\nSo, to recap: the C-iTRACE SO B-A offset is smaller than supported by available data in early deglaciation, while the IM is\nover 1000 years too large. By the end of the HS1, both the IM and SO simulations are old relative to the data, with the Iberian\nmargin simulated B-A showing no statistical change at all until the very end of the experiment associated with the Bolling-\nAllerod. Why is there an asymmetry in the bias in the model – and is it possible the experiment is fighting the bias? Did the\nauthors explore a control case with the non-freshwater forced iTrace? It would be interesting to know if these results are\nassociated with the freshwater forcing itself, or potentially a bias in the start-up condition?\n\nThe focus of this manuscript is on interpretation of the interior ocean radiocarbon record in the late deglaciation, and the\nauthors are advancing a rational alternative explanation for a confluence of factors that could drive an apparent decrease in\nAABW 14C age relative to the atmosphere while ventilation of the deep ocean in both the North Atlantic and Southern\nOcean slows. While as outlined above I think there could be a more robust exploration of the offsets between the\nobservational record and the C-iTRACE simulation, I don’t think that is a deal breaker for this hypothesis, and if correct it\nwould indeed shift a paradigm. The authors do advance a credible hypothesis to explain a potential fit between the model\nresults and evidence for enhanced Southern Ocean upwelling that has been associated with ventilation. However, while the\nauthors assert that the C-iTRACE results are in agreement with eNd records that also indicate little change in AABW\ncirculation through HS1 (e.g. line 138) I’m not actually sure that is a fair statement. It’s certainly\nnuanced; several of the eNd records cited do show limited change during HS1 relative to the large\nswing at the B-A, but the deepest North Atlantic records (e.g. core KNR197/10GGC-17) shows a\ndecrease relative to the LGM, while deep Southern Ocean records (e.g. RC11-83) shows an\nincrease (Poppelmeier et al., 2018, included in the compilation of Zhao et al., 2018, referenced in\nthis manuscript; Piotrowski et al., 2012). Other Southern Ocean eNd records in the Pacific sector\nalso show significant changes in eNd that seemingly evidence changes in AABW formation (e.g.,\nBasak et al., 2018; Du et al., 2018, referenced in this manuscript). These published observations\nhave been interpreted as strengthened AABW formation, in contrast to the weakening of AABW\ninferred from HS1 through C-iTRACE – do the authors have an alternative hypothesis to explain\nthese data and how they would fit in the proposed framework? Although the manuscript centrally\nhighlights the nuances of radiocarbon as a circulation tracer in the interior ocean, it seems as\nthough the C-iTRACE results are more in alignment with the published interpretation of the\nglobal synthesis of LGM radiocarbon records of Rafter et al., 2022 cited in line 88, as well\nas the deep abyssal Indo-Pacific reconstructions cited in Line 275.\n\nIn summary, this C-iTRACE simulation results presented in this manuscript are both interesting\nand informative, demonstrating that changes in surface ocean reservoir age at the time of\ndeepwater formation can have an outsized impact on the apparent/radiocarbon reservoir age of the\ninterior ocean that is decoupled from ocean circulation. While this observation in and of itself isn’t\nnew sensu stricto (e.g., Muglia and Schmittner, 2021), the potential application of this\nconcept to interpretation of the paradox set up between the C-iTRACE results (suggesting simultaneous\ndecreases in deep water formation in both the high latitude North Atlantic and Southern Ocean)\nand radiocarbon evidence in the deep Atlantic and Southern Ocean (suggesting a ‘bipolar radiocarbon ventilation seesaw’)\ncertainly is. My confidence in these\ninterpretations would be strengthened if the authors could offer an explanation for why the C-iTRACE results are different\nthan those of the LOVECLIM and UVIC models (e.g., Menviel et al., 2015). I also have concerns about the asymmetric\nbias in the SO versus IM data/model comparison of radiocarbon offsets between the interior ocean and atmosphere.\n\nMinor comments to Author:\nFig 1 – I think part F of the caption needs a correction. Would suggest adding units to the sub-plots or caption and define the\ngray bar in the caption.\nFig 2 – This is a hugely helpful figure in supporting your interpretations. This is a minor point, but it would be somewhat\nhelpful if the caption was in the same order as the experiments are explained in the methods. As for figure 1, there are no\nunits on any of the plots. The gray bar denoting HS1 is not defined in the caption.\nFig 3 – Again, units…\nExtended Data Figure 2 – This figure is a bit misleading. When presented together the SO and IB model/data comparison\ndoes seem to fall along a 1-1 ratio, albeit with significant scatter. However this hides the asymmetry of the offsets in the\nocean basins. What does this figure look like if you denote the SO data/model comparison points from the IB data/model\ncomparison points via different symbols, and can you comment on the origin of the inter-hemispheric asymmetric bias that\nseems obvious in the time series?\nExtended Data Figure 3 – would refer to reconstruction symbols as ‘points’ as opposed to ‘scatters’."}, "author_response": "Dear reviewers,\nWe are grateful for your time and invaluable feedback. In this letter, we address the\nreviewers’ comments and describe the resulting changes to the manuscript. The original reviewers’\ncomments are in red, our replies are in black, and the corresponding revised/new text in the\n5 manuscript to address the comments is in blue.\nIn this revision, the major changes are:\n(1) We discuss the different AABW response during Heinrich Stadial simulated by CESM in this\nstudy and LOVECLIM and UVIC in Menviel et al., (2015). We show that the opposite AABW\n10 responses can be reconciled by opposite surface buoyancy forcings applied in the Southern Ocean.\nIf similar warming in the Southern Ocean (thermal bipolar seesaw) is applied in UVIC model,\nit\nwill also produce a reduced AABW as in this study. Please refer to page 7 in the letter for details.\n(2) We discuss the model-data disagreement. The simulated radiocarbon ventilation age in the\nat LGM. The simulated radiocarbon ventilation age in the Southern Ocean is more or less in\nagreement with reconstructions. We emphasize that our model captures the opposite changes in\nthe abyssal radiocarbon ventilation ages in Iberian Margin and Southern Ocean qualitatively, and\nthe qualitative opposite abyssal radiocarbon ventilation changes between the two hemispheres\n20 together with the reduced Southern Ocean surface reservoir age infers reduced AABW during the\nearly deglaciation. Therefore, the “higher” bias in the simulated radiocarbon ventilation age in the\nIberian Margin does not affect the main conclusion in this study. Please refer to pages 14-16 in the\nresponse letter for details.\nReviewer #1:\nGu and colleagues present radiocarbon-enabled simulations performed with CESM of the early\nstage of the last deglaciation, testing whether the production rates of the Atlantic’s two dominant\n30 water masses, NADW and AABW, changed in tandem or in a compensating manner. The authors\nfind that the bipolar radiocarbon ventilation seesaw reconstructed for the deglacial deep Atlantic\ncan be attributed to changes in reservoir ages rather than alterations in transit times. Through a\nrange of well-designed sensitivity experiments the authors disentangle the different contributing\neffects and clearly show that the reconstructed radiocarbon data is in agreement with a concurrent\n35 reduction of both NADW and AABW during Heinrich Stadial 1.\nThe manuscript is well-written and clearly describes the well-designed and clean sensitivity\nexperiments, presenting a convincing argument that the model simulations can account for the\nreconstructions by simulating both weak NADW and AABW formation rates during HS1. The\nfindings are in line with a number of recent studies that use isotope-enabled models to better\n40 understand the deglacial changes of the most widely used proxies, which led to revised but more\nrobust interpretations of proxy timeseries. As such, I only have minor comments on the manuscript.\nWe thank the reviewer for the positive feedback.\nNevertheless, it may be worth considering study’s relatively focused scope. The broader\n45 implications on the large-scale climate change and carbon cycle are not addressed in the paper.\nThe revised deep water production rates and changes in global water mass fractions might hold\nclues for a better understanding of the initial CO2 rise during the last deglaciation, which still\n1\nremains not fully understood. This is not to suggest that the authors should add a lengthy discussion\nor further experiments into the current manuscript, which would be clearly beyond the scope of\n50 this study. The study, in its present form, is already a well-rounded piece of research. However,\nthese broader considerations might be relevant for the choice of journal, and Nature\nCommunications might have a too broad readership for the rather specialized topic at hand. Clearly,\nthis decision rests with the authors and editor, and should they deem this appropriate for Nature\nCommunications, I would be happy to see this manuscript published there.\nduring the early deglaciation has important implications on the deglacial carbon cycle but is\nbeyond the scope of current study. Understanding the deglacial ocean circulation change is the\nfirst step towards a better understanding of deglacial carbon cycle and the detailed mechanism of\nthe deglacial atmospheric CO rise is a research topic we will pursue in the future. In this\n2\n60 manuscript, we briefly describe the implication of the reduced abyssal ventilation on deglacial\ncarbon cycle as “While a weaker AABW overturning rate has been proposed to be linked to\nenhanced carbon sequestration in the abyssal ocean(Toggweiler et al., 2006), the simulated\nweaker AABW overturing rate does not necessarily contradict the observed rise in atmospheric\nCO during the early deglaciation. The simulated piston velocity in the Southern Ocean shows an\n2\n65 increase (Extended Data Fig. 7), indicating intensified outgassing in the early deglaciation.\nAdditionally, despite the decrease of the AABW overturning rate during HS1, AABW volume\nexpands in the Atlantic, while NADW volume decreases (Extended Data Fig. 2). NADW is\ncharacterized by a high air-sea exchange signature of dissolved inorganic carbon (DIC ),\nas\nefficiently absorbing atmospheric CO , whereas AABW is associated with low DIC , indicating\n2 as\n2\nobserved increasing atmospheric CO . Therefore, future work should focus on refining our\n2\nunderstanding of the impact of AABW changes on the deglacial carbon cycle.” in line 298-312.\nFigures 1-3 depict nearly the entire deglaciation until 13 ka. However, the manuscript only\ndiscusses the transition from the LGM to HS1. I would therefore strongly suggest to also only\nshow the timeseries until 15 ka, i.e., the end of HS1. This is because it might generally be confusing\nif the BA is also shown, and more importantly, the BA was characterized by very different ocean\n80 dynamics that are not fully captured by the model simulation, leading to a poor model-data match\n(especially in Fig. 1). Even though this is an independent issue, it currently slightly undermines\nhow convincing the study appears to the general reader, and I feel this is fixed easily by just not\nshowing the BA and YD.\nThanks for the suggestion. Figures 1-3 is now revised as suggested by showing results from\n85 19ka to 14.6ka. Time series in Extended Data Figures and Supplementary Figures are also revised:\nExtended Data Fig.3, Extended Data Fig.4\n(A) NADW (black) and AABW (red) convection rates (unit: Sv) in C-iTRACE. 231Pa/230Th\n90 records from the Bermuda Rise are overlaid as a proxy for NADW overturning rate. (B)\nSimulated deep ocean (below 3000m average) radiocarbon ventilation age (unit: yr) on the\nIberian Margin (solid line) compared with radiocarbon records (dots)(L. C. Skinner et al.,\n2014, 2021). (C) Simulated deep ocean (60°W-20°E, south of 60°S, below 3000m average)\nradiocarbon ventilation age (unit: yr) in the Southern Ocean (solid line) compared with\n95 radiocarbon records (dots)(Barker et al., 2010; Luke C. Skinner et al., 2010). (D) Simulated\nSouthern Ocean surface reservoir age (unit: yr) (average over 60°W-20°E, south of 60°S,\nwhich is the AABW formation region in the model based on the winter mixed layer depth in\nSupplementary Information Fig. S2) compared with observations(L. C. Skinner et al., 2019).\n(E) Simulated ideal age of the deep Iberian Margin (blue) and deep Southern Ocean (red)\n100 (unit: yr). (F) Sum of the Southern Ocean surface reservoir age anomaly and ideal age (dash)\nanomaly compared with simulated radiocarbon ventilation age anomaly (solid) of the deep\nIberian Margin (blue) and deep Southern Ocean (red) (unit: yr). (G) Simulated AABW%\nindicated by the idealized dye tracer (Methods) in the deep Iberian Margin (blue) and deep\nSouthern Ocean (red). The gray shading indicates the HS1 (17.5ka-14.7ka).\n105\nexperiments related to the abyssal radiocarbon age mechanism. (A) The atmospheric CO\n2\n110 (Unit in ppm). (B) the radiocarbon carbon abundance in the atmospheric CO . (C) Sea ice\n2\ncoverage in the AABW formation region (60°W-20°E, south of 60°). (D) Southern Ocean\nsurface reservoir age (average over 60°W-20°E, south of 60°S) (unit: yr). (E) Deep ocean\nventilation age in the Southern Ocean (unit: yr). (F) Deep ocean ventilation age on the Iberian\nMargin. Solid black lines are C-iTRACE (unit: yr). Dash red lines are experiment Fix_CO2.\nSolid green lines are experiment Fix_circulation&pCO2. The gray shading indicates the HS1\n(17.5ka-14.7ka).\n120 (A) Average zonal wind stress (unit: dyne/cm2) over 60°S-40°S in C-iTRACE (black) and\nexperiment SO_wind (green). (B) AABW overturning rate (unit: Sv) in C-iTRACE (black)\nand experiment SO_wind (green). (C) Density flux (black), thermal density flux (blue), haline\ndensity flux (red) and brine density flux (magenta) in the AABW formation region (60°W-\n20°E, south of 60°) (unit: 10-6 kg m-2s-1). (D) Sea ice fraction in the AABW formation region.\n125 (E) Sea surface temperature in the AABW formation region (unit: °C). The gray shading\nindicates the HS1 (17.5ka-14.7ka).\nMinor/technical points\n5\nand Poeppelmeier et al. (2023, doi:10.1038/s41561-023-01140-3) who showed that 14C\nventilation ages do not reliably indicate deep ocean advection rates.\nThanks for the suggestion. Muglia & Schmittner, 2021 was cited in the previous version\nand Pöppelmeier et al., 2023 is now added in the revised manuscript. These references are cited as\n135 “Modeling studies suggest that the radiocarbon ventilation age in the deep ocean is only weakly\naffected by true ventilation determined by the deep water convection rate(Muglia & Schmittner,\n2021). Instead, it is significantly dependent on the surface reservoir age, which is defined as the\nradiocarbon age difference between the ocean surface and the contemporary atmosphere,\nparticularly in the Southern Ocean(Khatiwala et al., 2019; Muglia & Schmittner, 2021), as well\n140 as water mass mixing (Muglia & Schmittner, 2021; Pöppelmeier et al., 2023)” in line 73-77.\nL121-126: Maybe again highlight here that the radiocarbon ventilation seesaw occurs in the\nsimulation without a deep water seesaw.\nThanks for the suggestion. This is now emphasized as “Therefore, in C-iTRACE, the\n145 radiocarbon ventilation seesaw occurs without a deep water seesaw” in line 134-135.\nL138: More recent studies (Poeppelmeier et al., 2022, doi:10.1016/j.quascirev.2022.107404; Du\net al., 2020, doi:10.1016/j.quascirev.2020.106396) suggest that Nd isotope records need to be\ninterpreted more carefully, as changes in weathering also a considerable role. I therefore suggest\n150 to phrase this a bit more carefully, maybe also highlighting that Nd isotope evidence remains also\nambiguous.\nThanks for the suggestions. The uncertainty of interpreting 𝜀 records as water mass\n𝑁𝑑\nmixing is mentioned in the revised text as “These limited changes in water mass composition in\nthe abyssal ocean are supported by the lack of substantial changes in 𝜀 records during the early\n𝑁𝑑\n155 deglaciation(Howe et al., 2016; Jonkers et al., 2015; Piotrowski et al., 2012; Roberts et al., 2010;\nZhao et al., 2019), although the interpretation of 𝜀 records as water mass mixing might be\n𝑁𝑑\ncomplicated by benthic fluxes (Du et al., 2020; Pöppelmeier et al., 2022; Zhao et al., 2019)” in\nline 147-148.\nThanks for the suggestion. This is now revised as “From 19ka to 15ka, the slightly\nincreased transit time (~557 years) is overwhelmed by the decreased reservoir age (~654 years) in\nthe Southern Ocean, while the significantly increased transit time (~1274 years) overwhelms the\ndecreased reservoir age (~654 years) in the Iberian Margin, leading to the bipolar radiocarbon\n165 ventilation seesaw” in line 168-171.\nL162: AABW production rate.\nThis is now revised as “the reduced AABW overturing rate leads to an increase in transient\ntime” in line 175. “AABW overturing rate” is used here to be consistent with the rest of the\n170 manuscript.\nL177: Maybe note that the thermal bipolar seesaw effects adds to the general global warming\ninitiated during the early deglaciation, leading to further sea-ice loss.\nThis is revised as “The thermal bipolar seesaw together with the general global warming\n175 due to the increase of the greenhouse gas leads to the warming in the Southern Ocean (Fig. 3E).\nin line 190-193.\nL270: Better highlight that this only holds true if changes in water mass mixing between NADW\n180 and AABW are very small, as in the early deglaciation. During later stages of the last deglaciation,\nwhen NADW formation substantially enhanced, any change in the AABW advection strength may\nbe overwhelmed by changes in water mass mixing.\nThanks for the suggestion. We agree that this only holds when the water mass composition\nstays almost unchanged. This is now revised as “Abyssal radiocarbon age provides a qualitative\n185 way to infer changes in the AABW overturning rate during the early deglaciation when the abyssal\nwater mass composition stayed almost unchanged” in line 285-287.\nL290: But AABW has a higher DIC concentration than NADW, which would indicate more carbon\nstorage at globally higher AABW fractions. How can this be reconciled?\n(Broecker & Maier-Reimer, 1992). Here we refer to the Figure 1 from Yu et al., 2019 (Figure R1\nbelow) to illustrate the difference between DIC and DIC_as. If without air-sea exchange, the\nbiology alone will lead to the vertical gradient of DIC and PO (Figure R1b), with atmospheric\n2\n195 ocean does not necessarily mean CO transferred from the atmosphere to the ocean and it could be\n2\nresulted from carbon redistribution in the ocean. If with biology and with air-sea exchange, the\nlow DIC in the surface ocean will leads to the transfer of atmospheric CO to the ocean (Figure\n4\ninfluenced by gas exchange. Since the impact of biology on DIC can be estimated from PO4 and\nexchange of CO (Broecker & Maier-Reimer, 1992)(Figure R1d). High DICas means more\n2\natmospheric CO invasion to the ocean. For preindustrial estimation, NADW features high DICas\n2\nand AABW feature low DICas, which means NADW absorbs atmospheric CO while AABW\n2\nreleases CO . Therefore, high DIC of AABW alone does not necessarily mean more carbon storge\n2\n205 and the DICas provides more accurate information related to past changes in atmospheric CO\n2\nlevel. This is now revised as “NADW is characterized by a high air-sea exchange signature of\ndissolved inorganic carbon (DIC ), efficiently absorbing atmospheric CO , whereas AABW is\nas 2\nassociated with low DIC , indicating CO outgassing(Broecker, 1982; Yu et al., 2019). Therefore,\nas 2\nalthough AABW features high dissolved inorganic carbon which might be dominated by carbon\n210 redistribution in the ocean, the low DICas of AABW suggests that AABW is inefficient in absorbing\natmospheric CO compared with NADW(Yu et al., 2019). Consequently, the decrease in NADW\n2\nvolume and increase in AABW volume suggest a reduced atmospheric CO uptake in the deep\n2\nocean, aligning with the observed increasing atmospheric CO ” in line 304-311.\n2\n7\nFig. 2: To increase clarity, please use more descriptive axes labels, i.e., ice fraction instead of\nIFRAC, SO Reservoir age instead of SO Rage, and spell out Iberian Margin.\nrefer to page 4 in this letter for details.\nFig. 3: Units are missing in a couple of panels.\nThanks for the suggestion. The units are added in the revised Figure 3 caption as “(A)\nSO_wind (green). (B) AABW overturning rate (unit: Sv) in C-iTRACE (black) and experiment\nSO_wind (green). (C) Density flux (black), thermal density flux (blue), haline density flux (red)\nand brine density flux (magenta) in the AABW formation region (60°W-20°E, south of 60°) (unit:\n10-6 kg m-2s-1). (D) Sea ice fraction in the AABW formation region. (E) Sea surface temperature\n230 in the AABW formation region (unit: °C).”\nThis manuscript highlights the Atlantic abyssal ventilation during the last deglaciation by\n235 analyzing model results from a isotope-enabled complex ocean model which providing direct\ncomparison between the reconstruction and modeling results. Numerical simulation using such\nmodel is quite limited and experiments setup in the manuscript is smart and requires quite amount\nof work. Hence,such work will make significant contribution in studying deglaciation global\nclimate change and abrupt climate events. Manuscript conclusion is concise and could be\n240 supported by analysis. The questions that I would like to address are listed below:\nWe thank the reviewer for the positive feedback.\n1. Considering Southern Ocean stratification is primarily determined by sea ice, What kind of\nprocess decelerate the AABW in sea_ice sensitivity experiment?\nthe surface buoyancy forcing which controls the AABW overturning rate is transient. This is now\nexplained more clearly as “Experiment Fix_sea_ice: the sea ice coverage for gas exchange is fixed\nat the LGM state (Fig.2C), with transient surface fluxes and atmospheric pCO2 and ∆14𝐶\n𝑎𝑡𝑚\n(Fig.2 A and B). The sea ice effect on the buoyancy forcing is included in the transient freshwater\n250 flux so that the physical circulation such as the AABW overturning rate is the same as C-iTRACE\n(Extended Data Fig. 6). Therefore, the difference between C-iTRACE and Fix_sea_ice shows the\neffect of insulation of gas exchange by sea ice on marine radiocarbon (Supplementary Information\nTable S2)” in line 364-370.\n255 2. In extended figure 1d, seems that increased model resolution in iPOP one degree HS1\nexperiment, AABW almost reverse its sigh compare to three degree run. What are the possible\nreasons would lead to such difference considering only resolution has changed? Will the\ndifference affect the final conclusion?\nSince the “AABW overturning rate is defined as the minimum of the global overturning\n260 streamfunction between 33°S and 10°S below 2000m” (line 395-396), the minimum value in Figure\n2d is -0.4Sv, which is still negative. Therefore, the AABW cell does not reverse the sign, but is\nalmost collapsed.\nC-iTRACE is the 3-degree ocean model (iPOP2) forced by the surface forcings\nfrom Community Climate System Model version 3 (CCSM3). iTRACE is the 1-degree fully\n265 coupled simulation by Community Earth System Model (CESM) (the ocean model is also iPOP2).\nCESM is an updated version of CCSM3. The similarity between C-iTRACE (Extended Data\nFigure 1 a and b) and iTRACE (Extended Data Figure 1 c and d) is that the ocean models is the\nsame (iPOP2). The differences between C-iTRACE and iTRACE are:\n(a) ocean model resolution: 3-degree for C-iTRACE and 1-degree for iTRACE;\n270 (b) ocean alone model (C-iTRACE) and fully coupled model (iTRACE);\n(c) model versions are different: C-iTRACE is forced by CCSM3 and iTRACE is using CESM.\nTherefore, in addition to the model resolution and model configuration, the model physics are\nslightly different. This is now described as “C-iTRACE experiment is a transient ocean-alone\nsimulation forced by the surface forcings from a fully coupled transient simulation\n275 using Community Climate System Model version 3 (CCSM3;TRACE21K)(Liu et al.,\n2009)(Methods)” in line 99-101, and “This in-phase change between NADW and AABW\noverturning rates is also produced by a fully coupled transient deglacial simulation with a higher\nmodel resolution using Community Earth System Model (CESM) (iTRACE)(He et al., 2021)” in\nline 109-111.\n9\nduring the early deglaciation is also in-phase, and the reduced AABW overturning rate is also\nassociated with reduced sea ice in the Southern Ocean (Figure R2 below). Both are consistent with\nC-iTRACE results discussed in this study.\nSouthern Ocean sea ice concentration (grey) in iTRACE. This is the Fig.1e and f in Zhu et al.,\n2022.\n3. Which atmospheric forcing is included in the control experiments? The author might add a few\n290 line in method section.\nThe surface forcings for the physical ocean-alone model is now added as “A transient\nsimulation of the last deglaciation (C-iTRACE) is simulated using iPOP2(Gu et al., 2019), which\nis forced by monthly surface forcings from a fully coupled simulation TRACE21K(Liu et al., 2009).\nC-iTRACE is forced by monthly surface momentum, heat and freshwater fluxes and surface sea\n295 ice fractions in TRACE21K, with surface temperature and salinity restoring to the monthly\naverage of TRACE21K output. The restoring time scale is 30 days for temperature and 60 days\nfor salinity. C-iTRACE reproduces the physical circulation in TRACE21K(Gu et al., 2019)” in line\n317-321. The atmospheric forcings for the marine ∆14𝐶 is described as “The atmospheric pCO\n2\nand ∆14𝐶 are prescribed by reconstructed records(Joos & Spahni, 2008; Reimer et al., 2013)\n𝑎𝑡𝑚\n300 (Fig. 2A and B)” in line 338-344.\nThis manuscript presents modeling results from the C-iTRACE simulation (Gu et al., 2019; 2020;\n305 2021), generated the isotope-enabled ocean component (iPOP2) of the NCAR Community Earth\nSystem Model (CESM1), exploring a potential alternative explanation for observational data (e.g.,\nNd isotopes, Pa/Th, 14C) that suggests Antarctic Bottom Water (AABW) ventilation was\nenhanced as North Atlantic Deep Water (NADW) convection slowed early in the last glaciation.\nThe authors postulate that simultaneous decreases in surface ocean reservoir age (e.g., the offset\n310 between the surface ocean and the atmosphere) occurred in both the Antarctic and high-latitude\nNorth Atlantic during Heinrich Stadial 1 (HS1). The resulting reduced surface ocean reservoir age\nat the point of bottom water formation decreases the radiocarbon reservoir ages, and apparent\nventilation age, in both AABW and NADW – even as deep convection slowed in both hemispheres\nin the C-iTRACE simulation. In the case of NADW, the authors postulate that the decrease in\n315 preformed radiocarbon reservoir age is overwhelmed by the increased transit time associated with\nthe reduced convection in the North Atlantic, but dominates the apparent ventilation age in the\nSouthern Ocean, leading to an apparent ‘bipolar radiocarbon ventilation seesaw’ in spite of\ntrue ventilation being reduced in both hemispheres.\nWe thank the reviewer for the summary.\nweakening of AABW formation during HS1, are distinct from simulations using LOVECLIM and\nthe UVIC Earth System Climate Model (i.e., Menviel et al., 2015). In understanding the potential\nsignificance of the C-iTRACE results, I believe it would strengthen the paper if the authors could\n325 elaborate on why these model predictions regarding the existence (or not) of a bipolar ventilation\nseesaw in early deglaciation are so distinct? The authors offer the favorable agreement of the C-\niTRACE simulation with deglacial North Atlantic proxy records as reason for confidence in these\nmodel results (e.g., lines 101-105; 216-219; 312-313). This is not a criticism of the novel ideas\nadvanced in the paper under review, but it is worth noting that iTRACE relies on significant, and\n330 to date unattributed, ‘pre-hosing’ to weaken AMOC prior to the Laurentide freshwater release of\nHeinrich Stadial 1 (HS1) to achieve the AMOC collapse that is in agreement with deglacial proxy\nrecords – so agreement with the proxy records is not an objectively independent outcome of\nprocesses captured by the CESM1, rather the simulation is forced to that agreement by design.\nThanks for the suggestion of comparing the CESM results with the LOVECLIM and UVIC\n335 models. Below we discuss the opposite AABW responses in CESM (this study) and in\nLOVECLIM and UVIC models (Menviel et al., 2015) are caused by different forcing method and\nAABW responses are consistent in different models if similar forcings are applied.\nIn CESM, AABW is reduced during Heinrich Stadials while in Menviel et al., (2015),\nAABW is increased in both LOVECLIM and UVIC models. In Menviel et al., (2015), negative\n340 freshwater forcing are applied to the Southern Ocean during Heinrich Stadials in LOVECLIM and\nUVIC models (Figure R3a and Figure R4a below). This negative freshwater forcing leads to\nincrease of surface salinity, and in turn the increase of AABW (Figure R3b and Figure R4b below).\nIn CESM, there is no artificial freshwater forcing applied in the Southern Ocean and the surface\nforcings are generated by the coupled model itself. Due to the thermal bipolar seesaw, the warming\n345 in the Antarctica leads to the decrease in the Southern Ocean sea ice (Figure 3), which is equivalent\nto positive freshwater forcing in CESM that is opposite to Menviel et al., (2015). The freshening\nof the Southern Ocean leads to the reduced AABW. Therefore, the simulated opposite AABW\nresponses in CESM and LOVECLIM/UVIC is caused by different forcings applied. This is now\nsummarized as “However, a stronger AABW is simulated during Heinrich Stadials by applying\n11\n350 negative freshwater fluxes to the Southern Ocean in LOVECLIM and UVIC models (Menviel et\nal., 2015; Menviel et al., 2018). This apparent opposite AABW responses in different numerical\nexperiments can be reconciled by opposite surface buoyancy forcings and UVIC model will also\nproduce a weakened AABW if similar surface forcing is applied (Buizert & Schmittner, 2015) as\nin C-iTRACE (Materials and Methods)” in line 111-116 and discussed in detail as “In C-iTRACE\n355 and iTRACE experiments by CESM, the simulated AABW overturning rate is weakened during\nHeinrich Stadial. However, other modeling studies suggest enhanced AABW overturning rate\nduring Heinrich Stadial in LOVECLIM and UVIC models(Menviel et al., 2015; Menviel et al.,\n2018). These opposite AABW responses is caused by different surface buoyancy forcings in\ndifferent studies. In reference 10 and 12, a negative freshwater forcing is applied to the Southern\nC-iTRACE and iTRACE simulated by CESM, no additional freshwater forcing is added to the\nSouthern Ocean, and the surface buoyancy flux is dominated by the reduced sea ice in response to\nthe Southern Ocean warming. Therefore, in CESM, warming in the Southern Ocean leads to\nreduced AABW overturning rate during Heinrich Stadial. If similar warming is applied in UVIC\n365 model, AABW will also show a decrease(Buizert & Schmittner, 2015), suggesting a robust\nweakening AABW response to the Southern Ocean warming” in line 390-401.\nHowever, if there is no artificial freshwater forcing added to the Southern Ocean in UVIC\nmodel, the warming in the Southern Ocean will also lead to the decrease of AABW (Figure R5\nbelow)(Buizert & Schmittner, 2015), which is the same as in CESM. Therefore, the reduced\nmodels. This is now emphasized as “The reduced AABW overturning rate resulting from warming\nin the surface Southern Ocean in C-iTRACE is also a robust feature in other climate models\n(Buizert & Schmittner, 2015; Snow et al., 2016) and aligns with observed and projected AABW\nweakening in the past and upcoming decades (Azaneu et al., 2013; Li et al., 2023)” in line 250-\n375 253.\nFigure R3. The Southern Ocean freshwater flux (a), AMOC (grey) and AABW (pink and blue) (b),\nand Antarctica temperature simulated by LOVECLIM. This is the Figure 2 in Menviel et al., (2015).\n12\nand Antarctica temperature simulated by UVIC. This is the Figure 3 in Menviel et al., (2015).\nFigure R5. The reduced AABW response to the warming of the Southern Ocean (yellow, orange\n385 and red curves) in UVIC model. This is the Figure 6e in Buizert & Schmittner, (2015).\nSince C-iTRACE is forced by monthly surface forcings from TRACE21K, we discusses\nthe freshwater forcings in TRACE21K (Liu et al., 2009). It is true that freshwater is added to the\nNorth Atlantic and Gulf of Mexico to weaken the AMOC in TRACE21K. However, the freshwater\n390 is added to mimic the melt water from the land ice sheet since there is no active land ice sheet\nmodule in the model. Especially, the magnitude and timing of this freshwater flux is based on the\nreconstruction of sea level (Peltier, 2004)(Figure R6 B below). With this freshwater forcing, the\nsimulated AMOC is in good agreement with 231Pa/230Th (Figure R6D), suggesting that the ocean\ncirculation is more or less realistic during the early deglaciation and laying a foundation for good\n395 model-data comparison in this study. In the revised manuscript, we describe the forcings in\nTRACE21K as “TRACE21K is a transient simulation under realistic insolation, greenhouse gas,\ncontinental ice sheet, and meltwater flux using CCSM3, which captures major climate changes\nduring the last deglaciation (Liu et al., 2009). Since the land ice sheet is prescribed in TRACE21K,\nthe timing and magnitude of the meltwater flux from the land ice sheet is based on the\n400 reconstruction of sea level (Peltier, 2004). With this freshwater added to the North Atlantic and\nGulf of Mexico, the simulated AMOC in TRACE21K is in good agreement with 231Pa/230Th records\n(McManus et al., 2004). There is no freshwater flux added to the Southern Ocean during the early\ndeglaciation.” in line 332-338.\n13\nreconstructed sea level is the grey curve in B and the estimated sea level change due to the\nfreshwater forcing in TRACE21K is the red curve in B. C shows the freshwater flux applied to the\nNorth Atlantic and Gulf of Mexico (red) in TRACE21K. D is the NADW overturning rate in\nTRACE21K (red).\nradiocarbon ventilation age changes between the LGM and HS1 ‘within data uncertainties’ (Line\n120), the simulation results presented in Figure 1 seem both qualitatively and quantitatively\ndistinct from the observational data. In the case of the Benthic – Atmosphere (B-A) 14C age\noffsets for the Iberian Margin (IM, the model results suggest little change in the deep ocean through early\ndeglaciation into HS1, contrasting with the increase observed in the data, and are overall\n**greater** than observations. In the case of the Southern Ocean (SO) model-data comparison,\nthe model results do show a decrease in 14C age B-A offsets through HS1 – almost imperceptible\nin magnitude relative to the proxy record. However in the SO case the modeled offsets are\n420 **lower** than observations during early deglaciation, becoming greater at the end of the\nsimulation largely because the model results are invariant while the proxy offset decreases by over\na thousand years.\nSo, to recap: the C-iTRACE SO B-A offset is smaller than supported by available data in early\ndeglaciation, while the IM is over 1000 years too large. By the end of the HS1, both the IM and\nstatistical change at all until the very end of the experiment associated with the Bolling-\nAllerod. Why is there an asymmetry in the bias in the model – and is it possible the experiment is fighting\nthe bias? Did the authors explore a control case with the non-freshwater forced iTrace? It would\nbe interesting to know if these results are associated with the freshwater forcing itself, or\n430 potentially a bias in the start-up condition?\nWe agree that the simulated radiocarbon ventilation age in the Iberian Margin is\nsystematically “higher” than the reconstruction (MD99-2334, SHAK03-6K, SHAK05-3K) (Figure\n1B). However, we cannot conclude that the simulated Southern Ocean radiocarbon age is\nsystematically “lower” than the reconstructions as the reviewer stated because data-data offset is\nmuch larger than the model-data offset. From Table R1 below, three available abyssal Southern\nthe data-data offset is ~1000years. From Figure 1C and Extended Data Figure 3B, the model-data\noffset mainly comes from the mismatch for core MD07-3076 (offset ~400 years), and the model\nvalues overall agree with the other two cores: TNO57-21 and MD97-2106. Therefore, it is unclear\nif the model values in the abyssal Southern Ocean is “lower” than reconstructions. The higher\nmodel values than reconstructions for the Iberian Margin might imply that AABW overturning\nduring the LGM (the start-up condition as the reviewer suggested) was weaker than reality, which\nleads to longer transit time and in turn higher radiocarbon ventilation age downstream in the North\nAtlantic in the simulation. This is now discussed as “although the magnitude of the simulated\n445 radiocarbon ventilation ages seems to have a systematic bias especially for the Iberian Margin\n(Fig. 1B). The higher radiocarbon ventilation age in the Iberian Margin might imply that AABW\noverturning was not strong enough during the LGM in C-iTRACE” in line 126-129.\nMD07-3076(Luke C. Skinner et TNO57-21(Barker et al., 2010) MD97-2106(Dai et al., 2021)\nal., 2010) (41°S, 8°E, 4981m) (45°S, 146°E, 3310m)\n(44°S,14.2°W, 3770m)\n3800 2449 1625\nTable R1. LGM (22ka-19ka) radiocarbon ventilation ages in the abyssal Southern Ocean\n450 reconstructions.\nHowever, in this study, we focus on the relative changes in each site and the good model-\ndata agreement lies in the “changes”, which is stated as “in agreement with the available compiled\nobservational radiocarbon ventilation age changes between LGM and HS1” in line 123-124. The\n455 reconstructions show robust opposite changes in Iberian Margin and Southern Ocean from LGM\nto HS1, although the magnitude of changes varies a lot among different sites, especially the\nSouthern Ocean sites (Table R2 below). The opposite signs have been correctly captured in the\nsimulation, which is stated as “Nevertheless, the bipolar radiocarbon ventilation seesaw is well\nsimulated in C-iTRACE, with the radiocarbon ventilation age exhibiting opposite changes between\n460 the abyssal Iberian Margin in the North Atlantic and the Southern Ocean: below 3km, the\nsimulated B-Atm age increases by 141 years in the Iberian Margin but decreases by 329 years in\nthe Southern Ocean from the LGM to HS1, roughly capturing the observational changes (Fig. 1 B\nand C, Extended Data Figs.2 and 4)” in line 129-134. The two Southern Ocean sites show a\ndifference larger than one order in the magnitude of the radiocarbon ventilation age decrease\n465 (Table R2 below), hindering a quantitative assessment of the model-data agreement.\nFurthermore, the qualitative increase in the SO reservoir age and opposite changes in the\nabyssal Iberian Margin and Southern Ocean suggest a reduced AABW overturning rate during the\nearly deglaciation, regardless of the quantitative magnitude of the changes. In this study, model\nserves as a tool to explore the possible explanations for the observed radiocarbon changes by\n470 carrying out different sensitivity experiments. This is discussed in the manuscript as “The iPOP2\nis used as a tool to explore the mechanism of the radiocarbon and AABW changes in this study\nand several sensitivity experiments are carried out to diagnose the contribution of pCO2, ∆14𝐶 ,\n𝑎𝑡𝑚\nand circulation to radiocarbon ventilation age” in line 347-349 and “If there is no significant\ndecrease in the Southern Ocean reservoir age, as in the sensitivity experiment Fix_CO2 (Methods,\nturn, increase in the radiocarbon ventilation age in both abyssal Southern Ocean and North\nAtlantic (Fig. 2E and F red). If AABW production rate remains the same, as in the sensitivity\nexperiment Fix_circulation (Methods) (Extended Data Fig. 6B blue), the reduced reservoir age in\n15\nthe Southern Ocean (Fig. 2D blue) will propagate to the abyssal ocean, leading to a reduced\n480 radiocarbon ventilation age in both the Iberian Margin and the Southern Ocean (Fig. 2E and F\nblue). Therefore, the observed reduced Southern Ocean reservoir age (Skinner et al., 2019) and\nthe bipolar radiocarbon ventilation seesaw in the abyssal ocean ( Skinner et al., 2014, 2021) imply\nan increased transient time in the Iberian Margin and, in turn, a weaker AABW overturning rate\nduring the early deglaciation” in line 173-183. Therefore, the conclusion in this study does not\n485 depend on the magnitude of the abyssal radiocarbon ventilation age changes and will not be\naffected by the bias in the simulated magnitude of the radiocarbon ventilation age in the Iberian\nMargin.\nIberian Margin MD99-2334 SHAK03-6K SHAK05-3K average\nObservation 227 486 751 488\nModel 141\nSouthern Ocean MD07-3076 TNO57-21 average\nObservation -1401 -58 -729.5\nModel -329\nTable R2. The radiocarbon ventilation age changes from the LGM (22ka-19ka) to HS1 (17ka-15ka)\n490 in reconstructions and our model.\nFor the question “Did the authors explore a control case with the non-freshwater forced\niTrace?”, the sensitivity experiment Fix-circulation is an experiment with no freshwater forcing as\nthe surface buoyancy flux is fixed at LGM. As described page 13 in this letter, freshwater flux is\n495 added to mimic the melt of land ice sheet. If there is no freshwater added to the North Atlantic,\nthe NADW overturning rate will stay unchanged, which does agree with the 231Pa/230Th records\n(McManus et al., 2004) and our current knowledge about the Atlantic circulation during HS1. In\nFix-circulation, NADW and AABW overturning rates stay at the LGM values (Extended Data\nFigure 6A), and the radiocarbon ventilation ages in the abyssal Iberian Margin and Southern Ocean\n500 show decreases (Figure 2 E and F), which does not agree with the observed opposite changes\nbetween two hemispheres. This is discussed as “If AABW overturning rate remains the same, as\nin the sensitivity experiment Fix_circulation (Methods) (Extended Data Fig. 6B blue), the reduced\nreservoir age in the Southern Ocean (Fig. 2D blue) will propagate to the abyssal ocean, leading\nto a reduced radiocarbon ventilation age in both the Iberian Margin and the Southern Ocean (Fig.\n505 2E and F blue)” in line 177-180.\nSo to summarize, revisions to address this model-data mismatch are:\n(1) We acknowledge that the simulated radiocarbon ventilation age in the Iberian Margin is higher\nthan reconstructions, possibly reflecting that the AABW overturing rate in the model is too weak\nat the LGM. We show that due to the large variability in the reconstructed radiocarbon ventilation\n510 ages in the Southern Ocean, the simulated radiocarbon ventilation age in the Southern Ocean\noverall agrees with reconstructions.\n(2) We clarify that the good model-data agreement in this study refers to the relative changes from\nthe LGM to HS1: model captures the reduced surface reservoir age decrease in the Southern Ocean\nand the opposite changes of radiocarbon ventilation age between Iberian Margin and Southern\n(3) We clarify that the model is a tool to explore different mechanism and the sensitivity\nexperiments reveal that the reduced surface reservoir age decrease in the Southern Ocean and the\nopposite changes of radiocarbon ventilation age between Iberian Margin and Southern Ocean\n16\nduring the early deglaciation infers a reduced AABW overturning rate. Therefore, the conclusion\n520 of this study is not affected by the “higher” bias in the simulated radiocarbon ventilation age.\nThe focus of this manuscript is on interpretation of the interior ocean radiocarbon record in the\nlate deglaciation, and the authors are advancing a rational alternative explanation for a confluence\nof factors that could drive an apparent decrease in AABW 14C age relative to the atmosphere\n525 while ventilation of the deep ocean in both the North Atlantic and Southern Ocean slows. While\nas outlined above I think there could be a more robust exploration of the offsets between the\nobservational record and the C-iTRACE simulation, I don’t think that is a deal breaker for this\nhypothesis, and if correct it would indeed shift a paradigm. The authors do advance a credible\nhypothesis to explain a potential fit between the model results and evidence for enhanced Southern\nthe C-iTRACE results are in agreement with eNd records that also indicate little change in AABW\ncirculation through HS1 (e.g. line 138) I’m not actually sure that is a fair statement. It’s certainly\nnuanced; several of the eNd records cited do show limited change during HS1 relative to the large\nswing at the B-A, but the deepest North Atlantic records (e.g. core KNR197/10GGC-17) shows a\n535 decrease relative to the LGM, while deep Southern Ocean records (e.g. RC11-83) shows an\nincrease (Poppelmeier et al., 2018, included in the compilation of Zhao et al., 2018, referenced in\nthis manuscript; Piotrowski et al., 2012). Other Southern Ocean eNd records in the Pacific sector\nalso show significant changes in eNd that seemingly evidence changes in AABW formation (e.g.,\nBasak et al., 2018; Du et al., 2018, referenced in this manuscript). These published observations\n540 have been interpreted as strengthened AABW formation, in contrast to the weakening of AABW\ninferred from HS1 through C-iTRACE – do the authors have an alternative hypothesis to explain\nthese data and how they would fit in the proposed framework? Although the manuscript centrally\nhighlights the nuances of radiocarbon as a circulation tracer in the interior ocean, it seems as\nthough the C-iTRACE results are more in alignment with the published interpretation of the global\n545 synthesis of LGM radiocarbon records of Rafter et al., 2022 cited in line 88, as well as the deep\nabyssal Indo-Pacific reconstructions cited in Line 275.\nFrom 20ka to 15ka, 𝜀 records from the deep North Atlantic and Southern Ocean do not\n𝑁𝑑\nvary much as shown in Figure R7 below, including the core KNR197/10GGC-17 and RC11-83\nmentioned by the reviewer. KNR197/10GGC-17 shows a decrease and RC11-83 shows an increase\n550 after ~15ka, consistent with the increase of AMOC (Figure R7a). We clarify this in the revised\ntext as “These limited changes in water mass composition in the abyssal ocean might be supported\nby the lack of substantial changes in 𝜀 records during the early deglaciation from 20ka to 15ka\n𝑁𝑑\n(Howe et al., 2016; Jonkers et al., 2015; Piotrowski et al., 2012; Roberts et al., 2010; Zhao et al.,\n2019)” in line 145-147.\n𝑁𝑑\nis complicated by other process such as sedimentary influences, this is also acknowledged as\n“although the interpretation of 𝜀 records as water mass mixing might be complicated by benthic\n𝑁𝑑\nfluxes (Du et al., 2020; Pöppelmeier et al., 2022; Zhao et al., 2019)” in line 147-148.\n17\n𝑁𝑑\nal., 2019).\nIn Rafter et al., (2022), the compiled data also show increased radiocarbon ventilation age\nin the deep Pacific, but the authors did not relate this to the changes of AABW overturning rate.\nexclusive to the North Atlantic. Recent reconstructions from the abyssal Indian Ocean (Bharti et\nal., 2022; Nisha et al., 2023) and North Pacific (Rafter et al., 2022; Stott et al., 2021) also exhibit\nan increased radiocarbon ventilation age during the early deglaciation” in line 292. In Rafter et\nal., (2022), the increased radiocarbon ventilation age is attributed to the reduction of NADW.\nradiocarbon change in the abyssal North Atlantic can be mainly explained by processes in the\nSouthern Ocean. This is discussed as “With the abyssal ocean dominated by AABW during the\nearly deglaciation, the radiocarbon changes in the abyssal ocean is mainly associated with\nchanges in AABW instead of NADW as suggested in previous studies (Rafter et al., 2022; Skinner\n575 et al., 2014)” in line 148-151.\nIn summary, this C-iTRACE simulation results presented in this manuscript are both interesting\nand informative, demonstrating that changes in surface ocean reservoir age at the time of\ndeepwater formation can have an outsized impact on the apparent/radiocarbon reservoir age of the\n580 interior ocean that is decoupled from ocean circulation. While this observation in and of itself isn’t\nnew sensu stricto (e.g., Muglia and Schmittner, 2021), the potential application of this concept to\ninterpretation of the paradox set up between the C-iTRACE results (suggesting simultaneous\ndecreases in deep water formation in both the high latitude North Atlantic and Southern Ocean)\n18\nand radiocarbon evidence in the deep Atlantic and Southern Ocean (suggesting a ‘bipolar\n585 radiocarbon ventilation seesaw’) certainly is. My confidence in these interpretations would be\nstrengthened if the authors could offer an explanation for why the C-iTRACE results are different\nthan those of the LOVECLIM and UVIC models (e.g., Menviel et al., 2015). I also have concerns about the asymmetric\nbias in the SO versus IM data/model comparison of radiocarbon offsets between the interior ocean and atmosphere.\nand in LOVECLIM and UVIC models in Menviel et al., (2015) and show that the reduced AABW\noverturning rate is a robust response to the Southern Ocean surface warming in both CESM and\nUVIC model. Please refer to page 11-13 in this letter for details.\nWe also discuss the model-data offset and show that this offset will not affect the\n595 conclusion in this study. Please refer to pages 14-17 in this letter for details.\nMinor comments to Author:\nFig 1 – I think part F of the caption needs a correction. Would suggest adding units to the sub-\nplots or caption and define the gray bar in the caption.\nanomaly and ideal age (dash) anomaly compared with simulated radiocarbon ventilation age\nanomaly (solid) of the deep Iberian Margin (blue) and deep Southern Ocean (red) (unit: yr)” in\nline 625-628.\nUnits are added in the y-axis labels as well as in captions.\n629-630. Please refer to page 3 in this letter for revised Fig.1 and caption.\nFig 2 – This is a hugely helpful figure in supporting your interpretations. This is a minor point, but\nit would be somewhat helpful if the caption was in the same order as the experiments are explained\n610 in the methods. As for figure 1, there are no units on any of the plots. The gray bar denoting HS1\nis not defined in the caption.\nThe experiments in Fig.2A legends and Fig.2 captions are revised in the same order as\ndiscussed in the manuscript. The units are added in the y-axis labels and in the captions. The gray\nbar is defined as “The gray shading indicates the HS1 (17.5ka-14.7ka)” in caption. Please refer to\n615 page 4 in this letter for revised Fig.2 and caption.\nFig 3 – Again, units…\nThe units are added in the caption as: “Fig. 3 | Time series related to the mechanism of\ndeglacial AABW overturning rate change. (A) Average zonal wind stress (unit: dyne/cm2) over\n620 60°S-40°S in C-iTRACE (black) and experiment SO_wind (green). (B) AABW overturning rate\n(unit: Sv) in C-iTRACE (black) and experiment SO_wind (green). (C) Density flux (black),\nthermal density flux (blue), haline density flux (red) and brine density flux (magenta) in the AABW\nformation region (60°W-20°E, south of 60°) (unit: 10-6 kg m-2s-1). (D) Sea ice fraction in the\nAABW formation region. (E) Sea surface temperature in the AABW formation region (unit: °C).”\n625 in line 644-651.\nExtended Data Figure 2 – This figure is a bit misleading. When presented together the SO and IB\nmodel/data comparison does seem to fall along a 1-1 ratio, albeit with significant scatter. However\nthis hides the asymmetry of the offsets in the ocean basins. What does this figure look like if you\n630 denote the SO data/model comparison points from the IB data/model comparison points via\n19\ndifferent symbols, and can you comment on the origin of the inter-hemispheric asymmetric bias that\nseems obvious in the time series?\nThanks for the suggestion. This figure is revised as suggested, with different symbols\nrepresenting North Atlantic (filled circles), Southern Ocean (asterisks), and regions in between\n635 (empty squares). Gray shadings are added to visualize if the model values are in the same sign as\nthe observational values since the sign of change is the key of model-data comparison as discussed\nin Page 15-16. From the revised figure, positive radiocarbon ventilation changes in the North\nAtlantic and negative radiocarbon ventilation changes in the Southern Ocean are clearly shown in\nboth observation and model. This is now added as “the bipolar radiocarbon ventilation seesaw is\n640 well simulated in C-iTRACE, with the radiocarbon ventilation age exhibiting opposite changes\nbetween the abyssal Iberian Margin in the North Atlantic and the Southern Ocean: below 3km, the\nsimulated B-Atm age increases by 141 years in the Iberian Margin but decreases by 329 years in\nthe Southern Ocean from LGM to HS1, roughly capturing the observational changes (Fig. 1 B\nand C, Extended Data Figs.2 and 4)” in line 129-134.\nbetween HS1 and LGM in the Atlantic and Southern Ocean (unit: yr). The colors of the dots\nindicate the depth of the observational sites. Sites north of 30°N are marked by filled circles. Sites\nsouth of 30°S are marked by asterisks. Sites from 30°S to 30°N are marked by empty squares. The\n650 observational information is listed in Supplementary Information Table S1. Gray shading\nindicates that the model values are in the same sign as observational values.\nFor the “the inter-hemispheric asymmetric bias”, we agree that the simulated values are\nlarger than reconstructions in the Iberian Margin, but due to the large variability in the Southern\nsmaller than the reconstructions. The higher radiocarbon ventilation age in the simulated Iberian\nMargin is probably related to weak AABW overturning rate at LGM in the model. And we also\ndiscuss that the opposite signs of the radiocarbon ventilation age changes between the abyssal\nSouthern Ocean and North Atlantic is the key to draw the conclusion that AABW overturning rate\n20\n660 is weakened during the early deglaciation, and this feature is well simulated in the model. Please\nrefer to pages 14-17 in this letter for details.\nExtended Data Figure 3 – would refer to reconstruction symbols as ‘points’ as opposed to ‘scatters’.\nThis is now revised to “high-resolution reconstruction (points with error bars)” in line 665.\n21\nAzaneu, M., Kerr, R., Mata, M. M., & Garcia, C. A. E. (2013). Trends in the deep Southern\nOcean (1958-2010): Implications for Antarctic Bottom Water properties and volume export.\nJournal of Geophysical Research: Oceans, 118(9), 4213–4227.\nhttps://doi.org/10.1002/jgrc.20303\ndeepening of the Atlantic overturning circulation during deglaciation. Nature Geoscience,\n3(8), 567–571. https://doi.org/10.1038/NGEO921\nBharti, N., Bhushan, R., Skinner, L., Muruganantham, M., Jena, P. S., Dabhi, A., & Shivam, A.\n(2022). 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Marine Reservoir Age Variability\nOver the Last Deglaciation: Implications for Marine Carbon Cycling and Prospects for\n760 1807–1815. https://doi.org/10.1029/2019PA003667\nSkinner, L. C., Freeman, E., Hodell, D., Waelbroeck, C., Vazquez Riveiros, N., & Scrivner, A.\nE. (2021). Atlantic Ocean Ventilation Changes Across the Last Deglaciation and Their\nCarbon Cycle Implications. Paleoceanography and Paleoclimatology, 36(2), 1–21.\nhttps://doi.org/10.1029/2020PA004074\nDeep Southern Ocean and Deglacial CO2 Rise. Science, (May), 1147–1151.\nSnow, K., Hogg, A. M., Sloyan, B. M., & Downes, S. M. (2016). Sensitivity of Antarctic Bottom\nWater to changes in surface buoyancy fluxes. Journal of Climate, 29(1), 313–330.\nhttps://doi.org/10.1175/JCLI-D-15-0467.1\nRespiration and Ventilation Change Hypothesis as a Mechanism for Changing Atmospheric\nCO2. Geophysical Research Letters, 48(3). https://doi.org/10.1029/2020GL091296\nToggweiler, J. R., Russell, J. L., & Carson, S. R. (2006). Midlatitude westerlies, atmospheric\nCO2, and climate change during the ice ages. Paleoceanography, 21(2), 1–15.\n775 https://doi.org/10.1029/2005PA001154\nYu, J., Menviel, L., Jin, Z. D., Thornalley, D. J. R., Foster, G. L., Rohling, E. J., et al. (2019).\nMore efficient North Atlantic carbon pump during the Last Glacial Maximum. Nature\nCommunications, 10(1), 2170. https://doi.org/10.1038/s41467-019-10028-z\nZhao, N., Oppo, D. W., Huang, K. F., Howe, J. N. W., Blusztajn, J., & Keigwin, L. D. (2019).\nGlacial–interglacial Nd isotope variability of North Atlantic Deep Water modulated by\nhttps://doi.org/10.1038/s41467-019-13707-z\nZhu, C., Zhang, J., Liu, Z., Otto-Bliesner, B. L., He, C., Brady, E. C., et al. (2022). Antarctic\nWarming during Heinrich Stadial 1 in a Transient Isotope-Enabled Deglacial Simulation.\nJournal of Climate, 35(22), 3753–3765. https://doi.org/10.1175/JCLI-D-22-0094.1\nWe are grateful for your time and valuable feedback. We have made a major revision to\nthe paper, addressing all the comments raised by the reviewers. The original reviewers’ comments\n5 are in red, our replies are in black, and the revised/new texts in the manuscript are highlighted in\nblue.\nThe major revisions include:\n(1) we add more details on the freshwater forcing scheme to alleviate the misunderstanding\nby Reviewer #3 because his/her comments neglect the large uncertainty in the sea level\n10 reconstructions and the location of the freshwater flux cannot be firmly constrained by\nreconstructions, which are crucial for deglacial simulations and are established in the\nliterature (details in Pages 16-17 in this letter). We also add a set of sensitivity experiments to\nshow a robust weakening of AABW no matter of the magnitude of freshwater forcing in the\nNorth Atlantic, further supporting that the comparison of sea level between models and\n15 reconstructions will not affect our main conclusion (details in Pages 18-19 in this letter). In the\nrevision, we acknowledge slightly larger amounts of freshwater in our model than those based on\nsea level estimations and show that our conclusion is robust regardless of the amounts of\nfreshwater forcing imposed.\n(2) we clarify the meaning of word “ventilation” because Reviewer #3 seems to mix up\n20 “AABW overturning rate” and “radiocarbon ventilation”, and we also clarify that the AABW\noverturning rate and AABW volume are different. With these clarifications, we show that our\nresults agree with diverse lines of evidence about the deglacial changes in AABW from the\nliterature, instead of being contradictory to evidence as suggested by Reviewer #3 (details in\nPages 10-11 in this letter);\n25 (3) we tone down our expressions as suggested by the editor by avoiding the usage of\n“realistic” and by revising “model captures changes in reconstructions” to “model captures the\nfirst-order changes in reconstructions”. We also emphasize that a model can never be exactly the\nsame as the reality, but a model can serve as a tool to understand the mechanisms of past changes\nin ocean circulation and proxy records.\nchallenging to accept the idea of reduced AABW overturning rate parallel to NADW weakening\nduring the early deglaciation. The prevailing paradigm is when NADW gets weaker, AABW gets\nstronger/the deep water production seesaw (Broecker, 1998; Piotrowski et al., 2008). However,\n35 unlike the NADW overturning, which can be reconstructed by North Atlantic 231Pa/230Th records\n(McManus et al., 2004; Ng et al., 2018), direct evidence for the history of AABW overturning\nrate remains scarce. While Reviewer #3 suggests proxies such as 𝜀 and oxygen provides\n\"#\ninformation for past AABW changes, these proxies either infer past volume of AABW instead\nof AABW overturning rate or are affected by processes other than overturing rate and have their\n40 own complications (details in Pages 10-11 in this letter). Furthermore, in our simulation, AABW\nalso shows a volume expansion in agreement with other proxy records mentioned by\nReviewer #3. We clarify that the AABW overturing rate and the AABW volume are two aspects\nof AABW changes and there is no clear relationship between these two aspects of AABW.\nFreshwater input in the North Atlantic -> reduced NADW overturning -> warming in the Southern\nOcean/retreat of sea ice around Antarctic -> reduced surface density in surface Southern Ocean\n1\n-> reduced AABW overturing rate -> increased AABW transit time -> increased deep-water\nradiocarbon ventilation ages on the Iberian Margin.\nor supported by reconstructions and model simulations. Therefore, we argue that a reduced AABW\nparallel to weakened NADW during the early deglaciation is a plausible scenario. Our study\npresents a motivation for future studies to investigate the evolution of AABW, which is\nimportant but also challenging to understand past ocean circulation changes.\n55\nThe authors have thoroughly revised the manuscript based on the reviewers comments. They have\nconvincingly answered all questions and comments raised, which has improved the manuscript\nsignificantly. I particularly appreciate the revised figures, which now only show the LGM to HS1\n60 transition, aligning now more closely to the discussion in the main text. Some minor improvements\nto the wording at some places could still be done. I therefore suggest accepting the paper, after the\nauthors have clarified some minor points that I have left, listed below.\nWe thank the reviewer for the comments and suggestions. We have revised the manuscript\nas suggested.\nin the TRACE21K simulation was derived from sea-level reconstructions. The good agreement of\nthe responding AMOC to this freshwater forcing with the North Atlantic Pa/Th record is then a\ncoincidence according to the authors. I am slightly surprised by this statement, since as far as I\n70 understand the setup of the simulation, the freshwater forcing was more of an ad hoc timeseries,\nthat is not directly constrained by the evolution of the continental ice volume during the last\ndeglaciation. The authors should clarify this.\nWe apologize that we did not explain this clearly in our previous reply. The amount of\nfreshwater added to the ocean is constrained by the sea level reconstruction, but we don’t know\n75 where to put the meltwater because this is not firmly constrained by reconstructions, presenting\nthe most challenging task to simulate the climate change during the last deglaciation (He, 2011).\nTo tackle this challenge, sensitivity experiments with the same magnitude of freshwater flux\nderived roughly from reconstructions but applied to different locations were carried out to test\nwhere to put the melt water in He (2011). The one with better agreement between simulated and\n80 reconstructed AMOC and Greenland temperature was chosen to form the final freshwater scheme\nin TRACE21K. Detailed discussion on the freshwater flux in TRACE21K can be found in Pages\n13-18 in this letter. The revised text reads as “TRACE21K is a transient simulation under\nreconstructed insolation, greenhouse gas, continental ice sheet using CCSM3, and the magnitude\nand location of the meltwater flux in TRACE21K is developed under the constraint of\n85 reconstructed sea level, Greenland temperature and AMOC(He, 2011). Although the meltwater\nscheme in TRACE21K can be further improved to reach better agreement with the recent sea level\nestimate,TRACE21K captures major climate changes during the last deglaciation, presenting a\nuseful tool for paleoclimate research(He et al., 2013; Liu et al., 2009)” in line 354-359.\ntitle?\nThanks for the suggestion. The title is now revised as “Reduced Antarctic Bottom Water\noverturning rate during the early last deglaciation inferred from radiocarbon records and a\ntransient simulation”.\nabyssal ventilation” to “Antarctic Bottom Water overturning rate” to avoid misunderstanding,\nbecause:\n(1) the Reviewer #3 seems to have misunderstanding of the word “ventilation”, which is meant to\nbe the water mass transit time from the surface to the ocean interior controlled by deep water\n100 overturning rates.\n(2) people may think “reduced Atlantic abyssal ventilation” refers to reduced NADW overturning\nduring the early deglaciation (McManus et al., 2004), which is not new and is not the focus of this\n3\nmanuscript. This manuscript is about the AABW overturning history during the last deglaciation,\nand infers a reduced AABW overturning rate which is novel.\nThis sentence is now revised to “The AABW transit time increase is larger than the\nSouthern Ocean surface reservoir age decrease in the abyssal North Atlantic but not in the abyssal\nSouthern Ocean, resulting in the radiocarbon ventilation seesaw” in line 43-46.\nThis sentence is now revised to “A key evidence that is used to supports this enhanced\nAABW overturning hypothesis lies in deglacial reductions in radiocarbon ventilation ages\nreconstructed from marine sediments” in line 61-62.\nthere are biases in the absolute ventilation ages.\nThis sentence is now revised to “we utilize an isotope-enabled transient ocean simulation\nthat effectively reproduces the overall trends in the reconstructed radiocarbon records” in line 93.\nThis is now revised to “C-iTRACE is a transient ocean-only simulation forced by...” in line\n101.\nThanks for the suggestion. The comparison to LOVECLIM simulation/Menviel et al., 2018\nis moved to section “Buoyancy-driven reduction in AABW overturning rate” and discussed as\n“Although stronger AABW overturning rate are simulated during Heinrich Stadials in previous\nstudies using LOVECLIM and UVIC models( Menviel et al., 2015; Menviel et al., 2018), these\n130 apparently opposite AABW responses during Heinrich Stadials in different numerical experiments\ncan be reconciled, in terms of physical mechanisms, by considering opposite surface buoyancy\nforcings (see details in Methods): a negative freshwater forcing/increasing density flux over the\nSouthern Ocean leads to the enhancement of AABW overturning rate in LOVECLIM and UVIC\nmodels(L. Menviel et al., 2015; Laurie Menviel et al., 2018), while a decreasing density flux due\n135 to sea ice retreat over the Southern Ocean leads to the decrease of AABW overturing in C-iTRACE\n(Figure 3). Therefore, freshening surface Southern Ocean results in weakening of AABW\noverturning rate and vice versa is a robust feature in different climate models. The reconstructed\nsea ice retreat and thus increased meltwater input during the early last deglaciation(Sadatzki et\nal., 2023) probably leads to a weakened AABW overturning rate, based on the above physical\n140 mechanisms” in line 241-352.\nL133: Please also give numbers for the change in the reconstructions.\nThanks for the suggestion. Due to the large uncertainty of the radiocarbon records\n(discussed in the first round of revision), it is difficult to give the exact values of the changes in\n145 reconstructions. However, the trends in reconstructions (decrease in Southern Ocean reservoir age\nand radiocarbon ventilation age, and increase in North Atlantic radiocarbon ventilation age) is\nclear and unquestionable. Therefore, in the revision, we revise our expression to emphasize that\nthe model captures the “trends” revealed by different radiocarbon reconstructions. This sentence\nis revised to “From the LGM to HS1, the simulated B-Atm age increases by 141 years at the Iberian\n4\nfirst-order increasing and decreasing trends in reconstructions from the Southern Ocean and\nIberian Margin respectively” in line 124-127.\nL249: If the effect is driven by the North Atlantic and propagated to the Southern Ocean, how can\n155 this then be in-phase? The SO response should then be delayed relative to the North Atlantic.\nWe intend to say there is no seesaw in NADW and AABW convections which is suggested\nin previous studies and AABW changes in the same direction as NADW. “in-phase” is revised to\n“parallel decrease” throughout the manuscript. This sentence is now revise to “Our results suggest\nparallel decreases in AABW and NADW convections during the early last deglaciation, driven by\n160 surface buoyancy forcings which are connected by the thermal bipolar seesaw” in line 255-257.\nL255: transport time or transit time, not transient time.\nIt is revised to “transit time”.\nprofound climate impact especially in the last degelaciation. However, reconstruction of AABW\nand NADW are sparse and controversial. Hence, it is important to investigate their property change\nfrom LGM to early Holocene. The experiments designed by the authors are appropriate and\nsufficient to support their final conclusions based on the response letters. Details e.g. calculation\n170 of thermol and haline contributions are provided in the manuscript which are helpful to\nunderstanding major arguments in the text. Last but not the least, I would expect some discussions\nof possible AABW and NADW states in other Heinrich stadials and change of these tipping\nelements in future.\nThanks for the suggestion.\nglobal warming and meltwater input around Antarctica (Li et al., 2023). This is cited as “The\nreduced AABW overturning rate during HS1 resulting from the surface Southern Ocean warming\nin C-iTRACE is also a robust feature in other climate models(Buizert & Schmittner, 2015; Snow\net al., 2016) and aligns with observed and projected AABW weakening in the recent and\n180 upcoming decades (Azaneu et al., 2013; Li et al., 2023)” in line 258-260.\nFor AABW changes in other Heinrich Stadials: in this revision, we analyzed the NADW\nand AABW responses to different freshwater hosing schemes with different locations and\nmagnitudes under preindustrial initial condition in the fully-coupled CESM (Zhu et al., 2017)\n(Figure R1/New Figure 4 below). The experiments are discussed in new section “Robust AABW\n185 weakening in response to North Atlantic freshwater input” in the Methods. These experiments\nshow that “With different magnitudes of reductions in NADW overturning rate, AABW shows a\nlong-term decreasing trend in all three idealized experiments, suggesting a robust parallel\nweakening of AABW and NADW in response to meltwater input in the North Atlantic” as stated in\nline 433-436. Based on these new results, we add “Sensitivity experiments with different\n190 magnitudes and locations of meltwater(Zhu et al., 2017) also consistently show weakening of\nAABW parallel to NADW slowdown” in line 260-262.\n6\n0.25Sv_North_Atlantic\n0.5Sv_North_Atlantic\n0.5Sv_Gulf_of_Mexico\nTime (year)\n0.25Sv_North_Atlantic\n0.5Sv_North_Atlantic\n0.5Sv_Gulf_of_Mexico\nTime (year)\nFigure R1/New Figure 4. NADW and AABW evolutions in idealized water hosing\nexperiments under preindustrial condition in CESM (Zhu et al., 2017). In experiment\n195 0.25Sv_North_Atlantic and 0.5Sv_North_Atlantic, 0.25Sv and 0.5Sv freshwater are added to 50°-\n70°N North Atlantic respectively. In experiment 0.5Sv_North_Atlantic. In 0.5Sv_Gulf_of_Mexico,\n0.5Sv freshwater is added to the Gulf of Mexico (15°-33°N, 255°-279°E).\n200\nI thank the authors for the consideration of the suggestions and questions stemming from my initial\nreview of this manuscript. I hope this second review is helpful.\ncontext regarding the limitations of the imposed boundary conditions and model utilized. This is\nan interesting study, which highlights longstanding but perhaps underrecognized wisdom (that\nmarine 14C is a complex tracer, and can reflect changes in watermass mixing, also known as ‘pre-\nformed age’, as well as true ‘ventilation age’) and presents an intriguing scenario in which AABW\n210 formation/ventilation could be reduced alongside NADW formation yet still show approximate\nsign agreement with the available 14C proxy records. However, I do not think this qualitative\nobservation, of itself, makes a strong case that the key conclusion (that AABW decreases\nsynchronous with NADW ventilation during deglaciation, resulting in a stagnant Atlantic Ocean\nduring HS1) is realistic.\noverturning rate is “realistic”. However, there are some misunderstandings on the “ventilation”,\nmodel freshwater scheme and the relationship between AABW overturning rate and AABW\nvolume in the comments, which potentially leads to the concern of Reviewer #3. In this revision,\nwe attempted to clarify these misunderstandings and show that our main result of the reduced\nabout “realistic” since models can never reproduce reality. However, models can be a useful tool\nto explore physical mechanisms. Our main conclusion of a weakened AABW production during\nthe early deglaciation is supported by different lines of evidence from reconstructions and\nmodeling results, and thus presents a plausible scenario for the deglacial AABW change, as\n225 summarized below:\n(1) If the abyssal ocean was filled by AABW, it follows that the increased radiocarbon age at the\nIberian Margin and the reduced surface reservoir age in the Southern Ocean revealed by\nreconstructions infers an increased AABW transit time, which suggests a reduced AABW\noverturing rate. This argument is supported by our C-iTRACE simulation and sensitivity\n230 experiments Fix_CO2 and Fix_circulation: with the decreasing Southern Ocean reservoir age\n(supported by reconstructions in Skinner et al., 2019), the increased deepwater radiocarbon\nventilation ages at the Iberian Margin (supported by reconstructions in Skinner et al., 2021) can\nonly be produced by a decrease in the AABW overturning rate, as discussed in line 172-183 in the\nrevised text.\n235\n(2) It is certain that that there was freshwater input to the North Atlantic during the early\ndeglaciation, although there is large uncertainty in its magnitude and location. New sensitivity\nexperiments with different magnitudes and locations of freshwater applied in North Atlantic\nshow a robust weakening of AABW overturning rate (Figure R1 above). Therefore, it is\n240 possible that the AABW overturning rate was reduced during the early deglaciation. The related\nrevision is “Sensitivity experiments with different magnitudes and locations of meltwater (Zhu et\nal., 2017) also consistently show weakening of AABW parallel to NADW slowdown (Fig.4)” in line\n260-262. Details in Page 18 in this letter.\n245 (3) Physical mechanism: reconstructions show warming in Southern Ocean (thermal bipolar\nseesaw)(He et al., 2013; Stocker & Johnsen, 2003) and retreat of sea ice around Antarctica\n(Sadatzki et al., 2023), which physically lead to the increase of surface buoyancy/decrease of\n8\nsurface density thus weakening of AABW overturning rate, supported by our and previous model\nsimulations (Buizert & Schmittner, 2015; Snow et al., 2016). The weakening of AABW overturing\nby decrease of surface density around the Antarctica is also supported by recent and projected\nAABW changes (Li et al., 2023). Therefore, it is physically grounded and supported by various\nrecords and numerical simulations that the thermal bipolar seesaw will lead to parallel changes in\nAABW and NADW overturning rate.\n255 (4) We cite a sortable silt record (Hall et al., 2001), which also support reduced AABW overturing\nrate during the early deglaciation. Details in lines 382-386 in this letter.\n(5) We clarify that the main result is about the AABW overturning rate during the deglaciation,\nwhich is different from the ventilation inferred from chemical proxy records such as\n260 dissolved oxygen (discussed in Page 11 in this letter) and is also different from the AABW\nvolume. Our results suggest a weaken AABW overturning rate with an expansion of the AABW\nvolume, which is compatible with previous studies suggesting AABW expansion during the\ndeglaciation (discussed in lines 362-374 in this letter). With these clarifications, our results do\nnot contradict the diverse lines of evidence suggested by Reviewer #3.\nsupported by radiocarbon records and sortable silt records and the physical mechanism for this\nAABW weakening can also be supported by different reconstructions and model simulations.\nTherefore, the weakened AABW is possible scenario during the early deglaciation. We\n270 encourage future studies to test the deglacial AABW evolutions which are important and\nchallenging. This is revised as “Therefore, radiocarbon records may not be linked directly to\n“deepwater production seesaw” during the early last deglaciation. Instead, the “thermal bipolar\nseesaw” favors parallel decreases in NADW and AABW overturning rates during HS1” in line\n303-305, “Nevertheless, we encourage further studies using next generation models that can more\n275 accurately represent Southern Ocean processes in the future” in line 311-312, and “Therefore,\nfuture studies should combine multi-proxy constraints and model-data comparisons to better\nreconstruct the deglacial history of AABW” in line 317-319,\nThis manuscript as currently drafted hinges on the premise that 14C-based reconstructions are the\n280 ‘key’ evidence for enhanced ventilation of the Southern Ocean/Antarctic Bottom Water (AABW)\nbeing a significant source of the aged CO2 released to the atmosphere at the end of the last glacial\nperiod. From revised manuscript Line 51-57: “The predominant hypothesis attributes this C-iTRACE\nresults are in agreement with eNd records that also indicate little change in AABW\ncirculation through HS1 (e.g. line 138) I’m not actually sure that is a fair statement. It’s certainly\nnuanced; several of the eNd records cited do show limited change during HS1 relative to the large\nswing at the B-A, but the deepest North Atlantic records (e.g. core KNR197/10GGC-17) shows a\n535 decrease relative to the LGM, while deep Southern Ocean records (e.g. RC11-83) shows an\nincrease (Poppelmeier et al., 2018, included in the compilation of Zhao et al., 2018, referenced in\nthis manuscript; Piotrowski et al., 2012). Other Southern Ocean eNd records in the Pacific sector\nalso show significant changes in eNd that seemingly evidence changes in AABW formation (e.g.,\nBasak et al., 2018; Du et al., 2018, referenced in this manuscript). These published observations\n540 have been interpreted as strengthened AABW formation, in contrast to the weakening of AABW\ninferred from HS1 through C-iTRACE – do the authors have an alternative hypothesis to explain\nthese data and how they would fit in the proposed framework? Although the manuscript centrally\nhighlights the nuances of radiocarbon as a circulation tracer in the interior ocean, it seems as\nthough the C-iTRACE results are more in alignment with the published interpretation of the global\n545 synthesis of LGM radiocarbon records of Rafter et al., 2022 cited in line 88, as well as the deep\nabyssal Indo-Pacific reconstructions cited in Line 275.\nFrom 20ka to 15ka, 𝜀 records from the deep North Atlantic and Southern Ocean do not\n𝑁𝑑\nvary much as shown in Figure R7 below, including the core KNR197/10GGC-17 and RC11-83\nmentioned by the reviewer. KNR197/10GGC-17 shows a decrease and RC11-83 shows an increase\n550 after ~15ka, consistent with the increase of AMOC (Figure R7a). We clarify this in the revised\ntext as “These limited changes in water mass composition in the abyssal ocean might be supported\nby the lack of substantial changes in 𝜀 records during the early deglaciation from 20ka to 15ka\n𝑁𝑑\n(Howe et al., 2016; Jonkers et al., 2015; Piotrowski et al., 2012; Roberts et al., 2010; Zhao et al.,\n2019)” in line 145-147.\n𝑁𝑑\nis complicated by other process such as sedimentary influences, this is also acknowledged as\n“although the interpretation of 𝜀 records as water mass mixing might be complicated by benthic\n𝑁𝑑\nfluxes (Du et al., 2020; Pöppelmeier et al., 2022; Zhao et al., 2019)” in line 147-148.\n17\n𝑁𝑑\nal., 2019).\nIn Rafter et al., (2022), the compiled data also show increased radiocarbon ventilation age\nin the deep Pacific, but the authors did not relate this to the changes of AABW overturning rate.\nexclusive to the North Atlantic. Recent reconstructions from the abyssal Indian Ocean (Bharti et\nal., 2022; Nisha et al., 2023) and North Pacific (Rafter et al., 2022; Stott et al., 2021) also exhibit\nan increased radiocarbon ventilation age during the early deglaciation” in line 292. In Rafter et\nal., (2022), the increased radiocarbon ventilation age is attributed to the reduction of NADW.\nradiocarbon change in the abyssal North Atlantic can be mainly explained by processes in the\nSouthern Ocean. This is discussed as “With the abyssal ocean dominated by AABW during the\nearly deglaciation, the radiocarbon changes in the abyssal ocean is mainly associated with\nchanges in AABW instead of NADW as suggested in previous studies (Rafter et al., 2022; Skinner\n575 et al., 2014)” in line 148-151.\nIn summary, this C-iTRACE simulation results presented in this manuscript are both interesting\nand informative, demonstrating that changes in surface ocean reservoir age at the time of\ndeepwater formation can have an outsized impact on the apparent/radiocarbon reservoir age of the\n580 interior ocean that is decoupled from ocean circulation. While this observation in and of itself isn’t\nnew sensu stricto (e.g., Muglia and Schmittner, 2021), the potential application of this concept to\ninterpretation of the paradox set up between the C-iTRACE results (suggesting simultaneous\ndecreases in deep water formation in both the high latitude North Atlantic and Southern Ocean)\n18\nand radiocarbon evidence in the deep Atlantic and Southern Ocean (suggesting a ‘bipolar\n585 radiocarbon ventilation seesaw’) certainly is. My confidence in these interpretations would be\nstrengthened if the authors could offer an explanation for why the C-iTRACE results are different\nthan those of the LOVECLIM and UVIC models (e.g., Menviel et al., 2015). I also have concerns about the asymmetric\nbias in the SO versus IM data/model comparison of radiocarbon offsets between the interior ocean and atmosphere.\nand in LOVECLIM and UVIC models in Menviel et al., (2015) and show that the reduced AABW\noverturning rate is a robust response to the Southern Ocean surface warming in both CESM and\nUVIC model. Please refer to page 11-13 in this letter for details.\nWe also discuss the model-data offset and show that this offset will not affect the\n595 conclusion in this study. Please refer to pages 14-17 in this letter for details.\nMinor comments to Author:\nFig 1 – I think part F of the caption needs a correction. Would suggest adding units to the sub-\nplots or caption and define the gray bar in the caption.\nanomaly and ideal age (dash) anomaly compared with simulated radiocarbon ventilation age\nanomaly (solid) of the deep Iberian Margin (blue) and deep Southern Ocean (red) (unit: yr)” in\nline 625-628.\nUnits are added in the y-axis labels as well as in captions.\n629-630. Please refer to page 3 in this letter for revised Fig.1 and caption.\nFig 2 – This is a hugely helpful figure in supporting your interpretations. This is a minor point, but\nit would be somewhat helpful if the caption was in the same order as the experiments are explained\n610 in the methods. As for figure 1, there are no units on any of the plots. The gray bar denoting HS1\nis not defined in the caption.\nThe experiments in Fig.2A legends and Fig.2 captions are revised in the same order as\ndiscussed in the manuscript. The units are added in the y-axis labels and in the captions. The gray\nbar is defined as “The gray shading indicates the HS1 (17.5ka-14.7ka)” in caption. Please refer to\n615 page 4 in this letter for revised Fig.2 and caption.\nFig 3 – Again, units…\nThe units are added in the caption as: “Fig. 3 | Time series related to the mechanism of\ndeglacial AABW overturning rate change. (A) Average zonal wind stress (unit: dyne/cm2) over\n620 60°S-40°S in C-iTRACE (black) and experiment SO_wind (green). (B) AABW overturning rate\n(unit: Sv) in C-iTRACE (black) and experiment SO_wind (green). (C) Density flux (black),\nthermal density flux (blue), haline density flux (red) and brine density flux (magenta) in the AABW\nformation region (60°W-20°E, south of 60°) (unit: 10-6 kg m-2s-1). (D) Sea ice fraction in the\nAABW formation region. (E) Sea surface temperature in the AABW formation region (unit: °C).”\n625 in line 644-651.\nExtended Data Figure 2 – This figure is a bit misleading. When presented together the SO and IB\nmodel/data comparison does seem to fall along a 1-1 ratio, albeit with significant scatter. However\nthis hides the asymmetry of the offsets in the ocean basins. What does this figure look like if you\n630 denote the SO data/model comparison points from the IB data/model comparison points via\n19\ndifferent symbols, and can you comment on the origin of the inter-hemispheric asymmetric bias that\nseems obvious in the time series?\nThanks for the suggestion. This figure is revised as suggested, with different symbols\nrepresenting North Atlantic (filled circles), Southern Ocean (asterisks), and regions in between\n635 (empty squares). Gray shadings are added to visualize if the model values are in the same sign as\nthe observational values since the sign of change is the key of model-data comparison as discussed\nin Page 15-16. From the revised figure, positive radiocarbon ventilation changes in the North\nAtlantic and negative radiocarbon ventilation changes in the Southern Ocean are clearly shown in\nboth observation and model. This is now added as “the bipolar radiocarbon ventilation seesaw is\n640 well simulated in C-iTRACE, with the radiocarbon ventilation age exhibiting opposite changes\nbetween the abyssal Iberian Margin in the North Atlantic and the Southern Ocean: below 3km, the\nsimulated B-Atm age increases by 141 years in the Iberian Margin but decreases by 329 years in\nthe Southern Ocean from LGM to HS1, roughly capturing the observational changes (Fig. 1 B\nand C, Extended Data Figs.2 and 4)” in line 129-134.\nbetween HS1 and LGM in the Atlantic and Southern Ocean (unit: yr). The colors of the dots\nindicate the depth of the observational sites. Sites north of 30°N are marked by filled circles. Sites\nsouth of 30°S are marked by asterisks. Sites from 30°S to 30°N are marked by empty squares. The\n650 observational information is listed in Supplementary Information Table S1. Gray shading\nindicates that the model values are in the same sign as observational values.\nFor the “the inter-hemispheric asymmetric bias”, we agree that the simulated values are\nlarger than reconstructions in the Iberian Margin, but due to the large variability in the Southern\nsmaller than the reconstructions. The higher radiocarbon ventilation age in the simulated Iberian\nMargin is probably related to weak AABW overturning rate at LGM in the model. And we also\ndiscuss that the opposite signs of the radiocarbon ventilation age changes between the abyssal\nSouthern Ocean and North Atlantic is the key to draw the conclusion that AABW overturning rate\n20\n660 is weakened during the early deglaciation, and this feature is well simulated in the model. 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The original reviewers’ comments\nare in red, our replies are in black\nIn the further revised manuscript Gu et al. have convincingly clarified the final points I raised in\nmy previous review, and I have no further remarks. In addition, the authors thoroughly addressed\nall comments and concerns by the other reviewers. Reviewer#3 remained critical about the\n10 employed boundary conditions for the simulations. Although uncertainties remain regarding the\nboundary conditions of the last deglaciation, especially with regard to freshwater fluxes, the\nauthors chose reasonable freshwater fluxes employed similarly in other studies and clearly state\nalso their limitations. The persistent conundrum that the largest freshwater fluxes (meltwater\npulses 1a and 1b) seem to have entered the ocean when the AMOC was in its recovery phase\n15 cannot be resolved here. This is also not the focus of the present study.\nTherefore, I congratulate the authors on this very interesting study, which further contributes to an\nimproved understanding of deglacial ocean dynamics.\nWe thank the reviewer for previous comments and suggestions.\nThe revised manuscript and additional figures/discussions addressed my previous concerns. The\nparallel weakening of AABW and NADW is convincing in model results. I therefore thanks for\nthe helpful information and satisfied.\nWe thank the reviewer for previous comments and suggestions.\n25\nI thank the authors for the consideration of the suggestions and questions stemming from\nmy initial review of this manuscript. I hope this second review is helpful.\nI continue to have significant concerns that the results presented in the manuscript lack\nimportant context regarding the limitations of the imposed boundary conditions and model\nutilized. This is an interesting study, which highlights longstanding but perhaps under-\nrecognized wisdom (that marine 14C is a complex tracer, and can reflect changes in\nwatermass mixing, also known as ‘pre-formed age’, as well as true ‘ventilation age’) and\npresents an intriguing scenario in which AABW formation/ventilation could be reduced\nalalongside NADW formation yet still show approximate sign agreement with the available\n14C proxy records. However, I do not think this qualitative observation, of itself, makes a\nstrong case that the key conclusion (that AABW decreases synchronous with NADW\nventilation during deglaciation, resulting in a stagnant Atlantic Ocean during HS1) is\nrealistic.\nThis manuscript as currently drafted hinges on the premise that 14C-based reconstructions\nare the ‘key’ evidence for enhanced ventilation of the Southern Ocean/Antarctic Bottom\nWater (AABW) being a significant source of the aged CO released to the atmosphere at the\n2\nend of the last glacial period. From revised manuscript Line 51-57: “The predominant\nhypothesis attributes this CO rise to enhanced ventilation of the abyssal ocean . Given the\n2 2–6\nabyssal ocean is primarily filled with the Antarctic Bottom Water (AABW) , the strengthening\n7,8\nof AABW overturning leads to improved abyssal ventilation, which is suggested to lead to the\natmospheric CO increase due to enhanced outgassing and reduced carbon sequestration in the\n2\nabyssal ocean on millennial time scale . The key evidence that supports this hypothesis lies in\n9–12\nthe observed reduction of radiocarbon ventilation age during this period,). While many 14C\nventilation records do support this hypothesis, they are far from the only proxy\nreconstructions to do so. Although, as for 14C, interpretation is nuanced (e.g. Du et\nal.,2022), Nd isotope reconstructions from the Pacific, Atlantic, and Southern Oceans\nstrongly support for early deglacial invigoration and expansion of AABW (e.g. Du et al.,\n2018; Basak et al., 2018; additional discussion highlighted in first review). However there\nare many parallel lines of evidence for northward AABW expansion, e.g. the glacial deep\nocean geometries utilizing proxies ranging from interior ocean oxygenation (e.g. Gottschalk\net al., 2020; Wang et al., 2024) to carbonate chemistry (e.g., Farrell, 1989; Farrell and Prell,\n1991; Gottschalk et al., 2015; Ronge et al., 2021). These are just a few among many\nexamples available in literature spanning over 40 years.\nGiven these diverse and converging lines of evidence, I would want to have very strong\nconfidence in both the boundary conditions and mechanics of the C-iTrace simulation\nbefore discounting the enhanced deglacial AABW ventilation hypothesis. I appreciate the\ninclusion of a review of the differences between CESM and the LOVECLIM and UVIC\nmodels – as someone whose paleoclimate expertise leans towards proxies over models,\nthis was very helpful. However, I remain concerned by the authors assertion that the\nfreshwater forcing used in the TRACE21k and/or iTRACE simulations are constrained by the\nbest available data for the location and extent of meltwater input through early\ndeglaciation.\nFrom line 333-337 of the revised manuscript:\n“A transient simulation of the last deglaciation (C-iTRACE) is simulated using iPOP ,\n22\nwhich is forced by monthly surface forcings from a fully coupled simulation TRACE21K .\nice sheet, and meltwater flux using CCSM3, which captures major climate changes during\nthe last deglaciation . Since the land ice sheet is prescribed in TRACE21K, the timing and\n26\nmagnitude of the meltwater flux from the land ice sheet is based on the reconstruction of\nsea level . With this freshwater added to the North Atlantic and Gulf of Mexico, the\n65\nsimulated AMOC in TRACE21K is in good agreement with 231Pa/230Th records .”\n“It is true that freshwater is added to the North Atlantic and Gulf of Mexico to weaken the\nAMOC in TRACE21K. However, the freshwater is added to mimic the melt water from the\nland ice sheet since there is no active land ice sheet module in the model. Especially, the\nmagnitude and timing of this freshwater flux is based on the reconstruction of sea level\n(Peltier, 2004)(Figure R6 B below). With this freshwater forcing, the simulated AMOC is in\ngood agreement with 231Pa/230Th (Figure R6D), suggesting that the ocean circulation is\nmore or less realistic during the early deglaciation and laying a foundation for good model-\ndata comparison in this study.”\nFollowing the response of the authors to my first review I did some more digging, in an\neffort to be sure my understanding (that freshwater forcing used in TRACE21K is not\nconsistent with either the timing or magnitude of the consensus RSL reconstructions) was\naccurate.\nI confirmed that the freshwater forcing of TRACE21k is significantly inconsistent with\ngeologic evidence for glacial meltwater input through deglaciation. I’ve created a plot here\nto illustrate the point (see Figure 1 below), showing the Sea Level Equivalent (SLE)\nmeltwater forcing in both Trace21k and iTrace, compared to the SLE relative to a 21ka\nbaseline from the Lambeck et al., 2014 reconstruction (an update relative to Peltier 2004,\nalthough not substantially dissimilar at this scale). This inconsistency is well known, and\nwas fairly openly addressed by (TRACE21K authors) He and Clark, 2022 (Nature Climate\nChange, doi.org/10.1038/s41558-022-01328-2). In this paper they present an alternative\nhosing experiment (TRACE21K-II) which removes all freshwater input during Meltwater\nPulse 1-A (the Bolling-Allerod warm period) – an approach they acknowledge is also\nproblematic, but with the net effect of bringing total freshwater input/SLE by the end of\ndeglaciation into agreement with geologic reconstructions such that Holocene AMOC\ndynamics are more realistic and consistent with proxy data. A direct quote from that\npublication (in describing the forcing of TRACE21K): “…the FW scheme in TraCE-21K-I\nwas designed to reproduce changes in the AMOC as suggested by proxies between\nthe Last Glacial Maximum (~21 ka) and onset of the Bølling warming, followed by a\nage is systematically “lower” than the reconstructions as the reviewer stated because\ndata-data offset is much larger than the model-data offset.” The available proxy data is\nadmittedly low resolution, but from Manuscript Figure 1C the only time when the\nreconstructions seem to definitively disagree by centuries (i.e. when there is coeval data\nfrom multiple cores) is around 18.5 ka – otherwise, and certainly in early H1, the records of\nMD07-3076 and TN057-21 seem statistically indistinguishable. Regardless, if proxy\nreconstructions are all indicating higher B-P ages than suggested by the model\nreconstruction until late H1, that would seem to still imply systematic offset. Perhaps it is\nstatistically irrelevant – could the authors establish that via some variant of a t-test (or\nperhaps there’s something more appropriate)?\nI accept the assertion of the authors that the major objective of the manuscript is to assess\nwhether or not the relative changes in the C-iTrace simulation could agree with the\nradiocarbon data without invoking an increase in AABW ventilation. While there are clearly\ndynamics in the proxy data that are not captured by these model results (i.e. the inflection\nin both Southern and Iberian Margin 14C data towards lower B-P differences in late H1), I\ncould believe the trends in early H1 are loosely qualitatively reproduced. However, while\nthe results of this C-iTrace experiment are interesting, I would remain very uncomfortable\nwith seeing this paper move forward with the implication that Southern Ocean ventilation\nis unlikely to be a meaningful source of atmospheric CO during the deglaciation. While\n2\nthere is nothing wrong (and much right!) with challenging paradigms, in this case there are\nmany diverse proxy lines of evidence countering these simulation results, in addition to\nstrong reason to question the accuracy with which the C-iTrace simulation captures\ndeglacial ocean dynamics. It is not that I think these results do not have value, only that\nthere should be accurate representation of their limitations in interpretation.\nThank you for your patience in digesting my review. I have done my best to be thoughtful\nand certainly do not want to be discouraging. I do think there is a potentially very\ninteresting result here – certainly one I have not seen before – in identifying that the iPOP2\nocean does not reproduce an increase in AABW ventilation during deglaciation when the\niTrace21k boundary conditions are imposed. It could be the boundary conditions of\nmeltwater input are the issue, but I think it’s very interesting that a hosing scheme known\nbe unrealistic but nonetheless implemented to drive the model to produce deglacial MOC\nvariability informed by the North Atlantic Pa/Th record apparently fails dramatically in the\nSouthern Ocean. Perhaps this also speaks to the physical dynamics of the model itself\n(i.e., why realistic hosing cannot produce changes in AMOC that agree with the Pa/Th proxy\nrecord through deglaciation)? I wonder if there may be a productive thread to pursue in\nidentifying why the CESM struggles in reconstruction of Southern Ocean processes and\nwhy it requires such unrealistic forcing to produce AMOC behavior in line with the best\navailable data reconstructions?"}]

Decision letter