Decoupled Antarctic and Subarctic export productivity under intensified Southern Hemisphere westerlies during the last interglacial

Domain: earth_science
Published: 2025-12-13
Editorial outcome: minor_revision

Abstract
The Last Interglacial (128–113 ka B.P.) provides key insights into climate-carbon interactions under warm, high-CO2 conditions. While Antarctic temperature-driven Southern Ocean ventilation has been proposed to modulate atmospheric CO2, the role of Southern Hemisphere westerlies (SHW) remains unclear. Here we synthesize 15 Antarctic and 5 subarctic Pacific export productivity records over two glacial-interglacial cycles. We find that Antarctic and subarctic productivity generally tracked Antarctic temperature and atmospheric CO2, but decoupled during Marine Isotope Stage 5, notably the Last Interglacial: high Antarctic productivity and CO2 persisted despite reduced subarctic productivity and over 6°C Antarctic cooling. We suggest that poleward-intensified SHW strengthened upwelling of Circumpolar Deep Water, sustaining Antarctic productivity and atmospheric CO2, while altered Southern Ocean deep-water supply to the subarctic Pacific likely suppressed upwelling, limiting CO2 release. These findings identify SHW as key in polar ocean dynamics and carbon cycling, with implications for future climate under projected SHW intensification.

Introduction
Physical and biological processes at high-latitude oceans play a crucial role in the global carbon cycle, influencing the uptake, storage, and transport of carbon, thereby modulating the Earth’s climate. In these regions, the Southern Ocean and the subarctic Pacific Ocean are particularly important1,2. Here, wind-driven upwelling or density-driven overturning bring nutrient- and CO2-rich deep waters to the surface, fueling phytoplankton growth and simultaneously releasing oceanic CO2 to the atmosphere3–5. Because phytoplankton incorporate nutrients and CO2 into biomass during photosynthesis, the balance between upwelled CO2 and the biologically fixed CO2 directly affects the net air-sea CO2 exchange. Thus, either an enhancement in export productivity or a reduction in vertical exchange would help to minimize the CO2 emission, thereby contributing to low atmospheric CO2 during glacial periods6–8.

Paleoceanographic records reveal a simultaneous decline in export productivity in both the Antarctic zone of the Southern Ocean and the Subarctic Pacific 2.7 million years ago, coinciding with intensified Northern Hemisphere glaciation and global cooling3. Over the past 800 ka, glacial-interglacial variations of similar magnitude were observed in both regions6,7,9, tracking Antarctic air temperature and atmospheric CO2 concentration10,11. The observed correlation among polar export productivity, Antarctic air temperature, and atmospheric CO2 has garnered widespread debate over whether it reflects a mechanistic linkage6,9 or mere coincidence3. Nonetheless, one point remains clear: the critical role of temperature in regulating polar ocean dynamics, ultimately affecting export productivity and global atmospheric CO2, is consistently emphasized5,6.

Marine Isotope Stage (MIS) 5 (128–71 ka B.P.) provides a well-documented interglacial interval for investigating ocean-carbon-climate interactions under elevated temperatures and atmospheric CO2 conditions11,12. During the Last Interglacial, the warmest interval (MIS 5e, 128–113 ka B.P13) in MIS 5, the Earth’s climate exhibited warming similar to future projections, yet atmospheric CO2 remained relatively stable at 270–280 ppm from 125 to 113 ka B.P., despite pronounced Antarctic temperature variations10,11,14,15. This asynchrony challenges the conventional relationship of a direct temperature-CO2 coupling. Increasing evidence points to the role of Southern Hemisphere westerlies4,16,17, which has been proposed to enhance deep-ocean upwelling and facilitate the oceanic carbon release on millennial timescales4. However, it remains unresolved whether the Southern Hemisphere westerlies played an equally, or perhaps more, influential role in oceanic vertical exchange and CO2 regulation during the Last Interglacial. This uncertainty particularly concerns the mechanisms influencing atmospheric CO2 controls during MIS 5d-5a, a period that followed the climatic optimum of the Last Interglacial but preceded the next glacial inception.

In this study, we examine the large-scale ocean-atmosphere processes influencing export productivity in the Antarctic zone and subarctic Pacific, with a focus on the role of Southern Hemisphere westerlies and their links to atmospheric CO2. To assess regional differences in export productivity and carbon cycling, we synthesize 15 export productivity records from 12 sediment cores in the Antarctic zone and 5 export productivity records from 10 sediment cores in the subarctic Pacific (see Fig. 1 and Methods; Supplementary Tables 1–2). Our results reveal that during MIS 5, Antarctic and subarctic Pacific export productivity decoupled, breaking the long-term covariation with atmospheric CO2. This decoupled evolution was most pronounced during the Last Interglacial, when Antarctic export productivity remained high and tracked atmospheric CO2, while subarctic Pacific export productivity dropped significantly, even as Antarctic air temperature declined by over 6°C. This asynchronous pattern suggests that poleward-intensified Southern Hemisphere westerlies, rather than Antarctic temperature alone, enhanced Circumpolar Deep Water upwelling, supporting high Antarctic export productivity and contributing to CO2 rise.

This study offers an alternative view to the traditional emphasis on Antarctic temperature as the dominant factor controlling atmospheric CO2 during past warm periods and underscores the critical role of Southern Hemisphere westerlies in influencing oceanic carbon cycling. Given projections of poleward-intensified westerlies under global warming, further investigation into their impact on polar ocean circulation and carbon cycles is crucial for understanding future climate change.

Results
Disrupted relationship between Antarctic and subarctic Pacific export productivity during MIS 5

Biogenic (or excess) barium, commonly referred to as bioBa, precipitates in seawater, and its flux to the seafloor is tightly linked to overall export productivity18. Likewise, the content and fluxes of biogenic opal have long been used as proxies for past export productivity, particularly in the diatom-rich Antarctic Zone of the Southern Ocean4,19. However, biogenic opal accumulation can be influenced by various factors such as diatom species composition and iron availability20,21, in addition to particle aggregation, differential dissolution, and post-depositional preservation effects22–25. To mitigate potential biases from a single proxy and strengthen our export productivity reconstructions, we developed independent stacks for biogenic opal (BSi content and flux) and biogenic barium (flux, concentration, and elemental ratios (Ba/Ti, Ba/Al, and Ba/Fe)). A combined stack integrating both proxies was constructed to provide a more comprehensive and robust qualitative framework. Each of the three stacks was independently reconstructed using two distinct methods, resulting in two versions for each stack type (see Methods).

To enhance the robustness of our compilation of the Southern Ocean export productivity, we applied a common age model based on the AICC 2012 age scale (see Methods; Supplementary Fig. 1). Chronological uncertainty (0-3.84 ka) is minor relative to the temporal scope of our analysis (Supplementary Tables 3–10).

Despite potential influences from local climate variability and differential preservation of biogenic opal and biogenic barium, the two versions of each stack exhibit exceptionally strong agreement, with Spearman’s correlation of ρ=0.94 for the biogenic opal stacks, ρ=0.98 for the biogenic barium stacks, and ρ=0.97 for the combined stacks (Supplementary Figs. 2, 3). All stacks exhibit synchronous glacial-interglacial patterns (Fig. 2a, b, and Supplementary Fig. 2), broadly consistent with previously published biogenic opal and barium records from the Antarctic Zone of the Southern Ocean26–29 (Supplementary Figs. 4–8), indicating that they capture the true underlying export productivity signals, not proxy-specific artifacts. Independent palaeoecological data on the relative abundance of Fragilariopsis kerguelensis within the permanent open-ocean zone south of the Polar Front in our study area30,31, which can be used as an indicator of productivity changes in the pelagic Southern Ocean, closely track the stacks (Supplementary Fig. 9), further supporting the interpretation that the reconstructed stacks capture key signals of Antarctic export productivity.

To facilitate comparison with Antarctic records, we compiled five independent export productivity reconstructions from the subarctic Pacific, using biogenic barium concentrations6,32–34 and sedimentary 231Pa/230Th as proxies35 and applying two stacking methods consistent with those used in Antarctic productivity (Fig. 1; see Methods and Supplementary Table 2). Unlike in the Southern Ocean, we excluded opal-based records because post-burial diagenetic alteration of biogenic silica can systematically underestimate subarctic Pacific productivity, as evidenced by discrepancies between opal and excess silica fluxes during older intervals (e.g., MIS 5)35. For some sites, detailed age-model data are not publicly available, limiting the possibility of further improving the chronology. Therefore, we retained published age models to maintain consistency and minimize additional uncertainty in the stack construction (Supplementary Table 2).

The two subarctic Pacific productivity stacks constructed using two independent methods show remarkably consistent patterns, with a high correlation of 0.97 (Supplementary Fig. 10). The temporal patterns of the stacks are consistent with published biogenic barium and 231Pa/230Th records from the subarctic Pacific6,32–34, with correlation coefficients exceeding 0.7 (Supplementary Figs. 11–13), supporting the reliability of the stacks as representative proxies and enabling inter-hemispheric comparisons across the glacial-interglacial cycles. Given potential chronological uncertainties in the stacks, which may affect the interpretation of millennial-scale variability, we further incorporated the high-resolution, astronomically tuned Ba/Al record from ODP Site 882 (Hole 882A) as a complementary dataset into our analyses6.

Generally, the Antarctic and subarctic Pacific export productivity stacks exhibit consistent patterns, broadly covarying with published Antarctic air temperature over the two glacial-interglacial cycles6,9,10 (Fig. 2a–c). Remarkably similar millennial-scale increases in export productivity were observed in both regions during Antarctic warming events36, such as Termination I and II (Fig. 2a–c). However, during MIS 5, the Antarctic and subarctic Pacific records exhibited a pronounced deviation from their long-term covariation (Figs. 2a–c and 3b). In particular, during the late Last Interglacial (125–113 ka B.P.), subarctic Pacific export productivity declined alongside an abrupt ~6°C drop in Antarctic air temperature, whereas Antarctic export productivity continued to rise post-Termination II and remained elevated6,9,10 (Fig. 2a–c). To investigate this anomaly, we applied linear regression across the full record. The regression fit is moderate when all data are included (R2=0.58, n=149), but it improves markedly once MIS 5 is excluded (R2=0.90, n=92; Fig. 3b). Notably, the largest deviation occurred during the late Last Interglacial, around 115–113 ka B.P, after which the deviation gradually diminished as Antarctic productivity converged toward the declining subarctic Pacific trend, with deviation persisting until MIS 5a, when the two regional productivity stacks finally reconverged (Fig. 3b).

Wind-driven upwelling, not temperature, controls Antarctic export productivity

Unlike the pelagic subarctic Pacific, which is unaffected by sea ice and experiences higher boreal summer sea surface temperature37, the Southern Ocean is highly influenced by variable sea ice extent. Several sediment cores used in this study are situated within the austral winter sea ice limit coverage, including Sites PC493, MD07-3134, U1537, U1361A, and PS1506-119,27,29,38. Sea ice, which limits sunlight vital for photosynthesis in phytoplankton, could potentially restrict export productivity38. However, despite the regrowth of sea ice after Termination II, Antarctic export productivity remained high during the late Last Interglacial and exhibited three distinct high peaks throughout MIS 5, suggesting that sea ice did not significantly affect regional export productivity39 (Fig. 4b and d). In the Subantarctic Zone, iron fertilization has been shown to enhance export productivity during ice ages, with increased deposition of iron-rich dust8,40,41. In contrast, iron fertilization does not appear to have been impactful in the Antarctic Zone38, which exhibits lower export productivity during glacial periods40.

In the modern Southern Ocean, export productivity in the Antarctic Zone of the Southern Ocean is dominated by diatoms such as Fragilariopsis kerguelensis, with silica-rich frustules that facilitate the formation and export of particulate organic carbon from the surface ocean30,31. The zone of high diatom productivity is largely located south of the Polar Front22,25, a region characterized by upwelling driven by Southern Hemisphere westerlies or buoyancy from heat fluxes4,5. Thus, the nutrient supply via upwelling is expected to play a crucial role in limiting the phytoplankton growth4,28, and the export productivity is widely regarded as a key indicator of upwelling4,26. Previous studies suggested that Antarctic air temperature plays an important role by directly affecting Southern Ocean surface buoyancy forcing over glacial-interglacial cycles5, which in turn influences upwelling intensity and nutrient supply for productivity. Hence, one would expect export productivity and Antarctic air temperature to exhibit a close relationship across the entire glacial-interglacial cycles10. We then tested this hypothesis using linear regression across the entire record, which yields a moderate fit (R2=0.76, n=149). Excluding MIS 5, however, results in a much stronger fit (R2=0.90, n=92; Fig. 3a), revealing that MIS 5 is an interval during which the relationship deviates from the long-term mean relationship. More importantly, during the late Last Interglacial, Antarctic export productivity remained elevated despite an abrupt 6°C drop in Antarctic air temperature (Fig. 4b and d), suggesting that Antarctic air temperature cannot fully account for the variability in Antarctic export productivity.

The Southern Hemisphere westerlies are crucial for promoting upwelling, and their poleward shift is known to intensify northward Ekman transport, facilitating the upwelling of nutrient-rich deep waters4,16. This mechanism is thought to influence export productivity over millennial timescales4,26, and likely operated in the same way during MIS 5. During Termination II, massive freshwater discharge and surface cooling in the North Atlantic substantially weakened the Atlantic Meridional Overturning Circulation (AMOC)42, reducing northward heat transport and, via the bipolar seesaw43,44, promoting heat accumulation in the Southern Ocean and Antarctic warming45. This resulting interhemispheric temperature contrast, with the Southern Hemisphere warmer than the Northern Hemisphere, appears to have triggered a southward shift of the Intertropical Convergence Zone (ITCZ)46 and poleward intensification of the Southern Hemisphere westerlies4, thereby enhancing Southern Ocean upwelling and surface productivity, consistent with the co-variation of Antarctic air temperature and export productivity10 (Fig. 4b and d). As the climate entered the early Last Interglacial, Antarctic air temperature records indicate peak temperatures10, while evidence suggests that AMOC had begun to recover42. However, proxy data show that surface waters in the North Atlantic, Labrador, and Nordic Seas remained cold and fresh, with reduced ventilation47, indicating that this recovery was incomplete and spatially heterogeneous, with the Southern Hemisphere remaining warm while the high-latitude Northern Hemisphere stayed relatively cold. Thus, the persistent hemispheric temperature contrast would have continued to sustain a bipolar seesaw-like state, reinforcing Southern Ocean upwelling and export productivity during the early LIG.

Orbital forcing likely further amplified changes in Southern Hemisphere westerlies, thereby sustaining Southern Ocean upwelling and high export productivity during the late last Interglacial. Previous studies indicate that summer insolation peaked at 65°N around 127 ka B.P. before declining, while it was at a minimum at 65°S and increased thereafter48 (Fig. 4a), creating an interhemispheric insolation contrast that amplified the meridional temperature gradient. Simultaneously, the declining obliquity during the late Last Interglacial would have steepened the meridional temperature gradient (Fig. 4a), further reinforcing the Southern Hemisphere westerlies17. This reorganization of the wind system closely resembles a prolonged positive phase of the Southern Annular Mode (SAM), the dominant mode of atmospheric variability in the Southern Hemisphere49–51. In modern observations and model simulations, a positive SAM phase is associated with enhanced upwelling of deep waters around Antarctica and reduced upper-ocean stratification52,53. By analogy, such a positive SAM-like circulation during the late Last Interglacial would have likely intensified the upwelling of nutrient-rich Circumpolar Deep Water (CDW), brought nutrients to the surface, thereby promoting productivity. This interpretation is supported by consistent evidence of intensified Antarctic Circumpolar Current (ACC)54,55, increased nitrate supply17,28, and higher export productivity28 in the Antarctic Zone of the Southern Ocean (Fig. 4d). Additionally, the significant southward shift of the Subtropical Convergence and ACC front provides further evidence of a more vigorous and expansive wind-driven circulation system during this period56,57 (Fig. 4e). These obliquity-driven changes, by steepening the meridional temperature gradient and reinforcing the Southern Hemisphere westerlies, likely reached their maximum impact around 113 ka, when obliquity was at its minimum and Antarctic export productivity most evidently diverged from temperature (Fig. 3a). Their influence appear to have persisted until MIS 5a, when Antarctic temperature and export productivity had largely returned to the linear trends observed across glacial-interglacial cycles outside MIS 5 (Fig. 3a).

Remote influence of Southern Ocean circulation on subarctic pacific export productivity

Although a strong coupling in export productivity between the Antarctic and subarctic Pacific over most of the past 160 ka was established by previous studies6,7 and our results (Fig. 2b, c), a significant weakening of this relationship during MIS 5 is identified. Notably, Antarctic export productivity remained high, likely due to intensified upwelling driven by poleward strengthened Southern Hemisphere westerlies, while a strong regression fit between subarctic Pacific export productivity and Antarctic air temperature was still observed (R2=0.77, n=149; Fig. 3c), implying a mechanistic connection or coincidence3,6,9.

For the subarctic Pacific, local processes such as enhanced upper-ocean stratification due to surface warming or freshening, reduced winter mixing, and changes in sea-ice extent may have influenced nutrient resupply and thus productivity35. However, our export productivity stack integrates sites spanning a wide range of hydrographic settings across both the western and eastern sectors, as well as marginal seas such as the Bering Sea. Some sites, such as pelagic ODP Site 882, were largely free from sea ice even during glacial times6, ruling out sea-ice cover as a major limitation for the observed productivity changes. Moreover, the co-variation between subarctic Pacific productivity (including Ba/Al at ODP Site 882) and Antarctic temperature is observed not only across interglacial-glacial cycles but also on millennial timescales during the Northern Hemisphere cooling6 (Fig. 2c). Given that the subarctic Pacific is unlikely to have been the primary driver of the climatic shifts documented in Antarctic ice cores, these co-variations may reflect a substantial influence of the Southern Hemisphere climate on subarctic Pacific biological productivity.

In the modern subarctic Pacific, where deep-water formation is absent58, ventilation relies largely on the lateral supply of nutrient- and CO2-rich abyssal water. This abyssal Pacific water, believed to comprise Antarctic Bottom Water (AABW) and recirculated North Atlantic Deep Water59 (NADW) that is contained within Lower CDW (LCDW) advected from the Southern Ocean. Together, these water masses form Pacific Deep Water in deeper layers of the subarctic Pacific60. Observations along the GP16 section of the East Pacific Rise indicate that west of 120°W, AABW contributes over 40% of the water column, while LCDW fractions vary patchily from 0 to 50% between 2000 and 5000 m61. Consequently, variations in AABW or LCDW export from the Southern Ocean could penetrate into the subarctic Pacific, potentially modulating nutrient availability and export productivity.

Compared with the glacial maxima (e.g., MIS 6), during rapid climate excursions that resulted in centennial to millennial-scale Northern Hemisphere cooling62,63, particularly across Terminations I and II, Antarctic warming linked to the bipolar seesaw was accompanied by an enhanced export of AABW64,65. Transient simulations support this rationale, indicating an acceleration of enhanced AABW intrusion to the subarctic Pacific66. In parallel, sortable-silt and magnetic-grain records document a southward shift of the ACC frontal system and weaker flow along its northern boundary67,68, likely limiting northward LCDW advection. The increased inflow of AABW, combined with reduced contributions of LCDW, promoted the accumulation of nutrient-rich deep water into the subarctic Pacific, and favored higher surface ocean productivity, as reflected in deglacial peaks in diatom productivity and Bio-Ba burial6,9 (Fig. 4c). Moreover, an independent upwelling index, calculated as the normalized opal mass accumulation rate at Site U1343 minus the normalized δ15N difference between Sites U1343 and 101269 also indicates enhanced deep-ocean ventilation during these intervals (Fig. 4c). This state likely persisted into the early Last Interglacial, prior to the full recovery of AMOC, under the continued dominance of AABW in the abyssal Pacific water mass properties.

By the late Last Interglacial, following AMOC recovery and enhanced ventilation of NADW and surface waters42,47, a strengthened upper overturning cell, driven by NADW and wind-induced CDW upwelling, coincided with a decline in AABW export64,65. A concurrent poleward migration of the ACC further restricted LCDW access to the abyssal Pacific67,68. This reorganization of deep-water sources reduced nutrient return to the Pacific surface and is consistent with the observed decline in productivity and deep-ocean ventilation6,9,69 (Fig. 4c). Overall, while Pacific Deep Water integrates contributions from multiple deep-water sources, subarctic productivity and ventilation change aligns most closely with variations in AABW volume transport over glacial, deglacial, and interglacial intervals6,9,64–66. Based on linear fits between Antarctic temperature and productivity, these observations suggest that Antarctic temperature may have remotely influenced subarctic Pacific ventilation and productivity primarily through its impact on AABW export. Yet, the precise mechanism remains to be elucidated.

Implications for ocean-atmosphere CO2 exchange in polar oceans

Currently, upwelling processes in both the Southern Ocean and subarctic Pacific significantly influence the exchange of CO2 between the ocean and atmosphere3,4, a process that plays a pivotal role in regulating atmospheric CO2 concentrations. A substantial amount of evidence suggests the Southern Ocean circulation’s key role in this regulation5–7, particularly over glacial-interglacial timescales, during which atmospheric CO2 levels closely correspond to Antarctic temperature70, distinct from temperature trends in mid- to high-northern latitudes71. Our results are consistent with this general framework but emphasize the role of wind-driven Southern Ocean upwelling, rather than Antarctic temperature itself, in modulating ocean-atmosphere CO2 exchange during MIS 5.

Compared to the Penultimate Glacial Maximum (e.g., MIS 6), the increased meridional thermal gradient between the hemispheres during Termination II triggered an intensification of the Southern Hemisphere westerlies4,43,44, promoting vigorous Southern Ocean upwelling and CO2 outgassing. Concurrently, enhanced AABW export coincided with stronger deep-ocean ventilation, supporting elevated CO2 outgassing in the subarctic Pacific6,9,69. This positive climate-carbon feedback amplified global warming and appears to have extended into the early interglacial, as reflected in the synchronous variability of Antarctic temperature, Antarctic and subarctic Pacific export productivity, and atmospheric CO2 (Figs. 4b–d and 4f, 5a, b). Proxy records, however, reveal that this tight coupling broke down during the late Last Interglacial (125–113 ka B.P.), showing atmospheric CO2 remained stable at ~270–280 ppm11,14,15 despite a >6°C Antarctic cooling10 (Fig. 2a, d). This striking offset challenges the conventional view, supported by numerous studies, that higher Antarctic temperatures during interglacial periods are typically associated with higher atmospheric CO2 levels10,72. Our statistical analysis further reveals that the decoupling between Antarctic temperature and CO2 is not confined to peak Last Interglacial but extends across the entire MIS 5, as evidenced by a moderate fit across the full record (R2=0.82, n=149) that strengthens markedly when MIS 5 data are excluded (R2=0.92, n=92; Fig. 3d). We therefore suggest that the conventional view of a tight Antarctic temperature-CO2 coupling oversimplifies the processes at play during MIS 5. Interestingly, our reconstructed export productivity, strongly linked to Southern Ocean upwelling4,26, exhibits a strong positive fit with atmospheric CO2 concentrations over the two full glacial-interglacial cycles (R2=0.92; n=149; Fig. 3e), confirming the persistent role of Southern Ocean upwelling in influencing ocean-atmosphere CO2 exchange.

Notably, during warm periods, high export productivity in the Antarctic zone of the Southern Ocean always coincided with elevated atmospheric CO26,7, even during the late Last Interglacial, when Antarctic temperatures dropped by more than 6°C (Fig. 2a, b, and d). This apparent paradox of elevated export productivity coinciding with increased atmospheric CO2 can be explained by an imbalance between upwelling supply and biological nutrient utilization7,17, as supported by low δ15N records indicating high surface nitrate supply during interglacial periods17,28. Vigorous upwelling may have continuously transported carbon- and nutrient-rich deep waters to the surface, exceeding the capacity of nutrients and CO2 assimilation through photosynthesis73. As a result, nutrient consumption was incomplete, constraining net biological CO2 fixation and allowing a portion of the upwelled carbon to escape to the atmosphere, a phenomenon analogous to that observed in contemporary high-nutrient, low-chlorophyll (HNLC) regions of the Southern Ocean. Thus, an HNLC-style imbalance between supply and utilization likely persisted during the Last Interglacial, maintaining elevated atmospheric CO2 even under high export productivity. This interpretation is further supported by the co-evolution of export productivity with increased nitrate supply17,28 in the Antarctic zone of the Southern Ocean and rising CO2 (Fig. 4d and f).

In contrast to the persistent role of Southern Ocean upwelling in influencing ocean-atmosphere CO2 exchange, the subarctic Pacific shows a weakened coupling from CO2 during MIS 5, as indicated by a moderate fit across the entire record (R2=0.66, n=149), and a substantially stronger fit after the exclusion of MIS 5 (R2=0.88, n=92; Fig. 3f). This decoupling parallels to the asynchronies observed among Antarctic versus subarctic Pacific export productivity, Antarctic export productivity versus Antarctic temperature, and Antarctic temperature versus atmospheric CO2, with maximum deviation around 115–113 ka (Fig. 3a, b, d and f).

In the Southern Ocean, a positive SAM-like westerlies pattern likely intensified Circumpolar Deep Water upwelling, thereby sustaining CO2 outgassing even as Antarctic temperatures declined (Fig. 5c). This interpretation is further supported by the co-evolution of export productivity with poleward-intensified Southern Hemisphere westerlies74, a strengthened ACC54, and rising atmospheric CO211 (Fig. 4d and f). In contrast, ventilation in the subarctic Pacific appears to have weakened, as indicated by a low deep-ocean ventilation69 (Fig. 4c), reducing the transfer of deep-ocean carbon to the surface (Fig. 5c). This inference is supported by the Ca/Al ratio at ODP Site 882, showing a lower deep ocean saturation state in the Subarctic Pacific6. Strengthened Southern Hemisphere westerlies during the late Last Interglacial enhanced Southern Ocean upwelling and carbon outgassing, likely forming positive climate-carbon feedback that may have initially amplified warming or prolonged the warm period. However, the stabilization of atmospheric CO2 at ~270–280 ppm, along with Antarctic cooling, suggests this feedback was eventually limited, possibly due to a new equilibrium established in winds and ocean circulation, to the finite nature of readily ventilated deep-ocean carbon reservoirs, or adjustments in Earth’s orbital parameters.

Given that the Last Interglacial is a period warmer than the pre-industrial by 1.0-1.5°C with comparable atmospheric CO2 concentrations, the contrasting responses between the Southern Ocean and the subarctic Pacific provide insights into how polar oceans may potentially respond to future CO2 changes in warming scenarios. Climate models project a reduced but stable AMOC75, alongside a poleward-downward spread of Circumpolar Deep Water76 and a >40% slowdown of Southern Ocean abyssal circulation driven by reduced AABW formation till 205076,77. Concurrently, climate simulations under scenarios of elevated greenhouse gas concentrations suggest a shift toward a more positive phase of the SAM, characterized by stronger, more poleward Southern Hemisphere westerlies78. As Southern Hemisphere westerlies continue to intensify79,80, upwelling in the Southern Ocean is expected to strengthen76,81, contributing to higher atmospheric CO2, amplifying and prolonging global warmth. In contrast, a slowdown of AABW, together with a concurrent poleward migration of the ACC that further restricts LCDW access to the abyssal Pacific, would be likely linked to reduced deep-water ventilation in the subarctic Pacific and limit ocean-atmosphere CO2 exchange. This future scenario, although projected to unfold over relatively short timescales, may be analogous to conditions during the Last Interglacial. Such potential parallels offer valuable insights into how changes in polar ocean-atmosphere interactions, driven by climate factors such as Southern Hemisphere westerlies and temperature variations, may evolve and subsequently affect atmospheric CO2 under warming scenarios.

Methods
Core material in the Southern Ocean

In this study, we have compiled 15 export productivity proxy records from 12 marine sediment cores from the Antarctic Zone of the Southern Ocean. These sediment cores include MD11-3353, PS75/083-1, PS1768-8, MD07-3134, U1537, TN057-14PC4, ODP 1094, PS75-072-4, PS1506-1, PC493, U1361A, and MD12-3394 (Supplementary Table 1). Notably, some of these cores contain multiple reconstructions of export productivity employing diverse methodologies. The data span an extensive period of 160 ka here, covering two glacial-interglacial cycles. Among the 15 proxy records, MD07-3134 and TN057-14PC, though not covering the LIG interval targeted in this study (spanning 92.5-0 ka B.P. and 80.6-6.6 ka B.P., respectively), provide detailed Holocene and millennial-scale productivity data, allowing for the compilation of a more comprehensive and robust Antarctic productivity stack. The sediment cores are situated between 50°S and 72°S, retrieved from an average water depth of 2976 m, with depths ranging from 1568 to 3713m. Sites MD11-3353, PS1768-8, PS75/83-1, PS75-072-4, PS75/83-1, TNO57-14PC, ODP 1094, and MD12-3394 are located in the Antarctic Zone of the Southern Ocean, north of the maximum winter sea ice line4,7,17,28,82–84. Sites MD07-3134, U1537, U1361A, and PS1506-1 are located within the winter sea ice limit, north of the modern maximum summer sea-ice line85, while Site PC493 is located close to the modern maximum summer sea ice limit29.

Age model of Southern Ocean sediment cores

Among the 15 export productivity proxy records, proxy records from Sites PS75/83-1, U1537, U1361A, and MD12-3394 are reconstructed using the AICC 2012 age scale17,26,82,86,87. In order to enhance the robustness of our compilation of the Southern Ocean export productivity, a common age model was applied. Thus, we have refined certain published age models, ensuring consistency across all proxy records using the AICC 2012 age scale. Specifically, the initially published chronology for Site MD11-3353 relied on the alignment of sea surface temperatures (SST) with the EPICA Dome C (EDC) δD record72,84. We have subsequently revised this age model on the AICC 2012 age scale88. Similarly, the chronology for Site PS1768-8 was refined through the correlation of SST to EDC δD record, also on the AICC 2012 age scale72,88,89. For Site MD07-3134 age scale, we enhanced and adjusted additional magnetic susceptibility (MS) of tie points to achieve the correlation to EDC dust records and East Antarctic Dronning Maud Land (EDML) nssCa2+ flux records on the AICC 2012 age scale39,40,88,90. We refined the chronology for Site TN057-14PC based on the alignment of the Eucampia Antarctica record with the EDC δD record on the AICC 2012 age scale4,88. The originally published chronology for ODP Site 1094 relied on the graphical alignment of N. pachyderma (s.) δ18Ο record with the EDC δD record7,72. We have updated this age model on the AICC 2012 age scale88. The originally published chronology for Site PS75-072-4 was established through the AMS14C and correlation of N. pachyderma (s.) δ18Ο record to the EDC δD record28,72. We have also refined this age model on the AICC 2012 age scale88. The originally published chronology for Site PS1506-1 was based on the correlation of N. pachyderma (s.) δ18Ο record to the EDC δD record27,72, we refined this age model on the AICC 2012 age scale88. The originally published age model for Site PC493 was established via tuning of both δ18O and δ18O data to the LR04 global δ18O stack29,91. We adjusted this age model by the alignment of both δ18O and δ18O data with the EDC δD record on the AICC 2012 age scale72,88. Additionally, we have incorporated estimates of age uncertainty. The chronological uncertainty associated with these records ranges from 0 to 3.84 ka, which is small in comparison to the temporal scope of our focused analysis. More details can be seen in Supplementary Fig. 1 and Supplementary Tables 1-9.

Southern Ocean export productivity reconstructions

Biogenic barium serves as a proxy for the export of organic matter from the surface ocean to the seafloor, providing a direct measure of export productivity18. In the Antarctic Zone of the Southern Ocean, diatom-derived biogenic opal offers an additional record of past productivity changes4,19. Thus, we have compiled data on both biogenic opal and biogenic barium proxy records, including the opal content, opal flux, barium flux and concentration, and various barium ratios such as Ba/Ti, Ba/Al, and Ba/Fe. We have developed a biogenic opal stack and a biogenic barium stack, each serving as a proxy for export productivity. Given the distinct nature of the biogenic opal and biogenic barium proxies—derived from different indicators such as opal content and opal flux for the opal stack, and barium concentration along with various barium ratios like Ba/Ti, Ba/Al, and Ba/Fe for the barium stack—these proxies are inherently independent of each other. Consequently, the combined use (biogenic opal+biogenic barium stack) provides a more comprehensive and robust qualitative framework to investigate past changes in export productivity in the Antarctic Zone of the Southern Ocean. Specifically, the biogenic opal stack incorporates opal content at Sites MD07-313438, U1537 (biogenic opal content is calculated by the correlation of Si/Al and biogenic opal content19), PS75/083-182, PS1768-883, and MD11-335384, as well as the opal flux data at Sites TN057-14PC4, ODP 10947, and PS75-072-428. Meanwhile, the biogenic barium stack incorporates barium concentrations from Site PS1506-192, barium flux data from Site PS1768-893, Ba/Ti ratio from Site PC49329, Ba/Al ratio from Site U1361A86, and Ba/Fe ratios from Sites ODP 10947, MD12-339417, and PS75-072-428. Collectively, these 15 proxy records have been integrated into a comprehensive combined (biogenic opal+biogenic barium) stack.

To develop biogenic opal, biogenic barium, and a combined (biogenic opal+biogenic barium) stack, we employed two different methods: the inverse standard deviation weighting method and principal component analysis (PCA). The integration of these approaches enhances our data reliability and reveals key patterns, leading to more robust and interpretable results.

The inverse standard deviation weighting approach is based on a well-established principle in quantitative data analysis, where variables are weighted inversely proportional to their variability (standard deviation). Such weighting approaches ensure more stable records contribute more strongly to the composite stack, thereby minimizing the influence of noisy records. To prepare the data for this analysis, we first resampled all 15 records at a 1 ka resolution, ensuring continuity in the data and filling the missing values. Following this refinement, each record was then normalized using the Max-Min normalization, defined as Xnorm = (Xsample - Xmin) / (Xmax - Xmin). This transformation rescales the data to the range [0,1], allowing for direct comparison of the magnitude and timing of changes in export productivity across different records. It also reduces biases in weight calculations caused by differences in data scale and improves the stability and consistency of the resulting weight distribution. Recognizing that the degree of dispersion directly reflects data stability, where a higher standard deviation corresponds to a greater fluctuation or dispersion, we calculated the standard deviation for each normalized record to quantify variability. Weight coefficients were then assigned as the reciprocal of the standard deviation of each normalized record (wi = 1/σi), so that more stable and reliable records contribute more to the final stack. The weights were then normalized to sum to 1, and weighted averages were calculated for the biogenic silica stack, the biogenic barium stack, and their combined stack (biogenic silica+biogenic barium). The result of biogenic opal, biogenic barium, and a combined (biogenic opal+biogenic barium) stack can be seen in Fig. 2, Supplementary Fig. 2 and Supplementary Data 194. The correlation of biogenic opal, biogenic barium, and a combined (biogenic opal+biogenic barium) stack with the associated export productivity records at different sites in the Antarctic Zone of the Southern Ocean can be seen in Supplementary Fig. 4.

PCA was employed to identify the main trends in the data, revealing the underlying patterns and relationships among the proxy records. This analysis was conducted using PAST v4.07 software95. First, we standardized all 15 export productivity records. This normalization process is crucial as it mitigates the influence of variables with larger ranges, allowing for a more accurate comparison of patterns across the proxy records. The individual normalized records were then resample data 1 ka resolution. Since many records did not span the full period of 0-160 ka, we addressed the missing values by employing mean value imputation. Following these steps, we performed the PCA analysis to extract the principal components. The result of principal components on related export productivity records is shown in Supplementary Figs. 2, 5, 694.

The PCA of the biogenic opal records, biogenic barium records, and the combined biogenic opal and biogenic barium records reveal a dominant trend in export productivity represented by the first principal component (PC1). This component also displayed high values during interglacial and lower values during glacials (Supplementary Fig. 2). PC1 of the biogenic opal records explains 53.614% of the total variance, while PC2 accounts for only 16.202% (Supplementary Fig. 5a). For the biogenic barium records, PC1 explains 68.203% of the total variance, with PC2 contributing just 11.496% (Supplementary Fig. 5b). In the combined records, PC1 accounts for 57.64% of the total variance, while PC2 explains only 11.654% (Supplementary Fig. 5c). Notably, all principal components except PC1 fall below the broken stick curve (Supplementary Fig. 5), indicating that only PC1 appears to carry enough signal for interpretation. Additionally, strong correlations between PC1 and the various variables further support the significance of PC1 in capturing the underlying dynamics of the dataset (Supplementary Fig. 6).

The most significant observation is that PC1 of the biogenic opal records, biogenic barium records, and their combined counterpart showed a high degree of overlap with the respective stacks generated using the inverse standard deviation weighting method (Supplementary Fig. 2). Given that the data are not normally distributed, Spearman’s correlation was applied, showing ρ values exceeding 0.94 across all three stacks (Supplementary Fig. 3). Importantly, both PC1 and the respective stacks correlate strongly with previously published biogenic opal and biogenic barium records in the Antarctic Zone of the Southern Ocean26–29 (Supplementary Figs. 4, 6–8). This consistency in findings enhances the reliability of the data, suggesting that both the PCA and inverse standard deviation weighting method capture the Antarctic export productivity patterns.

Subarctic Pacific export productivity stack

To facilitate comparison with the Southern Ocean records, we have additionally compiled 5 export productivity proxy records from 10 marine sediment cores located in the subarctic North Pacific using two independent methods consistent with those applied to Antarctic records, allowing direct comparison across datasets. These cores include ODP 8826, MR98-05-3PC34, JPC1733, ODP 88732, and a group of six closely spaced cores from the Juan de Fuca Ridge (JdFR) area35: AT26-19-05PC, AT26-19-09PC/06MC, AT26-19-12PC-TC/10MC, AT26-19-35PC, AT26-19-38PC, and AT26-19-39BB (Supplementary Table 2). Spatially, these cores are distributed between 44.96°N and 54.22°N, and from 164.98°E to 49.39°W, spanning a range of hydrographic settings across the subarctic Pacific, including both the western and eastern subarctic sectors and marginal seas such as the Bering Sea, with water depths ranging from 2209 to 5507m. Given the geographic proximity of the JdFR cores, which exhibit similar productivity signals and trends35, we adopted an existing regional productivity stack derived from these six sites, which provides a continuous record over the past 160 ka and serves as a representative index for this sector35. This approach ensures better temporal continuity and reduces redundancy in overlapping signals. Combined with records from other subarctic locations including JPC17 (118.8–0.6 ka)33, MR98-05-3PC (142–3.5 ka) 34, ODP 882 (149.7–8.7 ka)6, and ODP 887 (160-0 ka)32, these data enable the construction of a subarctic North Pacific productivity stack covering at least two full glacial-interglacial cycles.

Similar to the Southern Ocean, biogenic barium is widely used as a paleoceanographic proxy for export production in the subarctic Pacific. However, opal records from the subarctic Pacific may underestimate past productivity due to diagenetic alteration of biogenic silica after burial. This is evidenced by the observed divergences between opal and excess silica fraction fluxes during older intervals (e.g., MIS 5), when excess silica suggests higher productivity than opal35. In addition, although sedimentary 231Pa/230Th in the modern Pacific Ocean is largely influenced by opal scavenging96, it is relatively insensitive to preservation and diagenetic effects97,98, making it a robust tracer for reconstructing past changes in productivity35. Therefore, to reconstruct export productivity stack in the subarctic Pacific, we compiled records of both biogenic barium and 231Pa/230Th applying methods consistent with those used for the Antarctic records.

All sediment cores included in the compilation are well dated using well-established age models, primarily based on radiocarbon dating, magnetostratigraphy, biostratigraphy, and astronomical tuning (Supplementary Table 2). For some sites, detailed age-model data are not publicly available, so we used the original published age models to maintain consistency and avoid introducing additional uncertainties in the stack construction. While some spatial variability in export productivity exists among subarctic North Pacific sites, the stack constructed from multiple cores using two independent methods displays consistent patterns (Supplementary Data 294). The PCA of the subarctic Pacific records shows that the first principal component (PC1) captures the dominant trend in export productivity (EP), explaining 61.3% of the total variance, whereas the second component (PC2) explains only 14.8% (Supplementary Fig. 10). This pattern closely aligns with the stack generated using the inverse standard deviation weighting method, as well as the previously published biogenic barium and 231Pa/230Th records from the subarctic Pacific6,32–35 (Supplementary Fig. 10). To quantify this relationship, we calculated Pearson’s correlation coefficients, given that the subarctic data generally follow a normal distribution. The two stacks exhibit a strong correlation of 0.97 and maintain correlations above 0.7 with all previously published individual biogenic barium and 231Pa/230Th records. This consistency supports the reliability of the stacks as representative proxies, thereby enabling robust interhemispheric comparisons across glacial-interglacial cycles.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Dr Lijuan Lu and colleagues reconstruct changes in export productivity in the Southern Ocean and Arctic Pacific covering two glacial-interglacial cycles (last 160 Ka) using 15 export productivity records from 12 marine sediment cores from the Antarctic Zone of the Sothern Ocean and comparing them with subarctic Pacific EP. Authors argue that both Antarctic and Arctic export productivity followed similar patterns during most of the record expect for the Last Interglacial (128-113 kyrs). In particular, during the LIG, Antarctic export production and atmospheric CO2 was high and coupled with a decline in Antarctic air temperatures of ca. 6ºC, while Subarctic Pacific export productivity dropped. Authors argued that this decoupling was most likely driven by an intensification of the southern hemisphere Westerlies that stimulated upwelling of Circumpolar Deep Water that enhanced export productivity and the release of CO2 to the atmosphere.\n\nOverall, the manuscript is well-written and structured, and the figures are of high quality. Regarding the statistical treatment, some additional information is required while other analysis are beyond my expertise and therefore, I am unable to judge their validity. In regard to the scientific interest, the polar oceans play an essential role in the control of global atmospheric CO2, and model predictions point to a poleward intensification of westerlies in the coming decades. Therefore, new insights into the role of the Southern Ocean and Arctic Ocean during a period with analogies to our imminent future could be of great interest for a broad scientific audience.\n\nHaving said that, there are several important points that authors would have to deal with to prove their hypothesis and therefore before the manuscript can be considered suitable for publication in Nature Communications. Please find a list of the principal drawbacks of this paper listed below:\n\nBiogenic silica accumulation as a proxy for export productivity:\nThe interpretation of biogenic silica accumulation as a direct proxy for export productivity has been questioned by multiple authors. As indicated by De la Rocha et al. (2010): “Although there is a strong link between BSi production and BSi accumulation, variability and shifts in the ratios of BSi to POC during particle production, export, and preservation complicate the extrapolation of primary production from BSi accumulation (Pondaven et al., 2000; Ragueneau et al., 2000; DeMaster 2002; Nelson et al., 2002; Ragueneau et al., 2002; Moriceau et al., 2007).”\n\nAnd also in De la Rocha et al. (2010):: “…the lower glacial accumulation rates of BSi south of the APF in the Atlantic sector of the Southern Ocean can be entirely accounted for by shifts in diatom species composition and Fe availability and do not need to reflect lower glacial levels of primary production. Likewise, the higher glacial accumulation rates of BSi north of the APF may likely also not reflect higher glacial levels primary production”.\n\nMoreover, Assmy et al. (2013) also indicated that “It follows that high sedimentary silica accumulation rates are a proxy for iron-limited diatom assemblages rather than for high surface productivity, contrary to the prevailing view”.\n\nTherefore, in order to assess changes in export production would be important to discuss changes in diatom species composition (the main components of the Biogenic silica) rather than discussing the Biogenic Silica alone. Is diatom assemblage composition for those records available? Do diatom assemblages change? And if so, does the change in the diatom assemblage support the interpretations of the results? Although the present manuscript is not focussed on the diatom assemblage composition, some information in this regard could greatly help to support (or discard) the findings of the authors.\n\nTemperature as main control of export productivity:\nLine 176 “Assuming that temperature is the primary control on export productivity in the Antarctic Zone”. The main control of algal biomass accumulation and export productivity in the modern Antarctic Zone is iron availability (Boyd et al., 2000). While temperature is most likely an important factor controlling the distribution of phytoplankton functional groups and therefore efficiency of the biological pump, the primary role of temperature in the control of export production is not clear at all. This point has to be explained in detail in the manuscript. Please clarify.\n\nExport productivity in the modern Southern Ocean.\nLines 170-171 “Currently, the zone of maximum export productivity is located south of the Antarctic Polar Front (APF), within the Antarctic zone of the Southern Ocean”. This is statement is true for part of the western Pacific sector of the Southern Ocean based on the data from the AESOPS experiment. However, it cannot be considered a circumpolar feature of the Southern Ocean. For example, POC fluxes to the deep ocean are very similar between the Antarctic Zone, Polar Frontal Zone and Subantarctic Zone in the Australian sector of the Southern Ocean: in this region annual POC fluxes are slightly higher in the SAZ than in the AZ at 2000 m and 3800 m (1.4 and 1 g C m-2 yr-1 at 2000 and 3800 m at the SAZ versus 1.2 and 0.9 g C m-2 yr-1 at 2000 and 3800 m at the AZ). See Wynn-Edwards et al. (2020) and Rigual-Hernández et al. (2015). Please also look for information in the Atlantic sector. Based on all of the above, authors should be more cautious with their statement and explain in detail the differences in EP across sectors of the Southern Ocean. Also, quantifying the difference in EP between circumpolar systems would help the reader to understand what is the current difference in export production between different zonal systems. While total mass flux is quite different across zonal systems mainly due to differences in opal export, the magnitude in POC export is small.\n\nCorrelations: e.g. lines 211-212 and Fig. 4. It is not clear in the text if the variables in Fig. 4 are normally distributed. This information is important to determine which correlation coefficient would be more appropriate. Could authors explain in the text why Pearson correlation coefficient was used and not Spearman’s test? Please clarify and revised if necessary.\n\nMethods, lines 108-110: “inverse standard deviation weighting method employs the inverse of each variable’s standard deviation as weights to minimize the influence of high-variability variables on the results”. This methodology is beyond my expertise and therefore I am unable to evaluate if this is an adequate approach.\n\nFigure 5. Please explain in the text the meaning of the circle with a dot in the schematic (position of the main winds I assume).", "reviewer_2": "The manuscript by Lu et al. addresses a critical and timely question in paleoclimate: why Antarctic and subarctic Pacific EP became decoupled during the LIG despite generally co-varying with atmospheric CO2 and temperature on glacial–interglacial timescales. The study’s primary finding – that intensified, poleward-shifted SHW drove enhanced upwelling of nutrient- and CO2-rich deep water in the Antarctic Zone during the LIG, sustaining high Antarctic EP and contributing to high atmospheric CO2, while subarctic Pacific EP declined – is novel and significant. It challenges the traditional paradigm that Antarctic temperature was the dominant driver of Southern Ocean carbon release in warm periods. The authors provide an alternative mechanism whereby wind-driven ocean dynamics can override or modulate temperature-driven effects, which represents an important contribution to paleo-carbon cycle science and highlights the SHW as an underappreciated player in controlling past CO2 variability.\n\nOverall, this is an impressive and valuable manuscript and is squarely appropriate for Nature Communications, given its significant for paleoclimate and future climate understanding. I recommend publication after revision, addressing the concerns raised below. The most critical issues to resolve are clarifications of the proposed mechanism (especially regarding changes in deep ocean circulation and the contrasting behavior of the subarctic Pacific) and some enhancements to clarity in presentation. Thus, the revision can likely be done in a straightforward manner. I look forward to seeing a revised manuscript that incorporates these suggestions.\n\nSome key concerns:\nThe authors demonstrate high coherence among proxy types (opal vs. barium) and perform cross-correlations that show Antarctic EP, subarctic Pacific EP, and Antarctic temperature tracked each other closely for most of the past 160 kyr. For the critical LIG interval, the authors identify a breakdown of these correlations (the decoupling). However, I think this is not clearly shown qualitatively in figures, and I recommend quantifying the decoupling more explicitly. For example, the authors could report correlation coefficients between Antarctic and subarctic EP over specific intervals (glacial vs. interglacial vs. LIG) to statistically demonstrate that the LIG correlation is significantly lower. A running or windowed correlation analysis might also highlight when the divergence emerged.\n\nAnother methodological aspect that could use more explanation is how the subarctic Pacific export productivity record was derived or selected. The text references subarctic Pacific EP data with citations, but readers may benefit from a clearer description: e.g., is this a composite record from a particular site or region? Detailing the source and nature of the subarctic EP reference would help the audience understand its resolution and uncertainties relative to the Antarctic data. Additionally, if the subarctic record’s age model was also aligned to Antarctic chronology (perhaps via atmospheric CH4 or δ18O correlations at Termination II), that should be stated for transparency.\n\nThe authors argue that in the LIG, upwelling supply outpaced biological uptake, leading to CO2 release despite high productivity. This is an important nuance – usually high productivity draws down CO2, but only if nutrient utilization is nearly complete. In an intense upwelling scenario, even robust productivity can be overwhelmed by the carbon influx from the deep ocean, resulting in a net CO2 efflux. The manuscript should be commended for touching on this mechanism in the Intro and should ensure it is clearly reiterated in the Discussion when interpreting the late LIG CO2 “paradox.”\n\nOne point that would benefit from clarification is the contrast between Antarctic and subarctic Pacific regimes. The authors find that, unlike Antarctica, the subarctic North Pacific experienced a decline in productivity during the LIG cooling, remaining “tightly coupled” to Antarctic temperature changes. They attribute this to a “weakened Southern Ocean abyssal circulation” that suppressed nutrient upwelling in the subarctic Pacific, thereby limiting productivity and CO2 release in the north. This explanation is intriguing but somewhat under-explained. The implication is that global deep-ocean circulation patterns shifted such that, even as surface wind-driven upwelling intensified around Antarctica, the supply of deep nutrients to the North Pacific was curtailed – perhaps due to reduced overturning or deep water formation changes. This raises a few questions that the authors should address:\n• Mechanistic clarity: How exactly do the authors envision the chain of events linking SH westerly changes to North Pacific upwelling reduction? Is it that stronger upwelling in the Southern Ocean drew more deep water up there (and outgassed its CO2) before it could travel to the North Pacific, effectively “robbing” the North Pacific of nutrient-rich deep water? Or was there a coincident change in deep ocean stratification or Atlantic Meridional Overturning Circulation that left the North Pacific more stratified? The phrase “weakened Southern Ocean abyssal circulation” is a bit abstract – it would help if the authors could rephrase or expand this to describe the process in more concrete terms (e.g., reduced formation of Antarctic Bottom Water during the LIG or a bottleneck in deep-water exchange between the Southern Ocean and North Pacific). Since this point is central to explaining the Antarctic vs. subarctic decoupling, a clearer discussion is warranted. Even a schematic in a figure might help illustrate the hypothesized circulation changes.\n• Local vs. global factors in the subarctic: The authors might consider alternative or complementary explanations for the subarctic Pacific productivity decline. One possibility is increased upper-ocean stratification in the subarctic Pacific as global climate warmed into the LIG peak, reducing vertical nutrient mixing (warmer surface waters could strengthen the halocline, for instance). The text focuses on deep circulation, but could some of the subarctic decline simply be due to local oceanographic changes (like more stable water column or less winter mixing)? If there is literature or evidence on subarctic North Pacific stratification or nutrient changes in the LIG, referencing or discussing it would give a more balanced interpretation. The current interpretation is reasonable, but readers may wonder if the subarctic Pacific’s behavior is entirely governed by Southern Ocean deep-water supply or also by its own regional climate conditions.\n\nThe climatic and carbon cycle implications of this interpretation are important and should be explicitly stated. The authors touch on it in the abstract and intro: if westerlies indeed kept CO2 high during the LIG, this suggests a feedback wherein warming leads to stronger westerlies, which then lead to CO2 outgassing, potentially amplifying warming further. This is a positive feedback mechanism in the Earth system. The fact that CO2 stayed around 270–280 ppm while Antarctic temperature fell by ~6°C hints that other factors (perhaps declining Southern Hemisphere insolation or ice sheet regrowth) eventually outweighed the westerly-driven CO2 feedback in the LIG. The authors may want to comment on why CO2 did not continue rising indefinitely – presumably because the source of easily ventilated carbon was finite or because winds and ocean circulation reached a new equilibrium. A bit of discussion here would show the authors have considered the limits of the westerly wind effect in the natural system.\n\nIn terms of climate dynamics, I also have some comments to consider:\nOne aspect that could be emphasized is the parallel between the LIG scenario and the modern/future Southern Annular Mode (SAM) dynamics. The intensified, poleward-shifted westerlies the authors invoke are essentially a prolonged positive phase of the SAM (or a similar mode) under interglacial warmth. In modern observations and model simulations, a positive SAM (strengthened poleward westerlies) leads to enhanced upwelling of deep waters around Antarctica and reduced stratification, sometimes causing outgassing of CO2 from the Southern Ocean. The authors allude to future implications (with SHW expected to intensify under global warming), which is excellent. They could bolster this by explicitly noting that their paleo-findings are consistent with SAM-like behavior on long timescales. This not only places the result in a familiar framework for climate community but also underscores that such ocean-atmosphere coupling is already observed on shorter timescales today. If not already mentioned, integrating a brief mention of SAM or analogous wind-pattern changes would strengthen the connection to known climate modes.\n\nAlso, the idea of ocean-atmosphere feedbacks could be expanded. The authors have essentially identified a feedback loop: warmer climate (and/or changing insolation) → poleward-intensified westerlies → increased Southern Ocean upwelling → CO2 release → further warming globally. It would be valuable to articulate this clearly. It’s also worth discussing the time lag or sequence of events: Did the westerlies intensify because of the initial LIG warmth (e.g., due to a reduced equator-to-pole temperature gradient), thus later sustaining CO2 levels even as Antarctic temperatures started to drop? If so, we are observing a hysteresis or time-lag effect where wind-driven CO2 feedbacks prolong the warmth. Any insight the authors can provide (or speculate on, based on timing in their data) about the sequence – e.g., did Antarctic EP start rising before or just as CO2 started rising at Termination II, and did subarctic EP falter only after the Antarctic winds revved up? – would be intriguing context.\n\nThe manuscript touches on large-scale modes indirectly by discussing “abyssal circulation” and upwelling differences. To further integrate with global-scale ocean circulation, the authors might consider the role of the meridional overturning circulation (MOC) in their scenario. A subtle point: if Southern Ocean deep upwelling was enhanced, was there any compensating change in North Atlantic Deep Water production or global MOC strength? Some studies of past warm periods (and future projections) suggest that freshening or warming can alter North Atlantic circulation, which in turn might influence the volume of deep water reaching the Southern Ocean and eventually the North Pacific. While the focus here is rightly on Southern Ocean winds, acknowledging these broader connections would show comprehensive thinking. For instance, if the authors suspect a slowdown in some component of the MOC (hence “weakened abyssal circulation”), they could tie it to known possibilities like reduced Antarctic Bottom Water formation in a warmer climate. If any PMIP Last Interglacial model outputs or previous studies (e.g., on Atlantic vs. Pacific ventilation) align with their findings, they should be referenced to give the reader confidence that the proposed dynamics are realistic.", "reviewer_3": "This is a very interesting study that produced a stacked record of Antarctic Southern Ocean sediment proxy records of export productivity and compared this productivity stack with Antarctic temperature, atmospheric CO2, and Subarctic North Pacific export productivity variations over the last 160 kyr, with a specific interest in the Last Interglacial period. The study examines temporal variations in the relationships between these climatic parameters through comparison of time series and through direct correlation analysis and highlights that the positive linear correlation between atmospheric CO2, Antarctic temperature, and Antarctic and Subarctic productivity seem to break down during the late Last Interglacial. Based on this analysis, the authors conclude that the Last Interglacial period may have been associated with poleward migration of the Southern Hemisphere westerlies, which enhanced nutrient upwelling, productivity, and deep carbon release in the Antarctic Southern Ocean. In contrast, the authors suggest that the North Pacific experienced reduced abyssal upwelling, which caused a reduction in subarctic productivity and a decoupling between subarctic productivity and atmospheric CO2.\n\nThis work tackles the dynamic mechanism behind global carbon cycling in warmer-than-modern climate conditions, which is an interesting problem on its own and very relevant for accurate predictions of future climate behavior associated with anthropogenic warming. I appreciate the analytical approach of this study and commend the authors for the very good data accessibility provided in their Materials and Methods and supplemental tables. Although this is not the first study to support poleward migration of the westerlies during warmer-than-modern paleo-intervals, this is the first one to my knowledge that directly connects this mechanism to disparate changes in Antarctic and subantarctic export productivity across the Last Interglacial. However, I do have a few suggestions, including additional analysis of the data, that I believe would strengthen the impact of this work. Therefore, I recommend that the authors be given the opportunity to address the following comments before finalizing a publication decision for Nature Communications.\n\nGeneral Comments:\n(1) Broadly, the comparison between Antarctic export productivity records (synthesized into a stack of 15 different proxy time series from 12 different sediment cores) with the subarctic North Pacific export productivity records (indicated by two barium-based proxies from a single sediment core at Site 882) seems unbalanced. How well does export productivity at Site 882 reflect the entire subarctic North Pacific? The Antarctic records exhibit some degree of spatial variability (for example, ODP 1094 looks very different from U1537), would be expect the same in the North Pacific? Even if there are not additional North Pacific productivity records covering this interval (which I am not sure about), this manuscript would benefit from a discussion of how representative Site 882 is of the region based on more recent intervals, where additional productivity data is more likely to be available.\n\n(2) The argument that the dynamics observed during the Last Interglacial period represent a unique climate regime would be more compelling if the study also included analysis of how the Antarctic temperature, atmospheric CO2, Antarctic export productivity, and subarctic export productivity records compared across the Holocene/current interglacial period. The Antarctic productivity stack that this study has generated is well suited for this analysis because it covers the last 160 kyr completely. It seems the Site 882 productivity record does not span the Holocene, so perhaps another North Pacific productivity record could be used here for the comparison?\n\n(3) I appreciate the inclusion of the correlation plots in Figure 4 as a way to examine the data beyond the time series comparisons illustrated in Figures 2 and 3, with respect to differences between records during MIS6, T2 and the LIG. Panel 4d is especially helpful for illustrating the decoupling between Antarctic temperature and atmospheric CO2 during the LIG and Panels 4e and 4f very clearly suggest that atmospheric CO2 concentrations seem to be much more driven by Antarctic Zone productivity than by North Pacific productivity.\n\nHowever, the circled LIG data in panels 4a and 4b look to me more like a regime shift than a complete decoupling between Antarctic temperature and Antarctic productivity or between Site 882 productivity and Antarctic productivity (e.g., the data still look to be linearly correlated, though perhaps with a different slope and intercept). What correlation coefficients do you get when you examine just the LIG data, rather than the full 150-113 ka record? Are the LIG-labelled data points that fall closest to the 150-127 ka trendline the ones closest in age to T2, or are they from the middle of the LIG? It would also be helpful to see how data from MIS 3, 4, and 2 would plot on these graphs (at least in the supplement if it makes the main figure too crowded), to see how representative the MIS 6 and T2 relationships are of the last glacial cycle as well.\n\nSpecific Comments:\nLine 34: Suggest to rephrase to “…abrupt decline in the Antarctic air temperature stack (ATS)…” so unfamiliar readers will understand where the S is coming from in the acronym.\nLines 37-40: I think the conclusion of this sentence would be easier to follow if an additional step in the logic were explained (for example, please specify here why a tight coupling between subarctic Pacific export productivity and Antarctic temperature indicates suppressed abyssal upwelling).\nLine 91: Suggest to clarify here, rather than waiting until the methods section, that the 15 export productivity records are from 12 sediment cores (especially as there are only 12 orange dots in the map figure).\nLines 139- 141: Suggest to include the correlation coefficients for the full record here between the Antarctic proxy stack, the ATS and the subarctic records for consistency in comparison across time periods.\nLines 164-168: Suggest to rephrase to “in contrast, iron fertilization does not appear to have been impactful in the Antarctic Zone, which exhibits lower export productivity during glacial periods” to clarify this point.\nLines 176-178: It is my understanding that Ref 32 does not assert that temperature is the primary control on export productivity in the Antarctic zone, rather they suggest that low glacial productivity in this region may be caused by reduced wind-driven upwelling associated with a northward migration of the Southern Ocean fronts. Consequently, I suggest not using this reference to justify the first part of this sentence.\nLines 247-249: Suggest to continue to specify that “temperatures” refers to “Antarctic temperatures” here, to avoid confusion associated with asynchronous millennial-scale warming between Greenland and Antarctic temperature records (e.g., Blunier et al., 1998).\nLine 560 (Figure 3 caption): More detail is needed here regarding the upwelling index denoted (e.g., suggest to specify that it’s the subarctic upwelling index based on U1343 and Site 1012). Since the paper typically indicates what geochemical proxies are used to infer each parameter for productivity, ACC strength, etc…, I suggest to include the proxy basis of the upwelling index here as well (or perhaps where this is discussed in the main text).\nFigure 5: I found this figure very helpful for visualizing the different suggested circulation scenarios, but I can’t find a reference to this figure anywhere in the main text.\nLines 599-601: Please explain here why you included these two cores into the productivity stack, although they do not span the LIG range targeted by this study.\nLine 678: I assume records were normalized by dividing each by its mean. Could you please specify the exact approach used so unfamiliar readers can better follow along?\nLines 686-691: After looking through Figures S7 and S8, I wonder if this inverse dispersion weighting is artificially removing legitimate high frequency signal from the stacked record (leading to an over-smoothing). There are many Antarctic sediment cores in which sedimentation rates vary largely between productive climates and low-productivity climates (ODP 1094 is a good example), such that the ability of each record to capture millennial-scale changes varies temporally. There are also clear difference between sediment cores in their ability to capture high frequency variations in export productivity (for example, TN057-14PC4 captures much more resolution than PS75-072-4 during MIS3), does that mean TN057-14PC4 receives a lower weighting than PS75-072-4? Should it? Would the stacked export productivity records look very different if you weighted each record by sedimentation rate across a given interval, rather than inverse dispersion?\n\nAlso please note that, in general, Marine Isotope Stage labels are used throughout the figures, but do not appear to be mentioned or explained in the main text.\n\nReferences Mentioned:\nBlunier, T., Chappellaz, J., Schwander, J. et al. Asynchrony of Antarctic and Greenland climate change during the last glacial period. Nature 394, 739–743 (1998). https://doi.org/10.1038/29447"}, "author_response": "Thank you very much for your thorough and constructive review. We appreciate your positive comments on the writing and structure of our manuscript, as well as your recognition of the broader scientific interest of our study in the context of future climate change. In response to your comments, we have provided additional details on our statistical methods, including the use of Spearman’s or Pearson’s correlation based on data distribution, and a more thorough explanation of the inverse standard deviation weighting approach. We have also revised key sections of the manuscript and provided stronger evidence supporting our hypothesis. For example, we have added a discussion of changes in diatom species composition that further support biogenic silica accumulation as a reliable proxy for export productivity. A detailed point-by-point response is provided below.\n\nMain concern:\n1. Biogenic silica accumulation as a proxy for export productivity:\nThe interpretation of biogenic silica accumulation as a direct proxy for export productivity has been questioned by multiple authors. As indicated by De la Rocha et al. (2010): “Although there is a strong link between BSi production and BSi accumulation, variability and shifts in the ratios of BSi to POC during particle production, export, and preservation complicate the extrapolation of primary production from BSi accumulation (Pondaven et al., 2000; Ragueneau et al., 2000; DeMaster 2002; Nelson et al., 2002; Ragueneau et al., 2002; Moriceau et al., 2007).”\nAnd also in De la Rocha et al. (2010): “…the lower glacial accumulation rates of BSi south of the APF in the Atlantic sector of the Southern Ocean can be entirely accounted for by shifts in diatom species composition and Fe availability and do not need to reflect lower glacial levels of primary production. Likewise, the higher glacial accumulation rates of BSi north of the APF may likely also not reflect higher glacial levels primary production”.\nMoreover, Assmy et al. (2013) also indicated that “It follows that high sedimentary silica accumulation rates are a proxy for iron-limited diatom assemblages rather than for high surface productivity, contrary to the prevailing view”.\nTherefore, in order to assess changes in export production would be important to discuss changes in diatom species composition (the main components of the Biogenic silica) rather than discussing the Biogenic Silica alone. Is diatom assemblage composition for those records available? Do diatom assemblages change? And if so, does the change in the diatom assemblage support the interpretations of the results? Although the present manuscript is not focused on the diatom assemblage composition, some information in this regard could greatly help to support (or discard) the findings of the authors.\n\nResponse:\nThe reviewer raised a crucial issue regarding potential limitations of biogenic silica (BSi) accumulation as a proxy for export productivity, noting it can be significantly modulated by diatom species composition, iron availability, and preservation dynamics. We agree completely with the reviewer that BSi accumulation is not a direct, unambiguous proxy and thank them for highlighting this critical issue. To address this, we have taken a multi-pronged approach to ensure the robustness of our export productivity reconstructions:\n1) Explicit acknowledgement of limitations: We have revised the main text (lines 106-110) to explicitly state the potential complications with the BSi proxy, citing key literature:\n“The content and fluxes of biogenic opal have long been used as proxies for past export productivity, particularly in the diatom-rich Antarctic Zone of the Southern Ocean (Anderson et al., 2009). However, biogenic opal accumulation can be influenced by various factors such as diatom species composition and iron availability (e.g., De la Rocha et al., 2010; Assmy et al., 2013), in addition to particle aggregation, differential dissolution, and post-depositional preservation effects (Pondaven et al., 2000; Ragueneau et al., 2000; DeMaster, 2002; Nelson et al., 2002; Ragueneau et al., 2002; Moriceau et al., 2007).”\n2) Multi-proxy approach: To mitigate the biases of any single proxy, we developed independent stacks for biogenic opal (BSi content and flux) and biogenic barium ((flux, concentration and elemental ratios (Ba/Ti, Ba/Al, and Ba/Fe)). A combined stack integrating both proxies provides a more comprehensive and robust qualitative framework. The strong agreement (Spearman’s correlations of ρ = 0.94- 0.98) between these independent stacks via different methods (inverse weighting & principal component analysis (PCA)) strongly indicates they capture the true underlying export productivity signals, not proxy-specific artifacts. All stacks exhibit synchronous glacial-interglacial patterns, broadly consistent with previously published biogenic opal and barium records from the Antarctic Zone of the Southern Ocean, further supporting their reliability. Please see Fig.S2-S8 and lines 110-130 in Main Text.\n3) Validation with diatom assemblages data: As suggested, we incorporated independent paleoecological data. The abundance of Fragilariopsis kerguelensis, a dominant species in the permanent open-ocean zone of our study region and a key indicator of high productivity, closely tracks our reconstructed stacks (Fig. r1). This provides direct biological validation that our stacks reliably capture past changes in BSi-based export productivity. Please see lines 130-134 and Fig. S9 in Main Text.\n\nFig. r1 (new Fig. S9 of the revised manuscript) | Comparisons of biogenic opal and (opal+Ba) stack with the Fragilariopsis kerguelensis abundance records (Crosta et al., 2004; Abelmann et al., 2006), a dominant diatom species in the permanent open ocean zone.\n\n2. Temperature as main control of export productivity:\nLine 176 “Assuming that temperature is the primary control on export productivity in the Antarctic Zone”. The main control of algal biomass accumulation and export productivity in the modern Antarctic Zone is iron availability (Boyd et al., 2000). While temperature is most likely an important factor controlling the distribution of phytoplankton functional groups and therefore efficiency of the biological pump, the primary role of temperature in the control of export production is not clear at all. This point has to be explained in detail in the manuscript. Please clarify.\n\nResponse:\nThis is an excellent point that allowed us to clarify our hypothesis. We agree that on modern, seasonal timescales, iron limitation is paramount (Boyd et al., 2000). Our study focuses on glacial-interglacial timescales, where the mechanisms we explore involve temperature’s indirect role in modulating the physical supply of nutrients (including iron) via its control on buoyancy and upwelling (e.g., Watson et al., 2015). We have revised the text to clarify this distinction: the long-term correlation suggests temperature influences upwelling intensity and nutrient supply, but the MIS 5 decoupling shows that wind forcing can override this temperature-driven mechanism. The persistence of high productivity during Antarctic cooling is precisely the evidence that another driver (Southern Hemisphere Westerlies) played an important role during this interval:\n“Previous studies suggested that Antarctic air temperature plays an important role by affecting Southern Ocean surface buoyancy forcing over glacial-interglacial cycles (e.g., Watson et al., 2015), which in turn influences upwelling intensity and nutrient supply. Hence, one would expect export productivity and Antarctic air temperature to exhibit a close relationship across the entire glacial-interglacial cycles. We then tested this hypothesis using linear regression across the entire record, which yields a moderate fit (R2=0.76, n=149). Excluding MIS 5, however, results in a much stronger fit (R2=0.90, n=92; Fig. 3a), revealing that MIS 5 is an interval during which the relationship deviates from the long-term mean relationship. More importantly, during the late Last Interglacial, Antarctic export productivity remained elevated despite an abrupt 6 °C drop in Antarctic air temperature (Figs. 4b and 4d), suggesting that Antarctic air temperature cannot fully account for the variability in Antarctic export productivity.”\nPlease see lines 202-214 in Main Text.\n\n3. Export productivity in the modern Southern Ocean.\nLines 170-171 “Currently, the zone of maximum export productivity is located south of the Antarctic Polar Front (APF), within the Antarctic zone of the Southern Ocean”. This is statement is true for part of the western Pacific sector of the Southern Ocean based on the data from the AESOPS experiment. However, it cannot be considered a circumpolar feature of the Southern Ocean. For example, POC fluxes to the deep ocean are very similar between the Antarctic Zone, Polar Frontal Zone and Subantarctic Zone in the Australian sector of the Southern Ocean: in this region annual POC fluxes are slightly higher in the SAZ than in the AZ at 2000 m and 3800 m (1.4 and 1 g C m-2 yr-1 at 2000 and 3800 m at the SAZ versus 1.2 and 0.9 g C m-2 yr-1 at 2000 and 3800 m at the AZ). See Wynn-Edwards et al. (2020) and Rigual-Hernández et al. (2015). Please also look for information in the Atlantic sector. Based on all of the above, authors should be more cautious with their statement and explain in detail the differences in EP across sectors of the Southern Ocean. Also, quantifying the difference in EP between circumpolar systems would help the reader to understand what is the current difference in export production between different zonal systems. While total mass flux is quite different across zonal systems mainly due to differences in opal export, the magnitude in POC export is small.\n\nResponse:\nThe reviewer noted that modern export productivity is not uniformly high south of the Polar Front and exhibits significant regional variability, suggesting caution against over-generalization. We thank the reviewer for this important correction. Below, we provide additional context and the revisions made in the manuscript:\n1) Regional differences context: Observations and model-based estimates confirm that export productivity varies across sectors. Results from the Antarctic Environment and Southern Ocean Process Study (AESOPS; 1996-1998) in the Pacific sector showed that the highest annual fluxes of particulate organic carbon (POC; ~200 mmol m⁻² yr⁻¹) and biogenic silica (~1400 mmol m⁻² yr⁻¹) to the seafloor occurred in the southern Antarctic Circumpolar Current (61.5°S-65.5°S; Nelson et al., 2002). In contrast, in the Australian sector, long-term sediment trap data indicate that POC fluxes are comparable or even slightly higher in the Subantarctic Zone than in the Antarctic Zone (1.4 and 1.0 g C m⁻² yr⁻¹ vs 1.2 and 0.9 g C m⁻² yr⁻¹; Rigual-Hernández et al., 2015; Wynn-Edwards et al., 2020). Additionally, model-based estimates suggest that a large fraction of POC export is concentrated in a circumpolar belt aligned with the ACC and in coastal upwelling zones off Peru, Chile, and Namibia (Schlitzer, 2002). While we agree with the reviewer that a quantitative comparison of export productivity across circumpolar systems would be highly informative, this remains difficult due to the uneven spatial and temporal coverage of observations and methodological differences between sediment traps, satellite algorithms, and models.\n2) Revisions to manuscript text: While we provide detailed context on regional export productivity (AESOPS observations, other sectors, and model estimates), our study focuses on the Antarctic Zone of the Southern Ocean. Accordingly, we revised the sentence to avoid overgeneralization: the phrase “the zone of maximum export productivity is located south of the Polar Front” has been changed to “the zone of high diatom productivity is largely located south of the Polar Front,” to more accurately reflect the regional variability in export production patterns. Please see lines 194-199 in Main Text.\n\n4. Correlations: e.g. lines 211-212 and Fig. 4. It is not clear in the text if the variables in Fig. 4 are normally distributed. This information is important to determine which correlation coefficient would be more appropriate. Could authors explain in the text why Pearson correlation coefficient was used and not Spearman’s test? Please clarify and revised if necessary.\n\nResponse:\nWe thank the reviewer for this precise statistical guidance. We have re-examined the data distributions and adjusted our analysis accordingly：\nSouthern Ocean stacks: Data are not normally distributed. We now use Spearman’s correlation (ρ) to assess agreement between stacks and PC1, and between stacks and individual site records (see Figs. S3, S4, and S6 and the related captions, lines 579-580 in Main Text).\nSubarctic Pacific stack: Data are approximately normally distributed. We retain Pearson’s correlation for these comparisons (see Figs. S10 and S12 and the related captions; lines 632-636 in Main Text).\nFigure 3: We clarify that this figure presents linear regression analyses to better illustrate relationships between variables, which is distinct from correlation coefficients.\n\n5. Methods, lines 108-110: “inverse standard deviation weighting method employs the inverse of each variable’s standard deviation as weights to minimize the influence of high-variability variables on the results”. This methodology is beyond my expertise and therefore I am unable to evaluate if this is an adequate approach.\n\nResponse:\nWe have significantly expanded the description of this method in the Methods section (see lines 526-545 and 576-580 in Main Text) to enhance clarity:\n1) Principle: The inverse standard deviation weighting approach is based on a well-established principle in quantitative data analysis, where variables are weighted inversely proportional to their variability (standard deviation). Such weighting approaches ensure more stable records contribute more strongly to the composite stack, thereby minimizing the influence of noisy records.\n2) Data preparation: All records were resampled to 1 kyr resolution. Missing values were filled via linear interpolation. Each record was then normalized using Min-Max normalization, defined as X = (X –X )/(X – X ). This transformation rescales the data to the range [0, 1], allowing for direct comparison across datasets with different scales and units while reducing scale-related bias in weight calculations.\n3) Weight calculation & stack generation: The standard deviation of each normalized record was calculated, and initial weights were set as w = 1/σ. The final stack is the weighted average of all records, with weights normalized to sum to 1.\n4) Validation of the method: The results from this method show exceptionally strong agreement (ρ > 0.94) with the results from PCA, confirming the robustness of the approach and that it captures the dominant common signal.\nTo improve clarity for readers who are less familiar with this approach, we have added a more detailed explanation in the revised manuscript (see lines 526-545 and 576-580 in Main Text).\n\n6. Figure 5. Please explain in the text the meaning of the circle with a dot in the schematic (position of the main winds I assume).\n\nResponse:\nWe have updated the caption for Figure 5 to explicitly state: “Southern Hemisphere westerlies (a circle with a dot in the center)”.\n\nReviewer #2 (Comments to Author (shown to authors)：\nThe manuscript by Lu et al. addresses a critical and timely question in paleoclimate: why Antarctic and subarctic Pacific EP became decoupled during the LIG despite generally co-varying with atmospheric CO and temperature on glacial-interglacial timescales. The study’s primary finding – that intensified, poleward-shifted SHW drove enhanced upwelling of nutrient- and CO -rich deep water in the Antarctic Zone during the LIG, sustaining high Antarctic EP and contributing to high atmospheric CO , while subarctic Pacific EP declined – is novel and significant. It challenges the traditional paradigm that Antarctic temperature was the dominant driver of Southern Ocean carbon release in warm periods. The authors provide an alternative mechanism whereby wind-driven ocean dynamics can override or modulate temperature-driven effects, which represents an important contribution to paleo-carbon cycle science and highlights the SHW as an underappreciated player in controlling past CO variability.\nOverall, this is an impressive and valuable manuscript and is squarely appropriate for Nature Communications, given its significant for paleoclimate and future climate understanding. I recommend publication after revision, addressing the concerns raised below. The most critical issues to resolve are clarifications of the proposed mechanism (especially regarding changes in deep ocean circulation and the contrasting behavior of the subarctic Pacific) and some enhancements to clarity in presentation. Thus, the revision can likely be done in a straightforward manner. I look forward to seeing a revised manuscript that incorporates these suggestions.\n\nResponse:\nWe sincerely thank the reviewer for their encouraging comments and for highlighting the importance of our study for understanding past and future climate dynamics. These suggestions have greatly strengthened the mechanistic narrative of our paper. We have revised the manuscript accordingly, adding additional analyses to illustrate the regional decoupling of Antarctic and subarctic Pacific productivity during MIS 5, and integrating these findings with global meridional overturning circulation to clarify the underlying mechanism. We have also clarified how Southern Annular Mode (SAM)-driven Southern Ocean upwelling sustained CO levels despite Antarctic cooling during the late Last Interglacial, providing a mechanistic explanation for the “CO paradox.” Additionally, we have revised the manuscript’s structure, improved figure presentation, and carefully edited the text to enhance readability and precision. Below, we provide our responses to the reviewer’s comments in detail.\n\nSome key concerns:\n1. The authors demonstrate high coherence among proxy types (opal vs. barium) and perform cross-correlations that show Antarctic EP, subarctic Pacific EP, and Antarctic temperature tracked each other closely for most of the past 160 kyr. For the critical LIG interval, the authors identify a breakdown of these correlations (the decoupling). However, I think this is not clearly shown qualitatively in figures, and I recommend quantifying the decoupling more explicitly. For example, the authors could report correlation coefficients between Antarctic and subarctic EP over specific intervals (glacial vs. interglacial vs. LIG) to statistically demonstrate that the LIG correlation is significantly lower. A running or windowed correlation analysis might also highlight when the divergence emerged.\n\nResponse:\nTo better quantify the decoupling between Antarctic and subarctic Pacific export productivity (EP), we have conducted a comprehensive statistical analysis. We conducted linear regressions for key variable pairs over the entire 160-kyr record and then excluding MIS 5:\n1) Antarctic vs. Subarctic EP: The fit is moderate for the full record (R2 = 0.58, n=149) but becomes very strong when MIS 5 is excluded (R2 = 0.90, n=92) (Fig. r2b). This quantitatively demonstrates that MIS 5 is a clear outlier from the long-term relationship.\n2) Other pairs: We found similar decoupling for Antarctic EP vs. Antarctic temperature (R2: 0.76 full record vs. 0.90 excl. MIS 5), Subarctic EP vs. CO (R2: 0.66 full record vs. 0.88 excl. MIS 5), and Antarctic temperature vs. CO (R2: 0.82 full record vs. 0.92 excl. MIS 5) (Figs. r2a, r2d, and r2f). Conversely, the fit of Antarctic EP vs. CO remains strong throughout (R²=0.92) (Fig. r2e).\n3) Timing: The point of maximum divergence occurs during the late LIG (~115-113 ka), coinciding with the pronounced Antarctic cooling. These new analyses are presented in Fig.3 and discussed throughout the results and discussion (lines 169-175, 207-210, 365-374, 395-402 in the Main Text).\n\nFig. r2 (new Fig. 3 of the revised manuscript) | a) and b) illustrate cross plot of the Antarctic (biogenic opal+ biogenic barium) stack versus Antarctic temperature stack (ATS) and the subarctic Pacific export productivity stack, respectively; c) Cross plot of ATS versus subarctic Pacific export productivity stack; d), e), and f) are cross plots of atmospheric CO concentrations versus ATS, Antarctic (biogenic opal+ biogenic barium) stack and subarctic Pacific export productivity stack.\n\n2. Another methodological aspect that could use more explanation is how the subarctic Pacific export productivity record was derived or selected. The text references subarctic Pacific EP data with citations, but readers may benefit from a clearer description: e.g., is this a composite record from a particular site or region? Detailing the source and nature of the subarctic EP reference would help the audience understand its resolution and uncertainties relative to the Antarctic data. Additionally, if the subarctic record’s age model was also aligned to Antarctic chronology (perhaps via atmospheric CH₄ or δ¹⁸O correlations at Termination II), that should be stated for transparency.\n\nResponse:\nIn the revised manuscript, we have expanded the Methods section (lines 587-638) and Table S2 to provide full details of subarctic Pacific export productivity:\n1) Data sources: The subarctic Pacific export productivity signal we use is derived from a regional composite data set based on five independent export productivity reconstructions (biogenic barium concentrations and 231Pa/230Th) from ten sediment cores spanning the subarctic Pacific (Table r1). These cores include ODP 882, MR98-05-3PC, JPC17, ODP887, and a group of six closely spaced cores from the Juan de Fuca Ridge (JdFR) area: AT26-19-05PC, AT26-19-09PC/06MC, AT26-19-12PC-TC/10MC, AT26-19-35PC, AT26-19-38PC, and AT26-19-39BB. Spatially, these cores are distributed between 44.96°N and 54.22°N, and from 164.98°E to 49.39°W, spanning a range of hydrographic settings across the subarctic Pacific, including both the western and eastern subarctic sectors and marginal seas such as the Bering Sea, with water depths ranging from 2209 to 5507 m. Given the geographic proximity of the JdFR cores, which exhibit similar productivity signals and trends, we adopted an existing regional productivity stack derived from these six sites, which provides a continuous record over the past 160 kyr and serves as a representative index for this sector. This approach ensures better temporal continuity and reduces redundancy in overlapping signals.\n2) Methodology: We applied the same two stacking methods (inverse weighting & PCA) used for the Antarctic, ensuring consistency for interhemispheric comparison.\n3) Age model: For some sites, detailed age-model data are not publicly available, limiting the possibility of further improving the chronology. Therefore, we used the original published age models largely based on radiocarbon dating, magnetostratigraphy, biostratigraphy, astronomical tuning, or δ¹⁸O tie points, to maintain consistency and avoid introducing additional uncertainties in the stack construction (Table r1).\n4) Representativeness: The strong agreement (R = 0.97) between the two independent stacking methods and high correlations (R > 0.7) with previously published records from the region (Fig. S10-S13) confirm the stack’s robustness and regional representativeness.\n5) High-resolution record: We incorporated the high-resolution, astronomically tuned Ba/Al record from ODP Site 882 as a complementary dataset to better resolve millennial-scale variability (Jaccard et al., 2005).\nThese details have now been clarified in the revised manuscript (see Figs. S10-S13, Table S2, lines 136-157 and 587-638).\n\nTable r1 (new Table S2 in the revised manuscript). Details for sediment cores for reconstructing the subarctic Pacific export productivity\n\n3. The authors argue that in the LIG, upwelling supply outpaced biological uptake, leading to CO release despite high productivity. This is an important nuance – usually high productivity draws down CO , but only if nutrient utilization is nearly complete. In an intense upwelling scenario, even robust productivity can be overwhelmed by the carbon influx from the deep ocean, resulting in a net CO efflux. The manuscript should be commended for touching on this mechanism in the Intro and should ensure it is clearly reiterated in the Discussion when interpreting the late LIG CO “paradox.”\n\nResponse:\nWe have expanded the discussion (lines 376-393) to clarify this critical mechanism. The paradox is resolved by considering the balance between physical supply and biological consumption:\n“Notably, during warm periods, high export productivity in the Antarctic zone of the Southern Ocean always coincided with elevated atmospheric CO , even during the late Last Interglacial, when Antarctic temperatures dropped by more than 6 °C (Figs. 2a, 2b, and 2d). This apparent paradox of elevated export productivity coinciding with increased atmospheric CO can be explained by an imbalance between upwelling supply and biological nutrient utilization, as supported by low δ¹⁵N records indicating db high surface nitrate supply during interglacial periods. Vigorous upwelling may have continuously transported carbon- and nutrient-rich deep waters to the surface, exceeding the capacity of nutrients and CO assimilation through photosynthesis. As a result, nutrient consumption was incomplete, constraining net biological CO fixation and allowing a portion of the upwelled carbon to escape to the atmosphere, a phenomenon analogous to that observed in contemporary high-nutrient, low-chlorophyll (HNLC) regions of the Southern Ocean. Thus, an HNLC-style imbalance between supply and utilization likely persisted during the Last Interglacial, maintaining elevated atmospheric CO even under high export productivity. This interpretation is further supported by the co-evolution of export productivity with increased nitrate supply in the Antarctic zone of Southern Ocean and rising CO (Figs. 4d and 4f).”\n\n4. One point that would benefit from clarification is the contrast between Antarctic and subarctic Pacific regimes. The authors find that, unlike Antarctica, the subarctic North Pacific experienced a decline in productivity during the LIG cooling, remaining “tightly coupled” to Antarctic temperature changes. They attribute this to a “weakened Southern Ocean abyssal circulation” that suppressed nutrient upwelling in the subarctic Pacific, thereby limiting productivity and CO release in the north. This explanation is intriguing but somewhat under-explained. The implication is that global deep-ocean circulation patterns shifted such that, even as surface wind-driven upwelling intensified around Antarctica, the supply of deep nutrients to the North Pacific was curtailed – perhaps due to reduced overturning or deep water formation changes. This raises a few questions that the authors should address:\n• Mechanistic clarity: How exactly do the authors envision the chain of events linking SH westerly changes to North Pacific upwelling reduction? Is it that stronger upwelling in the Southern Ocean drew more deep water up there (and outgassed its CO ) before it could travel to the North Pacific, effectively “robbing” the North Pacific of nutrient-rich deep water? Or was there a coincident change in deep ocean stratification or Atlantic Meridional Overturning Circulation that left the North Pacific more stratified? The phrase “weakened Southern Ocean abyssal circulation” is a bit abstract – it would help if the authors could rephrase or expand this to describe the process in more concrete terms (e.g., reduced formation of Antarctic Bottom Water during the LIG or a bottleneck in deep-water exchange between the Southern Ocean and North Pacific). Since this point is central to explaining the Antarctic vs. subarctic decoupling, a clearer discussion is warranted. Even a schematic in a figure might help illustrate the hypothesized circulation changes.\n• Local vs. global factors in the subarctic: The authors might consider alternative or complementary explanations for the subarctic Pacific productivity decline. One possibility is increased upper-ocean stratification in the subarctic Pacific as global climate warmed into the LIG peak, reducing vertical nutrient mixing (warmer surface waters could strengthen the halocline, for instance). The text focuses on deep circulation, but could some of the subarctic decline simply be due to local oceanographic changes (like more stable water column or less winter mixing)? If there is literature or evidence on subarctic North Pacific stratification or nutrient changes in the LIG, referencing or discussing it would give a more balanced interpretation. The current interpretation is reasonable, but readers may wonder if the subarctic Pacific’s behavior is entirely governed by Southern Ocean deep-water supply or also by its own regional climate conditions.\n\nResponse:\nWe greatly appreciate the reviewer’s insightful comments on the contrasting productivity responses between the Antarctic and subarctic Pacific during the Last Interglacial, which have helped us clarify the mechanistic links between Southern Hemisphere westerlies and nutrient supply in the subarctic Pacific, as well as consider the potential influence of local factors:\n1) Mechanistic linking Southern Hemisphere westerlies to subarctic Pacific nutrient supply. Modern ocean circulation indicates that no new deep water is formed in the subarctic Pacific; nutrients there are largely supplied by lateral advection of nutrient- and CO -rich abyssal waters, which are composed of Antarctic Bottom Water (AABW) and recirculated North Atlantic Deep Water (NADW) incorporated into Lower Circumpolar Deep Water (LCDW), forming Pacific Deep Water (Talley, 2013). During the late Last Interglacial, intensified Southern Hemisphere westerlies and a southward shift of ACC enhanced CDW upwelling, increasing Southern Ocean productivity and CO outgassing. This occurred alongside reduced AABW export and restricted LCDW transport to the subarctic Pacific, which corresponds to weaker deep-ocean ventilation that limited nutrient supply, reducing surface productivity and CO exchange. A schematic illustrating the hypothesized circulation changes was added in Fig. r3. Please see Fig.5 and lines 294-337 and 395-412 in Main Text.\n2) Consideration of local factors on subarctic Pacific processes: Local processes such as enhanced upper-ocean stratification due to alterations in sea ice extent may have further influenced nutrient upwelling and productivity. Our compilation of five export productivity records from subarctic Pacific sites covers diverse hydrographic settings (44.96° N- 54.22° N, 164.98° E -49.39° W; Table r1). This dataset includes pelagic sites, such as ODP Site 882, which were largely free of sea ice even during glacial periods, indicating that sea-ice cover was not a major limitation. Moreover, the co-variation between subarctic Pacific productivity (Ba/Al at ODP Site 882) and Antarctic temperature across interglacial-glacial and millennial timescales indicates that Southern Hemisphere climate was likely the dominant influence on subarctic Pacific productivity, given that the subarctic Pacific is unlikely to have been the primary driver of the climatic shifts documented in Antarctic ice cores. Please see lines 279-292 in Main Text for more details.\n\n5. The climatic and carbon cycle implications of this interpretation are important and should be explicitly stated. The authors touch on it in the abstract and intro: if westerlies indeed kept CO high during the LIG, this suggests a feedback wherein warming leads to stronger westerlies, which then lead to CO outgassing, potentially amplifying warming further. This is a positive feedback mechanism in the Earth system. The fact that CO stayed around 270-280 ppm while Antarctic temperature fell by ~6°C hints that other factors (perhaps declining Southern Hemisphere insolation or ice sheet regrowth) eventually outweighed the westerly-driven CO feedback in the LIG. The authors may want to comment on why CO did not continue rising indefinitely – presumably because the source of easily ventilated carbon was finite or because winds and ocean circulation reached a new equilibrium. A bit of discussion here would show the authors have considered the limits of the westerly wind effect in the natural system.\n\nResponse:\nWe agree with the reviewer that the potential feedback involving warming, Southern Hemisphere westerlies, and atmospheric CO during the Last Interglacial is crucial for understanding Earth system dynamics. In response, we have revised the Discussion to clarify this positive feedback and its limits:\n1) Climate-carbon feedback mechanism: We have now emphasized a positive feedback loop: initial Antarctic warming via the bipolar seesaw during Termination II → stronger westerlies → enhanced Southern Ocean upwelling and subarctic Pacific deep water ventilation (AABW enhanced) → increased CO concentrations→ amplified warming, likely sustaining elevated CO into the early Last Interglacial. The relevant revision reads:\n“Compared to the Penultimate Glacial Maximum (e.g., MIS 6), the increased meridional thermal gradient between the hemispheres during Termination II triggered an intensification of the Southern Hemisphere westerlies, promoting vigorous Southern Ocean upwelling and CO outgassing. Concurrently, enhanced AABW export coincided with stronger deep-ocean ventilation, supporting elevated CO outgassing in the subarctic Pacific. This positive climate-carbon feedback amplified global warming and appears to have extended into the early interglacial, as reflected in the synchronous variability of Antarctic temperature, Antarctic and subarctic Pacific export productivity, and atmospheric CO (Figs. 4b-4d and 4f, 5a-5b).” Please see lines 351-359 in Main Text.\n2) Why the Feedback Did Not Persist: To address the reviewer’s question about why atmospheric CO concentrations did not continue rising indefinitely, we have now proposed that this feedback eventually weakened, presumably because the source of easily ventilated carbon was finite or because winds and ocean circulation reached a new equilibrium, or Earth’s orbital parameters altered. The relevant revision is as follows:\n“Strengthened Southern Hemisphere westerlies during the late Last Interglacial enhanced Southern Ocean upwelling and carbon outgassing, likely forming a positive climate-carbon feedback that may have initially amplified warming or prolonged the warm period. However, the stabilization of atmospheric CO at ~270-280 ppm, along with Antarctic cooling, suggests this feedback was eventually limited, possibly due to a new equilibrium in winds and ocean circulation, to the finite nature of readily ventilated deep-ocean carbon reservoirs, or adjustments in Earth’s orbital parameters.” Please see lines 412-419 in Main Text.\n\n6. In terms of climate dynamics, I also have some comments to consider:\nOne aspect that could be emphasized is the parallel between the LIG scenario and the modern/future Southern Annular Mode (SAM) dynamics. The intensified, poleward-shifted westerlies the authors invoke are essentially a prolonged positive phase of the SAM (or a similar mode) under interglacial warmth. In modern observations and model simulations, a positive SAM (strengthened poleward westerlies) leads to enhanced upwelling of deep waters around Antarctica and reduced stratification, sometimes causing outgassing of CO from the Southern Ocean. The authors allude to future implications (with SHW expected to intensify under global warming), which is excellent. They could bolster this by explicitly noting that their paleo-findings are consistent with SAM-like behavior on long timescales. This not only places the result in a familiar framework for climate community but also underscores that such ocean-atmosphere coupling is already observed on shorter timescales today. If not already mentioned, integrating a brief mention of SAM or analogous wind-pattern changes would strengthen the connection to known climate modes.\n\nResponse:\nWe sincerely appreciate the reviewer’s comments highlighting the parallels between the Last Interglacial and modern/future Southern Annular Mode (SAM) dynamics as a framework for interpreting past and future climate behavior. We have revised the manuscript to incorporate SAM-like dynamics in three key aspects:\n1) Link to SAM in Southern Hemisphere westerlies: During the late Last Interglacial, poleward-intensified Southern Hemisphere westerlies resemble a prolonged positive phase of the SAM. Such SAM-like circulation during the late Last Interglacial would have intensified the upwelling of Circumpolar Deep Water, delivering nutrients to the surface and contributing to elevated export productivity. Supporting evidence includes intensified Antarctic Circumpolar Current (ACC), increased nitrate supply, higher export productivity, and a southward shift of the Subtropical Convergence and ACC front. Please see lines 240-266 in Main Text.\n2) SAM-driven upwelling impacts on atmospheric CO : Some studies propose the Antarctic temperature-driven processes for atmospheric CO changes over glacial-interglacial timescales (Watson et al., 2015). Proxy records, however, reveal that this link weakened during the late Last Interglacial (125-113 kyr BP), as atmospheric CO remained stable at ~270-280 ppm despite Antarctic cooling of more than 6°C. Our reconstructed export productivity records suggest that SAM-like wind circulation intensified CDW upwelling and sustained CO outgassing. This wind-ocean-carbon linkage likely formed positive climate-carbon feedback that initially amplified warming or prolonged the warm period. However, the stabilization of atmospheric CO suggests that this feedback was eventually limited, possibly due to a new wind-ocean equilibrium, the finite nature of readily ventilated deep-ocean carbon reservoirs, or orbital variations. Please see lines 351-374 and 404-419 in Main Text.\n3) Implications for future SAM and CO dynamics: Climate projections indicate that Circumpolar Deep Water will spread poleward and downward by 2050, while a more positive SAM phase is expected, strengthening poleward Southern Hemisphere westerlies. Stronger westerlies are projected to enhance Southern Ocean upwelling, contributing to increased atmospheric CO . Although on shorter timescales, this scenario may parallel Last Interglacial conditions, providing insight into potential polar ocean-atmosphere feedbacks under future warming. Please see lines 424-441 in Main Text.\n\n7. Also, the idea of ocean-atmosphere feedbacks could be expanded. The authors have essentially identified a feedback loop: warmer climate (and/or changing insolation) → poleward-intensified westerlies → increased Southern Ocean upwelling → CO release → further warming globally. It would be valuable to articulate this clearly. It’s also worth discussing the time lag or sequence of events: Did the westerlies intensify because of the initial LIG warmth (e.g., due to a reduced equator-to-pole temperature gradient), thus later sustaining CO levels even as Antarctic temperatures started to drop? If so, we are observing a hysteresis or time-lag effect where wind-driven CO feedbacks prolong the warmth. Any insight the authors can provide (or speculate on, based on timing in their data) about the sequence – e.g., did Antarctic EP start rising before or just as CO started rising at Termination II, and did subarctic EP falter only after the Antarctic winds revved up? – would be intriguing context.\n\nResponse:\nWe have expanded the ocean-atmosphere feedback loop as: warming by bipolar seesaw during Termination II and early last Interglacial) → poleward-intensified westerlies → enhanced Southern Ocean upwelling and subarctic Pacific deep water ventilation (AABW enhanced) → increased atmospheric CO → amplified warming and orbital forcing change (late Last Interglacial)→positive SAM-like westerlies →increased Southern Ocean upwelling (Circumpolar Deep Water upwelling) →stable CO ). We also discuss its temporal evolution across three phases:\n1) Termination II and early Last Interglacial: Antarctic warming, driven by the bipolar seesaw mechanism, steepened the hemispheric temperature gradient and intensified the Southern Hemisphere westerlies. This enhanced Southern Ocean upwelling and AABW export, thereby increasing nutrient and CO supply from the deep Southern Ocean both locally and remotely. Regionally, this sustained high productivity and CO outgassing in the Southern Ocean. Remotely, the enhanced export of AABW advected into the subarctic Pacific, increasing abyssal nutrient content. This, together with enhanced deep-ocean ventilation, supported elevated productivity and CO outgassing in subarctic Pacific. Collectively, these processes in the Southern Ocean and subarctic Pacific have contributed to rising atmospheric CO , forming positive climate-carbon feedback that further amplified warming. Please see lines 220-238, 306-322, 351-359 in Main Text.\n2) Mid- to late LIG evolution: As obliquity declined, steepened the meridional temperature gradient further reinforced the Southern Hemisphere westerlies (Ai et al., 2020). Proxy evidence indicates that poleward strengthened ACC and weakened AABW export (Wu et al., 2020; Huang et al., 2020). Thus, wind-driven Circumpolar Deep Water upwelling remained active, supporting regional productivity and CO release. However, weaker AABW export and a poleward ACC shift likely limited nutrient supply to the subarctic Pacific. Together with a weakened upwelling index, this likely constrained surface productivity and ocean-atmosphere CO exchange. The wind-driven feedback likely prolonged warmth, yet the stabilization of atmospheric CO at ~270-280 ppm, suggests this feedback was eventually limited by new wind-ocean equilibrium, orbital variations, or the finite nature of readily ventilated deep-ocean carbon reservoirs. Please see lines 240-266, 324-337, and 404-419 in Main Text.\n3) Relevance to future climate: Model projections suggest that AABW-driven abyssal circulation may slow by more than 40% by 2050, accompanied by a poleward and downward expansion of Circumpolar Deep Water and a more positive SAM with poleward-intensified westerlies. As a result, Southern Ocean upwelling is projected to intensify, potentially enhancing CO outgassing and amplifying global warming. In contrast, a slowdown of AABW, together with a concurrent poleward migration of the ACC, would be likely linked to reduced deep-water ventilation in the subarctic Pacific and limited ocean-atmosphere CO exchange, echoing the spatial decoupling observed during the Last Interglacial. These potential parallels offer valuable insights into how Southern Ocean circulation has consistently influenced atmospheric CO levels in both past interglacial periods and projected future climate conditions. Please see lines 424-441 in Main Text.\n\n8. The manuscript touches on large-scale modes indirectly by discussing “abyssal circulation” and upwelling differences. To further integrate with global-scale ocean circulation, the authors might consider the role of the meridional overturning circulation (MOC) in their scenario. A subtle point: if Southern Ocean deep upwelling was enhanced, was there any compensating change in North Atlantic Deep Water production or global MOC strength? Some studies of past warm periods (and future projections) suggest that freshening or warming can alter North Atlantic circulation, which in turn might influence the volume of deep water reaching the Southern Ocean and eventually the North Pacific. While the focus here is rightly on Southern Ocean winds, acknowledging these broader connections would show comprehensive thinking. For instance, if the authors suspect a slowdown in some component of the MOC (hence “weakened abyssal circulation”), they could tie it to known possibilities like reduced Antarctic Bottom Water formation in a warmer climate. If any PMIP Last Interglacial model outputs or previous studies (e.g., on Atlantic vs. Pacific ventilation) align with their findings, they should be referenced to give the reader confidence that the proposed dynamics are realistic.\n\nResponse:\nIn the revised manuscript, we have included a clearer discussion of the meridional overturning circulation (MOC) accordingly, particularly regarding the link among Southern Ocean upwelling, NADW formation, ACC dynamics, and AABW export.\n1) Broadly speaking, the MOC consists of two interconnected cells: an upper cell driven by NADW formation and Southern Ocean wind-driven upwelling, and a lower abyssal cell maintained primarily by AABW formation. Our proxy reconstructions for Termination II and the early Last Interglacial indicate a marked intensification of Southern Ocean upwelling and Pacific ventilation consistent with enhanced AABW export (Huang et al., 2020; Hayes et al., 2014). This occurred concurrently with substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC; Böhm et al., 2015), primarily due to freshwater forcing and surface cooling in the North Atlantic, which suppressed NADW formation (Govin et al., 2012). Thus, the upper cell weakened while the abyssal cell strengthened, consistent with the bipolar seesaw hypothesis (Broecker, 1998).\n2) During the late Last Interglacial, the enhanced interhemispheric insolation contrast amplified the meridional temperature gradient. At the same time, declining obliquity further steepened this gradient, reinforcing the Southern Hemisphere westerlies. Proxy evidence shows stronger ACC speed (Wu et al., 2020), and enhanced Southern Ocean upwelling. At the same time, NADW recovered while AABW export declined (Hayes et al., 2014; Böhm et al., 2015; Huang et al., 2020). Together, these changes imply a transition toward an MOC dominated by a stronger NADW-driven upper cell and wind-induced Southern Ocean upwelling (strengthened ACC), while the abyssal AABW cell weakened.\n3) As already outlined in response to earlier comments above, climate models project a reduced but stable AMOC (Baker et al., 2025) and ~42% weaker AABW export by 2050, accompanied by a poleward and downward expansion of Circumpolar Deep Water (Li et al., 2023). Elevated greenhouse gas concentrations are also projected a more positive SAM, with poleward-intensified Southern Hemisphere westerlies (IPCC, 2007), further enhancing Southern Ocean upwelling (Chen et al., 2008). These projected changes suggest a future MOC structure resembling the late LIG, with a stronger wind-driven Southern Ocean upwelling (strengthened ACC) and stable NADW upper cell, and a weakened abyssal AABW cell.\nWe have added this discussion and relevant references to the revised manuscript. Please see lines 306-322, 324-330, and 424-436 in Main Text.\n\nReviewer #3 (Comments to Author (shown to authors)：\nThis is a very interesting study that produced a stacked record of Antarctic Southern Ocean sediment proxy records of export productivity and compared this productivity stack with Antarctic temperature, atmospheric CO , and Subarctic North Pacific export productivity variations over the last 160 kyr, with a specific interest in the Last Interglacial period. The study examines temporal variations in the relationships between these climatic parameters through comparison of time series and through direct correlation analysis and highlights that the positive linear correlation between atmospheric CO , Antarctic temperature, and Antarctic and Subarctic productivity seem to break down during the late Last Interglacial. Based on this analysis, the authors conclude that the Last Interglacial period may have been associated with poleward migration of the Southern Hemisphere westerlies, which enhanced nutrient upwelling, productivity, and deep carbon release in the Antarctic Southern Ocean. In contrast, the authors suggest that the North Pacific experienced reduced abyssal upwelling, which caused a reduction in subarctic productivity and a decoupling between subarctic productivity and atmospheric CO .\nThis work tackles the dynamic mechanism behind global carbon cycling in warmer-than-modern climate conditions, which is an interesting problem on its own and very relevant for accurate predictions of future climate behavior associated with anthropogenic warming. I appreciate the analytical approach of this study and commend the authors for the very good data accessibility provided in their Materials and Methods and supplemental tables. Although this is not the first study to support poleward migration of the westerlies during warmer-than-modern paleo-intervals, this is the first one to my knowledge that directly connects this mechanism to disparate changes in Antarctic and subantarctic export productivity across the Last Interglacial. However, I do have a few suggestions, including additional analysis of the data, that I believe would strengthen the impact of this work. Therefore, I recommend that the authors be given the opportunity to address the following comments before finalizing a publication decision for Nature Communications.\n\nResponse:\nWe thank the reviewer for the constructive feedback and the recognition of our analytical approach and our data, and the contribution of linking poleward Southern Hemisphere westerlies to contrasting Antarctic/subantarctic export productivity during the Last Interglacial. Following the reviewer’s suggestions, we carried out additional analyses of the data. Specifically, we compiled five independent subarctic Pacific export productivity records and constructed two composite stacks using the same methods as for the Antarctic records, confirming their regional representativeness. We further performed linear regressions extended to the Holocene, showing that MIS 5 exhibits a distinct interhemispheric productivity pattern, with the largest deviation during the Last Interglacial. Additional regressions of Antarctic temperature vs. Antarctic productivity, Antarctic temperature vs. CO , and subarctic Pacific productivity vs. CO over the past 160 kyr reveal a pronounced decoupling during MIS 5, highlighting its distinctive characteristics relative to other glacial-interglacial intervals and strengthening the overall impact of this work.\n\nGeneral Comments:\n1.Broadly, the comparison between Antarctic export productivity records (synthesized into a stack of 15 different proxy time series from 12 different sediment cores) with the subarctic North Pacific export productivity records (indicated by two barium-based proxies from a single sediment core at Site 882) seems unbalanced. How well does export productivity at Site 882 reflect the entire subarctic North Pacific? The Antarctic records exhibit some degree of spatial variability (for example, ODP 1094 looks very different from U1537), would be expect the same in the North Pacific? Even if there are not additional North Pacific productivity records covering this interval (which I am not sure about), this manuscript would benefit from a discussion of how representative Site 882 is of the region based on more recent intervals, where additional productivity data is more likely to be available.\n\nResponse:\nTo address the reviewer’s concern regarding the representativeness of export productivity for the broader subarctic Pacific region, we constructed two composite subarctic Pacific productivity stacks from five independent subarctic Pacific records using the same methods applied to Antarctic records (inverse standard deviation weighting method and PCA). The records spans both the western (e.g., ODP Site 882, MR98-05-3PC) and eastern (ODP887 and Juan de Fuca Ridge stack) sectors, and the Bering Sea (e.g., JPC17), covering diverse water depths (2209-5507 m) and proxies (biogenic barium concentration and 231Pa/230Th; Table r1).\nPCA shows that the first principal component (PC1) captures 61.3% of the total variance (Fig. r4), closely aligns with the stack generated using the inverse standard deviation weighting method (R = 0.97; Fig. r5), as well as the published subarctic Pacific productivity records (R> 0.7; Fig. r6), confirming regional representativeness. We also incorporated the high-resolution, astronomically tuned Ba/Al record from ODP Site 882 as a complementary dataset to better resolve millennial-scale variability.\nWe have incorporated the analyses into the revised manuscript, along with supporting Figs. S10-S12, Table S1, and text in Lines 136-157 and 587-638.\n\n2. The argument that the dynamics observed during the Last Interglacial period represent a unique climate regime would be more compelling if the study also included analysis of how the Antarctic temperature, atmospheric CO , Antarctic export productivity, and subarctic export productivity records compared across the Holocene/current interglacial period. The Antarctic productivity stack that this study has generated is well suited for this analysis because it covers the last 160 kyr completely. It seems the Site 882 productivity record does not span the Holocene, so perhaps another North Pacific productivity record could be used here for the comparison?\n\nResponse:\nWe agree that extending the analysis to the Holocene is valuable. Accordingly, we compiled five subarctic Pacific export productivity based on biogenic barium and 231Pa/230Th proxies covering the past 160 ka, thereby including the Holocene (Table r1). Using this expanded dataset, we constructed composite productivity stacks based on two independent methods consistent with those applied to Antarctic records (inverse standard deviation weighting and PCA). Both stacks show strong consistency with published subarctic Pacific records, enabling robust comparisons with Antarctic productivity and climate proxies during the Holocene. These new analyses and figures have been added to the revised manuscript (Fig.2, Figs. S10-S13, Lines 136-157 and 587-638).\n\n3. I appreciate the inclusion of the correlation plots in Figure 4 as a way to examine the data beyond the time series comparisons illustrated in Figures 2 and 3, with respect to differences between records during MIS6, T2 and the LIG. Panel 4d is especially helpful for illustrating the decoupling between Antarctic temperature and atmospheric CO during the LIG and Panels 4e and 4f very clearly suggest that atmospheric CO concentrations seem to be much more driven by Antarctic Zone productivity than by North Pacific productivity.\nHowever, the circled LIG data in panels 4a and 4b look to me more like a regime shift than a complete decoupling between Antarctic temperature and Antarctic productivity or between Site 882 productivity and Antarctic productivity (e.g., the data still look to be linearly correlated, though perhaps with a different slope and intercept). What correlation coefficients do you get when you examine just the LIG data, rather than the full 150-113 ka record? Are the LIG-labelled data points that fall closest to the 150-127 ka trendline the ones closest in age to T2, or are they from the middle of the LIG? It would also be helpful to see how data from MIS 3, 4, and 2 would plot on these graphs (at least in the supplement if it makes the main figure too crowded), to see how representative the MIS 6 and T2 relationships are of the last glacial cycle as well.\n\nResponse:\nWe thank the reviewer for this insightful comment. While the circled LIG data in Fig. 4a-b appear consistent with a regime shift, our regressions indicate otherwise. Across the full LIG (128-113 ka), correlations are very weak (Antarctic Temperature vs. productivity R2 = 0.07; Antarctic vs. Site 882 productivity R2 = 0.0008). A stronger fit (Antarctic Temperature vs. productivity) appears in the late LIG (125-113 ka; R2 = 0.66). Thus, the prevailing pattern during the LIG interval is a decoupling, rather than a simple change in regime shift.\nTo place the Antarctic productivity-temperature and Antarctic-subarctic Pacific productivity relationships in a broader context, we added new plots in Fig. r7, covering MIS 1-6 (including MIS 5e, LIG). From these plots, MIS 6 points fall close to MIS 2, 3, and 4, indicating that they are representative of glacial variability, while TI and TII data largely overlap, reflecting deglacial changes.\nExcluding MIS 5 greatly improves fits (R2 = 0.90; R2 Antarctic Temperature vs. productivity Antarctic-subarctic Pacific productivity = 0.91), whereas excluding MIS 1 and deglacial yields results similar to the full record (R2 : 0.76 full record vs. 0.80 excl. Antarctic Temperature vs. productivity Antarctic-subarctic Pacific productivity MIS 1 and deglacial; R2 : 0.58 full record vs. 0.56 excl. MIS 1 and deglacial). MIS 5 data begin to deviate starting around 125 ka and reach a maximum deviation around 115-113 ka.\nTaken together, these analyses indicate that MIS 5 as a whole, represents a unique interval of decoupling between Antarctic productivity and temperature or between Antarctic and subarctic Pacific productivity. Please see Fig. 3\n\nSpecific Comments:\n1. Line 34: Suggest to rephrase to “…abrupt decline in the Antarctic air temperature stack (ATS)…” so unfamiliar readers will understand where the S is coming from in the acronym.\n\nResponse:\nWe have rephrased the text as “over 6 °C Antarctica cooling” to make it easier for all readers to follow. Please see line 31 in Main Text.\n\n2. Lines 37-40: I think the conclusion of this sentence would be easier to follow if an additional step in the logic were explained (for example, please specify here why a tight coupling between subarctic Pacific export productivity and Antarctic temperature indicates suppressed abyssal upwelling).\n\nResponse:\nWe agree that specifying the mechanistic link would clarify the logic. In the revised abstract, we have added a concise statement indicating that altered the Southern Ocean deep-water supply to the subarctic Pacific likely suppressed upwelling, limiting CO release. A more detailed discussion of this mechanism is provided in the main text. Please see lines 33-35 and 294-337 in Main Text.\n\n3. Line 91: Suggest to clarify here, rather than waiting until the methods section, that the 15 export productivity records are from 12 sediment cores (especially as there are only 12 orange dots in the map figure).\n\nResponse:\nWe have revised the sentence to improve clarity. It now reads: “To assess regional differences in export productivity and carbon cycling, we synthesize 15 records from 12 sediment cores in the Antarctic zone and 5 records from 10 sediment cores in the subarctic Pacific.” Please see lines 81-85, Figure 1, and the related caption.\n\n4. Lines 139- 141: Suggest to include the correlation coefficients for the full record here between the Antarctic proxy stack, the ATS and the subarctic records for consistency in comparison across time periods.\n\nResponse:\nTo improve consistency across time periods, we have now included the linear regressions for the full record among the Antarctic export productivity stack, the ATS, and the subarctic Pacific export productivity records, which capture these relationships more comprehensively than simple correlation coefficients. Please see Figure 3, lines 169-175, 207-210, and 273-277 in Main Text.\n\n5. Lines 164-168: Suggest to rephrase to “in contrast, iron fertilization does not appear to have been impactful in the Antarctic Zone, which exhibits lower export productivity during glacial periods” to clarify this point.\n\nResponse:\nWe have rephrased the sentence as suggested to clarify this point. Please see lines 190-192 in Main Text.\n\n6. Lines 176-178: It is my understanding that Ref 32 does not assert that temperature is the primary control on export productivity in the Antarctic zone, rather they suggest that low glacial productivity in this region may be caused by reduced wind-driven upwelling associated with a northward migration of the Southern Ocean fronts. Consequently, I suggest not using this reference to justify the first part of this sentence.\n\nResponse:\nFollowing the reviewer’s suggestion, we have removed Ref 32 from the statement on temperature and revised the text accordingly. Please see lines 202-205 in Main Text.\n\n7. Lines 247-249: Suggest to continue to specify that “temperatures” refers to “Antarctic temperatures” here, to avoid confusion associated with asynchronous millennial-scale warming between Greenland and Antarctic temperature records (e.g., Blunier et al., 1998).\n\nResponse:\nThe text has been revised to specify “Antarctic temperatures” to avoid confusion. Please see lines 362-364 in Main Text.\n\n8. Line 560 (Figure 3 caption): More detail is needed here regarding the upwelling index denoted (e.g., suggest to specify that it’s the subarctic upwelling index based on U1343 and Site 1012). Since the paper typically indicates what geochemical proxies are used to infer each parameter for productivity, ACC strength, etc…, I suggest to include the proxy basis of the upwelling index here as well (or perhaps where this is discussed in the main text).\n\nResponse:\nWe have revised the main text and the caption of Figure 4 to clarify that the upwelling index, which represents the upwelling in the subarctic Pacific, is calculated as the normalized opal MAR at Site U1343 minus the normalized δ¹⁵N difference between Sites U1343 and 1012. Please see Fig. 4 caption and lines 317-320 in Main Text.\n\n9. Figure 5: I found this figure very helpful for visualizing the different suggested circulation scenarios, but I can’t find a reference to this figure anywhere in the main text.\n\nResponse:\nWe have added a reference to Figure 5 in the main text. Please see lines 359,406, and 411 in Main Text.\n\n10. Lines 599-601: Please explain here why you included these two cores into the productivity stack, although they do not span the LIG range targeted by this study.\n\nResponse:\nAlthough these two cores do not fully cover the targeted LIG interval, they include detailed productivity records from the Holocene and millennial-scale intervals. Including these records thus allowed us to compile a more comprehensive Antarctic productivity dataset, thereby enhancing the robustness and representativeness of our productivity stack. We think this approach strengthens the validity of our analysis and conclusions. Please see lines 450-454 in Main Text.\n\n11. Line 678: I assume records were normalized by dividing each by its mean. Could you please specify the exact approach used so unfamiliar readers can better follow along?\n\nResponse:\nEach record was using Min-Max normalization, defined as X = (X –X )/(X – X ). This transformation rescales the data to the range [0, 1], allowing for direct comparison of the magnitude and timing of changes in export productivity across different records. We have now explicitly stated this normalization procedure in the revised manuscript. Please see lines 532-537 in Main Text.\n\n12. Lines 686-691: After looking through Figures S7 and S8, I wonder if this inverse dispersion weighting is artificially removing legitimate high frequency signal from the stacked record (leading to an over-smoothing). There are many Antarctic sediment cores in which sedimentation rates vary largely between productive climates and low-productivity climates (ODP 1094 is a good example), such that the ability of each record to capture millennial-scale changes varies temporally. There are also clear difference between sediment cores in their ability to capture high frequency variations in export productivity (for example, TN057-14PC4 captures much more resolution than PS75-072-4 during MIS3), does that mean TN057-14PC4 receives a lower weighting than PS75-072-4? Should it? Would the stacked export productivity records look very different if you weighted each record by sedimentation rate across a given interval, rather than inverse dispersion?\n\nResponse:\nWe appreciate the reviewer’s careful consideration of our weighting method. Our inverse-dispersion weighting assigns higher weights to data with lower standard deviation and lower weights to more variable data, ensuring that stable and reliable records contribute more to the stacked record. For example, during MIS 3, TN057-14PC4 provides much higher resolution than PS75-072-4, yet it receives greater weight (standard deviation σ = 0.1683; w=0.0903) than PS75-072-4 (σ = 0.3084, w=0.0492 for opal flux;σ=0.2241, w=0.0678 for Ba/Fe). This approach does not diminish the role of high-resolution records. A principal component analysis of the compiled records shows that the first principal component (PC1) correlates strongly with our stacks (Fig. r8), supporting the robustness of our approach.\nFollowing the reviewer’s suggestion, we also tested sedimentation rate weighting. This method increases the amplitude of many peaks, likely because high-sedimentation intervals capture higher-frequency variations, including potential noise. In contrast, inverse dispersion weighting prioritizes more stable signals, providing a robust representation of overall export productivity trends. Importantly, both weighting methods produce relatively consistent large-scale patterns (Fig. r8), demonstrating the reliability of our approach.\n\n13. Also please note that, in general, Marine Isotope Stage labels are used throughout the figures, but do not appear to be mentioned or explained in the main text.\n\nResponse:\nWe have now added a brief explanation of the Marine Isotope Stage (MIS) labels in the main text to ensure consistency with the figures and improve clarity for readers."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have reviewed the authors' responses and the revisions in the manuscript and believe that the authors have made a great effort to respond to all of the reviewers' comments. Also, I find both the changes to the text and the new figures to be very appropriate. There is only one point that requires a small, but relevant, clarification. Fragilariopsis kerguelensis is not generally considered an indicator of high productivity (as indicated in the response to the reviewers). Other diatom species tend to dominate in highly productive environments such as coastal and shelf systems of the Antarctic continent. This does not mean that changes in the abundance of this species cannot be related to an increase in productivity. According to some studies (e.g. Nair et al., 2020 and other references in the text) increases in the flux of F. kerguelensis (the main vector of silica to the ocean floor) appear to be associated with greater nutrient input. Therefore, my only suggestion would be to clarify this point better in the text. The authors could indicate something like \"changes in the abundance of F. kerguelensis can be taken as an indicator of productivity changes in the pelagic Southern Ocean\" or something along those lines. Please also specifically mention in the text if authors are referring to absolute abundance and/or relative abundance of F. kerguelensis in the manuscript.\n\nI have no further suggestions from my side and would like to congratulate the authors on this very interesting work.", "reviewer_2": "Thanks authors for hard working on revising the manuscript. The authors’ revisions address all concerns. I recommend acceptance in Nature Communications.", "reviewer_3": "I am satisfied with the changes made in the revised manuscript and applaud the authors for their thorough incorporation of reviewer feedback and discussion. I believe these changes have made the manuscript stronger and, as such, I recommend this manuscript for publication."}, "author_response": "Reviewer #1 (Comments to Author (shown to authors)：\nI have reviewed the authors' responses and the revisions in the manuscript and believe that the authors have made a great effort to respond to all of the reviewers' comments. Also, I find both the changes to the text and the new figures to be very appropriate. There is only one point that requires a small, but relevant, clarification. Fragilariopsis kerguelensis is not generally considered an indicator of high productivity (as indicated in the response to the reviewers). Other diatom species tend to dominate in highly productive environments such as coastal and shelf systems of the Antarctic continent. This does not mean that changes in the abundance of this species cannot be related to an increase in productivity. According to some studies (e.g. Nair et al., 2020 and other references in the text) increases in the flux of F. kerguelensis (the main vector of silica to the ocean floor) appear to be associated with greater nutrient input. Therefore, my only suggestion would be to clarify this point better in the text. The authors could indicate something like \"changes in the abundance of F. kerguelensis can be taken as an indicator of productivity changes in the pelagic Southern Ocean\" or something along those lines. Please also specifically mention in the text if authors are referring to absolute abundance and/or relative abundance of F. kerguelensis in the manuscript. I have no further suggestions from my side and would like to congratulate the authors on this very interesting work.\n\nReference (please note that references are just a suggestion, there is no need to include more references if authors do not considered relevant)\nNair, A., Ghadi, P., Mohan, R., Manoj, M. C., Crosta, X., Shukla, S. K., and Thamban, M.: Glacial-interglacial flux and size variability of Fragilariopsis kerguelensis and Thalassiosira lentiginosa from the Indian sector of the Southern Ocean, Deep Sea Research Part II: Topical Studies in Oceanography, 178, 104746, https://doi.org/10.1016/j.dsr2.2020.104746, 2020.\n\nResponse:\nWe appreciate the reviewer’s constructive comment and helpful clarification. We have revised the text accordingly to clarify that Fragilariopsis kerguelensis can be used as an indicator of productivity changes in the pelagic Southern Ocean, rather than of high productivity in coastal or shelf environments. We have also specified that our discussion refers to its relative abundance. The revised sentence now reads as follows:\n“Independent palaeoecological data on the relative abundance of Fragilariopsis kerguelensis within the permanent open-ocean zone south of the Polar Front in our study area, which can be used as an indicator of productivity changes in the pelagic Southern Ocean, closely track the stacks (Fig. S9), further supporting that the interpretation that the reconstructed stacks capture key signals of Antarctic export productivity.”\n\nReviewer #2 (Comments to Author (shown to authors)：\nThanks authors for hard working on revising the manuscript. The authors’ revisions address all concerns. I recommend acceptance in Nature Communications.\n\nResponse:\nWe greatly appreciate your encouraging comments and positive recommendation for acceptance. We truly value the time and effort dedicated to reviewing and supporting our work.\n\nReviewer #3 (Comments to Author (shown to authors)：\nI am satisfied with the changes made in the revised manuscript and applaud the authors for their thorough incorporation of reviewer feedback and discussion. I believe these changes have made the manuscript stronger and, as such, I recommend this manuscript for publication.\n\nResponse:\nThank you very much for the supportive comments. We are pleased that our revisions have fully addressed your concerns and appreciate your acknowledgement that the manuscript is ready for publication."}]

Decision letter