Enhanced deglacial carbon transport by Pacific southern-sourced intermediate and mode water

Domain: earth_science
Published: 2025-06-05
Editorial outcome: accept

Abstract
Southern-sourced Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are currently major sinks of atmospheric CO2. During the last deglaciation, atmospheric CO2 levels increased significantly during two specific time periods, Heinrich Stadial 1 (H1) ~18–14.6 ka BP (thousand years ago before present) and the Younger Dryas (YD) ~12.8–11.5 ka BP. Model simulation and proxy data studies suggest that AAIW/SAMW was crucial in explaining these changes during H1 and YD, but its variability and properties in the Southeast Pacific Ocean are still largely unknown. Here, we present records of benthic foraminiferal carbon isotopes, Mg/Ca-based water temperatures, paleosalinity reconstructions, and sortable silt mean grain size variations over the last 30 thousand years from Ocean Drilling Program (ODP) Site 1233, in the Southeast Pacific Ocean, which is bathed in AAIW/SAMW. Our proxy data suggest an increased northward circulation of high pCO2/nutrient-enriched AAIW/SAMW during H1 and YD. Our data provides support for AAIW/SAMW as one of the important conduits for deglacial oceanic outgassing in the eastern equatorial Pacific upwelling.

Introduction
During the present global warming with vast human induced atmospheric CO2 increase, Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are major sinks for this anthropogenic CO2 1. These water masses are formed in the Southern Ocean in association with upwelling of nutrient enriched deep-water 2,3. As these waters are undersaturated with respect to heightened atmospheric CO2 from anthropogenic sources they absorb CO2 and transport it along with upwelled nutrients at intermediate depths towards low latitudes 1,3. In the equatorial Pacific, significant parts of this oceanic stored CO2 at intermediate water depths enters the Equatorial Undercurrent by way of diapycnal mixing processes and surfaces decades after its entrainment in the high southern latitudes 4,5. The eastern equatorial Pacific is one of the world’s major upwelling regions, where oceanic stored CO2 is released back into the atmosphere 6,7.

Prior to the industrial revolution, the last deglaciation (~18–11 ka BP) is the most recent time in the Earth’s history when global atmospheric CO2 levels markedly increased by ~80 ppm along with an average global warming of 3–6°C 8,9. Deglacial CO2 increases took place during two short intervals, during ~18–14.5 ka BP known as Heinrich Stadial 1 (H1) and during ~12.8–11.5 ka BP known as the Younger Dryas (YD) 8. In contrast to the modern, man-made atmospheric CO2 increase from emissions, this deglacial two-step increase in atmospheric CO2 resulted from Southern Ocean upwelling (and outgassing) of deep and “old” high pCO2 and nutrient-enriched water masses that had been isolated from the atmosphere for a long time 10,11. Several proxy-based studies including δ13C and radiocarbon activities (Δ14C) of planktonic and benthic foraminifera imply that AAIW/SAMW served as an important conduit of the glacial “old” high pCO2 water masses from the Southern Ocean toward the eastern Pacific tropical upwelling region and even further north 6,12–19. Indeed, during deglacial atmospheric CO2 rise high deglacial surface ocean pCO2 found in the eastern Pacific equatorial upwelling zone was proposed to partly originate from AAIW/SAMW 6.

However, the hypothesis that AAIW/SAMW was an important conduit of “old” (high pCO2 and nutrient-enriched) water during the last deglaciation has yet to be rigorously tested in the Pacific Ocean. Offshore southern Chile, the deglacial AAIW/SAMW evolution (including temperature and salinity) is mostly unknown. Existing datasets for AAIW/SAMW ventilation are either far outside its modern core 20 or are conflicting. For example, increased AAIW circulation was either suggested for the glacial or the deglaciation 21,22, and AAIW as a main conduit of deglacial “old” water masses was rejected and later confirmed 22–24. To fill this gap, we have developed multiproxy records of up to centennial scale (200 year resolution) from ODP Site 1233 (41°S; 74°27’W) covering the last 30 kyrs. This Site is close to the major modern formation region of AAIW, which in this area is identical to the densest variety of SAMW 2,25,26. Its water depth of 838 m puts it at the core of northward flowing AAIW/SAMW (Fig. 1a, b). Hence, we here refer to a combined AAIW/SAMW at our Site, since both water masses are indistinguishable using geochemical proxies 27 and SAMW was proposed to be a major source for eastern Pacific AAIW 28. We constructed AAIW/SAMW evolution using (1) benthic foraminiferal δ13C and Ba/Ca as indirect proxies for pCO2 contents and nutrients, (2) sortable silt mean grain sizes (SS), a direct proxy for paleocurrent speeds and thereby changes in AAIW/SAMW northward flow, and (3) combined benthic δ18O and Mg/Ca which tracks changes in the AAIW/SAMW temperature and salinity (see Methods and Supplementary Discussion).

Results
During the last deglaciation our high-resolution benthic Uvigerina peregrina δ13C record (hereinafter referred to as benthic δ13C) was not affected by surface productivity changes (see Supplementary Discussion and Supplementary Fig. 1). The smoothed record shows a major decrease of ~1.2‰, followed by a second decrease of ~0.6‰ that principally track H1 and YD, when global atmospheric CO2 levels sharply increased (Fig. 2a, g) and atmospheric δ13C decreased by ~0.3‰, as recorded in Antarctic ice cores 29 (Fig. 2a).

Low atmospheric δ13C during H1 has been interpreted as a direct consequence of upwelling (and outgassing) of deep, old water masses with low δ13C and high pCO2 in the Southern Ocean 29. The correlation of atmospheric δ13C (ref. 29) with our benthic δ13C tightly connects AAIW/SAMW δ13C with the outgassing of these deep low δ13C water masses (Fig. 2a). The fact that the amplitude of our benthic δ13C at 838 m water depth is much larger (2–4 times) than the changes observed in the atmosphere confirms that the low benthic δ13C is sourced from the deep ocean, and not from the surface ocean as an indirect consequence of outgassing in the Southern Ocean 30.

Multiple lines of evidence support our notion that the low deglacial benthic δ13C values at AAIW/SAMW depths reflect the entrainment of “old”, high pCO2/nutrient-enriched water masses into AAIW/SAMW as has been speculated before 6,12–19. Benthic δ13C are inversely correlated to Ba/Ca (this study; nutrient proxy) and Rhenium (Re) excess concentrations 21 (proxy for oxygenation) from the same site (see Supplementary Discussion and Supplementary Fig. 1). Further support for our interpretations stems from radiocarbon activities (Δ14C) of benthic foraminifera off the coast of Southern Chile at AAIW/SAMW depths (cores SO161-SL122; GeoB7162) 22,24 (Fig. 1a, b). Values deviate from the atmospheric Δ14C record during the deglaciation, and within AAIW/SAMW depths even reaching an extreme low value of ~−200‰ during H1 (GeoB 7162), marking the presence of “old” water masses 22,24. In contrast, core sites in the same ocean area but located outside and deeper than modern AAIW/SAMW are within Pacific Deep Water (PDW) 20. These sites do not show deglacial benthic δ13C decreases and even point to increases which is in line with a better ventilation of aged glacial PDW 20,24.

Our smoothed SS record shows considerable increases from ~14.25 μm to ~16.25 μm during H1 and high values of ~16 μm during YD, suggesting the intensification of AAIW/SAMW-related bottom currents at the intermediate depth core location (Fig. 2d). Such strengthening of AAIW/SAMW circulation together with low oxygen concentrations 21 (high Re) and low benthic δ13C water (this study; Fig. 2a) is best explained by the entrainment of “old” upwelled Southern Ocean waters into AAIW/SAMW (see also Supplementary Fig. 1). Without this assumption, a stronger circulation would likely lead to increased oxygenation 21, which contradicts the Re excess data.

The postulated acceleration of AAIW/SAMW current speeds is sustained by a model simulation for H1, showing an increased AAIW flow (and an atmospheric CO2 increase) in response to strengthened and poleward-displaced, Southern Hemisphere westerly winds (SWW) 31. However, this simulation and a related simulation of atmospheric and oceanic δ13C did not reproduce our observed negative δ13C anomalies at AAIW/SAMW depths, but rather in the upper ocean layer 30,31. Hence, we tested our observed AAIW/SAMW δ13C anomaly by running a Danish Center for Earth System Science (DCESS) model simulation forced with increased high-latitude ocean (i.e., Southern Ocean) vertical exchange for the H1 and YD periods (see Methods; Supplementary Discussion and Supplementary Fig. 2, 3). The results of the simulation for 800 m water depth qualitatively reproduces our observed negative δ13C anomalies in the South Pacific AAIW/SAMW, supporting the direct imprint of upwelled old (low δ13C) water masses in the Southern Ocean at AAIW/SAMW depths at Site 1233 (Fig. 3b). In addition, the results of this simulation for atmospheric pCO2 and δ13C show considerable agreement in timing and amplitude to the observed pCO2 and δ13C changes across H1 and YD (Fig. 3a, c).

Strengthened AAIW/SAMW circulation and low benthic δ13C between H1 and YD appear quasi-synchronous with the low δ13C of subsurface-dwelling N. dutertrei (habitat depth of ~100 water depth 32) and increased surface pCO2 based on boron isotopes from planktic foraminifera at eastern equatorial Pacific upwelling Site 1238 (ref. 6). This is accompanied by high opal accumulation rates at nearby Site V19-30 (ref. 33) (Fig. 2a–d). The synchroneity of events, provides support for the deglacial fueling of this region by increased flow of high pCO2/nutrient-enriched AAIW/SAMW 6. Indeed, AAIW/SAMW is in modern times a major southern source for the Pacific Equatorial Undercurrent and is estimated to account for 6–21% of its total transport 5,34. A significant amount of anthropogenic CO2 along with nutrients are transported by AAIW/SAMW (including the same density layer we track at Site 1233) into the Equatorial Undercurrent through diapycnal mixing and finally to the surface by upwelling 4,5,35. Additionally, direct upwelling of large amounts of AAIW/SAMW in the eastern equatorial Pacific was suggested from modern surface Δ14C data at the Peruvian upwelling and the Costa Rica Dome 36. Hence, it is plausible that principally during H1 and YD the increased AAIW/SAMW circulation which carried old and high pCO2 waters would have promoted the enhanced CO2 outgassing seen at eastern equatorial upwelling Site 1238 6 through one or both of these pathways (Fig. 2a, c, d). Also, other biochemical processes might have played a role for the increased outgassing during the deglacial. For instance, local mesopelagic nutrient recycling, a recently proposed major source for productivity in low latitudes lead to respiration of organic matter and therefore CO2 outgassing 37. Yet it is difficult to quantify this process for the deglaciation, however, we suspect an increase as is simulated for future global warming with rising SST 37. Deglacial outgassing in the eastern equatorial Pacific upwelling would have been complementary to vast CO2 outgassing in the Southern Ocean associated with global atmospheric CO2 increases during H1 and YD 8,10 (Fig. 2g).

The low deglacial N. dutertrei δ13C at ODP Site 1238 (ref. 6) could have been caused by surface exchange with the atmosphere 30. However, the close correspondence between high opal accumulation rates and increased CO2 outgassing at the surface, indicates that the N. dutertrei δ13C is associated with upwelling of high pCO2/nutrient-rich waters 6. Changes in the intensity of upwelling that could have impacted the above-described changes could be ruled out at ODP Site 1238 (ref. 6).

The paleocurrent speed proxy (SS) record broadly matches the radiocarbon age difference record between planktic and benthic foraminifera from deeper adjacent core MD07-3088 (ref. 24; Fig. 1a, b; 2d, e; 1536 m water depth), a record that reflects water mass stratification changes (small 14C difference = more vertical mixing). The deglacial acceleration of the AAIW/SAMW between H1 and YD is accompanied by an intensification of vertical mixing, and hence a decrease in stratification at core location MD07-3088 (ref. 24). Apparently, both processes were responding to the intensifying upwelling and mixing of deep-water masses in the Southern Ocean 10,38 (Fig. 2d–f), forced by increased and southward displaced SWW 10,31. At the same time there is evidence of reduced intermediate water mass stratification at sites along the Peruvian and Galapagos continental margins inferred from benthic δ13C and δ18O records between 400 and 1000 m water depths 12. It was speculated that AAIW/SAMW warming might have reduced the vertical temperature gradients in the Eastern Pacific, leading to reduced water mass stratification in the eastern equatorial Pacific upwelling region 12.

Our AAIW/SAMW temperature reconstructions at Site 1233 do not show a warming and instead imply rather cool temperatures during H1 and YD. Furthermore, a cooling of ~2°C is observed at the end of the Bølling-Allerød (BA; ~14.5–12.8 ka BP) which is known in the Southern Hemisphere as the Antarctic Cold Reversal (ACR) (Fig. 4c). Hence, the increased AAIW/SAMW current speeds during the deglaciation most likely led to an increased contribution of AAIW/SAMW at these intermediate levels northward to the Equatorial Undercurrent and thereby might explain weak intermediate water stratification in the regions cited above.

Our reported changes in proxy records across the last deglaciation could be influenced by vertical changes in the depth of AAIW/SAMW 39. For instance, a simulated global glacial shallowing of AAIW 40 could move Site 1233 out of the modern core of AAIW/SAMW and under increasing influence of lower Pacific Deep Water (PDW) (Fig. 1b). Our benthic δ13C and salinity reconstructions, however, in combination with existing proxy records for oxygenation from Site 1233 and from sites 1235 and 1234 from the upper and lower limit of modern AAIW/SAMW 21 (Fig. 1a, b) demonstrate that such shifts did not affect Site 1233 and that the changes we observe are instead principally related to changes in the properties and flow speed of AAIW/SAMW itself (see Supplementary Discussion).

The close correspondence between our AAIW/SAMW current speed reconstructions and benthic δ13C with negative benthic Δ14C excursions at core location MC19-GC31-PC08 off Baja California at 705 m water depth also sheds light on the ongoing controversy regarding the deglacial influence of AAIW there 16,18,19,41–44 (Fig. 1; 2a, d). Originally, these negative Δ14C excursions between H1 and YD were interpreted as the effect of the inflow of “old” Southern Ocean sourced water masses transported via AAIW to this site 16,18,19. This was contrasted by the view that regional hydrothermal vents might have caused the extreme low Δ14C off Baja California 43. A more widespread benthic Δ14C study in the eastern equatorial upwelling region, although lending support for AAIW as a source for moderate low Δ14C there, rather favoured North Pacific intermediate waters as a source for low Δ14C near Baja California, refuting the influence of regional hydrothermal vents 44.

The synchronization in time of our Site 1233 data with near Baja California benthic Δ14C data clearly supports the original interpretation 16,18,19. We provide evidence that the increased northward flow of “old” (low δ13C) AAIW/SAMW water masses between H1 and YD could have impacted Baja California causing negative Δ14C excursions there 16,18 (Fig. 2a, d). Support comes from neodymium isotope studies from the last deglacial from the same Baja California site (fish teeth) using a stereomicroscope. For Elemental analysis (Mg/Ca, Ba/Ca) ~7–20 specimens were selected and gently crushed between glass plates. If the sample material was ~>15 specimens, the crushed sample was separated into 2/3 used for Elemental analysis and into 1/3 which was used for stable isotope measurements. In cases when the sample material was smaller, 2-3 specimens were separated from the sample and used for stable isotope analysis. Stable isotope analyses were performed at GEOMAR Helmholtz Centre for Ocean Research Kiel on a Thermo Scientific MAT 253 with a Kiel IV Carbonate device. Standard Deviations (Analytical Precision 1σ) of Bremen standards measured during the sessions (n = 73) was ~0.04‰ for δ13C and ~0.09‰ for δ18O. Crushed samples for elemental analysis were cleaned according to standard cleaning procedure 53 (including reductive step with hydrazine) before analysis on a VARIAN 720–ES Axial ICP-OES at GEOMAR Helmholtz Centre for Ocean Research Kiel. Spectral lines for Mg/Ca ratios were 279 nm and 370 nm, and for Ba 455 nm. To monitor any remaining contamination source for the Mg/Ca (Ba/Ca) ratios through the presence of silicates and/or Mn-Fe oxyhydroxide coatings, Al (167 nm), Fe (238 nm), and Mn (257 nm) were measured. Al/Ca and Mn/Ca ratios were on average low and near detection limits and Fe/Ca ratios showed a median value of 0.1 mmol/mol indicating effective cleaning. However, when Al/Ca ratios were greater than >0.1 mmol/mol and were accompanied by relatively high Mg/Ca ratios we discarded these samples (including Ba/Ca values) as this might be an indication of contamination with terrigenous material 53. Mostly, these samples also showed higher Fe/Ca ratios (>0.1 mmol/mol). In one case, we decided to discard a sample based on high Fe/Ca. To correct a drift during the measurement sessions, the ECRM 752-1 standard was measured repeatedly. Blanks were inserted every 15th sample to monitor any external contamination. On average, analytical precision (1σ) for Mg/Ca ratios was ~0.04 mmol/mol.

Our benthic δ18O record largely parallels alkenone-derived SSTs from the same site 46 (age model updated according to our current age model; see Methods) and an Antarctic stacked δ18O record consisting of five Antarctic ice core cores that reflects changes in surface air temperatures 47 (Fig. 4a). During H1 SST warming (~18–16 ka BP), there is a dramatic benthic δ18O decline of ~1.5‰, followed by a plateau of values around 3.5‰ from the end of H1 across BA/ACR and then a decrease again of ~1‰ during the YD warming (Fig. 4a). These similarities between our benthic δ18O record and alkenone-derived SST 46 point to nearly synchronous deglacial changes at intermediate and in the surface waters.

The warming periods during H1 and the YD observed in alkenone-derived SST were interpreted as a combination of southward shifts of the SWW and Antarctic temperature increases, that directly affected atmospheric CO2 increases through upwelling and outgassing in the Southern Ocean 46. The BA/ACR, however, known as a cooling period in the Southern Hemisphere was described as a warm SST plateau, which did not suggest a rebound of the SWW back to the north 46 (Fig. 4a).

A recent combined model-data study based on available planktic foraminiferal δ18O records from the Southern Ocean support these deglacial changes in SWW, showing a ~5° poleward shift and ~25% strengthened SWW circulation 48 (Fig. 4b). As stronger and more southward SWW most likely intensified AAIW formation 31, it makes sense, that the deglacial AAIW/SAMW current speed increases at Site 1233 (SS) are in broad agreement with these poleward-displaced and strengthened SWW (Fig. 4a, b).

However, the comparison of our benthic Mg/Ca-derived temperatures (TMg/Ca) with alkenone-derived sea surface temperatures (SST) from the same site 46 shows an opposite development during H1 and at the end of the BA/ACR (Fig. 4a, c). During H1 (~18 and 16 ka BP), when alkenone-derived SST 46 increased most by ~4°C, benthic TMg/Ca stay relatively cold, and at the end of the BA/ACR when benthic TMg/Ca show a decrease of ~2°C (Fig. 4a, c), alkenone derived SST reached a warm plateau ~13°C (ref. 46; Fig. 4a, c).

This lack of deglacial warming of our benthic TMg/Ca creates an apparent mismatch with our decreasing benthic δ18O values (Fig. 4a, c). However, the benthic δ18O record does not exclusively reflect AAIW/SAMW temperatures, but instead depends on global ice volume, sea water temperature, and salinity. Our benthic δ18O record, corrected for these changes in global ice volume and temperature (see Methods) point to a lowering during H1, and at the end of BA/ACR with a total amplitude of ~1.3‰ (freshening), synchronous with relatively cold or cooling of benthic TMg/Ca, at times when the SWWs increased and shifted poleward 48 (Fig. 4b–d).

Hence, we argue that the freshening with relatively cold AAIW/SAMW temperatures during H1 and at the end of BA/ACR at Site 1233 were mainly driven by these changes in SWW through affecting the source region of AAIW/SAMW. This interpretation is supported by an alkenone-derived SST record from core GC-528 located in the Drake Passage 49 that shows deglacial cooling during the southward shift and strengthening of SWWs 48 that broadly resembles our cold AAIW/SAMW temperatures during H1 and at the end of BA/ACR (Fig. 4b, c, e).

Core GC-528 is reasonably representative for the source region of AAIW/SAMW bathing Site 1233 between ~50–60°S, just as modern AAIW formed at Drake Passage-like latitudes contributes to Atlantic Ocean AAIW and Southeast Pacific AAIW/SAMW that flows northward along the coast of Chile 25,50.

The tight interconnection between both ocean regions at different depth levels becomes especially evident by the comparison of our δ18OIVC-seawater values with GC-528 alkenone-derived SST 49 (Fig. 4d, e). Warm (cold) periods at GC-528 (ref. 49) coincide with more saline (fresher) AAIW/SAMW conditions at Site 1233 with notable AAIW/SAMW freshening at the end of BA/ACR.

Previously, it was argued that the deglacial poleward movement of SWW redirected the cold AAIW from the Pacific into the Atlantic, that might explain SST cooling at GC-528 (ref. 49). However, this would not explain synchronous cool AAIW/SAMW during H1 and cooling during the end of BA/ACR at Site 1233 (Fig. 4c, e).

Alternatively, we suggest that strengthening and poleward migration of the SWW enhanced upwelling of cold, deep-water masses and mixing between the polar and the subantarctic fronts nearby GC-528 that caused sea surface cooling in the AAIW/SAMW formation area, consequently affecting AAIW/SAMW temperatures at Site 1233. This cooling effect through upwelling/mixing is evidenced by the synchronous correlation between vertical mixing at MD07-3088 (ref. 24) and alkenone-derived SST at GC-528 (ref. 49) (Fig. 4e). Warm glacial alkenone-derived SST occurred when water masses were stratified, and SST dropped when the stratification decreased during the deglaciation. A general surface cooling effect from the Southern Hemisphere during the final BA/ACR, cannot be ruled out for having indirectly amplified AAIW/SAMW cooling at Site 1233, but seems rather unlikely, as the BA/ACR was not characterized as a surface cooling period at this Site 46. The high SST during the glacial at GC-528 (ref. 49) could have been amplified by regional meltwater stratification that might explain higher amplitudes there in comparison to Site 1233 AAIW/SAMW temperatures (Fig. 4c, e).

The freshening of AAIW/SAMW during H1 and at the end of BA/ACR at Site 1233 is best explained by increased freshwater input into the AAIW/SAMW formation area, most likely caused by increased precipitation rates due to the southward shift of the SWWs 12,51. In particular, during H1, this freshening was likely amplified by melting of sea ice or icebergs through vast Antarctic warming 12,51. During this time, increased sea-ice transport from strengthened SWWs likely intensified AAIW/SAMW formation rates 52. The formation of relatively fresh AAIW/SAMW during H1 and at the end of BA together with deep upwelling is not unlike the situation in the modern ocean, whereby upwelled relatively salty, Circumpolar Deep Water is transformed in part into relatively fresh AAIW/SAMW 52.

We constructed Southeast Pacific AAIW/SAMW properties close to its formation area at ODP Site 1233. Our benthic foraminiferal δ13C and sortable silt mean grain sizes (SS) showed prominent δ13C decreases and a strengthened northward AAIW/SAMW circulation between H1 and YD. These changes in combination with other proxy records from the same site suggested a strong northward flow of high pCO2 and nutrient-enriched AAIW/SAMW during H1 and YD, synchronous with the changes in the proxy records from the eastern Pacific equatorial upwelling region and even off Baja California. Our proxy evidence supports earlier assumptions about the far-reaching effects of AAIW/SAMW, which most likely increased outgassing of oceanic CO2 in the eastern equatorial Pacific upwelling region 6. Deglacial changes in our AAIW/SAMW temperature, salinity and current speed reconstructions were related to poleward shifted and strengthened SWW, which shaped the evolution and hence the properties of Southeast Pacific AAIW/SAMW.

Methods
Analysis of benthic foraminiferal stable isotopes (δ18O and δ13C), Mg/Ca, and Ba/Ca
For Mg/Ca and stable isotope analysis on benthic foraminifer Uvigerina peregrina we mostly used 20 cm3 of bulk sediment which was increased to 100 cm3 in several cases during the last 20 kyrs because of lack of enough specimens. Bulk samples from Site 1233 were freeze-dried, wet sieved over a 63 μm sieve and dried in an oven at 40°C. We then carefully selected benthic foraminifers from the >250 μm fraction using a stereomicroscope. For Elemental analysis (Mg/Ca, Ba/Ca) ~7–20 specimens were selected and gently crushed between glass plates. If the sample material was ~>15 specimens, the crushed sample was separated into 2/3 used for Elemental analysis and into 1/3 which was used for stable isotope measurements. In cases when the sample material was smaller, 2-3 specimens were separated from the sample and used for stable isotope analysis. Stable isotope analyses were performed at GEOMAR Helmholtz Centre for Ocean Research Kiel on a Thermo Scientific MAT 253 with a Kiel IV Carbonate device. Standard Deviations (Analytical Precision 1σ) of Bremen standards measured during the sessions (n = 73) was ~0.04‰ for δ13C and ~0.09‰ for δ18O. Crushed samples for elemental analysis were cleaned according to standard cleaning procedure 53 (including reductive step with hydrazine) before analysis on a VARIAN 720–ES Axial ICP-OES at GEOMAR Helmholtz Centre for Ocean Research Kiel. Spectral lines for Mg/Ca ratios were 279 nm and 370 nm, and for Ba 455 nm. To monitor any remaining contamination source for the Mg/Ca (Ba/Ca) ratios through the presence of silicates and/or Mn-Fe oxyhydroxide coatings, Al (167 nm), Fe (238 nm), and Mn (257 nm) were measured. Al/Ca and Mn/Ca ratios were on average low and near detection limits and Fe/Ca ratios showed a median value of 0.1 mmol/mol indicating effective cleaning. However, when Al/Ca ratios were greater than >0.1 mmol/mol and were accompanied by relatively high Mg/Ca ratios we discarded these samples (including Ba/Ca values) as this might be an indication of contamination with terrigenous material 53. Mostly, these samples also showed higher Fe/Ca ratios (>0.1 mmol/mol). In one case, we decided to discard a sample based on high Fe/Ca. To correct a drift during the measurement sessions, the ECRM 752-1 standard was measured repeatedly. Blanks were inserted every 15th sample to monitor any external contamination. On average, analytical precision (1σ) for Mg/Ca ratios was ~0.04 mmol/mol.

Calculation of Benthic temperatures, δ18O and ice volume corrected seawater values (δ18OIVC-seawater)
Bottom water temperatures were reconstructed using the core top based equation 54: Mg/Ca = 0.074 (±0.004) x T + 0.81 (±0.05) which translates into a temperature error of ~±0.9°C when applying our analytical precision of Mg/Ca analyses. We chose this equation as it provides the best fit of our core top benthic Mg/Ca-derived temperatures of on average 3.8°C (7 data points within the last 5.3 kyrs) with modern observed temperatures of 4.5°C at 830 m water depth near Site 1233 (ref. 55). It is known that the carbonate ion saturation of sea waters changed during the last deglaciation and is different between water masses and that this can affect benthic foraminiferal Mg/Ca ratios either during the precipitation process or post-depositionally through calcite dissolution 54,56. However, as has been shown, Mg/Ca ratios of Uvigerina peregrina are practically unaltered by these changes 54,56 supporting the robustness of our benthic Mg/Ca-derived temperatures.

Before calculating δ18OIVC-seawater values (δ18Oseawater) which serve as an approximation for past changes in salinities, the benthic δ18O record was interpolated because unfortunately, not all our Mg/Ca data were covered by δ18O data points due to lack of sample material. Interpolation between measured δ18O values were in the range of a few hundred years, and only once during the Holocene reach about 1 kyr. Due to interpolation between such short time intervals, we assume that the values are robust. δ18Oseawater values were then calculated using an established equation 57: T = 16.9 − 4.38 * (δ18Oforam − δ18Oseawater) + 0.1 * (δ18Oforam − δ18Oseawater)^2 after adding 0.27 ‰ for adjusting to the SMOW scale 58. Finally, values were corrected for changes in global ice volume to calculate δ18OIVC-seawater values. Here a high-resolution estimation of global ice volume change was used which is based on multiple observations of sea level changes over the last 35 kyrs 59. On average, the propagated error for our δ18Oseawater values is ~±0.26‰ and we estimate a similar error for our δ18OIVC-seawater values as we used the high-resolution and accurate ice volume estimation 59.

Grain size analyses and sortable silt mean grain size (SS) as paleocurrent proxy
For grain size analyses we used approx. 700–750 mg from the freeze-dried bulk sediment (20 cm3), and then chemical removal steps of organic material and calcium carbonate followed 60,61. To minimize any possible contamination with dust particles during removal steps, we decided to work under a laminar flow cabinet. Biogenic silica was not removed as opal concentrations were on average 3.7% (max. value of 5.2%) within the studied time interval 21,62 and therefore estimated as negligible 60. Organic material was removed by adding 20 mL H2O2 (35%) at about 70°C for 6 h or until no further reaction took place. CaCO3 was removed by adding 10 ml HCl (10%) to 100 ml MilliQ water at about 90°C for a few minutes. Subsequently, the acid sample solution was removed by repeated dilution with MilliQ water and decantation. Visual inspection of some samples after the removal steps with a microscope could not reveal any biogenic material. Finally, sample material was ultrasonicated and stored in a 0.02 M Polyphosphate solution. Before analysis, samples were again shortly ultrasonicated, filtered over a 100 μm membrane and analyzed on a Beckman Coulter Counter Multisizer 4e with an aperture tube of 100 μm at Pontifical Catholic University of Chile.

SS was then calculated according to the equation 63,64: SS = 1/V * Σ (Vi * di) Where i is the size bin reaching from i=190 = 10.02 μm to 400 = 59.75 μm. (The used grain size scale is justified as no grains were detected within 59.75 μm and 63 μm. Vi is the differential volume at the ith size bin, and di is mid-size of the ith size bin, and V is the total volume between i=190 and i=400. Because it has been shown that only a few data counts of the coarse end of the Sortable Silt scale can alter the SS result through their disproportional high weights compared to the small grain numbers 64, every sample was measured 2–6 times and an average value was calculated showing a standard deviation (1σ) of on average ~0.5 μm (150 samples). Additionally, all raw values of repeated analyses of the same samples were monitored to check consistency between single measurements. In 20 samples, we discarded single grain counts (1–4) in the coarse fraction (~30–60 μm) as they were not represented in the repeated analyses. Before rejecting these single grain counts, these samples usually showed an anomalous high standard deviation between 1.2 μm and 2.5 μm. Mismatches between repeated analyses of the same samples might have been caused by different errors. One might be the possibility that air bubbles were occasionally trapped within the system during measuring and were erroneously counted as grains. The other is the described phenomena of coincidence correction of measuring single grains, which can lead to artificial counts of grains caused by the software 64.

Age model
For an initial age control of Site 1233 we used published age control tie points during the Holocene based on tuning high-resolution Ca and magnetic susceptibility records to nearby 14C dated core GeoB3313-3 (Supplementary Fig. 4; ref. 65). For the period 8.7–32.8 ka BP we used 21 existing mixed planktic foraminiferal 14C AMS age control points 46,65–67. These 14C AMS age datums were originally calibrated based on different curves e.g., CALPAL 2004 January calibration curve (www.calpal.de), INTCAL04 (ref. 68), and Fairbanks 0107 (ref. 69). As all these calibration curves are now outdated, we updated the original 14C AMS age datums with Calib 8.20 software 70 using the current calibration curve for the Southern Hemisphere 71 (SHCal20; see Supplementary Table 1). Evidence from detailed tephrostratigraphy from southward located MD07-3088 indicates that reservoir ages most likely have considerably changed during the studied time-period 24,72. As a detailed tephrostratigraphic study does not exist from Site 1233, we applied variable reservoir ages for the surface ocean using a model simulation 73 and subtracted these from the initial datums before calibration (see Supplementary Table 1). Simulated average reservoir ages close by Site 1233 (41°S; 75°W) with a temporal resolution of 1 kyr were used for the critical time between 11 and 25 ka BP, and this resolution was lowered to 5 kyr for the Holocene and the remaining glacial period 73. Subsequently, a 3-point moving average smooth was calculated. Then, uncalibrated atmospheric 14C ages were determined for these simulated sea surface reservoir ages and applied to uncalibrated 14C age datums from Site 1233. The obtained results were compared with the sea surface reservoir age from an identified tephra layer (Lepué Tephra) at Site 1233 with a basal depth of 14.80 mcd, which eruption could be dated using land deposits indicating a 14C age (BP) of ~9588 years 74. Interpolation between planktic foraminiferal radiocarbon 14C ages (BP) at Site 1233 gives a marine age of ~10.07 ka BP for this tephra layer, resulting in a difference of 481 years compared with the land deposits, which is in accordance with the used simulated sea surface reservoir age of 554 years for this time (see Supplementary Table 1). The maximum difference between our used model-based surface reservoir age estimations 73 and these proposed for site MD07-3088 (ref. 24) is ~400 years. Compared to previous reported age models from the same Site 46,65–67 our revised age model has a maximum difference of ~1000 years during the deglaciation (Supplementary Fig. 4). As during the critical time period between 8 and 30 ka BP, there is every 1.1 kyrs a 14C AMS tie point, and the simulated surface reservoir ages were confirmed by one tephra layer, we assess the age model as robust. Indirect support for the updated age control comes from the timely match of our benthic δ18O and an Antarctic δ18O ice core stack 47 (Fig. 4a).

DCESS model simulation
The DCESS Earth system model 75 features modules for the atmosphere, ocean, ocean sediment, land biosphere and lithosphere, calibrated and tested against Earth system data and against output from more complex models. The model consists of one hemisphere, divided by 52° latitude into a low-mid latitude and a high latitude zone. The atmosphere module considers radiation balance, meridional transport of heat and water vapor between low-mid latitude and high latitude zones, and heat and gas exchange with the ice-free part of the ocean. Gases considered are carbon dioxide and methane for all three carbon isotopes, nitrous oxide and oxygen.

The model ocean is 270° wide, extends from the equator to 70° latitude and covers 70.5% of the Earth surface. Each ocean sector has 55 vertical layers with 100 m resolution. An ocean sediment sector is assigned to each ocean layer with widths taken from observed depth distributions. The ocean module has prescribed circulation and mixing. Model ocean tracers are temperature, salinity, oxygen isotope in water, phosphate, dissolved inorganic carbon for all three carbon isotopes, and alkalinity. Biogenic production of particulate organic matter depends on surface layer phosphate availability but with lower efficiency in the high latitude zone, due for example to iron limitation. The calcite to organic carbon rain ratio depends on surface layer temperature and carbonate ion concentration 76.

The semi-analytical, ocean sediment module considers calcium carbonate dissolution and oxic and anoxic organic matter remineralization. The sediment is composed of calcite, non-calcite mineral and reactive organic matter. The land biosphere module considers leaves, wood, litter and soil. Net primary production depends on atmospheric carbon dioxide concentration and remineralization rates in the litter and soil depend on mean atmospheric temperatures. Methane production is a small fraction of the soil remineralization. This module has been extended to include three dynamically varying vegetation zones and a permafrost component 77. The lithosphere module considers outgassing, weathering of carbonate and silicate rocks and weathering of rocks with old organic carbon and phosphorus.

For the simulation presented here we applied a approach 77 for the last glacial termination that was updated to better represent dust-climate feedbacks 78. For the period 25 ka BP to the present, we prescribed albedo forcing from changing ice sheet size as well as dust radiative forcing and iron fertilization of high latitude ocean biota as described in the above references.

We found agreement with observed low last glacial atmospheric pCO2 for the prescribed 25 ka BP forcing and with vertical exchange coefficient in the high latitude ocean (kvHL) above 1000 m reduced by 30% relative to pre-industrial levels (as calibrated before 75) and kvHL below 1000-2000 m set to low values of the low-mid latitude ocean. Depth-dependent kvHL in the model last glacial generates a model analogy to isolated deep water in the Southern Ocean at that time. This is meant to emulate the effects of some combination of weakened or equatorward-shifted SWW and increased stratification through brine-induced effects as have been suggested in various studies 31,48,79,80. This led to significantly increased deep ocean storage of 13C-depleted, dissolved inorganic carbon from remineralization of organic matter raining down from the ocean surface layer. In addition to the above mentioned forcing we also varied over time the size and distribution of kvHL to simulate storage/release of deep ocean carbon from/to the upper ocean and atmosphere (Supplementary Fig. 2). For the H1 and YD periods, we increased kvHL overall or most high latitude ocean depths to at or near preindustrial levels to emulate increased Southern Ocean upwelling drawn from greater depths as well as increased vertical exchange as would result from stronger SWW during these periods.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents new proxy data from their core ODP-1233: benthic isotopes and Mg/Ca (used for benthic CO2, T and salinity reconstructions) & sortable silt for bottom current speeds. The authors combine these with a wide array of similar proxies from this and other cores from the Eastern South Pacific and Drake Passage to build the case that AAIW flows sped up while keeping a signal of increased respired CO2 in the first half of the deglaciation. These 2 findings are in turn interpreted as fueling increased outgassing of CO2in the eastern equatorial Pacific (EEP) identified in previously published studies. This interpretation notably supports the earlier work and interpretations from Baja California (Marchitto et al. 2007) and the EEP (Martinez- Boti etal), but it also conflicts with numerous later the interpretations most notably that of De Pol-Holtz et al (2010) based on 14C results in a nearby core. This update on the debate is of wide interest.\nI’m excited to see this very detailed record that by its high resolution along improves on many records that have been published from this area. The most compelling proxy evidence is the sortable silt record showing an increase in bottom flow rates is a strong indicator of the sort of enhanced flow one would expect, and would expect to be necessary to carry any load of respired DIC (dissolved inorganic carbon) to the EEP and even farther to Baja California to be outgassed. This information alone is an important result. The sortable silt data supports their record of respired DIC signal (based on benthic 13C) that mimics that of deep planktics in the EEP (site 1238). They provide convincing evidence their benthic signal is due to enhanced respired DIC not atmospheric input. One can argue that the ~1.5 ‰ depletion seen in their benthic record could, account for a ~ 0.7 ‰ depletion in deep planktics in the EEP (site 1238; figure 1) even if it is not quantitative.\nDespite the strength of these data I find a few gaps in their arguments that should be cleared up.\nAs mentioned above: They state that the  14C records on the Chilean margin support their interpretations (De PolHoltz et al ) whereas the original study shows the  14C values on the Chilean margin are not low enough to have caused the very low signals in the low latitude North Pacific – the authors should reconcile their statements with that of previously published papers more thoroughly and quatiatively if possible, to support their interpretations\nMuralati et al (2010) show high oxygen in the same core used in this study (1233) through the period of low 13C interpreted as high respired DIC. Generally these parameters are anti-correlated. The authors should address how this can be reconciled.\nAdditionally, the 2010 study shows that increasing oxygenation that occurs after 16 ka which is later than the onset in changes in sortable silt and benthic 13C in site 1233. One would expect such changes to co-occur. Is this offset due to the changes the authors made to the core stratigraphy this should be addressed.\nThe authors have done a thorough job in re-doing the stratigraphy in the core. The authors should provide a figure of depth to age of the new stratigraphy and the old (on the same figure) so that readers can see how the revised/ improved stratigraphy affects the timing presented in this study relative to the timing in the several studies published on the same core.\nAlthough the authors have detailed the process in methods and provided tie points in the SOM the actual stratigraphy is not documented in the submission. I raise this because am concerned that the stratigraphy is based on 14C of mixed planktics given the statement (line 492), and from other studies, that the surface reservoir ages have changed considerably in this region during the time of most interest to this study. If the surface reservoir ages are changing so dramatically in the area (Siani et al and De Pol-Holtz et al). If they are using new stratigraphy it should be thoroughly documented with calendar years to depth (not provided) as well as 14C years (ages) to depth. How different is this stratigraphy from previous studies?\nThe benthic temperature (T) are interesting and exciting. However, the low resolution and lack of data in the period of H1 to the YD s is really much too low to make any strong conclusions about the temperature and the resulting salinity estimates in or salinity during this period. Importantly, the variability in the T data, but moreover the even greater variability in the resulting salinity calculations is so large that the “low salinity period” that they identify H1-YD period actually sits within the variability in the T estimates before this “event” and in the salinity before and after this event. One could equally strongly argue that the T in the H1-YD period is the same within variability to the LGM or in early H1 it appears perhaps warmer The 2 low salinity points on which they base their interpretations fall within the glacial variability. The 2 low S points may simply be the result of aliasing due to lack of data as much as it may indicate low salinity. Visually it is compelling but, is it accurate to base an assessment of water properties on 1-3 points across 3,000+ years. Arguably, the salinity at the start of H1 is high rather low.\nThe viability of this “trend” could be examined but running a smoothing cure through this data (as for the sotable sil) and or other statistical analyses (an error envelope). These should be done before making significant interpretations about salinity changes. This is perhaps the weakest of the main points made in the paper. However, this doesn’t detract from the other significant conclusions: even without the suggestion of freshening the paper could still be published on the merits of the other data presented.\nIn light of the freshening and cooling interpretations: The authors do not consider whether the signal in this core could represent heave in the location of AAIW. That is – what if AAIW was shallower (or deeper) than this core site in the LGM and what they are seeing is the depth of the AAIW center on this core for part of the deglaciation whereas it was deeper (or shallower) before/ after?\nSpecific comments:\nFigure 1 – it would be useful to put MD07-3088 on the vertical profile in panel B.\nLiz Sikes", "reviewer_2": "Karas and coworkers have compiled many pieces of evidence to support their argument that, during the most recent deglaciation, Antarctic Intermediate Water (AAIW) served as a conduit through which deep water that upwelled the Southern Ocean was transmitted to equatorial upwelling zones. These equatorial regions then served as a window through which CO2 was transferred from the deep ocean to the atmosphere. It is likely that they are correct, but there are alternative possibilities that should be considered before this conclusion is published.\nMy principal comment refers to the interpretation of sedimentary proxy data entirely in terms of changes in the flow rate and composition of AAIW. Could these same proxy records be interpreted as changes in the depth of the boundary between AAIW and other water masses, such as the Pacific Deep Water (PDW). Core ODP1233 was recovered from a depth of 833 m (manuscript Figure 1), within the gradient between AAIW and PDW (or Upper Circumpolar Deep Water, UCDW). Can a vertical shift in the depth of this boundary be responsible for the observed changes (Figures 2 and 3)?\nDisplacement of the SWW, invoked by Karas et al to explain the deglacial increase in upwelling of deep water in the Southern Ocean, would cause a change in the location of AAIW formation, the thickness of the AAIW water mass, and the depth of the boundary between AAIW and PDW (Danny Sigman and Mathis Hain, personal communication). Furthermore, Pahnke and Zahn (2005) noted that changes in bottom water composition at depths of AAIW could be related to changes in the depth of the boundary between AAIW and PDW (or UCDW) rather than to changes in the formation or composition of AAIW. Thus, one must consider changes in water mass structure, as well as changes in ocean circulation, in addition to changes in water mass composition, when interpreting the new records presented in this manuscript. Is the interpretation of Karas et al supported by the findings of Pahnke, even though the study sites are on opposite sides of the South Pacific Ocean? There are enough similarities between the approach of Pahnke and Zahn (2005) and the new study presented in this manuscript that I believe the paper by Karas et al would be stronger if it included a discussion of the data in Pahnke and Zahn.\nKaras et al may be correct in their interpretation, but the conclusions would be strengthened if they can rule out changes in water mass structure, leaving changes in water mass composition as the only reasonable interpretation.\nAlthough the main point that I would like the authors to address is described above, the statement that I object to most is in lines 151-153. I disagree completely with the implication that there was not much CO2 degassing from the Southern Ocean during the YD. Upwelling occurs south of the Antarctic Polar Front (APF), whereas the core sites studied by Shuttleworth et al (ref 26) are located near the Subtropical Front, barely within the Southern Ocean and far north of the zone of upwelling. We know that deep water reached the euphotic zone south of the APF because this was necessary to provide the dissolved Si that stimulated the opal fluxes shown in Fig 2f. Although the euphotic zone is sometimes deeper than the mixed layer, it is hard to imagine what physical conditions would allow deep water to reach the euphotic zone without a corresponding increase in CO2 release to the atmosphere. The statement in lines 151-153 is unnecessary and very likely inaccurate, so I recommend that it be deleted.\nThe manuscript is written very well. The only editorial change I would recommend is on line 456, where “CaCo3 was removed by adding 10 ml HCL” should be changed to “CaCO3 was removed by adding 10 ml HCl”.\nReferences (see Pahnke and Zahn 2005 but the others may be relevant too.)\n[Pahnke et al., 2008; Pahnke and Zahn, 2005; Pahnke et al., 2003]\nPahnke, K., S. L. Goldstein, and S. R. Hemming (2008), Abrupt changes in Antarctic Intermediate Water circulation over the past 25,000 years, Nature Geoscience, 1(12), 870-874", "reviewer_doi": "10.1038/ngeo360.\nPahnke, K., and R. Zahn (2005), Southern hemisphere water mass conversion linked with North Atlantic climate variability, Science, 307(5716), 1741-1746.\nPahnke, K., R. Zahn, H. Elderfield, and M. Schulz (2003), 340,000-year centennial-scale marine record of Southern Hemisphere climatic oscillation, Science, 301(5635), 948-952.", "reviewer_3": "This is an interesting paper that provides new insights from the SE Pacific into the role that AAIW plays in the 2 periods of CO2 flux during H1 and YD during the last deglacial. There is evidence from AAIW records in several other regions of the Southern Ocean– SW Pacific (Ronge et al., 2016; Ronge et al., 2021), Indian Ocean (Ronge et al., XXX) and also from the SE Pacific (Siani et al., 2013; Haddam et al., 2016) – which shows significant changes in the AAIW during the deglaciation as a result of the increased upwelling of old carbon at the Polar Front. The new record provided by Karas et al., provides higher resolution d13Cbenthic, Mg/Cabenthic, Ba/Cabenthic and Sortable Silt, of these deglacial changes in AAIW from the SE Pacific. The data test and ultimately support the previous hypotheses about the role of AAIW as a conduit for transporting the high CO2 waters to the EEP where d11B data show a release of CO2 during the deglaciation. It is great to see a good test of these hypotheses from the region where most of the AAIW is formed. This is a nice contribution to the literature around the deglacial CO2 fluxes.\nI appreciate that the authors have looked at the core in very high resolution – centennial scale. But the d13C and SS are very noisy. I wonder if it would help to have a bold line to show the running mean through the data as it is hard to see the longer term trends over the noise, especially for the benthic foram d13C data – at least remove the crosses on the data as it is too messy. I had to look hard to see the decline in the d13C during both H1 and YD. Why is the Ba/Ca data hidden in the supplementary data. I would show it in the main paper as this is part of the new data and you have the methods for it in the main paper. The SS is interesting showing a clear increase in current speeds in the deglaciation – this adds new insights into the processes going on, suggesting an increase in the AAIW currents (linked to the SWW – from Gray et al., 2023).\nThe main thing I struggled with in this paper was the model. It is unclear to me why the DCESS model results are hidden away in the supplementary section and not shown in the main paper. I would have liked to have seen more of a discussion around the models. Given previous models have not shown such a decline in d13C during the deglacial – why is this one different? This could be more of a discussion in the Supplementary.\nThe manuscript is generally well written, with just a few sections and sentences that need rephrasing to improve the flow and readability. Several of the figures could be improved and I have provided comments on them."}, "author_response": "We thank all three reviewers for thoughtful comments that\nimproved the manuscript. In the following, we address all\ncomments from the three reviewers. Comments from the\nreviewers are in small italic font size, our replies are in bold font\nsize and our revisions of the manuscript text are in quotation\nmarks:\nComments from Reviewer 1:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis paper presents new proxy data from their core ODP-1233: benthic isotopes and Mg/Ca (used for benthic\nCO2, T and salinity reconstructions) & sortable silt for bottom current speeds. The authors combine these\nwith a wide array of similar proxies from this and other cores from the Eastern South Pacific and Drake\nPassage to build the case that AAIW flows sped up while keeping a signal of increased respired CO2 in the\nfirst half of the deglaciation. These 2 findings are in turn interpreted as fueling increased outgassing of CO2in\nthe eastern equatorial Pacific (EEP) identified in previously published studies. This interpretation notably\nsupports the earlier work and interpretations from Baja California (Marchitto et al. 2007) and the EEP\n(Martinez- Boti etal), but it also conflicts with numerous later the interpretations most notably that of De\nPol-Holtz et al (2010) based on 14C results in a nearby core. This update on the debate is of wide interest.\nI’m excited to see this very detailed record that by its high resolution along improves on many records that\nhave been published from this area. The most compelling proxy evidence is the sortable silt record showing\nan increase in bottom flow rates is a strong indicator of the sort of enhanced flow one would expect, and\nwould expect to be necessary to carry any load of respired DIC (dissolved inorganic carbon) to the EEP and\neven farther to Baja California to be outgassed. This information alone is an important result. The sortable\nsilt data supports their record of respired DIC signal (based on benthic 13C) that mimics that of deep\nplanktics in the EEP (site 1238). They provide convincing evidence their benthic signal is due to enhanced\nrespired DIC not atmospheric input. One can argue that the ~1.5 ‰ depletion seen in their benthic record\ncould, account for a ~ 0.7 ‰ depletion in deep planktics in the EEP (site 1238; figure 1) even if it is not\nquantitative.\nWe appreciate that Reviewer 1 find our study interesting, important, and compelling.\nDespite the strength of these data I find a few gaps in their arguments that should be cleared up.\nAs mentioned above: They state that the 14C records on the Chilean margin support their interpretations\n(De PolHoltz et al ) whereas the original study shows the 14C values on the Chilean margin are not low\nenough to have caused the very low signals in the low latitude North Pacific – the authors should reconcile\ntheir statements with that of previously published papers more thoroughly and quatiatively if possible, to\nsupport their interpretations\nIn lines 96-99 we cited three studies, one of them is the study of De Pol-Holz et al. (2010), the\nresults of which were revised by a subsequent study of Siani et al. (2013). Siani et al. (2013)\nrecalculated 14C of water masses by applying variably surface reservoir ages for SO161-SL122.\nThey found local water masses to be about twice as old as found in the original study and to have\ndistinct deviations from the atmospheric 14C.\nAs citing the original study of De Pol-Holz et al. (2010) generates here confusion, we deleted it to\nbetter support our findings. In the following sentence we also provide quantitative information\nabout 14C changes.\n13C interpreted as high respired DIC. Generally these parameters are anti-correlated. The authors should\naddress how this can be reconciled.\nWe appreciate the comment of the reviewer but think that there is a misunderstanding. Muratli et\nal. (2010) showed higher oxygenation (lower Rhenium (Re) concentrations) during the last glacial,\nwhen our benthic 13C indicate higher values. During the last deglaciation, they showed lower\noxygenation, in line with our lower benthic 13C values (see Supplementary Figure 1). Together\nwith our Sortable Silt record we interpret these deglacial changes in our manuscript as an\nincreased northward circulation of “old” high pCO / nutrient enriched water masses.\nthe onset in changes in sortable silt and benthic 13C in site 1233. One would expect such changes to co-occur.\nIs this offset due to the changes the authors made to the core stratigraphy this should be addressed.\nFirst, we want to mention that as stated above, here may occurred a confusion because Muratli et\nal (2010) described lower oxygenation during the deglaciation, starting at around 17 ka. Second,\nas already speculated by the reviewer, the age model used by Muratli et al. (2010) slightly differs\nfrom our revised age model, however, to avoid offsets generated by different age models, we\nplotted all records from Site 1233 against core depth (mcd; see Supplementary Information Fig.1).\nWe now mention this in the Supplementary Information to avoid any confusion:\n“Note that Rhenium values were initially plotted against an outdated age model9 and for\nconsistency are here displayed against core depth.”\nWe further included our Sortable Silt record in Supplementary Information Figure 1c to better\nshow that deglacial increased current speeds co-occur with lower benthic 13C, and higher Ba/Ca,\nsuggesting the intensified transport of old (high pCO2 and low O2) nutrient rich waters:\nbe caused by its lower resolution compared to our records.\nThe authors have done a thorough job in re-doing the stratigraphy in the core. The authors should provide a\nfigure of depth to age of the new stratigraphy and the old (on the same figure) so that readers can see how\nthe revised/ improved stratigraphy affects the timing presented in this study relative to the timing in the\nseveral studies published on the same core.\nAs suggested by the reviewer, we inserted a new Supplementary figure 4 where we compare the\nupdated age model of this study vs. earlier chronologies on depth/age scales:\nAlthough the authors have detailed the process in methods and provided tie points in the SOM the actual\nstratigraphy is not documented in the submission. I raise this because am concerned that the stratigraphy is\nbased on 14C of mixed planktics given the statement (line 492), and from other studies, that the surface\nreservoir ages have changed considerably in this region during the time of most interest to this study. If the\nsurface reservoir ages are changing so dramatically in the area (Siani et al and De Pol-Holtz et al). If they are\nusing new stratigraphy it should be thoroughly documented with calendar years to depth (not provided) as\nwell as 14C years (ages) to depth. How different is this stratigraphy from previous studies?\nHere, we kindly want to mention that the Supplementary table 1 provided the depths (mcd) of Site\n1233 along with uncalibrated 14C years BP and the calibrated (calendar) years BP. Maybe the\nnomenclature of the original table was confusing and therefore we changed the column names to:\n“Original AMS 14C ages (ka)” to “Original AMS 14C ages (uncalibrated ka BP)”\n“Current age model (calibrated ages in ka)” to “Current age model (calibrated ages in ka BP)”\nThe differences between our revised stratigraphy and earlier studies are now shown in the new\nSupplementary Fig. 4. We inserted the following sentence to the methods section for better\nexplanation:\n“Compared to previous reported age models from the same Site35, 53-55 our revised age model\nhas a maximum difference of ~1000 years during the deglaciation (Supplementary Fig. 4). “\nThe benthic temperature (T) are interesting and exciting. However, the low resolution and lack of data in the\nperiod of H1 to the YD s is really much too low to make any strong conclusions about the temperature and\nthe resulting salinity estimates in or salinity during this period. Importantly, the variability in the T data, but\nmoreover the even greater variability in the resulting salinity calculations is so large that the “low salinity\nperiod” that they identify H1-YD period actually sits within the variability in the T estimates before this\n“event” and in the salinity before and after this event. One could equally strongly argue that the T in the H1-\nsalinity points on which they base their interpretations fall within the glacial variability. The 2 low S points\nmay simply be the result of aliasing due to lack of data as much as it may indicate low salinity. Visually it is\ncompelling but, is it accurate to base an assessment of water properties on 1-3 points across 3,000+ years.\nWe took into consideration the caveats depicted by Reviewer 1 related to our benthic temperature\nand salinity reconstructions. However, we want to make clear, in this respect, that the deglacial\ncooling and freshening is not only represented by 2 data points. In fact, H1 (18-16 ka) is covered by\n10 datapoints, clearly demonstrating a cooling of ~1 °C and freshening of ~1 ‰. Later, during the\nBølling Allerød (BA) (at ~13 ka BP), a further cooling and freshening is only documented by two\ndatapoints.\nFurther, we want to point out that it is rather irrelevant for our interpretation whether the\ndeglacial AAIW temperatures and/or salinities are not much lower compared to the last glacial.\nCentral for our interpretation is that AAIW temperatures are not showing the vast surface\nwarming trend at Site 1233 during H1 – BA (Fig. 4a,c). During this time period, they instead show a\nfreshening and slight cooling synchronous to SST cooling at GC-528 (Roberts et al., 2017).\nTo better explain this and clearly describe when our AAIW data support the interpretation of a\ncooling and freshening, we changed the text on several occasions:\n„This lack of deglacial warming of our benthic T which instead show a slight cooling,\nMg/Ca,\ncreates an apparent mismatch with our decreasing benthic 18O values (Fig. 4a, c).“\n„We argue that the freshening and cooling of AAIW we observe during H1 until BA at Site\n1233 most likely reflect climatic changes in the AAIW source region driven mainly by SWW\nchanges. This interpretation is supported by an alkenone-derived SST record from core GC-\n528 located in the Drake Passage38 that resembles our Site 1233 AAIW temperatures, both\nindicating a cooling over this time period, in stark contrast to sea surface warming at Site\n1233 (ref. 35) (Fig. 4a, c, e).“\nWarm (cold) periods at GC- 528 coincide with more saline (fresher) AAIW conditions at Site\n1233 with notable AAIW freshening during H1 and into BA associated with surface cooling\nat GC- 528 (ref. 38).“\nArguably, the salinity at the start of H1 is high rather low. The viability of this “trend” could be examined but\nrunning a smoothing cure through this data (as for the sotable sil) and or other statistical analyses (an error\nenvelope). These should be done before making significant interpretations about salinity changes. This is\nperhaps the weakest of the main points made in the paper. However, this doesn’t detract from the other\nsignificant conclusions: even without the suggestion of freshening the paper could still be published on the\nmerits of the other data presented.\nAccording to the Reviewer suggestion, we now included error bands for our benthic T and the\nMg/Ca\n18O\nIVC-seawater\nvalues in Fig. 3 to better indicate the robustness of our described trends:\ncore could represent heave in the location of AAIW. That is – what if AAIW was shallower (or deeper) than\nthis core site in the LGM and what they are seeing is the depth of the AAIW center on this core for part of the\ndeglaciation whereas it was deeper (or shallower) before/ after?\nThe reviewer raises the important aspect of possible heaves in AAIW during the deglaciation. To\naddress this point, we inserted a new paragraph in the manuscript and a new section in the\nSupplementary Information “Evidence for Site 1233 bathed by AAIW”, which explain this aspect in\ndetail. We argue that according to several proxy evidence from Site 1233 and close-by sites bathed\nwithin the margins of AAIW, prominent vertical changes of AAIW at Site 1233 were unlikely to\noccurred:\nInserted paragraph in the manuscript text:\n“Our reported changes in proxy records across the last deglaciation could be influenced by\nvertical changes in the depth of AAIW28. For instance, a simulated global glacial shallowing\nof AAIW29 could move Site 1233 out of the modern core of AAIW and under increasing\ninfluence of lower Pacific Deep Water (PDW) (Fig. 1b). Our benthic 13C and salinity\nreconstructions, however, in combination with existing proxy records for oxygenation from\nSite 1233 and from sites 1235 and 1234 from the upper and lower limit of modern AAIW17\n(Fig. 1a, b) demonstrate that such shifts unlikely affected Site 1233 and that the changes we\nobserve are instead principally related to changes in the properties and flow speed of AAIW\nitself (see Supplementary Discussion).”\n“Evidence for Site 1233 bathed by AAIW\nA multi model simulation suggested that global AAIW core water depth was shallower by\n~180-300 m during the Last Glacial Maximum (~20 ka BP) compared to modern, due to the\nnorthward expanded sea ice around Antarctica and the northward shift and reduction of the\nprecipitation band15. This simulated heave of global AAIW would move Site 1233 from its\npresent core of AAIW towards its deeper margin during the last glacial15,16 (see Fig. 1b).\nHowever, proxy data for the simulated sea ice extent crucial for AAIW depth are sparce\nduring the last glacial from the Pacific sector, especially from the Drake passage and it is\nknown that model simulations tend to overestimate glacial sea ice in these regions17.\nRadiocarbon 14C proxy evidence from intermediate depths from the Southeast Pacific does not\nsupport a deepening of AAIW core depth during the deglaciation but instead suggest a\nthickening of AAIW towards deeper waters of up to ~1500 m18.\nIf large heaves of AAIW would have occurred at Site 1233 during the studied time period,\nthis is testable as these would either have led to more influence of Pacific Deep Water (PDW)\nfrom below or from Equatorial Subsurface Waters (ESSW) from above. Both, PDW and\nESSW differ at modern from AAIW by having lower oxygen levels, lower 13C and higher\nsalinities16.\nOur benthic 13C values from Site 1233 show similar and even slightly higher values during\nthe last glacial compared with the Holocene (S-Fig. 1a; Fig.2a). This clearly contradicts a\nscenario of shallower AAIW during the last glacial compared to the modern15, as this would\nimply lower glacial benthic 13C values due to an increased PDW influence. In this light, also\nan increased glacial influence from above through low 13C ESSW can be ruled out. The\nlatter scenario is highly unlikely because it would imply a glacial deepening of AAIW of\nmore than 300 m compared to the modern16.\nFurther, during the last glacial and Holocene, oxygen proxy records of Rhenium (Re) and\nMangan (Mn) excess from sites 1233 (modern core depth of AAIW), 1235 (upper AAIW\nmargin), and 1234 (lower AAIW margin) indicate better oxygenation at Site 1233 compared\nto the other sites7. This conflicts with a hypothesized glacial shallowing of AAIW15 as this\nwould imply higher oxygen contents at shallower site 1235, whereas deeper sites 1233 and\n1234 would approach low oxygen PDW7,16. During the deglaciation, Site 1233 would sit\nwithin the center part of the AAIW even when considering simulated changes in global\nAAIW core depth15. A strengthened influence of PDW (or ESSW) can be ruled out during\nthis period, as our 18O values imply freshening. In the modern ocean and\nIVC-seawater\npresumably also during the last glacial, these water masses are saltier than AAIW15,16. Hence,\nwe are confident that Site 1233 was bathed by AAIW during the entire period studied. Still,\nsmall changes in the water depth of AAIW or temporal changes in the thickness of AAIW\nmight occurred but are beyond the scope of this study.”\nFinally, we also inserted the locations of Sites 1234 and 1235 we discuss in the manuscript text in\nfigure 1.\nSpecific comments:\nFigure 1 – it would be useful to put MD07-3088 on the vertical profile in panel B.\nDone\nComments from Reviewer 2:\nReviewer #2 (Remarks to the Author):\nrecent deglaciation, Antarctic Intermediate Water (AAIW) served as a conduit through which deep water\nthat upwelled the Southern Ocean was transmitted to equatorial upwelling zones. These equatorial regions\nthen served as a window through which CO2 was transferred from the deep ocean to the atmosphere. It is\nlikely that they are correct, but there are alternative possibilities that should be considered before this\nconclusion is published.\nMy principal comment refers to the interpretation of sedimentary proxy data entirely in terms of changes in\nthe flow rate and composition of AAIW. Could these same proxy records be interpreted as changes in the\ndepth of the boundary between AAIW and other water masses, such as the Pacific Deep Water (PDW). Core\nODP1233 was recovered from a depth of 833 m (manuscript Figure 1), within the gradient between AAIW\nand PDW (or Upper Circumpolar Deep Water, UCDW). Can a vertical shift in the depth of this boundary be\nresponsible for the observed changes (Figures 2 and 3)?\nDisplacement of the SWW, invoked by Karas et al to explain the deglacial increase in upwelling of deep water\nin the Southern Ocean, would cause a change in the location of AAIW formation, the thickness of the AAIW\nwater mass, and the depth of the boundary between AAIW and PDW (Danny Sigman and Mathis Hain,\npersonal communication). Furthermore, Pahnke and Zahn (2005) noted that changes in bottom water\ncomposition at depths of AAIW could be related to changes in the depth of the boundary between AAIW and\nPDW (or UCDW) rather than to changes in the formation or composition of AAIW. Thus, one must consider\nchanges in water mass structure, as well as changes in ocean circulation, in addition to changes in water\nmass composition, when interpreting the new records presented in this manuscript.\nKaras et al may be correct in their interpretation, but the conclusions would be strengthened if they can rule\nout changes in water mass structure, leaving changes in water mass composition as the only reasonable\ninterpretation.\nFirst, we want to thank Reviewer 2 for assessing our work. Second, we want to mention that also\nReviewer 1 raised the same concerns about possible vertical changes in the depth of AAIW. Hence,\na detailed discussion of this issue is already provided as response to Reviewer 1 on page 3. This\nincludes the insertion of a new paragraph in the main text and a new section in the\nSupplementary Information. For example, in the revised version we now discuss in detail all evidence that Site\n1233 bathed within AAIW during the period studied. According to Reviewer 2 we also now cite the\nstudy of Pahnke and Zahn (2005) to mention the possible role of vertical glacial/interglacial AAIW\nchanges.\nIs the interpretation of Karas et al supported by the findings of Pahnke, even though the study sites are on\nopposite sides of the South Pacific Ocean? There are enough similarities between the approach of Pahnke and\nZahn (2005) and the new study presented in this manuscript that I believe the paper by Karas et al would be\nstronger if it included a discussion of the data in Pahnke and Zahn.\nAccording to the suggestion of Reviewer 2, we now considered the study of Pahnke and Zahn\n(2005) when discussing possible vertical changes in AAIW (see above). We would like to point out,\nhowever, that the intention of our study is to focus on the southeast Pacific Ocean with our unique\nSite being very close to the AAIW formation area, and directly within the conduit of AAIW towards\nthe eastern Pacific tropical upwelling regions. The inclusion of a discussion of AAIW dynamics in\nthe southwest Pacific Ocean, as published by Pahnke and Zahn (2005), is to our mind beyond the\nscope of our manuscript. A direct relationship between southwest and southeast Pacific would\nremain highly speculative, as processes of AAIW formation/depth/thickness and northward\ncirculation might have been quite different in both ocean areas.\nAlthough the main point that I would like the authors to address is described above, the statement that I\nobject to most is in lines 151-153. I disagree completely with the implication that there was not much CO2\ndegassing from the Southern Ocean during the YD. Upwelling occurs south of the Antarctic Polar Front (APF),\nwhereas the core sites studied by Shuttleworth et al (ref 26) are located near the Subtropical Front, barely\nwithin the Southern Ocean and far north of the zone of upwelling. We know that deep water reached the\neuphotic zone south of the APF because this was necessary to provide the dissolved Si that stimulated the\nopal fluxes shown in Fig 2f. Although the euphotic zone is sometimes deeper than the mixed layer, it is\nhard to imagine what physical conditions would allow deep water to reach the euphotic zone without a\n7\ncorresponding increase in CO2 release to the atmosphere. The statement in lines 151-153 is unnecessary and\nvery likely inaccurate, so I recommend that it be deleted.\nWe followed the reviewer’s recommendation and deleted this sentence.\nThe manuscript is written very well. The only editorial change I would recommend is on line 456, where\n“CaCo3 was removed by adding 10 ml HCL” should be changed to “CaCO3 was removed by adding 10 ml HCl”.\nDone\nReferences (see Pahnke and Zahn 2005 but the others may be relevant too.)\n[Pahnke et al., 2008; Pahnke and Zahn, 2005; Pahnke et al., 2003"}]

Decision letter