This study investigated mechanisms of global ventilation age changes during the LGM and the last deglaciation based on two transient simulations. The topic is interesting and fits the CP. However, there are several concerns that should be addressed.

Major comments:

1. The “Introduction” section should be reorganized. The topic of the article is the underlying mechanisms for the younger ventilation during the LGM relative to the PD, in tandem with the oldest ventilation age during the last deglacial according to the IAGE. However, the present Introduction introduces too much about the ∆¹⁴C age, and the definition and application of the IAGE are lacking. Moreover, the significance of understanding the ventilation age during the LGM and last deglacial should be highlighted more clearly.

2. Line 168: How is the threshold of 70% defined? It is not clarified in the manuscript. Whether the evolution results would be different if using different values (such as 60%, 80%)?

3. Line 190-195: Authors indicated that the evolution of IAGE is more similar to the strength of AABW than GMOC. However, changes in the AABW transport may be a part of GMOC. In other words, the AABW transport and GMOC are interactive, and in the modelling who is the reason and who is the result? It makes me confused about the appearance of the GMOC across the manuscript. More discussions or clarifications about this point should be added.

4. Authors highlighted the importance of the AABW transport changes in regulating the ventilation age during the LGM and the last deglacial and indicated that the AABW transport changes are associated with sea ice and buoyancy flux over the Southern Ocean. However, this conclusion only appears simply in the Abstract and Summary, and analysis is lacking in the manuscript. More discussions about the mechanisms driving AABW transports should be added. At least, the evolution of sea ice during the LGM and last deglacial should be provided and the relationship between sea ice and AABW transport needs to be analyzed briefly. Moreover, the ultimate driving factors are also necessary to be discussed (i.e. the external forcings), maybe the role of freshwater injection or continental ice sheet during the LGM and last deglacial on the sea ice, on the AABW transport, and on the ventilation age should be discussed.

5. The simulated ventilation age during the LGM and last deglacial need to be compared with the various proxy reconstructions comprehensively in the manuscript. Please add some discussions on this point.

Minor comments:

1. Fig. 1 only provides the global mean IAGE and Pacific mean IAGE. What is about the Atlantic mean? Any opinion?

2. Line 124: suggest replacing “in contrast to” with “younger than”

3. Lines 126-127: This sentence is confusing. Pleas rewrite.

4. Lines 152-154: The description here is unexpected, and I suggest removing it or making it clear.

5. Line 170: The multiple sign is missing.

6. The order of subpanels in Fig. 6 and Fig. 7 should be rearranged, as Fig.6g-h and Fig. 7g-h appear earlier than Fig. 6a and 7a. (Line 244-245).

7. Fig. 6 and Fig. 7. The left string of figures may be “C-iTRACE”.

8. More quantified information should be added in the Abstract and Summary section (for example, the exact value of the ventilation age during the LMG and the last deglacial).

Li et al. analyse glacial-to-deglacial marine age tracer histories which where obtained in two previous simulations, C-iTRACE and iTRACE. C-iTRACE is an ocean-only simulation driven with forcing fields derived from the fully coupled TraCE-21ka simulation; iTRACE is a fully coupled climate simulation. Both simulations differ in their horizontal resolution of the oceans (3° in C-iTRACE vs 1° in iTRACE) as well as in their time domains regarding radiocarbon (¹⁴C) output (22-0 ka BP in C-iTRACE vs 20-11 ka BP in iTRACE). The authors consider "ventilation ages" of waters below 1 km depth in terms of ¹⁴C age differences with respect to either the atmosphere or the sea surface (both derived from simulated dissolved inorganic ¹⁴C concentrations), and in terms of artificial, ideal water ages (derived from numerical age tracers, "ideal ages" for short). They find systematic age differences between both approaches. According to C-iTRACE, ¹⁴C ventilation ages of the global ocean and the Pacific were somewhat higher during the last glacial maximum (LGM) than at the present day (PD). Ideal ages are almost always systematically younger than the ¹⁴C ages. Moreover, there are no significant differences of ideal ages between the LGM and PD, but ideal ages culminate near 13 ka BP. The results of simulation iTRACE are significantly different in that ideal ages are more than 1000 years younger during the LGM than at PD and do not peak near 13 ka BP. Combining their findings with numerical source water tracer distributions and simulated circulation fields, Li et al. attribute many of these features to the temporal evolution of Antarctic Bottom Water production.

General comments:

The paper is motivated by the differences between the temporal evolution of ¹⁴C ages and ideal ages shown in Figures 1 and 9. That ¹⁴C ages are higher than ideal ages is not new (e.g. Koeve et al. 2015). There are also other models showing for the LGM higher ¹⁴C ages but at the same time younger ideal ages compared to PD (e.g. Galbraith and de Lavergne 2019; see also the discussion by Skinner and Bard 2022). Apart from the introduction, ¹⁴C ages are not really discussed or compared with observations. In fact, this has been done by other authors (Gu et al. 2020, Zanowski et al. 2022). The focus of Li et al. is on the discussion of simulated ideal ages. Here, a systematic problem is that there is no way to validate ideal ages with observations from the past. Therefore, the results by Li et al. may be helpful to understand the model behaviour in the C-iTRACE and iTRACE simulations, but the added value for our understanding of real marine proxy records is limited.

Specific comments:

• Figure 1 and line 75: "incredibly identical" "BwP" ages. This is indeed incredible unless further details of this unpublished approach are provided
• Figure 1: "BP" (= 1950 CE) should be defined somewhere (maybe near line 23)
• Figures 1 and 9: It would be worthwhile to include the Atlantic to facilitate the understanding of Figs. 6 and 8
• Line 56: "AABW is defined as the minimum (...) from 2°S-70°S" – does it make sense in this context to consider 2°S which is far away from the source water = ventilation regions?
• Line 57: "ventilation time" should read "ventilation age"
• Lines 61-62: "both the B-A and B-P age are remarkably old[er] at the LGM than the present day". This is not really the case for B-P ages shown in Fig. 1.
• Line 79: There are no "true ventilation" proxies, see my comment above
• Line 120: "Ventilation" originally meant "oxygenation" of the deep sea; in this sense "ventilation ages" and "ideal ages" are not really the same for (chemical) oceanographers
• Figure 2 (h)-(i), lines 133 and 168: Contour lines in Fig. 2 represent the value of 0.8 but ideal ages are calculated where the percentages of AABW and NADW exceed 70%. This should be consistent (i.e., 0.8/80% or 0.7/70%)
• Figure 3 (a): What is the reason of the lag of ~2 kyears between the maxima of DYE_NA and DYE_S?
• Line 193: "decrease in AABW transport and increase in IAGE during the same period (14-13 ka)" is at odds with Figure 1
• Figure 4: The changes would become more obvious if the ages were scaled to values at 0 ka BP (i.e., if age anomalies were shown)
• Figure 4 / line 199: As the Pacific is connected with the Indian Ocean south of the equator, the Indo-Pacific MOC should be considered instead of the PMOC
• Figure 6, 7, 8, 11, 12, 13: Units are missing
• Line 235-236: "isopycnals (...) at 30°S exhibit minimal changes below 3.4 km" – this is at odds with what can be seen in Figs. 2 and 5
• Line 242: "The calculated northward AABW DWBC transport aligns exceptionally well the transport of model abyssal upper cell in each basin (Fig. 6g-h, Fig. 7g-h)". Do you mean "abyssal cell", "upper cell" or "abyssal and upper cell"? I don’t get that from Figs. 6 and 7.
• Line 255: Why is the (ideal) ventilation age of AABW typically much older than the age of NADW?
• Line 258-259: "more NADW sinking into deeper depths in the Arctic" – this is not in line with Fig. 3 (b) where the volume of DYE_NA remains almost constant
• Line 315: "The calculated southward DWBC and northward AABW DWBC are validated by the model MOCs" – this is a tautology
• Line 349: See my comment concerning line 75

References:

Koeve, W., Wagner, H., Kähler, P., and Oschlies, A.: ¹⁴C-age tracers in global ocean circulation models, Geoscientific Model Development, 8, 2079–2094, https://doi.org/10.5194/gmd-8-2079-2015, 2015.

Galbraith, E. and de Lavergne, C.: Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages, Climate Dynamics, 52, 653–679, https://doi.org/10.1007/s00382-018-4157-8, 2019.

Skinner, L. C. and Bard, E.: Radiocarbon as a Dating Tool and Tracer in Paleoceanography, Reviews of Geophysics, 60, e2020RG000720, https://doi.org/10.1029/2020RG000720, 2022.

Li et al. 2023 systematically examine the deglacial changes in true ocean ventilation age since the Last Glacial Maximum (LGM) based on the C-iTRACE and iTRACE transient simulations. Their results indicate a younger ventilation age during the LGM compared to the present day (PD) period, with older ventilation ages around 14 - 12 thousand years ago. The authors emphasize the significant role of Antarctic Bottom Water (AABW) in determining the global true oceanic ventilation age, and ventilation age is significantly older in the Pacific relative to the Atlantic. The modeling results could offer additional constraints on our understanding of deglacial ventilation evolution. Therefore, this study is a suitable fit for publication in Climate of the Past. However, there are several concerns that should be addressed, and revisions are recommended before this manuscript can be accepted for publication.

(1) The manuscript lacks a detailed description of ideal age tracer in these models, as readers in the community may not be familiar with C-iTRACE and iTRACE modeling. Therefore, it is necessary to provide a clearer context on model set up for idea age tracer (e.g. England, 1995).

(2) It would be worthwhile to discuss the importance of understanding the age of water and the ventilation in the introduction.

(3) It would be worthwhile to describe how the strength of the AABW physically affects the ventilation processes in two basins and, as AABW is part of the AMOC, why the AABW transport reduces during the deglaciation from a coupled view.

(4) Figure 4: What is driving the IAGE from the North Atlantic to get much older from 12 to 10 ka?

(5) No units in figures 6, 7, 8, 11, 12, and 13

(6) Line 76 and 39: Check grammar

(7) Line 8: Remove “postal code”

(8)Line 11: “CO2” to “CO₂”

(9)  Line 95 and Line 368: “Gu, Liu, Jahn, et al., 2019”, Remove “Liu, Jahn”

(10) What’s the meaning in Line 170: “8 10⁸ km³” and “7 10¹¹ yr km³”.

(11) Line 45: Remove the comma symbol in “Zanowski et al., (2022)”

Reference

England, M. H. (1995). The age of water and ventilation timescales in a global ocean. Journal of Physical Oceanography, 25, 2756–2777.