My main concerns are following.
1) In this study, the burial of land carbon by LGM ice sheets was simulated, which previously has not been done in an LGM equilibrium experiment set-up. However, it's not clear how this process is set up in GENIE. The authors attempt to justify the modelled larger LGM land carbon storage and smaller oceanic carbon storage. However, the arguments still seem weak, since many recent studies have suggested the opposite (for instance, Anderson et al., 2019, Global Biogeochemical Cycles).
2) Simulations were carried out with wide ranges of parameters, and some have captured the dramatic LGM CO2 drop. The relationship among different model variables, without explaining why these relations occur. After reading the whole manuscript, there were still so many question marks left in my mind. As readers of a scientific paper, we often like to see a question got answered within one paper. Thus, I have to say I personally didn't find it a pleasant reading experience.
In the response letter, the author argued: "We note that this paper was intended as part one of two related papers. This paper describing the relationships between ensemble outputs, and the second, currently being finalised for submission, describing dependencies on ensemble parameters to isolate mechanisms. ... " I would argue, that one only need to read the introduction and methodology of this paper and the analysis of the next paper, since the relations have to be mentioned when they are analysed. I do understand that it's not feasible to put everything in one huge paper. However, I don't agree that the value of this manuscript should be so much dependent on the next one.
My suggestion is to re-organise content of this manuscript and of the one in preparation such that in the first one, the methodology is described in detail, the behaviour of the physical states of the (sub)ensembles are described and EXPLAINED. In the second manuscript, the results concerning terrestrial and ocean carbon cycle are described and explained.
3) Section 3.2.6: Other paleo proxies
I think it's inappropriate to discuss extensively about carbon isotopes, which are not at all accounted for in this model. I got two main messages from this session. First, the authors attempt to argue that their modelled lower LGM oceanic carbon storage is possible, although this is not supported by many previous studies. However, I find the supporting arguments rather weak. See the specific comments below.
Second, the authors briefly summarise some previous work on 13C and 14C, and suggest some further simulations (see the specific comments). I would like to ask the authors the purpose of this text. Does it help achieving your research aim?
Page 1 Line 30 - Page 3 Line 5:
At the beginning of the introduction, the authors spend more than one page to review the hypotheses for the LGM CO2 drop compared to preindustrial. When reading this, I start to expect that this manuscript is about deepening the understanding of these mechanisms or the interplay among them. However, from Page 3 line 6, the authors state that their aim is to examine the sensitivity of LGM CO2 drop to model parameters. So why mentioning at all those mechanisms? At this stage, there seems no link between the mentioned mechanisms and the model parameters to be varied in GENIE. As stated in the comments to the first submitted manuscript, I appreciate that the authors give an comprehensive review of the mechanisms that governing the LGM atmospheric CO2 drop. However, it becomes excessive when it does not prepare the readers for the central topic of this study.
My suggestions for improvement is following:
1) Shorten the paragraphs on LGM CO2 drop mechanisms. Provide the links, if exist, between the mechanisms and the model parameters varied in this study.
2)The authors do state that "There is rarely any investigation of the impact of alternative assumptions regarding parameter values ..." I still think some words should be spent on what has been done in the (limited) previous studies.
Page 3 Line 15
It has not yet been implemented in GENIE in an equilibrium set-up, or it has not been implemented in other models?
Page 4 Line 29-31
Have the OL1 and VPC been explained in previous GENIE papers? If not, please provide more details about what do these parameters do. e.g., what happens if OL1 is increased.
Page 7 Line 10-11
What is effect of bioturbation, to increase vertical diffusivity in the sediment pore water? If so, the sediment equilibrium faster with bioturbation.
Page 8 Line 3-6
Is it the first time that the burial of terrestrial carbon under LGM ice sheet is implemented in GENIE? If yes, please provide here details about how it is done. If not, please provide references.
Page 12 Line 9-12
Here salinity rather than sea level is shown and is (mainly) described. So please consider change the title.
Page 16 Line 8
There was no "China" 21k before present. "Eastern Asia" instead?
Page 25 Line 23
I did not find observational data in Fig 11.
Page 30 Line 16-18
It has been mentioned many times that "We estimate that up to ~60 ppmv of ∆CO2". Please provide information about how this was done.
Page 33 Line 23-24
I find this argument weak. Indeed the LGM marine primary production is lower in the area of sea ice. However, both modelling and proxy data studies (e.g. Muglia et al., 2018, Earth and Planetary Science Letters, and references therein) have suggested that the LGM primary production in the Southern Ocean is higher compared to pi.
Page 34 Line 1-3
I would expect the opposite: LGM ocean delta_13C is lower when CaCO3 weathering flux is higher. Increased CaCO3 weathering flux results in the uptake of CO2 into the ocean. The atmospheric delta_13C is low (< -6 permil). Thus the uptake of atmospheric CO2 will decrease oceanic delta_13C.
Page 35 Line 5-6
There are other methods to estimate LGM ocean carbon inventory, e.g. based oxygen (see Anderson et al., 2019, Global Biogeochemical Cycles).
Page 35 Line 15-30
Instead of blaming missing processes in the model, why not just start analysing your ensemble results to investigate how oceanic oxygen, carbon storage, and so on, respond to different physical states?
1) When describing model results shown in a figure (or a table), it would be easier for the readers to follow if "what is shown in which figure(table)" is given at the beginning of the text. The authors have all the pictures in mind but the readers don't.
2) There are too many figures. Some figures could be combined. For instance, Fig 1 (a) and (b) can be combined into one figure. And in this way one can clearly compared different ensemble sets. In the case of Fig 1, the authors could also consider using nonlinear vertical axis such that the low frequencies are visible.
3) Fig 9
Please use the same colorbar range for (a) and (c), such that it's easier to compare between LGM and pi.