|I think changes made by the authors improved the manuscript considerably. There are still some aspects of the paper that I do not fully agree with, but I respect the decisions of the authors. I listed some comments to clarify my perspective on some of the points of disagreement, but I do not see these issues as an obstacle to the publication of the manuscript. |
One of my general comments was about the possibility of firn processes having something to do with the production of CO spikes. I gather from their responses that the authors disagree and see this as a process happening within the ice matrix after bubble closure. It may be a good idea to include a sentence to this effect to orient the reader.
I understand the statement about not having any direct evidence for CO production happening in the firn. However, some of the observations reported here are very difficult to explain solely with production after bubble closure. Specifically, there is no straight forward interpretation of the CO vs. the corresponding TOC data. New figures show seasonality in TOC and CO in the PLACE ice core, but the TOC in summer and winter layers appears to be within 10-20% of the mean levels and the lack of overall correlation between TOC and CO is puzzling. Thinking purely from this perspective, I don’t see why there would be no in situ production in the winter layers, for example. I agree with the need to better understand the fate of TOC in the firn and snow, but does this not imply that firn processes could have a role in the production of CO? The discussions regarding the fate of TOC and the relationship with NH4+ are snow/firn processes that the readers will struggle to connect to CO production in the ice after bubble closure. Perhaps, there is more that can be said to help make this connection? In my opinion, the strongest argument for accepting the composite 5th percentile of measurements as a likely atmospheric record is the agreement between different ice core measurements and firn records. The paper should emphasize this angle in the abstract and in the conclusions.
The authors chose not to include the Tunu record in the final composite history. My whole point in arguing for its inclusion was that this would not significantly alter the final composite record. The conceptual model included in the supplement (Fig. S12) is pretty helpful in demonstrating how the analytical smoothing impacts the measurements, but the average increase in CO is always less than 10 ppb and 3-4 ppb on average. The figures they included in the review also show that the composite record looks pretty much the same with or without inclusion of the Tunu record. In the end, I don’t really mind leaving out the Tunu record. If they believe there is potential for recovering a long term record at Tunu, it can be pointed out somewhere that artifacts due to analytical smoothing do not seem to drive the type of large long term trends apparent in the composite record and show Fig. R4 in the supplement.
Line by line comments/suggestions:
L13-15: This sentence gives the impression that the co-examination of CO and TOC records shows there is no CO production in winter layers. The strongest evidence for winter layers containing largely an atmospheric signal, hence the path towards recovering an atmospheric record from 5th percentile baselines, comes from the agreement between different ice core records in my opinion. I cannot readily infer from this that TOC in the winter layers does not produce CO in the ice while it does in summer layers because no possible explanation is provided for why it should not.
L85: “Burden” is commonly used to mean the total mass/moles of a gas in the atmosphere. Better to use mixing ratio, abundance, or levels instead.
L129: Rephrase “extracted along the sample line…”
L291: Ice core trends are not strictly “monotonic” even after 1875, especially for some of the cores; might be better to say “steady.”
L297: S17 instead of S15.
L324: Consider replacing “never expose directly freshly drilled cores to sunlight” with “never expose freshly drilled cores to direct sunlight.”
Lines 345-365, section 3.2.4: This 6 ppb appears to be a system blank. Is it subtracted from the discrete measurements? I could not see this info anywhere. It can probably be added to the caption of Fig. 3.
Line 426-427: The hypothesis here is not easy to understand. Do you mean only a fraction of TOC converts to CO and the excess above a certain threshold does not result in more CO production?
Line 450: Incorporating instead of “incorporates.”
Line 453: No need for “however.”
Line 467-468: I think you should put the baseline back in the figure otherwise it is hard to see this. You can show a version with a mean in the supplement.
Section 3.5.1: There is too much emphasis on Fig. 1, which has already been discussed. It would be better to point out the trends in Fig. 7, which is done in the conclusions but seems out of place there.