|Review of Sarnthein et al. “Plateaus and jumps in the atmospheric radiocarbon record – Potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis”|
I have reread the revised submission of Sarnthein et al., and their response to my comments and those of the other reviewer. I agree with the authors that globally synchronized chronologies of climate archives are a significant challenge to the community. The authors present the 14C plateau tuning as one (and currently the only) technique to achieve this, if marine sediment cores with sufficient abundance of foraminifera and sufficiently high sedimentation rates can be retrieved and are analyzed.
I am glad to see a shortening of some sections of the paper, a toning down of some statements on the 14C tuning method, and lists of major points which improves the paper structure and clarity. However, I still find the paper too long-winded in places. In fact, I find it hard to distill the main message(s) from it, and in particular to separate previous findings from new findings that result from the update in the Suigetsu age scale, and accordingly the 14C plateau-tuned surface ocean reservoir age estimates. Overall, although having substantial merit regarding the many different aspects discussed in the paper such as on the method (including those that were improved in the revision), it has not become more succinct as I hoped and as I think the paper would have benefited from. Below, I make some suggestions on how this and other points of the paper can be improved, and hope that the authors find them useful.
[Line numbers refer to manuscript with tracked changes in red] (cp-2019-127-author_response-version1.pdf)
Highlighting the major findings/points:
I welcome the added outline of the manuscript in lines 274-317 that simultaneously emphasizes the highlights of the manuscript. But paragraph 274-317 needs to be significantly shorted to one paragraph in my view. Any introduction of supplementary material is unnecessary here.
A novel aspect of the study seems to be the extension of the plateau tuning technique to 23-29 kyr BP, which allows to define new chronostratigraphic markers for that time interval. However, reservoir age estimates from this time interval are not discussed at all (the focus is on HS1 and the LGM), and seem to allow for sparse R age estimates only (Fig. S2) – the latter could be a matter of sediment quality. The reassessment of earlier records with the adjusted chronology for the LGM and younger time interval seems to support previous findings (judging from the references given to earlier work of the first authors’ group in the discussion). I think the paper would benefit from highlighting the knowledge gained form adjusting the chronology of the records rather than reiterating what these records as a whole show because this was done in numerous previous publications of the first authors’ group.. at least this could be done in a more concise way. Currently, it is hard to distill the main messages of the paper, in particular in the “Discussion and implications” section.
I welcome the authors’ effort of a synthesis paper and the reasoning behind “recaps of important results already published” in this study, but I think they should not necessarily be declared as “new” when they are not and when these have been published previously. When the authors write “These new features of MOC and the carbon cycle” (line 55) and “a new understanding of Ocean MOC during the LGM and its reversal during HS-1” (line 316) it is difficult to say what is exactly the novel piece of information here, also in comparison to the previous knowledge obtained from other proxies. This should be rewritten and/or more clearly carved out.
In those instances, where new insights can be gained and are introduced, I have to admit that it is really hard to follow the argumentation, in particular when the (raw) data is not discussed or described (as in the case of the Küssner or Ausin data). This applies for instance to line 526: “reveal that changes began ~1400 yr earlier in the north (Fig. S2)”, where it is unclear what is meant by “north”, how this number of 1400 yr came about, and what is meant by changes. The same applies to lines 529-532 or to line 709-710 “in particular due to a ‘thermal threshold’ (Abé-Ouchi, pers. comm.) overlooked in other model simulations.”.
I think some paragraphs are still long-winded and contain a lot of detail that are likely not necessary for the main message of that paragraph. This applies to the first and second paragraph of section 2.2. Also, I find the exercise performed in lines 440-464 to highlight the ocean carbon cycle influence on atmospheric 14C somewhat redundant. I think the same message (i.e. atmospheric 14C variability cannot be explained by production changes alone) can be conveyed by simply referring to existing literature, e.g. (Hain et al., 2014) or others; hence allowing to shorten the paragraph. In lines 486-491, I find the calculation of F-modern equivalent of a ventilation age change unnecessary. This can be removed in my view. Also the section on the influence of habitat changes on planktic foraminiferal 14C ages could be summarized in one sentence, to my mind (e.g. “The fact that 14C ages of co-existing planktic foraminifera were suggested to be influenced by habitat depths, in particular when comparing surface and sub-surface dwellers (refs), or seasonality (refs), makes a closer specification of model results as product of different seasonal extremes a further target.”).
Model-data comparison (Section 3.2.)
I am still confused why the study of Muglia et al., (2018) was chosen for model data comparison, and what the purpose of it is. This is in part because the background on the actual model simulation is missing (how were the reservoir ages estimated? How was the model forced?). Although some of this information is probably given in Muglia et al. (2018), in my view this information should be stand-alone in the paper. The comparison to previous surface reservoir modelling studies (Franke, Butzin) is quite limited and a reiteration of personal communications should be avoided. It seems that the authors have chosen the Muglia study because it seemingly reproduces the best match to their data, but what is the purpose then of the model-data comparison? Apart from highlighting the fact that models do not do a perfect job in reproducing the proxy data, in particular in epicontinental seas or in the Southern Ocean (which we knew before), the purpose of the model-data comparison is not very clear.
I still do not support the inclusion of the Ausin et al. unpublished data in the interpretation, and the Küssner et al. submitted, simply because this cannot be assessed by the reader at the moment and likely not at the time of publication. Simply referring to the pangaea database (in the replies to the reviewers) is not a best-practice to discuss the new data, explain how they have been obtained and how surface R were estimated, etc.. I expect a better solution here.
Line 199. Far superior evidence -> is not adjusted although described in the replies to reviewers.
Line 265-269 Move “We prefer the Suigetsu record to IntCal […] including carbonate-based speleothem and marine records.” to the preceding paragraph.
Line 415. Reference your earlier work here.
Line 486. There is no Fig. 4c that references the Küssner data subm.
Throughout manuscript: replace plankton-based with planktic foraminifera-based (planktic foraminifera are plankton but not all plankton is planktic foraminifera)