Dear Authors,

Thank you for providing careful and detailed responses to the comments and suggestions offered by the two reviewers.

The two reviewers recognized the scientific value and novelty of your manuscript. Agreeing with the two reviewers, I am pleased to tell you that you can now submit your revised manuscript to be considered for publication in Biogeosciences. However, given the large number of comments and some important issues raised by the reviewers, I have to recommend ‘reconsider after major revisions’.

I found that your indices of “biochemical transformations” based on FTICR-MS analysis might represent a key methodological approach. Although FTICR-MS has recently become a popular and common approach in various biogeochemical research fields, more detailed descriptions of the analytical procedures and accuracy, and its methodological limitations would help readers understand the unique approach employed in your study. First, please provide more details on the procedures of ‘sequential phase extraction protocol to remove salts as per Dittmar et al., 2008’ (e.g., SPE sorbent, eluent, recovery etc.) in addition to the recovery of WSOC commented by the first reviewer. Please refer to the papers that have warned the technical limitation of SPE (e.g., Anal Bioanal Chem (2016) 408: 4809–4819). Considering the limitation of FTICR-MS as a tool for quantifying molecular peaks, you might also need to discuss the limitation of your approach of comparing mass differences in peaks. For instance, it has been criticized that FTICR-MS results cannot accurately quantify changes in peak intensity between samples from some incubation experiments. Second, please think about whether presenting some representative van Krevelen diagrams (as shown in Fig S1) would help readers better figure out your approach of mass differences. You have only four figures in the main manuscript, so one additional figure would not add too much load, I think.

I would like to ask you to make all the changes easily identifiable in a marked-up manuscript and a point-by-point reply to all the comments offered by the two reviewers and myself. I would also suggest that you specify the line numbers of the revised parts in your responses to the reviewers’ and my own comments.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences