In this manuscript the authors present a new global ocean mercury model developed within the ocean component of CESM2. There have been a few efforts to develop such ocean mercury numerical models in the past, but this represents a significant step forward because of its coupling to marine ecosystem processes. It also complements existing CESM2 mercury modules focusing on the atmosphere, terrestrial processes, and river transport. The manuscript is well written, clearly organized and presents state-of-the-art description of the processes implemented in this ocean mercury module.  

Overall, this work represents a substantial contribution to modeling science within the scope of the journal. The work is of high quality, and I have only a few minor comments and suggestions for the authors to consider.

• The setup for initial conditions and spin-up is not described in the manuscript (page 3, line 95). Please clarify where do the initial conditions come from and the duration for the spin up. This comes up later in the manuscript (line 263: “the initial condition of the model features relatively high deep-ocean Hg0 ..”), so providing this context earlier would improve clarity.
• There are many figures presented in the form of maps. It would be helpful to include global mean values (and standard deviations), which could be added as text on the figures, especially for Figures 6-11 (species concentrations, deposition flux).
• The authors compare the results of this new ocean model to past modeling work, for example MITgcm-Hg and MERCY v2.0. Including a table summarizing the global oceanic mercury budget across these models would strengthen the comparison.
• Figure 5 typo in title (“reminization rate”)  

This manuscript presents POP2/Hg v1.0, a new global ocean mercury model embedded in the ocean component of CESM2 (POP2) and coupled to MARBL. The work is a worthwhile contribution: coupling mercury chemistry to a prognostic biogeochemistry library with separate soft and hard POC pools is a genuine advance over offline or climatology-driven predecessors, and the model reproduces several large-scale features of marine mercury. However, several issues must be resolved before the paper can be accepted. Most of these require clarification, correction, and modest added analysis rather than new simulations.

1. Lines 350 and 450: Section 4.1 reports a global Hg⁰ net evasion of ~4.43 Mg yr^-1 against an HgII deposition of 4.20 × 10³ t yr^-1, then calls this "near-equilibrium exchange." Taken literally, evasion (4.43 t) is ~1000× smaller than deposition (4200 t), which is the opposite of equilibrium. The Conclusion instead gives a net evasion of ~4.5 × 10³ t yr^-1, differing from Section 4.1 by three orders of magnitude. Published global ocean Hg⁰ evasion is on the order of thousands of Mg yr^-1, so "4.43 Mg yr^-1" appears to be a unit error. Please reconcile these numbers and re-examine the equilibrium statement.

1. Lines 263–265 and 334: Figure 11 is described as "year 10 (balanced state)", yet deep-ocean ventilation timescales are centuries to over a millennium. Consistent with this, the authors state that the elevated Hg⁰ at ~1000 m (up to 0.8 pM) is contributed by "the initial condition of the model". This undermines confidence in all 985/1000 m results (Hg⁰, HgII, Hg^p, MeHg, HgT), which may reflect the spin-up initial field rather than the model's biogeochemistry. Please justify the spin-up duration for the deep ocean, or explicitly qualify that deep-ocean fields are not equilibrated and interpret them accordingly.

1. Lines 365, 371, 375 versus 425–430: Section 4.2 reports surface HgII reduction at 1.53 × 10⁵ t yr^-1, intermediate net reduction at 1.74 × 10² t yr^-1, and deep net oxidation at 3.74 × 10³ t yr^-1. The sensitivity section then reports surface, mid-depth, and deep net reduction in "Mg yr^-1" with values of 8.22, 3.62/1.53, and 1.11/3.69. These cannot be reconciled with the Section 4.2 values (e.g., mid-depth 1.74 × 10² t yr^-1 vs. 3.62 Mg yr^-1). Please harmonize units and magnitudes throughout, ideally with a single flux-budget table.

1. Sections 3.1–3.4: The model–observation evaluation is entirely qualitative; no quantitative skill metrics are reported. The Results overlay pointwise observations on maps (Figs. 7, 10, 11) and describe agreement in words ("reasonably well," "good agreement"), but no correlation, RMSE, bias, or model-vs-observation scatter/statistics are given anywhere. For a model-description-and-evaluation paper, at least basic quantitative skill statistics against the compiled datasets (cited in Sect. 2.4) are expected and are computable from data already in hand. I recommend adding them.

1. Table 1 and Figure 6 caption: In Table 1, the symbols "OCRR," "hv," and "T_c/T_k" (reaction 7) are used but never defined in the text. In Figure 6, panel (a) atmospheric Hg⁰ "concentration" is labelled ng m^-2 d^-1 (a flux unit; concentration should be ng m^-3), and panel (b) HgII deposition is captioned mol m^-2 s^-1 while the colorbar reads pg m^-2 s^-1. Please define all symbols and correct the units.

1. Lines 187–189, 249, 320 (plus Eqs. 5–7): Duplicated text, typographical errors, and a POC/bioC notation mismatch. Lines 187–189 contain a near-verbatim duplicated sentence about Hg^p gravitational settling. Typos include "Hg0remain" and "below 0.009o4 pM". In addition, sorption (Eq. 5) partitions HgII onto POC only, whereas the sinking-velocity formulation (Eqs. 6–7) uses POC + bioC (living biomass); please clarify whether Hg^p is carried by living biomass and reconcile the notation. A careful language and consistency edit throughout is advised.