Berg and colleagues present a depth profile of sulfur geochemistry and target gene abundance for a ~10-meter-long core from Lake Cadagno sediments (Switzerland). This well-studied lake has moderately low concentrations of sulfate in deep porewater and experiences an influx of groundwater from below, making it a valuable intermediate case study between the ocean and very-low-sulfate lakes (e.g., Lake Superior). This dataset is likely to be of broad interest in the field. Before publication, however, there are several discussion topics that need additional attention, especially related to the interpretation of the organic S and C results and the deeper S-isotope trends. Detailed comments below are in manuscript order.

Line 111 – Please provide the essential details about your sampling procedure here so the manuscript is complete on its own without reference to Berg 2022. The reader needs be able to quickly understand e.g. that these are piston cores, at what coordinates and elevation, without an internet connection.

Line 230 – I was looking for a profile of TOC to make sense of the C:S ratio profiles before I found it in the Supplement. The x-axis for TOC concentrations appears to be missing on figure S1. Consider moving this to the main text, it’s very useful for thinking about the ratio profiles.

Fig. 2 – I realize that the x-axis in D is the full range of values observed, but it is not feasible to glean information about the relationships among CRS, HAS, and AVS in the main core at this scale. Please provide an additional zoomed-in scale or some other approach to make it possible to resolve d34S differences of a few permil. Similarly, the TC/TS range goes to 40 when the data max is 12 – it would help to adjust this so the data are easier to see.

Line 222–230 – I found it challenging to make sense of the humic acid sulfur data referenced only to total or total organic C. Values for humic acid carbon would be extremely helpful if they exist. Do we know at least roughly what proportion of TOC was extracted as HA?  Either way, a discussion is warranted about the relationship between HA extracts and total or residual (protokerogen-like) OC. How should the reader think about statistics like TOC:HAS when the ratios of C:S in HA, the ratio of C:S in non-soluble OC, and the relative abundances of those pools are all potentially changing? Additional discussion on this topic would also help translate this data to comparison sites, many of which report OC and OS from protokerogen or lipids rather than HAs. Exchanges of sulfur between HA, DOM, and this pool should likely also be considered.

Line 236 / Fig S2 – The comparison with 1991 data is potentially intriguing but insufficiently explained to be useful. A description of the sampling type and associated concentration data would make this much more valuable, perhaps as a Supplement section. Otherwise this data is undersupported. (Why were they able to get sulfate data when the current study was not? Different sampling volumes I presume?) Can the new sulfate data be overlain on the 1991 data for a more direct comparison?

Line 241 – is the support for this opening sentence the 1991 pocket dataset? Please tell us more about it! If this d34S sulfide trend is directly comparable, can it be included in Figure 2 so that it can be seen next to the AVS etc?

Line 370 – I don’t see abundant S0 at the deep redox transition in Fig 2A, please clarify.

Line 383 – The statement that HAS increases over the top 10 cm (without mention of its immediate decrease below) is a bit misleading. Please provide some explanation for the peak of HAS abundance between 5–15 cm that can account for both sides of the profile.

Line 411 – 413 – The description of organic sulfur sources is a bit tangled. Are you referring to the possibility of sulfate reducers using organic sulfate esters as a sulfate source in extremely low-S lakes? (Fakharee / Phillips should be cited if so). If sulfate esters are used for MSR, they could find their way into any product of reactions with sulfate, theoretically including pyrite as well as OS, and would not necessary be associated with specific OS functional groups. To understand the OS pool, the much larger effect is expected to be the incorporation of biogenic OS materials, either from PP or potentially also from secondary production by sediment microbes (Anderson and Pratt 1995 and many others).  

The discussion in 416-418 seems disconnected from the observations of HAS concentration in Figure 2. How do that data support (or not support) an argument about timing of HAS formation and stability, when peak HAS concentrations mostly experience loss at shallow depths? How might biogenic OS be related to this?

Line 438 – I was confused by the statement that “closed-system sulfate reduction is leading to similar δ34S fractionations as in surface sediments.” Closed-system processes don’t affect fractionation factors, but instead they affect the expression of those fractionations in the environment. It is not clear how closed-system distillation is creating consistent d34S distributions at depths with very different levels of system openness.

The argument for this closed-system control is, if I understand properly, the trend in CRS d³⁴S values between 600 and 800 cm depth above the groundwater source. This hypothesis needs to be explained in much greater detail to understand the mechanism being invoked and how other previous datasets might support it. Humic acid and AVS d34S values also tend light in that zone – how is that related? (and sulfate is light… why are these values not light just because they are offset from light sulfate?) Some simple calculations to compare offsets among the sulfur pools would be great. Alternative explanations for the CRS d34S trend in the deep zone should be considered.

General comments.

Berg et al. present an interesting and mostly original study on sulfur transformations and sulfur isotope partitioning into different inorganic and organic sulfur fractions in a previously well studied Swiss lake. The isotope geochemical study includes microbiological informations, which is nice, but of secondary importance, since it represents th present situation and not necessarily for the 'paleo'-ones that are responsible for the formation of benthic geochemical signatures. Lago Cadagno is well known for its unusual element cycling, including the development of euxinic conditions due to a groundwater derived carbonate-buffered substantial sulfate supply derived from gypsum and dolomite dissolution in the catchment area.

I suggest to change the title and remove 'active', since no rates measuremements of the current sulfur transition  have been carried out, and are only deduced from sediment solid phase parameters. Wath is missing is a competent discussion of the oecidative part of the sulfur cycle and relevant associatd sulfur cycling.

The presentation of data, for instance in the supplementary data, file has to be substantially improved. The lables at X axes of TIS, TOC etc. are missing, the lable at the d34S profiles from 1991 are not of publishable format/quality. In general, the basic geochemical data should be presented in the main text.

Detailed comments:

- I suggest to add the fundamental PhD thesis of Losher (1989) on S cycling in Lago di Cadagno into the introduction and reference list.

-L33:  ...catchment groundwater... instead of 'aquifer groundwater'

-L33: Is the d34S of lake water column sulfate really constant?

- L34: d34S values for dissolved sulfate in the deduced steep gradient are not measured, but only deduced. Therefore, this statement on apparent isotope enrichment factors is a substantial oversimplification

-L35: Due to the lack of pore water data, it stays questionable if the term 'closed system' really describes the boundary conditions

-L53: I am sure that there are really more basic studies from the 90s of the last century that should be cited here: e.g., Jorgensen, 1990, Thamdrup et al. etc.

-L57/58 ...cultures and marine sediments... References should include: Kaplan & Rittenberg (1964), Wortmann et al.  (Geology, 2001); Rudniki et al., (GCA, 2001)

-L66: add reference: Jorgensen et al. (GCA, 2004)

-L68: This is not a new result of Bryant et al., but has already been found out in the 80s of the last century and the 90s. You should refer to the original studies.

-L71: cite for closed system behaviour: Hartmann & Nielsen (1969 Geol. Rundschau, or 2012, Isot. Env. Health Stud.)

-L73: This statement is questionable. Whereas AVS may reflect system behaviour, pyrite often reflects dominantly the conditions provided by essentially open system conditions

-L78: Add Putschew et al. (OGC, 1996) and something from the 90s of the NIOZ group and more recently Amrani et al

-L80: 'substantially' instead of 'significantly'?

-L361: add Jorgensen et al. (2004, GCA)

-L364: ...disproportionation of sulfur intermediates... (Canfield & Thamdrup, 1994; Cypionka et al., 1998 FEMS)...

Question: I a repeated dispropotiontion cycling of sulfur intermediates really a plausible process in sediments? Canfield & Thamdrip (1994) proposed the model for an euxinic water column, the substantial role for benthic anaerobic sulfur cycling in competition to reductive processes is still to be shown

-L370: Which evidence of S° accumulation? Wjat would be the tole of S° reduction for isotope discrimination?

-L453: Is Fichtel et al. a suitable reference? The study belongs to a completely different type of system

-L455. Why refering to the ocean crust as aquifer, instead of much shallower submarine or limnic groundwater discharge sites(?)

-L474: I do not understand this statement. Why should memberhsip in an editorial board lead  for an academic trained person lead to a compettition of interests?

-L479: Didn't the co-authors also contributed to revision and editing of the ms? Who provided funding?

-Fig.2 Presentation should be improved: No combination of dissolved and solid phase species in one each plot; spread of the X axis for S isotope results