Referee comments Solid Earth se-2020-221

Authors: Fei Liang, Jun Niu, Adrian Linsel, Matthias Hinderer, Dirk Scheuvens, Rainer Petschick

Title:

Rock alteration at the post-Variscan nonconformity: implications for

Carboniferous-Permian surface weathering versus burial diagenesis

and paleoclimate evaluation

Referee: Reinhard Gaupp

1. General comments

The article addresses an interesting case of stacked lithologies with intermediate exposure surfaces with obviously different times of weathering alteration. The time lengths of exposure and geochemical-mineralogical alterations are not controlled by other independent evidence, like geochronology etc., but other factors of alteration are well constrained by the investigation. The authors attempt to evaluate thermal effects afer volcanic effusion with assumed “hydrothermal” alteration by fluids during burial history. 

The manuscript has an overall very good quality with a target interesting for a wide audience. I cannot fully support the author’s conviction that this case history is well suited to provide a workflow for data reliability analysis for paleoclimate research. But the elaborated workflow is helpful to unravel multistage low temperature (<200°C) overprints of (magmatic) rocks.

I suggest acceptance of the manuscript with minor corrections (see 2. and 3.)

1. Specific comments. Question issues
2. a) Chapter 5.4. Climate: The timing of Permian deglaciation cycles fixed into the climate curve of Roscher & Schneider 2006 is not a reliable base to evaluate the ages of the observed nonconformities (see Fig.12). This pretends the possibility to estimate the lengths of exposure to atmospheric influence and erosion.
3. b) The term “hydrothermal” is not clearly constrained in this article. Unfortunately this is often the case in the present papers. What evidence is given to define the fluid as “hydrothermal”? We should know the geothermal situation at the time of influx or mineralization and evaluate the deltaT to the observable mineralization or fluid inclusion data. Otherwise it is “possibly or likely hydrothermal”.
4. c) Mesozoic sedimentary cover of the investigated sequence: 600 to 1500m given in Line130; the minimum value of 600m is not justified by evidence within the preserved stratigraphy, and also by thermal consideration (>130°C in the Odenwald top basement)
5. d) Kaolinite in lithologies like the basalt. Table S1 does not include the mineralogy of the Rotliegend sediments. Can we exclude that kaolinite is a subrecent surface related weathering effect from petrography (present Telodiagenesis)? With an assumed maximum Mesozoic burial of the post-Variscan nonconformity of ca. 1500m, the illitisation of the small kaolinite contents would have occurred. This illitization of kaolinite (K-metasomatism?) would be supported by the assumed hydrothermal processes. Fig.2I shows adularia and kaolinite (replacing the adularia?)
6. e) A very interesting aspect of the study is the interpretation of K-Metasomatism. An increase in K in clastic deposits downsection in wells is observed frequently, with diverse attempts to get a grip on the sources and mechanisms. Metasomatism presumes the export from one volume to import in another volume of rock. In this study the increase of alkali elements is quantified for the gabbroic diorite and the basaltic andesite. Why should an export of potassium from the overlaying Rotliegend arkosic sediments be impossible? The Neogene to Quaternary weathering and erosion effect on the investigated section is only poorly touched.

1. 3. Technical corrections

Diverse annotations and comments, corrections are included in the manuscript (see pdf).

Here I present only few:

1. a) Line 118: Barruelian, compare Nelson & Lucas 2021: The Cantabrian and Barruelian substages…; in

 Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82._ please adjust to their suggestions (?)

1. b) Fig 11: "retrograde" trend is not explained beyond his figure caption. Please omit or explain.

Retrograde is a term in metamorphic petrology; does it apply here for K+ trends, metasomatism??

1. c) Line 445: we do not need subhumid climatic conditions to promote eventual flood events that create alluvial massflows. This occurs even in very arid conditions (Jahrtausend-Ereignisse).

1. Final assessment:
2. Does the paper address relevant scientific questions within the scope of SE? yes
3. Does the paper present novel concepts, ideas, tools, or data? Not really novel concepts, but interesting case study
4. Are substantial conclusions reached? Some conclusions are well based on evidence, others are less.
5. Are the scientific methods and assumptions valid and clearly outlined? yes
6. Are the results sufficient to support the interpretations and conclusions? In general yes, but see comments
7. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? yes
8. Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Most essential relevant literature is cited
9. Does the title clearly reflect the contents of the paper? I suggest to omit “and palaeoclimate evaluation”, since this part of the paper is least supported by evidence.
10. Does the abstract provide a concise and complete summary? Yes acceptable
11. Is the overall presentation well structured and clear? yes
12. Is the language fluent and precise? Good, but may have a native speaker looking at….
13. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? n.a.
14. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? See comments
15. Are the number and quality of references appropriate? See 7.
16. Is the amount and quality of supplementary material appropriate? Yes fine

This paper presents a very instructive and convincing example of two successive weathering events followed by diagenetic modifications. The study is well performed and presented and gives an overall impression of reliable data, discussion and conclusions.

The study includes a petrographic and a geochemical approach. In many such studies – as is also the case in this one – there is a problem in combining the petrographic evidence with the geochemistry. The petrography gives a very convincing information on the intensity of the weathering events, but when it comes to the geochemistry, the information seems to be strongly blurred by the much later diagenesis. The most common geochemical parameters for identifying the weathering intensity are CIA and PIA. Both suffer from uncertainty in estimating the loss of K (CIA) and Ca (CIA AND PIA). In this study there is an increase in K₂O compared to the protolith which seems to be well identified in both cases, and the authors have decided for “no loss” during weathering by choosing the K-value of the protolith for the CIA diagram (the equation used for K₂O_corr does not add any information as it can simply be reduced to K₂O_corr=K). Without a reliable estimate for K-loss during weathering, an important aspect of the study is lost. Similarly, Ca-loss cannot be well established when calcite and dolomite are present. At least the evaluation of the Ca-content should be made in the light of the presence of carbonates rather than apatite. The chois in this study has been to assume that Ca must be lower than Na.

The study concludes – with strong support from petrography – that the weathering resulted in formation of smectite, and that this smectite was (much) later transformed to illite as a result diagenetic transfer of K. But then: how did diagenesis influence the content of Na and Ca – the other key elements in relation to evaluation of weathering intensity. Was K lost during weathering and just more than fully replied during diagenetic transfer - or was it partly consumed by the illitic interlayers in the smectite – or simply not released from primary minerals in the protolith? Was Na and possibly some Ca consumed by smectite formation and only released as respond to the diagenetic illitization (transfer of K)? Or were they already lost by leaching during weathering?

Since the  evaluation of weathering intensity is discussed in relation to geochemical loss of elements the above aspects should be discussed in more detail