The Sedimentary Origin of Black and White Banded Cherts of the Buck Reef, Barberton, South Africa

Round 1

Reviewer 1 Report

This study presents a novel interpretation for the different rates of banding in Archean cherts, which is a long standing enigma. This work bares critical significance for our understanding of past sedimentary processes, together with how this potential paleoenvironmental record should be used in futur studies. I believe this article is highly recommended for publication.

The manuscript is generally well written, clear and somewhat concise. It doesn't bring much sophisticated data, which does not harm the article since its main contribution deals with a general reinterpretation of what was already known. We can however note the great effort in providing a new and detailed sedimentaological and petrographic description of the Buck Reef Chert, highly relevant for the new model of imbricated paleo climate cycles. The geochemical data provide good insights to the paleofluids responsible for chert formation, and the contrasted environments between black and white bands.

I suggest publication without revision (only minor typos)

I have no further comments to make on this excellent contribution, which I hope will be the beginning of exciting new research validating further this model, and using it as a refined chronostratigraphic technique, definitly missing for the study of the young Erath.

I've made just a few comments, and noted minor typos that are provided in the attached PDF.

Comments for author File: Comments.pdf

Author Response

 

Thank you very much for your review. We acknowledge that there were some grammatical errors in the submitted version and these have been corrected. The entire manuscript has now been checked carefully by a native English speaker (me).

Reviewer 2 Report

General Comments:

Ledevin et al. seek to provide new evidence for how Archean cherts form and conclude a chemical sedimentary origin driven by glacial-interglacial cycles that correspond to Milankovitch or orbital cycles. While I find the field, petrographic, and geochemical evidence to be sound and convincing, I think there could be other explanations aside from glacial-interglacial cycles that could be discussed such as the periodic exposure of emergent continental crust and increased riverine input. Overall, I recommend publication following a moderate revision.

Figure 13: Could these ratios be normalized to upper continental crust?

 

Figure 14 – as this is a single section, how confident are the authors that the decrease in Y/Ho reflect a truly global signal? Is this supported by geochemical analyses from additional sections?

 

There are a number of instances where closing parentheses are missing.

 

White chert veins are indicative of pervasive post-depositional silicification – what is the source of this silica? Are there any apparent controls on whether it manifests either homogenously or randomly – i.e., the lithology being silicified, fracture density, etc.?

 

Fig. 6 – Is it possible that the siderite or ankerite grains represent a minor, periodic detrital input during deposition? The rounded to sub-angular grains in Fig. 6C are reminiscent of detrital grains.

 

The alteration of black and white cherts is ascribed to climatic variations linked to Milankovitch cycles, namely glacial-interglacial cycles. However, given a general lack of glacial deposits from this period and previous estimates for high pCO₂and elevated temperatures in the Archean, is it possible that there are other environmental conditions that could account for changes in depositions but also correspond to Milankovitch cycles (i.e., shallowing of the basin, periodic exposure of continental crust and increased riverine input, changes in ocean circulation, etc.) or other external drivers such as fluctuations in volcanic outgassing and periods of increased acid rain? For instance, is it possible that while black cherts correspond to background silica precipitation, while white chert layers correspond to rapid precipitation induced by increased acid rain resulting from periodic increases in reduced volcanic gasses?

 

Line specific comments:

L26: Suggest changing “omnipresent” to widespread or common.

 

L36: “...best access to the Archean near-surface environment.”

 

L53: Suggest changing “..that crops out continuously for more than 30 km...” to “...that out crops continuously over more than 30 km...”

 

L74: Figure 2 caption. Suggest changing to: “ (c) and (d) The top of white layers underneath a denser black chert band commonly exhibit signs of soft-sediment deformation.”

 

L209: “On average, non-compacted...”

 

L221: Suggest changing “decomposed” to “digested”. Also, what type of bombs were used for the digestion?

 

L222: Change BHVO₂to BHVO-2 throughout.

 

L223: Suggest changing “theoretical” to “accepted”.

 

L247: Define REE+Y

 

L248: Change “in average” to “on average”.

 

L249: Define LILE and HFSE.

 

Figure 14 – Note in the caption that the colors correspond to black and white cherts as in Figure 12, and what the bars in the Pb/Th and Cr/Ti correspond to. 

 

Figure 15 – As both the x-axis and y-axis are normalized to Th, shouldn’t these strong positive correlations remain if the transition metals are plotted solely against each other (i.e., Co vs Cr, or Ni vs. Cr).

 

L287: Delete extra ( before references.

 

L403-407: What about Eu/Eu* anomalies? Most samples appear to have a positive Eu/Eu* anomaly indicating input for a high-T hydrothermal source.

 

L468: See also a recent paper by Lantink et al. (2019) in Nature Geoscienceabout Milankovich cycles corresponding to cyclicity in banded iron formations.

 

L497-499: How important would continental weathering be at 3.46 Ga given the lack of emerged continents? Furthermore, large geochemical compilations suggest depressed P cycling in the Precambrian (see Reinhard et al., 2017).

 

L568-570: Current estimates suggest that the rise of the first silicifying organisms was not until the Neoproterozoic – consistent with genomic and biomarker evidence, and a lack of microfossils in Archean cherts.

 

L578: What evidence is there for slightly alkaline river waters, especially given the likelihood that the atmosphere had substantially more CO₂, which would have led to acid rain.

 

Supplementary Information – for trace element analyses, what are the steel Paar bombs lined with? If they are unlined could these have contributed to the contamination of trace elements such as Ni, Co, Cr?

 

Supplementary Information –Suggest changing to: “The international standard BHVO-2 diluted 500 times and used in combination with the reference material RGM1...”

Author Response

see attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

See review file.

Comments for author File: Comments.pdf

Author Response

see attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Several of the later references do not correspond to their position in the reference list. For example, Konhauser et al. (2001) and Lantink et al. (2019)  in lines 584 and 708, respectively, do not correspond to the current order in the reference list. Please confirm and adjust appropriately.

Reviewer 3 Report

I'm happy with the changes the authors have made in regards to my comments from the first round of review. It's clear that they considered my concerns, particularly around the geochemical data, and made efforts to address them wherever possible.

The caption for Figure 14 is still lacking some important details: 1) a reference for the global increase in Y/Ho, 2) what do the shaded bands in the last two panels represent? 3) do the navy/yellow points represent black/white chert? 

There are still a few niggling typographical errors to correct (the ones I saw in a quick read are detailed below). Otherwise, I'm happy to see it go forward. 

Fig. 1 caption - the Buck Reef thickness number needs a hyphen

Line 622: necessary should be necessarily

Line 688: microbial (highlighted) is misspelled

Table 1 still reports the standard as BHVO-1 (should be -2, yes?)