Mineralogy of Miocene Petrified Wood from Central Washington State, USA
Round 1
Reviewer 1 Report
The authors have presented an exciting manuscript that expands the case studies of the petrification of woods and gives room for new thinking. The four examples from the Columbia River Basalt Group fill a gap in knowledge, as silicified woods from basalts are rarely addressed. Observations from recent basalt-dominated volcanic areas on Earth, such as Hawaii and Reunion, further show that due to the extremely high temperatures of the lava of over 1200 °C, organic matter rarely remains when embedded directly in basaltic flows. Therefore, only cavities are often formed and, when filled later, stone cores imprinted the former outer shape of the tree. Difficile cell preservation, on the other hand, is rare, and when it occurs, severe shrinkage is noted in the woody tissues. In the case of geologically younger vegetational elements, whose closest relatives are still alive, one could still determine how serious the changes of the tissues were during the taphonomic influence.
The results show that the silicification of wood is much more complex than described in the models published so far. It becomes clear that the very different geological site and framework conditions must be taken into account much more strongly than before and increasingly adequately. On the other hand, too early, one-sided generalizations are to be avoided since they are not applicable in diverse individual cases.
The deposition of various SiO2 forms, e.g., on cell walls and in cavities (cell lumens and fissures) could – as Dietrich et al. (2013) and Dietrich et al. (2015) have shown – also be accompanied by template effects of individual tissue components. Direct excretion of alpha-quartz has also been documented and is attributed to low SiO2 concentrations in mineralizing fluids (pH-triggered).
In contrast to older work on the silicification of woods, which states that opal is only preservable maximally back to the Eocene and older evidence is fully crystallized by diagenetic maturation, the proof of opal in the Permian of Tocantins/Brazil astonished me (Trümper et al. 2020). Nevertheless, this evidence is available, and previous compilations (cf. Fig. 24 of the manuscript) should be clarified. Compare your line 448.
Das Manuskript muss auf jeden Fall publiziert werden, braucht jedoch noch einige kleinere Korrekturen. Ich halte Minerals für ein sehr geeignetes Fachjournal und schlage „minor revision“ vor. Vor allem das Literaturverzeichnis braucht Überarbeitung, wie ich es auf der ersten Seite des Literaturverzeichnisses im pdf angedeutet habe.
The manuscript needs to be published but still needs some editorial revisions. I consider Minerals a very suitable journal and suggest "minor revision". Especially the bibliography needs revision, as I have indicated on the first page of the references in the pdf.
I have no severe problems with the manuscript's contents and look forward to seeing this study published soon. I have added some minor (rather) editorial issues directly in the pdf file. The figures are well done and informative, the results are convincing. Some suggestions (added in comments) should help improve the manuscript's discussion background. I hope the authors will find them helpful, but of course, they can handle them as they like. I will provide some papers that I have mentioned in my discussion and essentially contribute to the specific topic:
Dietrich, D., Lampke, T., Rößler, R. (2013): A microstructure study on silicified wood from the Permian Petrified Forest of Chemnitz. Paläontologische Zeitschrift 87, 397–407.
Dietrich, D., Viney, M., Lampke, T. (2015): Petrifactions and wood-templated ceramics: Comparisons between natural and artificial silicification. IAWA Journal, 36 (2), 167–185.
Trümper, S., Rößler, R., Götze, J. (2018): Deciphering silicification pathways of fossil forests: case studies from the Late Paleozoic of Central Europe. Minerals, 8, 432.
Trümper, S., Götze, J., Rößler, R. (2020): Siliceous Petrifactions in the Permian of the Parnaíba Basin, central-north Brazil: Sedimentary environment and fossilisation pathways. In: Iannuzzi, R., Rößler, R., Kunzmann, L. (eds): Brazilian Paleofloras: from Paleozoic to Holocene. 32 S.; Cham (Springer Nature Switzerland). https://doi.org/10.1007/978-3-319-90913-4_10-1
Trümper, S., Noll, R., Luthardt, L., Rößler, R. (2021): Environment and taphonomy of an intrabasinal upland flora from lower Permian volcaniclastic sediments. Neues Jahrbuch Geol. Paläont. Abhandlungen, 300 (3), 303–344.
Of course, I fully agree to disclose my identity.
Comments for author File: 
Comments.pdf
Author Response
Thanks for an encouraging and very helpful review. I especially appreciate the references to the Dietrich et al. (2013) and Trumper et al. (2020) papers. The reports of opal in Permian fossil wood is a big surprise. I have adopted your various suggestions for improving the manuscript. I appreciate your quick response, and I value your geologic insights.
Reviewer 2 Report
See attached file
Comments for author File: Comments.pdf
Author Response
We appreciate the detailed proofreading, and we have made changes to the various line numbers where improvements were suggested. Responses to other recommendations are more complex. The reviewer seems to be approaching this manuscript from the viewpoint of an “opal scientist” (to borrow a term from the review). Although much of the content is related to silica minerals, as authors we have set our sights on a broader perspective, where mineralogy is viewed in equal balance with paleobotany and plant anatomy. For that reason, we prefer to retain the original format rather than the suggested reorganization. However, we have strived to adopt the reviewer’s other recommendations.
Changes include expanding the introduction, adding many additional references, and increasing the information content in various sections. These additions include providing more detail about analytical methods, rewording the “amorphous” description for opal-A, and adding a brief discussion of sources for mineral-bearing groundwater. We continue to use “opal-A” in preference to “opal-AG”, but we now include an explanation of our reasoning. The reviewer objected to the use of the term “lepisphere” to describe microtextures characteristic of opal-A. This is a topic where there is not universal agreement, and we have included a discussion of the nomenclature options. For Australian scientists, precious opal has long been known to consist of ordered layers of opal-A nanospheres. For opalized wood in other regions, opal-A commonly occurs as aggregated particles that give rise to relatively large spheres. We have used informal nomenclature to refer to these spherical grains as microspheres to avoid the lepisphere confusion. We reserve lepisphere for opal-CT microstruc tures.
In regard to more general topics: We prefer to retain the Figure 24 phase diagram, which we originally described as “speculative”. The original source of the data (Mizutani 1970) explained that his interpretation was based on a combination of field observations and laboratory experiments. Our use of the “speculative” description was intended to mean that a single graph is unlikely to precisely explain all episodes of silica diagenesis. We have revised our wording, and include a reference to more recent data obtained from world-wide deep sea drilling that supports Mizutani’s observation that rates of diagenetic change can be correlated with burial depth (i.e., temperature) and geologic time. The fact that the reviewer was unfamiliar with the Mizutani 1970 plot supports our inclusion of that evidence.
The reviewer suggests the possibility that for fossil wood there is no diagenetic transformation between various silica minerals. The problem with that approach is that these transformations are well documented for other geologic environments (e.g., siliceous biogenic sediments and siliceous geothermal deposits), and the scarcity of opalized wood in Mesozoic and Paleozoic formations supports the likelihood that similar transformations can occur in silicified wood. The fact that it is difficult to observe evidence of these transitions in Miocene petrified wood of central Washington State is a key part of our research report. This topic is discussed in detail.
In summary, we greatly appreciate to constructive tone of the reviewer comments, and the manuscript is better because of them.
Round 2
Reviewer 2 Report
Many of my comments have been taken on board. Thankyou.
I shall address the revised paper by going through my original comments in turn. As I will note below, some of my comments were more stylistic than technical and I’ll leave them to the editors.
The Introduction is now much improved and I note that the reference list is updated with some very recent papers cited. The origin and nature of the water are now covered.
The nature of opal-A, and the Jones and Segnit classification are now discussed.
Various other minor things seem to have been repaired.
I now see the point that the authors are making about the relative strength of the XRD patterns of opal-A and opal-CT and difficulties experienced in only using XRD. Perhaps I should have been more explicit previously and pointed out that it is easy to differentiate them using Raman. The figure, however, still needs some work. I think that the d-spacings are off. I understand the rationale in using the same y-axis scale in figure 8 but I think it would look better expanded (with a “x10” next to the “opal-A”). The figure looks odd in relation to other papers on opal-A.
I am also still concerned about the possible peak at 36 degrees though happy to accept a comment like “similar to other examples of opal-A”. Line 517 implies a possible intermediate form.
Happy with the discussion about nomenclature of opal-A and opal-CT.
I have no real problems with including fig 24 after all it was published in a reputable journal, though as I pointed out, rather old. My principal concern is that diagenesis is inferred (as the authors note in line 488) rather than experimentally demonstrated. To reply to the authors’ comments I would say “little experimental evidence for diagenetic transformation” rather than “no diagenetic transformation”. The material from line 487 covers this well. Being a hard-core scientist, I tend to use words like “proposed”, “presumed” or “consistent with”…..
There are a few syntax problems, such as joined words and no space after a period, at least in the version that appears on my screen. This may be a pdf issue
Endnote has gone wrong with some of the new references.
Minor things I spotted:
Line 235 nearly amorphous? (this would offend the purists, it’s either amorphous or not) - with little structure?
Line 238 opal-AN
Line 240 opal-AG
Lines 256-257 needs a reference. See Wilson, M.J. The structure of opal-CT revisited. Journal of Non-Crystalline Solids 2014, 405, 68-75 for another opinion that attempts to reconcile XRD with spectroscopy.
Line 276 probably better to say “approximately” uniform (or equal). I’m not sure how quantitative you can make this
Line 780 journal name (and inconsistent with at least one other case)
Line 795 Liesegang
Author Response
We appreciate the reviewers attention to detail, and the constructive attitude. We have corrected the various small errors.
The "nearly amorphous" descriptions have been resolved by eliminating the use of "amorphous" to describe opal-A. The terminology is a bit confused for this material. Many past reports have described opal-A as being amorphous because it lacks a crystal lattice. However, stacking of the nanospheres can produce a small amount of XRD reflectance, The reviewer asserts that a material is either amorphous or not. A more common description is that materials are either amorphous or crystalline, and opal-A is certainly not crystalline.
This issue is related to the reviewer's recommendation to revise Figure 24 to show the opal-A pattern as having a 4-angstrom reflection. We decline that suggestion for two reasons. One reason is that I have studied a lot of opal-A from diverse locations. Australian opal-A typically shows that weak reflection, but not all opal-A specimens show that peak. I suspect that difference may because Australian opal has a much greater geologic age and formed under rather different geologic conditions from many other locations where opal-A has relatively young age. I therefore don't want to claim that a low intensity 4 angstrom peak is a defining characteristic of opal-A. The opal-A pattern from a Neogene Nevada sample that we used in Figure 24 is an example: there is no 4 angstrom peak, regardless of the intensity scale used for the pattern. There is a very slight rise in the diffuse pattern, but not the small peak that is observed in Australian opal-A.
Second, the point of that figure is to substantiate our statement that for samples that contain other silica phases, opal-A can not be recognized in XRD patterns. The reviewer's suggestion of adding a 10X view of the opal-A pattern is not appropriate for this situation, even for samples where a weak 4 angstrom peak might be present. For mixed-phase samples, regardless of the intensity scale that is used the opal-A pattern is obscured by the much larger reflections from other minerals.
