Comment on preprint of Franz et al.: A reinterpretation of the 1.5 billion year old Volyn

2 ‘biota’ of Ukraine, and discussion of the evolution of the eukaryotes, by Head et al. (2023)

Franz et al. respond to the comment by Head et.al (2023) on the discussion of the so-called 1.5 billion year old Volny biota, combined with a discussion on the evolution of eukaryotes. Head et al doubt the age of the fossils described and explain them as recent contamination by "museum dust" and some structures are interpreted as non-biological in origin. I already had the original paper by Franz et al 2023 for review and I had considerable problems understanding the preservation of the fossils presented. Some of them seem to have the character of pseudofossils - biomorphs - as described, for example, by Rouillard et al 2018. Head et al. obviously have similar problems and interpret the structures depicted as mentioned above. The arguments presented by Head et al. are largely comprehensible. In the manuscript now presented, Franz et al. attempt to refute the arguments of Head et al.. For me, Franz et al. convincingly describe that the sampling and sample preparation was lege artis. However, dust contamination is often unavoidable. I could imagine that the possible pollen grain (Fig. 6) is such contamination. Franz et al. deny this although the morphological similarity is considerable. The structure in Fig. 2b could also be a dust contamination. In chapter 3 they describe the structure of kerite and explain the presence at the margin in a beryl pseudomorph. They describe that the kerite structure was found in thin sections and is embedded in a C-H containing opal. I am puzzled by the opal, which cannot actually be preserved as such. The organic material has not been analyzed state of the art. It is not clear what the kerite actually is here. They refer here to their work from 2022 where pyrolysis methods were used coupled with a GC, they analyzed mostly aromatic structures and make it difficult to recognize a biological origin. Perhaps Raman spectroscopy would also help to characterize the kerite and also compound-specific d13C analyses. I am not quite sure what they call decayed OM here. In any case, I am convinced that the kerite is pristine and represents a thermally altered OM, but I do not believe that this organic matter is totally of biological origin. I agree with the arguments of Franz et al that the observed "trichomes" do not show biomineralization as Head et al suggest, but show diagenetic-metamorphic mineralization. However, this says nothing about the biogenic character of these structures. Franz et al have measured IR spectra and believe to have analyzed chitosan there - chitosan is formed by deacetylation of chitin. In Franz et al 2023 a FTIR spectrum is shown (Fig.13) in which kerite and chitosan are compared. I think the interpretation is not so convincing, further biogeochemical methods and comparisons would be necessary to confirm the original chitin character. There are also doubts about the age of the kerite, at least the Proterozoic age. Here, too, further dating would be helpful to make the Ar-Ar age measured by Franz et al plausible. Popov 2023 came to an early Paleozoic age. Due to the importance of the age classification of the Volny biota, it seems to me that further analytical action is needed. The origin of the kerite is being discussed and Fischer-Tropsch Type (FTT) processes in the context of serpentinization are, of course, a possibility. However, this requires ultrabasic mineral rocks, which are not to be expected in the immediate vicinity of granite. However, these organic fluids could have migrated, which can also be assumed. However, the very light δ13Corg values of around -50‰ are a problem and indicate an influence of methane metabolism. FTT experiments by McCollum and Seewald 2006 have shown a fractionation step of over 30‰, so that isotopically light Corg values are non-biologically possible. There is also still a need for action here. It cannot be ruled out that the kerite represents a mixed organic matter of FTT products and methane metabolism. There are still many open questions dealing with the so-called Volny biota and the validity as Proterozoic complex fossils is finally not convincing.

Review for the Reply to Comment on Franz et al 2023 A reinterpretation of the 1.5 billion year old Volyn ‘biota’ of Ukraine, and discussion of the evolution of the eukaryotes, by Head et al. (2023).

In the Reply the authors react quite positive about the comment by Head et al for bringing up the discussion about the biogenicity and the question of potential contamination, which was not raised in their previous and other publications describing the kerite (e.g. Franz et al 2017, 2022). In elder publications the kerite was assumed to derive from an abiotic formation (e.g. Ginzburg et al., 1987) or from a microbial mat of fossilized cyanobacteria (e.g. Gorlenko et al., 2000). In the following reply Franz et al discuss point by point 5 major aspects of concern, addressing why the kerite should not be considered ‘Museum dust’.

The first important aspect is, that the kerite was found in situ within in the pegmatite veins, partly intertwined with other minerals and also in abundant amounts which was document by the miners and further studies. In the recent article from Franz et al 2023 this aspect does not become clear enough, but it was mentioned and described in early studies (e.g. Franz et al 2017, 2022). Therefore, in the reply there is an extended description about the occurrence of the kerite and references of the first mentioning. Astonishingly kerite seemed present in kg-amount within cavities, but due to the explorative focus of the underground work, it did not receive much attention.

Later on, when morphologies of the specimens are discussed, Franz et al state, that Head et al just considered only 2 morphotypes in their ‘museum dust comparison’ out of several tenth of structures presented in the paper with very heterogenous morphologies. Potential dust contamination cannot be excluded for 1 sample, for the other samples Franz et al state, that they were packed in closed bags at the sampling site and just opened shortly before analyses.

A further discussion point addresses the fact that there are 2 quite similar morphologies (similarity to a pollen grain and to trichomes). Here, Franz et al, argue, that their mineralogical composition seems significantly different to their similar looking counterpart. In this context, Franz et al describe not only the composition and structure of the kerite from a geochemical and mineralogical point of view, but also the general mineralization pathway, the organic matter and mineral transformation leading to the final mineralogical composition of the fossilised matter and the surrounding minerals in more detail compared to the original article and also including maturation of the original matter. In my opinion, this part (lines 103- 249) is a strong asset for the overall understanding of the kerite formation. The major last point which is discussed is the the age of the kerite and the syngenetic formation with adjacent minerals, and minerals which were obviously formed later.

However, for the timely limited reader, it’s starting to get difficult to follow the different aspects and results of the kerite formation, even though I really liked the pros and cons of the discussion following on the original paper. In order to get a full picture of the kerite and its formation the reader is expected to go through all papers and follow up discussions.

Therefore, I recommend to Franz et al to compile a review about the kerite and the formation and include a overview with discussion points and images of potential look-a-likes and where they see the differences. 

All in all, I really liked reading this reply which was thoroughly written and which summarizes a lot of additional information about the kerite followed by a clear line of argumentation with detailed facts, why the authors are convinced about their conclusion that the kerite is fossilised, partly degraded and thermally altered Proterozoic organic matter. Therefore, even if a tiny grain of potential contamination cannot be excluded, the overall kerite material seems to be convincingly deposited during Proterozoic times in the pegmatite and is highly likely to be of biological, maybe even eukaryotic origin.