The Volyn biota (Ukraine) – 1.5 Ga old (micro)fossils in 3D-preservation, a spotlight on the ‘boring billion’
Abstract. The Volyn biota, fossilized organisms with a minimum age of 1.5 Ga, were found in cavities in granitic pegmatites from the Korosten pluton, NW Ukrainian shield. Fossilization was due to influx of hydrothermal fluorine-rich waters, which silicified the outermost part of the organisms, thus preserving the 3D morphology. Details of the morphology (investigated by scanning electron microscopy) show that the majority of the specimens is filamentous, of a large variety with diameters ranging from ~10 µm to ~200 µm, thin filaments with typical branching, thick filaments with ball-shaped outgrowths and dented surface. Filaments can be straight or conical, curvilinear or strongly curved, up to mm in length, some with a central channel. Some filaments show indications for segmentation, are grown as sessile organisms onto substrate; others show both intact ends, indicating growth in soft medium or floating in water. Objects with flaky morphology and agglutinating filaments are interpreted as fossil biofilms. Other objects are hollow and show a large variety of forms; spherical objects are scarce. Infrared spectroscopy indicates the presence of chitosan in one filament, electron microprobe analysis of nm-sized inclusions in filaments identified the presence of Bi(Te,S) minerals, and both observations are compatible with the interpretation of filaments as fungi-like organisms. Stable C- and N-isotope data of bulk samples are in the range of -31 to -47 ‰ δ13C/12C, and of +3 to +10 ‰ δ15N/14N, indicating possible methanogenic bacteria as part of the subsurface micro-ecosystem. The Volyn biota show that at 1.5 Ga fungi-like organisms lived in the continental deep biosphere, where complex forms of life existed, well above the microscopic level.
Gerhard Franz et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1116', Anonymous Referee #1, 30 Nov 2022
- AC1: 'Reply on RC1', Gerhard Franz, 07 Feb 2023
RC2: 'Comment on egusphere-2022-1116', Anonymous Referee #2, 26 Jan 2023
- AC2: 'Reply on RC2', Gerhard Franz, 07 Feb 2023
RC3: 'Comment on egusphere-2022-1116', Anonymous Referee #3, 27 Jan 2023
- AC3: 'Reply on RC3', Gerhard Franz, 07 Feb 2023
Gerhard Franz et al.
Gerhard Franz et al.
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The manuscript by Franz et al. (The Volyn biota (Ukraine) – 1.5 Ga old (micro)fossils in preservation, a spotlight on the ‘boring billion’) presents a very compelling dataset on a unique fossil assemblage of most probably fungal origin.
Overall, the manuscript is well written and the data are convincing, particularly the SEM images. However, I feel that in some cases the interpretations are not supported by the data. My main concern is the interpretation that this fossil assemblage represents a deep biosphere community.
I was not able to follow the author’s argument about the 150 m depth. Perhaps a cross section of the geologic structure could clarify this issue. Currently I do not see any compelling evidence for the claim that these organisms thrived at a depth of 150 m, in fact, I even see evidence for the opposite. In line 502f the authors interpret the intact ends of the filaments as indications for growth in a soft sediment or floating organisms. It is pretty hard to imagine floating organisms in depths >100 m unless there are fractures through which water can flow freely, like for example in Karst areas. Also, how should one imagine soft sediment at great depth, assuming compaction due to overburden? If the sediment is so soft that soft filaments can push themselves into it, then there are no fractures with floating water. And these filaments are much too large to assume that they can float freely through the pore space.
Although I am not a microbiologist myself, reading the term “methanogenic bacteria” makes me cringe. There are no methanogenic bacteria, only archaea. Please change throughout the text. Also, what do the authors mean with “anoxygenic”? I assume that “anoxic” is meant, at least that is the only term that would make sense to me. There are several other instances where I got the impression that the authors’ expertise lies outside the field of microbiology and thus, I would strongly recommend to include a microbiologist in the revision.
Given the geologic evidence it seems plausible that the fossil assemblage formed after the GOE and that atmospheric oxygen levels were in the single percent range around 1.5 Gy. However, I am at a complete loss as to how this fact can be used to infer that there were oxygen-depending ecosystems at great depth at that time. There isn’t even good evidence for such ecosystems today, with oxygen concentrations between 5 and 10 times higher. And coming back to my argument in the previous paragraph, how should such an ecosystem have looked like? Again, fractures that allow for rapid flow of water into great depths are highly unlikely in soft sediment and in soft, fine-grained sediment diffusion is the dominant mode of transport. It is not even remotely imaginable that oxygen diffused to 150 m depth, given its low concentration at the surface (a good example is given in Roy et al. Aerobic Microbial Respiration in 86-Million-Year-Old Deep-Sea Red Clay. Science. 2012;336(6083):922-5.)
Another point that argues against a deep biosphere setting is the mentioning of palynomorphs in line 668. How do should palynomorphs reach a depth of 150 m? Of course, they could have been deposited at the surface and then eventually got buried, but then they must be (much?) older than the estimated 1.5 Gy. This argument does seem to be out of place here.
Another statement that I am somewhat inclined to believe, but would like to see substantiated, is in line 484. Why does the age ague for microorganisms? The following sentences meander around, but the fundamental question here would be the first occurrence of eukaryotes.
The argument in line 710 ff does not really fit the scope of this study. There was subseafloor life much earlier in the Archean, but the cited study (Cavalazzi et al., 2021) describes a hydrothermal system that was most probably quite near to or even at the sediment surface and not buried at 150 m depth. While there are reports of abundant fungi in modern hydrothermal systems, none go as deep as 150 m. So I doubt that the Cavalazzi reference is useful here.
The mentioning of radioactivity as a potential energy source is intriguing, but like before, I would strongly recommend involving a microbiologist in the revision. Yes, there are highly radiation-resistant prokaryotes and even fungi, and given that the ecosystem was hosted in a U-Th-K-rich granitic-pegmatitic system, radiation must have played a role in shaping the community. However, I do not really see how this entire paragraph connects to the main study other than stating that life was possible despite high levels of radiation.
In closing I think that the SEM study is remarkable and warrants publication, but the interpretation that the fossil assemblage represents a terrestrial deep biosphere ecosystem is not well supported to say the least. I suggest major revisions and either removal of the deep biosphere part or much stronger arguments to support these claims.