Reduction spots and microbial life

[Rogue Embryo]

Was examining several of my rocks (sandstone & mudstone) that have light-coloured spots, such as the images of 3 rocks below. I read "Reduction Spots in the Mesoproterozoic Age: implications for life in the early terrestrial record" (Spinks, Parnell, Bowden) -- well, the little that I could understand -- and was wondering whether any of these 3 rocks might be an example of this. The spots in my rocks do not seem to have the "dark concretionary cores" that the article says are often present in the spots, but I was curious to know more. I'm having doubts but thought I'd ask. Thanks for any ideas.

 

Rock #1:

 

 

Rock #2:

 

 

Rock #3:

 

[ClearLake]

I have no idea, but I would first see if they look anything like what is shown in the paper you are reading.  Next I would make sure these are not just surface discoloration features.   Beyond that, try to determine if there is any different mineralogy between the light and dark areas.  These all look very geologic to me, but the Proterozoic is not an area of significant knowledge for me.

[Rogue Embryo]

Thanks for your response, @ClearLake.

 

1 hour ago, ClearLake said:

I would first see if they look anything like what is shown in the paper you are reading

 

The first rock, with its reddish hue, especially bears a resemblance to rocks whose "reduction spots" may be evidence of ancient microbial life:

 

"Reduction spots are common within continental red beds in the geological record. The method of formation of reduction spots is a subject of debate, but they are thought to be the results of the reducing nature of microbial life present in the sediment during burial, which caused localized reduction in sediment that was otherwise oxidized during diagenesis."

 

Maybe it's not possible to say with certainty, since there seems to be some debate about it, but I was curious about the phenomenon.

 

1 hour ago, ClearLake said:

Next I would make sure these are not just surface discoloration features.

 

I'm wondering whether one could say that reduction spots are by definition "discoloration features."

 

1 hour ago, ClearLake said:

try to determine if there is any different mineralogy between the light and dark areas

 

If I had access to a Scanning Electron Microscope, I would definitely try this. :-)

[ClearLake]

2 hours ago, Rogue Embryo said:

I'm wondering whether one could say that reduction spots are by definition "discoloration features."

True, but that is why I called it surface discoloration.  The point being, is it just on the surface (ie due to weathering or some sort of animal activity for example) or does it permeate into or through the rock.  To tell that you may have to scrub the surface with something hard (wire brush, steel wool, etc) or even better, crack one open to get a fresh surface.

 

2 hours ago, Rogue Embryo said:

If I had access to a Scanning Electron Microscope, I would definitely try this. :-)

While that would be nice, it is certainly not the only way to differentiate minerals.  Geologists have been doing it for a long time without SEM.  Again, a fresh surface is almost always better to look at minerals unless the crystals are large, which does not appear to be the case here.  Just a thought to consider in your search for an answer.

[Rogue Embryo]

23 minutes ago, ClearLake said:

True, but that is why I called it surface discoloration.  The point being, is it just on the surface (ie due to weathering or some sort of animal activity for example) or does it permeate into or through the rock.  To tell that you may have to scrub the surface with something hard (wire brush, steel wool, etc) or even better, crack one open to get a fresh surface.

 

It's interesting that some sources call these light-coloured areas "spots," but others say "spheroid." I wasn't thinking that the reduction spots suspected of being biomarkers are actually three-dimensional. So actually, what appears to be a "spot," might be the cross-section of a spheroid, and the dark, metallic core of the spheroid (an actual nodule in some cases) could lie deeper within the rock.

 

37 minutes ago, ClearLake said:

While that would be nice, it is certainly not the only way to differentiate minerals.  Geologists have been doing it for a long time without SEM.

 

True, but lacking the visual evidence of a dark core of the spheroid, which might contain a number of different metals (including radioactive ones), high-tech analysis might be the best way to detect such metals in very small amounts, according to the article that I cited.

 

Great suggestion to attempt to get a fresher surface than the weathered one.

[doushantuo]

e.g.:

Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere
Sean McMahon, Ashleigh v.S. Hood, John Parnell & Stephen Bowden

NATURE COMMUNICATIONS | (2018) 9:4505 | DOI: 10.1038/s41467-018-06974-9 | www.nature.com/naturecommunications

s41467-018-06974-9.pdf

 

edit : PDF (vide infra) added

Geochimica et Cosmochimica Acta, Vol. 60, No. 20, pp. 3897-3912, 1996
The size distribution of framboidal pyrite in modern sediments:
An indicator of redox conditions
R. T. WILKIN, H. L. BARNES, and S. L. BRANTLEY

Wilkin et al GCA(60)1996.pdf

 

B.A.Hoffman

Mineralogy and geochemistry of reduction spheroids in red beds

 

Mineralogy and Petrology(44)1991

 

 

 

 

 

 

Hofmann1991.pdf

Edited March 21, 2022 by doushantuo

[doushantuo]

More often than not,only detailed petrography can help solve the knotty problem of "why is this structure in this rock".

A well-constrained stratigraphy(e.g. with some radiometric ages available) might help shed light on the problem of "when" 

An understanding of the kinematics of formation is crucial,anyway

 

Edited March 21, 2022 by doushantuo

[Rogue Embryo]

Thanks for the references to more articles about reduction spheroids, @doushantuo.

 

I cracked open the reddish rock, and with a bit of sanding, several well-defined "halo bubbles" can be seen. I'm not seeing any dark cores yet.

 

 

Not sure whether these spheroids represent biomarkers, but this has been a fascinating detour from brachiopods and crinoids, and I learned a lot.

[hemipristis]

Reduced conditions (low pH, anaerobic) are characterized by darker  staining. Think anoxic marsh clay or gley wetland soils. White spots like I'm seeing herewould therefore be indicative of the opposite; i.e., aerobic (oxidizing), higher pH conditions

[Rogue Embryo]

 

Hi, @hemipristis,

 

As quoted above, from "Reduction Spots in the Mesoproterozoic Age: implications for life in the early terrestrial record" (Spinks, Parnell, Bowden):

 

"Reduction spots are common within continental red beds in the geological record. The method of formation of reduction spots is a subject of debate, but they are thought to be the results of the reducing nature of microbial life present in the sediment during burial, which caused localized reduction in sediment that was otherwise oxidized during diagenesis."

Edited March 21, 2022 by Rogue Embryo
typo

[hemipristis]

4 hours ago, Rogue Embryo said:

 

Hi, @hemipristis,

 

As quoted above, from "Reduction Spots in the Mesoproterozoic Age: implications for life in the early terrestrial record" (Spinks, Parnell, Bowden):

 

"Reduction spots are common within continental red beds in the geological record. The method of formation of reduction spots is a subject of debate, but they are thought to be the results of the reducing nature of microbial life present in the sediment during burial, which caused localized reduction in sediment that was otherwise oxidized during diagenesis."

I'm missing something here.  I'm not discounting the process.  What I'm saying is that the 'localized reduction" would oroduce darker zones in an otherwise medium- pr ligtcolored roc, not the other way around

[Rogue Embryo]

I've only heard of reduction spots occurring in red beds and producing "bleached" spheroids. Maybe someone here can address your question better than I.

[FranzBernhard]

2 hours ago, hemipristis said:

I'm missing something here.  I'm not discounting the process.  What I'm saying is that the 'localized reduction" would produce darker zones in an otherwise medium- to lightcolored rock, not the other way around.

That is something to think about it! Maybe it depends on the "power" of the pigment that is reduced? Hämatit is a very powerful pigment, the reduction products - maybe magnetite, perhaps a iron-rich clay mineral or a chlorite mineral? - are not as powerful pigments as hematite. Hence the lighter color of the reduction spot.

Franz Bernhard

Edited March 21, 2022 by FranzBernhard