Author Topic: What are the primary controls of speleothem occurance?  (Read 684 times)

Offline Kenilworth

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What are the primary controls of speleothem occurance?
« on: June 03, 2018, 10:39:31 pm »
Speleothems are somewhat unpredictable. In many cases, caves with apparently similar geologic formation and situation contain wildly differing amounts and types of speleothems. Even high-level joint-controlled passages, where speleothems are often best developed, are very inconsistent.

The factors; mineral composition of the bedrock, organic content of the overburden, the nuances of surface terrain and drainage, age of the passage, structure of the passage, airflow, &ect., are many. Which is most important? What can help an observer determine which may be the primary control in any particular passage?

Offline mikem

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Re: What are the primary controls of speleothem occurance?
« Reply #1 on: June 03, 2018, 11:03:28 pm »
The acidity of the water & the amount of time it spends in contact with the rock before reaching a point where it can deposit.

Mike

Offline Kenilworth

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Re: What are the primary controls of speleothem occurance?
« Reply #2 on: June 04, 2018, 12:00:15 am »
The acidity of the water & the amount of time it spends in contact with the rock before reaching a point where it can deposit.

Mike

Those are a couple of simple parts of the mechanics of deposition and explain rates of growth more than controls on incidence. I'm trying to understand large variances within the same geologic setting.

Offline pwhole

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Re: What are the primary controls of speleothem occurance?
« Reply #3 on: June 04, 2018, 12:33:02 am »
I would imagine that if a particular passage had a tendency toward creating speleothems - as in, enough time and acidity to pick enough calcite to re-deposit etc. - then any significant variance of quantity would most likely be primarily due to variance in the permeability of the limestone, and whether any local barriers or enablers to that were also present. For example, a large part of Peak Cavern in Castleton is well-decorated, but some sections are not, owing to the thick layer of impermeable lava which overlays those parts and prevents percolation water passing directly into the passages.

Offline Bob Mehew

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Re: What are the primary controls of speleothem occurance?
« Reply #4 on: June 04, 2018, 07:43:55 am »
My hypothesis is that the primary control is the concentration of CO2 in the water which arrives at the upper surface of the limestone.  I do so as the basic chemical equations for the growth of formations are:

CO2(gas) ⇋ CO2(aq)

CO2(aq) + H20 ⇋ H2CO3   

H2CO3 ⇋ HCO3- + H+   

HCO3- ⇋ CO32- + H+   

I conjecture that the level of CO2 in water is elevated by the process of decomposing organic material in the sub soil layers above that derived from the simple solution of atmospheric CO2 into the rain water. 

The solubility of Calcium is dependent upon the concentration of HCO3- being the (relatively) soluble species.  So with a higher level of CO2 in water, more dissolved Ca(HCO3)2 is taken into solution as the water percolates through the limestone.  When the water reaches the cavern, the amount of CO2 dissolved in water then decreases since the CO2 level in the cave atmosphere is at normal values (400ppm) so the HCO3- level drops and hence the CaCO3 "precipitates" out.
 
I have not yet made much headway into solving these equations so as to be able to predict levels of CO2 at the upper surface of the limestone.  But I have managed to calculate that higher levels of CO2 seen in some caves in the UK do not correlate with the level of precipitated CaCO3 / formations displayed.  This suggests that the assumption of atmospheric CO2 levels above the upper surface of the limestone are insufficient to permit the higher levels of CO2 seen in the atmosphere within some cave. 

I hope that makes some sense; in my defense I will observe that I gather this is cutting edge science.

Offline mikem

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Re: What are the primary controls of speleothem occurance?
« Reply #5 on: June 04, 2018, 08:42:54 am »
All of then affect deposition to varying degrees in different situations, if the flow through is too fast then you also won't get formations. I guess testing the chemical composition of non-calcite forming water might help answer the question - could other minerals inhibit calcification?

Offline Cave_Troll

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Re: What are the primary controls of speleothem occurance?
« Reply #6 on: June 04, 2018, 09:10:14 am »
like salt ?

Offline TheBitterEnd

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Re: What are the primary controls of speleothem occurance?
« Reply #7 on: June 04, 2018, 06:32:06 pm »
'Never argue with a fool, onlookers may not be able to tell the difference.' — Mark Twain

Offline Kenilworth

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Re: What are the primary controls of speleothem occurance?
« Reply #8 on: June 04, 2018, 09:49:45 pm »
Interesting Bob.
Thanks for the replies everyone.
Chemical composition aside, I'm guessing that other factors may be just as important. If little water reaches the cave passage for some structural reason (I'm thinking of a particular cave with no sandstone cap but also very little drip input) it doesn't really matter how saturated it is. Or if a surface valley lies over a passage in a dry climate, marked increase in speleothem density has been recorded, while areas of intense input in a wet climate may (as mike mentioned) not result in much deposition.

I am studying a large karst area approximately one mile wide and thirty miles long. The geology at a given elevation is more or less consistent throughout. Eyeball inspection has revealed no geological, hydrological, biological, or geographical reason for the extreme variations from intense deposition to almost none. If I were to go beyond the eyeball test, what tests would you suggest, followed by what others, should I fail to learn anything?

Offline Fulk

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Re: What are the primary controls of speleothem occurance?
« Reply #9 on: June 04, 2018, 10:59:55 pm »
Could the density of fractures in the rock play a part?

If a particular area is seamed with cracks and fissures this could allow water to percolate downwards through the rock readly, while if there are no (or few) fissures, water would be precented from, or severely inhibited in, percolating down through the rock.

Offline 2xw

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Re: What are the primary controls of speleothem occurance?
« Reply #10 on: June 04, 2018, 11:11:24 pm »
Interesting Bob. I wonder to what extent various acids etc other than H2CO3, resulting from the decomposition process, contribute to formation. Especially interesting in a peatland environment like some of the Dales caves. Does grouse moor burning accelerate rates of formation  :confused:

Offline Bob Mehew

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Re: What are the primary controls of speleothem occurance?
« Reply #11 on: June 05, 2018, 09:22:08 am »
Several questions which frankly are beyond my capacity to answer correctly.  But

a) flow or rather percolation through rock will influence the amount of take up since the faster the flow, the less time the water has in contact with the rock.  I have no feel for the rate of dissolution however and of course contact area will also be important.  (And state of metamorphosis of the limestone.)

b) magnesium carbonate is slightly more soluble than calcium carbonate.  So the presence of magnesium (or sodium) will suppress the take up of calcium since they will accommodate more HCO3 and CO3 ions.  (I think I have that right.)

c) I think it is reasonable to suggest that from differences between UK and European formations, that temperature is a significant parameter.  But it is not simple as the concentration of CO2 in water is inversely related to temperature.  Obviously you need a flow of water but given we are usually talking about the development of formations over centuries rather than years, I suspect some simple calculations will show you don't need a high flow rate (i.e. drips per hour). 

d) As pwhole mentioned with the example of Peak Cavern, clearly an impermeable layer above the limestone will stop water flow.     

e) I am not sure about other organic acids.  A quick flick through the Rubber Bible (a compendium of chemical and physical data) indicates some organic salts of calcium are less soluble whilst others are far more soluble than calcium carbonate.

All in all, for any given situation one parameter is going to be dominating (like no water or no fissures or no soil overburden).  It probably is rare when the set up is balanced between a range of parameters. 

What piqued my interest was reports of high CO2 levels in some caves suggests that there should be high deposition rates but the visual evidence does not support such deposition. 

I am trying to get hold of a copy of Speleotherm Science but I fear it will take some while.

Offline mikem

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Re: What are the primary controls of speleothem occurance?
« Reply #12 on: June 05, 2018, 10:10:34 am »
The figures & tables from the book are available at:
http://bcs.wiley.com/he-bcs/Books?action=index&bcsId=7193&itemId=1405196203

Mike

Offline Bob Mehew

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Re: What are the primary controls of speleothem occurance?
« Reply #13 on: June 05, 2018, 11:39:31 am »
Thanks.  By accident I found https://media.wiley.com/product_data/excerpt/03/14051962/1405196203-45.pdf is the first chapter.