I know it's a long shot but is there any way of dating clastic sediments in caves in the absence of organic material? We have a gravel band on top of clay/silt fill and it may be helpful to know roughly when the gravel was washed in as presumably there must have been active flow at that time.  Some of the gravel is calcited together and some is iron stained.

If the gravel was washed in from above ground, has a crystalline component and had been exposed to strong sunlight before ending up in the cave, then OSL (optically stimulated luminescence) _might_ give you a rough date but this is a bit of a long shot, you'd need to speak to a lab that does this and explain the nature of the deposit to them to get a definite answer on whether it would be worthwhile trying. Alternatively, the calcite cement might be dateable with Uranium series dating, but off the top of my head I don't know how large a sample you need and how clean it needs to be to work.

TheBitterEnd said:

in the absence of organic material

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How do you know there is no organic material? I believe pollen can be used to date sediments, but you won't see it with the naked eye.

Also magnetic orientation or similar of particles within the sediments. The Earths magnetic field has switched several times in the past and this can be used to date when the sediments were deposited. (At least if they are old enough, I guess.)

These things are some of the reasons why undisturbed sediments in places like the recent Water Icicle extensions are so valuable.

This testing all costs a fortune though. I think I heard someone say ?500 a sample recently. 

Mel.

Thanks for the replies, I had seen references to OSL but I think it's unlikely we would get access to such tests (and then, as you suggest, it seems to be fairly constrained) or other lab based dating techniques, due to cost as mmilner suggests.

Interesting thoughts about pollen and magnetic field reversal. The gravel looks typical of lake/river bed gravels so pollen and other microscopic organics may be present. Magnetic field reversal may not be so practical in this case because I guess the material has to be in the situation/orientation in which it formed for that to be measured.

Of course it would be nice to have some sort of rule-of-thumb but I doubt such a thing is possible.

mmilner said:

Also magnetic orientation or similar of particles within the sediments. The Earths magnetic field has switched several times in the past and this can be used to date when the sediments were deposited. (At least if they are old enough, I guess.)

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I'm not convinced by this. Assuming that we have the right conditions (very low flow with clay and ferric minerals being deposited together), firstly, the Brunhes?Matuyama reversal occurred 780,000 years ago, and very few of our caves are this old (although that brings us back to the discussion on the possible hypogenic origin of some of them); secondly the deposits would need to be lithified to fix the magnetic orientation; thirdly, all one could possibly deduce is how the poles were oriented at the time. It wouldn't tell you during which subchron the magnetic particles were oriented, and so couldn't give you a time range within which the sediment was deposited.

langcliffe said:

It wouldn't tell you during which subchron the magnetic particles were oriented, and so couldn't give you a time range within which the sediment was deposited.

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Hmm, I think you are right. I think it would only be of use in dating rocks, not sediments. My mistake.

My understanding is that the uranium series dating method only requires a very tiny sample nowadays. If you've got even thin bands of flowstone interbedded with your clastic sediment this is probably your best bet.

If you can justify doing it (which would be achieved most easily in collaboration with those academic cavers who are involved with this sort of thing) then you may well be able to obtain financial help from the BCRA's Cave Science & Technology Research Initiative. Have a look on the BCRA's website in the first instance and start a dialogue about it with those who really know what they're talking about.

Pollen is probably your best bet. It is pretty certain there will be some there... 

Which is cheapest?

Les W said:

Pollen is probably your best bet. It is pretty certain there will be some there... 

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If we're talking about palynology, then I can't see that it would help. First of all, as far as I am aware there are only decent studies of vegetation changes since the last glacial maximum (c. 18 Ka). Secondly, one would have to an existing dated analysis for the neighbourhood, which is unlikely. Thirdly, one would have to assume that the pollen assemblage found in the deposits was a good approximation for the pollen assemblages inspected in the existing analysis - which is also unlikely. Most detailed pollen analyses tend to be on lake bed and bog deposits, hence having a bias towards wind distributed pollen.

If the deposit contains quartz that has been exposed on the surface before burial in the cave, 10Be/26Al cosmogenic isotope analysis is possible, but it is very expensive and is only really useful for very old deposits.    As noted above, uranium-thorium disequilibrium dating on the calcite cement is a possibility and can be done on very small amounts of material - but at best it would only give you a minimum age on the fill (the calcite cement may be very much younger than the gravel fill).

DM

If there are layers of flowstone immediately above and below your layer of sediment this would give you two dates between which is the age of your sediment layer of interest.

But cost twice as much . . . .

Again, thanks to all for the input.

Pitlamp said:

If you can justify doing it (which would be achieved most easily in collaboration with those academic cavers who are involved with this sort of thing) then you may well be able to obtain financial help from the BCRA's Cave Science & Technology Research Initiative. Have a look on the BCRA's website in the first instance and start a dialogue about it with those who really know what they're talking about.

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It's good to know that such grants exist and I am tempted to have a go but I'm not sure I have the skills and experience (or confidence for that matter, and it's not really my dig...)

langcliffe said:

If we're talking about palynology, then I can't see that it would help.

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From a brief reading of pollen dating it looks like it is possible to just look at the predominant pollen types and fit them into a period from recent times back to the oldest dryas, because certain climate types gave rise to specific species -  Google "Pollen Zones"

TheBitterEnd said:

langcliffe said:

If we're talking about palynology, then I can't see that it would help.

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From a brief reading of pollen dating it looks like it is possible to just look at the predominant pollen types and fit them into a period from recent times back to the oldest dryas, because certain climate types gave rise to specific species -  Google "Pollen Zones"

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It wouldn't help for the three reasons that I've given.

You need a totally dated section from nearby, as although pollen types do reflect changes in climate, unfortunately it takes differing amounts of time for the vegetation to catch up in different areas;  different areas have different plant types anyway; and climates change in both directions, so populations wax and wane. So the best you can do is to compare the assemblage contained within the sediment with the assemblages found in your nearby section and find a match.

A couple of illustrative examples: birch reached it's maximum in Hockam Mere in Norfolk with a distinct spike at about 9.5 KA, but trended to a maximum 1.5 KA later in Abernethy Forest.  Oak has been common in Hockham Mere for 9 KA, but has only occasionally been seen in Abernethy. So you need detailed information about the pollen assemblages for the locality.

As I said previously, it would also be difficult to directly compare assemblage found in the section with those found in cave sediment, as the environments in which the pollen was deposited would be different, with different plants. Palynological studies tend to be done on bog and lake sediments where a decent period of time can be sampled, and these favour wind-deposited pollen, as many of the plants aren't in the immediate vicinity. In the case of streams which enter caves, they would contain a fuller spectrum of pollen and so the assemblage would be difficult to compare with those found in the cross-section.

Palynology is a very useful tool, especially as a quasi-indicator of climate change over a period. It is not an appropriate tool for dating cave sediment.

There are several ways of dating cave fills, but all have their drawbacks and most are expensive (usually hundreds of ?? per sample), and in general more than one sample is normally dated to make sure they are reliable. So, if you want go ahead, make sure you have something datable and a good reason for doing it! If you have a really good reason for dating, then you may be able to get a tame scientist to do the work for you.

1. Dating of any associated speleothem calcite is relatively quick and cheap these days using U-Th methods. Assuming good clean, non recrystallised calcite with no detrital contamination and usuable amounts of U, this is OK back to c. 500 ka. This is probably the best method for getting a reliable date, and only requires a very small sample (milligrams will normally do). It has been used extensively in the UK, for example a thick stal layer in the bank of gravel by the Bridge in GB has been dated to c. 50 ka, but this dates the timing of stal deposition, not the gravel fill per se. Several labs in the UK do this (Bristol, NIGL at the British Geological Survey). Older speleothems can be dated by the U-Pb method, but this relies on decent U concentrations and low 'common' Pb values.

2. Palaeomagnetic dating. This can be used on fine grained muds, but will only tell you if the sediments are normal or reversed polarity. The last reversal was c. 780ka. There are several caves in the UK which are old enough have reversed polarity sediments (GB for example), but most are not. It also requires quiet deposition in still water to enable the Fe-rich clay particles to orient themselves to the prevailing magnetic field, rather than water flow. There are several UK labs that can do this (eg Plymouth Uni). Under ideal conditions it may be possible to determine the secular variation, but this is unlikely. Worth doing if you think the sediment is old enough, as this is beyond the range of U series dating.

3. Cosmogenic isotopes (10Be/26Al). This relies on quartz that has been exposed on the surface before burial in the cave, as the isotope ratio will vary with time on burial. However, this is not cheap, and is best suited to older samples. It has been used in the USA to date the upper levels of Mammoth Cave back several million years, but has not been used much in the UK.

4. Palynology - this won't give an age unless you have some really good indicator pollen species (assuming you get any), and even then is open to interpretation.

5. OSL. This relies on quartz in the sediment being exposed to sunlight prior to emplacement in the cave, so has limited applications for cave sediemnts, unless they are unequivocally derived from the surface. Samples are about ?600 a pop.

6. Radiocarbon. Needs organic carbon, and prone to contamination from other sources of carbon (ie limestone). Only dates back to c. 40ka, so not really used for interior cave deposits.

7. Fauna. If you find any bones, they may provide a rough date (mammal assemblage zones), eg hippo is normally associated with Marine Isotope Stage 5e at c. 120 ka. You'll need Andy Currant at the BM!

Best way to date a cave fill is to have some speleothem encrusted mammal bones in it, ideally very old early hominid remains in it, adjacent to laminated clay intercalated with peat.... That way you'll get an archaeologist to do the dating for you!

I would try U-series on the calcite in the first instance. Its not cheap, but there are grants available, but you'll need a good proposal to get the funding.

Andy

Andy, thanks for the comprehensive response, I hope you don't mind but I have sent you a PM with a follow up question.