B S Alarm Sounding

[Rockwood]

Do those who question the reliability of radiometric dating on the basis that salts of the elements used are water soluble have a leg to stand on ? Something tells me it may be a true but irrelevant statement that is useful to their agenda.

[Mediospirifer]

I'm no expert, but I know a little about radiometric dating and how it works. Personally, (and I could be wrong; if so, I hope someone else will correct me!) I think the short answer to your question is "no, you're correct to think that's BS".

I do know that in some cases, such as potassium-argon dating, the decay product (Ar) is gaseous, therefore if the rock in question was water-permeable there would not be detectable Ar left in the sample. As far as dating goes, that would make the rock appear younger than it really is. In the case of uranium-lead dating in zircon crystals, the crystals are not water-permeable, so any water solubility of either element would be irrelevant. In both cases, the original rock would not have formed with the decay products inside--argon would have dissipated before the molten rock cooled, and crystallizing zircons exclude lead.

I suspect that the case you've heard this applied to most might be radiocarbon dating. That's certainly the one that I've heard the most complaining about from those who don't want to believe it! In that situation, I would say that water permeability removing carbon from the sample is irrelevant, as a diffusion process doesn't preferentially select one isotope over another. If carbon is being added to an object, it will be added in the modern ratio (at whatever time it occurs; the intrusion could have occurred several thousand years ago). The result of this will be that the sample looks younger!! (I say "sample" or "object" here because carbon dating isn't normally used on rocks, only organic remains. Very few rocks are young enough to still have carbon14 present)

One thing I am confident of: the experts in the field know about such potential problems and how to correct for them. I do know that radiocarbon dating, when compared to dendrochronology or ice cores, does tend to show younger ages than is obtained by counting layers.

Thanks for asking, and I hope this is helpful!

[Mediospirifer]

I'll just add, briefly:

The experts in dating methods typically look for pristine samples, regardless of the method used. They will also sample multiple locations to get a range of possible dates--for instance, they'll collect a few dozen zircons from an ancient lava flow, and throw out the ones that are damaged. I've seen electron microscope images of zircons to show that the area sampled within the crystal was deep enough in the crystal's interior, and far enough from any cracks, that the selected site can be considered trustworthy.

If the process was as unreliable as some people want to pretend, the results from a sampling run would be all over the map, or else not reproducible. The fact that the process is well understood and accepted as accurate by the scientific community tells me that when new results are announced from an established lab, they should be considered as accurate until someone else (who is also established in the field) tries to reproduce the results and comes up with a different date.

[verydeadthings]

It depends on what radiometric dating method you're using. As Mediospirifer has said, water solubility is not really an issue, to my knowledge, in uranium-lead or fission track dating. Even in open system cases where there is an exchange in isotopes/elements between the material and the environment, this can be modeled into the equations. Uranium-thorium dating in corals is an example where exchange with seawater is an issue. I'll just pull a few paragraphs from a paper I wrote for a coastal marine geology course a few years back. I've removed some unnecessary references from the original text.

Determining the age of older corals is often difficult because of diagenetic alteration and other variables. The most commonly used method of dating fossil corals is uranium-thorium dating. More specifically, this dating method utilizes the radioactive isotope 238U and two intermediate daughters in its decay chain, 234U and 230Th. The U-series chronometer is based on two conditions of disequilibrium: an extreme chemical fractionation between uranium and thorium during weathering processes, resulting in ocean waters with some uranium but virtually no thorium, and isotopic fractionation related to 238U decay in crystal lattices between 234U and 238U, resulting in ocean waters with excess 234U compared to 238U (Stirling and Anderson 2009).

When corals grow their carbonate skeletons they incorporate some uranium into their crystal lattice with the same isotopic composition as seawater. 234U in the aragonitic skeleton of the coral decays to 230Th with a half-life of approximately 245,000 years, and 230Th decays to 226Ra with a half-life of approximately 75,000 years. Values of 230Th are so low in the ocean that it is assumed that there is no 230Th in the aragonitic skeleton at the time of formation. As 234U decays, it approaches secular equilibrium with 230Th. These isotopic ratios can then be used to date the sample. As the system approaches secular equilibrium error ranges become larger and eventually any differences are undetectable. The upper limit of age dating using this method has already practically been reached, with the dating of a fossil coral from Henderson Island to approximately 600,000 years old (Anderson et al. 2010).

Two requirements for the accurate dating of corals with this method are that the coral has behaved as a closed system since incorporation of these radionuclides and that the 234U/238U ratio of seawater was not different when the coral was alive. The 234U/238U isotopic ratio is nearly constant in modern seawater (Hamelin 1991), but “…model calculations considering changes in riverine U flux into the ocean imply that permil- to percent-level variability in the marine 234U can potentially occur on Quaternary and even glacial-interglacial timescales due to a shift in the continental weathering regime” (Stirling and Anderson 2009).

The other requirement for accurate U-series ages is that the coral skeleton has behaved as a closed system since formation. This is often not the case, due to diagenetic alteration involving adsorption of nuclides from meteoric water, altering 234U/238U ratios. Traditionally, coral samples with ratios of 234U/238U that differed significantly from modern seawater values were discarded. More recently, open-system models have been developed in an attempt to date corals that have not behaved as a closed system (Stirling and Anderson 2009).

Andersen, Morten B., Claudine H. Stirling, Emma-Kate Potter, Alex N. Halliday, Steve G. Blake, Malcolm T. McCulloch, Bridget F. Ayling, and Michael J. O’Leary. 2010. “The Timing of Sea-Level High-Stands during Marine Isotope Stages 7.5 and 9: Constraints from the Uranium-Series Dating of Fossil Corals from Henderson Island.” Geochimica et Cosmochimica Acta 74 (12): 3598–3620.

Stirling, Claudine H., and Morten B. Andersen. 2009. “Uranium-Series Dating of Fossil Coral Reefs: Extending the Sea-Level Record beyond the Last Glacial Cycle.” Earth and Planetary Science Letters 284 (3): 269–283.

[Rockwood]

Thank you for the information. I apologize for the obtuse nature of the question. The professor in the video I had seen seemed to condemn the whole process on the basis that the radioactive elements commonly used all formed salts that were water soluble. I had a feeling it sounded more like part of the why you can't just . . . statement that one might expect near the beginning of an introduction to such a technical issue.

[Mediospirifer]

No apologies needed! There's a lot of BS (or simple misunderstanding) around.

I have to admit, when I read "The professor in the video..." saying this as a denigration of the whole process, my reflex response was "Professor of what? Theology?" I would be very surprised if the answer was "Physics"!

A PhD or professorship does not indicate expertise in all fields, after all, even if some BS artists like to act as if it does. I can think of several well-respected people in science who have spouted off complete BS in fields outside their own area of expertise.

[Doctor Mud]

I only have experience with radiocarbon dating, so I can talk about that method.

You always get a disclaimer with a radiocarbon date - the date provided is based on the carbon in the sample and the treatment process used before the sample was analysed.

There are many processes that can affect the carbon content of a substance or organism and these have to be kept in mind.

In the case of radiocarbon dating, the issue you refer to is mobility of salts in groundwater (carbonates). If you were to date a layer where younger carbonates in groundwater had percolated down and crystallised, this would affect your date and give you a younger date.

You can get dates that seem way too old from dating plants or animals that have been taking up old carbon. This happens in the ocean. If you were to radiocarbon date a living seashell, you might get a date that is several hundred years old. This is commonly corrected for with a marine reservoir effect correction. You can even get dates that are too old on human bones where the humans have a large quantity of fish or shellfish in their diet.

Aquatic plants can also give an anomalously old age because they can use dissolved salts (dissolved inorganic carbon or DIC). Think of a plant living in a deep lake with limestone in the catchment. The carbonates dissolve and flow into the lake and can be taken up by some aquatic plants. I try to avoid dating aquatic plants for this reason and prefer seeds from terrestrial plants that use atmospheric carbon.

As the others have alluded to, the way around this is to understand the system and correct for it. For radiocarbon there are pre-treatment methods to remove young carbon that is mobile in the ground water. This is usually an acid-base-acid treatment (referred to as AAA). The acid will remove carbonates and the base will remove humic ("juice" from decomposed organic matter) substances that have soaked into the sample.

I suspect that the young dates on dinosaur bones that have come out recently could come from precipitated carbonates. It could even come from minute traces of organics.

Anyway - the short answer is that we know that carbon is mobile and we use methods to prevent this affecting our results. Using multiple dating methods based on different assumptions is always a good move too (if you can afford it!).

Edited January 23, 2015 by Doctor Mud