Field of Science

Direct U-Pb Dating of Fossil Bone Material and Does the Presence of Paleocene Dinosaurs Even Matter?

Interesting paper, the focus of which is twofold: 1) demonstate the applicability of new U-Pb dating technology on fossil bone material; and 2) support the hypothesis that some non-avian dinosaurs survived into the earliest Paleocene. I'll need someone with more background on radioisotopic dating techniques to evaluate this paper for me, especially regarding aspects of Pb loss, diagenetic recrystallization, etc., but the implications are exciting if indeed this technique can be used to directly date fossil materials.

I think the idea of Paleocene dinosaurs will be debated for a long time, but really what does it matter?  It is not as if non-avian dinosaurs made it into the Paleocene, that they made it very far in.  Our base geologic timescales are based upon stratigraphic ranges of marine organisms, which are only now starting to be well calibrated. Correlation of these intervals between the marine and terrestrial realms is tricky at best and I would not be surprised if as we get more precision and accuracy in our dating techniques we find that there is a little slop around boundary defining events (i.e. extinctions of major terrestrial groups) and the exact dates defining the marine-defined boundaries. 

But does it really matter if the last non-avian dinosaurs died out entirely by the exact last day of the Cretaceous, or if a few locally squeezed into the earliest Paleocene?  These dates are so similar in relation to the overall earth timescale that they are pretty much simultaneous and really have no bearing on our ideas regarding the extinction of non-avian dinosaur. Same goes for the Triassic, if we find a phytosaur in the very earliest Jurassic strata is this really significant other than for general bean counting? I for one would not be surprised one bit if such a specimen was found; however, these data would not offer much in resolving the real question, which is why these major extinctions occurred in the first place.

Fassett, J.E., Heaman, L.M., and A. Simonetti. 2011. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico. Geology 39:159-162. doi: 10.1130/G31466.1

Abstract - Vertebrate fossils have been important for relative dating of terrestrial rocks for decades, but direct dating of these fossils has heretofore been unsuccessful. In this study we employ recent advances in laser ablation in situ U-Pb dating techniques to directly date two dinosaur fossils from the San Juan Basin of northwestern New Mexico and southwestern Colorado, United States. A Cretaceous dinosaur bone collected from just below the Cretaceous-Paleogene interface yielded a U-Pb date of 73.6 ± 0.9 Ma, in excellent agreement with a previously determined 40Ar/39Ar date of 73.04 ± 0.25 Ma for an ash bed near this site. The second dinosaur bone sample from Paleocene strata just above the Cretaceous-Paleogene interface yielded a Paleocene U-Pb date of 64.8 ± 0.9 Ma, consistent with palynologic, paleomagnetic, and fossil-mammal biochronologic data. This first successful direct dating of fossil vertebrate bone provides a new methodology with the potential to directly obtain accurate dates for any vertebrate fossil.

8 comments:

  1. From a scientific perspective it's certainly interesting, but I'd agree Bill that it basically doesn't matter.

    However fro the creationists and til-foil hat brigade it's ABSOLUTELY THE MOST IMPORTANT THING EVER !!!111 etc. since it PROVES SCIENCE IS WRONG and that DINOSAURS ARE STILL ALIVE! and that is all we are going to see about this story I suspect and we are going to face questions ad nauseum about this for all eternity. Joy.

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  2. I agree entirely with you Bill - even if a small population survived the K/P extinction it would have been of exceptionally limited macroevolutionary or macroecological significance. Besides, they miss the point, dinos got through anyhow, just with a more restricted body plan and range of body sizes. Quite trivial. I can't comment on the technique, but some geochronologists colleagues of mine have been rather scathing about it.

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  3. With respect to the method itself of dating fossil bone - it is problematic. The whole purpose of using zircons is that they're close to a "closed system" - they don't allow much ion exchange after crystallization. But even zircons are not fully closed systems - that is why we get lead loss and why there are methods such as thermal annealing plus chemical abrasion to deal with it.

    Fossil bone, and the minerals that infill the spaces in fossil bone, are horrible in this respect. They have a very open crystalline structure that allows a lot of ion exchange after burial and during diagenesis. There's a reason why we only use enamel and not fossil bone for stable carbon and oxygen isotopic analysis, and it's even more of a problem for uranium and lead. These infilling minerals (e.g., quartz and calcite) and bone are quite an open system - you have no idea how much younger than the depositional age the date really is. It’s the same problem for U-Pb ages from carbonate nodules.

    The problem is actually deeper than this. Any appreciable amounts of uranium in fossil bone has been introduced after deposition anyway! Uranium is deposited by saturated ground-waters, and this happens any time from thousands to millions of years after deposition of sediments (and therefore burial of the fossil). So you are getting an age of the emplacement of uranium, not depositional age, and there is no way to tell how far away from the depositional age you are. This is why we don't use U-Pb ages from uranium ores in the Chinle Formation, even though we could easily date them.

    So, it really doesn't matter that they heuristically happen to get a couple of samples that match up well with other ages - this is not a reliable method.

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  4. I forgot to mention one other thing:

    The fossil bone U-Pb ages they report are weighted mean ages, which is typical for U-Pb dates. But you'll notice that the individual spot ages form a whole range of dates, and do not form clusters. Statistically speaking, weighted means are only a valid method when you're data have a normal distribution around the mean. The data in this paper do not cluster around a mean whatsoever. They are getting false precision and accuracy by reporting the weighted mean rather than a more conservative measure.

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  5. I think your caveats are quite appropriate regarding the dating technique, but am uncertain how applicable they are in this instance. The bone from the underlying Kirtland Formation shows evidence for younger incursions of Uranium while preserving the underlying older signal that matches the age constraints from tuffs. If Pb leaching were present in the younger Ojo Alamo Sandstone, it has maintained its consistency with its own overlying Paleocene age constraint. In other words, according to 220mya's scenario the known Mesozoic U signal was reset past the Cretaceous constraints, but not so far as to give itself away by defying the overlying Paleocene Ar constraint. Possible of course, especially if there was no initial U uptake despite knowing other Mesozoic bones did uptake U (and my understanding of Rare Earth Element uptake in bone is at least partially driven by the reducing environment of the decomposing organic material--how long does this last postdeposition?), but seems a bit special pleading at first glance. The statistical argument you raise would definitely bear closer scrutiny I think.

    I do scratch my head at the lack of the younger signals seen in the Kirtland Formation bone. Presumably porosity differed in the two formations (although, you know, sandstone), some impermeable layer kept these two samples separate from the younger U incursion, or whatever, to preserve these different signals. Still, it would have been nice to have seen evidence for the same two 20-40 ma U incursions. Would have bolstered their case in my eyes.

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  6. Erratum: for Rare Earth Element read Uranium. Was thinking about another geochemical point and absent minded typed it.

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  7. Jason - I suspect that my rant lacked a bit of clarity.

    1. I'm not so much criticizing the accuracy of their specific results, as the generality of the method. The big problem is that Uranium emplacement in organic material is a diagnetic process, that happens a variable amount of time after deposition.

    2. Their dates are unrealistically precise - if they were a bit more honest about the uncertainty, the Paleocene age would have a lot more slop in it (and the uncertainty would include a chunk of the latest Cretaceous).

    You said: "according to 220mya's scenario the known Mesozoic U signal was reset past the Cretaceous constraints, but not so far as to give itself away by defying the overlying Paleocene Ar constraint. Possible of course...but seems a bit special pleading at first glance"

    Nope - not special pleading at all! Lead (Pb) loss is a pervasive concern. There are many many examples of it for zircon U-Pb work, and it would be even more symptomatic for open systems like bone, wood, etc. And many of these examples show that the age changes by only 1-5% depending on the severity of lead loss. Check out Mundil et al 2004 in Science for example. Again, I'm not saying their specific dates are wrong, but these problems make it an unreliable method unless you have other age data (and if you have precise U-Pb zircon and/or Ar/Ar dates, why bother?).

    I just noticed they also made no attempt to correct their reference Ar/Ar ages for the 1% decay constant bias when comparing to U-Pb ages (e.g., Renne et al. 2010 in Geochim. Cos. Act.). Given that the U-Pb bone ages are likely to be younger than the depositional age, failure to make this correction would make the Ar/Ar ages seem closer to the U-Pb bone ages than they actually are.

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  8. très bon blog,bravo de la qualité et du soin que vous y apportez!

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