tag:blogger.com,1999:blog-5519292617097628087.post196362637943147601..comments2024-01-02T16:09:12.886-07:00Comments on Chinleana: Direct U-Pb Dating of Fossil Bone Material and Does the Presence of Paleocene Dinosaurs Even Matter?Bill Parkerhttp://www.blogger.com/profile/05941940882532354219noreply@blogger.comBlogger8125tag:blogger.com,1999:blog-5519292617097628087.post-36677655548237058032011-01-27T01:19:32.472-07:002011-01-27T01:19:32.472-07:00très bon blog,bravo de la qualité et du soin que v...très bon blog,bravo de la qualité et du soin que vous y apportez!muondohttp://www.muondo.orgnoreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-89046503406529392712011-01-21T10:19:08.591-07:002011-01-21T10:19:08.591-07:00Jason - I suspect that my rant lacked a bit of cla...Jason - I suspect that my rant lacked a bit of clarity.<br /><br />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.<br /><br />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).<br /><br />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"<br /><br />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 <i>Science</i> 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?).<br /><br />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 <i>Geochim. Cos. Act.</i>). 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.220myahttps://www.blogger.com/profile/06403919493457640549noreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-39718575894012212742011-01-18T17:06:35.965-07:002011-01-18T17:06:35.965-07:00Erratum: for Rare Earth Element read Uranium. Was ...Erratum: for Rare Earth Element read Uranium. Was thinking about another geochemical point and absent minded typed it.Bilirubinhttps://www.blogger.com/profile/16404564345543884164noreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-42215154003108400972011-01-18T15:08:21.469-07:002011-01-18T15:08:21.469-07:00I think your caveats are quite appropriate regardi...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.<br /><br />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.Bilirubinhttps://www.blogger.com/profile/16404564345543884164noreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-43407343121731757402011-01-16T17:41:44.860-07:002011-01-16T17:41:44.860-07:00I forgot to mention one other thing:
The fossil b...I forgot to mention one other thing:<br /><br />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.220myahttps://www.blogger.com/profile/06403919493457640549noreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-63646497755052530602011-01-16T17:36:40.566-07:002011-01-16T17:36:40.566-07:00With respect to the method itself of dating fossil...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.<br /><br />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.<br /><br />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.<br /><br />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.220myahttps://www.blogger.com/profile/06403919493457640549noreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-79506419264949745182011-01-16T04:51:39.195-07:002011-01-16T04:51:39.195-07:00I agree entirely with you Bill - even if a small p...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.Paul Barrettnoreply@blogger.comtag:blogger.com,1999:blog-5519292617097628087.post-28529770201355459712011-01-16T03:08:55.457-07:002011-01-16T03:08:55.457-07:00From a scientific perspective it's certainly i...From a scientific perspective it's certainly interesting, but I'd agree Bill that it basically doesn't matter.<br /><br />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.Anonymousnoreply@blogger.com