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RE: Precise K-T boundary dating



> -----Original Message-----
> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of 
> Brad McFeeters
> Sent: Friday, October 23, 2015 11:03 AM
> To: dinosaur@usc.edu
> Subject: RE: Precise K-T boundary dating
> 
> When did ~66 Ma replace ~65 Ma, 

Just to be clear, this should be "when did the estimate of ~66 Ma replace the 
estimate of ~65 Ma"

> and why did that take so long to realize the mistake?  The 65 Ma date is very 
> entrenched in pop
> culture, unfortunately, and is even referenced in films as recent as Jurassic 
> World.
> Brad
> 
TL;DR answers: In 2008; because astrochronology and precision in both Ar- and 
U-based dating needed to reach sufficient levels; and
because popular culture (and professionals well entrenched in old habits) are 
slow to adopt some new changes.

Longer answer:

The key paper is https://eps.berkeley.edu/sites/default/files/500.pdf 

However, this was real a series stepwise discoveries, and has been happening 
over the last 10 years.

This had to do with recalibration of the Ar/Ar and K/Ar scale. Ar dating is 
cheaper, and is more widely useable in a greater variety
of geologic settings, than most other deep time radiometric dating techniques. 
But geochemists have been suspicious that it might
give slightly anomalous results: specifically, results that are just a little 
(~1%) too young.

U/Pb dates have been theoretically considered more secure, but are more 
expensive to run and are appropriate for fewer geological
settings. However, recent comparisons of U/Pb dates against 
astronomically-generated sedimentary cycles (for instance, the 2008
paper linked above, or this more recent one: 
http://www.unh.edu/esci/people/wcc/PDF%20of%20Papers/Machlus%20et%20al%20%202015.pdf)
has confirmed the accuracy of U/Pb dating.

(And just to point out that people get entrenched in their ways: the 
geochronologists in the
https://eps.berkeley.edu/sites/default/files/500.pdf paper still haven't 
updated to the proper chronostratigraphic nomenclature that
no longer uses the Tertiary!)
Hope this helps.

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu         Phone: 301-405-4084
Senior Lecturer, Vertebrate Paleontology
Office: Geology 4106, 8000 Regents Dr., College Park MD 20742
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
Phone: 301-405-6965
Fax: 301-314-9661              

Faculty Director, Science & Global Change Program, College Park Scholars
Office: Centreville 1216, 4243 Valley Dr., College Park MD 20742
http://www.geol.umd.edu/sgc
Fax: 301-314-9843

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        8000 Regents Drive
                        University of Maryland
                        College Park, MD 20742-4211 USA 


> > Date: Fri, 23 Oct 2015 08:03:18 -0400
> > From: tholtz@umd.edu
> > To: dinosaurtom2015@seznam.cz; dinosaur@usc.edu; bcreisler@gmail.com
> > Subject: RE: Precise K-T boundary dating
> >
> > Several issues to unpack here:
> >
> > 1) There is no "K/T" boundary. The Tertiary has been formally 
> > 'decommissioned'. It is the K/Pg boundary.
> >
> > 2) Do you mean "precise" or do you mean "accurate"? These are not the
> > same thing. Accurate is closest to the target; precise means "resolved to 
> > the smallest degree".
> >
> > 3) Like most Phanerozoic boundaries, the boundary is NOT based on
> > time. It is set on a marker in the geologic record. We then can
> > attempt to estimate the numerical age of that marker, but such estimates 
> > are subject to experimental error and (as is relevant
in
> this case) recalibration of the technique based on new information.
> >
> > Further information: in stratigraphy, what you technically define is
> > the base of the upper unit; the top of the lower one is therefore
> > contingent on your selection of the upper unit. So the bottom of the Danian 
> > (lowermost Stage of the Paleogene Series) is
defined,
> and the top of the Maastrichtian (uppermost Stage of the Upper Cretaceous 
> Series) stops at the base of the Danian.
> >
> > The formal base of the Danian is (as discussed here:
> > http://www.stratigraphy.org/GSSP/Danian.html) "the reddish layer at
> > the base of the 50cm thick, dark boundary clay in a tributary of the
> > Oued Djerfane, west of El Kef, Tunisia, where it coincides with the Iridium 
> > Anomaly fallout from a major asteroid impact." Thus,
at
> the GSSP for the Danian, the iridium anomaly is present in the boundary clay.
> >
> > Short version: geological boundary picked first; numbers estimated second.
> >
> > 4) Here are the dates in some papers published this year:
> >
> > Sprain et al.
> > (http://gsabulletin.gsapubs.org/content/127/3-4/393.short) found the impact 
> > layer at 66.043 ± 0.010 Ma (or ± 0.043 Ma
considering
> systematic uncertainties).
> > Schoene et al. (http://www.sciencemag.org/content/347/6218/182.full) found 
> > 65.968 ± 0.085 Ma.
> >
> > These are the **SAME NUMBER**, given the uncertainty.
> >
> > Neither actually dated at El Kef, but given that they dated material
> > which can be reasonably tied into the same event horizon present in 
> > Tunisia, that isn't bad.
> >
> > Hope this helps,
> >
> > Thomas R. Holtz, Jr.
> > Email: tholtz@umd.edu         Phone: 301-405-4084
> > Senior Lecturer, Vertebrate Paleontology
> > Office: Geology 4106, 8000 Regents Dr., College Park MD 20742 Dept. of
> > Geology, University of Maryland http://www.geol.umd.edu/~tholtz/
> > Phone: 301-405-6965
> > Fax: 301-314-9661
> >
> > Faculty Director, Science & Global Change Program, College Park
> > Scholars
> > Office: Centreville 1216, 4243 Valley Dr., College Park MD 20742
> > http://www.geol.umd.edu/sgc
> > Fax: 301-314-9843
> >
> > Mailing Address:        Thomas R. Holtz, Jr.
> >                         Department of Geology
> >                         Building 237, Room 1117
> >                         8000 Regents Drive
> >                         University of Maryland
> >                         College Park, MD 20742-4211 USA
> >
> > > -----Original Message-----
> > > From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On
> > > Behalf Of Poekilopleuron
> > > Sent: Friday, October 23, 2015 7:34 AM
> > > To: dinosaur@usc.edu; bcreisler@gmail.com; tholtz@umd.edu
> > > Subject: Precise K-T boundary dating
> > >
> > > Good day,
> > >
> > > I would like to ask, what is the current precise date of the K-T
> > > boundary - there are various dates given on internet, like 66,07
> > or 66,21
> > > or 66,038 million years. Which is the most relevant one? Also, is
> > > the K-T boundary and the Chicxulub impact the very same date or does
> > > it differ by a few hundred thousand years? Thank you, Tom
> >
>