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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

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
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
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