Astronomical Dating of the K-T Boundary
High-precision radio-isotopic dating techniques have advanced to the point where analytical uncertainties of 0.1% can be reached for rocks ranging in age from Paleozoic to Cenozoic. However, detailed comparison of independent radio-isotopic and astronomical dating techniques revealed discrepancies of ~1.0% or more in several Mediterranean Neogene sections, revealing fundamental limitations to our ability to reconstruct Earth history. To overcome this setback, astronomical and 40Ar/39Ar ages of tephras in marine deposits in Morocco were used to calculate an intercalibrated age for Fish Canyon sanidine (FCs), the most widely used standard in 40Ar/39Ar geochronology. The proposed intercalibrated age for FC sanidine of 28.201 ± 0.046 Ma (Kuiper et al., 2008) removes the age bias and would reduce the total uncertainty in the 40Ar/39Ar method from ~2.5% (or ~1600 kyr at 65 Ma) to <0.25% (or <165 kyr at 65 Ma).
An important application of the astronomically calibrated 40Ar/39Ar method is to provide constraints for the astronomical tuning of pre-Neogene sequences, down to the correlative 405-kyr earth-orbital eccentricity cycle. For example, the 405-kyr cycle can be identified in the K-T boundary section at Zumaia, Spain. Small-scale precession-related cycles are less well developed in the limestone beds of eccentricity related cycles, indicating that such beds correspond to eccentricity minima because eccentricity modulates precession amplitude. The K-T boundary corresponds to a 405-kyr eccentricity minimum and recalculated 40Ar/39Ar ages of tektites and in the IrZ-coal tephra beds allow to pinpoint the corresponding 405-kyr minimum in the astronomical solution. Using the average precession period at that time in combination with the number of precession related cycles, the astronomical age of the K-T boundary arrives at 65.957 or 65.940 Ma for the La2004 and Va03_R7 solutions, respectively. However, different interpretations of this interval around K-T are possible.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009