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Klaudia Kuiper, Vrije Universiteit Amsterdam (Netherlands)
Alan Deino, Berkeley Geochronology Center (United States)
Frits Hilgen, Utrecht University (Netherlands)
Wout Krijgsman, Utrecht University (Netherlands)
Paul Renne, Berkeley Geochronology Center (United States)
Jan Wijbrans, Vrije Universiteit Amsterdam (Netherlands)
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Accurate and precise measurement of geological time is a prerequisite for understanding Earth's history and the Geological Time Scale plays a crucial role in this endeavor. Calibration of the Geological Time Scale is achieved by independent radio-isotopic and astronomical dating, but these techniques yield discrepancies of ∼1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40Ar/39Ar ages of 6-7 Ma tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine (FCs), the most widely used standard in 40Ar/39Ar geochronology. Single crystals of sanidine from these astronomically dated tephras were dated exhaustively in two labs (BGC and VU) and yield essentially indistinguishable results. Using the tephras as standards, the age of FCs is solved as for an unknown. The solution depends on the total K decay constant, but is remarkably insensitive to the value used and to its uncertainty. In the present case we used a value of (5.463 ± 0.214) 10-10 yr-1, as derived by Min et al. (2000) from statistically rigorous evaluation of all available activity data. The resulting calibration produces a more precise and slightly older age of 28.201 ± 0.046 Ma and reduces the 40Ar/39Ar methods absolute uncertainty from ∼2.5 to 0.25 % (95% c.l.). We argue that this astronomically calibrated FCs age should be incorporated in the next standard Geological Time Scale to recalculate all other 40Ar/39Ar ages. The new age for FCs reconciles the 40Ar/39Ar geochronometer not only with the astronomical time scale, but also with the better-calibrated U/Pb system. Only in this way a mutually consistent age calibration of the GTS will be assured. Our integrated approach bears the bright prospect of generating a stable time scale with an unprecedented accuracy, precision and resolution that will not have to undergo significant changes any more.
For the moment, it provides tight constraints for the astronomical tuning of pre-Neogene successions, which we demonstrate by using recalculated published 40Ar/39Ar ages from the K-T boundary interval with our astronomical FCs age of 28.201 to pinpoint the 405 kyr eccentricity minimum correlative to the K-T boundary in the Zumaia section, Spain, resulting in a mutually consistent age of ~65.95 Ma for the K-T boundary.
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