ASTRONOMICAL CALIBRATION OF GEOLOGICAL TIME AND INTERCALIBRATION WITH 40AR/39AR AND U/PB DATING

Orbital tuning of climate proxy records to astronomical target curves provides a fully acceptable alternative method for absolute age calibration of geological time that is totally independent from radio-isotopic dating. Starting with the CLIMAP and SPECMAP projects, astronomical tuning of continuous deep-marine successions combined with integrated high-resolution stratigraphy now underlies the - age calibration of the - standard geological time scale for the Neogene and this tuning is already being extended into the Mesozoic. Tuning of Paleogene and Cretaceous sections including ODP cores is greatly facilitated by the recent publication of full numerical solutions for the Solar System and the presence of a very stable 400-kyr component in eccentricity as prime target. In this way a new generation of geological time scales with an unprecedented accuracy and resolution will be generated that is not prone to a promulgation of new time scales. Such an accurate high-resolution time scale is of prime importance not only for understanding astronomical climate forcing but also for unravelling fundamental problems in Earth Sciences such as the origin of 3rd order sequences and the ongoing debate about the tectonic versus climate control on the Messinian salinity crisis in the Mediterranean. In addition the new time scale will help solving fundamental astronomical issues such as the degree of chaotic behaviour of the Solar System. Of crucial importance for extending the tuning into the Mesozoic is the inter¬calibration of astronomical and 40Ar/39Ar time by single crystal sanidine dating of primary ash layers in astronomically dated sections. Based on data from the Melilla Basin, the intercalibration arrives at an astronomically calibrated age of 28.20 Ma for the FCT sanidine dating standard. The error is rather small because the uncertainties in the decay constants and the dating standards have been largely eliminated. Consequences of the intercalibration for the ages of major geological events such as at the P/E and K/T boundaries will be discussed also in the light of the ongoing tuning efforts. Evidently the K/T boundary interval marks the battleground in which the (geological) dating game will at least be partly decided.