RADIO-ISOTOPIC CALIBRATION OF THE PHANEROZOIC GEOLOGICAL TIMESCALE: PROBLEMS OF ACCURACY, PRECISION AND BIAS

Radio-isotopic dating of biostratigraphically controlled rocks, minerals and parts of mineral grains provides vital numerical age calibration of timescales for the evolution of the Earth and for the evolution of life on Earth. Stratigraphers, palaeontologists, and sedimentologists utilise isotopically generated numerical timescales to underpin interpretations of the age and tempo of various geological processes such as extinction and evolution rates, rates of sedimentation, duration and periodicity of cyclic sedimentation, and for establishing the “absolute” ages of formally defined geological boundaries and time duration of formal stratigraphical subdivisions of the Phanerozoic. This said, what is the best isotopic dating system for timescale calibration? The answer is that there is no single “best” system. Different isotopic systems offer different advantages and disadvantages of accuracy, precision and practical application and there is now clear documentation of systematic bias between some isotopic systems and between different methods using the same isotopic system. It is vital that an integrated multidisciplinary approach be taken in timescale calibration work, involving integrated biostratigraphic and radio-isotopic analyses of same samples. A multi-method approach to radio-isotopic dating, for example involving both U-Pb and Ar-Ar systematics and methods to date same samples, can provide the best of both worlds in terms of age (accuracy) and tempo (precision). However, care needs to exercised in interpreting numerical radio-isotopic age data. There are cases where isotopic ages from different isotopic methods and systems have been combined and integrated without due regard to known bias and variations in levels of accuracy and precision between these systems. This can, and has, led to erroneous interpretations in the geological literature. Recognition of bias between isotopic systems has even led to proposals of dual equivocal timescales for the same geological Period. Various examples illustrating problems of accuracy, precision and bias in radio-isotopic calibration of the Phanerozoic geological timescale will be presented.