The role of magnetostratigraphy in the development of geological time scales

Abstract

One of the major targets of geochronology is the “calibration” of chronostratigraphic boundaries (defined principally from biostratigraphic information) in terms of numerical ages derived from isotopic age data. Few isotopic age determinations correspond precisely with such chronostratigraphic boundaries. Furthermore the analytical (and other) uncertainties in isotopic age data are often greater than the duration of the age or zone whose boundaries need to be dated. Consequently, it is necessary to synthesize isotopic age data which are distributed both in space and time in order to obtain age estimates for the geological boundaries themselves. This synthesis requires the introduction of some linearizing operator on which to base the necessary interpolations or extrapolations. Commonly used linearizing operators involve assumptions of uniform rates of sedimentation and/or biological evolution. However, such assumptions are often highly questionable. An alternative and potentially more reliable linearizing operator utilizes the assumption of a uniform seafloor spreading rate at certain midocean ridges for certain periods in the (post mid‐Jurassic) geological past. The use of this operator for the Chronometric calibration of chronostratigraphic time scales is based upon the fact that successive polarity changes of the geomagnetic field are recorded simultaneously in the pattern of lineated magnetic anomalies formed by seafloor spreading and also in the paleomagnetic record of sedimentary sequences in which the chronostratigraphic boundaries can be defined. This approach provides a number of advantages over the use of more traditional linearizing operators, and these are reviewed in this paper. It has been used in several recent geological time scales. Its applicability depends upon a number of factors, including the reliability of the basic seafloor spreading polarity time scale and that of the paleomagnetic record in isotopically and/or biostratigraphically dated sequences. This paper reviews these topics, in particular the use of the constant spreading rate operator, and suggests that the full implications of “apparent spreading rate” changes which have been identified in previously published polarity time scales have not been fully addressed. Necessary remedial action is outlined, but the paper does not attempt to produce a further revision to the polarity time scale. A number of questions of terminology in magnetostratigraphy are discussed and the desirability of establishing a new rationalized or “systematized” magnetochron labeling scheme is reviewed.