Composite Time Lines: A Means to Leverage Resolving Power from Radioisotopic Dates and Biostratigraphy

Species origination and extinction events far outnumber radioisotopically dated events in the ancient stratigraphic record. In order to calibrate rapid rates of Mesozoic and Paleozoic change and to estimate the ages of paleobiologic events it would be ideal to have multiple dated events in single stratigraphic sections. This condition is rarely realized and the practical alternative is to build composite sections that combine information from many different locations. The compositing process takes advantage of all available evidence of relative age to produce high resolution time lines; i.e. ordered sequences of individual events whose average spacing is much finer than the duration of biostratigraphic zones and can approach the uncertainty intervals of the highest precision radioisotopic dates. Dated events are included in the compositing process from the outset. As a result the sequencing procedure is more efficient and the dated events find their optimal positions in the time line independent of any biostratigraphic zonal schemes. The sequencing procedures follow simple logical rules that may be learned from tiny data sets. When usefully large numbers of events are involved, however, the sequencing must be undertaken by computer and there is seldom a unique solution that best fits the field data. The range of positions in sequence that an event may occupy across the full set of equally best-fit solutions is a measure of the resolving power of the event. As new high-precision dates and detailed range charts continue to become available, the quality of the time lines will improve and they will become increasingly viable alternatives to zonal time scales in the older parts of the Phanerozoic.