Orbitally-tuned time scales and mapping the first appearances and extinctions of species in time and space

Efforts aimed at tuning the geologic time scale to variations inEarth’s orbit have taken us beyond the limits of radiometricdating and now allow us to make meaningful, quantitativeestimates and comparisons of oceanographic processes takingplace millions of years ago. An example of the usefulness of theorbitally tuned time scales is given by the ODP Leg 138 Sites inthe eastern equatorial Pacific. The complete, orbitally tunedsections recovered on this leg have provided biostrati-grapherswith a high- resolution chronostratigraphic framework withwhich to define the first and last appearances of species ateach site [Moore et al., 1993]. The geographic distribution ofthe sites allows us to map both the timing and spatial patternsof invasion and extinction of species. These maps not onlyprovide information on the process of evolution, migration, andextinction, they can also be related to water mass distributionsand near-surface circulation and indicate shifts in circulationpatterns through time. Maps of 28 diachronous radiolarianevents (first or last appearances) studied at the 11 ODP Leg 138