GSSP CHALLENGES AND SOLUTIONS: MARRIAGE OF BIOLOGICALLY TO GEOCHEMICALLY DETERMINED DEFINITIONS?

The core of the GSSP concept proposed over 40 years ago was the biochronological evolutionary approach that in regards to the GSSP principles provided the precise definition of the boundary, its reliable correlation, and stability of the Global Time Scale (GTS). The procedure of GSSP establishment is two-fold: the boundary is defined in the rock record, and the position of the boundary is determined by a biological evolutionary event within a chronocline that serves as a primary correlation tool. Thus the objective fixing of the “golden spike” in the rock record is kept semi-independent of the subjectiveness of taxonomic interpretation.

During last decade this concept has been strongly challenged. Several GSSPs that were proposed 20-30 years ago, when revisited in later studies have been found lacking. An examples: (1) Siphonodella sulcata - the index conodont species to define the D/C boundary was recently found below the established boundary; (2) no index fossils have been found at the current GSSP for the base of the Wenlokian Series of the Silurian; (3) new sets of series and stage names are proposed for the Ordovician and Cambrian because of the difficulty in global correlation. Thus, the major principle - the stability - of the GTS is devalued. A lesson is that even a highly refined taxonomic zonation (cf. conodonts) cannot provide the reliable/stable bases for modern chronostratigraphy because of the at times subjective and interpretive nature of taxonomy. Thus, new and more objective approaches are essential. The definition of a GSSP at a correlative marker horizon, such as an ash bed with zircons or other dateable mineral in a biostratigraphically constrained section that appears as close as possible to the traditional boundary can better serve the needs of the GTS. Recent advances in dating techniques (IDTIMS U-Pb and ⁴⁰Ar/³⁹Ar) provide unprecedented temporal resolution that rivals traditional biostratigraphy. The nature of the ash bed (instantaneous appearance in an undisturbed sequence) fulfills many of the principles of GSSP establishment. Once established in the ash bed, and volcanic minerals are radiometrically dated, the GSSP can be correlated through marine/continental/volcanic facies and provinces. All other traditional tools (bio-, chemo-, cyclostratigraphic) thus complement the radiometric one.