Shift in the paradigm for GSSP boundary definition

© 2020 International Association for Gondwana Research For over 200 years the use of biotic events as the basis for the establishment of chronostratigraphic boundaries has been the only approach successfully utilized for international and national chronostratigraphy. The traditional biostratigraphic method provides relatively high resolution, averaging 1 Ma or sometimes less. This biochronological evolutionary approach to the Global Boundary Stratotype section and Point (GSSP) utilizes biotic Primary Markers (PM), with a few exceptions, encompasses the integrated PM and other non-PM markers as the general principles for defining GSSP boundaries and is a reasonably reliable mechanism for global correlation and a relatively stable International Geologic Time Scale (IGTS). The biotic PM's, however, possessed several serious restrictions: the nature of biological taxonomy, climatic, sedimentary, environmental - and directly applicable within the tropics-subtropics only. Biotic evolution and radiogenic isotopes are the only systems in geologic time that encompass the direction of time. The latter possessed less restrictions than the former. The recent tendency to define GSSP's utilizing magnetic chrons, climatic events and geochemistry may work in the Cenozoic, but is useless in the Mesozoic and older sediments because their cyclic nature (repeatedness) and the need for a second, directional in time index (biostratigraphic or radioisotopic) to place the PM in the right position within the scale. I propose here to utilize volcanic ash beds as the best Primary Marker in geologic chronostratigraphy. The U-Pb system is one of the most dependable of the geochronologic systems because it relies on a simple and non-interpretive radioisotopic decay constant. The ash-bed GSSP as a lithological horizon is universal for the GSSP definition and can be correlated as an age in any facies (marine, lagoon and continental), regardless of paleoclimatic zones, paleoceanographic, geochemical, and most other geological factors. Even moderate level metamorphism (>900 °С) does not affect the U-Pb dating of zircons. The GSSP at the base of a volcanic ash bed (Primary Marker) could be established in a short working time and these ash beds can be integrated with the existing as well as new biostratigraphic, geochemical, magnetostratigraphic and astronomical data (Secondary Markers) to create a robust, accurate and highly useable time scale. Several potential GSSP's that could be established with the volcanic ash beds close to the traditional and/or historical boundaries serve as examples for this approach and include Devonian-Carboniferous, Moscovian-Kasimovian, Kasimovian-Gzhelian, and Sakmarian-Artinskian boundaries