WHAT’S A FEW CM’S BETWEEN FRIENDS? PALEOBIOGEOGRAPHIC DIACHRONEITY AND THE GSSP CONCEPT AT THE DEVONIAN-CARBONIFEROUS BOUNDARY

The discovery of Siphonodella sulcata below the GSSP Golden Spike for the base of the Carboniferous System in La Serre, France sparked a review of the Devonian-Carboniferous boundary stratotype section and point and prompted the formation of a working group to address the problem. As with most GSSPs erected during the last century, the focus was on a single-taxon biozone biostratigraphic ‘definition’ of the boundary level. This was due in large part because the concept of the Lineage Zone was given extraordinary status as the ‘gold standard’ for intercontinental chronostratigraphic correlation, so much so that the International Stratigraphic Guide went so far as to say that Lineage Zones even ‘approach chronostratigraphic units’. Unsurprisingly, the discovery of the key index fossil, S. sulcata below the GSSP level caused major concerns about the chronostratigraphic position of this boundary.

Now well into the 21^st Century, stratigraphers employs a wide range of tools for high-resolution intercontinental chronostratigraphic correlation that were either yet to be developed or were in their infancy when the initial push for GSSPs was underway. The objective of the GSSP concept, as laid out in the International Stratigraphic Guide, is to pick a position with as many globally identifiable markers as is possible and ABSOLUTELY NOT to mandate an over-reliance on a single chronostratigraphic tool. All chronostratigraphic tools are diachronous to some degree and the idea that any new species has a globally synchronous FAD is pure geofantasy.

The challenge to find either a new chronostratigraphic position, or a new stratigraphic section for the base Carboniferous GSSP is largely one of balance between stability in stratigraphy and the quest for precision. There are a range of major global biogeochemical changes near the current chronostratigraphic position of the base Carboniferous that may, in the long run, prove to be more precise tools for global chronostratigraphic correlation than the first occurrence of S. sulcata (or any single taxon) in any given section. However, moving the chronostratigraphic level of a systemic boundary destabilizes stratigraphy to a degree that many may find very difficult to accept. Striking this balance is the task of the International Subcommission on Carboniferous Stratigraphy.