The Carboniferous Timescale: State of the Art and Future Challenges

The Carboniferous is one of the most confusing and complicated geologic periods in terms of stratigraphic classification. The complex geological, biological, and climatic history of this time period has largely impacted and complicated the construction of a globally applicable Carboniferous timescale. To date, this timescale relies primarily on biostratigraphic tools, while the applications, although often promising, of other stratigraphical tools remain unevenly applied or are completly lacking. Improved calibration of is needed for the entire global Carboniferous timescale. Overall, the chronostratigraphic scale of the Carboniferous System was stabilized by the Carboniferous Subcommission (SCCS) during the late 1990s and early 2000s, with the formal establishment of two subsystems and seven stages. However, since then progress has stalled and only three GSSPs have been ratified for the Tournaisian, Visean and Bashkirian stages. The GSSPs for the bases of the Serpukhovian, Moscovian, Kasimovian and Gzhelian stages has yet to be formalized. Moreover, in collaboration with SDS, SCCS is working on the revision of the GSSP for the base of the system (=Tournaisian Stage). In contrast to the global situation, often long-established regional stratigraphic schemes allow very precise stratigraphic attributions in specific palaeogeographic or palaeoclimatic contexts. Notably, the Carboniferous terrestrial records are stratigraphically much better known compared to many Phanerozoic systems, largely due to the wealth of data generated in extensive coal mining worldwide. Paradoxically, the regional success seems to have a negative impact on the global efforts. The global chronostratigraphic scale, necessarily less precise than regional subdivisions, since based on stage-level resolution, relies heavily on primary markers from marine offshore settings, primarily conodonts. This reduces the applicability in shallow-marine, partly marine and terrestrial records, which are of major importance for understanding the Carboniferous World. After years of apparent still stand, SCCS has now formulated work plans to finalize the remaining GSSP definitions. At least two GSSPs, on being the base of the Gzhelian Stage, should be ratified before the next IGC in 2028, with one more on the way for ratification. The final missing GSSP for a Carboniferous stage, most likely the base of the Serpukhovian, is expected to be defined by 2030. However, the quest for these remaining golden spikes, has to be based on robust stratigraphic data and the ambition to make each boundary applicable for different settings and facies. This requires the shift from the focus on a single primary marker, to the correlation potential of the boundary across different facies and palaeocontinents, and thus into the existing regional stratigraphic schemes. Such a paradigm shift requires rethinking of stratigraphic practices, but promises a more accurate reflection of the complex reality of Carboniferous stratigraphy, ultimately leading to operational global Carboniferous timescale for the global geoscience community.