THE FALSE SENSE OF CHRONOSTRATIGRAPHIC CERTAINTY IN THE DEVONIAN

The latest version of the International Chronostratigraphic Chart shows a Devonian chronology characterized by a perhaps excessive precision. In this geological time scale (GTS2012), uncertainties on the absolute ages of the stage boundaries of the Middle and Late Devonian are extremely low, ranging between ±0.4 and ±1.6 Myr. Whereas, in GTS2010, the uncertainties on the exact same stage boundaries lied between ±2.5 and ±2.7 Myr. This spectacular improvement in the precision of the Middle and Late Devonian chronologies was, however, realized without the incorporation of any additional absolute dates of Devonian ash layers. The builders of the GTS2012 Devonian time scale reached the above-mentioned precision thanks to "a subjective chronostratigraphic scale that incorporates estimates of successive relative spans of conodont zones within each stage/substage" (Becker et al., 2012). However, the conodont biostratigraphy of the Devonian is often contested because of possible diachronity and low conodont yields in several reference sections. Therefore, the proposed high certainty on the absolute ages of the Middle and Late Devonian stage boundaries in GTS2012 is clearly overstated. As an alternative, astronomical cycles recognized in Devonian sedimentary archives can be used as a geochronometer. In the Paleozoic, the 405-kyr eccentricity cycle is best used for cyclostratigraphic purposes, as it is extremely stable and phase-coherent. By incorporating astronomical cycles in Devonian time scales, it becomes possible to bypass several problems that present-day chronologies face, such as poorly constrained biostratigraphies and a very limited number of absolute dates. For example, 405-kyr eccentricity cycles were observed in Frasnian deposits from western Alberta (Canada), which suggested that the Frasnian lasted for 6.7 ± 0.4 Myr and based on the same 405-kyr cycle, the Givetian from Belgium was determined to have lasted for 4.35 ± 0.56 Myr. The combination of this cyclostratigraphic information with the available absolute dates eliminates the subjective estimates of relative spans of conodont zones, applied in GTS2012. Moreover, the extremes of recognized eccentricity cycles provide a high-resolution series of correlation anchors across very different depositional environments.