Is An Orbitally Tuned Paleozoic Timescale Achievable?

Astrochronology has revolutionized Cenozoic chronostratigraphy, and is gradually being applied to older parts of the geologic timescale, however there are a number of substantial challenges that have limited the utility of this method for refining Paleozoic chronostratigraphy. First, the lack of deep-sea cores containing Paleozoic strata makes intercontinental correlation and global chronostratigraphy significantly more complicated than working in younger eras. Paleontological endemism, an incomplete epicontinental record, regional biological and lithological terminology (“state-line faults”), and a lack of integration between fields have all hindered Paleozoic efforts towards high-resolution chronostratigraphy. Second, the lack of sufficient radiometric data to reliably calibrate observed stratigraphic rhythms to temporal periods has served as a major obstacle to the development of accurate orbital time scales.

Detailed biostratigraphic and carbon isotopic data amassed from European and North American study sites demonstrates that high-resolution global correlations are now clearly within reach for portions of the Silurian. In addition, the recent development of an inverse method for quantitative identification and calibration of orbital signals preserved in the rock record (Average Spectral Misfit, or ASM) overcomes the second major challenge of insufficient radiometric data. Here we present the results of ASM analysis of a high-resolution δ¹³C_carb data set from the Silurian Visby Formation in Gotland (Sweden) that captures the onset of the early Sheinwoodian (Ireviken) positive carbon isotope excursion. Our analysis indicates that the null hypothesis (no orbital signal) can be rejected with a high degree of confidence (<0.2% probability) and provides the first segment of an orbitally-tuned Silurian timescale demonstrating that, although difficult and replete with challenges unique to the era, an orbitally-tuned Paleozoic timescale is indeed achievable.