BAYESIAN AGE MODELING OF THE LATE CAMBRIAN TIME SCALE AND THE DURATION OF THE SPICE CARBON ISOTOPE EXCURSION

The late Cambrian (Furongian Epoch) was one of the most dynamic times in Earth history; yet it remains one of the most poorly dated epochs in the Geologic Time Scale 2020 (GTS2020). This is due to the lack of precise depositional ages in strata that can be linked to global timescale boundaries. Because of this, the ages of internal stage boundaries and the timing, tempo and duration of changes to the Earth system during this time remain largely conjectural, including the prominent Steptoean Positive Isotopic Carbon Excursion (SPICE), and the potential link to trilobite mass extinctions, oceanic anoxia, and atmospheric oxygenation. The development of Bayesian age models has allowed for the incorporation of diverse datasets to improve temporal constraints on biological and geochemical processes preserved in the rock record. In stratigraphic sections where radioisotopic constraints are sparse, Bayesian age models allow for the novel integration of other age constraints, such as the occurrence of fossils, to fill this gap. Here we integrate recently reported U-Pb maximum depositional ages, Re-Os geochronology, and two new high-precision CA-ID-TIMS U-Pb zircon depositional ages linked to biostratigraphy from Wales, UK (Ogof-ddû: 490.097 ± 0.15 Ma and Bryn-llin-fawr: 487.292 ± 0.08 Ma), and trilobite biozonation to condition a Bayesian age model through a well-characterized upper Miaolingian through Furongian section at Smithfield Canyon, northern Utah, USA. Our modeling results place temporal constraints on the duration of the SPICE event and Laurentian trilobite biozones correlated to the global Cambrian GTS2020. This work represents the most rigorous attempt to numerically quantify the Furongian Epoch with an updated base age of the 494.5 +0.67/-0.58 Ma and a base age of the Ordovician Period of 487.3 ± 0.08 Ma, a ~30% reduction in the duration of the Furongian Epoch from the GTS2020. The duration of the SPICE is constrained to 2.3 ± 0.7, shorter than previous estimates of 3-4 Myr, updating hypotheses related to the cause(s) and consequences of this excursion. Finally, we show that our new approach of incorporating faunal succession into Bayesian age modeling is a useful means of constraining accumulation rates and possible hiatuses in sections with few radioisotopic ages.