Bentonite geochronology, marine geochemistry, and the Great Ordovician Biodiversification Event (GOBE)

Attribution of Ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in Ordovician biodiversification suggest the necessity of evaluating the relationships between K-bentonite deposition and increasingly high-resolution records of marine biogeochemical change. Globally, Ordovician strata preserve an extensive record of explosive volcanism — including the widely recognized Lower to Middle Ordovician Famatina K-bentonite suite in Argentina and the Upper Ordovician Millbrig–Deicke–Kinnekulle suite of North America and Europe. Here, we present high-resolution ID-TIMS U–Pb zircon ages of K-bentonites from measured sections of the San Juan Formation (Talacasto and Cerro La Chilca section) of the Argentine Precordillera. K-bentonites from the Argentine Precordillera provide stratigraphically consistent (i.e., younging upward) ages that range from 473.45 ± 0.70 Ma to 469.53 ± 0.62 Ma, and constrain the age of a low-magnitude (2‰), globally recorded, negative carbon-isotope excursion. Evaluation of the timing of K-bentonite deposition in the Argentina Precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change. From a regional standpoint, these ages provide critical direct evidence for a Dapingian to earliest Darriwilian age of the upper San Juan Formation at sampled localities. These ages are consistent with carbon-isotope data suggesting that the San Juan Formation in the region of its type section is coeval with only the base of the often-correlated Table Head Group of western Newfoundland. This data thus highlights the difficulties in using regional biostratigraphic data – particularly within erosionally truncated or otherwise diachronous units – to define the timeframe of carbon-isotope chemostratigraphy. New geochronological data also indicate that a discrete negative carbon-isotope excursion within the San Juan and Table Head formations is correlative to a globally recognized pre-MDICE negative excursion, and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker. San Juan Formation bentonites, however, cannot be discretely correlated with observed, environmentally significant changes in the Middle Ordovician marine geochemical records of carbon, sulfur, strontium, or sea surface temperature. These results suggest that the extent of volcanism represented by the Famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested.

► We provide new, high-resolution U–Pb ages for Middle Ordovician K-bentonites.
► Data permits detailed calibration of biostratigraphic and chemostratigraphic record.
► Chronology confirms diachroneity of the uppermost San Juan Formation, Argentina.
► We report a distinct, though environmentally variable, pre-MDICE negative excursion.
► Volcanism cannot be linked to change in Earth's biological or geochemical records.