New age constraints on the duration and origin of the Late Ordovician Guttenberg delta C-13(carb) excursion from high-precision U-Pb geochronology of K-bentonites

Perturbations to the global carbon cycle as recorded in the isotopic compositions of marine deposits have been commonly associated with major shifts in the climate and/or biologic activity, including mass extinctions. The Late Ordovician Guttenberg isotopic carbon excursion (GICE) is a large, globally correlative positive shift (similar to 3 parts per thousand) in the carbon isotopic composition of marine carbonates (delta C-13(carb)), but its driving mechanism(s) remains ambiguous. This is in large part due to uncertain correlations among Late Ordovician records, as well as complex and poorly constrained temporal relationships of abundant K-bentonite (altered volcanic ash) marker beds deposited in this time interval. Here, we provide new, high-precision U-Pb zircon geochronology by chemicalabrasion-isotope-dilution-thermal ionization mass spectrometry for K-bentonites bounding the GICE in the North American Midcontinent, including robust Pb-206/U-238 ages (reported with 2s analytical uncertainty) for two important regional markers: the Deicke (453.35 +/- 0.10 Ma) and Millbrig (453.36 +/- 0.14 Ma) K-bentonites. The new data from these K-bentonites directly constrain the duration of the GICE to less than 400 k.y. at two well-studied locations in eastern Missouri, United States. The abruptness of the GICE precludes relatively gradual tectonic mechanisms as possible drivers of the excursion and suggests more rapid environmental drivers, such as changes in eustatic sea level associated with pre-Hirnantian glacial activity.

Automated summary

By utilizing high-precision U-Pb zircon geochronology, the study successfully constrained the duration of the Late Ordovician Guttenberg isotopic carbon excursion (GICE) to less than 400,000 years at two locations in eastern Missouri, United States. The abruptness of the GICE suggests more rapid environmental drivers, such as changes in eustatic sea level associated with pre-Hirnantian glacial activity.