A HIGH PRECISION MARINE RECORD OF ENVIRONMENTAL CHANGE ACROSS THE JURASSIC-CRETACEOUS BOUNDARY, SOUTHERN PATAGONIA

The Jurassic–Cretaceous transition (~145 Ma) is an important boundary in earth history marked by ecological turnover (20% extinction), global tectonic break-up and volcanism, and a globally observed negative organic carbon isotope excursion. Yet, it remains one of the most poorly constrained Phanerozoic boundaries, still lacking a global boundary definition. This is due, in part, to limited global records of environmental perturbations linked to biotic and abiotic disruptions and concomitant radiometric dating. We conducted an interdisciplinary investigation of the depositional facies, stratigraphic architecture, high-precision geochronology, and chemostratigraphy of Jurassic-Cretaceous marine strata of the Rio Mayer Formation in the southern Patagonian Andes. Together, our data provide a detailed record of environmental change across the Jurassic–Cretaceous boundary from a mid- to high-paleolatitude southern hemisphere location. We compare our results with correlative records from Tethyan, Boreal, and other Panthalassic basins. Results show a protracted negative organic carbon isotope excursion (NCIE) across the Jurassic–Cretaceous boundary which correlates globally to other records of the Volgian Organic Isotopic Carbon Excursion (VOICE). New CA-TIMS U-Pb zircon dates of ash beds reveal a date of 144.50 +/- 0.07 Ma for the isotopic minimum of the VOICE and that the excursion may have persisted for nearly 5 Ma. This protracted NCIE represents one of the longest Mesozoic excursions, distinguishing it from other well-documented volcanically driven NCIEs and ocean anoxic events during the Mesozoic. The duration of the NCIE favors a geological mechanism which includes either a strongly negative carbon emission (e.g., methane or terrestrial organic matter) or the release of exceptionally large volumes of volcanic CO2. Correlative increases in weathering indices from major element concentrations are consistent with a long term increase in continental weathering and an Earth system which likely kept pace with release of strongly negative carbon during the latest Jurassic in order to avoid mass extinction. Lastly, the VOICE may also provide a useful calibration for delineating the Jurassic-Cretaceous boundary.