REVISING AGE MODELS FOR MIOCENE DEEP-SEA SEDIMENTS WITH U-PB ZIRCON GEOCHRONOLOGY

In order to understand the myriad climatic and biotic changes experienced by the Earth system during the Miocene, it is essential to develop accurate and precise chronologies that allow for a detailed alignment and integration of a variety of proxy records. Carbonate sediments from both surface outcrop and ocean drill cores have yielded much of our existing knowledge of Miocene climate history, with age models derived from some combination of magnetostratigraphy, biostratigraphy, and astronomical tuning. However, carbonates can be poor paleomagnetic recorders, biostratigraphic data may be obscured by poor preservation and other biotic and sedimentary processes, and hiatuses can disrupt efforts to tune many sections, thereby raising questions about the timing of events in marine and terrestrial sections.

Here, we target Miocene carbonate sections in outcrop and ocean drill core that contain interbedded volcanic ashes amenable to radioisotopic dating via U-Pb CA-ID-TIMS zircon geochronology, which yields Miocene ages with precision on the order of 10 ka, to refine age models previously constructed via biostratigraphy, magnetostratigraphy, and legacy ⁴⁰Ar/³⁹Ar geochronology. In the Lower Miocene Bisciaro Formation in the Contessa-Il Testimone section (Gubbio, Italy), our new ages from four ashes show that deposition occurred between ~22.3 and 20.1 Ma, rather than 22-17 Ma (Kasbohm et al., 2021). The current Geologic Time Scale (GTS 2020) relies on the Contessa section to date the base of G. altiaperturus, which we show to be at least 1 Ma older. Ages cited for other foraminiferal biostratigraphic markers (base of G. dehiscens, top of P. kugleri) in GTS 2020 are affirmed by our new geochronology, which may also be used to recalibrate prior and future paleoclimate studies based on Bisciaro sediments. We also present nine new U-Pb ages from ashes interbedded with Lower and Middle Miocene carbonates at ODP Site 1000 (Nicaragua Rise), yielding an absolute age calibration for a new high-resolution δ¹³C and δ¹⁸O record across the Miocene Climate Optimum. Our new Caribbean record can be integrated with astronomically tuned deep-sea records and other radiometrically dated records of the MCO to enhance our global understanding of the timing, tempo, and causes of this climatic perturbation.