The Oligocene-Miocene Transition in the Southwest Atlantic Ocean: Integration of bio- and cyclostratigraphy with stable isotope stratigraphy

Climate system reorganizations that occurred during the Oligocene-Miocene transition were linked with substantial changes in the ocean and biosphere. Specifically, ice expansion over Antarctica in the earliest Miocene was recorded in deep ocean sedimentary archives as the Mi1 isotopic event, depicted by benthic foraminiferal stable oxygen isotope (d¹⁸O) records (Miller et al., 1991). The Mi1 event is globally characterized by a ~0.5‰ positive d¹⁸O excursion (Miller et al., 1991; Zachos et al., 2001; Westerhold et al., 2020; Miller et al., 2020), whereas in the Equatorial Atlantic the magnitude of this excusion is typically ~1‰ (Greenop et al., 2019). This climate event occurs as a brief anomaly, in terms of rates and/or amplitudes, and is usually associated with a major disturbance in the global carbon cycle inferred from stable carbon isotope (d¹³C) data (~0,5‰ positive carbon excursion). Here we present an astronomically calibrated planktonic foraminiferal zonation, which is being tested by comparison with benthic foraminiferal d¹⁸O and d¹³C records at Well A, located in the Southwest Atlantic Ocean. The presence of the planktic foraminifera species Paragloborotalia pseudokugleri at the base of the succession, previously investigated by Gennari et al. (2017), together with the first appearance levels of P. kugleri and Globoquadrina dehiscens, and the presence of P. kugleri the top of the studied interval, indicate that the entire succession can be assigned to Zone O7 (Oligocene) and Subzone M1b (Miocene). Numerical ages were calibrated for these bioevents by tuning the gamma ray record of Well A to orbital eccentricity. Currently, we are checking the reliability of our stratigraphic framework by measuring benthic foraminiferal d¹⁸O and d¹³C records and comparing those to key records worldwide.