Revised astrochronology for the Lower to Middle Miocene of IODP site U1337. Implications for the Cenozoic megasplice (CENOGRID)

The Astronomical Time Scale (ATS) for the Cenozoic has seemingly been completed with the publication of the CENOzoic Global Reference benthic foraminifer carbon and oxygen Isotope Dataset (CENOGRID) and the standard Geological Time Scale 2020 (GTS2020). Over the years, major flaws in the astronomical tuning have been overcome, but some issues remain. Here we present a revised astrochronology from 15.5 to 18.7 Ma for the Lower - Middle Miocene of IODP Site U1337 in the equatorial Pacific, based on core photographs and gamma ray attenuation (GRA) records of Sites U1335-U1337. The GRA record of Site U1337 was statistically analyzed and tuned to La2004 eccentricity. For the Miocene Climatic Optimum (MCO), differences with the existing δ¹⁸O based tuning are relatively small. However, prior to the MCO, both our revised tuning and the published CaCO₃% based tuning of Sites U1335-U1336 start to deviate significantly from this tuning with a maximum discrepancy of ∼300,000 years. Our revised age model is important as the U1337 benthic δ¹⁸O record has been incorporated in CENOGRID. It is confirmed by high-resolution correlations to the tuned CaCO₃ records of Sites U1335-U1336 and to tuned proxy records of IODP Site 1264 from the eastern South Atlantic. The benthic δ¹⁸O record of this site can replace U1337 in CENOGRID or, alternatively, our revised age model can be applied to the U1337 δ¹⁸O record. Our study reveals that the tuning of the Cenozoic remains to be critically tested, using an integrated stratigraphic approach. Its completion is essential for portraying Earth's global temperature history and for the Earth Sciences as a whole. The study is part of continuing international efforts to complete the ATS and place regional climate records within a high-resolution astronomically dated framework.