MIOCENE PLANKTONIC FORAMINIFERAL STABLE ISOTOPES AND SEA SURFACE TEMPERATURES AT NORTH ATLANTIC SITE 608

The Miocene (22.8-5.3 Ma) was an epoch of general global cooling, punctuated by the Miocene Climatic Optimum (MCO), a warming that occurred at 17-14.7 Ma. We measured δ¹⁸O and δ¹³C values of two species of planktonic: Globoquadrina venezuelana (a thermocline dweller) and Globigerinoides quadrilobatus (a surface dweller) from the 250-300 μm and 212-250 μm size fractions, respectively. Ages were calibrated to the Geomagnetic Polarity Time Scale (Gradstein et al., 2012) using magnetochronology. Both species showed >1‰ increase in δ¹⁸O values in the Early Middle Miocene (ca. 14.8-12.8 Ma), with surface dwelling G. quadrilobatus showing a further 0.5 ‰ increase in the Late Miocene (ca. 8-7 Ma). These δ¹⁸O changes are larger than global d¹⁸O_seawater estimates of ~0.5 ‰ at this time (Cramer et al., 2011), suggesting surface and thermocline cooling of 1-2°C at this site. Benthic foraminiferal δ¹⁸O data (Miller et al,. 1991;Wright et al., 1992) and the Tex₈₆ paleotemperature proxy from Site 608 (Super et al., 2018) support this interpretation. We interpret there was a further surface-water cooling of ~1-2°C at 8 Ma, which is consistent with the data from UK’³⁷ at higher latitudes Site 982. Our work emphasizes the complex development of the meridional thermal gradient in the North Atlantic following the MCO.