STABLE ISOTOPE BASED TEMPERATURE RECONSTRUCTION IN THE NORTHERN NORTH ATLANTIC DURING THE EARLY-MIDDLE MIOCENE

The Miocene epoch contains two juxtaposed end-member climate states beginning with the Miocene Climate Optimum (17-14.8 Ma), the most recent interval of ice-free or near ice-free conditions and ending with the abrupt establishment of a permanent East Antarctic Ice Sheet, the Middle Miocene Climate Transition (MMCO). The North Atlantic plays a key role in the regulation of ocean water circulation and Earth’s climate and yet, few SST records exist for the Miocene subpolar North Atlantic. Biomarker (U^K’₃₇, TEX₈₆) SST estimates at subpolar ODP Site 982 (57.5°N) indicate that surface waters in the high latitudes remained warm (>20°C) until 8 Ma when 4°C cooling occurred. In contrast, foraminiferal-based estimates from subtropical and transitional DSDP Sites 563, 558, and 608 (33.6-42.8°N) indicate that the largest cooling (4°C) occurred at the end of the MCO between 14.8-12.8 Ma, recording little to no cooling after 8 Ma. We analyzed planktonic (G. bulloides, G. praebulloides) and benthic foraminifera (P. wuellerstorfi, S. tenuicarinata) from the Early to Middle Miocene (19.5 to 12.5 Ma) at Site 982 for ¹⁸O and ¹³C to test the biomarker-based reconstructions of SSTs in the subpolar North Atlantic.

Our results show that planktonic foraminifera from Site 982 record a 1.25‰ increase in δ¹⁸O values between 15.5-12.5 Ma. Accounting for ice volume difference using Miller et al. (2020) ice volume estimates, SSTs varied between 14-17°C during the MCO and cooled to 10-12°C with the MMCT. Our benthic foraminiferal δ¹⁸O values record a 0.5‰ increase reflecting a 2-3°C deep-water cooling following the MCO. In contrast, the U^K’₃₇ and TEX₈₆ based SSTs recorded modest cooling (<2°C) and remained >22°C through the MCO and subsequent Antarctic Ice sheet growth. Resolving disparity in SST proxies is important because they present two different and possibly incompatible scenarios. The biomarker-based estimates require an asymmetry in cooling with the subpolar North Atlantic maintaining subtropical conditions while the southern subpolar region cooled dramatically. We favor the stable isotope SST reconstruction because 1) the cooling coincides with CO₂ decreases and the growth of a large ice sheet on Antarctica and 2) the resulting SST estimates following the MCO are similar to the modern seasonal SSTs over Rockall Plateau (10-13°C).