Miocene Paleoceanography at Rockall Plateau Site 982

Abstract

The Miocene Epoch contains two juxtaposed end-member climate states beginning with the Miocene Climate Optimum (MCO) (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 during the Middle Miocene Climate Transition (MMCT) (14.8-12.8 Ma). The North Atlantic plays an important role in global ocean water circulation and Earth’s climate and yet, relatively few sea surface temperature (SSTs) records have been generated for the Miocene in the subpolar North Atlantic. Biomarker (U^K ’₃₇, TEX₈₆) SST records at subpolar Ocean Drilling Program (ODP) Site 982 (57.5°N) yield SST estimates which 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 during the MMCT between 14.8- 12.8 Ma, recording little to no cooling after 8 Ma. We analyzed planktonic (G. bulloides and G. praebulloides) and benthic foraminifera (P. wuellerstorfi, S. tenuicarinata) from the Early to Late Miocene (20 to 4.5 Ma) at Site 982 for δ¹⁸O and δ¹³C to evaluate SSTs, thermocline and deepwater evolutions, and to test the biomarker-based reconstructions of SSTs in the subpolar North Atlantic.

Our results show that planktonic foraminifera from Site 982 record a 1.45‰ increase in δ¹⁸O values between 15-13.8 Ma, reaching modern values by 11.75 Ma (1.55‰ Modern for G. bulloides). Accounting for ice volume difference using Miller et al. (2020) ice volume estimates, SSTs varied between 15-17°C during the MCO and cooled to 11-13°C with the MMCT, in line with the modern seasonal SSTs in the region (Modern winter 9°C, Modern Summer 13°C). 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 warm (>22°C) through the MCO and subsequent Antarctic Ice sheet growth. Resolving disparities between 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) and supports the preferred temperature ranges for the faunal species found in that interval.