Thermal evolution of the subtropical North Atlantic during the Miocene

Abstract:

During the Miocene, Earth’s climate transitioned to a colder state with the development of permanent ice sheets in Antarctica. The focus of our study is to understand the role of the ocean in heat transport by the Gulf Stream-North Atlantic Current by reconstructing and comparing the thermal structure at subtropical DSDP Site 558 (37.8°N) and transitional Site 608 (42.8°N) during the Miocene. We measured foraminiferal δ¹⁸O and δ¹³C of benthic P. wuellerstorfi/C. mundulus and planktonic G. venezuelana (thermocline) and G. quadrilobatus (surface dwelling) species to constrain deep, thermocline, and surface water thermal evolution. Previously published benthic records are identical at both locations implying ubiquitous changes in the North Atlantic deep ocean during the Miocene. Our new planktonic δ¹⁸O data show 0.5-0.75 ‰ larger decreases than in benthic species associated with the onset of the Middle Miocene Climatic Optimum (MCO; 17-14.8 Ma), suggesting a sea surface warming of at least 2-3°C in the subtropical North Atlantic. Following the MCO, during the middle Miocene Climate Transition (MMCT; 14.8-12.8 Ma), Site 608 records larger planktonic δ¹⁸O increases than Site 558 and thus more cooling outside of the subtropical gyre. Planktonic δ¹⁸O values at Site 608 continue to increase gradually from 10-8 Ma corresponding to the Late Miocene cooling, whereas planktonic δ¹⁸O values at Site 558 contrarily show steady decreases. Overall, planktonic δ¹⁸O trends in these sites begin to diverge following the MMCT and show a more prominent difference towards the end of the Miocene. We interpret the divergence in planktonic foraminiferal values between Sites 558 and 608 as intensification of subtropical North Atlantic gyre circulation in response to the MMCT and more cooling in the high latitudes.