Midlatitude cooling across the Oligocene-Miocene transition

Abstract. The transition from the Oligocene into the Miocene is marked by a significant positive benthic δ¹⁸O excursion, the Mi-1, which is interpreted to reflect a transient period of Antarctic ice sheet growth. Despite an increasing number of orbitally resolved benthic records reflecting deep ocean conditions, few comparable high-resolution records exist to evaluate whether this event also entailed changes in surface ocean temperatures either regionally in the Southern Ocean or globally. Here, we aim to evaluate the timing and amplitude of changes in surface ocean temperatures. We present new alkenone undersaturation temperatures from a site in the Southern Ocean on the Tasman Rise (ODP Site 1168), as well as from a site in the North Atlantic on the Newfoundland Margin (IODP Site U1406). Our results show a nearly 4 °C cooling in the Southern Ocean between 24 and 23 Ma in two steps. When benthic δ¹⁸O recovers to the lower values typical of the pre- Mi-1 excursion, there is only limited warming in average temperature between 23 and 22 Ma. In the North Atlantic our sampling captures only the second step of 2 °C cooling between 23.4 and 23.0 Ma, and average temperatures recover by nearly 2 °C by 22 Ma. These results suggest a consistent cooling of SST in both hemispheres during the Mi-1 glaciation. Bulk carbonate δ¹⁸O records from Site U1406 shows similar magnitude and timing of change as benthic δ¹⁸O from the site, and bulk carbonate from Site 1168 shows similar magnitude and timing of change as the deep South Atlantic Site 1264. Additionally, we generate surface ocean δ¹⁸O_sw estimates from alkenone SST and coccolith dominated bulk carbonate δ¹⁸O for both sites. Comparison of these estimates with benthic δ¹⁸O is consistent with significant deep ocean cooling during both the Mi-1 and Mi-1.1 (22.5 Ma) glacial intervals. Together, these records indicate that Mi-1 was associated with coherent surface ocean cooling across both hemispheres, supporting a tightly coupled response of the surface ocean, deep ocean, and Antarctic cryosphere during this major glaciation.