Stepwise mid-Cenozoic breakdown of sub-polar gyres and strengthening of the Antarctic Circumpolar Current

In the early-to-mid Cenozoic (66-34 Ma), Southern Ocean circulation was dominated by two subpolar gyres in the Atlantic-Indian and Pacific Oceans. These gyres transported surface water from the subtropics towards Antarctica. The Drake Passage and Tasmanian Gateway opening and widening during the late Cenozoic (34–0 Ma) gradually allowed circumpolar flow of the Antarctic Circumpolar Current (ACC) and the onset of complex oceanic frontal systems, which broke down the earlier subpolar gyres. Questions remain about the precise timing and nature of the onset of the ACC-system and the consequence for climate, ocean circulation and Antarctic ice volume. We hereby provide new insights into the Late Eocene-Miocene (37–5 Ma) oceanographic development by reconstructing surface ocean environment combining remains of organic walled dinoflagellate cysts (dinocysts) and organic biomarker (TEX₈₆and U^k’₃₇) sea surface temperature (SST) estimates from marine sedimentary drill cores from the southwestern South Atlantic (IODP Site U1536, ODP Site 696 and piston cores from Maurice Ewing Bank), southeastern Indian Ocean (ODP Site 1168) and southwestern Pacific (ODP Site 1172). We compare our results, together with available Southern Ocean records, with model experiments and tectonic reconstructions to deconvolve the effects of climate, ice volume and tectonic changes on Southern Ocean oceanography.

Late Eocene – Early Oligocene SSTs (37­–27 Ma) were broadly similar across the Southern Ocean (4–8°C latitudinal temperature difference), which we ascribe to persistent, strong subpolar gyral circulation influencing the sites. In the Late Oligocene (~26 Ma), progressive Antarctic-proximal cooling increased the SST gradient in the Australian-Antarctic gulf (>9°C). The timing of this Antarctic-proximal cooling coincided with sedimentary and kinematic reconstructions of Drake Passage deepening after 26 Ma, thus matching with ocean model experiments demonstrating that Drake Passage deepening weakened gyral circulation, enhanced thermal isolation and cooled Antarctic proximal waters. Throughout the Late Oligocene–Late Miocene (26–5 Ma) we record a continued contraction of the sub-polar gyre and southward migration of the subtropical gyre in the South Atlantic, with strengthening frontal systems and progressive cooling that first started in the southern South Atlantic. Although geographic coverage is sparse, our data shows for the first time the stepwise breakdown of subpolar gyres into the modern-like oceanographic regime with the development of strong frontal systems, latitudinal gradients and deep-water formation. We demonstrate, with modelling and geological data, that while climate and ice volume changes determine the strength of latitudinal SST gradients and position of ocean fronts on orbital time scales, gateway configurations play a large role in long-term trends.