Timing and nature of the deepening of the Tasmanian Gateway

Tectonic changes that produced a deep Tasmanian Gateway between Australia and Antarctica are widely invoked as the major mechanism for Antarctic cryosphere growth and Antarctic Circumpolar Current (ACC) development during the Eocene/Oligocene (E/O) transition (∼34–33 Ma). Ocean Drilling Program (ODP) Leg 189 recovered near‐continuous marine sedimentary records across the E/O transition interval at four sites around Tasmania. These records are largely barren of calcareous microfossils but contain a rich record of siliceous‐ and organic‐walled marine microfossils. In this study we integrate micropaleontological, sedimentological, geochemical, and paleomagnetic data from Site 1172 (East Tasman Plateau) to identify four distinct phases (A–D) in the E/O Tasmanian Gateway deepening that are correlative among ODP Leg 189 sites. Phase A, prior to ∼35.5 Ma: minor initial deepening characterized by a shallow marine prodeltaic setting with initial condensation episodes. Phase B, ∼35.5–33.5 Ma: increased deepening marked by the onset of major glauconitic deposition and inception of energetic bottom‐water currents. Phase C, ∼33.5–30.2 Ma: further deepening to bathyal depths, with episodic erosion by increasingly energetic bottom‐water currents. Phase D, <30.2 Ma: establishment of stable, open‐ocean, warm‐temperate, oligotrophic settings characterized by siliceous‐carbonate ooze deposition. Our combined evidence indicates that this early Oligocene Tasmanian Gateway deepening initially produced an eastward flow of relatively warm surface waters from the Australo‐Antarctic Gulf into the southwestern Pacific Ocean. This “proto‐Leeuwin” current fundamentally differs from previous regional reconstructions of eastward flowing cool water (e.g., a “proto‐ACC”) during the early Oligocene and thereby represents an important new constraint for reconstructing regional‐ to global‐scale dynamics for this major global change event.