No abstract

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.

The Southern Ocean is a key player in the climate, ocean, and atmospheric system. As the only direct connection between all three major oceans since the opening of the Southern Ocean gateways, the development of the Southern Ocean and its relationship with the Antarctic cryosphere has influenced the climate of the entire planet. Although the depths of the ocean floor have been recognized as an important factor in climate and paleoclimate models, appropriate paleobathymetric models including a detailed analysis of the sediment cover are not available. Here we utilize more than 40 years of seismic reflection data acquisition along the margins of Antarctica and its conjugate margins, along with multiple drilling campaigns by the International Ocean Discovery Program (IODP) and its predecessor programs. We combine and update the seismic stratigraphy across the regions of the Southern Ocean and calculate ocean‐wide paleobathymetry grids via a backstripping method. We present a suite of high‐resolution paleobathymetric grids from the Eocene‐Oligocene Boundary to modern times. The grids reveal the development of the Southern Ocean from isolated basins to an interconnected ocean affected by the onset and vigor of an Antarctic Circumpolar Current, as well as the glacial sedimentation and erosion of the Antarctic continent. The ocean‐wide comparison through time exposes patterns of ice sheet development such as switching of glacial outlets and the change from wet‐based to dry‐based ice sheets. Ocean currents and bottom‐water production interact with the sedimentation along the continental shelf and slope and profit from the opening of the ocean gateways.

Understanding the patterns and characteristics of sedimentary deposits on the conjugate Australian‐Antarctic margins is critical to reveal the Cretaceous‐Cenozoic tectonic, oceanographic, and climatic conditions in the basin. However, unraveling its evolution has remained difficult due to the different seismic stratigraphic interpretations on each margin and sparse drill sites. Here, for the first time, we collate all available seismic reflection profiles on both margins and use newly available offshore drilling data to develop a consistent seismic stratigraphic framework across the Australian‐Antarctic basins. We find sedimentation patterns similar in structure and thickness, prior to the onset of Antarctic glaciation, enabling the basinwide correlation of four major sedimentary units and their depositional history. We interpret that during the warm and humid Late Cretaceous (~83–65 Ma), large onshore river systems on both Australia and Antarctica resulted in deltaic sediment deposition offshore. We interpret that the onset of clockwise bottom currents during the early Paleogene (~58–48 Ma) formed prominent sediment drift deposits along both continental rises. We suggest that these currents strengthened and progressed farther east through the Eocene. Coevally, global cooling (<48 Ma) and progressive aridification led to a large‐scale decrease in sediment input from both continents. Two major Eocene hiatuses recovered by the Integrated Ocean Discovery Program site U1356A at the Antarctic continental slope likely formed during this preglacial phase of low sedimentation and strong bottom currents. Our results can be used to constrain future paleo‐oceanographic modeling of this region and aid the understanding of the oceanographic changes accompanying the transition from a greenhouse to icehouse world.