Antarctic ice sheet (AIS) growth and decay is strongly influenced by astronomical variations, yet it is not known why AIS response to this climate driver varies through time. Here we examine AIS variability from 34 to 5 million years ago through integration of geological records from the Antarctic margin and a novel assessment of sensitivity to changes in Earth's axial tilt (obliquity sensitivity) derived from the oceanic oxygen-isotope proxy for global ice volume. Three phases of AIS development are found: (1) ~34-24 Ma-a largely terrestrial ice sheet with low obliquity sensitivity; (2) 24 to 14 Ma-frequent ephemeral marine ice sheets with amplified obliquity sensitivity; and (3) 14 to 5 Ma-episodes of extensive marine ice sheet advance, persistent sea ice, and a general decrease in obliquity sensitivity. These phases are associated with decreasing atmospheric CO2 and progressively colder mean climate states. Our analysis suggests the AIS is most sensitive to obliquity forcing when it extends into marine environments and sea-ice extent is limited. We infer this is due to obliquity-driven changes in meridional temperature gradient that affect the position and strength of circum-Antarctic easterly flow, which enhances (or reduces) ocean heat transport across the Antarctic continental margin. Insight into causes of Antarctic ice sheet variability-over a range of time scales-is fundamental to our understanding of Earth system response to climate change. Large scale shifts in AIS volume and extent are controlled by changes in atmospheric CO2 1,2 and plate

Eocene onset of subduction in the western Pacific was accompanied by a global reorganization of tectonic plates and a change in Pacific plate motion relative to hotspots during the period 52–43 Ma. We present seismic-reflection and rock sample data from the Tasman Sea that demonstrate that there was a period of widespread Eocene continental and oceanic compressional plate failure after 53–48 Ma that lasted until at least 37–34 Ma. We call this the Tectonic Event of the Cenozoic in the Tasman Area (TECTA). Its compressional nature is different from coeval tensile stresses and back-arc opening after 50 Ma in the Izu-Bonin-Mariana region. Our observations imply that spatial and temporal patterns of stress evolution during western Pacific Eocene subduction initiation were more varied than previously recognized. The evolving Eocene geometry of plates and boundaries played an important role in determining regional differences in stress state.

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