The Eocene-Oligocene boundary climate transition: an Antarctic perspective

Ice sheet–free West Antarctica during peak early Oligocene glaciation

“…The first δ 18 O step occurred during the EOT [~34.4 to 33.7 million years ago (Ma)] ( 1 – 3 ), thought to be initiated by low summer insolation ( 3 ), whereas the second, more-prominent step—known as the early Oligocene oxygen isotope step (EOIS) ( 4 ) and commonly referred to as Oi-1 ( 1 , 3 )—marks the end of the EOT and the start of the Early Oligocene Glacial Maximum (EOGM) (~33.7 to 33.2 Ma) ( 1 , 5 ). Constraints on the size, location, and timing of growth of early Antarctic ice sheets, however, remain sparse ( 6 – 8 ), which prevents us from assessing their impact on past global climate dynamics, including ice-ocean-atmosphere interactions during phases of substantial change ( 2 ).…”

“…In the Ross Sea sector, presence of grounded ice directly at the coast has not been documented before ~33 Ma ( 13 ). For West Antarctica, it is debated whether there was major ice sheet buildup during the EOT-EOGM ( 6 , 14 , 15 ) or later during the late Oligocene–early Miocene interval ( 16 ). Numerical models suggest that the prominent EOIS at ~33.7 Ma ( 4 ) is best explained by Antarctic glaciation alone, likely with a partial ( 17 ) or full ( 7 ) West Antarctic contribution.…”