Evidence for a heavily glaciated Antarctica during the late Oligocene “warming” (27.8–24.5 Ma): Stable isotope records from ODP Site 690

Abstract

High δ18O values (>3.0 ‰) from a 9-kyr resolution benthic foraminiferal stable isotope record from the Ocean Drilling Program (ODP) Site 690B located on the Maud Rise, Antarctica indicate a heavily glaciated Antarctic continent during Late Oligocene (27.8-24.5 Ma). Values ranging 2.5-3.0 ‰ during interglacial periods and 3.0-3.6 ‰ during glacial intervals are consistent with an ice sheet near or larger than modern size. In addition, this record does not exhibit the long-term Late Oligocene warming trend seen in records from low-latitude drill sites. Oxygen isotope values from 26.0-24.5 Ma are comparable (ranging between 2.5 and 3.3 ‰) to values that preceded the δ18O event Oi2b at 26.7 Ma, indicating no significant glacial collapse occurred during the Late Oligocene. A gradient between ocean basins during the Oligocene has already been linked to the development of a modern, multi-layered ocean, and worked to bathe the low to mid-latitude, deep-sea records with a warmer water mass. We suggest this masked the significant Antarctic glaciation in low-latitude paleoceanographic records. Additionally, we propose a resolution for conflicting lines of evidence from some Antarctic proximal records suggesting significant glaciation and others suggesting reduced glaciation during the Late Oligocene by allowing a modern-sized ice sheet to grow on an Antarctic continent as more land surface area existed above sea-level during this time. This could allow at least some portions of the Antarctic coastline to remain ice-free during glacial minima while still maintaining modern or near-modern ice volume.