Controls on Glacial Facies Distribution in the AND-2A Core, Antarctica, Across the Middle Miocene Transition

In 2007, the ANtarctic Geological DRILLing Program (ANDRILL), recovered a 1138.54 m-deep hole (AND-2A) in Neogene strata including an expanded early and middle Miocene section (17-15 Ma), not previously recovered from the Antarctic continental shelf. This core provides the first opportunity to address Antarctica’s role in one of the most prominent oxygen isotopic shifts recognized in deep-sea cores at the middle Miocene climate transition, including the conditions that preceded it. The sedimentary record in AND-2A, therefore, provides a unique opportunity to unravel the impact of internal vs. external forcing factors on glacial dynamics. The facies associations in AND-2A display a strong cyclicity in the fourth and fifth order band width. Sixty glacial cycles were identified, each characterized by generally sharp-based conglomerates or diamictites at its base. Although there is significant variability in the character and vertical arrangements of facies, two different stacking patterns or motifs can be recognized. Motif 1 is diamictite dominated and represents glacially dominated depositional environments, including subglacial environments, with only brief intervals of ice-free coasts. Motif 2 includes a wider variety of facies and is more consistent with a very dynamic ice-sheet with facies characteristic of open marine to iceberg influenced depositional environments. Particle size data of the upper ~850 m of the core suggest that the short-term cyclicity in sedimentary facies is a result of changes in water depth and glacial proximity. Water depth at this site is controlled by the interplay of eustatic changes and glacio-isostatic processes. Although it is likely that changes in Earth’s orbital configurations ultimately produced the short-term cyclicity in facies associations, the long-term alternations between Motifs 1 and 2 are more difficult to explain. Ice advance at the middle Miocene transition predates rapid cooling of sea surface temperatures (SST) in the Southern Ocean, indicating that complex ice sheet to ocean feedbacks may have existed across the middle Miocene transition.