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The AND-2A drill hole (ANDRILL [Antarctic Geological Drilling Program] Southern McMurdo Sound Project), ~10 km from the East Antarctica coastline, records nearly 6 m.y. of sedimentation across the Miocene climatic optimum at a high-latitude site. Sedimentological studies of bedforms and particle size distributions indicate that the paleoenvironment was strongly affected by waves and currents, consistent with deposition in a glacially influenced neritic environment. We document abrupt shifts in mud percent within glacial-interglacial cycles ca. 17.8 Ma and between ca. 16.7 and 15.7 Ma that we attribute to the hydrodynamic effects of wave stirring tied to episodes of ice growth and decay. Although wave climate and geodynamic forcing of the paleobathymetry simultaneously affect wave stirring on a highlatitude shelf, both are ultimately controlled by the size of the ice sheet. The mud percent record displays cyclicity at short-eccentricity time scales (94-99 k.y.) and, unexpectedly, ice retreat phases interpreted from the particle size record coincide with eccentricity minima. We attribute the eccentricity-paced ice retreat phases during the late Early Miocene polythermal glacial conditions and the cool orbital parameters to marine ice sheet instability in response to changes in ocean circulation and heat transport. The particle size record of the AND-2A core provides unique near-field evidence for orbitally paced changes in high-latitude climate and ice volume during the Miocene climatic optimum and important insights into the mechanisms of ice sheet growth and decay in a period of global warmth. [ABSTRACT FROM AUTHOR]

In 2007, the Antarctic Geological Drilling Program (ANDRILL) drilled 1138.54 m of strata ~10 km off the East Antarctic coast, includ ing an expanded early to middle Miocene succession not previously recovered from the Antarctic continental shelf. Here, we pre sent a facies model, distribution, and paleoclimatic interpretation for the AND-2A drill hole, which enable us, for the first time, to reconstruct periods of early and middle Miocene glacial advance and retreat and paleo environmental changes at an ice-proximal site. Three types of facies associations can be recognized that imply significantly different paleoclimatic interpretations. (1) A diamictite-dominated facies association represents glacially dominated depositional environments, including subglacial environments, with only brief intervals where ice-free coasts existed, and periods when the ice sheet was periodically larger than the modern ice sheet. (2) A stratified diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-influenced depositional environments and is more consistent with a very dynamic ice sheet, with a grounding line south of the modern position. (3) A mudstone-dominated facies association generally lacks diamictites and was produced in a glacially influenced hemipelagic depositional environment. Based on the distribution of these facies associations, we can conclude that the Antarctic ice sheets were dynamic, with grounding lines south of the modern location at ca. 20.1--19.6 Ma and ca. 19.3--18.7 Ma and during the Miocene climatic optimum, ca. 17.6--15.4 Ma, with ice-sheet and sea-ice minima at ca. 16.5--16.3 Ma and ca. 15.7--15.6 Ma. While glacial minima at ca. 20.1--19.6 Ma and ca. 19.3--18.7 Ma were characterized by temperate margins, an increased abundance of gravelly facies and diatomaceous siltstone and a lack of meltwater plume deposits suggest a cooler and drier climate with polythermal conditions for the Miocene climatic optimum (ca. 17.6--15.4 Ma). Several periods of major ice growth with a grounding line traversing the drill site are recognized between ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma, with evidence of cold polar glaciers with ice shelves. The AND-2A core provides proximal evidence that during the middle Miocene climate transition, an ice sheet larger than the modern ice sheet was already present by ca. 14.7 Ma, ~1 m.y. earlier than generally inferred from deep-sea oxygen isotope records. These findings highlight the importance of high-latitude ice-proximal records for the interpretation of far-field proxies across major climate transitions. [ABSTRACT FROM AUTHOR]