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Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

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 fi rst 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 signifi cantly 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 stratifi ed diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-infl uenced 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 infl uenced 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 fi ndings highlight the importance of high-latitude ice-proximal records for the interpretation of far-fi eld proxies across major climate transitions.

We use all available chronostratigraphic constraints - biostratigraphy, magnetostratigraphy, radioisotopic dates, strontium-isotope stratigraphy, and correlation of compositional and physical properties to well-dated global or regional records - to construct a preliminary age model for ANDRILL SMS Project's AND-2A drillcore (77°45.488'S, 165°16.605'E, 383.57 m water depth). These diverse chronostratigraphic constraints are consistent with each other and are distributed throughout the 1138.54 m-thick section, resulting in a well-constrained age model. The sedimentary succession comprises a thick early and middle Miocene section below 224.82 mbsf and a condensed middle/late Miocene to Recent section above this. The youngest sediments are Brunhes age (

This Explanatory Notes section complements and supports four other documents, which collectively describe the process and procedures of scientific investigations employed during the ANDRILL SMS Project: (1) ANDRILL Southern McMurdo Sound Project - Scientific Prospectus (ANDRILL contribution 5 - Harwood et al., 2005); (2) a Science Plan summary of SMS Project research compiled from research proposals of Science Team menbers (available on the SMS Project ‘Science Drive’ – Harwood et al., 2007); (3) SMS Project Science Logistics Implementation Plan (SLIP) - draft documents developed and distributed to SMS team members prior to deployment; (4) Operations overview – Falconer et al., this volume). These five documents, and information presented (also available on the SMS Project ‘Science Drive’) at McMurdo Station during the initial morning meetings by co-chief scientists, staff scientist, media coordinator, curator and discipline team leaders, represent the essential elements of the full Science Logistics Implementation Plan (SLIP) for the SMS Project. Please also refer to the McMurdo Ice Shelf Project SLIP available at www.andrill.org (ANDRILL contribution 7 – Naish et al., 2005) for additional background information related to SMS science logistics and operations. These explanatory notes provide important background information on the nature of data present on the SMS Project ‘Science Drive’, including the on-ice report, other core characterization data, and documents and data that record the activities of the SMS Project Science Team.