Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica

Abstract

In 2007, the Antarctic Geological Drilling
\nProgram (ANDRILL) drilled 1138.54 m of
\nstrata ~10 km off the East Antarctic coast,
\ninclud ing an expanded early to middle Miocene
\nsuccession not previously recovered
\nfrom the Antarctic continental shelf. Here,
\nwe pre sent a facies model, distribution, and
\npaleoclimatic interpretation for the AND-2A
\ndrill hole, which enable us, for the fi rst time,
\nto reconstruct periods of early and middle
\nMiocene glacial advance and retreat and
\npaleo environmental changes at an ice-proximal
\nsite. Three types of facies associations
\ncan be recognized that imply signifi cantly
\ndifferent paleoclimatic interpretations. (1) A
\ndiamictite-dominated facies association represents
\nglacially dominated depositional environments,
\nincluding subglacial environments,
\nwith only brief intervals where ice-free coasts
\nexisted, and periods when the ice sheet was
\nperiodically larger than the modern ice sheet.
\n(2) A stratifi ed diamictite and mudstone facies
\nassociation includes facies characteristic of
\nopen-marine to iceberg-infl uenced depositional
\nenvironments and is more consistent
\nwith a very dynamic ice sheet, with a grounding
\nline south of the modern position. (3) A
\nmudstone-dominated facies association generally
\nlacks diamictites and was produced in a
\nglacially infl uenced hemipelagic depositional
\nenvironment. Based on the distribution of
\nthese facies associations, we can conclude that
\nthe Antarctic ice sheets were dynamic, with
\ngrounding lines south of the modern location
\nat ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma and during the Miocene climatic optimum, ca.
\n17.6–15.4 Ma, with ice-sheet and sea-ice minima
\nat ca. 16.5–16.3 Ma and ca. 15.7–15.6 Ma.
\nWhile glacial minima at ca. 20.1–19.6 Ma
\nand ca. 19.3–18.7 Ma were characterized by
\ntemperate margins, an increased abundance
\nof gravelly facies and diatomaceous siltstone
\nand a lack of meltwater plume deposits suggest
\na cooler and drier climate with polythermal
\nconditions for the Miocene climatic
\noptimum (ca. 17.6–15.4 Ma). Several periods
\nof major ice growth with a grounding line traversing
\nthe drill site are recognized between
\nca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma,
\nwith evidence of cold polar glaciers with ice
\nshelves. The AND-2A core provides proximal
\nevidence that during the middle Miocene climate
\ntransition, an ice sheet larger than the
\nmodern ice sheet was already present by ca.
\n14.7 Ma, ~1 m.y. earlier than generally inferred
\nfrom deep-sea oxygen isotope records.
\nThese fi ndings highlight the importance of
\nhigh-latitude ice-proximal records for the interpretation
\nof far-fi eld proxies across major
\nclimate transitions.