Late Cenozoic (13-0 Myr) Glacimarine Sedimentology, Facies Analysis, and Sequence Stratigraphy from the Western Ross Embayment, Antarctica: Implications for the Variability of the Antarctic Ice Sheets

Abstract

Sedimentary processes related to oscillations of the marine-based sector of Antarctic Ice Sheet (AIS) in the Ross Embayment over the past 13 Myr are examined at various timescales from stratigraphic records of glacial advance and retreat obtained from the McMurdo Sound region. An initial sedimentary model was developed from short (<2 m) sediment cores collected from beneath the present-day McMurdo Ice Shelf and seasonally open water in the Ross Island region. These cores document sedimentary processes associated with subglacial, ice shelf and open marine environments since the Last Glacial Maximum (LGM) in the Ross Sea Embayment. A radiocarbon chronology from these short cores implies that lift-off of grounded ice in the 900 m-deep marine basins surrounding Ross Island occurred by ~10,100 14C yr BP. Following lift-off, the ice shelf calving line retreated toward its present position. By ~8,900 14C yr BP, seasonally open marine conditions extended as far south as Ross Island. Glacial retreat was rapid and preceded the timing of Meltwater Pulse 1B. Since 8,900 14C yr BP, the calving line of the Ross Ice Shelf has remained pinned to Ross Island despite warmer-than-present temperatures during the mid-Holocene. Depositional models developed for the LGM to recent sediments were then applied to the interpretation of the 1284-m-long ANDRILL McMurdo Ice Shelf core (AND-1B) to documenting oscillations of the AIS in the Ross Embayment over the past 13 Myr. A sequence stratigraphic framework for grounding-line fluctuations of under a variety of glacial regimes, with three distinct types of glacimarine cycle (sequence motif) identified. Motif 1 (Pleistocene and Mid to early Late Miocene) is dominated by thick sub-glacial diamictite, deposited during glacial advance, with occasional thin interbeds of sparsely- to non-fossiliferous mudstone that marks an ice shelf setting during interglacial maxima. Motif 2 (Pliocene) comprises subglacial to glacimarine diamictite overlain by thin, proglacial deposits and capped with substantial beds of diatom-bearing mudstone or diatomite formed under open-marine conditions. Motif 3 (Late Miocene) extends from subglacial diamictite into a thick proglacial succession that includes a combination of stratified diamictite, graded sandstone, conglomerate, and rhythmically-stratified mudstone. The differences in these facies successions (motifs) are associated with the long-term evolution of the AIS in the Ross Embayment from a cold glacial regime with limited volumes of subglacial meltwater (Motif 1) to warmer styles (Motifs 2 and 3) of glaciation with increased subglacial meltwater discharge, before passing back to the cold style of glaciation that characterises the present-day AIS (i.e., limited subglacial meltwater). Each motif was interpreted on the basis of modern analogues of glacimarine sedimentation from a range of climatic/glacial settings, recording a fundamental change in the mass balance for the AIS in the Ross Embayment. For cold glacial regimes similar to the present day Antarctic Ice Sheets, ablation was largely controlled by calving at the marine margin and the melting of the underside of ice shelves by oceanic processes. For warmer regimes, in particular for Motif 3, ablation by melting was a significant influence on mass balance. This sedimentary model was then applied in detail to interpret the Pleistocene section of AND-1B (upper 150 m) with a chronostratigraphic interpretation constrained by sequence stratigraphy, 40Ar/39Ar dating of volcanic ashes, and magneto-stratigraphy. The glacimarine sequences in AND-1B drill core correlate one-to-one with cycles in the benthic delta 18 O record for the past ~0.8 Myr (Marine Isotope Stages 20-2), and are interpreted as recording fluctuations of the AIS in the Ross Embayment with a 100-kyr cyclicity. In this "100-kyr world", the AIS is relatively stable, with subglacial to grounding-zone sedimentation dominating at the AND-1B drill site, with only thin intervals of ice-shelf sedimentation during interglacials and little evidence for open-marine conditions during the Late Pleistocene "super-interglacials". An unconformity spans (~200 kyr) most of the Mid-Pleistocene Transition and is inferred to represent large scale expansion of AIS at ~0.8 Myr. Prior to this, Early Pleistocene glacial/interglacial cycles had a 40-kyr frequency, with interglacial periods characterised by open water deposits that contain volcanoclastic debris and diatomaceous sediments. This upper 150 m of AND-1B provides clear evidence for both a change in the frequency (40- to 100-kyr cycles), and a reduction in the sensitivity of a cooler marine-based AIS in the Ross Embayment.