The Andrill Programme McMurdo Ice Shelf Record of Late Pliocene to Pleistocene Climate Variability

During the austral summer of 2006-2007, a new Antarctic geological drilling programme, ANDRILL, successfully recovered a 1,285 m long record of strata (AND-1B) from beneath the McMurdo Ice Shelf sector of the Ross Ice Shelf. This record spans the last 14myr (Fig. 11.2; Naish et al., 2007).

11.2.5.1. Orbital-scale, glacial-interglacial sedimentary cycles

Forty unconformity-bound glacimarine cycles in the upper 600 m of the core record orbitally influenced fluctuations in the areal extent of the WAIS margin in the Ross Embayment, and the evolution of the whole Antarctic Ice Sheet from a period of relative global warmth in the Early Pliocene through a profound cooling step in deep-sea oxygen isotope records ~ 3-2.5 Ma, to the development of the present cold polar Antarctic thermal regime during the last ~ 1 million years (Naish et al., 2007, 2008).

The rocks were interpreted in terms of lithofacies associations - sediments representing specific environments of deposition. These ranged from open marine diatomites, mudstones and turbidites deposited during interglacials to ice-proximal massive and stratified diamictites, conglomerates and sandstones representing glacial periods. During glacial periods the ice sheet had a laterally extensive marine terminus extending well out into the Ross Sea beyond the drill site. In interglacials the drill site was covered by either an ice shelf (similar to present day), or lay in open water when the ice sheet margin had retreated onto the continent, with local deposition of marine diatoms, terrigenous mud and occasional debris from floating ice.

11.2.5.2. Chronology

A preliminary age model for the upper 700 m of drill core constructed from diatom biostratigraphy and radiometric ages on volcanic material allows a unique correlation between ~70% of the magnetic polarity stratigraphy and the Geomagnetic Polarity Time Scale (GPTS) (Wilson et al., 2007).

Figure 11.2: Lithological column of the upper 600 m of the AND-1B drill core recovered by the ANDRILL MIS Project. Correlation of biostrati-graphically constrained palaeomagnetic reversals (Wilson et al., 2007) with the oxygen-isotope time scale (Lisiecki and Raymo, 2005) are shown. Glacial surfaces of erosion mark boundaries of orbital-scale, glacimarine sedimentary cycles (Naish et al., 2008). Density, clast abundance and glacial proximity curves highlight the glacial and sedimentary cyclicity of G-I cycles. I, Ice Contact; P, Proximal Glacimarine; D, Distal Glacimarine; M,

Open Marine.

Figure 11.2: Lithological column of the upper 600 m of the AND-1B drill core recovered by the ANDRILL MIS Project. Correlation of biostrati-graphically constrained palaeomagnetic reversals (Wilson et al., 2007) with the oxygen-isotope time scale (Lisiecki and Raymo, 2005) are shown. Glacial surfaces of erosion mark boundaries of orbital-scale, glacimarine sedimentary cycles (Naish et al., 2008). Density, clast abundance and glacial proximity curves highlight the glacial and sedimentary cyclicity of G-I cycles. I, Ice Contact; P, Proximal Glacimarine; D, Distal Glacimarine; M,

Open Marine.

The age model provides several well-constrained intervals displaying relatively rapid (< 1 m/1,000yr) and continuous accumulation of sediment punctuated by several 0.5-1 myr stratal hiatuses representing more than half of the last 7 myr. Thus, the AND-1B record provides several highly resolved 'windows' into the Late Cenozoic development of the Antarctic Ice Sheets.

11.2.5.3. Implications for late Cenozoic ice sheet history

The Pliocene period (5-2 Ma) is characterised by a dynamic ice margin with interglacials displaying pelagic diatomite, implying high phytoplankton productivity in locally open water. A >80 m thick interval of diatomite between 370 and 460 m below sea floor (mbsf) represents an extended period of open water in the Ross Embayment and high phytoplankton productivity. Two erosional unconformities of greater than 100 kyrs dated at ~3.3 and 2.7 Ma enclosed by diamictites occur at ~285 and 250 mbsf, and are interpreted to represent stepwise cooling, expansion of the WAIS and glacial erosion in Western Ross Embayment. The timing leads and is coeval with the development of the NH ice sheets and evidence for ice expansion on other regions of the Antarctic continental shelf already discussed. (e.g. Rebesco et al., 2006).

Stratigraphic analysis and a new high-resolution integrated chronostrati-graphy (Wilson et al., 2007) for the overlying Late Pliocene AND-1B cycles, particularly the abrupt transitions from diamictites to biosiliceous deposits, implies rapid oscillations between glacial and marine environments with significant volume changes in the WAIS at a frequency of 41 kyr. Intriguingly, the facies also imply a cooler style of ice sheet compared with the Early Pliocene cycles, indicating reduced amounts of subglacial meltwater and local terrigenous sediment input from the McMurdo Sound area (McKay et al., accepted).

Naish et al (submitted) propose that between 3.5 and 2.5 Ma, high-latitude climate cooling drove both the WAIS and EAIS towards their present expanded cold polar state. Relatively warm, often terrestrially based ice margins were replaced by more permanent marine termini and the development of ice shelves. Mass balance changes affecting these colder ice sheets may have occurred primarily through calving processes at the marine margins, rather than melting. From ~ 2.6 Ma polar Antarctic ice volume changes may have became significantly influenced by NH, glacio-eustasy and this mechanism may account for much of the orbital variability of the WAIS since b 2.6 Ma.

A dramatic change in sedimentary cycle architecture occurs above an erosion surface at 82.74 mbsf in the AND-1B core, that is more or less coincident with the B-M polarity transition, 0.78 ka. Eight fluctuations in the proximity of WAIS margin, expressed as, polar-style sedimentary cycles, occur within the interval above the B-M boundary and correspond to an interval of eight, 100 kyr duration 818O oscillations between Marine Isotope Stages 20 and 2. These diamictite-dominated cycles indicate a predominance of a cold polar ice sheet over the drill site for most of the Late Pleistocene, which retreated to form an ice shelf during interglacial minima.

The erosional surface beneath these cycles removes about ~ 100 kyr of the upper part of the Matuyama Chron above a normal polarity interval of biosiliceous-bearing mudstone assigned to the Jaramillo Subchron (Wilson et al., 2007). An anorthoclase-bearing tuff within the mudstone has yielded a 40Ar/39Ar age of 1.014 7 0.004 Ma (Wilson et al., 2007) and indicates that this period of open marine pelagic sedimentation in western Ross Sea occurred during the warm interglacial of Marine Isotope Stage 31 providing the last evidence for an open Ross Embayment in the AND-1B record. The calving line of the Ross Ice Shelf appears not to have retreated south of its present interglacial position during subsequent 'warmer-than-Holocene super-interglacials' (e.g. Marine Isotope Stages 11, 9 and 5; Jouzel et al., 2007). These observations of Late Pleistocene relative stability of the Ross Ice Shelf seem incompatible with the geological evidence for Late Pleistocene collapse of the WAIS (e.g. Scherer et al., 1998), and remnant shorelines 20 m above present sea level during these interglacials (Hearty et al., 1999).

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