C42A-01
Chronostratigraphic constraints on Pliocene warmth and Pleistocene cooling in the ANDRILL-1B drillcore from beneath the McMurdo Ice Shelf, Antarctica.
In the 2006/2007 Austral summer, the ANDRILL project recovered a 1,285m Neogene drillcore (AND-1B) from the Victoria Land Basin beneath the McMurdo Ice Shelf (78°S), SW Ross Sea, Antarctica. Chronostratigraphic data available for the upper 700 m of the drillcore include diatom biostratigraphy, magnetostratigraphy, 40Ar/39Ar ages, 87Sr/86Sr ages and surfaces of erosion identified from physical appearance in the drillcore. The age data allow a relatively well-constrained age model to be constructed and allow direct correlation with the Geomagnetic Polarity Time Scale for ~70% of the upper 700m of the AND-1B record. The age model for the upper 700m of the drillcore indicates relatively rapid (up to 1m/k.y.) accumulation punctuated by several half to million year hiatuses representing more than half of the last 7 m.y. The Pliocene interval of the core is represented by ~450m of alternating diatomaceous and glacimarine strata comprising ~250m of Early Pliocene diatomaceous strata deposited relatively continuously between 4.9 and 4.2 Ma overlain by a more punctuated ~200m record of alternating diatomaceous and glacimarine strata deposited in the Late Pliocene (2.4-3.3 m.y.). The age model identifies several Late Pliocene 0.5-1 m.y. hiatuses in the AND-1B record coincident with glacial advance, grounding and erosion across the Western Ross Sea floor identified as glacial surfaces of erosion in the AND-1B drillcore and correlative with major regional unconformities in the seismic stratigraphic record of the Victoria Land Basin. Magnetic polarity tie points suggest that these hiatuses coincide with major Late Pliocene glacial isotope excursions with orbital-scale glacial variability recognised between major hiatuses. The upper ~150m of the AND-1B record includes a more strongly glacimarine succession which is dominated by diamicts, which indicate glacial advance and grounding in the western Ross Sea. Magnetic polarity stratigraphy and 40Ar/39Ar ages on volcanics between 100m and 150m indicate that orbital scale variability is still present in the Pleistocene record.
C42A-02
A Paleobathymetry Modelling Of The Ross Sea
Today the continental shelf of the Ross Sea is overdeepened (more than 500 m) and is characterised by glacial valleys incised by multiple advances of ice streams, draining both the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS). Post-rift backstripping modelling has been applied to depth contour maps of some principal unconformities, previously compiled in the frame of the ANTOSTRAT project (Cooper, Barker, Brancolini, 1995), in order to reconstruct the Ross Sea paleobathymetry through its Cenozoic evolution The depth contour maps have been restored after removing sediment load, assuming Airy isostatic compensation, and including constraints from DSDP site 270. The restored maps show the diachronous formation of glacial troughs and banks, in relation with the development of the Eastern and Western Ice Sheets. Major changes in the shape of the continental shelf and slope throughout the Cenozoic can be recognised: Until the late Oligocene the continental shelf was characterised by terrestrial areas (e.g. the Central high) outcropping above sea level and hosting valley glaciers or small ice caps. A continental shelf edge was not clearly developed yet, the eastern flank of the Central High appeared as an inclined ramp, dipping toward the ocean In the early Miocene glacial processes shaped the shallow sectors of the continental shelf, where small ice caps likely developed. Marine processes influenced the deposition in the deepest areas of the Eastern Basin, where no evidences of erosion and deposition from ice streams are found. The shelf profile was seaward dipping and not overdeepened yet. The first evidence of ice stream incision across the shallowest central area of the continental shelf is found in the Mid-Miocene (?16.2-?14 Ma) sequences. The deep incision has SW-NE direction and may be caused by ice streams draining from the Eastern Antarctic Ice Sheet. Sub-glacial delta features have been preserved along the trough-axis up to the paleo-shelf edge, where a trough-mouth fan formed. In the late Miocene-early Pliocene (?10 - ?4 Ma) the bathymetric profile of the continental shelf progressively evolved from seaward to landward dipping in the Eastern Ross Sea and reached a generally overdeepened configuration, similar to the present day profile. Depositional and erosional processes over the continental shelf were largely controlled by ice streams dynamics. Cooper, A. K., Barker, P. F., Brancolini, G., (Eds.), Geology and seismic stratigraphy of the Antarctic margin, Antarct. Res. Ser., vol. 68, 301 pp., atlas, CD-ROMs, AGU, Washington, D.C.
C42A-03
Major Mid-Miocene Climate Change In The Transantarctic Mountains
Independent lines of evidence from paleoecology, glacial geology and marine isotopes indicate major climate change in the Dry Valley sector of the Transantarctic Mountains (TAM) at c.14 Ma. A fossil assemblage of pollen and spores, freshwater diatoms, ostracods, mosses, and insect remains has been recovered from lacustrine sediments preserved in a small morainic lake basin in the western Olympus Range. The diatom assemblage indicates that the lake existed for >103yr and was ice-free during summers. Based on the moss and insect fossils the minimum mean summer temperature (MST- Dec-Feb) was 2°C but could have been as high as 5°C. Today at the site the MST is c. -15°C. The lake-marginal vegetation was a sparse tundra dominated by mosses and liverworts. Based on pollen, Nothofagus (southern beech) was part of the lowland regional vegetation and individual dwarfed shrubs may have grown on the slopes surrounding the lake basin. The age of the deposits is well-constrained by an 40Ar/39Ar age of 14.11 ± 0.11 Ma from an in situ volcanic ash within related lacustrine sediments. Based on an independent study of the glacial stratigraphy of the western Wright and McKelvey valleys, diamictites of a wet-based glacial regime had been replaced by those of cold-based regime by 13.85 ± 0.03 Ma. The drop in temperatures and the cessation of meltwater at c. 14 Ma would have caused the regional extinction of all plant and insect life with the exception of the hardiest of soil-dwelling organisms. Paleobotanical evidence indicates that Antarctica had likely been vegetated throughout the Cenozoic, with forests replaced by tundra during the early Oligocene. The mid-Miocene extinction marks the end of tundra in the interior of Antarctica and its replacement by the polar desert biota which exists today. Changes in δ18O and Mg/Ca ratios from different sectors of the Southern Ocean indicate sea surface temperature cooling and ice sheet growth between 13.8 - 14.2 Ma. The close correlation of events in these marine records with the glaciological and biological records from the Dry Valleys strongly suggests that they are part of a major hemispheric or global climatic event.
C42A-04
Stable or dynamic(?) East Antarctic Ice Sheet during warm paleoclimates: new aerogeophysical perspectives
Boundary conditions for the East Antarctic Ice Sheet (EAIS) such as subglacial topography, subglacial hydrology, the distribution of subglacial sediments, heat flux, crustal thickness, lithosphere rigidity, and tectonic structures can assist predictive ice sheet modelling addressing its role for future sea-level change. One of the key areas to assess the past, present and future stability of the EAIS is the Wilkes Subglacial Basin (WSB), in the backside of the Transantarctic Mountains. Dynamists for the EAIS predict that significant deglaciation occurred perhaps as recently as the Pliocene, allowing for major marine incursion into the WSB. Conversely stabilists have argued that since at least 14 Ma there has been a relatively stable ice sheet. This debate remains highly timely since predictions of the response of the EAIS to current global warming could utilise its past response as a template for the future. A major collaborative UK-Italian aerogeophysical survey was flown over the WSB during the 2005/06 field season to provide new boundary conditions for the EAIS. Over 60,000 line km of new data were collected, including airborne radar, aeromagnetic and airborne gravity. Our new-sub-ice topography significantly changes the previous view of the WSB as a broad shallow depression. Deep subglacial trenches flanked by mountain blocks and plateau-like features are now imaged. These tectonically controlled trenches remain well below sea-level even after isostastic rebound following glacial removal, much like the deep basins underlying highly dynamic ice streams of the West Antarctic Ice Sheet. The newly determined subglacial topography will be input into preliminary coupled-ice sheet\paleoclimate models to re-assess the stability of the EAIS. We will also present potential field signatures and preliminary models to tackle the contentious issue of possible sedimentary infill beneath parts of the WSB.
C42A-05
Large-scale ice-sheet modeling with horizontal stress gradients
The increased understanding of the specific roles played by longitudinal/membrane stresses in ice-sheets mean that an ice-sheet model which includes such stresses can now be specified. It must contain sufficient physics to be able to represent (i) grounding line motion and the grounding line boundary layer; (ii) the role of longitudinal stresses in propagating ice-stream transients; and (iii) the role of longitudinal/membrane stresses in delocalizing dissipative heating. Numerical computations using spectral methods comparing the results of 3D higher order approximations (i.e. Blatter-type models) and 2D vertically integrated equivalents (super-MacAyeal models) are presented, as well as full Stokes solutions. Vertically integrated works very well for flat bottomed ice-streams, but is less accurate in the presence of high-relief topography. The vertically integrated solutions require much computation. Spectral methods are inappropriate for models of the current and former ice-sheets. A finite difference implementation of the vertically integrated model is presented and applied to the issues listed above as well as to a specific case study of the post-glacial retreat of the Antarctic ice-sheet.
C42A-06
Conditions for a steady grounding line
To investigate the conditions under which a marine ice sheet may adopt a steady state profile, a finite element model which solves the full Stokes equations has been developed. The position of the grounding line is fixed, but the air-ice free surface and ice-sea free surface are unknown and part of the solution that we are seeking. Steady state solutions are sought for a range of basal conditions, mass fluxes, sea levels and back pressures. We demonstrate that the number of steady state solutions can be restricted by verifying that the simulations satisfy two contact inequalities. The contact conditions reflect that for physically acceptable solutions, the compressive normal stress at the base of the grounded ice should exceed water pressure, and that the shelf surface should not get into contact with the bedrock. Violation of either conditions would result in grounding line migration. We show that when ice slides over the bedrock and for a given sea level, only one combination of mass flux, grounding line thickness and back pressure, satisfies the contact conditions. When ice is frozen to the bedrock, however, a range of mass fluxes, grounding line thicknesses and back pressures satisfies the contact conditions for a particular sea level. The implication of these results for the instability hypothesis of marine ice sheets is addressed.
C42A-07
East Antarctic Ice Sheet: Stability Since the mid Holocene and Vulnerability to Sea Level Change
The East Antarctic Ice Sheet (EAIS), the world's largest, locks up approximately 60 m of eustatic sea level equivalent. Although thought to be less vulnerable than the Greenland or West Antarctic Ice Sheets, its continental scale means that small changes in its volume can have a major influence on global sea level. Cosmogenic exposure dating of glacial erratics in the Framnes Mountains indicates that the EAIS margin stabilised around 7-6,000 years ago, at a time when eustatic sea level approached near-modern levels. This is in contrast to the West Antarctic Ice Sheet, which continued to retreat throughout the Holocene. The similarity in timing between stability of the EAIS margin and the slowdown in post-glacial sea level rise supports a long- standing hypothesis, recently demonstrated by ice sheet modelling, that EAIS volume changes over glacial/interglacial cycles are dominantly forced by sea level changes. This is because the EAIS margin is presently too cold to experience much direct ablation through melting, and the dominant mode of ice loss is iceberg calving. The ice sheet is "interlocked by global sea level": Sea level fall allows for thickening and expansion of the ice margin. Conversely, sea level rise causes increased calving of the ice margin and retreat. The apparent sea-level control on the EAIS, now demonstrated by both ice sheet modelling and empirical evidence poses the question; Is the EAIS vulnerable to present-day sea level rise associated with thermal expansion of the oceans, melting valley glaciers and increasing ice fluxes in Greenland and West Antarctica? Or is the circa 1 m sea level rise predicted for this century too small to influence the EAIS? http://www.vuw.ac.nz/geo/people/andrew-mackintosh/index.html
C42A-08 INVITED
A need for more realistic ice-sheet models
The current generation of prognostic ice-sheet models fails to adequately capture rapid and non-linear responses of the polar ice masses to environmental forcings, such as dramatic increase in ice discharge following weakening and disintegration of buttressing ice shelves and ice tongues. While the Intergovernmental Panel on Climate Change acknowledged the potential importance of ice-dynamical effects, processes that could render the Greenland and Antarctic ice sheets more vulnerable to future warming are not incorporated into forecasts of future sea-level rise, primarily because of limited understanding of the processes involved. The challenge facing the glaciological community is to move beyond the prevailing "shallow ice paradigm" - which necessarily produces sluggish ice masses - and to develop the next generation of models that include non-linear processes in a more realistic manner. To achieve this goal, a concerted community effort will be needed that involves targeted data collection and interpretation for better process understanding, novel modeling approaches to incorporate processes acting over small spatial scales into models simulating the evolution of entire ice sheets, and model evaluation against current behavior as well as paleo-reconstructions. Many of the controlling processes are not well understood. Particular challenges include documenting and analyzing the response of grounded outlet glaciers and ice streams to ice-marginal forcings, and how this response may be modulated by bed topography and ice-dynamical flow adjustments. Fortuitously, ongoing ice shelf collapse in the Antarctic Peninsula as well as collapse of floating fjord glaciers in Greenland, offer the opportunity to study grounding-line instability at various stages of ice-shelf retreat and collapse. Coupled with advances in remote-sensing techniques that allow us to better constrain the geological nature of the bed, and geothermal heat supplied to the basal ice, major advances in understanding physical processes are possible through concerted multi- disciplinary efforts that involve both targeted field campaigns and theoretical developments and data interpretation. The objective of this contribution is to summarize the discussion ongoing in the glaciological community and to present possible directions for future research efforts.