Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing

Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved August 13, 2012 (received for review April 2, 2012)
September 17, 2012
109 (40) 16052-16056

Abstract

Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments—a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets.
Transitions between stable states of polar ice sheets are likely governed by thresholds, with rapid changes taking place during perturbation from one condition to the other (1). Establishing the rate at which such changes proceed, and the mechanisms that drive such transformations, commonly relies on geological data from previous glacial-interglacial transitions. Recent research suggests that previous interglacials may have brought about the repeated collapse of marine sectors of Antarctica (2, 3), with evidence from both hemispheres now indicating a dominant oceanic-forcing role in ice-sheet behavior both at present and in the recent past (4, 5). Yet geochronological records of the last glacial termination in Antarctica are ambiguous, with apparently contradictory records indicating either early (ca. 19–16 ka) or late (< 15 ka) retreat in different sectors (69), making it difficult to confidently infer the mechanisms which initiated and drove Antarctic deglaciation, or even to establish whether the southern hemisphere ice sheet receded synchronously with ice sheets in the north. Differences in the pattern and timing of LGM ice-sheet retreat in different sectors of Antarctica have been suggested to have arisen from either gravitationally induced regional variability of sea-level changes (10) or the stabilizing effects of grounding-zone sediment wedges (11), but these theories have yet to be verified against dated margin retreat positions at a continental scale. Here we propose and investigate a third possibility—that differences in the mechanism of ice-sheet flow played a critical role in controlling the rate and locations of ice-margin recession by governing the way that the ice sheet responded to deglacial sea-level rise and ocean warming.
Recent advances in our understanding of modern Antarctic ice-sheet dynamics indicate that a significant component of motion arises from enhanced flow—that is, movement as a consequence of basal sliding as well as internal deformation, even far into the interior (12). For paleoglaciological studies, the importance of these findings lies in the behaviors that arise from these different processes. An ice sheet flowing through viscous deformation alone cannot respond to environmental perturbations as rapidly, or as widely, as one whose motion comes from basal sliding and is controlled (at least in part) by longitudinal coupling (13, 14). In the context of Antarctic ice-sheet deglaciation, ocean forcings will therefore only bring about rapid and significant changes if the responding ice sheet is able to quickly propagate the changes taking place at the oceanic boundary further inland (15). In ice shelves and ice streams, where membrane stresses are dominant (16), flow perturbations are transmitted quickly along conduits of enhanced flow, and changes at marine margins are conveyed inland very effectively (14, 17). Where basal sliding is absent, this connectivity is orders-of-magnitude slower. Furthermore, the impact of external forcings on overall ice-sheet mass balance will also depend on the magnitude of horizontal ice flux (i.e., discharge). Because both velocity and discharge vary spatially, response of an ice sheet to far-field perturbations will be highly variable, even between neighboring catchments, and the lag between oceanic perturbations and their manifestation in geological archives far inland will depend on the proximity of sites to zones of enhanced flow (9).
Here we use a sophisticated ice-sheet model to simulate the geometry and dynamics of the LGM Antarctic ice sheet to (i) identify ice-sheet sectors dominated by enhanced ice flow, (ii) predict the response of this ice sheet to sea-level rise and ocean warming, and (iii) assess how these predictions compare to geological evidence of Antarctic deglaciation.

Results and Discussion

Geological data provide essential constraints to our ice-sheet modeling experiments. Pan-Antarctic cosmogenic surface exposure ages from bedrock samples and glacial erratics indicate that ice thicknesses close to the present coast were several hundred meters greater during the LGM than at present, and tapered inland (SI Appendix, Table S1), in agreement with inferences from ice cores (18). This terrestrial record constrains the vertical dimensions of the former ice sheet, whereas marine geologic interpretations from swath bathymetric and seismic surveys allow the lateral extent of the ice sheet to be reconstructed (19, 20).
Through systematic iteration of model parameters, we achieve an optimum simulation in which both the surface elevation and lateral extent of the ice sheet accords with the majority of geological data (Fig. 1A and SI Appendix, Table S1). Across much of East Antarctica, surface elevations of the simulated ice sheet are comparable to present values, although some inland ice divides are modeled to have been lower (Fig. 1B). Considerable thickening occurs around the coast to the extent that the present-day Filchner–Ronne, Ross, and Amery ice shelves are replaced by grounded ice that extends across much of the continental shelf (Fig. 1A). Modeled ice thicknesses over Ross Island are almost identical to empirically inferred values (21), but our simulation does not reproduce the ice-free McMurdo Dry Valleys, and so overestimates ice-surface elevations in this area. Surface exposure ages in the southern Ross Sea (22) and eastern Weddell Sea (23) embayments suggest that our modeled ice sheet is also too thick in these areas. However, the limited thickening implied by these empirical data, coupled with the greatly advanced grounding-line position interpreted from marine geological data, can only be reconciled with a surface slope of the LGM grounded ice sheet that is similar to that of the present ice shelf, requiring extremely low basal shear stress (< 15 kPa). This disagreement with the observations thus requires further investigation.
Fig. 1.
(A) Modeled ice-sheet surface and mismatch at sites (colored squares) where LGM elevation constraints from geological data are available (SI Appendix, Table S1), color-coded according to vertical legend. (B) Predicted ice thickness change from present. (A and B) Ice extent as interpreted from marine geological data (20, 44) shown with black lines (dashed denotes uncertain limit). Differences in interpretation exist in the Amundsen Sea, where the LGM margin has been defined both in the middle (44) and outer (20) shelf; our model is in best agreement with the latter. Shelf break (-1,000, -1,500, -2,000 m contours) shown in dark red; modeled LGM ice margin indicated by blue line. (CE) Surface profiles of the simulated LGM Antarctic ice sheet compared to present day (36) in Ross Sea, Weddell Sea, and Lambert–Amery sectors. Note the inland thinning and coastal thickening in all three cases, as well as isostatic depression of the bed in coastal areas.
Isostatic depression of the bed is greatest in the Weddell Sea sector, with lesser amounts of loading in the Ross Sea and Amery embayments (Fig. 1 CE)—consistent with global positioning system (GPS) measurements of present-day bedrock uplift in Antarctica (SI Appendix, Fig. S7).
Throughout the domain, modeled flowlines agree closely, but not perfectly, with LGM flow directions inferred from orientations of mega-scale glacial lineations (20, 24, 25) and from marine sediment core mineral provenance studies (26) (Fig. 2 and SI Appendix, Fig. S1). Highest modeled sliding velocities and maximum discharge rates occur in the Thiel/Crary Trough (eastern Weddell Sea), and in the eastern and central Amundsen Sea where Pine Island and Thwaites glaciers coalesce. Modeled discharge through the Amery Trough is also high, and sliding velocities here exceed 500 ma-1 in the outer trough. Although sliding velocity and discharge rates throughout the Ross Sea are lower and less well-partitioned than any of these three other outlets, fast flow influences a much larger sector of this embayment (Fig. 2). The geometry and pattern of flow of our steady-state LGM Antarctic ice sheet therefore fit well with both terrestrial and marine geological constraints, and accounts for a net increase in grounded ice volume of 2.702 × 106 km3 (6.67 m sea level equivalent). The physics and fine resolution of our model of the LGM Antarctic ice sheet permit us to resolve far greater spatial variability in the balance between dominant mechanisms of flow than previously simulated (27, 28). Where ice flow is primarily driven by viscous deformation in response to gravitational driving stress, basal velocities are close to zero, but where longitudinal coupling dominates the force balance of the modeled ice sheet, basal velocities are much higher. According to our model, basal sliding is the sole contributor to glacier motion in large sectors of West Antarctica and in coastal areas of East Antarctica (Fig. 3A). In the eastern Weddell Sea sector, ice draining the East Antarctic ice sheet is organized into discrete catchments that each nourish narrow, fast-flowing, conduits that anastamose around islands of slower ice (Fig. 2). A transect perpendicular to flow spanning 84 to 83 °S in this quadrant of the domain reveals that these fast-flowing conduits exhibit basal velocities that increase abruptly by up to three orders of magnitude across the creep-sliding transition (Fig. 3B). In contrast, surface velocity changes between fast and slow zones are typically half.
Fig. 2.
Surface velocity distribution and modeled flowlines in the simulated LGM Antarctic ice sheet. Dashed line (X–X) shows location of transect in Fig. 3B. Open squares are sites used for terrestrial geological constraint (SI Appendix, Table S1). Dates in red indicate sites of early-onset deglaciation; dates in purple indicate areas where later retreat occurred. Black filled circles show locations of calibrated (thousand years before present; Calib vs. 6.0) or uncalibrated (14C ka) radiocarbon ages. Shelf break shown with dark red contours; modeled LGM ice margin indicated in blue.
Fig. 3.
(A) Predicted occurrence of basal sliding at the LGM. Velocities normalized to surface values. (B) Surface and basal ice velocities along a transect perpendicular to flow in the eastern Weddell Sea sector (location shown in A and in Fig. 2). In areas where basal sliding dominates over viscous deformation, basal velocities approach or equal surface values.
To establish likely ice-sheet response to changing oceanic boundary conditions, we carried out a suite of sensitivity experiments involving sea-level and ocean temperature perturbations. The results show that highly partitioned flow, with abrupt lateral boundaries separating sliding and nonsliding ice, leads to spatially variable responses (Fig. 4). The fast-flowing ice-sheet outlets in our model respond instantly to perturbations at oceanic margins and propagate changes inland very rapidly—a dynamical sensitivity that is due largely to the effects of longitudinal coupling (14). Significantly, although our simulations show glacier acceleration confined to the conduits where enhanced flow occurs, changes in ice thickness are witnessed across far more extensive areas, reflecting a substantial drawing down of the surrounding ice-sheet surface. The conduits thus become the foci of greatest mass loss from the ice sheet. Assuming spatially uniform increases in sea level and oceanic heat flux, we identify that the outlets most susceptible to oceanic changes during deglaciation would likely have been those in the Thiel/Crary Trough, the eastern and central Amundsen Sea, the central Ross Sea, and to a lesser extent the Amery Trough (Fig. 4). In our model, these sectors respond more sensitively to oceanic forcings than slower-flowing or deformation-dominated sectors of the ice sheet, by accelerating and drawing down ice from their entire catchment areas. Geochronological data lend some support to this notion—relatively early deglaciation (> 16 ka) is recorded close to dynamic outlets in these areas (8, 9, 19, 29, 30), whereas retreat occurred later (< 15 ka) in neighboring, but significantly less mobile areas, including marginal areas of the Ross Sea (6, 7, 3134) (Fig. 2). Despite this encouraging agreement, however, we cannot entirely exclude the possibility that some coastal areas respond early simply because of their proximity to open ocean, or that some inland sites respond later because of their greater distance from the ice-sheet margin. A greater density of reliable retreat ages from around the continent may help test these hypotheses in the future.
Fig. 4.
(A) Response of ice-sheet thickness (ΔH) and velocity (Δu) after 1,000 y of an isochronous 25 m rise in sea level from the LGM lowstand, and (B) ice-sheet response when the same sea-level rise is accompanied by a stepped increase in oceanic heat flux of one-third the difference between present and LGM values as prescribed in our model. Grounded ice only is shown; ice shelves are modeled but omitted from display for clarity. Modeled grounding line at maximum LGM extent (gray dotted line) shown to highlight areas of recession. Coastal thickening of 10–20 m occurs in stable-margin areas as a consequence of slow diffusion of ice from inland areas. Oblique illumination (gray shading) is used to highlight areas of greater surface slope (Left) and steeper velocity gradients (Right).
Further experimentation may also help to determine whether the deglacial behavior of an LGM ice sheet that was out of equilibrium, continually adjusting to time-transgressive changes in ocean temperature, sea-level, and atmospheric conditions, would be the same as that of our steady-state ice sheet. If the LGM ice sheet were still expanding when deglaciation began, for example, it may have taken longer to respond to negative mass balance at the margin than an ice sheet that was already retreating, perhaps leading to delayed retreat in some areas. Nonetheless, in our steady-state experiments we find that the rate of grounding-line migration is controlled by the magnitude of the imposed forcings, whereas the regional pattern of ice margin retreat depends on the rate and volume of ice discharge from inland areas, the presence of pinning points, and the bathymetry of the embayments where ice loss takes place. Thus we infer that an indented margin characterized by calving bays that were fed by fast-flowing conduits, as predicted by our model in embayments such as the Ross Sea, is likely representative of the morphology of the retreating LGM ice-sheet margin, irrespective of whether deglaciation initiated from an equilibrium configuration or not.

Conclusions

By using an empirically constrained high-resolution ice-sheet model to simulate the Antarctic ice sheet at the LGM, and by forcing this model with oceanic perturbations, we conclude that spatial contrasts in Antarctic ice-sheet dynamics played a more important role in modulating southern hemisphere ice-sheet sensitivity to ocean forcings than previously realized, and may explain patterns of retreat not otherwise accounted for by regional enhancement of sea levels or grounding-line stabilization from sediment accumulation. That widespread ice-sheet thinning arises from localized acceleration of ice streams is particularly significant when considering recent observations of velocity increases in both Antarctic and Greenland ice sheets (35).

Materials and Methods

We use a three-dimensional, thermomechanical, continental ice-sheet model constrained by geological data that define lateral and vertical extents of the expanded Antarctic ice sheets around the time of the LGM. As in previous studies, we employ boundary distributions from modified BEDMAP topography (36), temperature and precipitation fields from gridded datasets (37, 38), and a spatially varying geothermal heat flux interpolation (39). Our model computes ice thickness and temperature changes, isostatic depression of topography, migration of grounding lines, and the growth of ice shelves. Interaction between modeled ice shelves and their surrounding ocean is accounted for using a mass balance determination based on heat flux across the ice-water boundary. We employ a stress balance that includes longitudinal (membrane) stresses (SI Appendix, Materials and Methods), impose boundary conditions (sea-level lowering, precipitation reduction, and atmospheric temperature perturbations) representative of the LGM, based on ice and marine sediment core isotopic deviations (40, 41) (SI Appendix, Fig. S5), and adjust model parameters affecting bed traction, ice rheology, and ice-shelf mass balance. In contrast to other studies (42), however, the geometry and dynamics of our modeled ice sheet are able to evolve naturally, because we do not prescribe grounding line or ice stream locations. Furthermore, we make use of parallel processing to implement our model at a uniquely high (5 km) resolution, achieving a 16–64 times increase in detail compared to other Antarctic simulations (27, 42, 43).

ACKNOWLEDGMENTS.

We are grateful to Ed Bueler, Constantine Khroulev, and Andy Aschwanden for help with the Parallel Ice Sheet Model, and to Tony Dale and Vladimir Mencl (University of Canterbury) for access to and assistance with the Bluefern Supercomputer. Tim Naish, Peter Barrett, Rob McKay and two anonymous reviewers are gratefully acknowledged for comments on previous versions of this manuscript. N.R.G. and A.N.M. acknowledge financial support from Victoria University Foundation Grant, "Antarctic Research Centre Climate and Ice-Sheet Modelling”. C.J.F. is supported by Australian Research Council Fellowships FL100100195 and FT120100004.

Supporting Information

Supporting Appendix (PDF)
Supporting Information

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Published in

Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 109 | No. 40
October 2, 2012
PubMed: 22988078

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Submission history

Published online: September 17, 2012
Published in issue: October 2, 2012

Keywords

  1. deglaciation
  2. ice-sheet modeling
  3. longitudinal coupling
  4. enhanced flow

Acknowledgments

We are grateful to Ed Bueler, Constantine Khroulev, and Andy Aschwanden for help with the Parallel Ice Sheet Model, and to Tony Dale and Vladimir Mencl (University of Canterbury) for access to and assistance with the Bluefern Supercomputer. Tim Naish, Peter Barrett, Rob McKay and two anonymous reviewers are gratefully acknowledged for comments on previous versions of this manuscript. N.R.G. and A.N.M. acknowledge financial support from Victoria University Foundation Grant, "Antarctic Research Centre Climate and Ice-Sheet Modelling”. C.J.F. is supported by Australian Research Council Fellowships FL100100195 and FT120100004.

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Nicholas R. Golledge1 [email protected]
Antarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand;
Christopher J. Fogwill
Climate Change Research Centre, University of New South Wales, Sydney, New South Wales 2052, Australia; and
Andrew N. Mackintosh
Antarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand;
Kevin M. Buckley
School of Engineering and Computing Science, Victoria University of Wellington, Wellington 6140, New Zealand

Notes

1
To whom correspondence should be addressed. E-mail: [email protected].
Author contributions: N.R.G. designed research; N.R.G. and K.M.B. performed research; K.M.B. contributed new reagents/analytic tools AND/OR performed research; N.R.G., C.J.F., and A.N.M. analyzed data; and N.R.G., C.J.F., and A.N.M. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

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    Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing
    Proceedings of the National Academy of Sciences
    • Vol. 109
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    • pp. 15967-16393

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