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Dynamics of the Last Glacial Maximum Antarctic Ice-Sheet and Its Response to Ocean Forcing

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Dynamics of the Last Glacial Maximum Antarctic Ice-Sheet and Its Response to Ocean Forcing Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing Nicholas R. Golledgea,1, Christopher J. Fogwillb, Andrew N. Mackintosha, and Kevin M. Buckleyc aAntarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand; bClimate Change Research Centre, University of New South Wales, Sydney, New South Wales 2052, Australia; and cSchool of Engineering and Computing Science, Victoria University of Wellington, Wellington 6140, New Zealand Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved August 13, 2012 (received for review April 2, 2012) Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet by governing the way that the ice sheet responded to deglacial is thought to have been initiated by changes in ocean heat and sea-level rise and ocean warming. eustatic sea level propagated from the Northern Hemisphere (NH) Recent advances in our understanding of modern Antarctic as northern ice sheets melted under rising atmospheric tempera- ice-sheet dynamics indicate that a significant component of tures. The extent to which spatial variability in ice dynamics may motion arises from enhanced flow—that is, movement as a con- have modulated the resultant pattern and timing of decay of the sequence of basal sliding as well as internal deformation, even far Antarctic ice sheet has so far received little attention, however, de- into the interior (12). For paleoglaciological studies, the impor- spite the growing recognition that dynamic effects account for a tance of these findings lies in the behaviors that arise from these sizeable proportion of mass-balance changes observed in modern different processes. An ice sheet flowing through viscous defor- ice sheets. Here we use a 5-km resolution whole-continent numer- mation alone cannot respond to environmental perturbations as ical ice-sheet model to assess whether differences in the mechan- rapidly, or as widely, as one whose motion comes from basal slid- isms governing ice sheet flow could account for discrepancies ing and is controlled (at least in part) by longitudinal coupling between geochronological studies in different parts of the conti- (13, 14). In the context of Antarctic ice-sheet deglaciation, ocean nent. We first simulate the geometry and flow characteristics of an forcings will therefore only bring about rapid and significant equilibrium LGM ice sheet, using pan-Antarctic terrestrial and mar- changes if the responding ice sheet is able to quickly propagate ine geological data for constraint, then perturb the system with sea the changes taking place at the oceanic boundary further inland level and ocean heat flux increases to investigate ice-sheet vulner- (15). In ice shelves and ice streams, where membrane stresses are ability. Our results identify that fast-flowing glaciers in the eastern dominant (16), flow perturbations are transmitted quickly along Weddell Sea, the Amundsen Sea, central Ross Sea, and in the conduits of enhanced flow, and changes at marine margins are Amery Trough respond most rapidly to ocean forcings, in agree- conveyed inland very effectively (14, 17). Where basal sliding is ment with empirical data. Most significantly, we find that although absent, this connectivity is orders-of-magnitude slower. Further- ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighbor- more, the impact of external forcings on overall ice-sheet mass ing areas leads to widespread thinning of entire glacier catchments balance will also depend on the magnitude of horizontal ice flux —a discovery that has important ramifications for the dynamic (i.e., discharge). Because both velocity and discharge vary spa- changes presently being observed in modern ice sheets. tially, response of an ice sheet to far-field perturbations will be highly variable, even between neighboring catchments, and the deglaciation ∣ ice-sheet modeling ∣ longitudinal coupling ∣ enhanced flow lag between oceanic perturbations and their manifestation in geological archives far inland will depend on the proximity of sites ransitions between stable states of polar ice sheets are likely to zones of enhanced flow (9). Tgoverned by thresholds, with rapid changes taking place dur- Here we use a sophisticated ice-sheet model to simulate ing perturbation from one condition to the other (1). Establishing the geometry and dynamics of the LGM Antarctic ice sheet to i the rate at which such changes proceed, and the mechanisms that ( ) identify ice-sheet sectors dominated by enhanced ice flow, ii drive such transformations, commonly relies on geological data ( ) predict the response of this ice sheet to sea-level rise and from previous glacial-interglacial transitions. Recent research ocean warming, and (iii) assess how these predictions compare suggests that previous interglacials may have brought about the to geological evidence of Antarctic deglaciation. repeated collapse of marine sectors of Antarctica (2, 3), with evi- Results and Discussion dence from both hemispheres now indicating a dominant ocea- nic-forcing role in ice-sheet behavior both at present and in the Geological data provide essential constraints to our ice-sheet recent past (4, 5). Yet geochronological records of the last glacial modeling experiments. Pan-Antarctic cosmogenic surface expo- termination in Antarctica are ambiguous, with apparently contra- sure ages from bedrock samples and glacial erratics indicate that dictory records indicating either early (ca. 19–16 ka) or late ice thicknesses close to the present coast were several hundred (<15 ka) retreat in different sectors (6–9), making it difficult meters greater during the LGM than at present, and tapered SI Appendix to confidently infer the mechanisms which initiated and drove inland ( , TableS1), in agreement with inferences from Antarctic deglaciation, or even to establish whether the southern ice cores (18). This terrestrial record constrains the vertical hemisphere ice sheet receded synchronously with ice sheets in the dimensions of the former ice sheet, whereas marine geologic north. Differences in the pattern and timing of LGM ice-sheet retreat in different sectors of Antarctica have been suggested Author contributions: N.R.G. designed research; N.R.G. and K.M.B. performed research; to have arisen from either gravitationally induced regional varia- K.M.B. contributed new reagents/analytic tools AND/OR performed research; N.R.G., bility of sea-level changes (10) or the stabilizing effects of ground- C.J.F., and A.N.M. analyzed data; and N.R.G., C.J.F., and A.N.M. wrote the paper. ing-zone sediment wedges (11), but these theories have yet to be The authors declare no conflict of interest. verified against dated margin retreat positions at a continental This article is a PNAS Direct Submission. scale. Here we propose and investigate a third possibility—that 1To whom correspondence should be addressed. E-mail: [email protected]. differences in the mechanism of ice-sheet flow played a critical This article contains supporting information online at www.pnas.org/lookup/suppl/ role in controlling the rate and locations of ice-margin recession doi:10.1073/pnas.1205385109/-/DCSupplemental. 16052–16056 ∣ PNAS ∣ October 2, 2012 ∣ vol. 109 ∣ no. 40 www.pnas.org/cgi/doi/10.1073/pnas.1205385109 Downloaded by guest on October 2, 2021 interpretations from swath bathymetric and seismic surveys allow fast flow influences a much larger sector of this embayment the lateral extent of the ice sheet to be reconstructed (19, 20). (Fig. 2). The geometry and pattern of flow of our steady-state Through systematic iteration of model parameters, we achieve LGM Antarctic ice sheet therefore fit well with both terrestrial an optimum simulation in which both the surface elevation and and marine geological constraints, and accounts for a net increase lateral extent of the ice sheet accords with the majority of geo- in grounded ice volume of 2.702 × 106 km3 (6.67 m sea level logical data (Fig. 1A and SI Appendix, Table S1). Across much of equivalent). The physics and fine resolution of our model of the East Antarctica, surface elevations of the simulated ice sheet are LGM Antarctic ice sheet permit us to resolve far greater spatial comparable to present values, although some inland ice divides variability in the balance between dominant mechanisms of flow are modeled to have been lower (Fig. 1B). Considerable thicken- than previously simulated (27, 28). Where ice flow is primarily ing occurs around the coast to the extent that the present-day driven by viscous deformation in response to gravitational driving Filchner–Ronne, Ross, and Amery ice shelves are replaced by stress, basal velocities are close to zero, but where longitudinal grounded ice that extends across much of the continental shelf coupling dominates the force balance of the modeled ice sheet, (Fig. 1A). Modeled ice thicknesses over Ross Island are almost basal velocities are much higher. According to our model, basal identical to empirically inferred values (21), but our simulation sliding is the sole contributor to glacier motion in large sectors of does not reproduce the ice-free McMurdo Dry Valleys, and so West Antarctica and in coastal areas of East Antarctica (Fig. 3A). overestimates ice-surface elevations in this area. Surface expo- In the eastern Weddell Sea sector, ice draining the East Antarctic sure ages in the southern Ross Sea (22) and eastern Weddell ice sheet is organized
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