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 oceanic-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 into discrete catchments that each nourish Sea (23) embayments suggest that our modeled ice sheet is also narrow, fast-flowing, conduits that anastamose around islands too thick in these areas. However, the limited thickening implied of slower ice (Fig. 2). A transect perpendicular to flow spanning by these empirical data, coupled with the greatly advanced 84 to 83 °S in this quadrant of the domain reveals that these fast- grounding-line position interpreted from marine geological data, flowing conduits exhibit basal velocities that increase abruptly by can only be reconciled with a surface slope of the LGM grounded up to three orders of magnitude across the creep-sliding transi- ice sheet that is similar to that of the present ice shelf, requiring tion (Fig. 3B). In contrast, surface velocity changes between fast extremely low basal shear stress (<15 kPa). This disagreement and slow zones are typically half. with the observations thus requires further investigation. To establish likely ice-sheet response to changing oceanic Isostatic depression of the bed is greatest in the Weddell Sea boundary conditions, we carried out a suite of sensitivity experi- sector, with lesser amounts of loading in the Ross Sea and Amery ments involving sea-level and ocean temperature perturbations. embayments (Fig. 1 C–E)—consistent with global positioning The results show that highly partitioned flow, with abrupt lateral system (GPS) measurements of present-day bedrock uplift in boundaries separating sliding and nonsliding ice, leads to spatially Antarctica (SI Appendix, Fig. S7). variable responses (Fig. 4). The fast-flowing ice-sheet outlets in Throughout the domain, modeled flowlines agree closely, but our model respond instantly to perturbations at oceanic margins not perfectly, with LGM flow directions inferred from orienta- and propagate changes inland very rapidly—a dynamical sensitiv- tions of mega-scale glacial lineations (20, 24, 25) and from marine ity that is due largely to the effects of longitudinal coupling (14). sediment core mineral provenance studies (26) (Fig. 2 and SI Significantly, although our simulations show glacier acceleration Appendix, Fig. S1). Highest modeled sliding velocities and max- confined to the conduits where enhanced flow occurs, changes in imum discharge rates occur in the Thiel/Crary Trough (eastern ice thickness are witnessed across far more extensive areas, re- Weddell Sea), and in the eastern and central Amundsen Sea flecting a substantial drawing down of the surrounding ice-sheet where Pine Island and Thwaites glaciers coalesce. Modeled dis- surface. The conduits thus become the foci of greatest mass loss charge through the Amery Trough is also high, and sliding velo- from the ice sheet. Assuming spatially uniform increases in sea cities here exceed 500 ma−1 in the outer trough. Although sliding level and oceanic heat flux, we identify that the outlets most sus- velocity and discharge rates throughout the Ross Sea are lower ceptible to oceanic changes during deglaciation would likely have and less well-partitioned than any of these three other outlets, been those in the Thiel/Crary Trough, the eastern and central EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES

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. (C–E) 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.

Golledge et al. PNAS ∣ October 2, 2012 ∣ vol. 109 ∣ no. 40 ∣ 16053 Downloaded by guest on October 2, 2021 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.

Amundsen Sea, the central Ross Sea, and to a lesser extent the boring, but significantly less mobile areas, including marginal Amery Trough (Fig. 4). In our model, these sectors respond more areas of the Ross Sea (6, 7, 31–34) (Fig. 2). Despite this encoura- sensitively to oceanic forcings than slower-flowing or deformation- ging agreement, however, we cannot entirely exclude the possibi- dominated sectors of the ice sheet, by accelerating and drawing lity that some coastal areas respond early simply because of their down ice from their entire catchment areas. Geochronological proximity to open ocean, or that some inland sites respond later data lend some support to this notion—relatively early deglacia- because of their greater distance from the ice-sheet margin. A tion (>16 ka) is recorded close to dynamic outlets in these areas greater density of reliable retreat ages from around the continent (8, 9, 19, 29, 30), whereas retreat occurred later (<15 ka) in neigh- may help test these hypotheses in the future. AB

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.

16054 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1205385109 Golledge et al. Downloaded by guest on October 2, 2021 A ∆H ∆u

∆ SL=+25m ∆ ohf=+0% B

∆ SL=+25m ∆ ohf=+33%

-650 -550 -450 -350 -250 -150 -50 0 100 -700 -500 -300 -100 100 300 500 700 ∆H (m) ∆u (m a-1)

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 explain patterns of retreat not otherwise accounted for by regio- the deglacial behavior of an LGM ice sheet that was out of equi- nal enhancement of sea levels or grounding-line stabilization librium, continually adjusting to time-transgressive changes in from sediment accumulation. That widespread ice-sheet thinning ocean temperature, sea-level, and atmospheric conditions, would arises from localized acceleration of ice streams is particularly be the same as that of our steady-state ice sheet. If the LGM ice significant when considering recent observations of velocity in- sheet were still expanding when deglaciation began, for example, creases in both Antarctic and Greenland ice sheets (35). it may have taken longer to respond to negative mass balance at the margin than an ice sheet that was already retreating, perhaps Materials and Methods leading to delayed retreat in some areas. Nonetheless, in our We use a three-dimensional, thermomechanical, continental ice-sheet model steady-state experiments we find that the rate of grounding-line constrained by geological data that define lateral and vertical extents of the migration is controlled by the magnitude of the imposed forcings, expanded Antarctic ice sheets around the time of the LGM. As in previous

studies, we employ boundary distributions from modified BEDMAP topogra- EARTH, ATMOSPHERIC,

whereas the regional pattern of ice margin retreat depends on the phy (36), temperature and precipitation fields from gridded datasets (37, 38), AND PLANETARY SCIENCES rate and volume of ice discharge from inland areas, the presence and a spatially varying geothermal heat flux interpolation (39). Our model of pinning points, and the bathymetry of the embayments where computes ice thickness and temperature changes, isostatic depression of ice loss takes place. Thus we infer that an indented margin char- topography, migration of grounding lines, and the growth of ice shelves. acterized by calving bays that were fed by fast-flowing conduits, as Interaction between modeled ice shelves and their surrounding ocean is ac- predicted by our model in embayments such as the Ross Sea, is counted for using a mass balance determination based on heat flux across the likely representative of the morphology of the retreating LGM ice-water boundary. We employ a stress balance that includes longitudinal ice-sheet margin, irrespective of whether deglaciation initiated (membrane) stresses (SI Appendix, Materials and Methods), impose boundary from an equilibrium configuration or not. conditions (sea-level lowering, precipitation reduction, and atmospheric temperature perturbations) representative of the LGM, based on ice and marine Conclusions sediment core isotopic deviations (40, 41) (SI Appendix, Fig. S5), and adjust By using an empirically constrained high-resolution ice-sheet model parameters affecting bed traction, ice rheology, and ice-shelf mass balance. In contrast to other studies (42), however, the geometry and dynamics model to simulate the Antarctic ice sheet at the LGM, and by of our modeled ice sheet are able to evolve naturally, because we do not forcing this model with oceanic perturbations, we conclude that prescribe grounding line or ice stream locations. Furthermore, we make use spatial contrasts in Antarctic ice-sheet dynamics played a more of parallel processing to implement our model at a uniquely high (5 km) important role in modulating southern hemisphere ice-sheet resolution, achieving a 16–64 times increase in detail compared to other Ant- sensitivity to ocean forcings than previously realized, and may arctic simulations (27, 42, 43).

Golledge et al. PNAS ∣ October 2, 2012 ∣ vol. 109 ∣ no. 40 ∣ 16055 Downloaded by guest on October 2, 2021 ACKNOWLEDGMENTS. We are grateful to Ed Bueler, Constantine Khroulev, for comments on previous versions of this manuscript. N.R.G. and A.N.M. and Andy Aschwanden for help with the Parallel Ice Sheet Model, and acknowledge financial support from Victoria University Foundation Grant, to Tony Dale and Vladimir Mencl (University of Canterbury) for access to "Antarctic Research Centre Climate and Ice-Sheet Modelling”. C.J.F. is and assistance with the Bluefern Supercomputer. Tim Naish, Peter Barrett, supported by Australian Research Council Fellowships FL100100195 and Rob McKay and two anonymous reviewers are gratefully acknowledged FT120100004.

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