Research ArticleGEOLOGY

Deglacial grounding-line retreat in the Ross Embayment, Antarctica, controlled by ocean and atmosphere forcing

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Science Advances  14 Aug 2019:
Vol. 5, no. 8, eaav8754
DOI: 10.1126/sciadv.aav8754
  • Fig. 1 Climate forcings and modeled grounded ice volume.

    Time series of the atmosphere forcings of (A) SAT and precipitation anomalies (°C relative to PD; % relative to PD) and (B) back pressure fraction offsets (λ; see Materials and Methods), (C) ocean thermal forcings of subshelf melt rate offsets (m year−1 relative to PD), (D) sea-level forcing (m), and (E) modeled grounded ice volume (m3). In (A), all time series correspond to both y axes because we use a temperature-precipitation scaling relationship for the precipitation forcing (7% °C−1). The blue bars in the panels correspond to the approximate timing of MWP-1A and MWP-1B, and the pink bar corresponds to the ACR. In (E), the six simulations forced with the TraCE-21ka SST (LOVECLIM) subshelf melt rate forcing in (C) are shown in dark blue (dark red). All other 24 simulations in the main model ensemble are shown in gray. BP, before the present.

  • Fig. 2 Grounding-line retreat scenarios.

    (A) Warm (LOVECLIM SAT/LOVECLIM 400m OT), (B) moderate (MPA SAT/MPA OT), and (C) cool (TraCE-21ka SAT/TraCE-21ka SST) grounding-line retreat scenarios. The black squares indicate marine sediment core locations with minimum age constraints of grounding-line migration from radiocarbon dating. The thin black line marks the grounding-line position in the eastern Ross Sea estimated by Bart et al. (14). (D) Smoothed bed elevation (m a.s.l.) used in the model simulations based on Bedmap2 (34). The gray lines show elevation contours of 200 m. The black line indicates the modern grounding-line position.

  • Fig. 3 Ice shelf formation.

    (A and B) Ice surface velocity (m year−1) before and after ice shelf formation of the moderate scenario (Fig. 2B). The modeled grounding-line position is shown in black. For context, the modern grounding (calving) line position is shown in pink (purple). IS, ice stream. (C) Floating ice shelf area (m2) of simulations forced with the MPA atmosphere forcing and different ocean forcings (orange lines) and simulations forced with the model-proxy ocean forcing and different atmosphere forcings (purple lines). The timing of (A) and (B) are indicated by the blue and pink bars in (C), respectively.

  • Fig. 4 Uncertainty due to climate forcing.

    SD of modeled ice thickness (m) in the Ross Embayment of the 36 climate-forced deglacial simulations (Table 1). The grid color indicates the time-averaged uncertainty of a given grid cell through the interval of 18 to 4 ka. The colored lines outline the areas that include the top 30% SD in ice thickness at a given time slice in 2-ka intervals from 14 to 6 ka. EAIS, East Antarctic Ice Sheet.

  • Fig. 5 Model-data comparison.

    Regional ice thickness anomalies relative to PD of the model simulations for the (A) NVL, (B) McMurdo, (C) STAM, and (D) MBL regions versus ice thickness anomalies of cosmogenic nuclide surface-exposure records of glaciers within each region. Individual model simulations are shown in gray, and the ensemble average is shown in black. Data for Tucker and Aviator glaciers were obtained from Goehring et al. (41). Data for Mt. Discovery and Mackay Glacier were obtained from Anderson et al. (40) and Jones et al. (38), respectively. The STAM glacier data were obtained from Spector et al. (39) and Todd et al. (37), and those of Mt. Rae were obtained from Stone et al. (36). All surface-exposure ages were recalculated following recent improvements to global production rate (53). Age uncertainties of the records are indicated by the horizontal bars. The inset in (A) shows the region locations (black box outline) and the glacier locations overlaid on the ensemble average of the final modeled RIS configuration (yellow shows floating ice, blue shows grounded ice, the thick black line shows modeled grounding-line position, white contours show surface elevation in 500-m intervals, the pink line shows observed modern grounding-line position, and the purple line shows observed modern calving line position). In the panels, the green line indicates the early retreat/Holocene readvance scenario (see Materials and Methods). CCR, Crater Cirque; SKF, Skua Basin; TAM, Transantarctic Mountain.

  • Table 1 Atmosphere-ocean forcing combinations applied to the ice sheet model.

    The top row and the leftmost column, respectively, refer to the six atmosphere forcings and the six ocean forcings applied to the ice sheet model. In total, 36 simulations of the ensemble are performed, with each table entry referring to an individual simulation. The atmosphere forcing is applied in the form of surface air temperature (SAT), precipitation (7.0%/°C), and sea ice resistance anomalies to a regional climatology from the TraCE-21ka and LOVECLIM transient climate simulations, the EDC and WDC ice core records, and an MPA of each atmosphere forcing. The ocean forcing is applied in the form of subshelf mass flux (melt rate) offsets based on the climate model simulations, two benthic ocean temperature (OT) reconstructions, and an MPA (LOVECLIM 400m T and two benthic OT reconstructions). We define three scenarios, namely, warm, moderate, and cool, based on LGM SAT anomalies and timing and amount of early deglacial ocean warming (Figs. 1 and 2). SST, sea surface temperature.

    LOVECLIM-1 SAT (full
    domain)
    “Warm atmosphere”
    WDC SATMPA SAT
    “Moderate atmosphere”
    EDC SATLOVECLIM-2 SAT2
    (coastal region only)
    TraCE-21ka SAT
    “Cool atmosphere”
    LOVECLIM 400m ocean T
    “Warm ocean”
    LOVECLIM SAT,
    LOVECLIM 400m T
    “Warm scenario”
    WDC SAT,
    LOVECLIM 400m T
    MPA SAT, LOVECLIM
    400m T
    EDC SAT,
    LOVECLIM 400m T
    LOVECLIM SAT2,
    LOVECLIM 400m T
    TraCE SAT,
    LOVECLIM 400m T
    TraCE-21ka 400m ocean TLOVECLIM SAT, TraCE
    400m T
    WDC SAT, TraCE
    400m T
    MPA SAT, TraCE
    400m T
    EDC SAT, TraCE
    400m T
    LOVECLIM SAT2, TraCE
    400m T
    TraCE SAT, TraCE
    400m T
    MPA ocean T
    “Moderate ocean”
    LOVECLIM SAT, MPA
    OT
    WDC SAT, MPA OTMPA SAT, MPA OT
    “Moderate
    scenario”
    EDC SAT, MPA OTLOVECLIM SAT2, MPA
    OT
    TraCE SAT, MPA OT
    Southern Ocean benthic ocean T
    (SOT) reconstruction (20)
    LOVECLIM SAT, SOTWDC SAT, SOTMPA SAT, SOTEDC SAT, SOTLOVECLIM SAT2, SOTTraCE SAT, SOT
    Global benthic ocean T (GOT)
    reconstruction (19)
    LOVECLIM SAT, GOTWDC SAT, GOTMPA SAT, GOTEDC SAT, GOTLOVECLIM SAT2, GOTTraCE SAT, GOT
    TraCE-21ka SST
    “Cool ocean”
    LOVECLIM SAT, TraCE
    SST
    WDC SAT, TraCE
    SST
    MPA SAT, TraCE SSTEDC SAT, TraCE
    SST
    LOVECLIM SAT2, TraCE
    SST
    TraCE SAT, TraCE SST
    “Cool scenario”

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaav8754/DC1

    Fig. S1. Sea-level forcing effect on ice sheet–ice shelf evolution.

    Fig. S2. High mantle viscosity/ocean forcing simulation.

    Fig. S3. Southern Ocean freshwater forcing effect.

    Fig. S4. LGM (20 ka) results of the subshelf melt rate (ssmr)–back pressure (bp) experiments.

    Fig. S5. PD (0 ka) results of the ssmr-bp experiments.

    Fig. S6. Moderate ocean/atmosphere simulation.

    Table S1. Subshelf mass flux–sea ice back pressure sensitivity experimental setup.

    Table S2. Model parameters used in the deglacial climate-forcing experiments.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Sea-level forcing effect on ice sheet–ice shelf evolution.
    • Fig. S2. High mantle viscosity/ocean forcing simulation.
    • Fig. S3. Southern Ocean freshwater forcing effect.
    • Fig. S4. LGM (20 ka) results of the subshelf melt rate (ssmr)–back pressure (bp) experiments.
    • Fig. S5. PD (0 ka) results of the ssmr-bp experiments.
    • Fig. S6. Moderate ocean/atmosphere simulation.
    • Table S1. Subshelf mass flux–sea ice back pressure sensitivity experimental setup.
    • Table S2. Model parameters used in the deglacial climate-forcing experiments.

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