Research ArticleGEOLOGY

Exceptions to bed-controlled ice sheet flow and retreat from glaciated continental margins worldwide

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Science Advances  13 Jan 2021:
Vol. 7, no. 3, eabb6291
DOI: 10.1126/sciadv.abb6291
  • Fig. 1 Distribution of sites (99) from which high-resolution bathymetric data are available and from which we have analyzed glacial landform assemblages.

    Sites in the Northern Hemisphere (A, B, and D) and Southern Hemisphere (C) are characterized by their regional topographic setting (troughs, fjords, basins, and banks). Numbers correspond to site numbers in table S1 and are referenced in subsequent figures. Topography and bathymetry are from ETOPO global terrain model. Last Glacial Maximum extent after (54, 55).

  • Fig. 2 Landform-based examples of glacial modes from glaciated continental shelves.

    (A) Thermal regime is classified as warm based, mixed warm and cold based, and indiscernible. For example, warm-based conditions are represented by sediment-based subglacial channels and mixed thermal regimes are indicated by rafted sediment blocks. (B) Ice flow regime is classified as streaming, nonstreaming, or indiscernible. For example, streaming is indicated by the presence of mega-scale glacial lineations, while nonstreaming, moderate flow is denoted by drumlins. (C) Retreat style is classified as inconsistent, consistent, readvance, or indiscernible. For example, inconsistent retreat is represented by variable size and spacing of grounding line landforms, consistent retreat is marked by regular size and spacing of grounding line landforms, and readvance is marked by a grounding zone wedge that overrides a prior retreat assemblage. The example of inconsistent retreat also shows an example of glacial lineations limited in size and extent used to inform our nonstreaming ice flow mode.

  • Fig. 3 Topographic control on glacial modes.

    (A) The number of glacial modes per site shows that basins host a higher diversity of glacial modes; however, the maximum modes per site is similar across all topographic settings. Boxplots depict the 25th to 75th percentiles as the filled box and median as the white line; whiskers show the minimum and maximum. The inset shows the number of sites and percentage of modes in each topographic setting for the full dataset. (B to D) The distribution of thermal regimes, ice flow regimes, and retreat styles within each topographic setting. The width of the bars scales with the percentage shown in the inset of (A).

  • Fig. 4 Bed slope and substrate geology control on ice flow and retreat.

    (A) Pie charts show the distribution of catchment-scale bed slopes for the full dataset and distributions of slopes for streaming and moderate flow modes. (B and C) Distribution of ice flow regimes and retreat styles for each bed slope type. (D) Pie charts show the distribution of regional substrate geology for the full dataset and the distribution of substrates for all streaming modes. (E and F) Distribution of ice flow regimes and retreat styles for each substrate type. The width of bars [(B), (C), (E), and (F)] scales with the percentage of modes within each classification type.

  • Fig. 5 Banks that support ice streams have lower amplitude and drain larger catchments than banks with nonstreaming, moderate flow.

    For example, Hayes Bank (site 87) is a low-amplitude bank (60-m relief over 30-km width), drains a portion of the >3 × 106 km2 Ross Sea catchment, and is overrun by mega-scale glacial lineations indicating streaming flow exceeding the capacity of the Glomar Challenger Basin (GC) to the west. Bathymetry in main panel is from GeoMapApp (Global Multi-Resolution Topography v. 3.7); inset bathymetry and ice cover are from the International Bathymetric Chart of the Southern Ocean. Boxplots depict the 25th to 75th percentiles as the filled box and median as the white line; whiskers show the minimum and maximum excluding outliers (crosses), which are defined as values exceeding 1.5× the interquartile range from either the minimum or maximum value.

  • Fig. 6 Enhanced lateral drag and high supply to the grounding line may help facilitate controlled, consistent retreat on reverse bed slopes.

    Among all consistent retreat cases, those on reverse slopes occur in steeper/narrower negative-relief topographic settings (i.e., may experience greater lateral drag) than cases on normal slopes. Reverse slope cases of consistent retreat are supplied by larger catchments than normal or variable slopes. Note that variable slopes show yet more lateral drag and the reverse slope catchment sizes show a large range; neither factor provides a complete explanation. Boxplots depict the 25th to 75th percentiles as the filled box and median as the white line; whiskers show the minimum and maximum excluding outliers (crosses), which are defined as values exceeding 1.5× the interquartile range from either the minimum or maximum value.

Supplementary Materials

  • Supplementary Materials

    Exceptions to bed-controlled ice sheet flow and retreat from glaciated continental margins worldwide

    Sarah L. Greenwood, Lauren M. Simkins, Monica C. M. Winsborrow, Lilja R. Bjarnadóttir

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