Research ArticleOCEANOGRAPHY

Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica

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Science Advances  09 Apr 2021:
Vol. 7, no. 15, eabd7254
DOI: 10.1126/sciadv.abd7254
  • Fig. 1 Maps of the survey region showing the Thwaites Glacier Tongue and Eastern Ice Shelf.

    Colors indicate bathymetry according to color bar, unsurveyed areas appear smoother as bathymetry from gravity inversion (7) is shown, and black solid line shows ice shelf and melange front in mid-February 2019. Orange dots indicate conductivity-temperature-depth (CTD) cast locations. (A) Bathymetry with notations indicating bathymetric features and areas discussed in the main text. Hatched regions show grounded ice, and colored lines show grounding line positions in the years indicated (1, 26) and the pinning point for Eastern Ice Shelf near the center of (A). White boxes outline areas shown in detail in (B) and (C), and orange lines in (B) to (D) show the autonomous underwater vehicle (AUV) mission paths. White stars indicate the irregular ridge separating Pine Island Bay from the eastern basin discussed in the text. (D) Ice-shelf front mapped with the multibeam echosounder overlaid on a grayscale Landsat 8 image of Thwaites ice front from 13 February 2019. The shear zone marking the boundary between Eastern Ice Shelf (EIS) and Thwaites Glacier Tongue (TGT) is indicated by blue text. Red stars mark the segments where average ice-shelf depths below sea level were calculated; numbers are the average ice-shelf face depths for that segment (see Methods), and numbers in parentheses are water column thickness (i.e., depth minus ice-shelf draft).

  • Fig. 2 Data from troughs T2 and T3 (Fig. 1).

    (A) Mission tracks for both troughs. (B) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram), colors as in (A). Black contours are potential density. (C) Dissolved oxygen (in milliliters per liter) versus conservative temperature θ (in degrees Celsius). (D) Dissolved oxygen (in milliliters per liter) versus absolute salinity (in grams per kilogram). Black lines in (B) to (D) are the mixing lines between CDW (red circle) and glacial melt (see Methods) (1618). (E and F) Conservative temperature θ (color bar) along the AUV track together with detided current velocity from the ship’s hull-mounted ADCP. Green arrows are average current velocity for a depth of 0 to 400 m, and black arrows are average current velocity for a depth of >400 m. For raw data, see fig. S5. Black arrow in lower right corner is scale arrow (5 cm/s). Ice shelf is indicated by blue transparent area, and depth contours are every 50 m. (E) Trough T2. (F) Trough T3. Note that most of the temperature variation in (E) and (F) are due to depth variations (see fig. S4 for depth and salinity along the AUV path)

  • Fig. 3 Visualization of different water masses in the surveyed area.

    Hydrographic data obtained by CTD [colors correspond to stations in (A)]. Pine Island Bay is indicated by PIB together with data obtained by the AUV (black is the mission in trough T3, and gray is the mission in trough T2). Solid black line shows the mixing line between CDW and ice shelf melt (see Methods) (16, 18, 40). (A) Map of the region. (B) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram). Dashed black line shows the freezing point at surface pressure (9), and solid contours show the potential density. (C) Dissolved oxygen (in milliliters per liter) versus absolute salinity SA (in grams per kilogram). (D) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram) for the zoomed parameter space indicated by blue box in (B). Solid contours show the potential density. (E) Dissolved oxygen (in milliliters per liter) versus absolute salinity SA (in grams per kilogram) for the zoomed parameter space indicated by blue box in (C). Blue arrows in (B) and (C) indicate the high meltwater concentrations discussed in the main text.

  • Fig. 4 Visualization of the deep water masses in the surveyed area.

    Hydrographic data obtained by CTD [colors correspond to stations in (A)]. Pine Island Bay is indicated by PIB together with data obtained by the AUV. Solid black line shows the mixing line between CDW and ice shelf melt (see Methods) (16, 18, 40). (A) Map of the region. (B) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram) for CTD data and AUV data from trough T2 (gray) for the zoomed parameter space indicated by red box in Fig. 3D. Solid contours show the potential density. (C) Dissolved oxygen (in milliliters per liter) versus absolute salinity SA (in grams per kilogram) for CTD data and AUV data from trough T2 (gray) for the zoomed parameter space indicated by red box in Fig. 3E. (D) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram) for CTD data and AUV data from trough T3 (black) for the zoomed parameter space indicated by red box in Fig. 3D. Solid contours show the potential density. (E) Dissolved oxygen (in milliliters per liter) versus absolute salinity SA (in grams per kilogram) for CTD data and AUV data from trough T3 (black) for the zoomed parameter space indicated by red box in Fig. 3E. Blue and red arrows indicate the two deep water types discussed in the main text.

  • Fig. 5 Deep water underneath the ice shelf front is lighter than water outside the ice shelf.

    (A) Map of trough T3 showing the AUV path color coded by latitude. Shaded region indicates the ice shelf front, and black contours are depth contours. (B) Conservative temperature θ (in degrees Celsius) versus absolute salinity SA (in grams per kilogram) for the AUV data points shown in (A), colors as in (A). Contours show potential density (9) relative to 900 m, and blue arrow indicates isopycnal mixing, i.e., water that has the same density but different temperatures and salinities. Green circles in (A) and (B) show the dense saline deep water found in trough T3 discussed in the main text. (C) Absolute salinity SA (in grams per kilogram) as a function of depth for the AUV data in trough T3 and the CTD data (colors indicate station as in Fig. 3). (D) Potential density (in kilograms per cubic meter) as a function of depth for the AUV data in trough T3 and the CTD data (colors indicate station as in Figs. 3 and 4). Red and blue arrows indicate the two deep water masses discussed in the main text from Pine Island Bay and Thwaites Trough. Dissolved oxygen versus θ and SA is shown in fig. S6.

  • Fig. 6 Suggested pathways and mixing area of the water underneath Thwaites ice tongue and Eastern Ice Shelf inferred from the data presented.

    Red arrows indicate main pathways of warm salty water, blue arrow indicates outflow of meltwater-laden fresher water, and red dashed arrow indicates possible warm salty inflow below the range of the ship-borne ADCP. Blue shading shows bathymetric troughs, and purple lines indicate grounding zones. The two arrows from Pine Island Trough indicate that it is not possible from the present dataset to identify which part of this region sources the deep water.

Supplementary Materials

  • Supplementary Materials

    Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica

    A. K. Wåhlin, A. G. C. Graham, K. A. Hogan, B. Y. Queste, L. Boehme, R. D. Larter, E. C. Pettit, J. Wellner, K. J. Heywood

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