Part 2—Reconstruct Ice Stream Flow

In this part of the activity you will use GeoMapApp to investigate the following problems:

  • What was the maximum extent of grounded ice during the Last Glacial Maximum (LGM)?
  • What was the location and direction of paleo-ice stream flow in the Ross Sea during the LGM?
  • What is the relationship between paleo-ice streams and trough location in the Ross Sea?
  • Do paleo-ice streams align with current ice streams in the WAIS?
You will use the skills you learned in the previous section to create your own images and interpretations of data to solve these key questions. As you work, save images and make notes of the steps you take to create them. These notes and images will assist you in writing your final report, described in Part 3. Make sure to review the criteria and scoring before you begin your research.

Step 1–Recognize mega-scale glacial lineations

If you are re-starting GeoMappApp, set up your base maps and overlays as shown in the image below. Include the Antarctic Coastline, Antarctic Ice Shelf Edge, and Ice Flow Velocities.
  1. To determine just how far grounded ice extended in the Ross Sea at the Last Glacial Maximum (≈20,000 years before present) you will use multibeam sonar data that reveals trace evidence of glacial advances. In particular, you will be searching for the geomorphic features known as mega-scale glacial lineations (MSGLs). These large features are thought to be formed under fast-flowing grounded ice.
    Take a moment to study the following two images:

    The image on the left is a Google Earth satellite image of a region in northern Canada showing MSGLs formed during the LGM when ice covered large parts of North America and Europe. The image on the right is a multi-beam composite of an area in the Ross Sea basin south of the current position of the floating ice shelf. Note that both images are presented at the same scale and cover approximately 50km2. These formations are direct evidence that grounded ice moved over this section of seafloor. Additionally, the direction of the paleo-ice stream is shown by the orientation of the scars.
  2. As you search the Ross Sea for MSGLs be on the lookout for iceberg furrows! Make sure that you don't confuse the two. Iceberg furrows are arc-shaped gouges in the seafloor. They can often be found over running the MSGLs. This makes sense. These features are formed by large icebergs that break off the leading edge of the ice shelf and have such deep drafts that ocean currents push them like giant ploughs over the sea floor. This can only take place after the grounded ice and the floating shelf have retreated.

Step 2–Simplify the search for MSGLs

  1. To make your search for MSGLs easier you can place a layer of multi-beam sonar transects on your map image. From the drop down menus at the top of your GeoMapApp screen select Portals > Multibeam Swath Bathymetry. Wait a few seconds for the data to load.
  2. Notice the black track lines of the multi-beam swaths. Think back to your readings on multi-beam sonar. Consider the following questions:
    • Why are so many tracks back and forth parallel transects similar to the path of a lawn mower?
    • Why do you think there are no transects in the southern parts of the Ross Sea?
    • Why do you think the swaths are concentrated on the continental shelf region north of the ice rather than far out to sea?
  3. Next, select the Zoom tool from the tool bar and click on an area where the tracks are very dense. These areas will have the greatest multi-beam coverage since the swaths of images made with each pass will have overlapping bathymetry data. Remember to be patient as each magnification loads. You may need to reposition your mouse as you zoom in.
  4. As you zoom in, use the Pointer tool from the extreme left of the toolbar (looks like a black arrow) and click on one of the lines to select it. This will allow you to access reference information for your report by next clicking the Cruise info button that appears to the right of the window.

    • The page that pops up will have the number, and will display the dates and principal investigators of the cruise.

    • Click the Supporting Information button under the map on the IEDA page will in order to find the scientific publications that resulted from this expedition. Search these out for review and possible inclusion in your own paper's reference section.
  5. Continue to zoom in on an area of interest to you as you search for MSGLs. Once you have zoomed in to about 128x (this number is shown in the extreme right of the toolbar just past the coordinates under the menu) you should be able to recognize MSGLs if they are present. Don't forget that you must be patient after each click of the zoom tool to ensure that the map has time to focus! You can reposition the map view by selecting the Pan tool from the menu and then clicking and dragging the image to center it on a set of MSGLs. Note: practice moving around the map, turning the tools on and off as needed.
  6. Once you have found a part of the seafloor with lineations select the Profile tool as you did in the previous section of the activity. Use the Profile tool to drag a transect line parallel to the lineations you have found. Notice how the start and stop coordinates of the line are shown in the pop-up window with the graphical profile.
  7. Minimize all your GeoMapApp windows. (Do not close the program)

Step 3– Launch a spreadsheet and collect location data

  1. Launch a spreadsheet program, such as Excel, and create a new workbook document. Name it MSGL. Label the first three columns: "MSGL #", "LAT", and "LONG". In the first and second cell below "MSGL#" enter 1, to indicate your the start and stop points of your first measurement.
  2. In the third and fourth cells below "MSGL#" enter 2 and so on through 10. You are now ready to collect data!
  3. Bring your GeoMapApp window and profile back to the front. In the Profile window you will see a longitude and latitude to the right of the button labeled "Enter Start". These are the coordinates of the start of your transect line along the MSGL feature. Below that you will see the coordinates of the end of your line.
  4. Copy the starting longitude by highlighting it and using your computer's copy function (Mac command+c, PC control+c). Paste this value (make sure you have the correct sign and the number) into the first cell under "LONG". This should be adjacent to the first "1" under the "MSGL#"column. Next, copy and paste the latitude value and place it in the first cell under "LAT". Repeat this process by copying and pasting the "LONG" and "LAT" values from the end of your line in the second "1" row under "MSGL#". Make sure to save your spreadsheet file as you go!
  5. Your task is to repeat this procedure as you search the Ross Sea basin and collect data on the locations of MSGLs. Each time you find an area with MSGLs, use the Profile tool to place a line parallel to the lineations and then take coordinate data from that line to put into your spreadsheet. Each numbered profile line will have two entries in your spreadsheet: longitude and latitude at the start and at the end of the line you drew.

Step 4 - Import spreadsheet data to GeoMapApp and plot mega-scale glacial lineations on a map

Once you have collected sufficient data on MSGL locations and entered them into Excel (or other spreadsheet program) you can place these locations on your map as icons. This will allow you to see the minimum extent of grounded ice during the last glacial maximum (LGM). Additionally, since each MSGL that you located has a start and stop coordinate you will be able to show the direction they point and hence the direction of paleo-ice stream flow.
  1. To begin, use the Opacity slider to make the Multibeam Swath Bathymetry layer transparent. Next zoom out until you can see the entire Ross Sea area.
  2. Use your mouse and hold down the left click button to highlight and select the entire MSGL Spreadsheet. Make sure to include the column headers as they will be used in GeoMapApp. Use Ctrl + C or other command to copy the entire spreadsheet to you clipboard. Minimize the spreadsheet.
  3. On your GeoMapApp screen select File > Import Table or Spreadsheet > From Clipboard. The spreadsheet you highlighted will now be imported automatically.
  4. Click OK to import your spreadsheet data.
  5. The next window that appears will allow you to double check that the icons you are creating on the map are in the correct place. You should ensure that "Latitude Column" is "LAT" from your spreadsheet and Longitude Column is "LONG" from your spreadsheet. This should be the default. Choose OK at the bottom of the window.

    You should now see icons on the map window along with a copy of your spreadsheet attached to the bottom of the map. The spreadsheet data should show up in the window automatically. If it does not you will need to repeat the appropriate steps above and try again. (Important Note: If you select "close" from the Tool Box at the right the entire data set will be deleted and you will have to re-import!)
  6. To differentiate between the different MSGL start and stop points you can color-code them.
    • Click the Color button on the Tool Box at the right of your GeoMapApp window.
    • In the window that opens, choose color by MSGL#.
    • Now the starting and stopping points of the different lines you made parallel to the MSGLs are shown in different colors.
  7. You will now be able to connect the dots and see which points go together. Assuming your search for MSGLs was thorough, you can also visualize the extent of the former grounding line. You know that since MSGLs are created by ice in contact with the seafloor, the grounding line was at least as far north as your most northerly points on the map!
  8. You should also be able to compare the orientation of the matching color icons and determine the direction of paleo-ice stream flow and whether the continental shelf was scoured by uniform glacial process or if the effects of ice streams was concentrated in certain regions. Finally, you can begin to infer whether paleo-ice streams (as evidenced by MSGLs) correlate to the location of modern ice streams as shown in the Rignot (2011) data. Zoom to an extent that allows you to view all of your paleo-ice stream (MSGL) locations. Use the profile tool that you learned about earlier (top tool bar) to transect the Ross Sea. Make sure your line crosses the MSGL data points and is perpendicular to them.
  9. Move your cursor along the graphical profile as you did before and determine whether the paleo-ice streams of fast moving grounded ice correlate to deep troughs in the continental shelf or the shallower banks.
  10. To save map images select File > Save Map Window as Image/Grid File. You will be prompted to choose the file extension that you prefer and destination folder. Make sure to give your file a name that will allow you to find it later.