Part 3—Locate a Suitable Core in the Southern Ocean

In Part 3, you will be guided to a core location in the Southern Ocean. This core is particularly well suited for three reasons: the core came from a tectonically quiet region; the sediments were extracted above rocks that meet our "older than 60 million years old" criteria; and, the core was extracted from a depth less than 3000 meters. The depth provides us with representative foraminifera, crucial in identifying the Paleocene-Eocene Thermal Maximum (PETM) boundary.

Step 1 Rotate the Map to Antarctica View

  1. If necessary, re-launch Virtual Ocean and add the Plate Boundaries and Seafloor Crustal Ages datasets. (If you are unsure how to do this, return to the instructions in Part 2, Steps 2 and 3.)
  2. Rotate Earth, so that Antarctica is at the center of your image.

  3. Choose Portals > Ocean Floor Drilling.
  4. When the dataset loads, notice cores that fit our criteria: away from plate boundaries and from sediments above rocks that are 60 to 100 million years old.
  5. Using your cursor, identify a core (actually, it is a set of three cores) at about -65.456 Latitude and 1.145 Longitude. The core is in a part of the Southern Ocean very close to the Weddell Sea. Click on the core(s).

Step 2 Locate Core 113-690B

Notice that when you clicked on the map, core ID 113-690A was highlighted in the core data table. The table may be hard to locate, it is most likely hidden under the main screen. Move it to where you can see all the data and the map at the same time. Also, notice there are two accompanying cores: 113-690B and 113-690C. The core you are going to examine further is 113-690B.

  1. Click on the core ID number 113-690B in the table to highlight the data.

  2. Scroll across the table to see the information that is available about this core. The following information is available about the core:
    • the core's location (Latitude and Longitude),
    • depth to seafloor (2,914 meters),
    • how deep into the sediment the core penetrated (213.4 meters, as opposed to 7.7 meters for core "A"),
    • the Janus Link, which provides additional notes and details about the core, and
    • "false" (meaning not yet identified) information about foraminifera species.

Step 3 Observe the Age Depth Plot and Automated Vane Shear Graph

  1. Make sure core ID 113-690B is highlighted in the table.
  2. Turn off both Plate Boundaries and Seafloor Crustal Ages by clicking the "check box" on the left-hand side of the Layer Manager.
  3. Click on both the view age-depth model and view down-core measurements icons, located at the top of the IODP Drill Holes data table.
  4. Two images appear. One is an age-depth plot (ADP) and the other is a graph of automated vane shear (AVS). Move your cursor up and down on the graphs, and observe that the red line moves on both graphs simultaneously.
  5. The Age Depth Plot (or ADP) is a best fit line of correlation (LOC) of sediment age based on known appearance and extinction of taxa (primarily foraminifera). The red boxes in the ADP graph are the range of age and depth for each taxon that is used to calibrate the age of the sediments. The range of age is part of the uncertainty of using biostratigraphic markers, like the extinction of a certain taxon or its appearance. The range of depth is due to the sampling resolution in the core. There is a longer range for lower sampling resolution.

    The automated vane shear (AVS) tests are performed on sediments to determine their shear strength. Changes in shear strength relate to changes in carbonate content. This shear strength is indirectly an indicator on the abundance or lack of carbonate-rich foraminifera deposits. The more carbonate, the greater the shear strength. Carbonate rocks are hard and clay is soft and easily sheared.

  6. On the graphs, look for the first occurrence of a rapid change in shear strength, from high strength to low strength. This is a strong indication of a rapid extinction of carbonate-rich foraminifera, most likely due to rapid changes in ocean temperature. At what depth does this first rapid change in shear strength occur?
  7. If you look at the corresponding ADP, you will notice a series of numbers. Look for the number set in the middle of the group of numbers at the bottom of the window, which is most representative of the desired age. At what age, in million of years ago (MA), does this rapid change in carbonate content occur?

    You just found a core that showed rapid change in carbonate content (extinction of foraminifera), which has been attributed to the Paleocene-Eocene Thermal Maximum.
Up to now, foraminifera have been discussed in general terms. However, there are specific foraminifera that thrive in certain ocean conditions. When those conditions change, some foraminifera die off, while others that are more tolerant of the new conditions begin to establish themselves. Thus, at the PETM, when Earth underwent a global warming, we would expect to see a foraminifera species, tolerant of warmer, more saline conditions, emerge in the core fossil record.

Two of those species are Acarinina praepentacamerata and Morozovella velascoensis.

In Part 4, you will search the database of ocean cores for both Acarinina praepentacamerata and Morozovella velascoensis, to determine if only the Southern Ocean underwent a warming or whether the other oceans did as well, indicating an overall global warming.