Initial Publication Date: August 18, 2014

Analyzing Sediment Cores

Part C: Dating the Sediment Core: More Evidence to Support Your Hypothesis

Now that you have made some observations about the sedimentary features in the core, it's time to determine the age of the sediments and establish a timeline for the core section. The relative ages of cores are determined onboard the JOIDES Resolution by examining both the Earth's paleomagnetic record and microfossils preserved within the cores.

The Paleomagnetism Lab

As you learned earlier from Dr. Maureen Davies, magnetic minerals are like microscopic compasses that become aligned with the Earth's magnetic field at the time the sediments are deposited. Deep sea sediments provide scientists like Dr. Davies with a detailed record of the Earth's paleomagnetic record through time and can be used to help determine ages of sediment cores.

The JOIDES Resolution has a wealth of advanced lab equipment on board, including a cryogenic magnetometer (shown above) that measures the orientation of magnetic mineral grains in rocks. Magnetometers measure the inclination of magnetic minerals, which is the angle between the mineral grain and the surface of the Earth. Inclination measurements vary between -90◦ and 90◦. Magnetic minerals that have positive inclinations point down and represent periods of normal polarity periods of time in the past in which the direction of the Earth's magnetic field was the same as the present direction., while minerals that have negative inclinations point up and represent periods of reversed polarity periods of time in the past in which the Earth's magnetic field was in the opposite direction from the present orientation.. Normal polarity means that the magnetic field was in the same orientation as today, whereas reversed polarity means that the magnetic field was the opposite of today.

The last full magnetic reversal occurred approximately 780,000 years ago, and it's known as the Brunhes-Matuyama magnetic field reversal. This means that a compass on Earth (like iron-bearing minerals in sediments!) 800,000 years ago would have pointed south, and after the reversal it would have pointed north again. Full magnetic field reversals don't happen in an instant, but rather take several hundred to a few thousand years to complete.

Watch the following video to learn more about Earth's magnetic field.

Magnetic Field Reversal from The Discovery Channel

Digging into the Data: Using the Earth's Magnetic Field to Tell Time

Magnetic field reversals have occurred throughout geologic time and have provided scientists with a "barcode" record of normal and reversed polarity. This barcode pattern is reflected in the Geomagnetic Polarity Timescale (GPTS) shown below (Mankinen and Wentworth, 2003). Black bars represent periods of normal polarity and white bars represent periods of reversed polarity. Multiple data sources were used to construct the GPTS - the age intervals were obtained from radiometric dating of volcanic rocks, and the duration normal and reversed polarity sequences were determined using sequences of deep sea sediments.

Checking In

  1. How many magnetic field reversals have occurred in the past 3 million years? Do magnetic field reversals occur in regular time intervals?
  2. When was the last time the Earth's magnetic field reversed direction?
  3. Refer back to your geologic time scale from Lab 5. Which geologic periods are associated with the Matuyama-Gauss reversal that occurred ~ 2.6 million years ago? What geologic epochs occur at this magnetic field reversal?


Analyzing Paleomagnetic Data: Estimating the Age of the Sediments

You will now practice interpreting paleomagnetic data collected during Expedition 341. The plots below show the inclination values of the same section of core you analyzed in "The Core Lab" visualization in Part A of this activity. Positive inclination values (0◦ to 90◦) represent periods of normal polarity, while negative inclination values (0◦ to -90◦) represent periods of reversed polarity. Using the data, you will create a "barcode" of normal and reversed polarity from that you will compare to the Geomagnetic Polarity Timescale (GPTS) to help determine the ages of sediments from "The Core Lab."

Lab Procedure

  1. Use the blank "Polarity" column to the right of the data to record periods of normal polarity (shade in) and reversed polarity (leave blank). Shade in periods of normal polarity, and leave periods of reversed polarity blank. Use the worksheet provided by your teacher or download it here (Acrobat (PDF) 206kB Nov4 21).
  2. Interpret the data by matching the barcode pattern that you have drawn to the GPTS. Keep in mind that the thickness of the bars that you have drawn may be different from the thickness of the bars on the GPTS due to different rates of sediment deposition. Draw dashed lines from the polarity barcode pattern that you have drawn across to the GPTS to match up the ages of the sedimentary layers and answer the questions that follow.

Stop and Think

8. According to the data, what is the approximate age of the sedimentary sequence you described in the Core Lab Visualization, from ~260 to ~ 360 meters below the seafloor?

9. During which geologic epoch(s) was this sedimentary sequence deposited? (Hint: Refer to your Miocene Timeline)

10. Refer back to the geologic timescale you created in Lab 5. What significant events were happening on Earth at this time?

11. What additional information might be needed to verify the ages of the sediments?