Analyzing Sediment Cores

Part E: Oxygen Isotopes: A Proxy for Sea Surface Temperature

Now that you have gathered visual descriptive information and have estimated the age of a section of deep water seafloor sediments from The Gulf of Alaska, it's time to compare your findings to the global climate record. As you have learned, the Earth has cycled between ice ages and warm periods, but what evidence do scientists have for these extreme fluctuations? Studying the chemistry of marine organisms provides scientist with a proxy of the temperature on Earth thousands, or even millions of years ago! As you will learn, oxygen isotopes are one of the most important proxies for deciphering past climates.

Watch the following video as Exp. 341 scientist, Dr. Alan Mix, explains how oxygen isotopes can be used to predict what temperatures on Earth were millions of years ago:

Checking In

  • Oxygen-18 and Oxygen-16 are two important isotopes of oxygen. What is the difference between these isotopes?
  • How does the composition of oxygen in the ocean change during an ice age?
  • How can scientists determine the composition of oxygen in the ocean at any given point in geologic history?
  • Scientists graph the ratio of 18O/16O as a proxy for past sea surface temperature (SST). Would you expect this ratio to be higher during a glacial period, or interglacial period? Explain.

Data Spotlight: The Oxygen Isotope Record and Global Sea Surface Temperature

The graph below shows the ratio of 18O/16O, also called the delta 18O record a synonym for the ratio of 18O to 16O. Several different kinds of data have been plotted on the graph:

  • Black line - Shows the delta 18O record obtained from benthic foraminifera in deep sea sediment cores collected from the northeastern Pacific Ocean.
  • Red line - Represents sea surface temperatures derived from the delta 18O record.
  • Horizontal dashed line - Shows the modern-day 18O/16O ratio of the world's oceans.

Notice that the delta 18O axis is shown on the left side of the figure, and the axis is inverted so that values increase downward. Examine the graph and answer the questions that follow.

Stop and Think

15. Compared to modern levels, how has the delta 18O record changed over the past 8 million years?

16. How has the sea surface temperature (SST) of the world's oceans changed over the past 8 million years?

17. What is the relationship between delta 18O values and sea surface temperature (SST)?

18. Based on the ages of the sediments in core U1417B, how were the delta 18O values in the world's oceans changing at the time these sediments were deposited?

19. What does the delta 18O record tell you about how the climate may have been changing in S. Alaska when these sediments were being deposited?