Oxygen Isotopes

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Stable isotopes of oxygen in sea water, and explores ways in which this has changed during the Quaternary. Both text and figures draw heavily on the water-based oxygen isotope part of a previously published essay on stable oxygen and carbon isotopes in foraminifera (Rohling and Cooke, 1999). In contrast to the current text, which offers. Oxygen is made up of two isotopes: Oxygen - 16 (also known as 16O 8 protons + 8 neutrons; a “light” oxygen); The relative amounts are expressed as either 18O/16O or δ18O Oxygen - 18 (aka 18O 8 protons + 10 neutrons; a “heavy” oxygen). 16 and 18 are atomic masses of isotopes 16O and 18O.

  1. Oxygen Isotopes Paleoclimatology
  2. Natural Abundance Of Oxygen Isotopes
  3. Oxygen Isotopes Abundance

Characterizing the relative amounts of different isotopes of an element can help identify where a sample came from.

Oxygen atoms have 8 protons and 8 electrons and exist as three main isotopes, which vary in the number of neutrons: 16O (8 neutrons), 17O (9 neutrons), and 18O (10 neutrons). The relative abundance of these stable isotopes can be expressed as ratios. 17O/16O is a measure of how much 17O there is compared to 16O (relative to a known standard and multiplied by 1000, because these are very small differences). Similarly, 18O/16O indicates how much 18O there is compared to 16O.

Natural processes separate the isotopes from each other on Earth, typically according to the mass of the isotopes. At different places on the planet, the amounts of the isotopes may differ, but their ratios will be the same. Therefore, their ratios fall along a line with a particular slope when plotted on a graph. The isotopic composition of nearly all rock samples from Earth fall along this line, which has a slope of 0.5.

Objects formed in different parts of the solar system contain different proportions of 16O, 17O, and 18O, so their rocks fall on different lines. This graph compares the oxygen isotope ratios for rock samples from the Moon, meteorites from Mars, and meteorites from asteroid Vesta, as compared to rock samples from Earth, represented by a line. Studies of other isotopes including silicon, chromium, titanium, and tungsten in Moon samples also show similarities with Earth samples.

Take Notes

Based on the evidence, construct an argument on whether data about isotopic ratios support the giant impact hypothesis.

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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:

An interview with Dr. Alan Mix. Alison Mote Download Video interview with Dr. Alan Mix(MP4 Video 31.8MB)Details

Checking In

Oxygen-18 and Oxygen-16 are two important isotopes of oxygen. What is the difference between these isotopes?
Oxygen-18 is a heavier isotope with 8 protons and 10 neutrons, while Oxygen-16 is a lighter isotope with 8 protons and 8 neutrons.
How does the composition of oxygen in the ocean change during an ice age?
During an ice age there is more Oxygen-18 in ocean water. This occurs because Oxygen-16 is lighter and is more easily evaporated from the oceans. During ice ages, Oxygen-16 is transferred from the atmosphere to the land as snow and becomes trapped in the ice caps.
How can scientists determine the composition of oxygen in the ocean at any given point in geologic history?
Microscopic organisms, called foraminifera, have shells made of calcium carbonate (CaCO3) and they make their shells with oxygen from ocean water. By measuring the ratio of oxygen-18 to oxygen-16 in their shells, scientists can determine the relative concentration of oxygen isotopes in the ocean at the time foraminifera were living.
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.

Oxygen Isotopes Paleoclimatology

The 18O/16O ratio would be larger during glacial periods because ocean water would be enriched in 18O, while more 16O would be trapped in the ice caps. If foraminifera are making their shells during this time the concentration of 18O in their shells will be greater.

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

The graph below shows the ratio of 18O/16O, also called the . 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.
  • Natural Abundance Of Oxygen Isotopes

    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.

    Oxygen isotope graph. Source: IODP

    Stop and Think

    1. Compared to modern levels, how has the delta 18O record changed over the past 8 million years?
    2. How has the sea surface temperature (SST) of the world's oceans changed over the past 8 million years?
    3. What is the relationship between delta 18O values and sea surface temperature (SST)?
    4. 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?
    5. 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?

    Oxygen Isotopes Abundance