Which of the following correctly compares the oxygen isotopes 10 and 180?
O
A. 160 has 16 neutrons, whereas 180 has 18 neutrons.
O
B. 160 has 8 neutrons, whereas 180 has 10 neutrons.
O
O
c. 160 has 8 protons, whereas 180 has 10 protons.
D. 160 has 16 protons, whereas 180 has 18 protons.
SUBMIT

 Oxygen Isotopes Appendix I: p. 359-361; Ch. 6.2 p. 100-101 • How can we tell the temperature in the geological past? • What are isotopes? • Compare two isotopes of oxygen in ice cores, coral reefs, and sediments.

Introduction to Isotopes Atoms of all matter on Earth are made up of a nucleus and electrons. Atoms with the same number of electrons and protons, but different numbers of neutrons, are called  

Isotope Geochemistry Isotopes are forms of a chemical element that have the same atomic number but differ in mass.  Oxygen is made up of two isotopes: Oxygen - 16 (also known as 16O8 protons + 8 neutrons; a “light” oxygen);   Oxygen - 18 (aka 18O8 protons + 10 neutrons; a “heavy” oxygen).  16 and 18 are atomic masses of isotopes 16O and 18O.  The relative amounts of these two isotopes in a sample of water, ice, rock, plant, human, etc. is a function of climate/environment  The relative amounts are expressed as either 18O/16O or δ18O  16O ~99.8% 18O ~0.2%  18O/16O = 1/400 = 0.0025    

Oxygen isotope ratio (δ18O) δ18O (in ‰) = [18O/16O)sample – (18O/16O)standard]×1000 / (18O/16O)standard  

Why does δ18O decrease with T?   The concentration of 18O in precipitation decreases with temperature. This graph shows the difference in 18O concentration in annual precipitation compared to the average annual temperature at each site. The coldest sites, in locations such as Antartica and Greenland, have about 5 percent less 18O than ocean water. (Graph adapted from Jouzel et. al., 1994) Source: NASA Paleoclimatology webpage

Isotope Fractionation Enrichment process (isotope fractionation): The lighter 16O evaporates first. The heavier 18O condense out first.  18O depleted, 16O enriched  δ18O more negative    18O enriched, 16O depleted  δ18O more positive  

Isotope Fractionation  Enrichment process (isotope fractionation): The lighter 16O evaporates more easily. The heavier 18O is easier to condense out.

Typical δ18O Values         In the modern tropical oceans, 0 to –2‰ surface waters, +3 to +4‰ deep waters  In ice sheets, –30‰ in Greenland, –55‰ in Antarctica

Oxygen isotope ratios as a thermometer Precipitation has less 18O than ocean, why? 18O content of Precipitation at the give latitude decreases with decreasing temperature. Why? The less 18O found in the glacier ice, the colder the climate.

Long-term δ18O trend in the deep ocean as measured from the calcite shells of foraminifera    Two factors: (1)  Changes in deep- ocean temperatures (2)  Growth of ice sheets on land (16O enriched)

Oxygen isotope ratios as a thermometer more O18 depleted ocean water suggests more rainfall, less ice, warmer T. Less O18 depleted ocean water suggests more ice and colder T.

Summary: •  What is isotope? –  Atoms with the same number of electrons and protons, but different numbers of neutrons. •  ? –  Temperature: colder temperature  more O18 depletion (more negative δO18) –  Condensation  preferentially remove O18 (heavier isotope) –  Evaporation  preferentially remove O16 (higher isotope) •  ? –  Pastearth’.

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