Isotope Chemistry in Oceanography

GEOL 1033

(Lesson 24, especially p. 161 in the Study Guide)

Ppt file 103-21

Isotopes• Isotopes = same atomic number (= # protons in nucleus),

but different atomic weight (= # protons + # neutrons in nucleus)

• Number of neutrons in nucleus varies, so atomic weight varies accordingly

• Physical properties will vary slightly from isotope to isotope of the same element

• Some isotopes are unstable, i. e., radioactive• Used to study some oceanographic phenomena

(Sr-90 example)• Two commonly used isotopes are

– Carbon-14

– Oxygen-18

Carbon-14• Used to date carbon-bearing materials, e. g., shells,

wood, etc.

• Carbon isotopes12C is the most common13C is uncommon14C is rare & radioactive

– Forms from 14N in upper atm.– From cosmic ray bombardment– Decays back to 14N with a half-life of 5 730 years

• Used for dating up to 50 k years (usually) to about 100 k years (special circumstances) ago

Half-Life Concept of a Radioactive Isotope

• Time it takes for each successive half of the remaining amount of the isotope to decay

• Example: if a ratio (R) of 14C (parent material) to 14N (daughter product) is 14C / 14N,

then (1/8) ÷ (7/8) = (0.125) ÷ (0.875) = 0.143,

so the age is 3 x 5730 y = 17 190 y.

• Example: <10 k y old shallow-water shell beds of the outer continental shelf were dated this way, indicating lower sea levels during and soon after the continental glaciers of the last ice age began to recede.

T = 0 y T = 5 730 y T = 2 x 5 730 y T = 3 x 5 730 y

All 14C 1/2

1/4 1/8

7/83/41/2

R = 0.143

Oxygen-18• Oxygen isotopes:

16O = most common = about 99.7% of total O17O = very little amount18O = least abundant, but “heaviest”

• Ratio of 18O / 16O is a temperature indicator and a measure of glacial ice volume.

• If you analyze tropical foraminiferal tests (=shells) deposited on the deep-sea floor during a past ice age, – Ratios show higher 18O level, meaning higher concentration in

oceans at that time (when the forams were alive)– Evaporation of water from oceans favours the lighter isotope – Lighter isotope becomes preferentially concentrated in

continental glaciers after precipitation.– Heavier isotope stays behind & is concentrated in seawater.

• The normal ratio is restored in later foram tests when glaciers melt and return the water to the ocean basins

Using 180 for Paleoclimate Studies• Comparison of 18O/16O ratios for Atlantic Ocean surface waters to

calculated summertime solar insolation in the northern hemisphere for the last 350 000 years

Using 180 for Correlation Studies

• Correlating sediments in Pacific and Caribbean cores for last 700 k y

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