Lab 6 Homework ANSWERS
GEO 302C
If the answers to this lab were not addressed in class, check out Ruddiman.
Warm up True and False:
1. (4 points) True or False? 12C is the most common isotope of carbon on Earth.
2. (4 points) True or False? Earth’s obliquity ranges from 20 to 30 degrees. Earth’s
obliquity ranges from about 22.2 to 24.5 degrees with a period of 41,000 years
3. (4 points) True or False? The Earth’s tilt causes seasons
4. (4 points) True or False? Earth’s eccentricity controls the extent to which insolation
changes between summer and winter. Insolation changes between summer and winter
is a fancy way to describe the differences between seasons. Earth’s obliquity is what
controls the seasons. See Ruddiman p. 175 for a good picture.
5. (4 points) True or False? When Earth’s climate is considered an “icehouse,” there are no
greenhouse gases in Earth’s atmosphere. During “icehouse” climates, there are still
greenhouse gases (e.g., carbon dioxide, methane) in Earth’s atmosphere; however,
the concentration of greenhouse gases is much lower during icehouse climates than
during greenhouse climates. (“None” is usually a flag for a false statement.)
6. (5 points) Define perihelion (one sentence or less). (2 points) What is the date of perihelion
today? Quoting Ruddiman: “The point in Earth’s slightly eccentric orbit at which it is
closet to the Sun.” Today perihelion occurs on January 3, which results in a less-severe
winter than would occur if aphelion occurred during winter.
7. (5 points) Define aphelion (one sentence or less). (2 points) What is the date of aphelion
today? Quoting Ruddiman: “The point in Earth’s slightly eccentric orbit at which it is
farthest from the Sun.” Today aphelion occurs on July 4. If we were closest to the sun
during the northern hemisphere summer, our summer would be more extreme.
8. (6 points) What causes the date of aphelion and perihelion to change?
Short answer:
Precession
Medium-length answer:
Axial precession (the wobble of Earth’s axis of rotation) and orbital precession (the wobble
of Earth’s orbit around the Sun) combine to shift the seasons during which Earth reaches
aphelion and perihelion. This combined effect has a period of 23,000 years.
How does eccentricity get in the mix here? Changes in the eccentricity of Earth’s orbit
affect the STRENGTH of the effect of precession on insolation. When Earth’s orbit is
nearly circular, the distance between Earth and the Sun at aphelion is very similar to the
distance between Earth and the Sun at perihelion. So if eccentricity is low (that is, if
Earth’s orbit is very nearly circular) then precession of the equinoxes doesn’t have a big
effect on climate.
9. (5 points) Define δ13C. (5 points) How does the δ13C of living vegetation differ from that of
the ocean? (5 points) Why does it differ?
NOTE! δ13C is NOT the same thing as 13C!
13
C is an isotope of carbon. A 13C atom has an atomic weight of 13. It is heavier than 12C,
the light isotope of carbon.
A δ13C value is the difference between the ratio of 13C to 12C in a sample and the ratio of
13
C to 12C in a standard reference ratio. (See the text box on Ruddiman p. 243 for the
mathematical formula and a more informative discussion about δ13C.) When you say that a
sample has become more “enriched” in 13C, you’re saying that the δ13C (“delta 13 carbon”)
value has gotten higher. So when one sample (e.g., an ocean sediment core) has a higher
δ13C value than another sample (e.g., a chunk of coal), the sample with the higher δ13C
contains more 13C atoms for every 12C atom. Stated another way:
The more positive the δ13C value, the more 13C-enriched a sample is.
The more negative the δ13C value, the more 13C-depleted a sample is.
(Note that if a sample (e.g., a chunk of coal) is depleted in 13C, it is by definition enriched in
12
C.)
Why do we care about the relative enrichment or depletion of 13C? Photosynthesis
fractionates carbon. This means that the process of photosynthesis (in which plants use
carbon dioxide and water to capture energy from the sun) changes the ratio of 13C to 12C in
a sample. Although plants CAN use 13CO2 during photosynthesis, they “prefer” to use
12
CO2. (Plants are “light” eaters – they “prefer” the light isotope of carbon to the heavy
isotope.) So photosynthesis is the primary process that causes δ13C values to vary.
10. (10 points) Define δ18O. (5 points) If a layer of sea-floor sediment has a positive δ18O value,
what does that mean about Earth’s climate at the time that the sediment was deposited?
NOTE! δ18O is NOT the same thing as 18O!
18
O is an isotope of oxygen. 18O means you’ve got an oxygen atom and that oxygen atom
has an atomic weight of 18. It is heavier than 16O, the light isotope of oxygen, which has an
atomic weight of 16. The heavy oxygen contains more neutrons than the light oxygen.
A δ18O value is the difference between the ratio of 18O to 16O in a sample and the ratio of
18
O to 16O in a standard reference ratio. (See the text box on Ruddiman p. 151 for the
mathematical formula and a more informative discussion about δ18O.) When someone says
that one ocean-sediment core is more “enriched” in 18O than a second core, they’re saying
that the δ18C (“delta 18 oxygen”) value in the first ocean-sediment core is higher than that
in the second. A sample that is 18O-enriched (that is, a sample with a more-positive δ18O
value) contains more 18O atoms for every 16O atom. Restated in a different way:
The more positive the δ18O value, the more 18O-enriched a sample is.
The more negative the δ18O value, the more 18O-depleted a sample is.
(Note that if a sample (e.g., a chunk of coal) is depleted in 18O, it is by definition enriched in
16
O.)
So what? Evaporation and condensation fractionate oxygen isotopes. This means that the
processes of evaporation and condensation change the ratio of 18O to 16O in a sample.
Evaporation favors the light isotope of oxygen (16O). Condensation favors the heavy isotope
of oxygen (18O). Note that water containing an 18O atom still evaporates and water
containing a 16O atom still condenses. But because water containing an 18O atom is heavier
than water containing an 16O atom, it is EASIER for the light water to evaporate than it is
for the heavy water to evaporate. Correspondingly, it is EASIER for the heavy water to
condense than it is for the light water to condense.
If deep-ocean sediments have positive δ18O values, then the total ratio of 18O to 16O in the
ocean has changed. Because there is so much water in the ocean, the only way to
appreciably change the average δ18O value of the entire well-mixed ocean is to change the
quantity of ice stored on land. (Ice is depleted in 18O [and thereby enriched in 16O];
correspondingly, ice stored in continental ice sheets has a negative δ18O value.) Positive
δ18O values in ocean-core sediments mean that lots of 18O-depleted water was stored in
continental ice sheets. It is reasonable to assume that if lots of ice is stored in glaciers,
Earth’s average surface temperature at the time that that ocean sediment was deposited
was colder than it is today.
11. (3 points) When was the Last Glacial Maximum?
About 20,000 years ago. (Acceptable answers include anything near 20,000.)
Acceptable answers include:
18,000 14C years before present.
21,000 calendar years before present.
12. (2 points) How far south did the Laurentide ice sheet extend at the last glacial maximum? (5
points) How do scientists know this?
To approximately 37 degrees N (into the Midwest United States). Acceptable answers here
included drawings of the location of the Laurentide ice sheet, “Great Lakes”, etc.
Scientists know this for several reasons, one of which is the 14C dating of glacial moraines
(sediments deposited at the edge of an ice sheet). (N.B.: Carbon-14 is a third, radioactive
isotope of carbon that is very rare but that can be used to figure out how old things are.)
Other acceptable answers include CLIMAP model reconstructions, reconstructions using
glacial striations, etc.
13. (5 points) Why are the air bubbles in ice cores younger than the ice that surrounds them?
See Ruddiman p. 236.
14. (15 points; 5 each) Give three sources of evidence that lead scientists to believe that, 100
million years ago, Earth’s climate was warmer than it is today.
Answers can include, but are not limited to:
1. Fossils of warm-adapted animals (e.g., dinosaurs) found in places that were at high
latitudes 100 million years ago
2. Fossils of warm-adapted vegetation found in places that were at high latitudes 100
million years ago
3. 100-million-year-old marine sediment (limestone) deposits (e.g., the Edwards
Aquifer limestone here in Austin), which are many meters above sea level today,
showed that sea level was much higher than it is today.