OCG123, Spring 2002
Answers to third hourly exam, Monday 8 April 2002
Chapters 7–9
Instructions:
Read over the entire exam first. Begin
with the questions you feel most confident about. Pace yourself so that you will
have time enough to try everything. If you don’t know how to get all the way
to an answer, go as far as you can and state how you would approach the rest.
And keep your handwriting neat! Use the back of the sheets if necessary. Show
your work so that I can give you all the partial credit you deserve.
Definitions (2 points each; 20
points total)
1. Marine biological pump—The
mechanism of transferring organic matter and nutrients from upper waters to
deeper waters by the settling of fecal matter and dead organisms.
2. Primary producer—A plant that
photosynthesizes organic matter that can be used by consumers (animals).
3. Reduced carbon—Carbon combined
mainly with hydrogen, nitrogen, or other carbon atoms. Organic matter is a
common form of reduced carbon.
4. Terminal moraine—A ridge of
sediment deposited at the end of a glacier as it is no longer advancing.
5. Hydrothermal vent—A crack in the
seafloor near mid-oceanic ridges, through which reduced materials from deeper
levels are released.
6. Impact degassing—The release of
water, carbon dioxide, and other volatile materials directly into the atmosphere
as meteors, asteroids, or comets strike the earth’s surface.
7. Detritus—Mineral matter of various
sizes in sediments that was transported and deposited without being dissolved.
[N.B. The classical definition differs somewhat from the one in our book, and is
roughly “Loose material, such as rock fragments or organic particles, that is
a direct product of disintegration.” I gave credit for the organic answer.]
8. Uraninite—The insoluble oxide of
uranium (UO2) that has U at its lower oxidation state of +4. Its
presence in marine sediments means that the continental material originally
containing the U was weathered in the presence of low concentrations of oxygen.
9. Interplanetary dust particles—Small
particles that entered the earth’s atmosphere from space.
10. Redbeds—Sedimentary deposits
whose grains are coated with the colored mineral hematite, Fe2O3.
The iron’s high oxidation state (+3) signifies that it was weathered in the
presence of high atmospheric oxygen.
Short
answers (8 points each; 40 points total)
1. The atmosphere contains 10 Gton of
carbon in methane. About 0.5 Gton enters and leaves each year at steady state.
(a) Calculate the residence time of methane in the atmosphere. (b) Can methane
move between the hemispheres before being removed, and why?
(a) 10 Gton/0.5 Gton y-1 =
20 y residence time of methane in the atmosphere
(b) Methane can move repeatedly between
the hemispheres because the interhemispheric mixing time is a year or so, much
shorter than methane’s residence time.
2. (a) Explain the sequence of steps by
which atmospheric CO2 weathers crustal rocks. (b) Why does weathering
of silicate rock deplete CO2, whereas weathering of carbonate rock
does not?
(a) Weathering.
(1) CO2
dissolves in rainwater to form the weak carbonic acid, H2CO3.
(2)
The carbonic acid partially ionizes to form H+ and HCO3-.
(3)
The H+ attacks the minerals in the rocks and dissolves them.
(4)
The dissolved materials flow to the oceans in streams and rivers.
(b) Weathering of silicate rock depletes
CO2 because it converts continental silicate rock to marine carbonate
sediments, with the carbonate coming from the CO2. Weathering of
carbonate rock and creation of carbonate sediments does not deplete atmospheric
CO2 because the carbonate in the sediments comes ultimately from the
original carbonate in the rocks. (The precipitation of carbonate rocks releases
the CO2 that was used to weather the rocks.)
3.
(a) How old is the earth? 4.5–4.6 by (billion years)
(b)
When did the heavy bombardment period end? About 3.8 bya
(c)
When did life on earth arise? At or before 3.5 bya
(d)
When did atmospheric oxygen rise? 2.0–2.2 bya
(e)
When did an effective ozone shield appear? At or before about 2.1 bya, i.e.,
very soon after oxygen rose.
4. Explain the major differences between
the early atmosphere and today’s atmosphere. (a) The early atmosphere was
denser, with pressures up to ten times current levels. (b) It was weakly
reducing, with very high concentrations of CO2 (up to several bars),
methane, hydrogen, etc., and much lower concentrations of oxygen.
5. (a)
What was the likely cause of the Huronian glaciation of about 2.2 to 2.4 bya? The
greenhouse gas methane (CH4) being oxidized by the oxygen that was
just forming.
(b)
What was the likely cause of the Last Proterozoic glaciation, when most of the
continents were grouped together near the equator? Loss of CO2 by
rapid weathering of the silicate rock on the continents that were assembled at
the equator at that time. The rapid weathering would have been caused by the
warmth and humidity of the tropical regions.
(c)
What was the likely cause of the Permo-Carboniferous glaciation, when the great
coal deposits were being laid down? Sequestering of large amounts of carbon
in the sediments. That carbon would not have been able to be oxidized back to
carbon dioxide.
(d)
What was the likely cause of the warmth of the Mesozoic Period? Unusually
rapid spreading of oceanic plates and release of volcanic gases, including CO2.
Longer
answers (20 points each; 40 points total)
1. Nearly all banded-iron formations (BIFs)
were laid down before 1.9 billion years ago. Assume that the iron in BIFs exists
mainly as Fe3+ (Fe2O3, hematite) interspersed
by chert (SiO2). Explain in detail how the BIFs were formed and what
this tells us about the rise of oxygen in the atmosphere. Be sure to include in
your answer the continental shelf vs. the deep ocean, and Fe2+ vs. Fe3+.
The BIFs were laid down in the ocean.
Their fine particle size means that their Fe had to be transported as soluble Fe2+,
which in turn means that it had to be weathered from the continents under
conditions of low oxygen. Once the dissolved Fe reached the sea, it had to be
oxidized to the insoluble Fe3+. That presumably happened near the
surface, where some oxygen was available from microorganisms and UV radiation
was available. The Fe in question got near the surface because it was in water
over the shallow continental shelves rather than in the deep ocean basins, an
idea confirmed by the very preservation of the BIFs because they would have
disappeared by subduction if they had been laid down in the deep ocean. In the
shallow shelf waters the Fe2+ would have been regularly brought
nearer the surface as the waters mixed seasonally. There they could be oxidized
to Fe3+ and the BIFs created. The net result of all this would have
been to transport soluble iron to the sea, oxidize it to insoluble iron, and
deposition on the continental shelves. This process would have been halted as
soon as the concentrations of atmospheric oxygen increased enough to create
insoluble Fe3+ by weathering.
2. Recall Figure 9-15, which shows that
δ13C of carbonate in marine sediments has remained nearly
constant at 0‰ since 4 bya and that δ13C of organic carbon in
marine sediments has remained nearly constant at -25‰ over the same period.
Construct the series of steps in reasoning that shows that atmospheric oxygen
must have arisen because the oxidation state of the mantle increased. Be sure
that your answer includes (a) the two possible reasons why oxygen increased, (b)
which of these reasons is supported by the figure, and (c) and possible ways to
decrease the sinks of atmospheric oxygen.
For 10 points extra credit, explain why
the C in sedimental organic carbon is 25‰ lighter than the C in sedimental
carbonate carbon.
Basic answer.
(a)
Atmospheric oxygen could have increased because its sources increased or because
its sinks (losses) decreased. (This answers assumes that oxygen was being
created steadily since 4 bya.)
(b)
Since marine photosynthesis of dissolved CO2 to organic C creates a
ratio of 13C/12C in the organic material that differs in
one direction from the CO2 and leaves the residual dissolved CO2
with a ratio that differs in the other direction, changes in rates of
photosynthesis (in effect sources of oxygen) will change the 13C/12C
in both substances with time. But Figure 9-15 shows that the rate was constant,
however, because the two 13C/12C ratios remained constant.
This means that oxygen was being produced at near-constant rates since 4 bya.
(c)
Since the rate of production didn’t increase, the rate of loss must have
decreased.
(d)
The rate of loss could have been affected by changes in either of the two major
sinks for oxygen, oxidation of reduced continental materials and oxidation of
reduced volcanic gases.
(e)
The first mechanism, oxidation of reduced surficial continental materials, could
not have decreased because it wasn’t active then. (It requires higher
concentrations of oxygen than existed back then.)
(f)
The second mechanism, decreased oxidation of reduced volcanic gases, requires
not just reduction in the overall release of the gases, but a change in the
proportions of the reduced gases, i.e., an increase in the oxidation state of
the mantle.
(g)
The proposed mechanism, creation and escape
of hydrogen gas as Fe2+ in hydrated basalt was oxidized to Fe3+
while being subducted, fits all the requirements nicely.
Extra credit. The C in
sedimental organic material is lighter than the CO2 from which it is
formed because plants find it easier to grab the lighter 12CO2
than the heavier 13CO2 during photosynthesis. (Don’t you
prefer to lift lighter objects?) Making organic material from the lighter carbon
leaves behind the heavier carbon in the dissolved CO2, which then
goes on to form heavy carbonate sediments.