Lab 6 30 points Name: __________________
Due Date: Monday April 8 (Hand in before class) Lab: M W 1 2
Late assignments will be docked 5 points each day late.
This assignment covers some of the important concepts from Chapter 9. Short answers are acceptable for many of the questions. For questions marked with “*”, please answer using complete sentences and correct grammar. This assignment will be posted on the web later today so you can copy and paste it into word, if you like.
Chapter 9 The Chemical Environment: Evolution of the atmosphere
1. Describe the three theories on the origin of life given in the text.*
For details see text.
1) Chemical sysnthesis on earth
surface
Miller-Urey experiment - formation of amino acids and proteins from inorganics,
reducing atmospheric conditions and electrical discharge. These building blocks
eventually turned into life forms over time.
RNA World – RNA formed before proteins from abiotic
precursors.
2) Organic compounds were formed in space and transported to earth
3) Chemical synthesis in oceanic hydrothermal vents
Which explanation seems most plausible to you and why?* (4 points) Answers will vary
2. Geologic Evidence of the Rise of Oxygen (18 points)
a. What are the three oxidation states of iron?
Elemental, Ferrous (Fe2+) and Ferric (Fe3+)
b. Which state is soluble in water? Ferrous (Fe2+)
c. What happens when the soluble form of iron is exposed to oxygen?*
It loses electrons and is oxidized: Fe2+
-> Fe3+
Note that for Fe to react with oxygen to form FeO, oxygen has to be in its ionic
form O2-
which is different from its dominant state in the atmosphere at O2
d. What three minerals are found in BIFs? magnetite
(Fe3O4), hematite (Fe2O3), and chert (SiO2)
e. Where in the ocean did BIFs form? Continental
shelves How do we know this?*
If it had been formed on the deep sea it would have been subducted.
Also the waters had to be oxygenated for the dissolve iron (FE2+) to
precipitate out (Fe3+ form).
f. Why can we “safely conclude that the deep oceans must have been anoxic
prior to 1.9 billion years ago, during the time that BIFs formed”?*
Because of the large volume of BIFs, iron must
have been transported in its soluble form (Fe2+). If the oceans were not anoxic,
the iron would have precipitated out. Iron
probably derived from weathering of continental rock and from mid-oceanic
hydrothermal vent fluids. What does BIF formation imply about atmospheric
oxygen level?* BIF
formation implies, that the oceanic waters were oxygenated at least to some
degree when they were formed. Oxygen
would have present mostly in surfaces waters either due to UV radiation that
would break down water molecules and oxidize the iron, from biogenic production
of oxygen by photosynthetic cyanobacteria in surface waters, or by wind induced
upwelling which may have occurred along continental shelves. BIFs did not occur
before 1.9 billion years ago, so before this time the ocean must have been
anoxic. Because the amount of dissolved gasses in the ocean waters equilibrates
with the atmosphere, we know that little to no oxygen must have been present in
the atmosphere.
g. What form of uranium is soluble in water? U6+ the
oxidized form
h. What is uraninite? UO2 What ion forms it? U4+
i. You have one sedimentary rock that is 3 billion years old and another that is
1.1 billion years old. How do the
uranium components in the two rocks differ?*
The older rock would contain uraninite in detrital form, whereas the
younger rock would probably not contain uraninite (if it did only small
quanitity and would not be in detrital form.
j. How was the weathering of pyrite different prior to 2.2 billion years ago
than it is today?* Pyrite
would have been weathered and stayed in detrital form. Today the pyrite would be
oxidized with its sulfur changin to sulfate and iron oxidized to Fe3+.
k. Paleosols older than 2.2 billion years have lost significant amounts of iron
compared to younger (<1.9 billion year old) palesols. Explain why this is
so.*
When oxygen had only a minimal concentration in the atmosphere, the iron in the
paleosols would have been soluble and removed by weathering processes.
The iron exposed to oxygen would form the insoluble Fe3+ and stayed in
the soil.
l. What are redbeds? Redbeds are reddish-colored sandy
and silty sediments. Where do they form? In arid
regions such the American Southwest.
m. What do redbeds indicate?* Redbeds indicate
oxidizing conditions at the time of their formation because the redbeds are
coated with hematite (Fe2O3) which is an oxidized form of iron.
n. To summarize this section, what geological evidence do we have for the rise
of oxygen? (Be concise.)
- BIFs formed only after 1.9 bya
- Detrital uraninite deposited only before 2.2 bya
- Detrital pyrite deposited only before 2.2 bya
- Iron-depleted
paleosols that are all older than 2.2 bya
- Iron-rich
paleosols younger than 1.9 bya
- Formation of redbeds after 2.2 bya
a. What does the graph suggest about CO2 levels over
earth’s history?*
Sorry about this one. I thought I had made a typo should have been 02 levels.
The graph implies that carbon burial had remained fairly constant with time.
Therefore the source of oxygen (from carbon burial) also can be inferred to have
remained constant – or at least as large (if not larger) as it is today.
b. Why do the delta13C differ for carbonates and organic carbon?*
The13C for carbonates reflects the isotopic composition of bicarbonate in the
seawater. Recall from Chapter 7 (page 144) that most carbonate is formed by
remains of organism that had incorporated the bicarbonate into their shells.
The 13C content of dissolved bicarbonate depends on the relative burial
rates of organic carbon and carbonates. When the burial rate of isotopically
light (low C13) organic carbon is high, the remaining bicarbonate in the oceans
becomes heavy. When the burial rate
of (lowC13) is low, the dissolved bicarbonate becomes iostopically lighter. For
organic carbon, instead of the bicarbonate determining whether its light or
heavy isotopically, the availability of carbon dioxide in the atmosphere
determine the isotopic ratio. Remember that organisms preferentially uptake C12
instead of C13 when CO2 is plentiful (Chapter 8 p. 168)?
On Figure 9-15, the organic carbon does show greater variability – in part
because of the greater natural variability in these processes and greater
disturbance rates of organic carbon (better food source for benthic organisms)
than carbonates.
c. How are O2 and CO2 atmospheric levels linked?*
They are link by the following photosynthetic and respiration equation:
Photosynthesis -> CO2 + H20 <-> CH2O + O2 <- respiration
d. When did cyanobacteria start producing oxygen? 3.5 bya When did atmospheric oxygen become apparent? 2.2 bya What are the various explanations for the delayed rise in oxygen levels?* Its is thought that the sinks for oxygen were much larger in the past than they are today. Today volcanic gases consist largely of H20 and CO2 (note both are greenhouse gases) and reduced sulfur (which forms sulfate aerosols and has a short term cooling effect). However, these gases may have occurred in different concentrations in the volcanic explosions past. Its possible that the mantle of the earth was more reduced (less oxidized) in the past than today. Greater emission of reduced gases from volcanoes would represent a greater oxygen sink in the past. Weathering rates in the past were also likely to be greater than occur today. Before oxygen became present in the atmosphere, the weathering rate of rocks would have been much slower. These rates would have increased with the rise of oxygen until a steady state was reached over time.
Please proofread and edit your work. Make sure your name is on the typed pages.