Geoscience Reference
In-Depth Information
500
Phanerozoic
Proterozoic
400
300
200
100
0
2.5
modern seawater
2.0
1.5
1.0
modern river water
0.5
0.0
0
500
1000
1500
2000
Age (millions of years)
Figure 10.3. Concentrations and isotopic compositions of molybdenum over the last two
billion years. Data from a variety of sources through a compilation kindly made available by
tais Dahl with additional data from sahoo et al (2012).
animals. One possible way to explain the emergence of larger animals
would be an increase in atmospheric oxygen concentrations. We will
therefore follow this line of reasoning and try to deal with three related
issues: (1) If oxygen rose, how high did it go? (2) hat caused the rise?
(3) hat might be the relationship between rising oxygen and animal
evolution?
Let's begin by taking on the first issue. Independent of any consider-
ations about animals and their needs for oxygen, can we say anything
about late Neoproterozoic atmospheric oxygen levels? Unfortunately,
our geochemical tool kit is not yet developed enough to provide a spe-
cific answer, but with some careful reasoning, we can maybe come up
with some rough estimates. Let's start with the molybdenum isotopes.
hile it's true that molybdenum isotopes point to an increase in the
removal of molybdenum from the oceans into oxic marine settings in
 
 
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