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100
90
80
70
marine clastics
60
coal basin
50
other cont. clastics
40
30
20
10
0
0
100
200
300
400
500
Age (millions of years)
Figure 11.1. Distribution of sediment types through the Phanerozoic eon. Data from A. B.
ronov as summarized in Berner and Canfield (1989).
We need one small additional detail. After some initial model runs, it
seemed that some feedback in the sulfur cycle would be important. That
was my job, and my modest contribution to the study. I figured out that
an oxygen dependence on sulfur burial in marine sediments would be
reasonable. This idea was explored in chapter 5, and the basic idea is that
as atmospheric oxygen concentration lowers, there should be a spread
of anoxic, sulfidic, conditions in the oceans. This leads to an increase
in pyrite burial rates and hence an increase in rates of oxygen liberation
to the atmosphere. The effect is a negative feedback; enhanced rates of
sulfur burial under low oxygen conditions helps oxygen from getting
too low.
To get some sense for the model results, rates of organic carbon burial
calculated from the sediment abundance trends are shown in igure 11.2.
These are also compared with organic carbon burial rates calculated
from modeling carbon isotopes, as Bob did in his first study. To my eye
anyway, there is a remarkable similarity between these two calculation
 
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