Geoscience Reference
In-Depth Information
Figure 2.31. Plot of R(CO
2
) vs. time based on proxies, compared with geological models
(adapted from Berner, 2004).
Proxies have been used to estimate CO
2
levels during the Phanerozoic.
According to Berner (2004),
''Methods include determining (1) the
d
13
C of carbonates in paleosols; (2) the
stomatal density of fossil leaves; (3) the degree of fractionation of carbon
isotopes of specific compounds secreted by phytoplankton and preserved in
sedimentary rocks; and (4) the boron isotopic composition of marine carbonate
fossils. Each of the methods has its own problems, but if certain precautions are
taken, they provide reasonable estimates of ancient CO
2
levels.''
In addition, various proxies have yielded the curve of R(CO
2
) vs. time as
shown in
Figure 2.31
.
According to Berner (2004),
''There are problems with all of the methods of CO
2
estimation, and the
[R(CO
2
)] curves are not intended to be used as an accurate CO
2
measure (as is
sometimes mistakenly done), but rather as a suggestion of how CO
2
has changed
over the Phanerozoic. New advances
...
will undoubtedly cause modifications
...
.''
Rothman (2002) used the difference between the
d
13
C of bulk organic matter
and calcium carbonate (
d
13
C) to calculate the value of atmospheric CO
2
concen-
tration during the Phanerozoic as shown in
Figure 2.32
.
Rothman's estimates for
R(CO
2
) are much smaller than those of Berner (2004). Rothman acknowledged
this difference but felt that his estimates were justified. However, Berner (2004)
Search WWH ::
Custom Search