Geol. 656 Isotope Geochemistry
Chapter 10
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solved CO2; Pagani et al. (1999) determined this from the isotopic composition of planktonic fora-
minifera shells in the same layer. Combining paleotemperature estimates based on δ18O discussed in
the previous section with estimated [CO2]aq estimated using equation 10.04, they estimated atmospheric
CO2 through most of the Miocene and late Oligocene (Figure 10.40). The results were surprising be-
cause they showed that CO2 has been near its pre-industrial modern level throughout most of the Mio-
cene. Thus the cooling that occurred in the late Miocene seen in Figure 10.38 was not due to decreasing
atmospheric CO2 as was widely suspected. PCO2 does appear to declined sharply at the Oligocene-
Miocene boundary, coinciding with a known glacial event, but otherwise there is relationship to appar-
ent climate change over this period. As we shall see in the next chapter, boron isotopic measurements
largely confirm these results.
We should also caution that these results do not negate the now well-established control that CO2 ex-
erts on climate. We have seen that atmospheric CO2 correlates strongly with temperature through the
Pleistocene climate extremes. And, as we shall see, atmospheric CO2 was higher in the early Tertiary
and Cretaceous when climate was warm. As of 2011, atmospheric CO2 stood at over 390 ppmv. That
this is higher than it has been at any time over the last 25 million years is very much reason for concern.
The Phanerozoic Carbon Isotope Record and Models of Atmospheric CO2
Figure 10.41 shows the Phanerozoic variation of δ13C in and δ34S in marine carbonates, both of which
presumably record the isotopic composition of seawater at the time of deposition. (This long-term re-
cord misses some short-term events, such as dramatic negative shifts in δ13C associated with the Permo-
Triassic and Cretaceous-Tertiary extinctions that are captured by detailed sampling such as that shown
in Figure 10.39.) The record does reveal a gradual increase in δ13C and decrease in δ34S through much of
the Paleozoic and a more dramatic increase in δ13C (and decrease in δ34S) associated with the expansion
of land plants and high rates of
burial of organic carbonic in
the Carboniferous.
We expect the isotopic com-
positions of carbon and sulfur
to be linked because burial and
erosion of reduced sediment
(organic carbon and sulfide)
affect the concentration of at-
mospheric oxygen. Because of
mass balance, photosynthesis
and subsequent burial of or-
ganic carbon increases δ13C in
the ocean-atmosphere system
and also increases atmospheric
O2 concentration. The latter
enhances sulfide oxidation.
Sulfide (and other forms of re-
duced sulfur) is isotopically
light, so when it is oxidized to
sulfate and added to the ocean,
it lowers seawater δ34S. This
mechanism accounts for the
shift to more positive δ13C and
more negative δ34S in the Car-
boniferous, when the terres-
Figure 10.41. Isotopic compositions of carbon and sulfur in the oceans
through Phanerozoic time. δ34S from Kampschute and Strauss (2004);
δ13C from the compilation of Berner (2006a).
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