Geoscience Reference
In-Depth Information
those who search for nontraditional explanations to “standard wisdom.”
hen Joe, or someone else, finds an anaerobic sterane synthesis path-
way, I will happily change my views.
There's also the issue of the oxygen “whiffs” that we discussed in the
last chapter. Here, in my opinion, Joe sidesteps the evidence. He doesn't
really explain how molybdenum can be liberated from the continents
and into the oceans in the absence of oxygen. Also, why are the molyb-
denum enrichments associated with other evidence for oxidative weath-
ering on the continents like enrichments in rhenium? It seems to me
that the best way to explain the geologic evidence is to accept that cya-
nobacteria evolved before the GOE, as we discussed in the last chapter.
Therefore, we are challenged to look to other reasons for the rise of oxy-
gen at the GOE.
Maybe we can gain some additional insight if we look again at the
controls of oxygen as explored in
chapter 5
. If you recall, oxygen is
liberated to the atmosphere, ultimately, from the burial of pyrite sulfur
and organic carbon in sediments. Perhaps we can see some evidence for
this in the geologic record? The sulfur story is interesting, and we will
take it up in the next chapter, but take my word for it here, there is no
evidence that a massive burial of pyrite caused the GOE. hat about
carbon then? To explore this we need to return to isotopes. The basic
story is as follows. There are two major chemical forms of carbon in
nature. There's so-called inorganic carbon, like the CO
2
in the atmo-
sphere and the bicarbonate ion (HCO
3
-
) in natural waters,
4
and then
there's organic carbon, the stuff of life. Inorganic carbon enters the ocean
mostly from rivers and mostly in the bicarbonate form and it leaves the
ocean as either organic carbon, the remains of life, or as some type of
calcium carbonate mineral; think of shells, corals, and limestone. Quite
simply, what comes into the oceans must ultimately leave. Carbon comes
into the oceans as inorganic carbon and it leaves as either organic or
inorganic carbon.
Now here's where the isotopes come in. Inorganic carbon comes into
the ocean with a ratio of C-13 to C-12 atoms that we usually assume
hasn't changed much through time. As we explored in
chapter 6,
the
organic carbon produced by carbon-ixing organisms like cyanobac-
teria and algae (these are the main primary producers of organic matter
today in the sea) contains more C-12 than the inorganic carbon from
