Geoscience Reference
In-Depth Information
excursion in Earth history. This excursion was first fully appreciated in
1996 by Dick Holland and his colleague Juha Karhu from the University
of Helsinki. 5 They viewed the excursion, and its associated burial pulse
of organic carbon, as the source of oxygen driving the GOE. Problem
apparently solved. However, if you look again carefully at the graph,
you can see that things don't quite add up. Recent and better dating
now puts the Lomagundi isotope excursion after, rather than during,
the GOE. Rats, it made so much sense. We are forced to look for an-
other cause.
To introduce the next possible cause we start with those fleeting whiffs
of oxygen from the late Archean Eon, as discussed in the last chapter.
During these whiffs, it seems as if the atmosphere experienced periodic
pulses of oxygen, only to see them vanish again. We also offered in the
last chapter a tentative explanation for the whiffs, suggesting that at
this time in Earth history, the flux of reducing gases from the mantle
was close to the flux of oxygen liberation from organic carbon and py-
rite burial. Most of the time the volcanic flux was in excess, but occa-
sionally, the balance tipped toward an excess in oxygen liberation gen-
erating a whiff of oxygen to the atmosphere.
Let's pursue this line of logic, but to do it right, we need to start way
back, toward the beginning of Earth time. Indeed, we need to go back
to before the beginning of the rock record, to a time when we can only
use our wits and make our best guesses. hat we want to know is the
rate at which oxygen-reactive gases, mainly hydrogen (H 2 ), spewed out
of volcanoes when Earth was really young.
How does one even hazard a guess? Well, let's start with today. We
have some idea of how much hydrogen gas comes out of volcanoes, at
least within a factor of probably 2 to 3. The degassing rate of hydrogen
will depend on the chemistry of the mantle, but as we touched upon in
the last chapter, this has probably not changed too much through most
of Earth history, so we won't worry more about that. The hydrogen de-
gassing rate should also depend on the rate at which the viscous mate-
rial in the mantle convects or mixes. In chapter 1 we explored how this
process of mantle convection drives plate tectonics, which in turn drives
the recycling of materials at the Earth surface and basically allows Earth
to be a pleasant place for life.
 
Search WWH ::




Custom Search