Oxygen by Canfield Donald E

Author:Canfield, Donald E.
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2013-01-14T16:00:00+00:00

Figure 8.3. Isotopic composition of inorganic carbon showing the Lomagundi Isotope Event spanning from about 1950 to 2300 million years ago. Also shown is the GOE.

We can measure the ratio of C-13 to C-12 atoms in organic matter from old rocks, as Minik Rosing did in rocks from Isua, Greenland (chapter 6), and since inorganic carbon is also removed as limestones (and shells after the evolution of animals), we can measure the ratio of C-13 to C-12 in inorganic carbon too. Thus, we can assemble a record of the C-13 to C-12 ratio of both organic carbon and inorganic carbon through time. If you remember from chapter 6, we generally discuss these carbon isotope ratios as δ13C values, as we will here.

Now let’s look at the data (fig. 8.3) and focus on the time of the GOE (remember, this was around 2.30 to 2.35 billion years ago). Indeed, δ13C of inorganic carbon becomes highly elevated around this time, and this period of elevated values has been dubbed the Lomagundi isotope excursion. By all appearances, it is the biggest carbon isotope 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 another 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 pyrite burial. Most of the time the volcanic flux was in excess, but occasionally, the balance tipped toward an excess in oxygen liberation generating 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. What we want to know is the rate at which oxygen-reactive gases, mainly hydrogen (H2), 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.

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