Geoscience Reference
In-Depth Information
tially very sensitive. Suppose, for example, that we have a mineral that
oxidizes halfway, or maybe only 10%, in a low-oxygen environment.
Although partially oxidized, this mineral might still be preserved in a
river deposit, as we explored above, and would indicate low oxygen. An
oxidation product, however, would also be liberated, and if discovered
by some curious scientist, it would demonstrate that although low in
concentration, some oxygen was still present.
A promising element to explore is molybdenum (Mo), and both
groups focused on this. On early Earth, in the absence of oxygen,
molybdenum was likely present in rocks mostly as molybdenum sulfide
(MoS
2
) or as a minor component in pyrite. These forms of molyb-
denum are chemically reduced and stable in the absence of oxygen.
In the presence of oxygen, however, molybdenum sulfide phases are
readily oxidized to a water-soluble and mobile form, the molybdate ion
(MoO
4
2-
). This ion, once formed, is carried by rivers to the sea. There-
fore, with no oxygen in the atmosphere, there is no molybdate transfer
to the oceans and thus negligible molybdate in seawater. Add oxygen,
though, and the concentration of molybdenum in the ocean will rise.
Both teams looked for molybdenum enrichments in the ancient sedi-
ments they explored, and both found them (
ig. 7.3
). The element rhe-
nium (Re) behaves similarly to molybdenum, and both teams also found
enrichments in this element as well. Therefore, each team independently
discovered that oxygen reacted with compounds at the surface of Earth
during a time when the mass-independent sulfur isotope record suggests
very low concentrations of atmospheric oxygen.
Does this mean that James was wrong? No it doesn't. It implies, rather,
that Mo and Re can be oxidized and mobilized into rivers at lower con-
centrations of oxygen than those required to make the mass-independent
isotope effect go away. It also implies, and this is important, that cyano-
bacteria were around 2.5 to 2.65 billion years ago to produce the oxy-
gen. Combined with the biomarker evidence for steranes presented in
chapter 6,
we now have two independent lines of evidence for cyano-
bacteria during a time when concentrations of atmospheric oxygen were
still quite low. Ariel Anbar referred to this late Archean oxygen pulse as
a “whiff” of oxygen. This name has stuck, but the question is, why only
a whiff? If cyanobacteria were around, why were atmospheric oxygen
concentrations so low?
