Geoscience Reference
In-Depth Information
the O
2
content of the atmosphere rose, the oxidation of FeS
2
dur-
ing weathering must have increased dramatically. The H
2
SO
4
gen-
erated during FeS
2
oxidation must have increased the total rate
of chemical weathering and, hence, the rate at which PO
4
- 3
was
delivered to the oceans. At the same time, H
2
S and HS
-
in the
near-surface oceans were probably oxidized to SO
4
- 2
. This process
lowered the pH of river and seawater and perhaps made PO
4
- 3
more available for photosynthesis.
Thus, Holland argues that the acid generated during pyrite oxidation
in response to the GOE would have enhanced the weathering process.
It's well known that most rocks will dissolve faster under acidic condi-
tions, but important here are those specific minerals within the rocks
containing phosphorus. An important mineral in this group is apatite
[Ca
5
(PO
4
)
3
(OH,F,Cl)], basically the same stuff as our teeth and bones.
As with our teeth when overly exposed to carbonated beverages, the
GOE, in the view of Dick Holland, produced massive apatite decay on
the Earth surface. This, in turn, liberated enormous quantities of phos-
phorus to the oceans. This huge phosphorus input stimulated primary
production resulting in high rates of organic matter burial that pro-
duced the super-sized Lomagundi carbon isotope excursion we discussed
in the last chapter. In a rather unusual inversion of cause and effect,
the Lomagundi excursion, which was once thought to be the cause of
the GOE, now becomes a consequence.
Also, the organic matter accumulating in rocks up to the GOE will
have been accompanied by key nutrients like nitrogen and phosphorus.
The oxidation of this organic matter after the GOE would have released
these nutrients as well, providing another source of phosphorus to the
oceans. Maybe the liberation of this phosphorus was also aided by the
acid generated during the oxidation of pyrite in organic-rich rocks. Over-
all, I rather like the idea that the accelerated liberation of phosphorus
accumulated in organic-rich rocks after the GOE helped trigger the Lo-
magundi excursion. I like it because it provides a reason for the start of
the Lomagundi as well as a reason for its end.
You might reasonably ask why we need a reason for the end. This is
because of the operation of the rock cycle. We already learned in previ-
ous chapters that sadly, the geologic record of really old rocks is poor.
