Geoscience Reference
In-Depth Information
Gaia hypothesis of James Lovelock.
9
I won't get into the details here,
but in its most basic form, the Gaia hypothesis posits that organisms
have had an active role in shaping the chemistry of the environment.
This was implicit in Bob's and my earlier model, as organisms and par-
ticularly the evolution of land plants played a major role in influencing
organic carbon abundance in rocks, which influenced oxygen levels.
Organisms were more explicit in Bob's recent isotope models because
they directly influenced the carbon cycle in a variety of ways.
10
Instead of relying on carbon and sulfur isotopes to fundamentally
drive rates of oxygen production, the COPSE model is driven by a num-
ber of external factors, both geological and biological in nature. The
factors included rates of metamorphic and volcanic outgassing, rates of
tectonic uplift, land plant evolution, the enhancement of land plants on
weathering, the location of organic carbon burial in the oceans (deep or
shallow seas), and the increase in solar luminosity through the Phanero-
zoic Eon.
11
Indeed, many of these drivers can be found in Bob's models,
but the COPSE model differs significantly from Bob's in some of its
feedbacks. For example, the COPSE model imposes an oxygen sensi-
tivity to the oxidation rate of sulfides and organic carbon weathering on
land. This is something that the early Kump and Garrels model incor-
porated (recall the discussion between Karl Turekian and Bob Garrels
in
chapter 5
), but Bob removed that sensitivity from his models long
ago. The COPSE model also has a wildfire feedback (as also explored
in
chapter 5)
to keep atmospheric oxygen concentrations from getting
too high. The model also keeps track of nutrients in the oceans, mainly
phosphorus, to regulate organic matter burial in sediments. In the end,
rather than using the carbon and sulfur isotope curves as drivers, the
COPSE model is tuned to try and recover these curves as closely as pos-
sible.
12
These curves are an integral part of the geologic record, and if
they cannot be recovered, at least in their broad features, then there is
likely something suspect with the model.
Results from the COPSE model are shown together with the GEO-
CARBSULF model in
igure 11.4.
Both models have some strong simi-
larities, particularly the increase in oxygen concentration accompanying
the rise of land plants and the subsequent burial of coal in swamps, as
discussed above, but there are important differences as well. The main
difference is the low levels of oxygen the COPSE model produces early
