8/10/2019 Over Geological Periods
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Over geological periods, atmospheric CO2 levels are controlled
by the geochemical cycle.
Modelling is most accurate for the Phanerozoic, because of its
plentiful and relatively well preserved
sedimentary rock record. GEOCARB balances the major sinks for
CO2burial of organic matter, and
silicate weathering combined with subsequent burial of
carbonates (Eqns 6.16.4)with the major
sources of the gas
oxidative weathering of kerogen and the thermal breakdown of
both kerogen
and carbonates (including volcanic emissions).
temperature and atmospheric CO2 levels are linked.For example,
weathering of C-containing rocks
affects CO2 levels and hence temperature, but is itself
influenced by temperature through the rate
of weathering and palaeogeography (the elevation of land and its
location relative to warm, humid
zones).
Among the least well constrained parameters are those related to
weathering caused by plants,
which was modelled to start at 380350Ma, with angiosperms
dominating the process by 80 Ma
(Berner 1998). The rate of plant-induced weathering varies with
atmospheric CO2 levels and may begreater for angiosperms than
gymnosperms (Volk 1989), but these influences are poorly
quantified
at present. Other factors that influence modelled CO2 levels
significantly are land uplift (leading to
erosion and weathering) and volcanic degassing rates
(particularly during increased tectonic activity
in the Cretaceous; Larson 1995).
However, it appears that CO2 concentrations were very high
during the Palaeozoic, fell sharply
during the Devonian (when plants colonized the land), rose
moderately during the Mesozoic and
then graduallydeclined throughout the late Mesozoic and
Cainozoic.
Conversely, an increase in mid-ocean ridge volcanicity would
cause 87Sr/86Sr to fall and
atmospheric CO2 concentration to increase.
The rate of chemical weathering appears to have been relatively
constant since the mid-Proterozoic
(Holland 1984), which suggests that the absence of plant-induced
weathering prior to land plants
becoming widespread in the Devonian must have been compensated
for by higher levels of
atmospheric CO2 (Berner 1990, 1991).However, the degree of
elevation of atmospheric CO2 levels
may have been offset by contributions of the gas from microbial
respiration in pre- Silurian soils
(Keller & Wood 1993), and by the fact that temperature may
be a more important control on silicate
weathering than soil CO2 levels (Brady & Carroll1994).
