Geoscience Reference
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that increase in standing carbon biomass came an
incremental flux of C
org
to marine sediments.
As the bryophytes evolved, there would have
been a substantial increase in the standing carbon
biomass in subaerial habitats with respect to that
of the microbial mat flora. The rationale behind
this assertion is the observation that peats today
are formed by peat moss (Sphagnum) and that the
models of coal accumulation in the past are based
on the notion that coals are primarily accumulations
of buried peat. There is therefore an actualistic
model of sedimentary organic accumulation of geo-
logically significant proportions that is afforded by
plants at the bryophyte grade of evolution. An incre-
mental increase in the retention of C
org
occurred as a
result of the evolution of subaerial bryophyte-grade
plants in the Cambrian. This represents the first sig-
nificant change in the distribution of carbon biomass
since the long-term establishment of the Neoproter-
ozoic equilibrium of the microbial mat phase.
The establishment of a subaerial, thallophytic
vegetative cover in the Cambrian may not have sig-
nificantly altered the previously established Pre-
cambrian level of chemical weathering, because
bryophytes do not possess true roots that would
have deeply penetrated parent rock. The effect of
vegetative cover, with respect to chemical weather-
ing, may have been both to produce organic acids
and retain environmental water at the surface, effec-
tively trapping acidic water at the surface of parent
rock. The organic acids produced by bryophyte-
grade plants may have been more effective at indu-
cing weathering reactions in parent rock than in the
previous microbial landscape, but there is no com-
pelling reason to make such an assumption. We
therefore may assume that any increase in chemical
weathering during the Cambrian and first half of
the Ordovician due to this effect would have been
nil. However, as seen in Figure 3 where we have
plotted the estimated origin times for each of
the model stages in vegetative cover along with
the data from GEOCARB III, there is substantial
pCO
2
decline beginning in the Middle Cambrian.
This implies that the evolution of the thalloid bryo-
phyte phase of terrestrialization substantially con-
tributed to the increased weathering of parent rock.
Results: The pCO
2
drawdown and the
decline of the Acritarcha
Strother (2008) has pointed out that the pCO
2
curves
for the Palaeozoic based on the GEOCARB models
of Berner and his co-workers (Berner & Kothavala
2001; Berner 2004) closely precede the acritarch
decline through the terrestrialization interval.
The GEOCARB model data is very sensitive to
the effects of increased weathering of parent rock
due to the rise of land plants and, to some extent,
this has been directly tested in modern settings
p
CO
2
and the Evolution of Terrestrial Vegetation
30
thalloid bryophytes
25
20
tracheophytes
lignophytes
15
10
5
0
Pre
C
O
S
D
M
I
P
P
J
K
N
C
600
500
400
300
200
100
0
Geological Time (Ma-Period)
Fig. 3. pCO
2
and the evolution of terrestrial vegetation. Estimated origin times are plotted with the GEOCARB III
data showing the relation between plant evolution and its relation to the atmospheric CO
2
model. Cryptospores and
smaller plant fragments support the origin of thalloid bryophytes by the end of the Early Cambrian (523-513 Ma).
Tracheophyte fossils are found at the beginning of the Homerian (423-426 Ma); if it turns out that trilete spores are
indicative of the tracheophytic condition then their origin would plot at least back to the Ordovician-Silurian boundary
(c. 440 Ma) and perhaps even earlier (Steemans et al. 2009). The origin of lignophytes is plotted as Frasnian, the time
of the first recorded true forests (Meyer-Berthaud et al. 2010).
