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slightly younger material from the central Appala-
chians (Driese & Mora 2001). We can therefore
assume that, by the Siluro-Devonian boundary, the
effects of increased weathering due to the presence
of a rhizosphere would just have begun to have an
impact on the global carbon cycle.
Rooting on the scale of depths up to 1 m have
been described from rocks of Emsian age (Elick
et al. 1998). Rooting of this depth should be corre-
lated with the evolutionary origin of arborescence,
which is known with confidence somewhat later
in the Givetian-Frasnian (Stein et al. 2007). The
palaeobotanical evidence indicates that the effects
of both increased rooting and the increased retention
of woody biomass on the continents would have
been felt by Frasnian time. Also, as trees continued
to evolve and expand, the effects of forestation
would have continued to increase throughout the
Famennian.
Table 1 provides a summary of the estimates
for each of the four successive terrestrial vegetative
floras. There are two components to the relation
between these successive stages in vegetative cover
and their effect on pCO
2
in the atmosphere. The first
is the retention of standing carbon biomass. This
is effectively the amount of carbon that has been
transferred from the atmosphere via fixed carbon
from oxygenic photosynthesis to biomass retained
on the surface. There is an additional and, in
reality, more important component: the buried C
org
that accumulates during geological time due to the
burial of vegetative biomass that accumulates in
clastic sediments and is eventually incorporated
into the lithosphere.
The accumulation of buried carbon has a long
history which is documented well into the Archaean
(Reimer et al. 1979; Shidlowski et al. 1979). Once
this component of the carbon cycle was established,
the evolution of subaerial microbial ecosystems
would only have incrementally increased the flux
of C
org
to the lithosphere. The palaeontological
record of cyanobacteria shows essentially modern
forms as early as 2.0 Ga (Hofmann 1976). Disparity
of the cyanobacteria, as recorded in the systematics
of cherty microbiotas, does not show a progressive
increase through the remainder of the Precambrian.
It has been argued recently that the major groups of
cyanobacteria diversified by 2450 to 2100 Ma
(Tomitani et al. 2006). There is little indication in
the fossil record of the cyanobacteria of a progress-
ive increase in the intensity of weathering, in the
accumulation of biomass or in the burial of C
org
as
a result of progressive evolution of cyanobacterial
mats through Proterozoic time. It is also difficult
to ascertain, based on the fossil record, when subaer-
ial cyanobacterial mats established themselves on
the continents. Kennedy et al. (2006) used changing
clay mineralogy as a proxy for terrestrial weathering
through the Neoproterozoic. They concluded that
beginning around 750 Ma, siliciclastic sediments
do record a progressive increase in weathering
intensity, implying a concomitant increase in
microbial cover ('land biota') on the land surface.
Their results could be explained either by an
increase in subaerial microbial mat activity or by
the evolution of new elements such as thalloid
bryophytes. However, given that the fossil record
of cryptospores does not extend below the upper-
most Lower Cambrian, we would suggest that a
gradual expansion of cyanobacterial mat ecosys-
tems into continental habitats would have been the
more likely cause.
Standing carbon biomass and the resultant flux to
sediments to form buried C
org
would have had an
incremental increase from the origin of embryo-
phytes (or their progenitors) some time in the
Cambrian. This is based on the observations of
Strother & Beck (2000) who first demonstrated the
presence of cryptospores in rocks of Middle Cam-
brian age. There is evidence of a bryophyte-grade
cover as early as the late Early Cambrian (Strother
et al. 2004; Strother 2008) based on cryptospores
which have wall ultrastructure related to liverworts
(Taylor & Strother 2008). We cannot say that these
early bryophyte-grade floras possessed plants with
upright axial sporophytes (because these fossils
are not found until the Wenlock), but we can
assert that thalloid plants that were more complex
than microbial mat dwellers appeared on land
prior to the Middle Cambrian. This bryophyte-grade
morphology generated an incremental increase in
stranding carbon biomass on Earth's surface. With
Table 1. Estimated parameters for each of the four successive terrestrial vegetative phases. The average height
is a simple metric of differences in biomass and the rhizosphere depth. In addition, it is used to give a basic
impression of the potential impact of changes associated with each of the successive floras
Vegetation phase
Origination (Ma)
Average height (cm)
Rhizosphere depth (cm)
5 10
2
10-10
2
Lignophytes
375-385
Tracheophytes
423-426
2
0.1
2 10
21
Thalloid Bryophytes
513-523
0.0
1 10
21
Microbial Mats
2200-1000
0.0
