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Table 2. Regression coefficients between the GEOCARB and acritarch genus-richness curves. The acritarch
genus data is progressively shifted to the left in 10 Ma increments and the regression coefficient is
re-calculated to find the closest fit, which occurs at - 10 Ma
Offset (1
t
)
0 Ma
210 Ma
220 Ma
230 Ma
240 Ma
250 Ma
R
0.89
0.95
0.93
0.81
0.89
0.83
r
2
0.79
0.91
0.87
0.66
0.79
0.69
organic carbon. This is not a new observation. The
relation between the composition of the gases in
the atmosphere, photosynthesis and the burial of
organic carbon (in the form of Carboniferous coal)
was noted as early as 1844 by Spencer (1904). It
acknowledges an important aspect of the relation
between the biosphere and the carbon cycle, which
is that the biosphere has played a significant r
ˆ
le
in the distribution of carbon on the face of the
Earth and in the atmosphere. It is therefore reason-
able to expect that the evolution of progressively
more complex terrestrial vegetative floras would
have had a direct effect on the composition of
the atmosphere.
Each of the proposed stages in the progressive
development of vegetative cover corresponds to a
subsequent drop in palaeo-CO
2
levels as established
in the GEOCARB III model (Fig. 3). This is not an
expected result of the estimates given in Table 1,
because the primary effect of terrestrialization
should not have been felt until the rise of the
forests during the latter part of the Devonian. In
fact, that section of the curve is determined by incor-
porating increased weathering estimates from the
rise of forests in the Late Devonian. However, it
seems a remarkable coincidence that both prior
periods of the most significant drops in the pCO
2
model begin at the time of the origin of each succes-
sive vegetative phase.
The proposed origin of thalloid bryophyte cover,
as evidenced by the first record of cryptospores, rep-
resents perhaps the more tenuous correlation
because it is not based on a recovery of macroscopic
plants in the fossil record but can only be inferred
from the palynological record. However, the obser-
vation that the decline in atmospheric CO
2
began
during the Cambrian supports the notion that this
was a significant period in plant evolution. The
origin of multicellular plants (of a bryophytic
grade) probably began the stage in Earth history
when the bulk distribution of C
org
associated with
photosynthetic organisms became greater in terres-
trial habitats than in the oceans. This is most cer-
tainly the case today when total terrestrial C
org
is
estimated at 1800 Pg (Brovkin et al. 2002) and
living biomass is 600-800 Pg (Woodwell et al.
1978) as compared to phytoplankton biomass of
about 0.25-0.65 Pg (Falkowski & Raven 2007).
Secondly, there is no palaeobotanical model
for the existence of a significant rhizosphere prior
to the Silurian; the influence of rhizosphere depth
on the Cambro-Ordovician portion of the atmos-
pheric CO
2
curve would perhaps have been nil.
This could mean that if there was a feedback
between palaeo pCO
2
and the rise of a punctuated
vegetative cover beginning in the Cambrian, it was
associated with an increase in terrestrial biomass
and not in weathering. But this result seems to indi-
cate that the bryophytes may also be important in
providing organic acids and a soil cover that substan-
tially increasedweathering rates on a global scale. At
the very least, this should be taken into consideration
when modelling the evolution of terrestrialization.
The decline of pCO
2
and its effect on the
large phytoplankton of the Early
Palaeozoic oceans
Our data in Figure 4 demonstrate that the standing
diversity of acritarchs (genus-level taxon richness)
is highly correlated with the decline in Palaeozoic
pCO
2
as modelled by Berner & Kothavala (2001).
The correlation is highest when the acritarch
values are subjected to a 10 Ma linear transform,
which has the effect of mapping the acritarch
curve into the rCO
2
curve. This lag, 1
t
, is about
10 Ma which seems biologically reasonable in
terms of a progressive forcing of phytoplankton
extinctions as pCO
2
declined.
Is it possible that the decline in pCO
2
could have
caused the acritarch decline? Are the large phyto-
plankton, in an evolutionary sense, capable of res-
ponding to shifts in CO
2
availability? In this case
we can demonstrate that the phylogeny of the
extant phytoplankton indicates that past speciation
events include responses to CO
2
availability. In
addition, there is an experiment (Collins & Bell
2004) showing that extant algal populations res-
ponded to changing levels of CO
2
in ways that
included heritable changes. Their result showed
clearly that changes in levels of pCO
2
have the poten-
tial to lead to speciation in extant algae populations.
Tortell (2000) first pointed out that the extant
algae preserve evidence of selection for inorganic
carbon (C
i
) uptake over geological time. This idea
