Geoscience Reference
In-Depth Information
equilibrium and the pH of the oceans. By the end of
the Mississippian, the global carbon cycle had rea-
ched a new near steady state in which both pCO
2
and ocean pH were reset to essentially modern
levels. This dynamic component of the system, the
change in pCO
2
over time, is the direct effect of
the terrestrialization process. This paper examines
its r
ˆ
le as a causal agent in first-order evolutionary
dynamics during this interval.
We compare GEOCARB III model data of pCO
2
with the rise of the land plants based on recent
palaeobotanical discoveries that indicate a Lower
Cambrian origin to the land plants. While the con-
nection between the origin of trees and the dramatic
decline of pCO
2
during the Devonian is both well
known and built into the model (Algeo et al. 1995,
2001; Algeo & Scheckler 1998; Berner 2001),
GEOCARB III shows pCO
2
beginning to decline
in the Cambrian long before the origin of trees.
The effect of the rise of the land plants on pCO
2
includes both changes in the rates of weathering of
parent rock and on the burial of refractory carbon
(Berner 2001). Prior work on comparing the rise
of land plants to the pCO
2
decline is focused primar-
ily on events that occurred during the Devonian and
Mississippian (Algeo et al. 1995, 2001; Algeo &
Scheckler 1998; Berner 2001). However, it could
be that the influence of prevascular land plants
may have started a cumulative process that began
the CO
2
decline which accelerated during the
Devonian.
We next examine the potential perturbations to
the phytoplankton of the mid-Palaeozoic marine
realm as CO
2
(aq) declined and as particulate
organic matter (pom) and dissolved organic matter
(dom) delivery to the shallow shelf increased nutri-
ent flux to the oceans. These are the two most
significant changes to the marine environment that
occurred as a direct result of the terrestrialization
process. They represent primary signals that could
have potentially widespread evolutionary outcomes,
particularly with respect to the phytoplankton.
We use the fossil record of acritarchs as a proxy
for the large phytoplankton of the Palaeozoic
seas, following Strother (2008). Even although the
acritarchs are a polyphyletic group, their sheer
numbers (in terms of taxon richness and individual
abundance within sampled fossil populations)
provide a robust way to measure change in marine
palaeoecosystems. Studies on general trends in
Palaeozoic phytoplankton (Strother 1996, 2008)
and more detailed, period-level analyses (Servais
et al. 2004; Vecoli & Le H
´
riss
´
2004; Mullins &
Servais 2008) provide examples of the value of
acritarchs in deciphering palaeoecological trends
in the fossil record. These works make use of mod-
erately large databases of taxonomic occurrences.
We continue in this manner in this paper, relying
principally on the Palynodata database (Palynodata
Inc. & White 2008) as the source of
taxon
distribution values.
Methods: Creating the CO
2
and acritarch
diversity curves
The pCO
2
data from GEOCARB III (Berner &
Kothavala 2001) is available at ftp://ftp.ncdc.noaa.
gov/pub/data/paleo/climate_forcing/trace_gases/
phanerozoic_co2.txt. The rCO
2
values are plotted in
10 Ma bins, which are roughly similar to the stage-
level bins used for our taxonomic data. The resol-
ution of these two curves is therefore similar when
plotted simultaneously.
The basic method to produce the acritarch taxon
(genera, species) richness distributions is to pro-
gressively clean taxon distribution data extracted
from the Palynodata database. Developed originally
as an extension of the work of Gerhard Kremp, this
database was supported during the 1990s by a con-
sortium of commercial sources, paid subscribers
and the Canadian Geological Survey. Last updated
in 2006, Palynodata is available as an unsupported
download as Geological Survey of Canada
Open File 5793 at http://geopub.nrcan.gc.ca/more-
info_e.php?id=225704 (Palynodata Inc. & White
2008). Records of acritarch occurrences extracted
from the Palynodata database were progressively
filtered to produce a curve that includes only well-
characterized acritarch species dated to the level
of series or better (Strother 2008). This was done
to remove questionable or illegitimate taxa as des-
cribed in the Fensome et al. (1990) index, invalidly
named taxa, certain poorly constrained typically
long-ranging genera, taxa left in open nomenclature
and synonymous taxa. The use of the above filtering
set removed 558 genera (52%) and 3012 species
(54%) from the acritarch taxa used in the global
analysis. The filtered equal weight taxon distri-
bution values are plotted in Figures 1 and 2.
The absolute ages in Palynodata are based on
an outdated timescale. We therefore used the rela-
tive, time-stratigraphic series and stage names as
the basis of stratigraphic occurrence for acritarch
taxa. The conversion of time-stratigraphic units to
absolute dates was carried out using the values
available on the website www.stratigraphy.org as
of December 2008.
Results: Assessing the rise of land plants
during the terrestrialization interval
The effect of the rise of land plants on the carbon
cycle has been twofold: (1) causing a drawdown
of pCO
2
due to the sequestration of C
org
in refrac-
tory organic matter trapped in plant biomass, litter,
