Geoscience Reference
In-Depth Information
The effects of terrestrialization on marine
ecosystems: the fall of CO
2
PAUL K. STROTHER
1
*, THOMAS SERVAIS
2
& MARCO VECOLI
2
1
Palæobotany Laboratory, Weston Observatory of Boston College, Department of Geology
and Geophysics, 381, Concord Road, Weston, Massachusetts 0493, USA
2
Universit´ Lille 1, FRE 3298 CNRS G´osyst´mes, Laboratoire de Pal´ontologie,
Cite Scientifique, F-59655 Villeneuve d'Ascq, France
*Corresponding author (e-mail: strother@bc.edu)
Abstract: The rise of land plants during the early Palaeozoic had profound effects upon sub-
sequent Earth history and evolution. The sequestration of standing biomass and carbon burial
caused a primary shift in the distribution of active carbon within the biosphere and surficial
Earth systems. This manifested itself in a dynamic decline in pCO
2
during Silurian-Devonian
time, affecting both terrestrial and marine ecosystems. We examined first-order correlations
between terrestrialization and pCO
2
by comparing the GEOCARB III data with time-constrained
fossil events in the early evolution of land plants. We compared the same GEOCARB III data with
the species/genus richness of lower Palaeozoic acritarchs. The correlation between the rise of
woody plants and pCO
2
is built into the GEOCARB model for the Late Devonian and later, but
pCO
2
begins to decline in the Cambrian long before the origin of woody trees (lignophytes).
The influence of early phases in plant evolution may be seen in a two-stage pCO
2
decline corre-
sponding to fossil evidence for the origin of thalloid bryophytes in the Middle Cambrian and the
origin of tracheophytes near the Ordovician-Silurian boundary. The decline of the acritarchs
shows a highly correlated lag of about 10 Ma with respect to the pCO
2
decline. The relation
between pCO
2
and acritarch species richness suggests a tight coupling between the evolution of
the marine phytoplankton and atmospheric CO
2
, supporting previous suggestions that pCO
2
was
a significant causal factor in the near extinction of acritarchs by the end of the Devonian.
There are two periods in Earth's history when bio-
logical innovation in photosynthetic organisms pro-
foundly and irreversibly altered the global chemical
environment. The first occurred with the evolution
of oxygenic photosynthesis, which permanently
shifted the redox chemistry of Earth's surface
by 2.2 Ga (Holland 1994; Rye & Holland 1998).
The second period, beginning with the origin of
wood in plants, permanently shifted the distribution
of active carbon species within the global carbon
cycle by the end of the Mississippian. By fixing
carbon into the biologically inert cell walls of trac-
heids that constitute wood, the rise of a forested
landscape transferred carbon from the atmosphere
into terrestrial plant biomass. The subsequent
increase in soil litter and eventual burial of that
inert organic matter effectively trapped carbon for
long enough to permanently reduce the mass of
CO
2
in the atmosphere. The evolution of a forested
landscape is, of course, only part of the complex
interconnected nature of the phenomenon of terres-
trialization. Because of its fundamental result - the
drawdown and resetting of equilibrium levels
of atmospheric CO
2
- it is however a component
of
subsequent environmental and biological evolution
on Earth.
The shift in atmospheric CO
2
that occurred
during the middle part of the Palaeozoic is a prime
example of biologically driven environmental
evolution affecting both terrestrial and marine eco-
systems. Since the mass of CO
2
gas in the atmos-
phere must be in equilibrium with dissolved
CO
2
(aq) in the surface oceans, on the geological
timescale any shift in atmospheric CO
2
must be
matched by a corresponding shift in dissolved
CO
2
(aq) in the oceans. Dissolved CO
2
(aq) in the
oceans is a terminal component of the bicarbonate
buffer that controls global pH, however; as pCO
2
dropped in the atmosphere, there therefore must
have been a corresponding increase in ocean pH.
Most geochemists think of the cause and effect
the other way around: it is the pH-dependent solu-
bility of oceanic CO
2
(aq) that determines the con-
centration of CO
2
in the atmosphere (Peng &
Broecker 1980). This may be true today in terms
of HCO
3
2
as controlling the steady-state buffer, but
during the terrestrialization interval in geological
time, it was the drawdown of atmospheric CO
2
that was driving both the ocean-atmosphere
terrestrialization that has profoundly altered
