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Australia: The Land Where Time Began |
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Atmospheric Carbon Dioxide - A 300-Million-Year record from Plant
Cuticles
According to the author1 geochemical proxies are used to
reconstruct past atmospheric CO2 levels in order to better
understand the link between atmospheric CO2 and climate over
geological time (Berner, 1997; Pearson & Palmer, 2000; Ekart et al.,
1999; Pagani, Freeman & Arthur, 1999). A broad picture of the variation
of atmospheric CO2 throughout the Phanerozoic, that covers
the last 544 My, has been provided by these records, though there are
still some inconsistencies and gaps requiring attention. In this article
Retallack presents evidence, based on stomatal abundance in fossil
leaves from 4 plant genera closely related to the Ginkgo of the present,
for a continuous record spanning 300 My. He reconstructed atmospheric CO2
concentrations based on the known relationship between the number of
stomata on a leaf and CO2 concentrations in the growing
season. The record he constructed indicated that over the last 300 My
there were 2 episodes in which there were intervals when the atmospheric
CO2 concentrations were low, less than 1,000 ppmv, and both
of these intervals coincided with known ice ages occurring in the
Neogene 8-1 Ma and Early
Permian, 290-275 Ma. The levels
of atmospheric CO2 were high, 1,000-2,000 ppmv, 250-65 Ma,
most of the
Mesozoic, though in this period
there were intervals during which there were excursions in the
atmospheric CO2 levels above 2,000 ppmv. The results of the
author's study are consistent with some past CO2 record
reconstructions (Berner, 1997; Pearson & Palmer, 2000), and
palaeotemperature records (Veizer, Godderis & François, 2000), though
they suggest the reconstructions based on carbon isotope proxies (Ekart
et al., 1999; Pagani, Freeman & Arthur, 1999) may be compromised
by isotopically light methane that was released as episodic outbursts
(MacLeod & Wing, Krull et al., 2000). According to the author1
water vapour, CO2 and methane were involved in greenhouse
climate warming over the last 300 My.
Over the past 200 years of industrial fossil fuel consumption as
atmospheric levels of CO2 have risen, plants have responded
by decreasing the density of stomates on their leaves (Pagani, Freeman &
Arthur, 1999; Beerling, McElwain & Osborne, 1998). In this study
Retallack has used the relationship, which is well-known, between
concentration of CO2 and stomatal density as a
palaeobarometer of atmospheric CO2 during the growth of
fossil plant leaves. Stomatal density has, however, been related to
differences in insolation, water stress and the position of stomata
within leaves, which also affect the size of the cell, and thus stomatal
density. The use of stomatal index (percentage stomata over stomata plus
epidermal cells), instead of stomatal density, stomata per unit area
(Berner, 1997), in order to minimise the effects of these competing
variables: the habitats of fossil leaves that have been studied, such as
mostly humid, lowland, fluvial and swamp environments that are suitable
for the preservation of plant cuticles (Meyen, 1987), have been
suggested to further limit such competing variables. These plants grew
in substrates that were peaty, nutrient-poor and siliceous, underwent
periods of flood disturbance and were in open vegetation early in the
ecological succession following disturbance, as is indicated by
sedimentological and taphonomic studies of several of the studied fossil
sites (Meyen, 1987; Retallack, 1999). The differences between trees of
different sex in
Gingko: trees that produce
pollen have branches reaching upwards, and are therefore lit evenly,
whereas horizontal branches tend to shade branches that are lower down
in trees that produce ovules. Counts of stomatal index obtained from SEM
images of leaves from a male and female tree that were recovered from
sites in Eugene, Oregon, show identical stomatal index (mean SI was 8.6
± 0.4; Fig. 1).
The different SI among species growing in the same location is the most
serious obstacle to the use of SI as a CO2 palaeobarometer.
Application of this technique have, therefore, focused on particular
species such as
Quercus patraea, which is
known in the fossil record as far back as the Miocene, 10 Ma (Kürschner
et al., 1996). In this paper the
Ginkgo genus has primarily
been used, which has a fossil record of leaf cuticles that are swell
preserved back to at least the Triassic period 229 Ma (Anderson &
Anderson, 1989). The Pteridosperm genera
Lepidopteris (Permian to
Triassic),
Tatarina (Permian) were
chosen for several reasons. Where they occur in the same places they
have similar SI (7.6 ± 1.4) and
Lepidopteris africana SI
(6.5 ± 1.7) at Little Switzerland (Late Triassic, South Africa (Anderson
& Anderson, 1989)), and
Tatarina conspicua (SI 5.5 ± 1.1) and
Lepidopteris sp. (SI 6.3 ±
0.9 at Baizovka (Late Permian, Russia (Gomankov & Meyen, 1986).
Furthermore, there is a relationship between ginkgoes and Peltasperm
seed ferns, as has been revealed by analysis of their fossilised
reproductive structures (Mayen, 1987). All of the 4 genera have stomata
that are morphologically similar. The cuticles of their cells are darker
(because they are thicker) than are those of other epidermal cells.
Subsidiary cells that are arranged in an even ring each have a hollow
papilla that is oriented in such a way that they occlude the stomatal
pit. This kind of stomate is also found in cycads such as
Dioon, and conifers such
as
Torreya, Agathis and
Thuja (Florin, 1931). A degree of functional equivalence is
given by such partial papillar occlusion to the stomatal apparatus of
Ginkgo, Lepidopteris,
Tatarina and
Rachiphyllum that is
absent in other fossil genera such as
Dicroidium, Pachypteris or
Baiera (Anderson &
Anderson, 1989).
Illustrations of fossil cuticle that had been published previously were
used in this study, except for the 2 species from the Triassic (250-245
Ma) from near Sydney Australia. A current geological time scale provided
the numerical ages of the fossil cuticles (Gradstein
et al., 1995),
but published accounts of the cuticles provided relative age. It was
demonstrated by the compilation of Retallack that SI in Permian (275-296
Ma)
Rachiphyllum and
Lepidopteris was
comparable with Neogene (0-8 Ma) Ginkgo. Such high SI (>9) is a
reflection of low levels of atmospheric CO2 that was
predicted for the Permian by models of sedimentary mass balance (Berner,
1997). From the role of CO2 as a greenhouse gas cool climates
would be predicted (Beerling
et al., 1998) and the Neogene and the Permian are both known from evidence of
tillites, stratified pavements, and debris that had been ice-rafted to
be times when there were large polar ice caps (Retallack, 1999). It is
indicated by these 2 endpoints that plants in the Permian were similar
to modern ones in their relationship between stomatal index and
atmospheric CO2, and there has been no long-term drift in
this proxy measure of atmospheric CO2.
It has been found that fossil plant CI can be calibrated to levels of
atmospheric CO2 from greenhouse experiments that used
Ginkgo biloba (Beerling,
McElwain & Osborne, 1998).
Ginkgo biloba herbarium
specimens that ranged in age back to 1888 were also counted for stomatal
index and levels of CO2 for each taken from global estimates
for their year of collection (Kürschner et al., 1996). It is indicated by
herbarium and greenhouse results, together with additional greenhouse
studies that are in progress (Royer, Berner & Hickey, 2000), that the
response of SI to concentrations of CO2 is markedly
nonlinear. Greenhouse (Meyen, 1987) and herbarium data were fitted with
a quadratic equation for the concentration of CO2: [CO2]
I = 155.17 (SI)2 – 2897.8 (SI) + 13773, where r2 =
0.96 (adjusted to sample size of 8). Retallack used this transport
function in order to estimate past partial pressures of CO2
from individual measurements of SI, and smoothed by a 5-point moving
average to give a curve comparable to past estimates of atmospheric CO2.
According to Retallack the comparison of the new results with estimates
from the mass balance model is good, given the coarse resolution (10 Myr
intervals) of the model in comparison with the present study (mean
sample gap = 3.6 ± 5.4 Myr). Also, high CO2 levels shown for
the cuticular estimates during the early Tertiary comparable to those by
high-resolution (mean sample gap 1.8 ± 2.7 Myr) boron isotope studies of
marine foraminifera, though if correction for secular changes of
riverine boron input into the ocean was carried out, these estimates
could be reduced substantially (Lemarchand
et al., 2000).
Evidence from petrographic and chemical studies of palaeosols
(Retallack, 1999; Retallack, Bestland & Fremd, 2000), the migration of
thermophilic organisms to high latitudes (Thomas, Zachos & Bralower,
2000; Retallack, 1999), and oxygen isotope analysis of marine skeletal
remains from different palaeolatitudes (Veizer, Godderis &
Francois, 2000;
Thomas, Zachos & Brawler, 2000),
support the increased warmth and weathering that occurred during times
when high CO2 has been demonstrated in this study. There are
also indications from palaeosols and palynology that times of high CO2
were times when the palaeoclimate was humid and, conversely, that times
of cool intervals were dry. E.g., the abundance of fossil pollen that is
believed to be evidence of aridity in many parts of Asia (Vakrameev,
1991), there are rises at all 4 times of low CO2 and cooling
during the Jurassic and Cretaceous. From the Eocene to the Oligocene the
low CO2 and cooling, which is indicated by the cuticular
record, there is evidence for synchronous palaeoclimatic drying in the
palaeosol record of central and western North America (Retallack,
Bestland & Fremd, 2000).
Many transient excursions to very high CO2 (>2,000 ppmv) are
shown by the cuticular time series. It is indicated by high resolution
studies of the SI minimum in fossil
Ginkgo and cycad leaves across the Triassic-Jurassic (200 Ma) in
Greenland and Sweden that a transient spike in CO2 was
coincident with excursion to carbon that was isotopically lighter (δ13Corg)
of the same leaves, and mass extinction (That claimed
Lepidopteris among others)
(McElwain, Beerling & Woodward, 1999).
Following mass extinction of the earliest Triassic (250 Ma), and
faunal overturn of the early Jurassic (190 Ma), Early Cretaceous (117
Ma), and Late Palaeocene (55 Ma), other CO2 and carbon
isotopic transients, have been related to catastrophic outbursts of
methane that was isotopically light from marine permafrost and
reservoirs of marine methane hydrate (Thomas, McElwain & Bralower, 2000;
Krull, Retallack, Campbell & Lyon, 2000; Jahren
et al., 2001).
At the Cretaceous-Tertiary boundary there was yet another spike of CO2
that coincided with an excursion to carbon isotopes values that were
lighter, on land as well as in the sea, asteroid impact in Yucatan,
which claimed dinosaurs and ammonites (Arens & Jahren, 2000). In the
cuticular time series transient CO2 maxima represent strong
perturbations in the carbon cycle.
Between cuticular estimates of atmospheric CO2 and estimates
that are based on the carbon isotopic composition of palaeosol
carbonates, which indicate CO2 minima (which includes even
concentrations that are negative) at 250 Ma (earliest Triassic, 190 Ma
(Early Jurassic), 117 Ma (Early Cretaceous) and 55 Ma (Latest Palaeocene
(Ekart, et
al., 1999).
According to Retallack the timing of these events may be a clue to the
timing of these discrepancies, as carbon isotopic studies indicate that
these were times of catastrophic release of methane that was
isotopically light from permafrost and marine gas-hydrate reservoirs
(Thomas, Zachos & Bralower, 2000, Krall
et al., 2000; Jahren & Arens, 2001). Methane is oxidised
in 2-7 years to carbon dioxide once it is in the atmosphere, which
retains the unusual isotopic signature of gas-hydrate reservoirs that
are isolated from global surficial systems (Khalil (ed.), 2000). The
atmospheric addition of methane isotopes that are isotopically light may
also explain why the palaeosol isotopic palaeobarometer (Ekart
et al., 1999) and high-resolution
carbon isotopic studies of deep oceanic organic matter (Pagani, freeman
& Arthur, 1999) failed to detect the high CO2 levels and the
warm palaeoclimate of the Middle Miocene, which is evident from the few
Miocene results that were presented in this paper, as well as from
studies of foraminifera (McGowran & Li, 1997), plants (Utescher,
Mossbrugger & Ashraf, 2000), palaeosols (Schwartz, 1997) and the oxygen
isotopic composition of marine shells (Veizer, Godderis & Francois,
2000; McGowran & Li, 1997). The observation that carbon isotopic
palaeobarometers fail to show high CO2 at times of warm
palaeoclimate has led to the suggestion that climatic warming and
increasing atmospheric CO2 concentrations were decoupled
(Veizer, Godderis & Francois, 2000; Cowling, 1999). If carbon isotopic
CO2 palaeobarometers are compromised by episodic methane
bursts, as suggested by this study, then the accepted role of CO2,
methane and water vapour remains intact for at least the past 300 Myr
(Veizer, Godderis & Francois, 2000).
Retallack, G. J. "A
300-Million-Year Record of Atmospheric Carbon Dioxide from Fossil Plant
Cuticles." Nature 411 (// 2001):
287-90. |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |