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Australia: The Land Where Time Began |
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Mesozoic and Early Cainozoic Climate Linked to atmospheric
Carbon Dioxide
In the Quaternary
period the relationship between carbon dioxide (CO2) and
climate has been investigated extensively, but the role of CO2
during changes of temperature during the remainder of the history of the
Earth is less clear (Veizer et al.,
2000). It has been particularly difficult to interpret the range of
geological evidence for cool periods during the high CO2
Mesozoic ‘green house
world’ (Royer et al., 2004;
Royer et al., 2006) of high
concentrations of CO2 that has been indicated by models
(Berner, 2004) and fossil soils (Ekart, Cerling, Montañez & Tabor,
1999). In this paper Fletcher et
al., present high resolution records of Mesozoic and early
Cainozoic atmospheric CO2
from a combination of carbon-isotope analyses of nonvascular plant
(bryophyte) fossils and theoretical modelling (Fletcher et
al., 2005; Fletcher et
al., 2006). Atmospheric CO2
concentrations are indicated by these records to have risen from ~420
ppmv in the
Triassic (about 200 Ma) to a peak in the middle
Cretaceous
(about 100 Ma). Then atmospheric concentrations of CO2
declined to ~680 ppmv by 60 Ma. These variations are shown by
time-series comparison to coincide with large climate shifts in the
Mesozoic (Dromart et al.,
2003; Wilson et al., 2002;
Shouten et al., 2003), which
contrasts with earlier suggestions of cimate-CO2 decoupling
during this interval (Veizer, Godderis & François, 2000). On several
occasions these reconstructed concentrations of atmospheric CO2
drop below the threshold for the initiation of glaciations (DeConto &
Pollard, 2003) and therefore help to explain the occurrence of cold
intervals in a ‘greenhouse world’ (Royer,2006).
For a number of reasons the fundamental role of CO2 as a
driver of past warm climates in the Mesozoic and early Cainozoic has
remained obscure. Widely disparate estimates of CO2 for this
critical phase in the history of the Earth (Royer, 2006) have been
yielded by different proxies, such as stomatal indices of fossil leaves
and palaeosols suggest, in general, that CO2 levels were
thousands of ppmv higher than fossil leaves (Retallack, 2001).
Furthermore, it is not clear whether mismatches between oxygen-isotope
(δ18O) that are based on palaeoclimate records and ancient
estimates (Veizer, Godderis & François, 2000) of CO2, from
either proxies or numerical carbon-cycle models (Royer et
al., 2004; Royer, 2006;
Berner, 2004), reflect genuine decoupling between CO2 and
climate or are an artefact that is associated with difficulties of
reconstructing accurately these critical features of the history of the
Earth (Kump, 2002). However, evidence for brief cool pulses, that are
typically <1 Myr, that punctate climates that are generally warm in the
Jurassic and Cretaceous is challenging the prevailing high CO2
Mesozoic ‘greenhouse world’ paradigm (Royer, 2006) that is indicated by
palaeosols (Ekart, 1999) and geochemical carbon cycle models (Berner,
2004),
In this paper Fletcher et al.,
report a series of CO2 estimates for the ancient atmosphere
as far back as >200 Ma, by use of a new method that was based on the
strong dependency of carbon-isotope fractionation (Δ13C) in
terrestrial bryophytes on atmospheric CO2 during
photosynthetic carbon uptake (Fletcher et
al., 2005; Fletcher et
al., 2006). This approach is
the terrestrial analogue of the marine phytoplankton CO2
proxy (Freeman & Hayes, 1992), used successfully to reconstruct
glacial-interglacial CO2 changes from peat cores that dated
to the Pleistocene (White et al.,
1994). In terms of function, Δ13C approaches the maximum
value (30‰) expressed by the primary carboxylating enzyme
carboxylase/oxygenase (RuBisCo), and the concentration of CO2
at the site of fixation (Ci)
approaches the external concentration of CO2 (Ca)
(Fletcher et al., 2005;
Fletcher et al., 2006).
Because Ci is set by the
combined influence of net photosynthetic CO2 uptake (A) and
resistance to the inward diffusion of CO2 (r),
Δ13C is proportional to Ci/Ca, where
Ca – Ci
= A x r.
r is not subject to stomatal
control in bryophytes, as it is in the leaves of vascular plants, and Δ13C
varies with Ca and
the photosynthetic demand for CO2 by RuBisCo (Fletcher et
al., 2005; Fletcher et
al., 2006).
In this study ancient concentrations of CO2 were estimated on
the basis of 93 δ13C measurements on 61 liverwort gametophyte
compression fossils. The samples were from 12 localities on 5 continents
and spanned 150 Myr. Conversion of δ13C measurements to
palaeoatmospheric concentrations of CO2 is achieved after
calculating Δ13C to account for variations in the δ13C
of atmospheric CO2, and inverting a mechanistic mathematical
model that has been well validated (BRYOCARB)
describing the CO2 dependency of Δ13C in
bryophytes. Fossil Δ13C values that have been calculated lie
within the range that is sensitive to variations in CO2, with
the exception of those for the (Ypresian) specimens from the Palaeocene.
The balance between the photosynthetic CO2 draw-down and the
supply of diffusional CO2 from the atmosphere to the tissues
is determined by the Bryocarb models Δ13C as a function of Ca/Ci.
Photosynthesis is represented with established biochemical theory for
assimilation of CO2 and accounts explicitly for the
interactive effects of irradiance, O2 and temperature
(Fletcher et al., 2006) (see
Supplementary Information for derivation of values). The supply of
atmospheric CO2 to the site of photosynthesis is modelled by
analogy with Ohm’s law (Fletcher et
al., 2006). In liverworts
with pores, where r is
regulated by varying density of pores and morphology, BRYOCARB
p
calibrates Δ13C change. However, some groups of liverworts do
not have pores and so cannot regulate in this manner. In this study a
second version of the model, BRYOCARBNP,
to calibrate Δ13C changes in liverworts that have no pores,
with a prescribed resistance and a lower maximum rate of carboxylation,
that is RuBisCo limited, (Vcmax; see supplementary
Information) (Fletcher et al.,
2005). For each CO2 estimate uncertainties are characterised
by deriving probability density functions (PDFs) by the use of large
(25,000) ensemble Monte Carlo simulations to integrate uncertainties
into inferred and measured input variables for both versions of BRYOCARB).
The new reconstruction of atmospheric CO2, that spans
approximately ⅓ of the Phanerozoic eon, which covers the past 450 Myr,
in this study exhibits trends that are coherent throughout the Mesozoic
and early Cainozoic. These trends are primarily a reflection of the
systematic shifts that are driven by CO2 of up to 5‰ of
fossil bryophyte Δ13C; a secondary role is played by other
environmental inputs in the determination of the pattern. Estimates of
CO2 that are derived from fossil bryophyte Δ13C
calibrated by using either BRYOCARB
or
BRYOCARBNP
both rise from comparatively low concentrations in the Triassic and
Early Jurassic to a peak of ~1,139 ppmv in the Middle Cretaceous,
following which they decline toward 680 ppmv in the early Cainozoic.
Using BRYOCARBNP
calibrated Δ13C leads to CO2 estimates that are
approximately 60 ppmv higher than those using BRYOCARBP,
as the increased resistance to inwards diffusion of CO2 meant
that a higher Ca is required to produce
Ci/Ca
inferred from fossil Δ13C. A singular high CO2
estimate during the Ypresian (48.6-55.8 Ma 2,300-4,700 ppmv) is the only
exception to a coherent trend which might be a reflection of the reduced
sensitivity of the bryophyte proxy at high CO2 and should
therefore be regarded as less certain than the others. According to
Fletcher et al. they have
omitted this from their subsequent time-series analyses until it is
understood better by investigation of further fossil material, though
they recognise that it coincides with the peak of warmth during the
Cainozoic.
In this study the records of Bryophyte CO2 were used to
evaluate the role of this greenhouse gas in the evolution of warm
climates in the Mesozoic and early Cainozoic by calculating the
temperatures that were forced by CO2 for comparison with
independent palaeoclimate records. In this study cross-correlations were
computed between the changes that resulted in mean global surface
temperature (ΔT) and tropical ocean surface temperature series that were
adjusted for pH that were based on the δ18O of marine calcium
carbonate fossils (Veizer, Godderis & François, 2000; Ridgwell, 2005),
after accounting for uncertainties in ΔT that were due to imperfect
knowledge of vital effects (Veizer, Godderis & Françoise, 2000),
alteration that occurred after deposition (Wilson, Norris & Cooper,
2002) and the δ18O of ocean water (Roche, Donnadieu, Pucéat &
Paillard, 2006). The major pattern that was revealed by the pH-adjusted
δ18O record, that is supported by other evidence, that shows
cooler Middle Jurassic (167-160 Ma) (Dromart et
al., 2003) and warmer
climates in the Cretaceous (Wilson, Norris & Cooper, 2002; Shouten et
al., 2003), which suggests
that it is a useful metric for evaluating the greenhouse forcing by
calculated CO2 trends. Agreement between calculated changes
in ΔT was quantified by the use of a
hierarchical curve reconstruction technique that accommodated the
irregular spacing of CO2/ΔT
determinations and uncertainties in their estimation and dating. The
same approach was also used in the evaluation of CO2 as a
driver of climate change using an extensive
compilation of estimates of CO2
that were derived from studies of fossil leaves and palaeosols (Royer,
2006).
It was indicated by the results that bryophyte and stomatal histories
reproduce the average climate of the Mesozoic, which yields average
global temperatures within 0.7oC of the temperature that had
been estimated in the pH-adjusted δ18O climate record.
Calculated temperatures with palaeosol CO2 histories were, in
contrast, 2.0-2.5oC higher than the marine record. It was
revealed by cross-correlations that only ΔT
patterns that were calculated from CO2 histories of fossil
bryophytes were correlated positively with ΔT
from the marine δ18O record (medium coefficient = 0.2-0.5).
Positive correlations with independent records of climate were not
produced by the stomatal or palaeosol proxies. It was suggested by
Fletcher et al. that the
capacity for the bryophyte proxy to describe overall temperatures as
well as the magnitude of climate change by evidence of forcing by CO2
of major climate shifts, though this suggestion requires the assessment
of reliability of each of the 3 proxies that were used to reconstruct
ancient concentrations of atmospheric CO2.
Therefore, they benchmarked proxy performance against predictions of
numerical carbon cycle models in order to identify those that are most
consistent with variations that are well known in the long term sources
and sinks for CO2. Comparisons were made with a series of 3
histories of CO2 that were simulated by using a coupled
carbon-oxygen-sulphur cycle geochemical model (Berner, 2006) that
incorporated different representations of the weathering of basalt in
order to describe the range of field observations (Dessert et
al., 2003; Taylor, 2000).
Treatment of all other source and sink terms remained unchanged in each
of the following 3 simulations:
1.
Standard weathering (Berner, 2006) (GEOSW),
,
2.
Moderate weathering that was linked to variations in the production and
exposure of volcanic rocks (Berner, 2006) assuming values that are
unusually low for basalt-seawater (BSW) reaction rate (GEOVW)
and,
3.
As 2 but with enhanced weathering of basalts (Dessert et
al., 2003) and very low rates
of (BSW), or with moderate weathering and reasonable BSW rates (GEOVW).
Proxy performance was defined with summary statistics for absolute
differences in proxy-model CO2 values that were integrated
over the Mesozoic and early Cainozoic, and correlation coefficients
between proxy-model CO2 time series.
Fossil bryophytes are indicated by these comparisons to provide a record
of fluctuations that is more coherent in the atmospheric CO2
content of the Earth over the Mesozoic and early Cainozoic than either
of the other 2 existing proxies that cover the same interval. Fossil
bryophytes and the CO2 concentrations that have been
estimated by stomatal proxy are lower than the concentrations in all of
the 3 model simulations though this underestimation was reduced
progressively from ~-750 ppmv to only ~-90 ppmv with increasing emphasis
being placed on the weathering of volcanic silicates. Contrasting with
this, the CO2 values were overestimated considerably by the
palaeosol proxy, increasing from ~+590 ppmv to ~+1,370 ppmv across the
same series of simulations. The case for variations in the production
and exposure of basalts as playing a prominent role in the regulation of
CO2 levels on a timescale of multimillion years (Dessert et
al., 2003; Berner, 2006), is
strengthened by the closest agreement between 2 out of 3 proxies with
GEOEVW.
It is indicated, however, by cross-correlation analyses that the
bryophyte proxy is the only one that was both a good fit and exhibited a
positive correlation in the trends in CO2 simulated with GEOEV
and GEOEVW.
It is suggested by this that the bryophyte proxy has accuracy and
captures some patterns of the histories of CO2 simulated by 2
versions of the geochemical model. The other 2 proxies do not fulfil
both criteria simultaneously. The stomatal CO2 proxy record
correlated best with GEOSW CO2 predictions, though this
simulation gave the largest CO2 difference. The palaeosol
proxy CO2 record did not correlate strongly with any of the
model simulations, and had concentrations of CO2 that
differed from model results.
It was concluded by Fletcher et
al. that forcing by CO2 played an important role in
Mesozoic and early Cainozoic climate change, and that earlier claims for
a decoupling in the CO2-climate change relationship at this
critical phase in the history of the Earth (Veizer, Godderis & François,
2000) are premature. Changes in the CO2 greenhouse effect
that are driven by geochemical controls on the long-term climate cycle
(Berner, 2004; Berner, 2006), thereby contributing an explanation for
climates of the Jurassic and Cretaceous without the need to invoke the
influence of cosmic rays on cloud cover and planetary albedo (Wallmann,
2004). Furthermore, reconstructed concentrations of CO2 in
the study (500-1,300 ppmv) coincide with the threshold for the
initiation of glaciations as determined by global climate modelling
calibrated for mid-Cainozoic conditions (DeConto & Pollard, 2003)
(560-1,120 ppmv). CO2 histories that are derived from
palaeosols (Ekart et al.,
1999) and earlier geochemical carbon cycle models (Berner, 2004) exceed
the threshold by several thousand ppmv, though the exact range is likely
to be dependent on the particular climate model. Therefore, the new CO2
reconstructions in this study offer a resolution for the high CO2
‘greenhouse world’ paradox by explaining better the apparent
susceptibility of the Earth system to experience brief discrete cool
events during the Mesozoic (Royer, 2006).
Fletcher, B., et al. (2007). "Atmospheric carbon dioxide linked Mesozoic
and Early Cenozoic climatic change." Nature Geoscience 1:
43-48.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |