Australia: The Land Where Time Began |
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Triassic climates — State of the art and perspectives
According to Preto et al. a
non-zonal pattern which was dictated by a strong global monsoonal
system, the effects of which were most evident in the
Tethys realm,
characterised the climate of the
Triassic
Period. The aggregation of the supercontinent
Pangaea was complete by
the Triassic, and the formation of this supercontinent is related to
this strong monsoonal regime, and oscillations of climate existed within
this framework. Preto et al.
suggest that the harsh hot-house conditions were characteristic of the
Late Permian, and
they also suggest that it is possible the climatic conditions
precipitated the mass extinction event at the close of the Permian. They
also suggest these difficult conditions were probably maintained during
the Early Triassic and may account for the impoverishment of the
distinctive faunal and floral associations of the Lower Triassic.
Carbonate production remained high, at least in the western Tethys
realm, though metazoan reef builders were probably the most affected by
the climatic conditions at this time. Locally, episodes of humidity
characterised the Middle Triassic, though their geographic distribution
has not yet been clarified. An episode of increased rainfall has been
documented worldwide, the Carnian Pluvial Event, was the most
distinctive climate change that occurred within the Triassic. A number
of hypotheses have been proposed for its causes: circulation changes in
the ocean or atmosphere that were driven by plate tectonics; the maximum
continental aggregation leading to a peak of the global monsoon; or
triggering by the eruption of a large igneous province. The subsequent
Carnian and Norian seem to have been climatically stable, though
recently minor climate changes have been described from these periods of
time. Climatic changes, warming and increased rainfall in particular,
are also associated with the mass extinction event at the close of the
Triassic, though most of the evidence for this has been found in the
northern parts of the Central
Atlantic Magmatic Province, and the global climate change pattern at the
Triassic/Jurassic boundary is not yet resolved. Preto et
al. say there are many facets
of the Triassic climate that are intriguing and need further research.
So far palaeoclimate studies of the Triassic have been carried only
locally by the use of different proxies. In order to correctly depict
the temporal and geographic patterns of the Triassic climate the proxies
require inter-calibration.
Summary
It is not simple to reconstruct the climate of the Triassic as there
were many oscillations to and from humid conditions, at least in the
region of the Tethys, and it is difficult to assess the geographical
distribution of the climatic zones, to a large extent as a result of the
global monsoonal system that is believed to have been stronger than that
of the present (Wang, 2009). According
to Preto et al. an
understanding of the temporal and spatial pattern of this climatic
variability is still some way from being achieved, with possible
exception of the system boundary intervals, probably a result of the
diverse proxies that have been used to date to reconstruct
palaeoclimates of the Triassic. This review has highlighted the way in
which different parts of the Triassic were studied by using evidence
from either palaeontology or sedimentology, not more than a few works
attempting a more inclusive approach, with palaeontological, isotopic or
modelling studies (e.g., Kutzbach & Gallimore, 1989; Korte et
al., 2005; Hochuli & Vigran,
this issue; Kiessling, 2010-this issue). Preto et
al. say the record of
Triassic palaeoclimate, that is rather dispersed, needs to be brought
together, possibly by switching to more objective proxies and
inter-calibrating the geochemical, palaeontological and sedimentological
datasets. Multidisciplinary works in key localities that are
representative of different climatic zones still remain unavailable.
Also, there is insufficient information on the timing of climatic
events, as well as their geographical distribution.
Some degree of variability of climate can be observed throughout the
Triassic, even in time intervals that have traditionally been considered
to be stable, though the strongest climate changes appear to have
occurred during the Early Triassic and the middle of the Carnian.
Included in this volume are important reviews of carbonate systems from
the Triassic (Kiessling, 2010-this issue; Stefani et
al., 2010-this issue), which
highlight the large amount of information that is available on carbonate
platforms from the Triassic. The story of the production of carbonate
during the Triassic needs to be revised in light of the reassessment of
the time scale of the Triassic, especially with reference to the Lower
Triassic. It has been considered that the Lower Triassic was a time of
reduced carbonate production (Payne et
al., 2004), characterised by
a gap in the reef building organisms and many Lazarus taxa, such as
calcareous algae and echinoderms, that reappear at a later time in the
Lower Triassic (e.g., Broglio Loriga et
al., 1983; Wignall et
al., 1998) or in the Middle
Triassic (e.g., Gaetani et al.,
1981; Senowbari-Daryan et al.,
1993). Net rates of carbonate accumulation are also to be recalculated
as the duration of the Induan stage has been shortened as a result of
recent geochronological studies, with the result that some localities,
such as the central Dolomites, in 80-90 m/Myr, a figure that is close to
or higher than that of recent carbonates. Therefore, after the
end-Permian extinction event, there is not a crisis of carbonate
production, though there is a crisis of reef-building biota, which is in
agreement with the general observations of Kiessling et
al. (2003).
Several recent studies have focused on the Carnian Pluvial Event (see,
in this issue, Kozur &Bachmann; Roghi et
al.). Preto et
al. say this event is clearly
a milestone in the climatic and sedimentary evolution of the Triassic.
An hypothesis that the Carnian Pluvial Event might have been a climate
and oceanographic perturbation that was similar in both its causes and
its effects on the carbon cycle, and physico-chemical modifications of
waters in the deep ocean, to Oceanic Anoxic Events (OAEs) in the
Jurassic and Cretaceous, or to
Palaeocene-Eocene Thermal Maximum (PETM), triggered by a large
igneous province eruption (Furin et
al., 2006; Rigo et
al., 2007). The Carnian
Pluvial Event may become an important analogue for climate change in the
future, if this is true, and specifically it corresponded to an
atmospheric CO2 maximum, as it is the case for several
similar events in the
Mesozoic and Cainozoic.
Though in contrast to the PETM and all OAEs from the
Jurassic and
Cretaceous,
the Carnian Pluvial Event occurred in an “aragonite seas” phase (Stanley
& Hardie, 1998; Stanley, 2008), i.e., with a seawater chemistry that was
similar to that of the present. Consequently, the Carnian Pluvial Event
could be a better analogue than, e.g., the PETM for the understanding of
ocean acidification driven by climate and it consequences on carbonate
producers in shallow water.
The recent reassessments of the time scale of the Triassic require also
that the climate history of the Late Triassic be re-examined, as well as
a different interpretation of the carbonates from the Early Triassic.
Specifically, the Norian stage is characterised by a stable climate (but
see Berra et al., 2010-this
issue), now corresponds to a duration of about 20 Myr. The stability of
the Norian is therefore a long-standing one, though Preto et
al. suggest it is probably
only apparent, as a result of the lack of palaeoclimatic data.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |