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Australia: The Land Where Time Began |
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Phanerozoic Climate Modes - The Cool and Warm Modes
The Cool
Mode – Early
Carboniferous to Late
Permian As a result of intensive study into the glacial
phase of the
Palaeozoic
since its recognition much is known about this episode in Earth history.
It is now known that glaciations at this time were centred on polar and
sub-polar latitudes, that they developed in parts of Laurentia and
Gondwana, and
that their inception was related regionally to orogenic activity, both
collisional and extensional, with ice sheets developing in the central
parts of continents, and that there was waxing and waning of glaciers
that were in response to palaeolatitudinal changes. Relatively little is
known of many other aspects of the Cool Mode of the late Palaeozoic.
These included such things as the full duration of the development of
glaciers, the climates in non-glaciated parts of the Earth, and why the
glaciers melted.
The Warm
Mode – Late Permian to Middle
Jurassic It now appears that the previously held
generalisation concerning the entire Mesozoic, which has been
interpreted as a time of a warm, arid interval, does not account for the
significant variations within the era. It appears that much of the Early
Mesozoic was a warm
time, with extensive aridity developing only after the Middle
Triassic, and
significant cooling beginning in the Middle Jurassic. A Warm Mode is
taken here to extend from the End of Gondwana and Asian ice-rafting in
the Later Permian (Kazanian) through part of the Middle Jurassic, to the
end of the Aelenian, following which ice-rafting again became common at
sites at high latitudes (Frakes & Francis, 1988). Mesozoic warming
resumed following a cool though arid time from the Middle Jurassic to
roughly the mid-Cretaceous.
The Cool
Mode - Middle Jurassic to Early
Cretaceous The Jurassic and Cretaceous have been considered to
be intervals that were very warm, with low temperature gradients, high
latitudes with temperate conditions and extensive deposition of
evaporite. It is indicated by a great deal of data that this is an
over-simplification, and that some parts of the Earth were actually
quite cool during this period. Freezing conditions near the poles are
suggested by reports of ice-rafted deposits at high latitudes at
intervals during the latter part of the Jurassic and continuing into the
Early Cretaceous, Bajocian to Albian, and that there may possibly have
been temperate glaciers (Kemper, 1987; Frakes & Francis, 1988, 1990). It
therefore appears that the equator-to-pole temperature gradient may have
been greater than previously suspected. It also seems marked seasonality
may have been a prominent feature of climate during this interval. The
designation of this interval, from the late Middle Jurassic to the Early
Cretaceous as a Cool Mode (183-105 Ma, Bajocian to mid-Albian) is
therefore based on the presence of ice at high latitudes, at least
seasonally. There have been no reports of deposits that can be
identified as being ice-rafted from the latter half of the Cretaceous.
The climate appears to have changed to being much warmer conditions
globally during the Middle Cretaceous. As a result Frakes et
al. have divided the
Cretaceous Period into 2 different climate modes. According to Frakes et
al. palaeoclimate information
for the latter part of the Jurassic, from the Bajocian onwards, is much
sparser than for later times. The scarcity of oceanic oxygen isotope
data has made a detailed analysis of temperature for short-term
intervals impossible as a result of the rarity of ocean floor sediments
from the Jurassic, as is possible for the Late Cretaceous and Tertiary.
Climate information for the Jurassic is mostly from the continental
areas. Physiognomic analysis for the margins of
leaves has
yielded a great deal of information for the Late Cretaceous and Tertiary
that is only possible for
angiosperm plants which are therefore a valuable source of
information for the Jurassic climate. Gymnosperm plant fossils from the
Jurassic cannot be used in this manner, so it is only after the
angiosperms had arisen that climate information could be gained from
plant fossils. For the Early Cretaceous more information, such as
isotope data, is available. The Warm
Mode – Late Cretaceous to early Tertiary One of the warmest times of the late Phanerozoic
was the period from the Middle Cretaceous, the mid-Albian, to the
mid-Early Eocene, approximately 105-55 Myr. Frakes et
al. suggest the average
global temperature was about 6oC higher than at present
(Barron, 1983), which allowed the polar regions to be free of permanent
ice. Vertebrates and forests were present at both poles as the
temperatures were high enough at that time, particularly at the time of
the peak warmth in the Early Eocene. Contrasting with the mid-Jurassic
to the Early Cretaceous there is no known evidence of seasonal ice, in
the form of ice-rafted deposits, at high latitudes from the Cenomanian
to the Maastrichtian, though during the Palaeocene there may have been
seasonal ice at the northern high latitudes. Seas were at globally high levels during the
mid-Cretaceous, with extensive areas covered by shallow shelf seas,
which favoured moderate climates and increased evaporation and
precipitation (Arthur, Dean & Schlanger, 1985). It is believed that at
that time the oceans were more stratified than at the present, and with
not much vertical mixing, and large quantities of sediments that were
organically rich were deposited in anoxic bottom waters. The developing
Atlantic Ocean, in
particular, had restricted circulation, though more open circulation was
established by the latter part of the Cretaceous, and the deposition of
black shales had diminished. In the Late Cretaceous sea levels reached
their peaks for the Mesozoic-Cainozoic, after which sea levels gradually
dropped (Haq et
al., 1987).
The sea levels regressed from the continents during the latest
Cretaceous to expose large continental areas, which led to the
establishment of seasonal, continental climates (Hays & Pittman, 1973).
However, it is indicated by refined
palaeotemperature data that the reinterpretation of the thermal
tolerances of fossil plants that there were distinct warmer and cooler
phases within the warm trend, which were particularly highlighted at
high palaeolatitudes. This includes the Cretaceous-Tertiary transition
and the associated climatic perturbations. Though it has been proposed
there was a cooling trend with a decline of mean temperature of 0.2/Myr
for the Late Cretaceous (Lasaga, Berner & Garrels, 1985) and it is
indicated by the subsequent warming in the Early Tertiary that this warm
mode persisted beyond the Cretaceous. It was only in the late Early
Eocene that the major global cooling began when there was a change in
the dynamics of the ocean circulation and the appearance of permanent
ice at the poles. The Cool
Mode of the Cainozoic – Early Eocene to Late Miocene Cooling in the Early Eocene, 55 Ma, was the
beginning of the early stages of the
Cainozoic Cool Mode.
From that time onwards the climate of the Earth gradually cooled from
the Warm Mode of the late Cretaceous to early Tertiary to the cool
glacial climates of the present. Enhancement of climate zonation and the
development of a thermally stratified ocean are some of the important
changes that occurred in this phase. The earliest part of the cooling of
the Cainozoic is not recorded simply by the presence of an ancient
glacial deposit, unlike the record of the Palaeozoic. During the
Tertiary, any direct evidence of extensive glacial ice at the poles is
scarce, the main reason being that in these regions the rocks are now
covered by ice. Positive evidence for the presence of ice, that is at
least seasonal, is provided by the presence of ice-rafted debris in deep
ocean cores. The oxygen isotope record of calcareous
foraminifera in the ocean document the principal evidence for cooling in
the Tertiary. It is this which illustrates the decline in the
temperature of the ocean and the increase of ice at the poles, mainly at
the South Pole, which has occurred since about 55 Ma onwards. The same
cooling trend is reflected by the climate evidence from fossil plant
assemblages. The crucial transformation phase from non-glacial to
glacial conditions, which is recorded by several geological parameters,
is illustrated by this trend, which makes this documentation of the
cooling during the Tertiary important. Frakes et
al suggest that this trend or
transformation can be used as a model for determining the history of
glacial accumulation during the former Cool Modes, such as those
occurring in the Palaeozoic, for which the data are less reliable.
Several shorter intervals of cooling and warming
are imposed on this major cooling phase, including the sharp decline of
temperatures at the boundary of the
Eocene-Oligocene,
as well as during the late Middle
Miocene, and
warming phases that occurred in the Eocene, which have been suggested to
have possibly been related to an enhanced greenhouse effect that was the
result of increased CO2 levels. The spread of temperate
vegetation to high latitudes near the poles is the best documentation of
these warming phases. It remains difficult to resolve the temperature
ranges that have been interpreted from the continental floral and ocean
isotope records, though both reveal the dominant cooling trend.
Continental temperatures that have been derived from plant assemblages
are generally somewhat higher than those derived from ocean isotopes. In
the climate history of the Tertiary, tectonic events have also played an
important role, particularly uplifting changing topography and changes
in the circulation of the ocean. According to Frakes et
al. the dynamics of the
Tertiary climate have been modelled (Barron, 1983) with the aim of
examining if changes to palaeogeography, such as the Tibetan Plateau
uplift or the expansion of the
Pacific, was the
principal cause of the cooling trend, as has been proposed previously
(Frakes & Kemp, 1972; Barron, Sloan & Harrison, 1980). It is suggested
by the modelling results, however, that the long-term cooling trend
probably resulted from the interaction of a series of events, including
the opening of ocean gateways and the development of new ocean currents,
as well as variation in the concentrations of
Atmospheric CO2. Several cycles of various orders have
been recognised because of the large amount of detailed data available
for the Cainozoic. The Cool
Mode of the late Cainozoic – Late Miocene to Holocene Major palaeoclimatic and palaeoceanographic events
occurred in the late Cainozoic. The latest part of the Miocene was the
most critical time with a number major events occurring:
·
Ocean surface waters cooled globally at
middle and high latitudes;
·
Cold
Antarctic waters
migrated northwards;
·
The
Antarctic ice sheet expanded;
·
There was a major fall in sea level;
·
The Mediterranean Basin was isolated and
desiccated;
·
Carbon isotopes trended towards lighter
isotopes. Major warming and major sea level rise occurred in
the Early
Pliocene,
which was followed in the Late Pliocene by a deterioration of the
climate and a gradual intensification of
glaciation in the
Northern hemisphere which led in the Late
Pleistocene
to climate changes that are visible in the 100,000 year cycle. The most
complete record for defining global change during development of a Cool
Mode, especially as it relates to glacial cyclicity that is orbitally
driven, is available for the late Cainozoic. Climate
Change – Causes and chronology Frakes et
al.,
say seeking the cause of climate change is exciting and difficult, as is
shown by the long history of enquiry and the scarcity of firm
conclusions. Frakes et
al.
say it is safe to say that of the many hypotheses that have been
advanced to explain local or temporally short changes, or to provide a
global framework for change over geological time, that no single
hypothesis takes account of all the variables as they are known at
present, though in many of these hypotheses there are elements of truth.
An integrated approach has been utilised that considers the atmosphere,
the hydrosphere, the biosphere and the solid Earth in an attempt to help
with understanding of the climate system. The tendency of this approach
to be historical is said by Frakes et
al. to be justifiable in
order to make use of geological data that bears on the temporal and
spatial variability of processes that are central to the system. They
say there is a constant need to re-evaluate concepts, in view of the
flood of new information and, given the growing awareness of the
complexities of the climate system, which many more attempts will likely
need to be made before a thorough understanding of the way in which the
system has worked over geological time is achieved.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |