Australia: The Land Where Time Began |
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Phanerozoic Climate Modes The
Phanerozoic – 600 million years if climate oscillations · Warm Mode: Early Cambrian – Late Ordovician
·
Cool Mode:
Late Ordovician – Late
Silurian
·
Warm Mode: Late Silurian to Early
Carboniferous
·
Cool Mode:
Late Carboniferous – Late
Permian
·
Warm Mode: Late Permian – Middle
Jurassic
·
Cool Mode:
Middle Jurassic – Early
Cretaceous
·
Warm Mode: Late Cretaceous – early
Tertiary
·
Cainozoic Cool Model:
Early
Eocene –
Late
Miocene
·
Late Cainozoic Cool Model: Late
Miocene – Holocene
The Warm Mode, Early Cambrian-Late Ordovician
The Cool
Mode – Late Ordovician to Early Silurian According to Frakes et al. the Cool Mode of the
Ordovician-Silurian is ranked 3rd after the Cool Modes of the
Palaeozoic and the late Cainozoic, it is the most extensive and
intensive Cool Mode of the Phanerozoic period of time. The effects of
the glaciation were predominantly in Africa and the displaced terranes
derived from there, as well as in South America. In North and Central
Africa a major ice sheet developed, the age of glacial deposits being
belied to probably cover 35 My, as opposed to a minimum of 65 My for the
late Palaeozoic. The glaciation of the Ordovician-Silurian appears to
have been restricted to land masses at high latitudes in the Southern
Hemisphere, in spite of this widespread evidence of glaciation, with
evidence of cooling effects being discerned only with difficulty
elsewhere. Climates of
the past The authors1 say that of the many
reasons for investigating the earth’s climatic history the most
important is to gain knowledge of how the climate has evolved over the
last 600 million years. In order to work out the mechanisms of global
climate change and set boundary conditions for numerical models, with
the aim of predicting future changes of climate, it is necessary to
understand past climates. In the short term future improved climate
predictability can be included among the advantages that can be gained
in the applied sense, such as predicting the distribution of commodities
that are economically significant, including petroleum, phosphorite,
bauxite and other sedimentary minerals that have accumulated in various
locations that are climatically controlled redox reactions. The global mean surface temperature of the Earth
depends on a number of factors that include its orbital parameters, the
Sun’s luminosity and the distance of the Earth from the Sun, as well as
the albedo of the surface of the Earth (planetary albedo) and the
reflectivity of clouds, and the composition and dynamics of the
atmosphere and hydrosphere. The dominant greenhouse gas of the atmosphere of
the Earth is CO2, the atmospheric content of which has
changed over time as a response to changes in the rates and patterns of
tectonic activity that result in the heating of the sedimentary
reservoir of the Earth to the point at which carbon that is sediment
bound is converted to CO2 gas that is released back to the
surface with volcanic fluids. The result of this is that over geologic
time these changes led to variations of the climate of the Earth. There is a tendency for the atmospheric carbon
dioxide concentrations to decrease over geological time as carbon is
cumulatively buried in sediments. Another process that changes over
geological time is solar luminosity that has been increasing, which
counterbalances the declining trend of the amount of atmospheric CO2
(leading to cooling), the result of which is that tectonic activity is
linked to the non-organic part of the carbon cycle. The organic part of
the carbon cycle is linked to the climate through the atmospheric level
of CO2 reduction by the activities of vegetation and
phytoplankton. The climate of the Earth also responds to planetary
orbital parameters, such as the obliquity eccentricity and cycles of
precession. The other planets in the solar system influence the
Earth by causing these parameters to undergo cyclic change as a result
of their influence on the orbit of the Earth. The authors1
suggest it is likely polar ice caps are caused to wax and wane by the
seasonal and latitudinal distribution solar insolation as a result of
orbital cycles. The tectonic-scale climate trends are superimposed by
orbital climate oscillations. Within the climate system of the Earth the
redistribution of incoming solar radiation is a function of several
climate parameters, such as surface albedo, the distribution of land and
sea, the amount of cloud cover, etc. For the evolution of the climate in
the long-term the Phanerozoic the carbon cycle may be the
most important factor. Regional and seasonal climate changes that
were related to the internal storage and distribution of energy, for
example, have resulted from such things as opening and closing of ocean
gateways, changes in the distribution of land and sea, transport of
ocean heat etc., have also caused significant changes in the climate
over the Phanerozoic. Over the last 600 million years the climate of the
Earth has been a long way from static, passing from extremely cold
phases and glaciation to periods during which the global climate was
warm and arid. This is demonstrated by physical and biological evidence
from sedimentary sequences at individual sites, or from comparisons
between contemporaneous sites at locations from around the Earth.
Therefore, Devonian reefs at a site in Western Australia indicated warm
conditions, which were followed in the Carboniferous and Permian, cold
conditions, which were indicated by glacial deposits. The authors1
say that whether or not a global climate change is indicated depends on
if the site had changed its location relative to the thermal gradients
of the Earth along lines of latitude. It is now possible to deal with this factor of
continental motion for the Phanerozoic as palaeogeographic maps,
developed in the 1960s and 1970s, are available that are based on the
palaeomagnetism of rocks. As a result of decreasing resolution of age
data with the increasing age of the deposits, the problem of
contemporaneity of geological sites and the reliability of their
palaeoclimatic information increases with the age of the deposits,
though the resolution is sufficient in many instances to allow the
establishment of an approximate synchroneity, which therefore allows the
definition of climate events on a regional, continent-wide or global
scale, and the ability to compare the climate of an interval with the
climate event of another interval. Global-scale climate change has
clearly been demonstrated through the Phanerozoic. The authors1 have concentrated on the
last 600 My in this book, so climates earlier than 600 Ma are discussed
briefly as outlines. For most of the Precambrian there is a lack of
evidence of cold climates leading to common acceptance of the
Precambrian being a time of prevailing warm climates. The wide
occurrence of shelf carbonate sediments from the Precambrian,
particularly the sequences from the Proterozoic, supports the assumption
of prevailing warm climates. In the early part of the Precambrian, about
2,300 Ma, tillites and other glacial features in Huronian rocks found in
North America, together with approximate correlates in southern Africa
and western Australia, indicate a major cold episode. Following this
cold episode the presence of carbonate rocks suggest that warm climates
had returned. The glacial episode of the Late Precambrian occurred
between about 860 Ma and 600 Ma, the glacial debris from this glaciation
being described from all continents except Antarctica. It has been common to concentrate on a particular
geological Period in most of the previous palaeoclimatological studies,
mention of the climate prevailing before or after a particular interval
being slight. The authors1 suggest this leads to potential
for missing important, influential processes and events that set the
stage for the climates under study, therefore their book has divided the
Earth’s climate history into climate modes, i.e., time intervals in
which similar climates prevailed, in particular, the palaeoclimate
history has been divided into Cool Modes and Warm Modes. According to the definition of the authors1
the Cool Modes were times of ‘global refrigeration’ when ice was present
on the Earth. Included in this category was a range from intense
glaciation when large permanent ice caps were present on the polar
regions, to times at which high latitude regions were cold seasonally,
though were cold enough for ice to form during winter. Tillites,
striated pavements and clasts that are grooved (see Frakes, 1979) record
ancient glaciations, though the record for seasonal ice is more
indistinct and controversial. The best evidence of seasonal ice is the
presence in fine grained mudrocks of ice-rafted dropstones, though the
presence of dropstones with no other obvious evidence of glaciation,
interpretation becomes more difficult. The basis for the authors’1 recognition
of this interval as a Cool Mode is reports of ice-rafted deposits that
were laid down at high latitude regions between the Early Jurassic to
the Early Cretaceous, which is at odds with the view that is generally
accepted as one of the earliest time in the history of the Earth. The
authors1 say that evidence has now begun to accumulate from
previous reports, the authrs’1 own field observations, from
geochemical data and climate modelling, which suggests cool polar
climates at these times. The authors1 say they believe that
over the past years the idea has prevailed that the Mesozoic was a time
of warm climates has tended to be accepted without question, which has
influenced the interpretation of climate data towards the global warmth
for the Mesozoic, and that they were somewhat biased in their deliberate
search for evidence of cold climate. They say that future palaeoclimate
research will determine if they are correct about cool climates in the
Mesozoic. The authrs1 defined Warm Modes as times
when climates were globally warm, the evidence for this being abundant
evaporites, geochemical data, faunal distributions, etc., and the
presence of little or no polar ice. The modes as defined do include
brief intervals when climates are contrasting as they span intervals of
about 150 million years. The Warm Mode that covered the Late Silurian to
the Early Carboniferous includes a South American glaciation that was
very brief. As this event was very localised it was believed that it did
not justify the erection of another Cool Mode or the lengthening of
another. It is suggested that factors may have acted to prevent this
small glaciation developing into an event that was more global, these
factors possibly being the same factors that enforced the Warm Mode.
Also, evidence has been found of cooler, though
probably not freezing, climates in the Warm Mode from the Late
Cretaceous to the Early Tertiary, and in the Cool Mode of the Late
Jurassic to the Early Cretaceous there are signs of intervals that are
relatively warm. Conclusions In this study the authors1 have divided
the climate history of the Phanerozoic into 4 Warm Modes and 4 Cool
Modes. The Warm Mode from the Late Cretaceous to early Tertiary was
probably the warmest of the Warm Modes, with the Warm Mode of the
Silurian-Devonian and the early Mesozoic ranked equal second. The lack
of information from high latitudes has meant that the position of the
Cambrian-Ordovician is uncertain. During the Late Palaeozoic and the
Late Cainozoic were times when the most extreme cold climates prevailed,
which was followed by Ordovician-Silurian, which was more intensely cold
than the middle Mesozoic and the anomalous local cooling of the Late
Devonian. According to the authors1 an important conclusion
of this ranking is that a clear trend to overall warming or cooling in
the last 570 My is not displayed by the Phanerozoic climates that had
such a high level of variation, all that can be said is that it can be
characterised as alternating warm and cool intervals over a long period.
Using as a baseline the mean annual global temperature of the present,
most ancient climates are shown to be relatively warm, as the Earth is
at the present in an interglacial phase. With regard to the mean global humidity over the
Phanerozoic the Carboniferous to Early Permian and the early Tertiary
appear to have been the wettest, with the driest likely during the
Triassic-Jurassic and the Devonian. As with temperature, there is no
apparent trend towards increasing or decreasing global aridity during
the Phanerozoic. The authors1 caution that there are many
uncertainties in estimates of both temperature and humidity, and as
conclusions originate from diverse kinds of analyses of rock and fossil
types and distributions, this results in these uncertainties. It has become apparent from the recent large
increases in information about the Earth’s palaeoclimatic history that
past climates have varied more than was previously recognised. The Early
Cretaceous and the late Cainozoic are times when this is particularly
evident. Other cool modes are very likely to have been characterised by
similar variations, and in strata from the Palaeozoic several variations
of comparatively long wavelength have been detected. The authors1
suggest greater variability on both short and long wavelengths in
sequences that were deposited in Warm Modes may be revealed by further
work. Enormous changes in the climate system are
signified by the abrupt shifts that have been found to have occurred
between modes, while it is indicated by relative homogeneity of the
internal record of each mode, which persisted for tens of millions of
years, a large level of inertia in the system.
Latest
Precambrian glaciation – age and distribution A poor framework for the dating and distribution of
glacial deposits from the Late Precambrian is all that has been
established by earlier research, and therefore the synchroneity versus
diachronism has not been resolved for these deposits.
It becomes apparent when the geochronological data are assembled
that glaciation existed for at least 230 My, ~800-~570 Ma. These data
which are taken from Hambrey and Harland (1981), are of variable
quality, and several are based on estimates from stratigraphy. Also the
age data are limited to Africa, Asia, Australia and Europe. When the results are considered in detail, the
continents have individual patterns, e.g., Africa contributes all dates
of pre-800 Ma, and when the African dates are combined with those from
Asia, all the dates younger than 610 Ma. The Tiddiline Tilloid of Morocco (615-580 Ma:
Leblanc, 1981), the Ibeleat Group in Mauritania (630-595 Ma; Clauer &
Deynoux, 1987), in central China (620-600 Ma), the Louquan Formation,
and the Churochnaga Tillite in the northeastern Urals (650-630 Ma;
Chumakov, 1981). A range of ~650 - ~ 580 Ma is given by these dates for
glaciation on 3 separate continents, and an interval that is narrow
enough to suggest synchronism of these respective glaciations. It cannot
yet be determined if the glacial deposits of North and South America,
that have similar stratigraphic positions, correspond to this event. An attempt was made at a global reconstruction of
the continents in the Late Precambrian, 675 Ma (Morel & Irving, 1978)
that resulted in a continental mass that straddled the equator, Pangaea,
though it reached to the middle and high latitudes in a sector which
included China and northern
Gondwana (the
western part of Australia, East Antarctica, India, northeast Africa and
the Arabian Peninsula). According to the authors1 the
glaciation in the Chinese localities might be explained by this
positioning at high latitudes, the glaciations of west Africa appear to
have been near the equator. In the Ural Mountains the tillites were
located at middle latitudes in another reconstruction (Bond, Nickerson &
Kominz, 1984) and (Piper 1987). It has been well established, however,
that the pole was located at a position near North Africa in the
Cambro-Ordovician as well as at earlier times (e.g., Morel & Irving
1798, figs. 7 & 16). This implies a period during which there was rapid
polar wander/drift which could account for the glaciation of West
Africa. The whole of the glacial mode of the Late Precambrian is not
encompassed by such an explanation in which glaciation arises from
poisoning at high latitudes. There is also another complication, it is
shown by palaeomagnetic results that older glacial beds show low
latitude formation (Embleton & Williams, 1986; Sumner, Kirschvink &
Runnegar, 1987). This discussion (above) relates only to the
tillites of the Late Precambrian, constituting evidence that the authors1
find convincing, for glaciation occurring immediately before the Warm
Mode of the Early Palaeozoic. There is, however, some evidence
indicating the glaciation continued into the Cambrian. It has been noted
that possible glacial deposits occurred in West Africa in the Cambrian,
and in North and South America, Asia and Europe, problematic deposits
for which the age is less certain. The authors1 have been
influenced to place the start of the Warm Mode at ~ 560 Ma, in the Early
Cambrian.
Lithological indicators Evaporites The Cambrian was a time during which there was
major halogen accumulation (e.g., Meyerhoff, 1970; Zharkov, 1981),
especially in the Early Cambrian, a time when deposition of evaporites
reached a level of 39 % of all Palaeozoic evaporites, making it the most
significant interval in this regard. Evaporites were very abundant in
the Early Cambrian, though they were not widespread around the world.
Most of the strata deposited in the Early Cambrian were in the Siberia
and southern Asian region between Saudi Arabia and India, though there
are also evaporite deposits from this period in Australia, Morocco and
North and South America. The remainder of the Warm Mode was chacterised
by very low deposits in Asia, Australia, Europe, and North America. According to some reconstructions (Smith, Hurley &
Briden, 1981) and (Ziegler et
al.,
1979) evaporites from the Early Cambrian in Asia and South America are
located in southern latitudes (<25o), and the occurrences in
southern Asia appear to be located on the west coast of Gondwana, <30o
latitude, locations that were ideal for the formation of desert
conditions. In Morocco the Early Cambrian evaporites are anomalous as
they occur at palaeolatitudes of about 50o, and were located
on the east coast where it might be expected that there were humid
conditions. In the Late Proterozoic deposition of evaporites expanded
from latitudes that were somewhat lower to latitudes of about 25o
in the Warm Mode, then in the latest Ordovician retreated again. The
abundant evaporites from the Early Cambrian in Asia were deposited at
latitudes that were slightly higher than earlier and later evaporites.
Carbonates Throughout the Warm Mode carbonates that were
deposited subaqueously on the continents accumulated at rates that were
relatively slow and gradually decreasing, the rate of deposition of
carbonate in the Ordovician reaching about half the average Phanerozoic
deposition rate (Ronov, 1980; Hay, 1985). Reef structures were
restricted to North America, Australia and Antarctica in the Cambrian,
but in the Ordovician they were more widespread, which included
Australia and North America (but not Antarctica), as well as Europe and
the Asian blocks. It was concluded (Webby, 1984) that the best
explanation for these distributions is by several fluctuations of
climate. In the Early Palaeozoic non-skeletal aragonite was uncommon,
and the distribution/abundance of ooliths has not yet been quantified. When the latitudinal distribution of carbonates
formed in the Warm Mode is considered this mode is characterised by
carbonates that are widespread up to relatively high latitudes. This is
shown best by the Asian blocks, where it is dated, up to ~ 30o
in the Vendian [Ediacaran]
to ~45o in the Early and Middle Ordovician, Archaeocyathid
reefs, as well as other prominent build-ups of carbonate extended to ~
20o in the Cambrian. It is suggested by the authors1
that new types of reefs, composed of corals and other taxa originating
in the Middle Ordovician, may have reached up to 40o in
Baltica. A compilation of the distribution of major rock
types supports the above features (Ziegler et
al., 1981a). Carbonates of
the Cambrian are shown to be abundant up to ~ 50o
palaeolatitudes by Ziegler et
al.
(1981a). Also seen in this study, a distinctive feature in the Early
Palaeozoic is the abundance of Carbonates from the Cambrian up to ~ 50o
palaeolatitudes, which differs from most other times in the Palaeozoic.
Other
indicators In chert-phosphate pairs from the Late Cambrian
calculations of oxygen isotope composition suggest that the sea surface
temperature (SST) at low latitudes were about 50-60o (Karhu &
Epstein, 1986). Data from carbonate cements from the Ordovician
(Lindström, 1984; Popp, Anderson & Sandberg, 1986) also suggest warmth.
It is not known if such extremely high isotopic values would be
explained by the effects of low salinity, and the authors1
say they view the problem with caution until such time as supporting
data becomes available. The authors1 caution that it must be
remembered that the distributions of the great mass of evaporites from
the Early Cambrian cannot strictly be used to determine
palaeotemperatures as evaporites are formed wherever the air temperature
gradient exceeds that in the water column. At any temperature above 0oC
this may occur, though the effectiveness of the evaporation rises with
the temperature. Therefore it was not necessary for evaporitic regimes
to be located in warm zones during the Early Cambrian, though it does
appear that along the western margins of the Asian block at this time
the appropriate combinations of rainfall, topographic and wind
conditions for arid climates were well developed at these locations.
This relates to the fact that there are not many
calcretes or other indicators of humid climates that have been found in
the record from the Early Phanerozoic. A small number of lateritic
profiles are know that date to the Middle and Late Cambrian of Laurentia
(Chafetz, 1980; Van Houten, 1985), and limited to a few occurrences of
deposits from the Vendian [Ediacaran]
to Early Cambrian of Asia, (central Siberia; Bardossy, 1979),
bauxites are known. All of these would have formed at palaeolatitudes of
less than 20o, which suggests that during this Warm Phase low
latitude zones were of mixed character, comprised of both arid and humid
climates. This suggests a mixed character for the low latitude zones
during the Warm Mode, with both arid and humid climates.
In North America in the Cambrian and Ordovician the
shelf-carbonate sequences a prominent characteristic appears to have
been cyclicity (Aitkin, 1966). A study of these ‘Grand Cycles’ in the
northern Appalachians attributed their formation to variations in the
sea level rate rises in the Middle to Late Cambrian (Chow & James,
1987), a eustatic cause being based on 3 distinct Grand Cycles that were
correlative across North America: with 1 occurring in the late Middle
Cambrian and in the Late Cambrian, 2 others. For North America through
the Cambrian 12 were suggested (Palmer, 1981). Eustatic curves were
produced by Vail, Mitchum & Thompson, 1977) and (Hallam, 1984b), but
they are nor in sufficient detail to confirm either chronology, though
they were in agreement with irregular cycles of black shale that have
been recognised (Leggett et
al.,
1981). Evidence of
climate from palaeontology The speed at which early shelled organisms spread
and diversified has weakened the climatic inferences from palaeontology
of the Palaeozoic (Sepkoski, 1979). There were at this time, more than
at any other times in the history of the Earth, an abundance of
environmental niches and impediments to diversification was at a
minimum, and evolutionary rates among fossil groups were at their most
variable. A result of this is that knowledge of the diversity in the
Early Palaeozoic is rendered less useful as a guide to climate than at
any other time. The data for echinoderms provides an example of the
variation of diversity (Sprinkle, 1981), the number of genera and
species being initially low in the Early Cambrian and then a general
diversity increase in the Middle Cambrian and a decrease of diversity
long before the earliest known appearance of predators of echinoderms.
According to the authors1 it is not certain if this decrease
in diversity was the result of a change in climate or the environment
changing in some other way. Tropical to subtropical climates have predominantly
been inferred for the Cambrian, the climate of Europe being inferred as
temperate (Ziegler et
al.,
1979). The Ordovician represents a time when a major change in the
organic composition of the oceans occurred. Included in this change was
the rise and spread of the brachiopods, corals, as well as other groups,
and an increase in the total diversity that was unparalleled.
Contributing to the change in diversity were taxa displacement and
environmental changes that were widespread (Sepkoski, 1981). It is not
easy to interpret the nature of the environmental changes from the fauna
that have been preserved, though it is possible to distinguish shallow
marine provinces (Palmer, 1972, 19789; Jell, 1974; Ziegler et
al., 1979). It has been
suggested that the best characterisation of the pre-glacial Ordovician
is as warm and cool temperate realms (Berry, 1979), with Baltica being
included in the latter. A different interpretation has been suggested
(Spjeldnaes, 1981) which explains the Middle and Late Cambrian faunas by
global warmth, which was followed by an irregular, progressive cooling
of the Ordovician through the Llandeilian to the onset of glaciation of
the following Cool Mode. Other
factors that may be related to climate According to the authors1 it appears it
was a time of moderate continental volcanicity and major marine
transgression (Vail et
al.,
1977; Ronov, 1980; Hallam, 1984b). A post-glacial eustatic sea rise has
been suggested to have been initiated within the Early Cambrian (Harland
and Rudwick, 1964; Mathews & Cowie, 1979). The authors1 say
this widespread event might have occurred between 590 and 560 Ma and
therefore indicate the close of the glaciation of the Early Cambrian,
though the chronology is poorly established. The authors1 suggest that
tectono-eustatic events reflect sea level fluctuations that have been
postulated to have occurred at other times in the Warm Mode. It was
recognised by Hallam that at the end of the Early Cambrian there was a
regression that was followed soon after by a transgression. Hallam
placed the succeeding drop in sea levels at the end of the Cambrian,
though this major culmination, which was of comparable level to that at
the close of the Cretaceous, has been placed by Vail et
al., in the Early Ordovician.
An alternative suggestion (McKerrow, 1979) favours a culmination in the
Late Ordovician following several oscillations. In the Cambrian oolitic
ironstones were scarce, though in the Early to Middle Ordovician they
were abundant (Van Houten, 1985), the abundance suggesting a correlation
with the high stand sea level. The times of transgressions and eustatic high
stands that have been assumed have been defined by the use of shales
that are organic-rich. These shales are concentrated in the Middle and
Late Cambrian, and the latter half of the Ordovician in Europe
(Thickpenny & Leggett, 1987) and in parts of the Early Cambrian and the
Tremadocian–late Llandeilian were characterised by conditions that were
oxidised and there was a paucity of organic matter (Leggett et
al.., 1981). Oceanic
upwelling resulted in increased productivity and this is a possible
explanation for about half of these dark shale occurrences which have a
tendency to be located along the west coast of continents, in the
Cambrian at least (Parrish, 1982; Ziegler & Humphreville, 1983).
Enormous deposits of phosphorite accumulated in the
Late Proterozoic and Early Cambrian, following which there was a steady
decline in accumulation until the Late Ordovician (Cook & McElhinny,
1979). Phosphorites from the Lower Cambrian are prominent in the Asian
blocks, Europe and Australia (Cook & Shergold, 1986). Most of these are
near the equator where they might be attributed to coastal upwelling at
low latitudes, though the phosphorites from Australia reach ~ 35o
latitude, according to reconstruction of the Early Cambrian (Smith et
al., 1981). The authors1
suggest that the relationship between the genesis of
phosphorite/upwelling and times when the global climate was cold, that
has been often quoted, would therefor appear to not hold for the Warm
Mode of the Early Palaeozoic. It is suggested by other evidence that
upwelling was not common; the sedimentary chert record is sparse in the
Cambrian-Ordovician (Hein and Parrish, 1987). During this Warm Mode the oceans were isotopically
distinct being very light in carbon;
13C values in carbonates
were more unique in the Early Proterozoic, as they were consistently
negative, with a range from about -1 – 0. In the Cool Mode that followed
there was an increase to positive values. During this Warm Mode
fluctuations in the carbon isotope record included an increasing trend
through in the Cambrian and a sharp decrease to the Phanerozoic minimum
near the start of the Ordovician. Thereafter
13C increased
regularly throughout the Ordovician (Holser, 1984). Detailed studies
(Popp et
al., 1986) supported
the broad latter trend. The expected reversed trend is shown by sulphur
isotopes.
Latest
Precambrian glaciation – age and distribution A poor framework for the dating and distribution of
glacial deposits from the Late Precambrian is all that has been
established by earlier research, and therefore the synchroneity versus
diachronism has not been resolved for these deposits.
It becomes apparent when the geochronological data are assembled
that glaciation existed for at least 230 My, ~800-~570 Ma. These data
which are taken from Hambrey and Harland (1981), are of variable
quality, and several are based on estimates from stratigraphy. Also the
age data are limited to Africa, Asia, Australia and Europe. When the results are considered in detail, the
continents have individual patterns, e.g., Africa contributes all dates
of pre-800 Ma, and when the African dates are combined with those from
Asia, all the dates younger than 610 Ma. The Tiddiline Tilloid of Morocco (615-580 Ma:
Leblanc, 1981), the Ibeleat Group in Mauritania (630-595 Ma; Clauer &
Deynoux, 1987), in central China (620-600 Ma), the Louquan Formation,
and the Churochnaga Tillite in the northeastern Urals (650-630 Ma;
Chumakov, 1981). A range of ~650 - ~ 580 Ma is given by these dates for
glaciation on 3 separate continents, and an interval that is narrow
enough to suggest synchronism of these respective glaciations. It cannot
yet be determined if the glacial deposits of North and South America,
that have similar stratigraphic positions, correspond to this event. An attempt was made at a global reconstruction of
the continents in the Late Precambrian, 675 Ma (Morel & Irving, 1978)
that resulted in a continental mass that straddled the equator, Pangaea,
though it reached to the middle and high latitudes in a sector which
included China and northern Gondwana (the western part of Australia,
East Antarctica, India, northeast Africa and the Arabian Peninsula).
According to the authors1 the glaciation in the Chinese
localities might be explained by this positioning at high latitudes, the
glaciations of west Africa appear to have been near the equator. In the
Ural Mountains the tillites were located at middle latitudes in another
reconstruction (Bond, Nickerson & Kominz, 1984) and (Piper 1987). It has
been well established, however, that the pole was located at a position
near North Africa in the Cambro-Ordovician as well as at earlier times
(e.g., Morel & Irving 1798, figs. 7 & 16). This implies a period during
which there was rapid polar wander/drift which could account for the
glaciation of West Africa. The whole of the glacial mode of the Late
Precambrian is not encompassed by such an explanation in which
glaciation arises from poisoning at high latitudes. There is also
another complication, it is shown by palaeomagnetic results that older
glacial beds show low latitude formation (Embleton & Williams, 1986;
Sumner, Kirschvink & Runnegar, 1987). This discussion (above) relates only to the
tillites of the Late Precambrian, constituting evidence that the authors1
find convincing, for glaciation occurring immediately before the Warm
Mode of the Early Palaeozoic. There is, however, some evidence
indicating the glaciation continued into the Cambrian. It has been noted
that possible glacial deposits occurred in West Africa in the Cambrian,
and in North and South America, Asia and Europe, problematic deposits
for which the age is less certain. The authors1 have been
influenced to place the start of the Warm Mode at ~ 560 Ma, in the Early
Cambrian.
Warm Mode –
Early Cambrian-Late Ordovician Beginning at the end of the Precambrian glaciation,
probably in the earliest part of the Cambrian, the first warm mode
began, as definite glacial deposits from the Cambrian have not been
found. The continents were mostly dispersed in the zones of low latitude
in ~100 My of the early Palaeozoic Warm Mode. And there were no known
glacial deposits. The initiation of glaciers in North Africa indicates
the termination of the warm phase in the Late Ordovician (Caradocian).
Major problems have been encountered in attempts to explain the
glaciation of the Precambrian to (?Late) Cambrian at low latitudes, and
the previous glaciation termination and the initiation of the Early
Palaeozoic Warm Mode.
Lithological indicators - Evaporites The Cambrian was a time during which there was
major halogen accumulation (e.g., Meyerhoff, 1970; Zharkov, 1981),
especially in the Early Cambrian, a time when deposition of evaporites
reached a level of 39 % of all Palaeozoic evaporites, making it the most
significant interval in this regard. Evaporites were very abundant in
the Early Cambrian, though they were not widespread around the world.
Most of the strata deposited in the Early Cambrian were in the Siberia
and southern Asian region between Saudi Arabia and India, though there
are also evaporite deposits from this period in Australia, Morocco and
North and South America. The remainder of the Warm Mode was chacterised
by very low deposits in Asia, Australia, Europe, and North America. According to some reconstructions (Smith, Hurley &
Briden, 1981) and (Ziegler et
al.,
1979) evaporites from the Early Cambrian in Asia and South America are
located in southern latitudes (<25o), and the occurrences in
southern Asia appear to be located on the west coast of Gondwana, <30o
latitude, locations that were ideal for the formation of desert
conditions. In Morocco the Early Cambrian evaporites are anomalous as
they occur at palaeolatitudes of about 50o, and were located
on the east coast where it might be expected that there were humid
conditions. In the Late Proterozoic deposition of evaporites expanded
from latitudes that were somewhat lower to latitudes of about 25o
in the Warm Mode, then in the latest Ordovician retreated again. The
abundant evaporites from the Early Cambrian in Asia were deposited at
latitudes that were slightly higher than earlier and later evaporites.
Carbonates Throughout the Warm Mode carbonates that were
deposited subaqueously on the continents accumulated at rates that were
relatively slow and gradually decreasing, the rate of deposition of
carbonate in the Ordovician reaching about half the average Phanerozoic
deposition rate (Ronov, 1980; Hay, 1985). Reef structures were
restricted to North America, Australia and Antarctica in the Cambrian,
but in the Ordovician they were more widespread, which included
Australia and North America (but not Antarctica), as well as Europe and
the Asian blocks. It was concluded (Webby, 1984) that the best
explanation for these distributions is by several fluctuations of
climate. In the Early Palaeozoic non-skeletal aragonite was uncommon,
and the distribution/abundance of ooliths has not yet been quantified. When the latitudinal distribution of carbonates
formed in the Warm Mode is considered this mode is characterised by
carbonates that are widespread up to relatively high latitudes. This is
shown best by the Asian blocks, where it is dated, up to ~ 30o
in the Vendian [Ediacaran] to ~45o in the Early and Middle
Ordovician, Archaeocyathid reefs, as well as other prominent build-ups
of carbonate extended to ~ 20o in the Cambrian. It is
suggested by the authors1 that new types of reefs, composed
of corals and other taxa originating in the Middle Ordovician, may have
reached up to 40o in Baltica.
A compilation of the distribution of major rock
types supports the above features (Ziegler et
al., 1981a). Carbonates of
the Cambrian are shown to be abundant up to ~ 50o
palaeolatitudes by Ziegler et
al.
(1981a). Also seen in this study, a distinctive feature in the Early
Palaeozoic is the abundance of Carbonates from the Cambrian up to ~ 50o
palaeolatitudes, which differs from most other times in the Palaeozoic.
Other
indicators In chert-phosphate pairs from the Late Cambrian
calculations of oxygen isotope composition suggest that the sea surface
temperature (SST) at low latitudes were about 50-60o (Karhu &
Epstein, 1986). Data from carbonate cements from the Ordovician
(Lindström, 1984; Popp, Anderson & Sandberg, 1986) also suggest warmth.
It is not known if such extremely high isotopic values would be
explained by the effects of low salinity, and the authors1
say they view the problem with caution until such time as supporting
data becomes available. The authors1 caution that it must be
remembered that the distributions of the great mass of evaporites from
the Early Cambrian cannot strictly be used to determine
palaeotemperatures as evaporites are formed wherever the air temperature
gradient exceeds that in the water column. At any temperature above 0oC
this may occur, though the effectiveness of the evaporation rises with
the temperature. Therefore it was not necessary for evaporitic regimes
to be located in warm zones during the Early Cambrian, though it does
appear that along the western margins of the Asian block at this time
the appropriate combinations of rainfall, topographic and wind
conditions for arid climates were well developed at these locations.
This relates to the fact that there are not many
calcretes or other indicators of humid climates that have been found in
the record from the Early Phanerozoic. A small number of lateritic
profiles are know that date to the Middle and Late Cambrian of Laurentia
(Chafetz, 1980; Van Houten, 1985), and limited to a few occurrences of
deposits from the Vendian [Ediacaran]
to Early Cambrian of Asia, (central Siberia; Bardossy, 1979),
bauxites are known. All of these would have formed at palaeolatitudes of
less than 20o, which suggests that during this Warm Phase low
latitude zones were of mixed character, comprised of both arid and humid
climates. This suggests a mixed character for the low latitude zones
during the Warm Mode, with both arid and humid climates.
In North America in the Cambrian and Ordovician the
shelf-carbonate sequences a prominent characteristic appears to have
been cyclicity (Aitkin, 1966). A study of these ‘Grand Cycles’ in the
northern Appalachians attributed their formation to variations in the
sea level rate rises in the Middle to Late Cambrian (Chow & James,
1987), a eustatic cause being based on 3 distinct Grand Cycles that were
correlative across North America: with 1 occurring in the late Middle
Cambrian and in the Late Cambrian, 2 others. For North America through
the Cambrian 12 were suggested (Palmer, 1981). Eustatic curves were
produced by Vail, Mitchum & Thompson, 1977) and (Hallam, 1984b), but
they are nor in sufficient detail to confirm either chronology, though
they were in agreement with irregular cycles of black shale that have
been recognised (Leggett et
al.,
1981). Evidence of
climate from palaeontology The speed at which early shelled organisms spread
and diversified has weakened the climatic inferences from palaeontology
of the Palaeozoic (Sepkoski, 1979). There were at this time, more than
at any other times in the history of the Earth, an abundance of
environmental niches and impediments to diversification was at a
minimum, and evolutionary rates among fossil groups were at their most
variable. A result of this is that knowledge of the diversity in the
Early Palaeozoic is rendered less useful as a guide to climate than at
any other time. The data for echinoderms provides an example of the
variation of diversity (Sprinkle, 1981), the number of genera and
species being initially low in the Early Cambrian and then a general
diversity increase in the Middle Cambrian and a decrease of diversity
long before the earliest known appearance of predators of echinoderms.
According to the authors1 it is not certain if this decrease
in diversity was the result of a change in climate or the environment
changing in some other way. Tropical to subtropical climates have predominantly
been inferred for the Cambrian, the climate of Europe being inferred as
temperate (Ziegler et
al.,
1979). The Ordovician represents a time when a major change in the
organic composition of the oceans occurred. Included in this change was
the rise and spread of the brachiopods, corals, as well as other groups,
and an increase in the total diversity that was unparalleled.
Contributing to the change in diversity were taxa displacement and
environmental changes that were widespread (Sepkoski, 1981). It is not
easy to interpret the nature of the environmental changes from the fauna
that have been preserved, though it is possible to distinguish shallow
marine provinces (Palmer, 1972, 19789; Jell, 1974; Ziegler et
al., 1979). It has been
suggested that the best characterisation of the pre-glacial Ordovician
is as warm and cool temperate realms (Berry, 1979), with Baltica being
included in the latter. A different interpretation has been suggested
(Spjeldnaes, 1981) which explains the Middle and Late Cambrian faunas by
global warmth, which was followed by an irregular, progressive cooling
of the Ordovician through the Llandeilian to the onset of glaciation of
the following Cool Mode. Other
factors that may be related to climate According to the authors1 it appears it
was a time of moderate continental volcanicity and major marine
transgression (Vail et
al.,
1977; Ronov, 1980; Hallam, 1984b). A post-glacial eustatic sea rise has
been suggested to have been initiated within the Early Cambrian (Harland
and Rudwick, 1964; Mathews & Cowie, 1979). The authors1 say
this widespread event might have occurred between 590 and 560 Ma and
therefore indicate the close of the glaciation of the Early Cambrian,
though the chronology is poorly established. The authors1 suggest that
tectono-eustatic events reflect sea level fluctuations that have been
postulated to have occurred at other times in the Warm Mode. It was
recognised by Hallam that at the end of the Early Cambrian there was a
regression that was followed soon after by a transgression. Hallam
placed the succeeding drop in sea levels at the end of the Cambrian,
though this major culmination, which was of comparable level to that at
the close of the Cretaceous, has been placed by Vail et
al., in the Early Ordovician.
An alternative suggestion (McKerrow, 1979) favours a culmination in the
Late Ordovician following several oscillations. In the Cambrian oolitic
ironstones were scarce, though in the Early to Middle Ordovician they
were abundant (Van Houten, 1985), the abundance suggesting a correlation
with the high stand sea level. The times of transgressions and eustatic high
stands that have been assumed have been defined by the use of shales
that are organic-rich. These shales are concentrated in the Middle and
Late Cambrian, and the latter half of the Ordovician in Europe
(Thickpenny & Leggett, 1987) and in parts of the Early Cambrian and the
Tremadocian–late Llandeilian were characterised by conditions that were
oxidised and there was a paucity of organic matter (Leggett et
al.., 1981). Oceanic
upwelling resulted in increased productivity and this is a possible
explanation for about half of these dark shale occurrences which have a
tendency to be located along the west coast of continents, in the
Cambrian at least (Parrish, 1982; Ziegler & Humphreville, 1983).
Enormous deposits of phosphorite accumulated in the
Late Proterozoic and Early Cambrian, following which there was a steady
decline in accumulation until the Late Ordovician (Cook & McElhinny,
1979). Phosphorites from the Lower Cambrian are prominent in the Asian
blocks, Europe and Australia (Cook & Shergold, 1986). Most of these are
near the equator where they might be attributed to coastal upwelling at
low latitudes, though the phosphorites from Australia reach ~ 35o
latitude, according to reconstruction of the Early Cambrian (Smith et
al., 1981). The authors1
suggest that the relationship between the genesis of
phosphorite/upwelling and times when the global climate was cold, that
has been often quoted, would therefor appear to not hold for the Warm
Mode of the Early Palaeozoic. It is suggested by other evidence that
upwelling was not common; the sedimentary chert record is sparse in the
Cambrian-Ordovician (Hein and Parrish, 1987). During this Warm Mode the oceans were isotopically
distinct being very light in carbon;
13C values in carbonates
were more unique in the Early Proterozoic, as they were consistently
negative, with a range from about -1 – 0. In the Cool Mode that followed
there was an increase to positive values. During this Warm Mode
fluctuations in the carbon isotope record included an increasing trend
through in the Cambrian and a sharp decrease to the Phanerozoic minimum
near the start of the Ordovician. Thereafter
13C increased
regularly throughout the Ordovician (Holser, 1984). Detailed studies
(Popp et
al., 1986) supported
the broad latter trend. The expected reversed trend is shown by sulphur
isotopes.
1.
Frakes, L. A., et al. (1992). Climate
modes of the Phanerozoic, Nature Publishing Group.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |