Australia: The Land Where Time Began

A biography of the Australian continent 

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

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.

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.

Sources & Further reading

1.      Frakes, L. A., et al. (1992). Climate modes of the Phanerozoic, Nature Publishing Group.

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 08/03/2015
Home
Journey Back Through Time
Geology
Biology
     Fauna
     Flora
Climate
Hydrology
Environment
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading