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