A Quick Look at the Cretaceous World David Lillis – 13 May 2020 e-mail: [email protected] The Extent of the Cretaceous The Cretaceous Period began about 144 million years ago and terminated with the well- known asteroid impact some 66 million years ago. Geologists subdivide it into 12 stages, each defined by particular rock formations, fossils and sediments at a specific locality called the type area. Several of these type areas are located in France (e.g. Cognac, France, is the type area for the Coniacian Stage). These stages lasted for several million years each, and much geological, climatic and evolutionary change took place within each of them. Figure 1 gives the time frames of the twelve Cretaceous stages. Figure 1: The Cretaceous and its twelve stages The Cretaceous stages vary in duration but average somewhat less than seven million years. The Aptian stage has the greatest duration, at about 12 million years; while the shortest stage is the Santonian, at below three million years. The final stage is known as the Maastrichtian (approximately 6.1 million years in duration), the stage that ended with the famous asteroid impact and mass extinction of dinosaurs and other life forms. Significant rock formations or the earliest appearance of particular organisms define both the lower and upper boundaries of these stages. For example, one marker for the base of the Maastrichtrian is the earliest occurrence of a marine mollusc, the ammonite Pachydiscus fresvillensis. 1 Figure 2 shows a fossilised Pachydiscus fresvillensis from Madagascar, dated at about 69 million years. Figure 2: A Pachydiscus fresvillensis fossil, about 69 million years old The Cretaceous lies within the Mesozoic Era, which we can think of as the age of dinosaurs. The Mesozoic encompassed the Triassic, Jurassic and Cretaceous periods, and lasted for almost 180 million years. In turn, the Mesozoic era lies within the Phanerozoic eon, the expanse of time since the beginning of the Cambrian period to now (about 541 million years in total) and that is characterised by teeming life on Earth). This eon comprises the Palaeozoic, Mesozoic and Cenozoic eras. The Cretaceous was followed by the Paleogene Period, lasting to approximately 23 million years ago. Figure 3 shows the configuration of Earth’s land and oceans in the late Cretaceous (94 million years ago). Figure 3: The configuration of Earth’s land and oceans in the late Cretaceous 2 We see that Africa and South America were much closer than they are now, but Europe and North America were much closer too. Over the period of approximately 79 - 80 million years of the Cretaceous, the general configuration of today’s continents emerged, the Pacific and Atlantic oceans were formed and the Indian Ocean emerged from the Tethys Ocean. Previously, the Earth’s land had consisted largely of two continents, Laurasia in the north and Gondwana in the south, a configuration that persisted into the Jurassic (the period immediately preceding the Cretaceous). These continents were almost completely separated by the Tethys Ocean, and Laurasia and Gondwana had already started to separate, a process known as ‘continental drift’. Though drift rates may seem slow over a human lifetime, when nature has millions of years to play with, the distances moved by continents become enormous. For example, the current drift is approximately 2.5 cm per year, or 2.5x10-5 km per year. This does not seem a lot but, if we consider a very long period of time, such as the duration of the Cretaceous (80 million years), the distance grows in this case to 2000 km. Drift rates have varied over geologic time but, during particular phases of the Cretaceous, have reached as much as 17 cm per year in certain parts of the ocean basins. Earth’s magnetism was relatively stable during the Cretaceous. In particular, magnetic reversals involve flipping of the Earth’s magnetic field, such that the north and south poles essentially swap places. Generally, this phenomenon occurs somewhat randomly every few million years. Currently, the magnetic north points roughly towards the geographic north pole – known as ‘normal polarity’. However, no magnetic reversals are observed in the geomagnetic record from about 120 million years ago to about 83 million years ago. This phase of extended normal magnetic polarity is known as the Cretaceous Normal. Cretaceous Sea Levels and Reduced Currents The world's sea levels were much higher than at present and, for much of the Cretaceous, land took up only approximately 18% of the Earth's surface, compared with 28% today. However, despite a reduced area of land, the total habitable area of the Earth may have reached its greatest during the Cretaceous because a warm world fostered widespread life. The oceans were about 100 to 200 metres higher in the Early Cretaceous (145 - 100.5 million years ago) and about 200 to 250 metres higher in the Late Cretaceous (100.5 million - 66 million years ago) than at present. High sea levels may have resulted from water affected by the growth of mid-oceanic ridges. Ocean building was widespread at this time of splitting of the mega-continents. The production of new oceanic crust over the last 80 million years is estimated at approximately 20 million cubic kilometres per million years. However, about 120 million years ago, the production of new crust rose to approximately 35 million cubic kilometres per million years and remained at about this level for the next 40 million years. Thus, an array of new and expanding ocean ridges and their additional volume of rock displaced water and caused sea level rise. 3 The atmosphere was much warmer than today and it was warm even at the poles. The temperature gradient from the equator to the poles was much less than now, so that there was less seasonality and less atmospheric circulation (winds) than today. Reduced wind led to weaker oceanic currents than today, reduced up-welling and long periods of relatively stagnant deep oceanic waters. That relative stagnation led to anoxia (oxygen deprivation), exactly the conditions for the formation of dark shales that today yield oil (e.g. oil deposits of Saudi Arabia and the wider Middle East). Shale is a fine-grained sedimentary rock, made of clay minerals and other minerals, including quartz and calcite. Dark shales are dark in colour because they comprise significant amounts of unoxidized carbon (organic matter that yields oil) and are often deposited in stagnant waters with little oxygen. Seasons and Climate During the Cretaceous the Earth began to experience more well-defined seasons, involving colder winters and warmer summers. Both fossil evidence and computer models of the Cretaceous climate suggest that at times the global average temperature was up to 15 degrees Celsius higher than today. Such changes in temperature gave rise to rapid evolution of plant life. The climate was generally warmer and more humid than today, probably because of very active volcanism associated with high rates of seafloor spreading. Seafloor spreading releases the greenhouse gas, carbon dioxide, the warming effect of which is greater than the cooling effect of the release of sulphur dioxide, which partially blocks the transmission of sunlight. Thus, the poles were covered by forest rather than ice sheets. Evidence of forests in both polar regions is extensive and the existence of large forests cloaked in darkness, in some places for nearly six months every year, is a very interesting scenario and so different to what we have today. The climate of the Cretaceous was possibly warmer than at any other time over the 541 million years from the Cambrian Period to now. In addition, the climate was more even in that the difference between the temperature at the poles and the Equator was about 50% that of the present. Evidence from fossil plants suggests that tropical climates pertained as far as 45o North. In addition, temperate climates (climates with moderate rainfall over the year, mild-to-warm summers and cool-to-cold winters) characterised the poles. Models of Earth’s climate for the mid-Cretaceous take account of the positions of the land masses and the distribution and extent of the existing oceans. They indicate weaker atmospheric circulation than today. Though the Early Cretaceous was somewhat cooler than the Late Cretaceous, for most of that period the Earth’s surface was a greenhouse environment. Cretaceous Carbon Cycles and Climate Change Estimated Cretaceous carbon dioxide levels are very consistent with climate variation from the geologic record. Mid-Cretaceous atmospheric carbon dioxide levels were at times up to five times the present level, indicating a surface temperature of 20–21 degrees Celsius. The 4 warmth of the Cretaceous resulted from tectonism which emitted additional carbon dioxide. However, the Cretaceous also saw enhanced organic carbon burial (sequestration), which tended to decrease atmospheric carbon dioxide levels. Burial involved the formation of carbonates such as limestones and chalks, and organic carbon in the form of oil, coal, gas and organic-rich rocks. These carbon reservoirs took carbon from the atmosphere and influenced the long term Cretaceous climate significantly. For much of the Cretaceous the organic carbon burial rate was greater than burial rates since then, and included three major peaks that correspond to sustained phases of anoxia (oxygen depletion) in the oceans, leading to the formation of oil and coal. Thus, the Cretaceous climate reflected an interplay between emission of carbon dioxide from tectonic activity and burial of carbon. Factors that influenced this interplay included the relative positions of the continents, a lack of mountain ranges by comparison to today, runoff from rivers, the radiation of flowering plants and their eventual predominance over conifers, and the radiation of plankton and other carbon-bearing forms into deep waters.