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Cretaceous

Cretaceous

In geological time, the (from Latin creta meaning “chalk”) is the third and last period of the era; it follows the Jurassic period and precedes the Paleogene. The Cretaceous extended from 145 to 65 million years ago, and it presents 12 globally recognized subdivisions (ages): Berriasian, Valanginian, Hauterivian, Barremian, Aptian, and Albian (Lower Cretaceous); and Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian (Upper Cretaceous). The beginning of the Cretaceous is not marked by a significant mass and, by international consen­sus, is located coinciding with the lowest occur­rence of the ammonite Berriasella jacobi in the fossil record. The end of the Cretaceous (also called the K-T boundary) is marked by the mineralogical and geochemical anomalies related to the impact of the Chicxulub bolide, and with the Cretaceous- Paleogene catastrophic mass extinction event.

The Cretaceous period differed from our pres­ent world in several major respects: It was a much warmer world with high oceanic volcanic activity and sea levels higher than those of today. Moreover, there were complex biological continental and marine communities dominated by large reptiles such as the dinosaurs.

Continental Breakup

By the beginning of the Cretaceous, the old supercontinent Pangea divided into two large continents: the northern Laurasia (including present- day North America, Europe, and Asia), and the southern (including present-day South America, Africa, Antarctica, Australia, and India- Madagascar). They were separated by an east-west equatorial seaway known as the Tethys Ocean. During the Cretaceous, there was a second phase of continental breakup. In Laurasia, the drifting of the continents caused the separation between North America and Europe and the birth of the North Atlantic Ocean. Moreover, began to break into four large pieces: South America, Antarctica-Australia, Africa, and India-Madagascar. The progressive drifting of these continents opened the South Atlantic and Indian oceans. These plate tectonic movements during the Cretaceous were concurrent with a period of unusual seafloor spreading. For example, the volume of mid-oceanic crust produced in the Aptian-Campanian interval was almost three times greater than in the Jurassic or periods. The high oceanic volcanic activity caused flooding of the continents, especially around the Cenomanian-Turonian when the sea level was about 200 meters higher than at present, and one third of present-day Earth’s land area was submerged. During that time, Europe was an archipelago.

Climate Change

The abundant emission of volcanic greenhouse gases (mainly carbon dioxide) into the atmosphere­ocean system caused a global warming during the middle part of the Cretaceous, about 120 to 80 mil­lion years ago. This interval is considered an exam­ple of ice-free greenhouse climate conditions. Fossil records suggest a gentler latitudinal temperature gradient becoming the high-latitude climate that was one of the warmest in the earth’s history. For example, Upper Cretaceous dinosaurs and palm trees were present in the Arctic Polar Circle, in Antarctica, and in southern Australia; Cenomanian breadfruits flourished in Greenland; and crocodiles and turtles inhabited the Turonian-Coniacian Canadian Arctic. Although there is evidence of some cooler episodes in the Lower Cretaceous, geo­logical and geochemical data support that the global annual mean temperature during the Cretaceous was 6°C to 14°C higher than at present.

The absence of high-latitude ice caps during most of the Cretaceous contributed to the maintenance of considerably higher sea levels, forming numerous Tethyan shallow seaways in southern Laurasia platforms. Warm waters coming from the Tethys were transported northward, warming the polar regions. In contrast to what occurs at present, oceanic circulation during the Cretaceous was driven mostly by warm, saline deep water derived from low-latitude areas with high evaporation. The locus of deepwater formation was not the cold polar waters, which occupy the deep ocean at present, but the warm Tethyan waters. These conditions were favorable for the development of the well-known oceanic anoxic events (OAEs) that occur only during short intervals of very warm climate that is characterized by high levels of carbon dioxide. OAEs stratigraphic records consist of widespread, episodic deposits of organic- rich shales in the ocean basins. The characteristic anomalous accumulation of organic matter in these rocks corresponds to conditions of low levels of dissolved oxygen (anoxia), high biological productivity (atmospheric carbon dioxide led to increased weathering of the continents and greater delivery and availability of nutrients in the marine realm), and poorly oxygenated deep water. It is thought that mid-Cretaceous OAEs were the result of an excess of carbon dioxide in the atmosphere­hydrosphere system from volcanic eruptions, and of the alteration of deepwater circulation patterns.

Effects on the Biosphere

Tectonic forcing of climate and ocean fertility had a profound impact on terrestrial and marine ecosystems and on the evolution of life during the Cretaceous. The breakup of the Laurasia and Gondwana super­continents led to increased regional differences in flora and fauna between the resultant continents. The mild climatic conditions favored the development of ecto- thermic animals like reptiles, amphibians, or fish, whose internal body temperature is the same as the temperature of their surroundings. Dinosaurs are gen­erally reckoned to have been the dominant terrestrial vertebrates from the Upper Triassic through the K-T boundary. Some Cretaceous dinosaurs are popularly known, including Tyrannosaurus, Triceratops, and Velociraptor. The large sauropods (quadrupedal her­bivorous dinosaurs) that had dominated in the Jurassic period declined during the Cretaceous, being replaced in importance by the iguanodontids, such as Iguanodon. Pterosaurs were common for most of the Cretaceous, though not in the last millions of years due to ecological competition with new types of birds. During the Cretaceous, giant crocodile and new insect and mammal groups appear, as well as the first flowering plants (angiosperms).

Mild climatic conditions and the existence of extended shallow tropical seas strongly affected the evolution of marine communities. Plesiosaurs, mosa­saurs, and other marine reptiles coexisted with rays, sharks, groups of modern fishes, and with ammo­nites and belemnite cephalopods. Rudist and inocer- amid bivalves, echinoderms, and benthic foraminifera were abundant in the bottoms of the Cretaceous seas. Floating at the sea surface, microorganisms with calcareous test, such as coccolithophores or planktic foraminifera, were ubiquitous. The gradual accumulation of the minute calcite plates and test on the sea bottom formed a large quantity of chalk, a soft, white, porous sedimentary rock that is very abundant in the Cretaceous.

Growing interest in the Cretaceous climate is largely a product of the current concern over modern human-induced global warming. Studying the complex Cretaceous world offers a good opportunity to understand and appreciate how the biosphere responds to climate change in terms of migration, extinction, adaptation, diversifica­tion, and organic evolution.

See also Chicxulub Crater; Dinosaurs; Evolution, Organic; Extinction; Extinction and Evolution; Extinctions, Mass; Fossil Record; Geologic Timescale; ; Global Warming; K-T Boundary; Paleontology; Pangea; Permian Extinction; Plate Tectonics

 

Further Readings

Barrera, E., & Johnson, C. C. (Eds.). (1999). Evolution of the Cretaceous ocean-climate system [Special paper]. Geological Society of America, 332.

Gradstein, F. M., Ogg, J. G., & Smith, A. G. (Eds.). (2004). A geologic time scale 2004. Cambridge, UK: Cambridge University Press.

Skelton, P. (Ed.). (2003). The Cretaceous world. Cambridge, UK: Cambridge University Press.

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