Tertiary Period

Paleogeography

The present-day configuration of the continents and oceans on Earth is the result of a complex sequence of events involving the growth and rearrangement of Earth’s tectonic plates that began almost 200 million years ago. By the beginning of the Tertiary, the supercontinent of Pangea had been fragmenting for more than 100 million years, and the geometry of the continents and oceans had assumed an essentially modern aspect with several notable exceptions. The fragmentation and dispersal of the Southern Hemisphere supercontinent known as Gondwana, which had begun in the early part of the Mesozoic Era (252 million to 66 million years ago), continued into the Cenozoic. Australia began to separate from Antarctica roughly 55 million to 56 million years ago during the late Paleocene Epoch. The initial subsidence of the South Tasman Rise, which occurred about 35 million years ago during the late Eocene Epoch, resulted in a shallow but inexorably widening oceanic connection between the Indian and Pacific oceans. It was this progressive separation of the two continents that led to the development of the Antarctic Circumpolar Current, a current that sweeps around Antarctica and thermally isolates it from the effects of warmer waters and climates to the north. This current was strengthened further and assumed its modern form as Antarctica and South America separated and thus formed the Drake Passage. There is much debate over when this opening actually occurred. Some experts state that the Drake Passage opened as early as the Eocene about 41 million years ago, whereas others maintain that this event took place as late as the boundary between the Oligocene and Miocene epochs about 23 million years ago.

The collision of India and southern Asia began between 50 million and 40 million years ago, during the Eocene Epoch, and continues today. The collision produced two main geologic results. First, it began to block the westward-flowing Tethys seaway near the Equator, a process completed with the junction of Africa and Asia near present-day Iran roughly 16 million to 14 million years ago. Second, the creation of the Himalayas and the Plateau of Tibet, which resulted from the collision, altered global climates by changing patterns of weathering (and thus the transfer rate of carbon to the atmosphere) as well as wind circulation. India’s collision with southern Asia also altered patterns of oceanic productivity by increasing erosion and thus nutrient runoff to the Indian Ocean.

The present-day Mediterranean Sea is a geologically recent descendant of a portion of the Tethys seaway. About six million years ago, during the Messinian Age, the western remnant of the Tethys seaway was subject to a brief paroxysm, known as the Messinian salinity crisis, that lasted approximately 270,000 years and saw the entire basin virtually isolated from the world ocean. The basin experienced severe desiccation and the precipitation of vast deposits of evaporites (such as salt and gypsum) up to several kilometres in thickness. The Atlantic Ocean subsequently refilled the basin from the west at the beginning of the Zanclean Age. Geologic evidence suggests that water rushing through a channel cut near Gibraltar filled some 90 percent of the Mediterranean Sea within two years. Some scientists contend that sea levels may have risen 10 metres (about 33 feet) per day within the basin during the period of peak flow. The Mediterranean basin has undergone significant geologic evolution during the most recent five million years. About one million years ago this part of the Tethys was transformed into the Mediterranean Sea by the elevation of the Gibraltar sill. Consequently, the Mediterranean basin became isolated from deep oceanic bottom waters, and the present-day pattern of circulation developed.

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In the Northern Hemisphere the fragmentation of the northern supercontinent of Laurasia, which occurred as the result of the separation of Eurasia from North America and Greenland, was accomplished with the final opening of the Norwegian-Greenland Sea about 55 million years ago during the Eocene Epoch. (The seaway between the two landmasses was open at various times during the Jurassic and Cretaceous periods.) Prior to the Eocene, the Greenland-Scotland Ridge formed the Thulean Land Bridge, a continental connection that allowed the exchange of terrestrial mammals between western Eurasia and eastern North America. The subsidence of this ridge during the early Eocene allowed the exchange of surface water between the Arctic and Atlantic oceans. The termination of the Thulean land connection led to the development of separate patterns of evolution among terrestrial vertebrates in Europe and North America (see evolution: Geographic speciation).

On the Eurasian continent itself, the Ural Trough (or Turgai Strait), a marine seaway that linked the Tethys with the Arctic region but also constituted a barrier to the east-west migration of terrestrial faunas, was terminated by regional uplift some 29 million years ago during the Oligocene. The resulting immigration of Eurasian land animals into western Europe, and the consequent changes that occurred in terrestrial vertebrates, is known among vertebrate paleontologists as the Grande Coupure (French: “Big Break”).

The Bering Land Bridge, which united Siberia and Alaska, served as a second connection between Eurasia and North America. This link seems to have been breached by the Arctic and Pacific oceans between five and seven million years ago, allowing the transit of cold water currents and marine faunas between the Pacific and Atlantic oceans. The Atlantic and Pacific were also linked by the Central American seaway in the area of present-day Costa Rica and Panama. This seaway, extant since the first half of the Cretaceous Period, prevented the interchange of terrestrial fauna between North and South America; however, for a brief interlude during the Paleocene, a land connection may have existed between North and South America across the volcanic archipelago of the Greater Antillean arc, and some scholars have argued that land bridges between the two continents may have existed for short periods during the Late Cretaceous and again during the late Miocene. The seaway was closed by the elevation of the Central American isthmus between 5.5 million and 3 million years ago. This event had two significant geologic results. First, the emergence of the isthmus permitted a major migration in land mammal faunas between North and South America—the so-called Great American Interchange—which allowed ground sloths and other South American immigrants to move into North America as far as California, the Great Plains, and Florida. In addition, some North American mammals (such as cats, horses, elephants, and camels) migrated as far south as Patagonia. Second, the emergence of the isthmus deflected the westward-flowing North Equatorial Current toward the north and enhanced the northward-flowing Gulf Stream. This newly invigorated current carried warm, salty waters into high northern latitudes, which contributed to increased rates of evaporation over the oceans and greater precipitation over the region of eastern Canada and Greenland. This pattern eventually led to the formation and development of the polar ice cap in the Northern Hemisphere between 4 million and 2.5 million years ago. Deflection of the Equatorial Current also changed circulation patterns throughout the Caribbean, Gulf of Mexico, and western North Atlantic, which may have altered patterns of oceanic productivity in the region, resulting in significant evolutionary changes (extinctions and originations) in marine faunas.