سنوزوئیک
Cenozoic
Cenozoic (Cainozoic) is the youngest
and the shortest of the three Phanerozoic geological eras. It represents the
geological time (and rocks deposited during that time) extending from the end of
the Mesozoic Era to the present day.
The geological concept of the
Cenozoic, as the youngest era of the Phanerozoic, was introduced by J. Phillips
in 1941. He considered it to be a unit equivalent to the Tertiary, a term that
was still in use from G. Arduino's classification of Primary, Secondary, and
Tertiary rocks, introduced in
Modern time scales include all of
the past 65 million years of geological history in the Cenozoic Era. The
distinction of the Cenozoic Era from older eras has been traditionally based on
the occurrence of fossils showing affinities to modern organisms, and not on any
particular lithostratigraphic criteria.
See also: Index fossil; Paleontology
Subdivisions
Traditional classifications
subdivide the Cenozoic Era into two periods (Tertiary and Quaternary) and seven
epochs (from oldest to youngest): Paleocene, Eocene, Oligocene, Miocene,
Pliocene, Pleistocene, and Holocene. The older five epochs, which together
constitute the Tertiary Period, span the time interval from 65 to 1.8 million
years before present. The Tertiary is often separated into two subperiods, the
Paleogene (Paleocene through Oligocene epochs, also collectively called the
Nummulitic in older European literature) and the Neogene (Miocene and Pliocene
epochs). These subperiods were introduced by M. Hornes in 1853. The Quaternary
Period, which encompasses only the last 1.8 million years, includes the two
youngest epochs (Pleistocene and Holocene). Holocene is also often referred to
as the Recent, from the old Lyellian classification. Recent stratigraphic
opinions are leaning toward abandoning the use of Tertiary and Quaternary (which
are seen as the unnecessary holdovers from obsolete classifications) and in
favor of retaining Paleogene and Neogene as the prime subdivisions of
Cenozoic. See also: Eocene;
Holocene; Miocene; Oligocene; Paleocene; Pleistocene; Pliocene
Tectonics
Many of the tectonic events
(mountain-building episodes or orogenies, changes in the rates of sea-floor
spreading, or tectonic plate convergences) that began in the Mesozoic continued
into the Cenozoic. The Laramide orogeny that uplifted the Rocky Mountains in
North America, which began as early as Late Jurassic, continued into the
Cretaceous and early Cenozoic time. In its post-Cretaceous phase the orogeny
comprised a series of diastrophic movements that deformed the crust until some
50 million years ago, when it ended abruptly. The Alpine orogeny, which created
much of the
In the
In the intracontinental region of
the Tethys between Europe and
In the Indian Ocean, perhaps the
most significant event during the Cenozoic was rapid movement of the Indian
plate northward and its collision with
Another major long-term affect of
the tectonic uplift of Tibetan Plateau, which is dated to have been significant
by 40 million years ago, may have been the initiation of the general global
cooling trend that followed this event. The uplifted plateau may have initiated
a stronger deflection of the atmospheric jet stream, strengthening of the summer
monsoon, and increased rainfall and weathering in the
The convergence between
Oceans and
climate
The modern circulation and vertical
structure of the oceans and the predominantly glacial mode that the Earth is in
at present was initiated in the mid-Cenozoic time. The early Cenozoic was a
period of transition between the predominantly thermospheric circulation of the
Mesozoic and the thermohaline circulation that developed in the mid-Cenozoic. By
the mid-Cenozoic the higher latitudes had begun to cool down, especially in the
Southern Hemisphere due to the geographic isolation of
The overall history of the Cenozoic
oceans is marked by a long-term withdrawal of the seas from epicontinental and
coastal oceans and the accretion of ice on the polar regions. The ice buildup
may have been partly favored by the more poleward position of the landmasses and
the eventual thermal isolation of the Antarctic continent.
In the early Paleogene, for the
first time a deep connection between the North and
The most prominent feature of the
surface circulation in the early Cenozoic was the westward flowing circumglobal
Tethys Current, which dominated the oceanic scene in the tropical latitudes. As
the Indian plate approached the Asian mainland in the early Cenozoic, it
progressively restricted the flow of Tethys Current to its north. In the middle
Eocene, when the general drop in global sea level and the first encounter of the
Indian plate with
A paleogeographic event of major
import for the overall Cenozoic oceanic patterns was the breaching of the
straits between Antarctica and South America at the
All climatic indicators point to a
general warming trend through the Paleocene, culminating in a period of peak
global temperatures at the close of Paleocene. The warm climates continued into
the early Eocene interval. The latitudinal and vertical thermal gradients in the
late Paleocene–early Eocene were low, and mean surface temperature was around
10°C (50°F) in the higher latitudes and 20°C (70°F) in the tropics. Terrestrial
flora and fauna also corroborate the peak warming of this interval. For example,
the Arctic
Studies have revealed a prominent
carbon-isotopic shift in global carbonate reservoir that coincides with the
latest Paleocene peak warming. This has been ascribed to the breakdown of
deposits of methane hydrates on continental margins and catastrophic release of
methane into the water and atmosphere due to rapid warming of the bottom waters.
In the latest Paleocene, bottom water temperature increased rapidly (in less
than 10,000 years) by as much as 4°C (7.2°F), with a coincident prominent
enrichment of 12C isotope of the global carbon reservoir. The isotopic changes
are accompanied by important biotic changes in the oceanic microfauna and are
synchronous in the oceans and on land. This rapid and prominent isotopic shift
cannot be explained by increased volcanic emissions of carbon dioxide, changes
in oceanic circulation, or terrestrial and marine productivity alone. Increased
flux of methane from gas-hydrate sources into the ocean-atmosphere system and
its subsequent oxidation to carbon dioxide is held responsible for this isotopic
excursion in the inorganic carbon reservoir. High-resolution data support the
gas-hydrate connection to latest Paleocene abrupt climate change. Evidence from
two widely separated sites from the low- and high-latitude
The Eocene time is also
characterized by higher global sea levels and increased oceanic productivity and
carbonate deposition on the shelves and banks. The climate became more extreme
in the late Eocene and through the Oligocene, when the latitudinal contrast
increased due to development of ice in the polar regions. Himalayan uplift that
produced the obstruction of the Tibetan Plateau in the path of the jet stream in
the late Eocene may have been an important contributory factor to the cooling
trend of the mid and late Cenozoic. Vertical thermal gradients also steepened
once the cold bottom waters outflow began from the higher latitudes.
The late Cenozoic is characterized
by further accentuation of the oceanographic and climatic patterns that were
initiated in the early Cenozoic. By Miocene time the Tethyan connection between
the Indian Ocean and the Mediterranean Sea had been broken. This event modified
the circulation patterns in the North Atlantic and the Mediterranean, which for
the first time began to resemble their modern analogs. Neogene climatic proxies
(such as stable isotopes in cores and glacial records on land) show evidence of
considerable climatic fluctuations. Six major climatic deterioration events have
been identified in the Miocene-Pliocene record. These events were also
associated with enhanced surface circulation, bottom erosion, and aridity on
land over North Africa. The cooler early Miocene climates were followed by a
climatic optimum in mid-Miocene, to be in turn followed by a significant
deterioration in climate, which has been ascribed to a major enlargement of the
ice sheets on Antarctica.
In the late Miocene, the
Mediterranean suffered a salinity crisis following a sea-level fall and the
isolation of the basin. The growth of ice caps in the Miocene eventually led to
the fall of sea level below the depth of Gibraltar Sill, isolating the
Mediterranean Basin. The lack of connection to the open Atlantic and excess
evaporation led to high salinities and deposition of voluminous quantities of
evaporites in a relatively short time. The Mediterranean was reconnected to the
Atlantic in the early Pliocene, allowing cold deep waters to suddenly spill over
the subsided Gibraltar Sill. The Black Sea was also converted into an alkaline
lake during the salinity crisis when the Mediterranean inflow was cut off. The
early Pliocene reconnection with the Mediterranean was once again followed by
isolation of the Black Sea, this time as a fresh-water lake. These conditions
lasted into the Quaternary, when the reconnection to the Mediterranean was
established through the Bosporus Straits around 7000 years ago, ushering in the
present-day conditions. See also:
Saline evaporites
Another important threshold event of
the late Cenozoic was the closing of the connection between the Central Atlantic
and Pacific oceans at the Isthmus of Panama. The connection was operative until
the mid-Pliocene, when tectonic events led to its closure some 3 million years
ago. The closure most likely led to more vigorous Gulf Stream flow due to
deflected energy, displacing the stream northward to its present-day position.
The modern circulation patterns in the Caribbean also date back to this event.
Another major climatic-oceanographic event of the Cenozoic was the development
of an extensive ice cap on the Arctic. Although there is some evidence of ice
cover in the Arctic since the Oligocene, evidence of a significant amount of ice
accumulation is only as old as mid-Pliocene, some 3 million years ago,
coincident with the closing of the Panama isthmus. The deflection of the Gulf
Stream northward due to the latter event may have provided the excess moisture
needed for this accumulation.
The Quaternary climatic history is
one of repeated alternations between glacial and interglacial periods. At least
five major glacial cycles have been identified in the Quaternary of northwestern
Europe. The most recent glacial event occurred between 30,000 and 18,000 years
ago when much of North America and northern Europe was covered with extensive
ice sheets. The late Pliocene and Pleistocene glacial cyclicity led to repeated
falls in global sea level as a result of sequestration of water as ice sheets in
higher latitudes during the glacial intervals. For example, the sea level is
estimated to have risen some 110 m (360 ft) since the end of the last glacial
maximum. As a by-product of these repeated drops in sea level and movement of
the shorelines toward the basins, large deltas developed at the mouths of the
world's major drainage systems during the Quaternary. These bodies of sand and
silt constitute ideal reservoirs for hydrocarbon accumulation. See also: Delta; Paleoclimatology
Life
At the end of the Cretaceous a major
extinction event had decimated marine biota and only a few species survived into
the Cenozoic. The recovery, however, was relatively rapid. During the Paleocene
through middle Eocene interval, the overall global sea-level rise enlarged the
ecospace for marine organisms, and an associated climatic optimum led to
increased speciation through the Paleocene, culminating in high marine
diversities during the early and middle Eocene. Limestone-building coral reefs
were also widespread in the tropical-temperate climatic belt of the early
Cenozoic, and the tropical Tethyan margins were typified by expansive
distribution of the larger foraminifera known as Nummulites (giving the
Paleogene its informal name of the Nummulitic period). See also: Nummulites
The late Eocene saw a rapid decline
in diversities of marine phyto- and zooplankton due to a global withdrawal of
the seas from the continental margins and the ensuing deterioration in climate.
Marine diversities reached a new low in the mid-Oligocene, when the sea level
was at its lowest, having gone through a major withdrawal of seas from the
continental margins. The climates associated with low seas were extreme and much
less conducive to biotic diversification. The late Oligocene and Neogene as a
whole constitute an interval characterized by increasing partitioning of
ecological nichesinto tropical, temperate, and higher-latitude climatic
belts,and greater differentiation of marine fauna and flora.
The terminal Cretaceous event had
also decimated the terrestrial biota. Dinosaurs, which had dominated the
Mesozoic scene, became extinct. Only a few small shrewlike mammalian species
survived into the Paleocene. In the absence of dinosaurian competition, mammals
evolved and spread rapidly to become dominant in the Cenozoic. The evolution of
grasses in the early Eocene and the wide distribution of grasslands thereafter
may have been catalytic in the diversification of browsing mammals. Marsupials
and insectivores as well as rodents (which first appeared in the Eocene)
diversified rapidly, as did primates, carnivores, and ungulates. The ancestral
horse first appeared in the early Eocene in North America, where its lineage
evolved into the modern genus Equus,only to disappear from the continent in the
late Pleistocene. A complete evolution of the horse can be followed in North
America during the Cenozoic. Increase in overall size, reduction in the number
of toes, and increasing complexity of grinding surface of the molars over time
are some of the obvious trends. Hominoid evolution began during the Miocene in
Africa. Modern hominids are known to have branched off from the hominoids some 5
million years ago. Over the next 4.5 million years the hominids went through
several evolutionary stages to finally evolve into archaic Homo sapiens about 1
million years ago. Truly modern Homosapiens do not enter the scene until around
100 thousand years ago. See also:
Dinosauria; Fossil humans; Mammalia; Organic evolution
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B. U. Haq and F. W. B van Eysinga, Geological Time Table, Elsevier,
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K. J. Hsü (ed.), Mesozoic and
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C. Pomerol, The Cenozoic Era: Tertiary and Quaternary, 1982
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M. E. Raymo and W. F. Ruddiman, Tectonic forcing of late Cenozoic climate, Nature, 359:117–122, 1992
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