HOME

TheInfoList



OR:

The term Middle Miocene disruption, alternatively the Middle Miocene extinction or Middle Miocene extinction peak, refers to a wave of
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed an ...
s of terrestrial and aquatic life forms that occurred around the middle of the
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent ...
, roughly 14
million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
, during the
Langhian The Langhian is, in the ICS geologic timescale, an age or stage in the middle Miocene Epoch/Series. It spans the time between 15.97 ± 0.05 Ma and 13.65 ± 0.05 Ma (million years ago) during the Middle Miocene.GeoWhen (2007) The Langhian was ...
stage of the Miocene. This era of extinction is believed to have been caused by a relatively steady period of cooling that resulted in the growth of ice sheet volumes globally, and the reestablishment of the ice of the
East Antarctic Ice Sheet The East Antarctic Ice Sheet (EAIS) is one of two large ice sheets in Antarctica, and the largest on the entire planet. The EAIS lies between 45° west and 168° east longitudinally. The EAIS holds enough ice to raise global sea levels by and ...
(EAIS). Cooling that led to the Middle Miocene disruption is primarily attributed to orbitally paced changes in oceanic and atmospheric circulation due to
continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of p ...
. These may have been amplified by CO2 being pulled out of the
Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
by organic material before becoming caught in different locations like the
Monterey Formation The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands ...
. This period was preceded by the Miocene Climatic Optimum, a period of relative warmth from 18 to 14 Ma.


Effects

One of the primary effects of the climatic cooling that took place during this time period was the growth of the East Antarctic Ice Sheet (EAIS). Significant sections of ice on the Antarctic continent are believed to have started growth at the beginning of the Middle Miocene disruption and continued to expand until about 10 Ma. This growth has been attributed primarily to orbitally paced changes in oceanic and atmospheric currents, with possible amplification by a significant drop in atmospheric carbon dioxide (ppm): Atmospheric CO2 fell temporarily from about 300 to 140ppm as estimated by the relationship between atmospheric levels of CO2 and pH levels in the ocean determined by boron isotopic levels in calcium carbonate. One of the primary indicators for the significant global ice sheet growth is the higher concentration of 18O found in benthic foraminifera from oceanic sediment cores during this time period. During periods of ice sheet growth, the lighter 16O isotopes found in ocean water are drawn out as precipitation and consolidate in ice sheets while a higher concentration of 18O is left behind for foraminifera to utilize. One of the other primary effects of the climatic cooling during the Middle Miocene was the biotic impact on terrestrial and oceanic lifeforms. A primary example of these extinctions is indicated by the observed occurrence of
Varanidae The Varanidae are a family of lizards in the superfamily Varanoidea within the Anguimorpha group. The family, a group of carnivorous and frugivorous lizards, includes the living genus '' Varanus'' and a number of extinct genera more closely re ...
,
chameleon Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...
s,
Cordylidae Cordylidae is a family of small- to medium-sized lizards that occur in southern and eastern Africa. They are commonly known as girdled lizards, spinytail lizards, or girdle-tail lizards. Cordylidae is closely related to the family Gerrhosaurida ...
, Tomistominae,
Alligatoridae The family Alligatoridae of crocodylians includes alligators, caimans and their extinct relatives. Phylogeny The superfamily Alligatoroidea includes all crocodilians (fossil and extant) that are more closely related to the American alligator ...
, and giant turtles through the Miocene Climatic Optimum (18 to 16 Ma) in Central Europe (45-42°N palaeolatitude). This was then followed by a major and permanent cooling step marked by the Mid Miocene disruption between 14.8 and 14.1 Ma. Two crocodilians of the genera '' Gavialosuchus'' and '' Diplocynodon'' were noted to have been extant in these northern latitudes prior to the permanent cooling step, but then became extinct between 14 and 13.5 Ma. Another indicator that would lead to extinctions is the conservative estimate that temperatures in the Antarctic region may have cooled by at least 8o C in the summer months 14 Ma. This Antarctic cooling, along with significant changes in temperature gradients in Central Europe as indicated by Madelaine Böhme's study on ectothermic vertebrates, provide evidence that plant and animal life needed to migrate or adapt in order to survive.


Suggested causes

The primary causes for the cooling that came out of the Middle Miocene Climatic Optimum are centered around significant changes in both oceanic circulation, as well as changing atmospheric CO2 levels. Oceanic circulation changes are defined by increases in Antarctic Bottom Water (AABW) production, the halting of saline water delivery to the
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
from the
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
and additional
North Atlantic Deep Water North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning circulation) of the world's oceans involves the flow of warm surface waters from th ...
(NADW) production. Falling CO2 concentrations in the atmosphere has been linked to drawdown of the gas into organic material deposited along continental margins like the
Monterey Formation The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands ...
of coastal
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the mo ...
. These sites of CO2 drawdown are thought to have been extensive enough to drop atmospheric concentrations in CO2 from about 300 to 140ppm and lead to processes of global cooling that helped in the expansion of the EAIS. An additional suggested cause for the Middle Miocene disruption has been attributed to a shift from a solar insolation cycle that is obliquity dominated to one that is dominated by eccentricity (see
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypoth ...
). This change would have been significant enough for conditions near the Antarctic continent to allow for glaciation.


Extinction event

The Middle Miocene disruption is considered a significant extinction event and has been analyzed in terms of the importance of there being a possible periodicity between extinction events. A study from Raup and Sepkoski found that there is a statistically significant mean periodicity (where P is less than .01) of about 26 million years for 12 major extinction events. There is debate whether this potential periodicity is caused by some set of recurrent cycles or biologic factors.


References


Further reading

*


External links


Middle Miocene global map
{{ExtEvent nav . Miocene events Langhian