The Middle Miocene Climatic Transition (MMCT) was a relatively steady period of climatic cooling that occurred around the middle of the Miocene, roughly 14 million years ago (Ma), during the Langhian stage, [1] and resulted in the growth of ice sheet volumes globally, and the reestablishment of the ice of the East Antarctic Ice Sheet (EAIS). [2] The term Middle Miocene disruption, alternatively the Middle Miocene extinction or Middle Miocene extinction peak, refers to a wave of extinctions of terrestrial and aquatic life forms that occurred during this climatic interval. This period was preceded by the Middle Miocene Climatic Optimum (MMCO), a period of relative warmth from 18 to 14 Ma. [3] Cooling that led to the Middle Miocene disruption is primarily attributed CO2 being pulled out of the Earth's atmosphere by organic material before becoming caught in different locations like the Monterey Formation. [4] These may have been amplified by changes in oceanic and atmospheric circulation due to continental drift. [1] Additionally, orbitally paced factors may also have played a role. [5]
One of the primary effects of the climatic cooling that took place during this time period was the growth of the EAIS, [6] [4] termed the East Antarctic Ice Sheet Expansion (EAIE). [7] A thermal shift from wet to cold-based glaciers is recorded from the Transantarctic Mountains about 13.94 Ma, reflecting a mean annual temperature drop of 25-30 °C. [8] 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. [9] 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 140 ppm as estimated by the relationship between atmospheric levels of CO2 and pH levels in the ocean determined by boron isotopic levels in calcium carbonate. [2] 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. [10] The >180° phase reversal in the 41-kyr obliquity cycle around 14.0 to 13.8 Ma has also been interpreted as a signal of the EAIE. [7]
During the MMCT, the latitudinal precipitation gradient declined in Europe, though it increased during short term warming periods superimposed on the broader cooling trend, [11] whereas the seasonality of mean temperature increased. [12] Global cooling during the MMCT caused aridification in North Africa and South Asia. [13] In the Columbia River Basalt Group (CRBG), the cessation of kaolin-producing pedogenic processes occurred at the start of the MMCT and has been used as a proxy marker for the end of the MMCO. [14] Southwestern Australia exhibited the most arid conditions it had witnessed over any interval of the Miocene, while northwestern Australia was also hyperarid. [15] In the Qaidam Basin, silicate weathering sharply decreased around 12.6 Ma, indicating a major aridification event. [16]
The primary cause of the cooling that came out of the MMCO was changing atmospheric CO2 levels. [1] 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 of coastal California, an explanation known as the Monterey Hypothesis. [17] 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. [2]
Organic carbon burial on land, evidenced by widespread formation of lignite deposits at this time, also contributed heavily to the reduction in pCO2. [18]
Another hypothesis is that increased silicate weathering of the uplifting Himalayas caused the MMCT, but this is contradicted by geological evidence from the Indus River system. [19]
As well significant changes in greenhouse gas concentrations, alterations to ocean circulation brought about major climatic and biotic changes. Oceanic circulation changes that took place during the MMCT are defined by increases in Antarctic Bottom Water (AABW) production, the halting of saline water delivery to the Southern Ocean from the Indian Ocean, and additional North Atlantic Deep Water (NADW) production. [10] The reduction in water transport from the warm Indian Ocean to the cool Southern Ocean is believed to be responsible for the increase in AABW production. [20] The Tethys Seaway is believed to have closed around this time, exacerbating the disruptions of ocean circulation patterns that caused the MMCT. [1] The cooling of the Southern Ocean was coupled to the growth of the EAIS. [21]
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). This change would have been significant enough for conditions near the Antarctic continent to allow for glaciation. [5]
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 David Raup and Jack 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. [22]
A sharp drop in carbonate production, known as the Miocene Carbonate Crash (MCC), occurred during the early Tortonian, shortly after the cooling event; this event is generally regarded to have been induced by the changes in thermohaline circulation resulting from the Middle Miocene disruption. [23] Changes in the intensity and seasonality of the Indian monsoon have been suggested to have brought about this change in ocean circulation. [24] Another hypothesis for the crash involves the shrinkage and shoaling of the Central American Seaway, limiting water mass exchange between the Atlantic and Pacific Oceans. [25] Evidence for this event is known from the Indian Ocean, Pacific Ocean, Atlantic Ocean, Caribbean Sea, and Mediterranean Sea, suggesting the decline of carbonate-producing marine organisms was a global phenomenon. [23]
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, chameleons, Cordylidae, Tomistominae, Alligatoridae, 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. [26] 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. [27]
Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.
The Eocene is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek Ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.
The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern marine invertebrates than the Pliocene has. The Miocene is preceded by the Oligocene and is followed by the Pliocene.
The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago (Mya) to the beginning of the present Quaternary Period 2.58 million years ago. The Neogene is sub-divided into two epochs, the earlier Miocene and the later Pliocene. Some geologists assert that the Neogene cannot be clearly delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes (1815–1868). The earlier term Tertiary Period was used to define the span of time now covered by Paleogene and Neogene and, despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use.
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from Ancient Greek ὀλίγος (olígos) 'few' and καινός (kainós) 'new', and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.
The Paleogene Period is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the first part of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use. Paleogene is often abbreviated "Pg", although the United States Geological Survey uses the abbreviation "Pe" for the Paleogene on the Survey's geologic maps.
Approximately 251.9 million years ago, the Permian–Triassicextinction event forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras. It is Earth's most severe known extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. It is also the greatest known mass extinction of insects. It is the greatest of the "Big Five" mass extinctions of the Phanerozoic. There is evidence for one to three distinct pulses, or phases, of extinction.
The Paleocene–Eocene thermal maximum (PETM), alternatively ”Eocene thermal maximum 1 (ETM1)“ and formerly known as the "Initial Eocene" or “Late Paleocene thermal maximum", was a geologically brief time interval characterized by a 5–8 °C global average temperature rise and massive input of carbon into the ocean and atmosphere. The event began, now formally, at the time boundary between the Paleocene and Eocene geological epochs. The exact age and duration of the PETM remain uncertain, but it occurred around 55.8 million years ago (Ma) and lasted about 200 thousand years (Ka). The entire warm period lasted for about 200,000 years. Global temperatures increased by 5–8 °C.
The Late Ordovician mass extinction (LOME), sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 million years ago (Ma). It is often considered to be the second-largest known extinction event just behind the end-Permian mass extinction, in terms of the percentage of genera that became extinct. Extinction was global during this interval, eliminating 49–60% of marine genera and nearly 85% of marine species. Under most tabulations, only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. Despite its taxonomic severity, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event, also known as the Frasnian-Famennian extinction, which occurred around 372 million years ago, at the boundary between the Frasnian stage and the Famennian stage, the last stage in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct. A second mass extinction called the Hangenberg event, also known as the end-Devonian extinction, occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period.
The Andean-Saharan glaciation, also known as the Early Paleozoic Ice Age (EPIA), the Early Paleozoic Icehouse, the Late Ordovician glaciation, the end-Ordovician glaciation, or the Hirnantian glaciation, occurred during the Paleozoic from approximately 460 Ma to around 420 Ma, during the Late Ordovician and the Silurian period. The major glaciation during this period was formerly thought only to consist of the Hirnantian glaciation itself but has now been recognized as a longer, more gradual event, which began as early as the Darriwilian, and possibly even the Floian. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.
The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma and is ongoing. Although geologists describe this entire period up to the present as an "ice age", in popular culture this term usually refers to the most recent glacial period, or to the Pleistocene epoch in general. Since Earth still has polar ice sheets, geologists consider the Quaternary glaciation to be ongoing, though currently in an interglacial period.
The late Paleozoic icehouse, also known as the Late Paleozoic Ice Age (LPIA) and formerly known as the Karoo ice age, was an ice age that began in the Late Devonian and ended in the Late Permian, occurring from 360 to 255 million years ago (Mya), and large land-based ice-sheets were then present on Earth's surface. It was the second major icehouse period of the Phanerozoic.
The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition (EOT) or Grande Coupure, is the transition between the end of the Eocene and the beginning of the Oligocene, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago. It was marked by large-scale extinction and floral and faunal turnover, although it was relatively minor in comparison to the largest mass extinctions.
Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: greenhouse and icehouse Earth. Both climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur as alternate phases within an icehouse period and tend to last less than 1 million years. There are five known Icehouse periods in Earth's climate history, which are known as the Huronian, Cryogenian, Andean-Saharan, Late Paleozoic, and Late Cenozoic glaciations. The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes from tectonic plate dynamics. Greenhouse and icehouse periods have played key roles in the evolution of life on Earth by directly and indirectly forcing biotic adaptation and turnover at various spatial scales across time.
The natural history of New Zealand began when the landmass Zealandia broke away from the supercontinent Gondwana in the Cretaceous period. Before this time, Zealandia shared its past with Australia and Antarctica. Since this separation, the New Zealand landscape has evolved in physical isolation, although much of its current biota has more recent connections with species on other landmasses. The exclusively natural history of the country ended in about 1300 AD, when humans first settled, and the country's environmental history began. The period from 1300 AD to today coincides with the extinction of many of New Zealand's unique species that had evolved there.
The Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced with a Paleozoic fauna rich in suspension feeder and pelagic animals.
Amelia E. Shevenell is an American marine geologist who specializes in high-latitude paleoclimatology and paleoceanography. She is currently a Professor in the College of Marine Science at the University of South Florida. She has made notable contributions to understanding the history of the Antarctic ice sheets and published in high-impact journals and, as a result, was awarded full membership of Sigma Xi. She has a long record of participation in international ocean drilling programs and has served in leadership positions of these organizations. Shevenell served as the elected Geological Oceanography Council Member for The Oceanography Society (2019-2021).
Global paleoclimate indicators are the proxies sensitive to global paleoclimatic environment changes. They are mostly derived from marine sediments. Paleoclimate indicators derived from terrestrial sediments, on the other hand, are commonly influenced by local tectonic movements and paleogeographic variations. Factors governing the Earth's climate system include plate tectonics, which controls the configuration of continents, the interplay between the atmosphere and the ocean, and the Earth's orbital characteristics. Global paleoclimate indicators are established based on the information extracted from the analyses of geologic materials, including biological, geochemical and mineralogical data preserved in marine sediments. Indicators are generally grouped into three categories; paleontological, geochemical and lithological.
The Middle Miocene Climatic Optimum (MMCO), sometimes referred to as the Middle Miocene Thermal Maximum (MMTM), was an interval of warm climate during the Miocene epoch, specifically the Burdigalian and Langhian stages.
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