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39-454: The length of time that the MECO spanned is disputed, although it is known to have lasted from around 40.5 to 40.0 Ma. Depending on location and methodology, the event's duration has been variously estimated at 300, 500, 600, and 750 kyr . The MECO was globally synchronous and observed in both marine and terrestrial sequences. The global mean surface temperature during the MECO was about 23.1 °C. In

78-462: A Latin root. This standards - or measurement -related article is a stub . You can help Misplaced Pages by expanding it . This time -related article is a stub . You can help Misplaced Pages by expanding it . Silicate weathering The carbonate–silicate geochemical cycle , also known as the inorganic carbon cycle , describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation , and

117-521: A calcium ion, Ca , a bicarbonate ion, HCO 3 , and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals. The chemical pathway is as follows: River runoff carries these products to the ocean, where marine calcifying organisms use Ca and HCO 3 to build their shells and skeletons, a process called carbonate precipitation : Two molecules of CO 2 are required for silicate rock weathering; marine calcification releases one molecule back to

156-416: A challenge to modelers who have tried to relate the rate of outgassing and subduction to the related rates of seafloor change. Proper, uncomplicated proxy data is difficult to attain for such questions. For example, sediment cores, from which scientists can deduce past sea levels, are not ideal because sea levels change as a result of more than just seafloor adjustment. Recent modeling studies have investigated

195-464: A favourable temperature, this highly diverse flora reverted to pre-MECO levels of biodiversity after the hothouse concluded. Coastal southeastern Australia was dominated by mesothermal rainforests, although whether or not this flora was already present before the MECO remains up for debate. Kyr The abbreviation kyr means "thousand years". kyr was formerly common in some English language works, especially in geology and astronomy , for

234-552: A greenhouse effect. With its thin atmosphere, Mars' mean surface temperature is 210 K (−63 °C). In attempting to explain topographical features resembling fluvial channels, despite seemingly insufficient incoming solar radiation, some have suggested that a cycle similar to Earth's carbonate-silicate cycle could have existed – similar to a retreat from Snowball Earth periods. It has been shown using modeling studies that gaseous CO 2 and H 2 O acting as greenhouse gases could not have kept Mars warm during its early history when

273-406: A negative feedback that helped restore global temperatures to their pre-MECO state after the warming ended. However, deoxygenation was not globally ubiquitous; South Atlantic sites such as South Atlantic Ocean Drilling Program Site 702 show no evidence of any shift towards dysoxic conditions. The enhanced formation of glauconites in some studied sections across the MECO is believed to in part reflect

312-408: A possible solution to the faint young Sun paradox . The carbonate-silicate cycle is the primary control on carbon dioxide levels over long timescales. It can be seen as a branch of the carbon cycle , which also includes the organic carbon cycle , in which biological processes convert carbon dioxide and water into organic matter and oxygen via photosynthesis . The inorganic cycle begins with

351-505: Is called ocean acidification . One should not assume that a carbonate-silicate cycle would appear on all terrestrial planets . To begin, the carbonate-silicate cycle requires the presence of a water cycle. It therefore breaks down at the inner edge of the Solar System's habitable zone . Even if a planet starts out with liquid water on the surface, if it becomes too warm, it will undergo a runaway greenhouse , losing surface water. Without

390-571: Is converted to carbonic acid , which increases weathering. Tectonics can induce changes in the carbonate-silicate cycle. For example, the uplift of major mountain ranges, such as Himalayas and the Andes , is thought to have initiated the Late Cenozoic Ice Age due to increased rates of silicate weathering and draw down of carbon dioxide . Seafloor weather is linked both to solar luminosity and carbon dioxide concentration. However, it presented

429-468: Is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or soda springs , which are natural springs that contain carbon dioxide gas or soda water: This final step returns the second CO 2 molecule to the atmosphere and closes the inorganic carbon budget . 99.6% of all carbon on Earth (equating to roughly 10 billion tons of carbon) is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in

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468-406: Is sensitive to factors that change how much land is exposed. These factors include sea level , topography , lithology , and vegetation changes. Furthermore, these geomorphic and chemical changes have worked in tandem with solar forcing, whether due to orbital changes or stellar evolution, to determine the global surface temperature . Additionally, the carbonate-silicate cycle has been considered

507-498: The Precambrian - Cambrian boundary would have allowed more efficient removal of weathering products from the ocean. Biological processes in soils can significantly increase weathering rates. Plants produce organic acids that increase weathering . These acids are secreted by root and mycorrhizal fungi , as well as microbial plant decay . Root respiration and oxidation of organic soil matter also produce carbon dioxide , which

546-480: The Tethys Ocean , sea surface temperatures (SSTs) have been estimated at 32-36 °C. Water temperatures off what is now Liguria rose by about 4-6 °C, while the seas off southwestern Balkanatolia warmed by 2-5 °C. The northwestern Atlantic experienced a 3 °C increase in upper ocean temperatures. In the southwestern Pacific , SSTs rose from an average of about 22 °C to 28 °C. Deep ocean temperatures were about 9 °C at

585-451: The abyssal zone . The MECO was marked by a notable rise in atmospheric carbon dioxide concentrations. At their peak, p CO 2 values may have reached as high as 4,000 ppm. One possible cause of this rise in p CO 2 was the collision of India with Eurasia and formation of the Himalayas that was occurring at this time, which would have metamorphically liberated large quantities of

624-441: The unit of 1,000 years or millennium . The "k" is the unit prefix for kilo- or thousand with the suffix "yr" simply an abbreviation for "year". Occasionally, the "k" is shown in upper case , as in "100 Kyr"; this is an incorrect usage. "kyr" itself is often considered incorrect, with some preferring to use "ky". ISO 80000-3 recommends usage of ka (for kiloannum ), which avoids the implicit English bias of "year" by using

663-589: The Sun was fainter because CO 2 would condense out into clouds. Even though CO 2 clouds do not reflect in the same way that water clouds do on Earth, it could not have had much of a carbonate-silicate cycle in the past. By contrast, Venus is located at the inner edge of the habitable zone and has a mean surface temperature of 737 K (464 °C). After losing its water by photodissociation and hydrogen escape , Venus stopped removing carbon dioxide from its atmosphere, and began instead to build it up, and experience

702-479: The amount of organic matter making its way to the ocean depths. Large benthic foraminifera, however, thrived, which contributed to the large increase in platform carbonate deposition observed across the warming event. Silicoflagellates, diatoms, and radiolarians flourished as silicic acid was supplied to the oceans in greater quantities than before. The increase in iron transport into the oceans, causing populations of magnetotactic bacteria to grow. The MECO coincided with

741-411: The atmosphere for longer. This may have come about as a result of continental rocks having become less weatherable during the very warm Early Eocene and Early Middle Eocene; by the time of the MECO, few areas of silicate rock potent enough to absorb significant amounts of carbon dioxide would have remained. The MECO warmth may have been sustained through a further inhibition of silicate weathering following

780-508: The atmosphere. The calcium carbonate (CaCO 3 ) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor. The final stage of the process involves the movement of the seafloor. At subduction zones , the carbonate sediments are buried and forced back into the mantle . Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO 2 to form CaSiO 3 and CO 2 , which

819-419: The available carbonate ions, which presents an obstacle to the carbon carbonate precipitation process. Put differently, 30% of excess carbon emitted into the atmosphere is absorbed by the oceans. Higher concentrations of carbon dioxide in the oceans work to push the carbonate precipitation process in the opposite direction (to the left), producing less CaCO 3 . This process, which harms shell-building organisms,

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858-502: The carbonate-silicate cycle has a stabilizing effect on the Earth's climate, which is why it has been called the Earth's thermostat. Aspects of the carbonate-silicate cycle have changed through Earth history as a result of biological evolution and tectonic changes. Generally, the formation of carbonates has outpaced that of silicates, effectively removing carbon dioxide from the atmosphere. The advent of carbonate biomineralization near

897-457: The conditions enabling the MECO's persistent global warmth. Planktonic foraminifera underwent a major biotic turnover; acarinids were greatly reduced in diversity and morozovellids went extinct. The range of the planktonic foraminifer Orbulinoides beckmanni , a species well adapted to warm waters, expanded to higher latitudes during the MECO. Benthic foraminifera exhibited a decline due to enhanced respiration of pelagic heterotrophs, limiting

936-417: The cycle but even more generally, it serves as a critical negative feedback loop between carbon dioxide levels and climate changes. For example, if CO 2 builds up in the atmosphere, the greenhouse effect will serve to increase the surface temperature, which will in turn increase the rate of rainfall and silicate weathering, which will remove carbon from the atmosphere. In this way, over long timescales,

975-399: The decrease in marine oxygen content, as this disinhibited the mobility of iron and its ability to be incorporated to make glauconite. There is evidence of ocean acidification occurring during the MECO in the form of major declines in carbonate accumulation throughout the ocean at depths of greater than three kilometres. Acidification affected the entire water column, extending as far as

1014-424: The dominant biomes in what is now the middle Black Sea region of northern Anatolia. The plant diversity of Patagonia increased by 40% during the MECO, largely due to the southward migration of neotropical plants that mixed with the established temperate Gondwanan flora. Neotropical lineages that today only occupy the tropics reached the southernmost end of South America. Nourished by abundant carbon dioxide and

1053-447: The form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere. Changes to the surface of the planet, such as an absence of volcanoes or higher sea levels, which would reduce the amount of land surface exposed to weathering can change the rates at which different processes in this cycle take place. Over tens to hundreds of millions of years, carbon dioxide levels in the atmosphere may vary due to natural perturbations in

1092-551: The greenhouse gas, although the timing of metamorphic carbon release is poorly resolved. Enhanced rates of seafloor spreading and metamorphic decarbonation reactions around the region between Australia and Antarctica , combined with increased volcanic activity in this region, may also have been a source of the carbon injection into the atmosphere. Yet another hypothesis implicates increased continental arc volcanism in what are now Azerbaijan and Iran for this surge in atmospheric greenhouse gas levels. Some analyses have also found that

1131-751: The oceans. Extensive eutrophication is recorded from the Tethys, North Atlantic , South Atlantic, and Southern Oceans . A decline in seawater oxygen content occurred during the MECO in the Tethys Ocean. Dysoxic conditions in the Tethys lasted for about 400-500 kyr according to geochemical study of the Alano site in northeastern Italy. Evidence from the Southern Ocean indicates deep water deoxygenation developed in this marine region too. Organic carbon burial rates skyrocketed in these oxygen-poor waters, which may have acted as

1170-409: The onset of warming via enhanced clay formation. Milankovitch cycles have been suggested to have played a role in triggering MECO warmth. The MECO coincided with a minimum in the 2.4 Myr eccentricity cycle that occurred around 40.2 Ma. This 2.4 Myr eccentricity minimum coincided with a minimum in the 400 kyr eccentricity cycle; the simultaneous occurrence of these eccentricity minima likely fomented

1209-520: The paleo-record. Human emissions of CO 2 have been steadily increasing, and the consequent concentration of CO 2 in the Earth system has reached unprecedented levels in a very short amount of time. Excess carbon in the atmosphere that is dissolved in seawater can alter the rates of carbonate-silicate cycle. Dissolved CO 2 may react with water to form bicarbonate ions, HCO 3 , and hydrogen ions, H . These hydrogen ions quickly react with carbonate, CO 3 to produce more bicarbonate ions and reduce

Middle Eocene Climatic Optimum - Misplaced Pages Continue

1248-436: The peak of the MECO. On the shallow shelf around Seymour Island, temperatures warmed by ~5 °C. The North American continental interior warmed more pronouncedly, by 9 °C from 23 °C ± 3 °C to 32 °C ± 3 °C at the peak of the MECO, followed by a decline of 11 °C after the MECO. In Western North America , lakes became markedly less saline . The Pyrenees became hotter and drier. North-central Turkey , then part of Balkanatolia,

1287-508: The production of carbonic acid (H 2 CO 3 ) from rainwater and gaseous carbon dioxide. Due to this process, normal rain has a pH of around 5.6. Carbonic acid is a weak acid , but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates. These substances break down into dissolved ions as a result. For example, calcium silicate (CaSiO 3 ), or wollastonite , reacts with carbon dioxide and water to yield

1326-500: The replacement of lamniform elasmobranchs with carcharhinids in the medium to large predator guild. In North America, the MECO marked the high point of the Middle-Late Eocene mammalian assemblage. MECO warmth catalysed the faunal turnover leading to the rise of crown-group carnivorans to prominence in the continent's terrestrial ecosystems. In Balkanatolia, lower montane forests and warm, humid lowland rainforests were

1365-437: The requisite rainwater, no weathering will occur to produce carbonic acid from gaseous CO 2 . Furthermore, at the outer edge, CO 2 may condense, consequently reducing the greenhouse effect and reducing the surface temperature. As a result, the atmosphere would collapse into polar caps. Mars is such a planet. Located at the edge of the solar system's habitable zone, its surface is too cold for liquid water to form without

1404-414: The rise in atmospheric p CO 2 was more limited than previous studies have suggested, instead proposing that the observed warming was caused by a much greater sensitivity of the Earth's climate to changes in p CO 2 relative to today. Diminished negative feedback of silicate weathering may have occurred around the time of the MECO's onset and allowed volcanically released carbon dioxide to persist in

1443-618: The role of seafloor weathering on the early evolution of life, showing that relatively fast seafloor creation rates worked to draw down carbon dioxide levels to a moderate extent. Observations of so-called deep time indicate that Earth has a relatively insensitive rock weathering feedback, allowing for large temperature swings. With about twice as much carbon dioxide in the atmosphere, paleoclimate records show that global temperatures reached up to 5 to 6 °C higher than current temperatures. However, other factors such as changes in orbital/solar forcing contribute to global temperature change in

1482-448: The transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism . Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism . On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature. The rate of weathering

1521-402: Was wet and warm. Continental Asia was once thought to have experienced intense aridification during the MECO, though more recent research has shown that this took place after the MECO, when global average temperatures resumed dropping. Continental weathering increased with rising temperatures. Marine biological productivity surged as enhanced hydrological cycling delivered more nutrients to

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