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102-601: The Cenomanian and Turonian stages were first noted by D'Orbigny between 1843 and 1852. The global type section for this boundary is located in the Bridge Creek Limestone Member of the Greenhorn Formation near Pueblo, Colorado , which are bedded with the Milankovitch orbital signature . Here, a positive carbon-isotope event is clearly shown, although none of the characteristic, organic-rich black shale

204-463: A carbon sink on geologic timescales, the uptick in sequestration of carbon dioxide by the lithosphere may have helped to stabilise global temperatures after global temperatures soared. Particularly so at high latitudes, where the increase in weatherability was very pronounced. Although some early studies suggested the marine biodiversity decline observed during the Cenomanian-Turonian transition

306-405: A cause of OAE2. Enrichments in zinc further bolster and reinforce the existence of extensive hydrothermal volcanism, as do extreme negative δCr excursions. The absence of geographically widespread mercury (Hg) anomalies resulting from OAE2 has been suggested to be because of the limited dispersal range of this heavy metal by submarine volcanism. A modeling study performed in 2011 confirmed that it

408-482: A continuous supply of magma to a hotspot. As the overlying tectonic plate moves over this hotspot, the eruption of magma from the fixed plume onto the surface is expected to form a chain of volcanoes that parallels plate motion. The Hawaiian Islands chain in the Pacific Ocean is the archetypal example. It has recently been discovered that the volcanic locus of this chain has not been fixed over time, and it thus joined

510-414: A core mantle heat flux of 20 mW/m , while the cycle time (the time between plume formation events) is about 2000 million years. The number of mantle plumes is predicted to be about 17. When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and to undergo widespread decompression melting to form large volumes of basalt magma. It may then erupt onto

612-534: A decline in reactive phosphorus species within OAE2 sediments. The mineralisation of seafloor phosphorus into apatite was inhibited by the significantly lower pH of seawater and much warmer temperatures during the Cenomanian and Turonian compared to the present day, which meant that significantly more phosphorus was recycled back into ocean water after being deposited on the sea floor during this time. This would have intensified

714-405: A decline in rotaliporids and heterohelicids, a zenith of schackoinids and hedbergellids, a 'large form eclipse' during which foraminifera exceeding 150 microns disappeared, and the start of a trend of dwarfism among many foraminifera. This phase also saw an enhanced oxygen minimum zone and increased productivity in surface waters. Phase III lasted for 100,000 to 900,000 years and was coincident with

816-472: A drawdown of seawater molybdenum , and molecular biomarkers of green sulfur bacteria . Although euxinia was not uncommon in the latter part of the Cenomanian, it only expanded into the photic zone during OAE2 itself. OAE2 began on the southern margins of the proto-North Atlantic, from where anoxia spread across the rest of the proto-North Atlantic and then into the Western Interior Seaway (WIS) and

918-536: A high Sr/ Sr ratio. Helium in OIB shows a wider variation in the He/ He ratio than MORB, with some values approaching the primordial value. The composition of ocean island basalts is attributed to the presence of distinct mantle chemical reservoirs formed by subduction of oceanic crust. These include reservoirs corresponding to HUIMU, EM1, and EM2. These reservoirs are thought to have different major element compositions, based on

1020-405: A long thin conduit connecting the top of the plume to its base, and a bulbous head that expands in size as the plume rises. The entire structure resembles a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the conduit faster than the plume itself rises through its surroundings. In the late 1980s and early 1990s, experiments with thermal models showed that as

1122-612: A lower temperature. Mantle material containing a trace of partial melt (e.g., as a result of it having a lower melting point), or being richer in Fe, also has a lower seismic wave speed and those effects are stronger than temperature. Thus, although unusually low wave speeds have been taken to indicate anomalously hot mantle beneath hotspots, this interpretation is ambiguous. The most commonly cited seismic wave-speed images that are used to look for variations in regions where plumes have been proposed come from seismic tomography. This method involves using

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1224-527: A mantle plume postulated to have caused the breakup of Eurasia and the opening of the North Atlantic, now suggested to underlie Iceland . Current research has shown that the time-history of the uplift is probably much shorter than predicted, however. It is thus not clear how strongly this observation supports the mantle plume hypothesis. Basalts found at oceanic islands are geochemically distinct from mid-ocean ridge basalt (MORB). Ocean island basalt (OIB)

1326-463: A negative feedback of increased atmospheric oxygenation and wildfire activity that decreased chemical weathering, a feedback which operated on a much longer timescale. Enhanced phosphorus recycling would have resulted in an abundance of nitrogen fixing bacteria , increasing the availability of yet another limiting nutrient and supercharging primary productivity through nitrogen fixation . The ratio of bioavailable nitrogen to bioavailable phosphorus, which

1428-643: A network of seismometers to construct three-dimensional images of the variation in seismic wave speed throughout the mantle. Seismic waves generated by large earthquakes enable structure below the Earth's surface to be determined along the ray path. Seismic waves that have traveled a thousand or more kilometers (also called teleseismic waves ) can be used to image large regions of Earth's mantle. They also have limited resolution, however, and only structures at least several hundred kilometers in diameter can be detected. Seismic tomography images have been cited as evidence for

1530-553: A number of mantle plumes in Earth's mantle. There is, however, vigorous on-going discussion regarding whether the structures imaged are reliably resolved, and whether they correspond to columns of hot, rising rock. The mantle plume hypothesis predicts that domal topographic uplifts will develop when plume heads impinge on the base of the lithosphere. An uplift of this kind occurred when the North Atlantic Ocean opened about 54 million years ago. Some scientists have linked this to

1632-399: A plume developed into a weakly defined hypothesis, which as a general term is currently neither provable nor refutable. The dissatisfaction with the state of the evidence for mantle plumes and the proliferation of ad hoc hypotheses drove a number of geologists, led by Don L. Anderson , Gillian Foulger , and Warren B. Hamilton , to propose a broad alternative based on shallow processes in

1734-404: A positive feedback loop in which phosphorus is recycled faster into anoxic seawater compared to oxygen-rich water, which in turn fertilises the water, causes increased eutrophication, and further depletes the seawater of oxygen. The influx of volcanically erupted and chemically weathered sulphate into the ocean also inhibited phosphorus burial by increasing hydrogen sulphide production, which hinders

1836-567: A sediment core in the Tarfaya Basin is indicative of the main positive carbon isotope excursion occurring during a prolonged eccentricity minimum. Carbon isotope shifts smaller in scale observed in this core likely reflected variability in obliquity. Ocean Drilling Program Site 1138 in the Kerguelen Plateau yields evidence of a 20,000 to 70,000 year periodicity in changes in sedimentation, suggesting that either obliquity or precession governed

1938-503: A separate causal category of terrestrial volcanism with implications for the study of hotspots and plate tectonics. In 1997 it became possible using seismic tomography to image submerging tectonic slabs penetrating from the surface all the way to the core-mantle boundary. For the Hawaii hotspot , long-period seismic body wave diffraction tomography provided evidence that a mantle plume is responsible, as had been proposed as early as 1971. For

2040-502: A significant faunal turnover, and post-OAE2 tethysuchians tended to inhabit warmer environments compared to pre-OAE2 tethysuchians. Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates , including certain planktic and benthic foraminifera , mollusks , bivalves , dinoflagellates and calcareous nannofossils . Planktonic foraminifera suffered from

2142-641: A single province separated by opening of the South Atlantic Ocean), and the Columbia River basalts of North America. Flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean. The narrow vertical conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing

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2244-508: A total duration of 820 ±25 ka. The level is also known as the Bonarelli Event because of 1-to-2-metre (3 ft 3 in to 6 ft 7 in) layer of thick, black shale that marks the boundary and was first studied by Guido Bonarelli in 1891. It is characterized by interbedded black shales, chert and radiolarian sands and is estimated to span a 400,000-year interval. Planktonic foraminifera do not exist in this Bonarelli Level, and

2346-448: A type section according to more modern standards. This palaeogeography article is a stub . You can help Misplaced Pages by expanding it . This article about stratigraphy is a stub . You can help Misplaced Pages by expanding it . Mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle , hypothesized to explain anomalous volcanism. Because

2448-447: Is 16:1 in the present, fell precipitously as the ocean transitioned from being oxic and nitrate-dominated to anoxic and ammonium-dominated. A potent feedback loop of nitrogen fixation, productivity, deoxygenation, nitrogen removal, and phosphorus recycling was created. Bacterial hopanoids indicate populations of nitrogen fixing cyanobacteria were high during OAE2, providing a rich supply of nitrates and nitrites. Negative δ15N values reveal

2550-462: Is a strong thermal (temperature) discontinuity. The temperature of the core is approximately 1,000 degrees Celsius higher than that of the overlying mantle. Plumes are postulated to rise as the base of the mantle becomes hotter and more buoyant. Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting . This would create large volumes of magma. This melt rises to

2652-454: Is consistent with a system that tends toward equilibrium: as matter rises in a mantle plume, other material is drawn down into the mantle, causing rifting. In parallel with the mantle plume model, two alternative explanations for the observed phenomena have been considered: the plate hypothesis and the impact hypothesis. Since the beginning of the 21st century, a paradigm debate "The great plume debate" has developed around plumes, in which

2754-536: Is enriched in trace incompatible elements , with the light rare earth elements showing particular enrichment compared with heavier rare earth elements. Stable isotope ratios of the elements strontium , neodymium , hafnium , lead , and osmium show wide variations relative to MORB, which is attributed to the mixing of at least three mantle components: HIMU with a high proportion of radiogenic lead, produced by decay of uranium and other heavy radioactive elements; EM1 with less enrichment of radiogenic lead; and EM2 with

2856-413: Is known to have been synchronous with the marine transgression associated with OAE2, although the loss of freshwater floodplain habitat has been speculated to have possibly resulted in the demise of some freshwater taxa. In fossiliferous rocks in southwestern Utah , a local extirpation of some metatherians and brackish water vertebrates is associated with the later marine regression following OAE2 in

2958-481: Is more diverse compositionally than MORB, and the great majority of ocean islands are composed of alkali basalt enriched in sodium and potassium relative to MORB. Larger islands, such as Hawaii or Iceland, are mostly tholeiitic basalt, with alkali basalt limited to late stages of their development, but this tholeiitic basalt is chemically distinct from the tholeiitic basalt of mid-ocean ridges. OIB tends to be more enriched in magnesium, and both alkali and tholeiitic OIB

3060-438: Is posited to exist where super-heated material forms ( nucleates ) at the core-mantle boundary and rises through the Earth's mantle. Rather than a continuous stream, plumes should be viewed as a series of hot bubbles of material. Reaching the brittle upper Earth's crust they form diapirs . These diapirs are "hotspots" in the crust. In particular, the concept that mantle plumes are fixed relative to one another and anchored at

3162-490: Is possible that a LIP may have initiated the event, as the model revealed that the peak amount of carbon dioxide degassing from volcanic LIP degassing could have resulted in more than 90 percent global deep-ocean anoxia. Later on, when anoxia became widespread, the production of nitrous oxide , a greenhouse gas about 265 times more potent than carbon dioxide, drastically increased because of elevated nitrification and denitrification rates. This powerful positive feedback mechanism

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3264-483: Is present. It has been estimated that the isotope shift lasted approximately 850,000 years longer than the black shale event, which may be the cause of this anomaly in the Colorado type section . A significantly expanded OAE2 interval from southern Tibet documents a complete, more detailed, and finer-scale structures of the positive carbon isotope excursion that contains multiple shorter-term carbon isotope stages amounting to

3366-482: Is that material and energy from Earth's interior are exchanged with the surface crust in two distinct and largely independent convective flows: The plume hypothesis was simulated by laboratory experiments in small fluid-filled tanks in the early 1970s. Thermal or compositional fluid-dynamical plumes produced in that way were presented as models for the much larger postulated mantle plumes. Based on these experiments, mantle plumes are now postulated to comprise two parts:

3468-479: Is the type locality . Also it can be defined as "The particular sequence of strata chosen as standard of reference of a layered stratigraphic unit." When a stratigraphic unit is nowhere fully exposed, the original type section may be supplemented with reference sections covering the full thickness of the unit. A reference section may also be defined when the original type section is poorly exposed, or for historical units which were designated without specifying

3570-661: Is what may have enabled extremely hot temperatures to persist in spite of the supercharged organic carbon burial associated with anoxic events. Large-scale organic carbon burial acted as a negative feedback loop that partially mitigated the warming effects of volcanic discharge of carbon dioxide, resulting in the Plenus Cool Event during the Metoicoceras geslinianum European ammonite biozone. Global average temperatures fell to around 4 °C lower than they were pre-OAE2. Equatorial SSTs dropped by 2.5–5.5 °C. This cooling event

3672-703: The Chagos-Laccadive Ridge , the Louisville Ridge , the Ninety East Ridge and Kerguelen , Tristan , and Yellowstone . While there is evidence that the chains listed above are time-progressive, it has been shown that they are not fixed relative to one another. The most remarkable example of this is the Emperor chain, the older part of the Hawaii system, which was formed by migration of the hotspot in addition to

3774-458: The Rotalipora cushmani planktonic foraminifer biozone, has been argued to be another example supporting this hypothesis of regular oceanic anoxic events governed by Milankovitch cycles. The MCE took place approximately 2.4 million years before the Cenomanian-Turonian oceanic anoxic event, roughly at the time when an anoxic event would be expected to occur given such a cycle. Geochemical evidence from

3876-608: The Sudbury Igneous Complex in Canada are known to have caused melting and volcanism. In the impact hypothesis, it is proposed that some regions of hotspot volcanism can be triggered by certain large-body oceanic impacts which are able to penetrate the thinner oceanic lithosphere , and flood basalt volcanism can be triggered by converging seismic energy focused at the antipodal point opposite major impact sites. Impact-induced volcanism has not been adequately studied and comprises

3978-531: The Yellowstone hotspot , seismological evidence began to converge from 2011 in support of the plume model, as concluded by James et al., "we favor a lower mantle plume as the origin for the Yellowstone hotspot." Data acquired through Earthscope , a program collecting high-resolution seismic data throughout the contiguous United States has accelerated acceptance of a plume underlying Yellowstone. Although there

4080-404: The lower mantle under Africa and under the central Pacific. It is postulated that plumes rise from their surface or their edges. Their low seismic velocities were thought to suggest that they are relatively hot, although it has recently been shown that their low wave velocities are due to high density caused by chemical heterogeneity. Some common and basic lines of evidence cited in support of

4182-512: The Bohemian Cretaceous Basin during the peak of oceanic anoxia. Phosphorus liberation in the pore water environment, several centimetres below the interface between seafloor sediments and the water column, enabled the precipitation of phosphate through biological mediation by microorganisms. Strontium and calcium isotope ratios both indicate that silicate weathering increased over the course of OAE2. Because of its effectiveness as

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4284-499: The Bonarelli Level's deposition and exhibited extensive proliferation of radiolarians, indicative of extremely eutrophic conditions. Phase IV lasted for around 35,000 years and was most notable for the increase in the abundance of hedbergellids and schackoinids, being extremely similar to Phase II, with the main difference being that rotaliporids were absent from Phase IV. Phase V was a recovery interval lasting 118,000 years and marked

4386-468: The Cenomanian-Turonian boundary is one of the main carbon isotope events of the Mesozoic. It represents one of the largest disturbances in the global carbon cycle from the past 110 million years. This δC excursion indicates a significant increase in the burial rate of organic carbon, indicating the widespread deposition and preservation of organic carbon-rich sediments and that the ocean was depleted of oxygen at

4488-620: The Cenomanian-Turonian boundary of Wunstorf , Germany, reveal the uncharacteristic dominance of a calcareous nannofossil species, Watznaueria , present during the event. Unlike the Biscutum species, which prefer mesotrophic conditions and were generally the dominant species before and after the C/T boundary event; Watznaueria species prefer warm, oligotrophic conditions. In the Ohaba-Ponor section in Romania ,

4590-521: The Cretaceous period. Even before OAE2, during the late Cenomanian, tropical sea surface temperatures (SSTs) were very warm, about 27-29 °C. The onset of OAE2 was concurrent with a 4-5 °C rise in shelf sea temperatures. Mean tropical SSTs during OAE2 have been conservatively estimated to have been at least 30 °C, but may have reached as high as 36 °C. Minimum SSTs in mid-latitude oceans were >20 °C. This exceptional warmth persisted until

4692-498: The Cretaceous. Bivalves declined significantly in diversity during the leadup to the δC org peak of OAE2. Rudist bivalves suffered high extinction rates combined with low origination rates during OAE2. Ammonoids suffered during the crisis, though anoxia was not the main driver of their declines in diversity. Ammonoid diversity losses were primarily concentrated in the seas around Europe; elsewhere, they were negligibly affected. The diversity of trace fossils sharply plummeted during

4794-476: The Earth's core, in basalts at oceanic islands. However, so far conclusive proof for this is lacking. The plume hypothesis has been tested by looking for the geophysical anomalies predicted to be associated with them. These include thermal, seismic, and elevation anomalies. Thermal anomalies are inherent in the term "hotspot". They can be measured in numerous different ways, including surface heat flow, petrology, and seismology. Thermal anomalies produce anomalies in

4896-676: The Pacific, an unradiogenic osmium spike began about 350 kyr before the onset of OAE2 and terminated around 240 kyr after OAE2's beginning; the osmium isotope data from a highly expanded OAE2 interval in southern Tibet show multiple osmium excursions with the most pronounced one lagging the onset of OAE2 by ≈50 kyr that was probably related to the ocean connectivity change at ~94.5 Ma. Osmium data also reveal that three distinct pulses of intense volcanism occurred ~60, ~270, and ~400 kyr after OAE2's onset, prolonging it. Positive neodymium isotope excursions provide additional indications of pervasive volcanism as

4998-676: The South Atlantic, as well as in the Chalk Sea. Benthic foraminifera suffered noticeable losses. The benthic foraminifera Gavelella berthelini and Lingulogavelinella globosa dominated during deoxygenated conditions in Poland. The alterations in diversity of various marine invertebrate species such as calcareous nannofossils are reflective and characteristic of oligotrophy and ocean warmth in an environment with short spikes of productivity followed by long periods of low fertility. A study performed in

5100-473: The South Atlantic, were able to remain partially oxygenated at least intermittently. Indeed, redox states of oceans vary geographically, bathymetrically and temporally during OAE2. It has been hypothesised that the Cenomanian-Turonian boundary event occurred during a period of very low variability in Earth's insolation, which has been theorised to be the result of coincident nodes in all orbital parameters. Barring chaotic perturbations in Earth's and Mars' orbits,

5202-482: The Turonian-Coniacian boundary. One possible cause of this hothouse was sub-oceanic volcanism. During the middle of the Cretaceous period, the rate of crustal production reached a peak, which may have been related to the rifting of the newly formed Atlantic Ocean. It was also caused by the widespread melting of hot mantle plumes under the ocean crust , at the base of the lithosphere , which may have resulted in

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5304-631: The Turonian. Among mammals, diversity changes likely reflect shifting ranges and changes in ecology rather than a true extinction event. Whatever the nature and magnitude of terrestrial extinctions at or near the Cenomanian-Turonian boundary was, it was most likely caused mainly by other factors than eustatic sea level fluctuations. The effect of the ecological crisis on terrestrial plants has been concluded to have been inconsequential, in contrast to extinction events driven by terrestrial large igneous provinces. However, while terrestrial plants did persist even during

5406-426: The beginning of the Cenomanian-Turonian boundary event. The recovery interval after the anoxic event's conclusion features an abundance of Planolites and is characterised overall by a high degree of bioturbation . At the time, there were also peak abundances of the green algal groups Botryococcus and prasinophytes, coincident with pelagic sedimentation. The abundances of these algal groups are strongly related to

5508-507: The bottom of the mantle transition zone at 650 km depth. Subduction to greater depths is less certain, but there is evidence that they may sink to mid-lower-mantle depths at about 1,500  km depth. The source of mantle plumes is postulated to be the core-mantle boundary at 3,000  km depth. Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with adiabatic gradients above and below this boundary. The core-mantle boundary

5610-428: The bulbous head expands it may entrain some of the adjacent mantle into itself. The size and occurrence of mushroom mantle plumes can be predicted by the transient instability theory of Tan and Thorpe. The theory predicts mushroom-shaped mantle plumes with heads of about 2000 km diameter that have a critical time (time from onset of heating of the lower mantle to formation of a plume) of about 830 million years for

5712-415: The burial of phosphorus through sorption to iron oxyhydroxide phases. OAE2 may have occurred during a peak in a 5-6 Myr cycle governing phosphorus availability; at this and other peaks in this oscillation, an increase in chemical weathering would have increased the marine phosphorus inventory and sparked a positive feedback loop of increasing productivity, anoxia, and phosphorus recycling that was only ended by

5814-589: The club of the many type examples that do not exhibit the key characteristic originally proposed. The eruption of continental flood basalts is often associated with continental rifting and breakup. This has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins. The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts. These basalts, also called ocean island basalts (OIBs), are analysed in their radiogenic and stable isotope compositions. In radiogenic isotope systems

5916-429: The core-mantle boundary (2900 km depth) to a possible layer of shearing and bending at 1000 km. They were detectable because they were 600–800 km wide, more than three times the width expected from contemporary models. Many of these plumes are in the large low-shear-velocity provinces under Africa and the Pacific, while some other hotspots such as Yellowstone were less clearly related to mantle features in

6018-466: The core-mantle boundary would provide a natural explanation for the time-progressive chains of older volcanoes seen extending out from some such hotspots, for example, the Hawaiian–Emperor seamount chain . However, paleomagnetic data show that mantle plumes can also be associated with Large Low Shear Velocity Provinces (LLSVPs) and do move relative to each other. The current mantle plume theory

6120-463: The core-mantle boundary. Lithospheric extension is attributed to processes related to plate tectonics. These processes are well understood at mid-ocean ridges, where most of Earth's volcanism occurs. It is less commonly recognised that the plates themselves deform internally, and can permit volcanism in those regions where the deformation is extensional. Well-known examples are the Basin and Range Province in

6222-438: The correlation between major element compositions of OIB and their stable isotope ratios. Tholeiitic OIB is interpreted as a product of a higher degree of partial melting in particularly hot plumes, while alkali OIB is interpreted as a product of a lower degree of partial melting in smaller, cooler plumes. In 2015, based on data from 273 large earthquakes, researchers compiled a model based on full waveform tomography , requiring

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6324-408: The dominance of ammonium through regenerative nutrient loops in the proto-North Atlantic. In the present day, sulphidic waters are generally prevented from spreading throughout the water column by the oxidation of sulphide with nitrate. However, during OAE2, the inventory of seawater nitrate was lower, meaning that chemolithoautotrophic oxidation of sulphides with nitrates was inefficient at preventing

6426-480: The end of the 'large form eclipse' that began in Phase II; heterohelicids and hedbergellids remained in abundance during this phase, pointing to continued environmental disturbance during this phase. Earth pronouncedly warmed just before the beginning of OAE2. The Cenomanian-Turonian interval represents one of the hottest intervals of the entire Phanerozoic eon, and it boasted the highest carbon dioxide concentrations of

6528-513: The epicontinental seas of the Western Tethys. Anoxic waters spread rapidly throughout the WIS due to marine transgression and a powerful cyclonic circulation resulting from an imbalance between precipitation in the north and evaporation in the south. Anoxia was especially intense in the eastern North Sea, evidenced by its very positive δC values. Thanks to persistent upwelling, some marine regions, such as

6630-518: The equivalent of 3 million hours of supercomputer time. Due to computational limitations, high-frequency data still could not be used, and seismic data remained unavailable from much of the seafloor. Nonetheless, vertical plumes, 400 C hotter than the surrounding rock, were visualized under many hotspots, including the Pitcairn , Macdonald , Samoa , Tahiti , Marquesas , Galapagos , Cape Verde , and Canary hotspots. They extended nearly vertically from

6732-524: The event lasted for a shorter time: only ~600 kyr. Biodiversity patterns of planktic foraminifera indicate that the Cenomanian-Turonian extinction occurred in five phases. Phase I, which took place from 313,000 to 55,000 years before the onset of the anoxic event, witnessed a stratified water column and high planktonic foraminiferal diversity, suggesting a stable marine environment. Phase II, characterised by significant environmental perturbations, lasted from 55,000 years before OAE2 until its onset and witnessed

6834-441: The exceptional warmth, the Plenus Cool Event facilitated a notable expansion of angiosperm-dominated savanna ecosystems. Stratotype A stratotype or type section in geology is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the standard of reference for unlayered rocks

6936-513: The expansion of oxygen minimum zones; those that dwelt in deeper waters were especially hard hit. In Whadi El Ghaib, a site in Sinai, Egypt , the foraminiferal community during OAE2 was low in diversity and dominated by taxa that were extremely tolerant of low salinity, anoxic water. In the southeastern Indian Ocean, off the coast of Australia, the planktonic foraminifer Microhedbergella was highly abundant, while Heterohelix thrived in reducing waters in

7038-433: The increase of both the oxygen deficiency in the water column and the total content of organic carbon. The evidence from these algal groups suggest that there were episodes of halocline stratification of the water column during the time. A species of freshwater dinocyst — Bosedinia —was also found in the rocks dated to the time and these suggest that the oceans had reduced salinity. No major change in terrestrial ecosystems

7140-418: The large-scale burial of organic carbon. Within the OAE2 positive δC excursion, short eccentricity scale carbon isotope variability is documented in a significantly expanded OAE2 interval from southern Tibet; periodic negative δC excursions paced by the short eccentricity cycle are easily detectable in southwestern Utah too. The phosphorus retention ability of seafloor sediments declined during OAE2, revealed by

7242-653: The main basic causes involved in the contribution of the event. The timing of the peak in trace metal concentration coincides with the middle of the anoxic event, suggesting that the effects of the LIPs may have occurred during the event, but may not have initiated the event. Other studies linked the lead (Pb) isotopes of OAE-2 to the Caribbean-Colombian and the Madagascar LIPs. An osmium isotope excursion coeval with OAE2 strongly suggests submarine volcanism as its cause; in

7344-531: The mantle source. There are two competing interpretations for this. In the context of mantle plumes, the near-surface material is postulated to have been transported down to the core-mantle boundary by subducting slabs, and to have been transported back up to the surface by plumes. In the context of the Plate hypothesis, subducted material is mostly re-circulated in the shallow mantle and tapped from there by volcanoes. Stable isotopes like Fe are used to track processes that

7446-436: The model. The unexpected size of the plumes leaves open the possibility that they may conduct the bulk of the Earth's 44 terawatts of internal heat flow from the core to the surface, and means that the lower mantle convects less than expected, if at all. It is possible that there is a compositional difference between plumes and the surrounding mantle that slows them down and broadens them. Mantle plumes have been suggested as

7548-619: The ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction . An acceleration of the hydrological cycle induced by warmer global temperatures drove greater fluxes of nutrient runoff into the oceans, fuelling primary productivity. The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures. Extreme hothouse conditions encouraged ocean stratification . Boundary sediments show an enrichment of trace elements, and contain elevated δC values. The positive δC excursion found at

7650-501: The oceans, the emission of SO 2 , H 2 S, CO 2 , and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. Evidence of ocean acidification can be gleaned from δCa increases coeval with the extinction event, as well as coccolith malformation and dwarfism. Lithologies characterised by low calcium carbonate concentrations predominated during intervals of carbonate compensation depth shoaling. Ocean acidification

7752-498: The onset of the oceanic anoxic event and is thus cannot be used to consistently demarcate its beginning. Selby et al. in 2009 concluded the OAE 2 occurred approximately 91.5 ± 8.6 Ma, though estimates published by Leckie et al. (2002) are given as 93–94 Ma. The Cenomanian-Turonian boundary has been refined in 2012 to 93.9 ± 0.15 Ma. The total duration of OAE2 has been estimated at 0.9 Myr, 0.82 ± 0.025 Myr, or 0.71 ± 0.17 Myr. At high latitudes,

7854-409: The originally subducted material creates diverging trends, termed mantle components. Identified mantle components are DMM (depleted mid-ocean ridge basalt (MORB) mantle), HIMU (high U/Pb-ratio mantle), EM1 (enriched mantle 1), EM2 (enriched mantle 2) and FOZO (focus zone). This geochemical signature arises from the mixing of near-surface materials such as subducted slabs and continental sediments, in

7956-601: The plate motion. Another example is the Canary Islands in the northeast of Africa in the Atlantic Ocean. Helium-3 is a primordial isotope that formed in the Big Bang . Very little is produced, and little has been added to the Earth by other processes since then. Helium-4 includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium and thorium . Over time, helium in

8058-566: The plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots , such as Hawaii or Iceland , and large igneous provinces such as the Deccan and Siberian Traps . Some such volcanic regions lie far from tectonic plate boundaries , while others represent unusually large-volume volcanism near plate boundaries. Mantle plumes were first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971 and 1972. A mantle plume

8160-412: The plume hypothesis has been challenged and contrasted with the more recent plate hypothesis ("Plates vs. Plumes"). The reason for this is that the mantle-plume hypothesis has not been suitable for making reliable predictions since its introduction in 1971 and has therefore been repeatedly adapted to observed hotspots depending on the situation. Over time, with the growing number of models, the concept of

8262-499: The presence of Watznaueria barnesae indicates warm conditions, while the abundances of Biscutum constans , Zeugrhabdotus erectus , and Eprolithus floralis peak during cool intervals. Sites in Colorado , England , France , and Sicily show an inverse relationship between atmospheric carbon dioxide levels and the size of calcareous nannoplankton. Radiolarians also suffered heavy losses in OAE2, one of their highest diversity losses in

8364-540: The presence of radiolarians in this section indicates relatively high productivity and an availability of nutrients. In the Western Interior Seaway , the Cenomanian-Turonian boundary event is associated with the Benthonic Zone, characterised by a higher density of benthic foraminifera relative to planktonic foraminifera, although the timing of the appearance of the Benthonic Zone is not uniformly synchronous with

8466-459: The sense of columnar vertical features that span most of the Earth's mantle, transport large amounts of heat, and contribute to surface volcanism. Under the umbrella of the plate hypothesis, the following sub-processes, all of which can contribute to permitting surface volcanism, are recognised: In addition to these processes, impact events such as ones that created the Addams crater on Venus and

8568-547: The shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate. The base of the mantle is known as the D″ layer , a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle and may contain partial melt. Two very broad, large low-shear-velocity provinces exist in

8670-529: The simultaneous occurrence of nodes of orbital eccentricity , axial precession , and obliquity on Earth occurs approximately every 2.45 million years. Numerous other oceanic anoxic events occurred throughout the extremely warm greenhouse conditions of the Middle Cretaceous, and it has been suggested that these Middle Cretaceous ocean anoxic events occurred cyclically in accordance with orbital cycle patterns. The mid-Cenomanian Event (MCE), which occurred in

8772-707: The source for flood basalts . These extremely rapid, large scale eruptions of basaltic magmas have periodically formed continental flood basalt provinces on land and oceanic plateaus in the ocean basins, such as the Deccan Traps , the Siberian Traps the Karoo-Ferrar flood basalts of Gondwana , and the largest known continental flood basalt, the Central Atlantic magmatic province (CAMP). Many continental flood basalt events coincide with continental rifting. This

8874-425: The speeds of seismic waves, but unfortunately so do composition and partial melt. As a result, wave speeds cannot be used simply and directly to measure temperature, but more sophisticated approaches must be taken. Seismic anomalies are identified by mapping variations in wave speed as seismic waves travel through Earth. A hot mantle plume is predicted to have lower seismic wave speeds compared with similar material at

8976-562: The spread of euxinia. A marine transgression in the latest Cenomanian resulted in an increase in average water depth, causing seawater to become less eutrophic in shallow, epicontinental seas. Turnovers in marine biota in such epicontinental seas have been suggested to be driven more so by changes in water depth rather than anoxia. Sea level rise also contributed to anoxia by transporting terrestrial plant matter from inundated lands seaward, providing an abundant source of sustenance for eutrophicating microorganisms. A phosphogenic event occurred in

9078-411: The surface and erupts to form hotspots. The most prominent thermal contrast known to exist in the deep (1000 km) mantle is at the core-mantle boundary at 2900 km. Mantle plumes were originally postulated to rise from this layer because the hotspots that are assumed to be their surface expression were thought to be fixed relative to one another. This required that plumes were sourced from beneath

9180-724: The surface. Numerical modelling predicts that melting and eruption will take place over several million years. These eruptions have been linked to flood basalts , although many of those erupt over much shorter time scales (less than 1 million years). Examples include the Deccan traps in India, the Siberian traps of Asia, the Karoo-Ferrar basalts/dolerites in South Africa and Antarctica, the Paraná and Etendeka traps in South America and Africa (formerly

9282-525: The theory are linear volcanic chains, noble gases , geophysical anomalies, and geochemistry . The age-progressive distribution of the Hawaiian-Emperor seamount chain has been explained as a result of a fixed, deep-mantle plume rising into the upper mantle, partly melting, and causing a volcanic chain to form as the plate moves overhead relative to the fixed plume source. Other hotspots with time-progressive volcanic chains behind them include Réunion ,

9384-473: The thickening of the oceanic crust in the Pacific and Indian Oceans . The resulting volcanism would have sent large quantities of carbon dioxide into the atmosphere, leading to an increase in global temperatures. Greenhouse gas release was further increased by the degassing of organic-rich sediments intruded into by volcanic sills. Several independent events related to large igneous provinces (LIPs) occurred around

9486-469: The time of OAE2. A multitude of LIPs were active during OAE2: the Madagascar , Caribbean , Gorgona, Ontong Java , and High Arctic LIPs. The abundance of LIPs at this time reflects a major overturning in mantle convection. Trace metals such as chromium (Cr), scandium (Sc), copper (Cu) and cobalt (Co) have been found at the Cenomanian-Turonian boundary, which suggests that an LIP could have been one of

9588-416: The time. Depletion of manganese in sediments corresponding to OAE2 provides additional strong evidence of severe bottom water oxygen depletion. An increase in the abundance of the planktonic foraminifer Heterohelix provides further evidence still of anoxia. The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins. The proto-North Atlantic in particular

9690-556: The upper atmosphere is lost into space. Thus, the Earth has become progressively depleted in helium, and He is not replaced as He is. As a result, the ratio He/ He in the Earth has decreased over time. Unusually high He/ He have been observed in some, but not all, hotspots. This is explained by plumes tapping a deep, primordial reservoir in the lower mantle, where the original, high He/ He ratios have been preserved throughout geologic time. Other elements, e.g. osmium , have been suggested to be tracers of material arising from near to

9792-480: The upper mantle and above, with an emphasis on plate tectonics as the driving force of magmatism. The plate hypothesis suggests that "anomalous" volcanism results from lithospheric extension that permits melt to rise passively from the asthenosphere beneath. It is thus the conceptual inverse of the plume hypothesis because the plate hypothesis attributes volcanism to shallow, near-surface processes associated with plate tectonics, rather than active processes arising at

9894-457: The uprising material experiences during melting. The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water-soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water-soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs sink as far as

9996-576: The western USA, the East African Rift valley, and the Rhine Graben . Under this hypothesis, variable volumes of magma are attributed to variations in chemical composition (large volumes of volcanism corresponding to more easily molten mantle material) rather than to temperature differences. While not denying the presence of deep mantle convection and upwelling in general, the plate hypothesis holds that these processes do not result in mantle plumes, in

10098-431: Was a hotbed of carbon burial during OAE2 as it was in later, less severe anoxic events. Though anoxia was prevalent throughout the interval, there were transient periods of reoxygenation during OAE2. Sulphate reduction increased during OAE2, causing euxinia , a type of anoxia defined by sulphate reduction and hydrogen sulphide production, to occur during OAE2, as revealed by negative δCr excursions, positive δMo excursions,

10200-406: Was exacerbated by a positive feedback loop of increased heterotrophic respiration in highly biologically productive waters, elevating seawater concentrations of carbon dioxide and further decreasing pH. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO 2 content of the oceans could have increased organic productivity in

10302-407: Was insufficient at completely stopping the rise in global temperatures. This negative feedback was ultimately overridden, as global temperatures continued to shoot up in sync with continued volcanic release of carbon dioxide following the Plenus Cool Event, although this theory has been criticised and the warming after the Plenus Cool Event attributed to decreased silicate weathering instead. Within

10404-578: Was not a real extinction but instead represented an artifact of preservation, recent work confirms that significant extinctions were experienced by vertebrates, invertebrates, and microbes. The event brought about the extinction of the pliosaurs , and most ichthyosaurs . Coracoids of Maastrichtian age were once interpreted by some authors as belonging to ichthyosaurs, but these have since been interpreted as plesiosaur elements instead. Dolichosaurids became rare after OAE2, whereas mosasauroid diversity bloomed in its aftermath. Tethysuchians experienced

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