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100-512: The Snowball Earth is a geohistorical hypothesis that proposes during one or more of Earth 's icehouse climates , the planet's surface became nearly entirely frozen with no liquid oceanic or surface water exposed to the atmosphere . The most academically mentioned period of such a global ice age is believed to have occurred some time before 650 mya during the Cryogenian period, which included at least two large glacial periods ,

200-448: A "shallow-ridge hypothesis" involving the breakup of Rodinia, linking the eruption and rapid alteration of hyaloclastites along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events. Historical geology During

300-482: A banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits. For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ferrous oxide ) could accumulate before it met an oxidant that would precipitate it as ferric oxide . For

400-473: A buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation. Earth's surface is very depleted in iridium , which primarily resides in Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During

500-470: A case where the "feed-back" action is positive in contrast to negative feed-back action, which they mentioned only in passing. Harold Stephen Black 's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black: Positive feed-back increases the gain of the amplifier, negative feed-back reduces it. According to Mindell (2002) confusion in the terms arose shortly after this: ...   Friis and Jensen had made

600-405: A case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as 200 Ma can be riddled with difficulties. The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at

700-498: A circular argument. This makes reasoning based upon cause and effect tricky, and it is necessary to analyze the system as a whole. As provided by Webster, feedback in business is the transmission of evaluative or corrective information about an action, event, or process to the original or controlling source. Self-regulating mechanisms have existed since antiquity, and the idea of feedback started to enter economic theory in Britain by

800-485: A classic in feedback control theory. This was a landmark paper on control theory and the mathematics of feedback. The verb phrase to feed back , in the sense of returning to an earlier position in a mechanical process, was in use in the US by the 1860s, and in 1909, Nobel laureate Karl Ferdinand Braun used the term "feed-back" as a noun to refer to (undesired) coupling between components of an electronic circuit . By

900-456: A consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle". His studies of phosphorus deposits and banded iron formations in sedimentary rocks made him an early adherent of the snowball Earth hypothesis postulating that the planet's surface froze more than 650 Ma. Interest in the notion of a snowball Earth increased dramatically after Paul F. Hoffman and his co-workers applied Kirschvink's ideas to

1000-434: A deliberate effect via some more tangible connection. [Practical experimenters] object to the mathematician's definition, pointing out that this would force them to say that feedback was present in the ordinary pendulum ... between its position and its momentum—a "feedback" that, from the practical point of view, is somewhat mystical. To this the mathematician retorts that if feedback is to be considered present only when there

1100-478: A detailed knowledge of geological history. Layers of rock, or strata , represent a geologic record of Earth's history. Stratigraphy is the study of strata: their order, position, and age. Structural geology is concerned with rocks' deformational histories. Fossils are organic traces of Earth's history. In a historical geology context, paleontological methods can be used to study fossils and their environments, including surrounding rocks, and place them within

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1200-414: A given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism. When sedimentary rocks form, magnetic minerals within them tend to align with Earth's magnetic field . Through the precise measurement of this palaeomagnetism, it is possible to estimate

1300-502: A higher value and counterbalance the effects of photosynthesis. The mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant CO 2 from the atmosphere to form carbonic acid , which would fall as acid rain . This would weather exposed silicate and carbonate rock (including readily attacked glacial debris), releasing large amounts of calcium, which when washed into

1400-455: A large influx of positively charged ions , as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The δ C isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle—such a low value could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from

1500-569: A literal interpretation of Christian scripture , was that of a young Earth shaped by catastrophic events. Hutton, however, depicted a very old Earth, shaped by slow, continuous change. Charles Lyell further developed the theory of uniformitarianism in the 19th century. Modern geologists have generally acknowledged that Earth's geological history is a product of both sudden, cataclysmic events (such as meteorite impacts and volcanic eruptions ) and gradual processes (such as weathering, erosion, and deposition). The discovery of radioactive decay in

1600-419: A nutrient elicits changes in some of their metabolic functions. Feedback is also central to the operations of genes and gene regulatory networks . Repressor (see Lac repressor ) and activator proteins are used to create genetic operons , which were identified by François Jacob and Jacques Monod in 1961 as feedback loops . These feedback loops may be positive (as in the case of the coupling between

1700-582: A process, whereas the positive feedback loop tends to accelerate it. The mirror neurons are part of a social feedback system, when an observed action is "mirrored" by the brain—like a self-performed action. Normal tissue integrity is preserved by feedback interactions between diverse cell types mediated by adhesion molecules and secreted molecules that act as mediators; failure of key feedback mechanisms in cancer disrupts tissue function. In an injured or infected tissue, inflammatory mediators elicit feedback responses in cells, which alter gene expression, and change

1800-461: A result, late in his career, he speculated about the possibility of global glaciation. Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and those of other continents where low- latitude glacial deposits are found, have remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for

1900-413: A similar anomaly could be explained by the impact of a large meteorite . Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if

2000-413: A similar energy-balance model predicted three stable global climates, one of which was snowball Earth. This model introduced Edward Norton Lorenz 's concept of intransitivity , indicating that there could be a major jump from one climate to another, including to snowball Earth. The term "snowball Earth" was coined by Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning

2100-524: A single discipline an example of feedback can be called either positive or negative, depending on how values are measured or referenced. This confusion may arise because feedback can be used to provide information or motivate , and often has both a qualitative and a quantitative component. As Connellan and Zemke (1993) put it: Quantitative feedback tells us how much and how many. Qualitative feedback tells us how good, bad or indifferent. While simple systems can sometimes be described as one or

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2200-400: A snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed,

2300-490: A snowball Earth. Tropical continents are more reflective than open ocean and so absorb less of the Sun's heat: most absorption of solar energy on Earth today occurs in tropical oceans. Further, tropical continents are subject to more rainfall, which leads to increased river discharge and erosion. When exposed to air, silicate rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in

2400-514: A succession of Neoproterozoic sedimentary rocks in Namibia and elaborated upon the hypothesis in the journal Science in 1998 by incorporating such observations as the occurrence of cap carbonates . In 2010, Francis A. Macdonald, assistant professor at Harvard in the Department of Earth and Planetary Sciences, and others, reported evidence that Rodinia was at equatorial latitude during

2500-441: A sufficient input of iron could provide the necessary conditions for BIF formation. A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments; such interbedding has been suggested to be an artefact of Milankovitch cycles , which would have periodically warmed the seas enough to allow gas exchange between the atmosphere and ocean and precipitate BIFs. Around

2600-481: A sugar molecule and the proteins that import sugar into a bacterial cell), or negative (as is often the case in metabolic consumption). On a larger scale, feedback can have a stabilizing effect on animal populations even when profoundly affected by external changes, although time lags in feedback response can give rise to predator-prey cycles . In zymology , feedback serves as regulation of activity of an enzyme by its direct product(s) or downstream metabolite(s) in

2700-418: A system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to feed back into itself. The notion of cause-and-effect has to be handled carefully when applied to feedback systems: Simple causal reasoning about a feedback system is difficult because the first system influences the second and second system influences the first, leading to

2800-574: A true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth. In addition, glacial sediments of the Port Askaig Tillite Formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments. The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and

2900-399: Is also found in certain behaviour. For example, "shame loops" occur in people who blush easily. When they realize that they are blushing, they become even more embarrassed, which leads to further blushing, and so on. The climate system is characterized by strong positive and negative feedback loops between processes that affect the state of the atmosphere, ocean, and land. A simple example is

3000-461: Is an actual wire or nerve to represent it, then the theory becomes chaotic and riddled with irrelevancies. Focusing on uses in management theory, Ramaprasad (1983) defines feedback generally as "...information about the gap between the actual level and the reference level of a system parameter" that is used to "alter the gap in some way". He emphasizes that the information by itself is not feedback unless translated into action. Positive feedback: If

3100-548: Is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where solar radiation is most direct. Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a supervolcano , a reduction in the atmospheric concentration of greenhouse gases such as methane and/or carbon dioxide, changes in Solar energy output , or perturbations of Earth's orbit . Regardless of

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3200-426: Is one, tend to preferentially incorporate the lighter C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae , tend to be very slightly depleted in C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower C/C ratio within organic remains and a higher ratio in corresponding ocean water. The organic component of

3300-453: Is the car; its input includes the combined torque from the engine and from the changing slope of the road (the disturbance). The car's speed (status) is measured by a speedometer . The error signal is the difference of the speed as measured by the speedometer from the target speed (set point). The controller interprets the speed to adjust the accelerator, commanding the fuel flow to the engine (the effector). The resulting change in engine torque,

3400-1378: Is the study of past climates recorded in geological time. Feedback Collective intelligence Collective action Self-organized criticality Herd mentality Phase transition Agent-based modelling Synchronization Ant colony optimization Particle swarm optimization Swarm behaviour Social network analysis Small-world networks Centrality Motifs Graph theory Scaling Robustness Systems biology Dynamic networks Evolutionary computation Genetic algorithms Genetic programming Artificial life Machine learning Evolutionary developmental biology Artificial intelligence Evolutionary robotics Reaction–diffusion systems Partial differential equations Dissipative structures Percolation Cellular automata Spatial ecology Self-replication Conversation theory Entropy Feedback Goal-oriented Homeostasis Information theory Operationalization Second-order cybernetics Self-reference System dynamics Systems science Systems thinking Sensemaking Variety Ordinary differential equations Phase space Attractors Population dynamics Chaos Multistability Bifurcation Rational choice theory Bounded rationality Feedback occurs when outputs of

3500-512: Is used to boost poor performance (narrow a gap). Referring to definition 1, some authors use alternative terms, replacing positive and negative with self-reinforcing and self-correcting , reinforcing and balancing , discrepancy-enhancing and discrepancy-reducing or regenerative and degenerative respectively. And for definition 2, some authors promote describing the action or effect as positive and negative reinforcement or punishment rather than feedback. Yet even within

3600-503: Is well-constrained, and the signal is demonstrably original. Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis, many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, many sedimentary features traditionally associated with glaciers can also be formed by other means. Thus

3700-427: The Cryogenian period with glacial ice at or below sea level, and that the associated Sturtian glaciation was global. The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. According to modelling, an ice–albedo feedback would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° to 30° of

3800-529: The Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48 ‰— δ C values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments. Isotopes of boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation . This may indicate

3900-489: The Sturtian and Marinoan glaciations . Proponents of the hypothesis argue that it best explains sedimentary deposits that are generally believed to be of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean, and they emphasize

4000-417: The biosphere , most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of

4100-406: The biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at the same time. The best that can be done is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so. The first two points are often the source of contention on

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4200-483: The carbon cycle . A gradual rise of the proportion of the isotope C relative to C in sediments pre-dating "global" glaciation indicates that CO 2 draw-down before snowball Earths was a slow and continuous process. The start of snowball Earths are marked by a sharp downturn in the δC value of sediments, a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans. In January 2016, Gernon et al. proposed

4300-403: The centrifugal governors used in steam engines. He distinguished those that lead to a continued increase in a disturbance or the amplitude of a wave or oscillation, from those that lead to a decrease of the same quality. The terms positive and negative feedback are defined in different ways within different disciplines. The two definitions may be confusing, like when an incentive (reward)

4400-544: The latitude (but not the longitude ) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator, although the accuracy of this reconstruction is in question. This palaeomagnetic location of apparently glacial sediments (such as dropstones ) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at

4500-401: The steam engines of their production. Early steam engines employed a purely reciprocating motion , and were used for pumping water – an application that could tolerate variations in the working speed, but the use of steam engines for other applications called for more precise control of the speed. In 1868 , James Clerk Maxwell wrote a famous paper, "On governors", that is widely considered

4600-450: The 17th century, Nicolas Steno was the first to observe and propose a number of basic principles of historical geology, including three key stratigraphic principles: the law of superposition , the principle of original horizontality , and the principle of lateral continuity . 18th-century geologist James Hutton contributed to an early understanding of the Earth's history by proposing

4700-461: The 18th century, but it was not at that time recognized as a universal abstraction and so did not have a name. The first ever known artificial feedback device was a float valve , for maintaining water at a constant level, invented in 270 BC in Alexandria , Egypt . This device illustrated the principle of feedback: a low water level opens the valve, the rising water then provides feedback into

4800-445: The 1940s onwards was centred around the study of circular causal feedback mechanisms. Over the years there has been some dispute as to the best definition of feedback. According to cybernetician Ashby (1956), mathematicians and theorists interested in the principles of feedback mechanisms prefer the definition of "circularity of action", which keeps the theory simple and consistent. For those with more practical aims, feedback should be

4900-399: The atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium. A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of

5000-476: The atmosphere. As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis. Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth. Additional factors that may have contributed to

5100-513: The authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations. Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles. There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date

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5200-501: The biology of the Proterozoic eon. The major contributions from this work were: (1) the recognition that the presence of banded iron formations is consistent with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a completely ice-covered Earth—specifically, the accumulation of CO 2 from volcanic outgassing leading to an ultra- greenhouse effect . Franklyn Van Houten's discovery of

5300-487: The breakup of Rodinia that exposed many of these flood basalts to warmer, moister conditions closer to the coast and accelerated chemical weathering, is also believed to have caused a major positive shift in carbon isotopic ratios and contributed to the beginning of the Sturtian glaciation. During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of C to C. Close analysis of

5400-722: The closest dated bed to the Port Askaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth. The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's albedo, which would result in positive feedback for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback

5500-522: The cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this ' negative feedback ' process limits the magnitude of cooling. During the Cryogenian, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This caused ice to advance beyond

5600-443: The current state and inputs are used to calculate a new state which is then fed back and clocked back into the device to update it. By using feedback properties, the behavior of a system can be altered to meet the needs of an application; systems can be made stable, responsive or held constant. It is shown that dynamical systems with a feedback experience an adaptation to the edge of chaos . Physical systems present feedback through

5700-419: The deposition of cap carbonates. The thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. An alternative mechanism, which may have produced

5800-491: The difficulty of escaping an all-frozen condition. Several unanswered questions remain, including whether Earth was a full " snowball " or a " slushball " with a thin equatorial band of open (or seasonally open) water. The Snowball Earth episodes are proposed to have occurred before the sudden radiations of multicellular bioforms known as the Avalon and Cambrian explosions ; the most recent Snowball episode may have triggered

5900-450: The end of 1912, researchers using early electronic amplifiers ( audions ) had discovered that deliberately coupling part of the output signal back to the input circuit would boost the amplification (through regeneration ), but would also cause the audion to howl or sing. This action of feeding back of the signal from output to input gave rise to the use of the term "feedback" as a distinct word by 1920. The development of cybernetics from

6000-467: The equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover. Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove three things: This last point is very difficult to prove. Before the Ediacaran ,

6100-604: The evolution of multicellularity. Long before the idea of a global glaciation was first proposed, a series of discoveries occurred that accumulated evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson, who found ancient glacier-reworked material ( tillite ) in Islay , Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding, which came to be known as " Reusch's Moraine ,"

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6200-405: The feedback, combines with the torque exerted by the change of road grade to reduce the error in speed, minimising the changing slope. The terms "positive" and "negative" were first applied to feedback prior to WWII. The idea of positive feedback already existed in the 1920s when the regenerative circuit was made. Friis and Jensen (1924) described this circuit in a set of electronic amplifiers as

6300-430: The general form An example of such a reaction is the weathering of wollastonite : The released calcium cations react with the dissolved bicarbonate in the ocean to form calcium carbonate as a chemically precipitated sedimentary rock. This transfers carbon dioxide, a greenhouse gas, from the air into the geosphere , and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into

6400-429: The geologic time scale. Sedimentology is the study of the formation, transport, deposition, and diagenesis of sediments . Sedimentary rocks , including limestone, sandstone, and shale, serve as a record of Earth's history: they contain fossils and are transformed by geological processes, such as weathering, erosion, and deposition, through deep time. Historical geology makes use of relative dating in order to establish

6500-474: The glacial origin of many of the key occurrences for snowball Earth has been contested. As of 2007, there was only one "very reliable"—still challenged—datum point identifying tropical tillites, which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the Sturtian glaciation is accumulating. Evidence of possible glacial origin of sediment includes: It appears that some deposits formed during

6600-606: The glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes. In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. From this data and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued that an ice age occurred that

6700-442: The groups of molecules expressed and secreted, including molecules that induce diverse cells to cooperate and restore tissue structure and function. This type of feedback is important because it enables coordination of immune responses and recovery from infections and injuries. During cancer, key elements of this feedback fail. This disrupts tissue function and immunity. Mechanisms of feedback were first elucidated in bacteria, where

6800-400: The ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had, in fact, never happened, as his model offered no way to escape from such a feedback loop. In 1971, Aron Faegre, an American physicist, showed that

6900-410: The late 19th century and the development of radiometric dating techniques in the 20th century provided a means of deriving absolute ages of events in geological history. Geology is considered a historical science; accordingly, historical geology plays a prominent role in the field. Historical geology covers much of the same subject matter as physical geology, the study of geological processes and

7000-425: The lithified sediments will remain very slightly, but measurably, depleted in C. Silicate weathering , an inorganic process by which carbon dioxide is drawn out of the atmosphere and deposited in rock, also fractionates carbon. The emplacement of several large igneous provinces shortly before the Cryogenian and the subsequent chemical weathering of the enormous continental flood basalts created by them, aided by

7100-516: The lower to upper layers of Cryogenian BIFs may reflect an increase in ocean acidification, as the upper layers were deposited as more and more oceanic ice cover melted away and more carbon dioxide was dissolved by the ocean. Opponents of the hypothesis suggest that the rarity of the BIF deposits may indicate that they formed in inland seas. Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's Black Sea ;

7200-426: The metabolic pathway (see Allosteric regulation ). The hypothalamic–pituitary–adrenal axis is largely controlled by positive and negative feedback, much of which is still unknown. In psychology , the body receives a stimulus from the environment or internally that causes the release of hormones . Release of hormones then may cause more of those hormones to be released, causing a positive feedback loop. This cycle

7300-437: The movement of lithospheric plates has structured the Earth throughout its geological history. Weathering , erosion , and deposition are examples of gradual geological processes, taking place over large sections of the geologic time scale. In the rock cycle , rocks are continually broken down, transported, and deposited, cycling through three main rock types: sedimentary , metamorphic , and igneous . Paleoclimatology

7400-460: The mutual interactions of its parts. Feedback is also relevant for the regulation of experimental conditions, noise reduction, and signal control. The thermodynamics of feedback-controlled systems has intrigued physicist since the Maxwell's demon , with recent advances on the consequences for entropy reduction and performance increase. In biological systems such as organisms , ecosystems , or

7500-447: The ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice. Near the end of a glaciation period, a reestablishment of gas exchange between the ocean and atmosphere oxidised seawater rich in ferrous iron would occur. A positive shift in δFe IRMM-014 from

7600-468: The ocean would form distinctively textured layers of carbonate sedimentary rock. Such an abiotic "cap carbonate" sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis. However, there are some problems with the designation of a glacial origin to cap carbonates. The high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic and dissolve any carbonates contained within—starkly at odds with

7700-459: The onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with methane in the atmosphere , oxidizing it to carbon dioxide, a much weaker greenhouse gas, and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic. Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation,

7800-470: The other type, many systems with feedback loops cannot be shoehorned into either type, and this is especially true when multiple loops are present. When there are only two parts joined so that each affects the other, the properties of the feedback give important and useful information about the properties of the whole. But when the parts rise to even as few as four, if every one affects the other three, then twenty circuits can be traced through them; and knowing

7900-415: The output of one affecting the input of another, and vice versa. Some systems with feedback can have very complex behaviors such as chaotic behaviors in non-linear systems, while others have much more predictable behaviors, such as those that are used to make and design digital systems. Feedback is used extensively in digital systems. For example, binary counters and similar devices employ feedback where

8000-518: The oxidation of Earth's atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide. The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen -rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit

8100-428: The parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel. An example of this is insulin oscillations . Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, positive and negative do not imply that the feedback causes good or bad effects. A negative feedback loop is one that tends to slow down

8200-437: The polar regions. Once ice advanced to within 30° of the equator, a positive feedback could ensue such that the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered. Polar continents, because of low rates of evaporation , are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from

8300-502: The properties of all the twenty circuits does not give complete information about the system. In general, feedback systems can have many signals fed back and the feedback loop frequently contain mixtures of positive and negative feedback where positive and negative feedback can dominate at different frequencies or different points in the state space of a system. The term bipolar feedback has been coined to refer to biological systems where positive and negative feedback systems can interact,

8400-470: The rate of cooling of Earth's core , it is possible that during the Proterozoic, the magnetic field did not approximate a simple dipolar distribution, with north and south magnetic poles roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Palaeomagnetic data would then have to be re-interpreted, as

8500-598: The same distinction Black used between "positive feed-back" and "negative feed-back", based not on the sign of the feedback itself but rather on its effect on the amplifier's gain. In contrast, Nyquist and Bode, when they built on Black's work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition. Even before these terms were being used, James Clerk Maxwell had described their concept through several kinds of "component motions" associated with

8600-499: The same time as the Gaskiers glaciation . Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building orogeny releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes

8700-486: The sedimentary minerals could have aligned pointing to a "west pole" rather than the north magnetic pole . Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north magnetic pole would occur around

8800-556: The sequence of geological events in relation to each another, without determining their specific numerical ages or ranges. Absolute dating allows geologists to determine a more precise chronology of geological events, based on numerical ages or ranges. Absolute dating includes the use of radiometric dating methods, such as radiocarbon dating , potassium–argon dating , and uranium–lead dating . Luminescence dating , dendrochronology , and amino acid dating are other methods of absolute dating. The theory of plate tectonics explains how

8900-428: The signal feedback from output is in phase with the input signal, the feedback is called positive feedback. Negative feedback: If the signal feedback is out of phase by 180° with respect to the input signal, the feedback is called negative feedback. As an example of negative feedback, the diagram might represent a cruise control system in a car that matches a target speed such as the speed limit. The controlled system

9000-749: The snowball period could only have formed in the presence of an active hydrological cycle . Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition. It is considered possible that ice streams such as seen in Antarctica today could have caused these sequences. Further, sedimentary features that could only form in open water (for example: wave-formed ripples , far-traveled ice-rafted debris and indicators of photosynthetic activity) can be found throughout sediments dating from

9100-508: The snowball-Earth periods. While these may represent " oases " of meltwater on a completely frozen Earth, computer modelling suggests that large areas of the ocean must have remained ice-free, arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models. There are two stable isotopes of carbon in sea water: carbon-12 (C) and the rare carbon-13 (C), which makes up about 1.109 percent of carbon atoms. Biochemical processes, of which photosynthesis

9200-413: The system, closing the valve when the required level is reached. This then reoccurs in a circular fashion as the water level fluctuates. Centrifugal governors were used to regulate the distance and pressure between millstones in windmills since the 17th century. In 1788, James Watt designed his first centrifugal governor following a suggestion from his business partner Matthew Boulton , for use in

9300-468: The theory of uniformitarianism , which is now a basic principle in all branches of geology. Uniformitarianism describes an Earth formed by the same natural phenomena that are at work today, the product of slow and continuous geological changes. The theory can be summarized by the phrase "the present is the key to the past." Hutton also described the concept of deep time. The prevailing conceptualization of Earth history in 18th-century Europe, grounded in

9400-431: The time the sediments were deposited. It is not clear whether this implies a global glaciation or the existence of localized, possibly land-locked, glacial regimes. Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator. Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on

9500-417: The timing of C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic. Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert . In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to

9600-571: The top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary limestone or dolomite metres to tens of metres thick. These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry. These cap carbonates have unusual chemical composition as well as strange sedimentary structures that are often interpreted as large ripples. The formation of such sedimentary rocks could be caused by

9700-455: The trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen equator as cold as modern Antarctica. Global warming associated with large accumulations of carbon dioxide in

9800-414: The ways in which they shape the Earth's structure and composition. Historical geology extends physical geology into the past. Economic geology , the search for and extraction of fuel and raw materials , is heavily dependent on an understanding of the geological history of an area. Environmental geology , which examines the impacts of natural hazards such as earthquakes and volcanism , must rely on

9900-536: Was reported by Hans Reusch in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection (at the time) of continental drift . Douglas Mawson , an Australian geologist and Antarctic explorer, spent much of his career studying the stratigraphy of the Neoproterozoic in South Australia, where he identified thick and extensive glacial sediments. As

10000-421: Was so extreme that it resulted in marine glacial rocks being deposited in the tropics. In the 1960s, Mikhail Budyko , a Soviet climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness ( albedo ) of

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