Keynote lectures/Magnetic field reversals
The magnetic field of the Earth is known to undergo magnetic field reversals.
The observed magnetic profile for the seafloor across the East Pacific Rise tends to agree with a profile calculated from the Earth's known magnetic reversals.
Cooled crustal rock moving outward from the Mid-Atlantic ridge retains a record of the Earth's magnetic field.
“Planets which generate magnetic fields in their interiors ... are surrounded by invisible magnetospheres. ... [I]n many respects, the magnetosphere of Venus is a scaled-down version of Earth’s. ... Earth’s magnetosphere is 10 times larger [than that of Venus]”
Theoretical magnetic field reversals
On the right is an image of a model for the formation of magnetic striping on Earth.
Here's a theoretical definition:
Def. a polarity reversal of the global magnetic field of an astronomical object or body is called a magnetic field reversal.
Charge is usually thought of as a property of matter that is responsible for electrical phenomena, existing in a positive or negative form.
Def. "the quantity of unbalanced positive or negative ions in or on an object; measured in coulombs" is called charge, or electric charge.
In the figure on the right "the 1D parabola tracks the spin excitation (spinon)."
The interactions of charges is fundamental to subluminal physics. The transformation of an electron to a photon and back is the key to electromagnetic propagation.
"The electric-sun hypothesis assigns the solar body the role of anode - that of the higher-potential electrode - in a cosmical electric discharge."
The "Sun is not an electrically isolated body in space, but the most positively charged object in the solar system, the center of a radial electric field."
The solar dynamo is the physical process that generates the Sun's magnetic field. The Sun is permeated by an overall dipole magnetic field, as are many other celestial bodies such as the Earth. The dipole field is produced by a circular electric current flowing deep within the star, following Ampère's law. The current is produced by shear (stretching of material) between different parts of the Sun that rotate at different rates, and the fact that the Sun itself is a very good electrical conductor (and therefore governed by the laws of magnetohydrodynamics).
Regarding "the stability of the dynamical behaviour of axisymmetric α2ω dynamo models in rotating spherical shells as well as spheres [...] the spherical dynamo models are more stable in the following senses:
- [minimize] chaotic behaviour and
- are robust with respect to changes in the functional form of α. [Yet]
- are capable of producing chaotic behaviour for certain ranges of parameter values and
- possess, in the combined "space" of parameters and boundary conditions, regions of complicated behaviours, [...] regimes in which small changes in either the dynamo parameters or the boundary conditions can drastically change the qualitative behaviour of the model."
For an axisymmetric mean field dynamo, the "standard mean field dynamo equation [...] is of the form"
where u is the mean velocity, is the mean magnetic field, t is time. "The quantities α (giving rise to the α effect) and ηt (the turbulent magnetic diffusivity) appear in the process of parameterization of the second order correlations 〈u' x B'〉 between the fluctuations u' and B' by"
A "functional form for α [may be] given by"
where "the exact functional (and in general precise tensorial) forms of α, and in principle also of ηt, are complicated and not well understood in the solar and stellar settings."
The images at the top right of this lecture show the magnetic field lines of the poloidal field Bp and contours of the toroidal field Bt for a solution showing temporal chaos in an axisymmetric spherical shell dynamo.
Secular "rotational stability [may be] in response to loading using the fluid limit of viscoelastic Love number theory. [...] an uncompensated surface mass load [...] of any size would drive true [rotational] polar wander (TPW) that ultimately reorients the load to the equator." The "equilibrium pole position is a function of the lithospheric strength, [with] significantly larger predicted TPW for planets with thin lithospheres. [...] nonaxisymmetric surface mass loads and internal (convective) heterogeneity, even when these are small relative to axisymmetric contributions, can profoundly influence the rotational stability. Indeed, [...] nonaxisymmetric forcing initiates an inertial interchange TPW event (i.e., a 90° pole shift)." A two-step process, depending on the mass loading could place the rotational pole from one end, to the equator, then to the other end.
"Stratospheric sudden warmings (SSWs) are extreme events in the polar stratosphere that are both caused by and have effects on the tropospheric flow. This means that SSWs are associated with changes in the angular momentum of the atmosphere, both before and after their onset. Because these angular momentum changes are transferred to the solid Earth, they can be observed in the rate of the Earth's rotation and the wobble of its rotational pole." An "anomaly in the orientation of the Earth's rotational pole, up to 4 times as large as the annual polar wobble, typically precedes SSWs by 20-40 days. The polar motion signal is due to pressure anomalies that are typically seen before SSW events and represents a new type of observable that may aid in the prediction of SSWs. A decline in the length of day is also seen, on average, near the time of the SSW wind reversal and is found to be due to anomalous easterly winds generated in the tropical troposphere around this time, though the structure and timing of this signal seems to vary widely from event to event."
There are "two new finite rotation poles from visual-fitting, for chron C33 in the Bellingshausen Sea sector."
"The pattern of focal mechanisms and plate motion studies suggest that [the CAPricorn and AUStralian diffuse plate boundary] is made of two disjoint zones, on either side of the CAP/AUS rotation pole."
The "redistribution of magnetic polarities in the inner heliosphere during [a 10.5-month period of maximum solar activity] can be simply described by a gradual 180 degree rotation of the dipole axis from near-alignment with one solar rotational pole to the other."
"Good agreement is seen between the 14C (Fig. 8a) and 10Be (Fig. 8b) records, which confirms they are indeed measuring changes of the GCR flux, since their respective transport processes from the atmosphere to archive are completely different. After its formation, 14C is rapidly oxidised to 14CO2 and then enters the carbon cycle and may reach a tree-ring archive. On the other hand, 10Be attaches to aerosols and eventually settles as rain or snow, where it may become embedded in a stable ice-sheet archive. The correlation between high GCR flux and cold North Atlantic temperatures embraces the Little Ice Age, which is seen not as an isolated phenomenon but rather as the most recent of around ten such events during the Holocene. This suggests that the Sun may spend a substantial fraction of time in a magnetically-quiet state."
"We have postulated complex thermonuclear reactions occurring deep inside the stars as the source of stellar energy and the natural progression of these reactions as the basis of stellar evolution. Yet our first-hand knowledge of stellar structure is limited, consisting largely of surface observations. How certain, then, is our understanding of the processes governing synthesis of the elements deep within fiery stellar cores?"
"The standard stellar theory depicts the main sequence as a relatively simple, steady-state period in a star’s evolution. Thus, any failure of the standard theory to predict the present behavior of the sun could indicate a serious flaw in our stellar physics."
"We believe that there is disturbing and controversial evidence that such flaws may exist. Part of the evidence is provided by the earth’s climatic history, and part by a contemporary experiment that directly monitors the thermonuclear reactions in the solar core. The evidence suggests that variations in the rate of solar energy generation occur, perhaps induced by periodic mixing of the core."
"According to the standard model, the solar luminosity, or the rate at which the sun radiates electromagnetic energy, has remained constant apart from a monotonic increase of 30 per cent over the lifetime of the sun. This increase tracks the rises in the temperature and helium-4 abundance of the solar core as its supply of hydrogen is depleted."
"To the extent that the earth’s geologic and biologic history provides a record of the solar luminosity, we can check the predictions of the standard model. There appear to be a number of inconsistencies. The low initial luminosity predicted by the standard model suggests a primordial climate for the earth quite different from today’s, yet the paleoclimatic record shows no evidence for any significant climatic evolution."
"Sunspot activity has waxed and waned in a regular eleven-year cycle since 1715. In the preceding seventy years, termed the Maunder Minimum, sunspot activity was nearly absent, and, according to European records, persistently cold weather took its toll on crops. Corroborating evidence for a quiescent sun during the Maunder Minimum exists in the reduced brightness and extent of the sun’s corona, a diminished number of auroral displays, and an increased abundance of carbon-14 in the atmosphere."
A "diminished number of auroral displays, and an increased abundance of carbon-14 in the atmosphere. [...] result from a decreased emission of charged particles by the sun."
The "warm Twelfth Century Grand Maximum and the cold Sporer Minimum of 1450 to 1540, also are correlated with periods of increased or decreased solar activity."
The "eleven-year cycle of solar magnetic field reversals, which govern sunspot activity, is thought to be maintained by dynamo action associated with convection and rotation. Thus, the climatic anomalies of the past millenium may indicate merely some lack of detail in standard stellar theory rather than a basic flaw. However, these phenomena do demonstrate that variations in the solar output have terrestrial consequences. A more provocative question then becomes whether there exist some climatic tests of solar behavior over the longer time scales that might characterize possible changes in the solar core, where the basic process of energy generation occurs."
"Evidence of long-term climatic variability is found in the repeated advance and retreat of continental glaciers and in the quasi-periodic occurrence of major glacial epochs. The strong correlation between stages of continental glaciation and the periods (10,000 to 100,000 years) of the earth’s orbital parameters (its eccentricity, obliquity, and precession) suggest that these changes are governed by the earth’s orbital geometry rather than solar phenomena."
"In the last billion years major glacial epochs lasting several million years have occurred regularly, separated by warmer periods lasting several hundred million years. The latest glacial epoch, the Pleistocene, began just three million years ago, and the proximity of its onset indicates that the present is an atypical time in the earth’s climatic history."
"The duration of the glacial epochs is comparable to the thermal diffusion time of the solar core. Their spacing corresponds to a fundamental hydrogen-burning scale, the time required for the ratio of helium-3 to hydrogen [...] to reach equilibrium over an appreciable fraction of the solar core. These observations have stimulated development of a number of nonstandard models in which variations in the solar output are coupled to these thermal and nuclear time scales."
"The duration and spacing of the transient mixing stages nicely match those of the glacial epochs. It is also widely believed that reduction of the sun’s luminosity by 5 per cent would induce major climatic changes and that periodic mixing, by softening the long-term luminosity increase, would yield a primordial value more acceptable than that of the standard model."
Extinct "Martian rivers indicate an ice-age climate for Mars coincident with the earth’s Pleistocene epoch, which further suggests the existence of extraterrestrial controls."
The "suggested mode of solar variability leaves unexplained other glacial phenomena, such as the steady cooling of the oceans in the ten million years preceding the Pleistocene epoch."
Because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind.
"Venus and the Earth have similar radii and estimated bulk compositions, and both possess an iron core that is at least partially liquid. However, despite these similarities, Venus lacks an appreciable dipolar magnetic field."
This "absence is due to Venus’s also lacking plate tectonics for the past 0.5 b.y. (1 b.y.=109 yr). The generation of a global magnetic field requires core convection, which in turn requires extraction of heat from the core into the overlying mantle. Plate tectonics cools the Earth’s mantle; on the basis of elastic thickness estimates and convection models, [...] the mantle temperature on Venus is currently increasing. This heating will reduce the heat flux out of the core to zero over ~1 b.y., halting core convection and magnetic field generation. If plate tectonics was operating on Venus prior to ca. 0.5 Ga, a magnetic field may also have existed. On Earth, the geodynamo may be a consequence of plate tectonics; this connection between near-surface processes and core magnetism may also be relevant to the generation of magnetic fields on Mars, Mercury and Ganymede."
The lack of an appreciable Earth-like dipolar magnetic field "cannot be explained by the planet's slow rotation".
In "the absence of plate tectonics, the mantle on Venus cannot cool rapidly enough to drive core convection and a geodynamo."
"Planetary magnetic fields are produced by motion in a conductor, usually the planet’s iron core. Such motion may be due to either thermal convection or compositional convection, driven by core solidification".
"The maximum heat flux that can be extracted from the core without thermal convection is given by"
"where k and α are the thermal conductivity and expansivity, g is the acceleration due to gravity, T is the core temperature, and Cp is the specific hear capacity. [...] Fc is in the range 11-30 mW·m-2. Thermal convection will cease if the heat being extracted from the core is less than Fc; in the absence of core solidification, the geodynamo will halt. Compositional convection may continue [...], but will certainly halt if the heat flux out of the core drops to zero or below (i.e., the core starts heating up). The rate at which the core loses heat is controlled by the temperature difference between core and mantle and, thus, on the rate at which the mantle is cooling".
Any "field reversal [may be] linked to biological extinction7–12 [...] the reversal record of the past 165 Myr [shows that a] stationary periodicity of 30 Myr emerges (superimposed on the non-stationarities already established by others5), which predicts pulses of increased reversal activity centred at 10, 40, 70,… Myr BP."
A "recently observed 15 Myr periodicity is probably a harmonic of the 29.5-30.5 Myr period. The calculations do not confirm an inherent magnetic reversal property of the earth. The reversals may arise from tectonic events or from impacts from extraterrestrial objects."
"The precession peaks found in the δ18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate."
"The Earth's geomagnetic field reverses its polarity at irregular time intervals. [It] is not clear whether a reversal is a deterministic (low dimensional) or a random (high-dimensional) process; the duration-frequency distribution of the polarity time intervals resembles those generated by random processes, but many models suggest that a geomagnetic field reversal can be the outcome of a deterministic dynamics, that of the convection in the Earth's outer core. [The] limited size of the magnetic reversal data (282 points) and the poor convergence of the correlation integrals make a quantitative assessment of low-dimensional chaos impossible."
"Earth's magnetic field is generated by fluid motion in the liquid iron core. Details of how this occurs are now emerging from numerical simulations that achieve a self-sustaining magnetic field. Early results predict a dominant dipole field outside the core, and some models even reproduce magnetic reversals."
"Regeneration of the Earth's magnetic field by convection in the liquid core produces a broad spectrum of time variation. [...] the amplitude of convective fluctuations in the core [is predictable], and establish a physical connection to the rates of magnetic reversals and excursions."
The graph in the center shows a comparison of the observed magnetic profile for the seafloor across the East Pacific Rise against a profile calculated from the Earth's known magnetic reversals, assuming a constant rate of spreading.
"Heinrich Layers [HLs] are found in the North Atlantic Ocean as well-constrained markers of catastrophic iceberg surges from the Pan-Atlantic ice sheets during the last glacial cycle. Their physical and geochemical characteristics [...] are predominantly due to the source sediments of the ice-rafted debris (IRD) on the one hand (magnetic susceptibility, color, carbonate content) and the response of the palaeo-environment on the other hand (carbonate content, foraminiferal assemblage)."
"Sediment cores in the Porcupine Seabight (West off Ireland) have shown the presence of Heinrich Events [Hs] without the diagnostic changes in magnetic susceptibility (MS) [...] the concentration of ice-rafted debris (commonly referred to as the fraction > 150 μm) increases towards the culmination of HL2, marked by an increase in MS, [X-ray fluorescence] XRF Ca and the percentage of N. pachyderma s."
The "zone where the density increases is marked by a cloud of fine and highly dense particles surrounding the IRD. [The] fine clayey “background” matrix throughout the core [consists] of zoned dolomites. [...] the mineralogical analyses [suggest] a predominant volcanic source for the magnetic susceptibility. [Both] XRF Fe and Ti show significant decreases near the HL culmination".
Chronology of climatic events on the upper right of importance for the Last Glacial Period (~last 120,000 years) is recorded in polar ice cores, and approximate relative position of Heinrich events, initially recorded in marine sediment cores from the North Atlantic Ocean. Light violet line: δ18O from the NGRIP ice core (Greenland), permil (NGRIP members, 2004). Orange dots: temperature reconstruction for the NGRIP drilling site (Kindler et al., 2014). Dark violet line: δ18O from the EDML ice core (Antarctica), permil (EPICA community members, 2006). Grey areas: major Heinrich events of mostly Laurentide origine (H1, H2, H4, H5). Grey hatch: major Heinrich events of mostly European origine (H3, H6). Light grey hatch and numbers C-14 to C-25: minor IRD layers registered in North Atlantic marine sediment cores (Chapman et al., 1999). HS-1 to HS-10: Heinrich Stadial (HS, Heinrich, 1988; Rasmussen et al., 2003; Rashid et al., 2003). GS-2 to GS-24: Greenland Stadial (GS, Rasmussen et al., 2014). AIM-1 to AIM-24: Antarctic Isotope Maximum (AIM, EPICA community members, 2006). Antarctica and Greenland ice core records are shown on their common timescale AICC2012 (Bazin et al., 2013; Veres et al., 2013).
The image second down on the right suggests that lithic proportion of sediments deposited during H3 and H6 is substantially below that of other Heinrich events.
The image on the left shows the ratio of calcium versus strontium in a North Atlantic drill core (blue; Hodell et al., 2008) compared to petrologic counts of "detrital carbonate" (Bond et al., 1999; Obrochta et al., 2012; Obrochta et al., 2014), the mineralogically-distinctive component of Hudson Strait-derived IRD. Shading indicates glaciations ("ice ages").
|Hemming (2004)||Bond & Lotti (1995)||Vidal et al. (1999)|
|H1,2 are dated by radiocarbon; H3-6 by correlation to GISP2.|
In the section above the most recent geomagnetic polarity reversal occurred 780,000 b2k.
"Paleomagnetic samples were obtained from cores taken during the drilling of a research well along Coyote Creek in San Jose, California, in order to use the geomagnetic field behavior recorded in those samples to provide age constraints for the sediment encountered. The well reached a depth of 308 meters and material apparently was deposited largely (entirely?) during the Brunhes Normal Polarity Chron, which lasted from 780 ka to the present time."
"Three episodes of anomalous magnetic inclinations were recorded in parts of the sedimentary sequence; the uppermost two we correlate to the Mono Lake (~30 ka) geomagnetic excursion and 6 cm lower, tentatively to the Laschamp (~45 ka) excursion."
"Some 41,000 years ago, a complete and rapid reversal of the geomagnetic field occured. Magnetic studies on sediment cores from the Black Sea show that during this period, during the last ice age, a compass at the Black Sea would have pointed to the south instead of north."
"[A]dditional data from other studies in the North Atlantic, the South Pacific and Hawaii, prove that this polarity reversal was a global event."
"The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today's configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today's field."
"The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast."
"During this period, the field was even weaker, with only 5% of today's field strength. As a consequence, Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure."
"This is documented by peaks of radioactive beryllium (10Be) in ice cores from this time, recovered from the Greenland ice sheet. 10Be as well as radioactive carbon (14C) is caused by the collision of high-energy protons from space with atoms of the atmosphere."
"The polarity reversal [...] has already been known for 45 years. It was first discovered after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central, which differed significantly from today's direction of the geomagnetic field. Since then, this geomagnetic feature is known as the 'Laschamp event'."
The "new data from the Black Sea give a complete image of geomagnetic field variability at a high temporal resolution."
"Right now, the magnetic pole, which happens to be close to the geographical North Pole, is shifting some 50 km a year from Canada in the direction of Russia. And the magnetic field has significantly decreased in some places."
"The biggest of these weak spots lies in a large area reaching from South Africa to South America: over the South Atlantic, airline passengers are subjected to radiation 1,000 times higher than on other air routes at flying altitude. At these latitudes, the crew of the International Space Station receives 90 percent of it‘s radiation dosis, despite the fact that it is staying there only about ten minutes every day."
The "floors of thousand years old clay huts in Africa [...] are keeping an imprint of the magnetic field as it was in prior centuries – thanks to a local custom: the inhabitants of the banks of Limpopo-River, at the border of South Africa with Zimbabwe and Botswana, used to ritually burn down their huts when there was a drought."
"The heat of the fire, [reached above 1,000° C], transformed the clay: when the fire died out and the soil cooled down, magnetic minerals contained in the clay shifted position to align themselves with the magnetic field such is as it was reigning at the time. Moreover, the presence of particles with a measurable rate of radioactive decay make it possible to determine accurately the age of these huts."
The record points "to a weak, old magnetic field: between 1000 and 1500 [AD, 1,000 to 500 b2k], the [Earth's] magnetic field had decreased in that area by ca 30 per cent."
The "African discovery is actually good news. It shows that the [Earth‘s] magnetic field undergoes bouts of weakness more often than had been thought, and the present slowing down doesn’t seem to be particularly significant."
The "field recovered temporarily after a weak phase in the Middle-Ages, before it went into a crisis again 180 years ago."
"The clay floors of the African huts reveal this magnetic field weakness of the Middle-Ages thanks to their magnetic particles: they froze into place, still pointing North, yet their angle, called inclination, varies in unusual ways."
"Under South Africa, the South Atlantic and South America, at the edge of the [Earth‘s] core at a depth of some 3,000 kilometers, pictures reveal a zone of low speed – there, the rocks must be made up in a special way. A relation with the weak geomagnetic field comes to mind: probably that the flow of liquid iron which produces the magnetic field is changing it's course at this spot."
"Roughly, the fluctuations of the geomagnetic field can be followed back for millions of years. The Earth’s solidified lava rocks too contain magnetisation. The examination of such rocks shows that the Earth‘ magnetic field reverses its poles on average every few hundreds of thousands of years."
"More than 2,500 years ago in the ancient Near East, the Earth's geomagnetic field was going gangbusters."
"During the late eighth century B.C., a new study finds, the magnetic field that surrounds the planet was temporarily 2.5 times stronger than it is today."
"About 3,000 years ago, a potter near Jerusalem made a big jar. It was meant to hold olive oil or wine or something else valuable enough to send to the king as a tax payment. The jar's handles were stamped with a royal seal, and the pot went into the kiln."
"Over the next 600 years, despite wars destructive enough to raze cities, potters in the area kept making ceramic tax jars, each one stamped with whatever seal represented the ruler du jour."
"Albert Einstein defined this problem as one of the five most enigmatic issues in modern physics, and it still is, because the mechanism that creates the magnetic field is not well understood."
"All those years ago, as potters continued to throw clay, the molten iron that was rotating deep below them tugged at tiny bits of magnetic minerals embedded in the potters' clay. As the jars were heated in the kiln and then subsequently cooled, those minerals swiveled and froze into place like tiny compasses, responding to the direction and strength of the Earth's magnetic field at that very moment."
The "jars indicate that in the late 8th century B.C., the core went a little crazy. The intensity of the magnetic field spiked to about double what it is today."
"It was the strongest it's been, at least in the last 100,000 years, but maybe ever. We call this phenomenon the Iron Age spike."
"Then, it weakened quickly after 732 B.C.E., losing about 30 percent of its intensity in just 30 years."
The "Earth could undergo big changes in magnetic intensity — the poles are thought to reverse about every 200,000 to 300,000 years. But in between those times, people assumed there wasn't much going on."
An entity of magnetic field reversals is Bernard Brunhes who discoverered the Earth's magnetic field reversals.
"He was the first to highlight in 1905 the phenomenon of inversion of the magnetic field of the Earth, observing volcanic rocks (lava flows) in the Massif Central which preserve the memory of the magnetic field direction dating from the time of these flows (Pontfarein casting, near Saint-Flour, Cantal)."
Def. "a geologic era within the Proterozoic eon; comprises the Tonian, Cryogenian and Ediacaran periods from about 1000 to 544 million years ago, when algae and sponges flourished" is called the Neoproterozoic.
"In the central Flinders Ranges the 4.5 km thick Umberatana Group encompasses the two main phases of glacial deposition (see Thomas et al., 2012). The carbonaceous, calcareous and pyritic Tindelpina Shale Member, of the interglacial Tapley Hill Formation, caps the Fe-rich diamictite and tillite formations of the Sturt glaciation. The upper Cryogenian glacials of the Elatina Formation are truncated by the Nuccaleena Formation at the base of the Wilpena Group and the Ediacaran System."
At "the end of a period called Ediacara, in which there already existed forms of simple multicellular life resembling jelly-fish, the magnetic field of Earth reversed itself several times in a short lapse of time. [These] reversals [may have] occurred at a rate 20 [times] faster on average than in the past million years."
“Earth’s magnetic field underwent a period of hyperactive reversals. One can deduce from this that the magnetic field of Earth must have been weaker on average during several episodes over a period of several million years."
The "cosmic ray bombardment occurring then would have been sufficient to significantly damage the ozone layer, reducing it by some 40 % on average all over the planet. And less ozone means decreased protection against ultraviolet rays for species living at the surface of Earth, on land and in the oceans. Curiously, these febrile reversals coincide with [...] the crisis of the Kotlinian, which decimated the fauna of Ediacara wholesale, right before the Cambrian Explosion."
"The Kotlinian Crisis, as it is known, saw widespread extinction and put an end to the Ediacaran Period. During this time, large (up to meter-sized) soft-bodied organisms [such as in the image on the left], often shaped like discs or fronds, had lived on or in shallow horizontal burrows beneath thick mats of bacteria which, unlike today, coated the sea floor. The slimy mats acted as a barrier between the water above and the sediments below, preventing oxygen from reaching under the sea floor and making it largely uninhabitable."
"The Ediacaran gave way to the Cambrian explosion, 542 million years ago: the rapid emergence of new species with complex body plans, hard parts for defense, and sophisticated eyes. Burrowing also became more common and varied, which broke down the once-widespread bacterial mats, allowing oxygen into the sea floor to form a newly hospitable space for living."
"Organisms with the ability to escape UV radiation would be favored in such an environment. This flight from dangerous levels of UV light might explain many of the evolutionary changes that occurred during the Late Ediacaran and Early Cambrian."
"Creatures with complex eyes to sense the light and the ability to seek shelter from the radiation—for example, by migrating into deeper waters during the daytime—would have been more successful. The growth of hard coatings and shells would afford additional UV protection, as would the capacity to burrow deeper into the sea floor."
"In turn, these changes may have opened up new environments. The development of shells, for example, helps creatures colonize intertidal areas, protected not only from UV rays but also stronger waves and the risk of drying out."
"In 2004, the Global Stratotype Section and Point (GSSP) for the terminal Proterozoic was placed near the base of the Nuccaleena Formation in Enorama Creek in the central Flinders Ranges [in the image on the right], thus establishing the Ediacaran System and Period (Knoll et al., 2006). As the Nuccaleena Formation has not been accurately dated, a date of c. 635 Ma from near-correlative levels in Namibia and China is presumed for the base of the Ediacaran (Hoffmann et al., 2004; Condon et al., 2005; Zhang et al., 2005)."
- Magnetic field reversals affect the local magnetic fields within crystallizing liquids.
- Magnetic field reversals may cause the ionosphere to contact the surface of the Earth during the reversal period.
- Magnetic field reversals may allow increased radiation to irradiate the upper crustal rocks to significant depths during the reversal period.
- Subject to the origin of magnetic field reversals, they may cause significant increases in volcanic activity during the reversal period.
- Magnetic field reversals of the Sun occur with the sunspot cycle which may have its origins in enhanced electron current from Jupiter and Venus when perihelion is coincident.
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