The text and charts on this page come from a special feature on Mass Extinction Cycles in the January 7, 2009 issue of the Unified Cycle Theory Newsletter:

After a research team led by Luis Alvarez published their paper Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, interest in the causes of extinctions increased rather dramatically in the scientific community.[Alvarez et al., 1980]  The Alvarez team theorized that the great dinosaur extinction 65 million years ago resulted from an asteroid impact in the Yucatan Peninsula.

The 1984 publication of Periodicity of Extinctions in the Geologic Past broadened the interest by introducing the idea that extinctions occurred at periodic intervals of 26-to-30 million years.[Raup & Sepkoski, 1984]

After those introductory papers, proposed causes of extinctions expanded to include the following:
            Solar system movements through the Milky Way's galactic plane.
            Solar system periodic movement through the Milky Way's spiral arms.
            Periodic episodes of volcanic eruptions.
            Continental drift mechanics.
            Gamma ray bursts from supernova explosions.
            Hydrogen sulfide emissions from the oceans.
            Methane gas emissions.
            Global warming and cooling cycles.
            Sea level changes.
            Asteroid impacts.

Writing for NASA in an article entitled The Great Dying, Patrick Barry describes the greatest extinction of the last 550 million years.  Often called the Permian-Triassic extinction, 9 in 10 marine species and 7 in 10 land species vanished around 251 Ma.[Barry, 2002]  Life on Earth almost came to an end.  Barry describes the NASA-funded research led by geologist Luann Becker from the University of California, Santa Barbara:

"Scientists have suggested many possible causes….  Undaunted, Becker led a NASA-funded science team to sites in Hungary, Japan and China where such rocks still exist and have been exposed. There they found telltale signs of a collision between our planet and an asteroid 6 to 12 km across -- in other words, as big or bigger than Mt. Everest….  Deep inside Permian-Triassic rocks, Becker's team found soccer ball-shaped molecules called 'fullerenes' with traces of helium and argon gas trapped inside.  The fullerenes held an unusual number of 3He and 36Ar atoms -- isotopes that are more common in space than on Earth.  Something, like a comet or an asteroid, must have brought the fullerenes to our planet.  Becker's team had previously found such gas-bearing buckyballs in rock layers associated with two known impact events: the 65 million-year-old Cretaceous-Tertiary impact and the 1.8 billion-year-old Sudbury impact crater in Ontario, Canada.  They also found fullerenes containing similar gases in some meteorites.  Taken together, these clues make a compelling case that a space rock struck the Earth at the time of the Great Dying.  But was an asteroid the killer, or merely an accomplice?  Many scientists believe that life was already struggling when the putative space rock arrived.  Our planet was in the throes of severe volcanism.  In a region that is now called Siberia, 1.5 million cubic kilometers of lava flowed from an awesome fissure in the crust.  World geography was also changing then.  Plate tectonics pushed the continents together to form the super-continent Pangea and the super-ocean Panthalassa. Weather patterns and ocean currents shifted.  Many coastlines and their shallow marine ecosystems vanished.  Sea levels dropped.  'If life suddenly has all these different things happen to it,' Becker says, 'and then you slam it with a rock the size of Mt. Everest -- boy!  That's just really bad luck.'"[Barry, 2002]

Since the pioneering work of Luis Alvarez, David Raup, and John Sepkoski over 25 years ago, a consensus quickly emerged that sudden environmental changes caused extinctions.  However, with a large number of seemingly independent events all happening at the same time, pinpointing the precise cause of mass-extinctions proved difficult.  But the Unified Cycle Theory offers a solution.  All of these seemingly unrelated environmental influences, in fact, aren't coincidences at all.  The Permian-Triassic extinction didn't result from incredibly bad luck, as Becker suggested.  Instead, it resulted from a dramatic cyclical reversal of a 2.46-gyr EUWS cycle that theoretically peaked 267.9 million years ago.[Puetz, 2009]

And the 2.46-gyr cycle exerted its influence in a way similar to the other EUWS cycles - the greater its frequency, the greater its impact on Earth's environment.  The EUWS cycles certainly originate from outside our planet.  That's because, in additional to influencing volcanic activity and global temperatures, these mysterious waves modulate cycles in asteroid impacts.  In the process, the 2.46-gyr cycle created the illusion observed by Becker - that Earth encountered incredibly bad luck at the time of the Permian-Triassic extinction.  By creating multiple correlations from their influence, the EUWS cycles tempted a great majority of scientists into erroneous cause-and-effect mistakes.  A case in point follows.

Shanan Peters, a geologist from the University of Wisconsin-Madison, recently focused on sea level changes in a paper entitled Environmental Determinants of Extinction Selectivity in the Fossil Record.  Peters believes that changes in shelf habitats, related to rising and falling sea levels, plays a primary role in extinction cycles of marine species.  Peters wrote:

"These results do not preclude a role for biological interactions or unusual physical events as drivers of macroevolution, but they do suggest that the turnover of marine shelf habitats and correlated environmental changes have been consistent determinants of extinction, extinction selectivity, and the shifting composition of the marine biota during the Phanerozoic eon."[Peters, 2008]

Nonetheless, claiming that sea level changes cause extinction cycles seems far-fetched.  Peters' argument that ocean levels correlate with extinction cycles is believable - and certainly true.  However, correlation and cause aren't synonymous.  The immediate question that comes to mind is…. what caused sea levels to fluctuate?

For the past few decades, climatologists have measured delta Oxygen-18 (‰) in glacial ice-cores and ocean sediments as an indirect way of measuring global temperature oscillations.  Based on this long history, it's clear that global temperature cycles modulate glacial volume on land and ice in the polar-regions.  And as glacial volumes oscillate, sea levels rise and fall.  Hence, rather than claiming that sea level fluctuations cause marine extinctions, it seems more accurate to state that global climate regulates extinction cycles.  For example, ocean temperatures correlate with atmospheric temperatures, with a slight lag.[Puetz, 2009]  At the end of this chain, ocean temperature changes could play a more critical role than ocean habitats in triggering extinctions.

Of course, by shifting focus to global climate, a new question emerges…. what caused climate oscillations?  In fact, going back to the preceding list (and as Barry already noted), most of the potential causes for mass-extinctions occurred simultaneously around 250 Ma.  To fully understand extinction cycles, a better model must be developed explaining why periodic volcanic eruptions, continental drift mechanics, hydrogen sulfide emissions, methane gas emissions, global climate cycles, sea level changes, and asteroid impacts all correlate with each other.  Attempting to associate extinctions to any individual factor seems dangerous considering that they all correlate with extinctions to varying degrees.[Puetz, 2009]

In the past, scientists involved with extinction cycle research have liberally applied cause-and-effect associations.  It's easy to agree that these researchers have found important correlations.  However, it's more difficult agreeing with their cause-and-effect conclusions.  From this point forward, concentrate more on the correlations (rather than the conclusions) as we delve further into extinction cycles.

Adding to the potential causes, geologist Yukio Isozaki from the University of Tokyo introduces geomagnetism as a factor in a publication entitled Plume Winter Scenario for Biosphere Catastrophe: The Permo-Triassic Boundary Case.  Isozaki wrote:

"The so-called end-Paleozoic mass extinction was double-phased; one at the Guadalupian-Lopingian boundary (G-LB, ca. 260 Ma) and the other at the Changhsingian/Induan boundary (P-TB sensu stricto, ca. 252 Ma). …  The end-Paleozoic global environmental change was likely triggered by the episodic activity of a mantle plume, not through basaltic but rhyo-dacitic volcanism.  The felsic nature of the volcanism suggests not only highly explosive eruption but also extensive delivery of air-borne ash.  Alkaline magmatism, delivered directly from a mantle plume, may have played an important role in the G-LB and P-TB extinction-relevant environmental turmoils.  The 'Plume Winter' scenario is proposed here to explain the unique geological phenomena of the end-Paleozoic event.  When a plume head hits the bottom of a pre-existing continental lithosphere, alkaline volcanism of felsic to intermediate composition, often accompanying kimberlite/carbonatite, occurs in a highly explosive manner prior to a voluminous CFB eruption in a later stage.  The violent volcanism directly causes 1) extensive ash fall, 2) formation of a dust/aerosol-screen in the stratosphere, and 3) acid rain, and these are followed by severe destruction of the photosynthesis-supported global food web both on land and in the sea through 4) less insolation and 5) low temperature."[Isozaki, 2007]

This brief excerpt from Isozaki's abstract reveals three important items that help clarify this complicated issue:
          1) While most researchers concentrate on the area around 251 Ma for the Permian-Triassic extinction, Isozaki places its origin sometime around 260 Ma.
          2) The alkaline volcanism of a superplume indicates that geomagnetism may be involved in extinction cycles.
          3) Rather than simple cause-and-effect scenarios, Isozaki shows that mass extinctions most likely result from a chain reaction of events - maybe even going three or four levels deep in the chain.

In an article entitled Did Magnetic Blip Trigger Mass Extinction, science-writer Michael Reilly provides further details about Isozaki's hypothesis:

"By the time the dust had settled on the Permian-Triassic mass extinction 250 million years ago, 90 percent of life on the planet had been snuffed out.  Now a new theory suggests the catastrophe was set in motion 15 million years earlier, deep in the Earth.  On the edge of the molten outer core, a plume of super-hot material began rising through the mantle, upsetting convection in the core and throwing the planet's magnetic field into disarray.  The weakening of Earth's magnetic field exposed the surface to a shower of cosmic radiation, says Yukio Isozaki of the University of Tokyo.  He believes the radiation broke nitrogen in the atmosphere into ions that acted as seeds for clouds enshrouding the planet….  The superplume disrupted the magnetic field and put a strain on creatures living on the surface, but it was only the beginning.  Five million years later it reached the surface, Isozaki said, and the hot material punched through the crust, erupting as three successive super volcanoes….  Today the remnants of those volcanoes are scattered through India, China and Norway. On their own they were too small to do much harm, but together Isozaki thinks they cooled the climate even further, launching an extinction as bad as the one that would kill the dinosaurs 185 million years later.  Then, 10 million years later, the Permian-Triassic extinction struck….  Isozaki thinks both 'punches' were caused by the same superplume.  Ten million years after the smaller volcanoes blew their tops, a much larger volcano, the Siberian Traps, erupted, launching the worst killing in the planet's history."[Reilly, 2008]

Up to this point, attention centered exclusively on the Permian-Triassic extinction.  While it wrought the greatest destruction during the past 550 million years, the 251 Ma extinction wasn't alone.  Since the Cambrian Explosion, numerous other mass-extinctions caused enough death to be classified as major events.  Hermann Pfefferkorn, a geologist from the University of Pennsylvania, identified five major extinction events during the Phanerozoic Eon.  The age at the beginning of each line corresponds to a red box in Chart MOD6 - indicating when the extinction occurred.
 

443 Ma - Near the Ordovician-Silurian boundary.
364 Ma - Within the Late Devonian.
251 Ma - At the Permian-Triassic boundary.
206 Ma - Near the Triassic-Jurassic boundary.
65 Ma - At the Cretaceous-Tertiary boundary.[Pfefferkorn, 1999]

Importantly, Pfefferkorn provides this observation about the stability of Earth's environment:

"The mass extinction at the Permian-Triassic boundary can be described as a major environmental disturbance.  As such it can be seen as an extreme point in a continuum of disturbances that … occur at nearly any scale of space and frequency.  It has been recognized by ecologists for some time that most environments, even if they appear to be very stable, consist of a patchwork of former disturbances."[Pfefferkorn, 1999]

In addition to the major events, the time-table of other minor mass-extinctions follows.  The numbers between 1 and 13 correspond to the black numbers in Chart MOD6 - indicating when these extinctions occurred.
          [1]   542 Ma - End-Ediacarian extinction. [Amthor, 2003]
          [2]   517 Ma - End Botomian.[Marusek, 2005]
          [3]   495 Ma - Dresbachian. [Wignall & Hallam, 1997]
          [4]   428 Ma - Ireviken Event.[Munnecke et al., 2003]
          [5]   423 Ma - Mulde Event.[Jeppsson & Calner, 2007]
          [6]   420 Ma - Lau Event.[Jeppsson & Aldridge, 2000]
          [7]   416 Ma - End Silurian.[Macquarie, 2008]
          [8]   299 Ma - Late Carboniferous.[Pfefferkorn, 1999]
          [9]   260 Ma - End Middle Permian.[Taylor, 2004]
          [10]  183 Ma - Toarcian Extinction.[Wignall & Hallam, 1997]
          [11]  145 Ma - End Jurassic.[Raup & Sepkoski, 1984]
          [12]  117 Ma - Aptian Extinction.[Archangelsky, 2001]
          [13]   34 Ma - Eocene-Oligocene Extinction.[Retallack et al., 2004]

Chart MOD6 presents a theoretical model of all EUWS cycles with frequencies between 30.4-myr and 7.39-gyr.  The vertical grid-lines indicate theoretical peaks of the 30.4-myr EUWS cycle.  Theoretical turning points for larger EUWS cycles are marked with arrows - with one exception.  No arrows were placed above the 268 Ma grid-line - which corresponded to a 2.46-gyr theoretical peak.  In the lower-left portion of the chart, the triple green arrows represent a 821-myr theoretical low - corresponding to the last Snowball Earth episode around 670 Ma.[Kasting and Howard, 2006]  The double-arrows represent 274-myr theoretical peaks and troughs, while single-arrows indicate 91.3-myr theoretical turning points.

Once again, Chart MOD6 shows that fact matches predictions made by the Unified Cycle Theory rather closely.  During the last billion years, the two greatest global temperature extremes matched almost identically to major EUWS points.  The coldest period came near the 821-myr theoretical low at 670 Ma - coinciding with a Snowball Earth episode.  As it should be expected, with most of Earth covered in ice, few life-forms survived the Snowball Earth episode.  At that time, the diversity of life reached a minimum.  But as Earth warmed, opportunities for life opened.  Life forms multiplied as the Cambrian warming took hold.  Then, as the 2.46-gyr theoretical peak approached at 268 Ma, global temperatures ascended to their highest level in the past billion years.[Frakes et al., 1992]  Then, immediately after the 268 Ma theoretical peak, temperatures started to decline, and the greatest mass extinction event of the past billion years struck.

The 2.46-gyr and 821-myr cycles dominate environmental conditions on Earth; however, their sub-cycle offsprings also exert considerable influence on climate and extinctions.  In fact, to put these cycles into their proper perspective, they should be viewed as evolution-extinction cycles.  During their up-phases, they promote evolution.  During their down-phases, they encourage extinctions.

This can be seen easily with the 274-myr cycle in Chart MOD6.  Theoretical 274-myr peaks occurred at 542 Ma (red double- arrow), at 268 Ma, and the next one will arrive in six million years (red double-arrow at 6-AP).  Notice how the bulk of the mass-extinctions occurred during spans of theoretical peak-to-trough for the 274-myr cycles.  That is, extinctions were concentrated in the 137 million year spans following 542 Ma and at 268 Ma.

Chart MOD6 also shows the influence of the 91.3-myr sub-cycle.  Four of the five "major mass-extinctions" occurred very close to theoretical peaks of the 91.3-myr EUWS cycle.  (The major extinctions appear in the chart as the red numbers 443, 364, 251, 206, and 65.)  The only exception came at the time of the Triassic-Jurassic extinction at 206 Ma.  And in that particular case, the model projected equivalent peaks for the 30.4-myr peak at 207 Ma and the 91.3-my peak at 177 Ma.  Hence, a major extinction coming at 206 Ma should not be viewed as extremely unusual in this particular case.

Finally, the 30.4-myr sub-cycle reveals itself through the spacing of the extinctions near the 30.4-myr grid-lines in Chart MOD6.  The 30.4-myr EUWS cycle acts as the core modulator of the mass extinction cycle - with concentration and intensity ratcheted higher at 91.3-myr, 274-myr, 821-myr, and 2.46-gyr intervals.  The 30.4-myr cycle is the same frequency that Raup & Sepkoski statistically identified in 1984, and then discarded.  Amazingly, even though a subsequent standard autocorrelation analysis confirmed its significance, the pair rejected the 30-myr cycle in favor of a 26-myr cycle.  In rejecting the 30-myr cycle, they wrote:

"The correlogram showed statistically significant autocorrelation (P < 0.01) for cycles between 27 and 35 myr.  However, we do not consider this conclusive because autocorrelation gives undue weight to the long intervals of background extinction between peaks."[Raup & Sepkoski, 1984]

In spite of their mistake on its frequency, when it comes to insight, these pioneers understood extinction cycles far better than most others since.  In their groundbreaking publication, Periodicity of Extinctions in the Geologic Past, the pair realized the difference between Earth's environmental symptoms and the true origin of the extinction cycles, as the following passage indicates:

"If periodicity of extinctions in the geologic past can be demonstrated, the implications are broad and fundamental.  A first question is whether we are seeing the effects of a purely biological phenomenon or whether periodic extinction results from recurrent events or cycles in the physical environment.  If the forcing agent is in the physical environment, does this reflect an earthbound process or something in space?  If the latter, are the extraterrestrial influences solar, solar system, or galactic?  Although none of these alternatives can be ruled out now, we favor extraterrestrial causes for the reason that purely biological or earthbound physical cycles seem incredible, where the cycles are of fixed length and measured on a time scale of tens of millions of years.  By contrast, astronomical and astrophysical cycles of this order are plausible even though candidates for the particular cycle observed in the extinction data are few."[Raup & Sepkoski, 1984]

Raup & Sepkoski correctly identified two important characteristics of the extinction cycle.  (a) It propagates from an extraterrestrial origin (the EUWS cycles), and (b) the 30.4-myr cycle acts as its primary modulator.

Recently, in an abstract entitled Cycles in Fossil Diversity, Robert Rohde and Richard Muller found a 62±3-myr cycle to be statistically significant.[Rohde and Muller, 2005]  Interestingly, this 62-myr cycles has gained a popular following as the regulator of the extinction cycle - almost obscuring recognition of the 30-myr cycle.

While the 62-myr extinction cycle is certainly valid, it merely functions as a secondary harmonic to the primary 30.4-myr cycle.  The explanations below detail the secondary nature of the 62-myr cycle.

Alvarez, L.W.; Alvarez, W.; Asaro, F.; Michel, H.V., [1980].  Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.  Science, 208 (June), 1095-1108.

Amthor, J.E. [2003]. Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian Boundary in Oman.  Geology 31: 431-434. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2.

Archangelsky, S., [2001].  The Tico Flora (Patagonia) and the Aptian Extinction Event. Acta Paleobotanica 41(2), 2001, pp. 115-22.

Barry, P.L. [2002].  The Great Dying.  Science@NASA, sponsored by Science and Technology Directorate at NASA's Marshall Space Flight Center.
 http://science.nasa.gov/headlines/y2002/28jan_extinction.htm

Frakes, L.A., Francis, J.E., Syktus, J.I. [1992].  Climate Modes of the Phanerozoic.  Cambridge University Press.  ISBN-13: 9780521021944; ISBN-10: 0521021944.

Isozaki, Y., [2007].  Plume Winter Scenario for Biosphere Catastrophe: The Permo-Triassic Boundary Case.  Dept. of Earth Sciences and Astronomy, Univ. of Tokyo, Japan.  In Yuen, D., Maruyama, S., Karato, S. and Windley, B.F. (eds.), Superplume: beyond plate tectonics. pp. 409-440, Springer, Berlin.

Jeppsson, L., Aldridge, R.J., [2000].  Ludlow (Late Silurian) Oceanic Episodes and Events.  Journal of the Geological Society 157 (6): 1137.

Jeppsson, L., Calner, M. [2007].  The Silurian Mulde Event and a Scenario for Secundo Events.  Earth and Env. Science Transactions of the Royal Society of Edinburgh 93 (02): 135-154. doi:10.1017/S0263593300000377.

Macquarie Univ., [2008].  Earth's Evolving Environments: Extinction Events.  Department of Earth and Planetary Sciences.
 http://www.es.mq.edu.au/courses/GEOS272/downloads/extinction_events_lecture.pdf.

Marusek, J.A., [2005].  The Cosmic Clock, The Cycle of Terrestrial Mass Extinctions.  Lunar and Planetary Science XXXVI (2005).  http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1009.pdf

Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Paleozoic and Proterozoic events". Palaeogeography, Paleoclimatology, Palaeoecology 195 (1): 99-124. doi:10.1016/S0031-0182(03)00304-3.

Peters, S.E., [2008].  Environmental Determinants of Extinction Selectivity in the Fossil Record.  Nature 454: 626. doi:10.1038/nature07032.

Pfefferkorn, H.W., [1999].  Recuperation from mass extinctions.  Proceedings of the National Academy of Sciences, USA, 1999 November 23; 96(24): 13597-13599.

Puetz, S.J., [2009].  The Unified Cycle Theory: How Cycles Dominate the Structure of the Universe and Influence Life on Earth. Outskirts Press, Denver, Colorado; ISBN: 978-1-4327-1216-7.

Retallack, G.J.; Orr, W.N.; Prothero, D.R.; Duncan, R.A.; Kester, P.R.; Ambers, C.P., [2004].  Eocene-Oligocene Extinction and Paleoclimatic Change Near Eugene, Oregon.  GSA Bulletin; July/August 2004; v. 116; no. 7/8; p. 817-839; doi: 10.1130/B25281.1.

Reilly, M., [2008].  Did Magnetic Blip Trigger Mass Extinction?  Discovery News.
http://dsc.discovery.com/news/2008/12/12/magnetism-extinction.html

Taylor, P.D., [2004].  Extinctions in the History of Life.  Cambridge University Press. ISBN-10: 0521842247

Wignall, P.B.; Hallam, A. [1997].  Mass Extinctions and Their Aftermath. Oxford University Press; pp. 164-5.