Terrestrial mass extinctions, cometary impacts and the Sun's motion perpendicular to the galactic plane

Generation of a galactic chronology with impact ages and spiral arm tangents

Resolving the role of galactic processes in Solar System/Earth events necessitates a robust temporal model. However, astrophysical theory diverges with models varying from long-lasting spiral density waves with uniform pattern speeds and arm structures to others with fleeting and unpredictable features. Here, we address those issues with (1) an analysis of patterns of impact periodicity over periods of 10 to 250 million years (Myr) using circular statistics and (2), an independent logarithmic spiral arm model fitted to arm tangents of 870 micron dust. Comparison of the impact periodicity results with the best-fit spiral arm model suggests a galactic period of 660 Myr, i.e. 165 Myr to pass from one arm to the next in a four spiral arm model, with the most recent arm passage around 52 million years ago (Ma). The oldest impact ages imply that the emerging galactic chronology model is robust for at least the last 2 Gyr. The arm-passing time is consistent with spectral analyses of zircons across 3 Gyrs. Overall, the model provides a temporal framework against which to test hypotheses of galactic mechanisms for global events such as mass extinctions and superchrons.

Theory and classification of mass extinction causation

Theory regarding the causation of mass extinctions is in need of systematization, which is the focus of this contribution. Every mass extinction has both an ultimate cause, i.e. the trigger that leads to various climato-environmental changes, and one or more proximate cause(s), i.e. the specific climato-environmental changes that result in elevated biotic mortality. With regard to ultimate causes, strong cases can be made that bolide (i.e. meteor) impacts, large igneous province (LIP) eruptions and bioevolutionary events have each triggered one or more of the Phanerozoic Big Five mass extinctions, and that tectono-oceanic changes have triggered some second-order extinction events. Apart from bolide impacts, other astronomical triggers (e.g. solar flares, gamma bursts and supernova explosions) remain entirely in the realm of speculation. With regard to proximate mechanisms, most extinctions are related to either carbon-release or carbon-burial processes, the former being associated with climatic warming, ocean acidification, reduced marine productivity and lower carbonate δ13C values, and the latter with climatic cooling, increased marine productivity and higher carbonate δ13C values. Environmental parameters such as marine redox conditions and terrestrial weathering intensity do not show consistent relationships with carbon-cycle changes. In this context, mass extinction causation can be usefully classified using a matrix of ultimate and proximate factors. Among the Big Five mass extinctions, the end-Cretaceous biocrisis is an example of a bolide-triggered carbon-release event, the end-Permian and end-Triassic biocrises are examples of LIP-triggered carbon-release events, and the Late Ordovician and Late Devonian biocrises are examples of bioevolution-triggered carbon-burial events. Whereas the bolide-impact and LIP-eruption mechanisms appear to invariably cause carbon release, bioevolutionary triggers can result in variable carbon-cycle changes, e.g. carbon burial during the Late Ordovician and Late Devonian events, carbon release associated with modern anthropogenic climate warming, and little to no carbon-cycle impact due to certain types of ecosystem change (e.g. the advent of the first predators around the end-Ediacaran; the appearance of Paleolithic human hunters in Australasia and the Americas). Broadly speaking, studies of mass extinction causation have suffered from insufficiently critical thinking-an impartial survey of the extant evidence shows that (i) hypotheses of a common ultimate cause (e.g. bolide impacts or LIP eruptions) for all Big Five mass extinctions are suspect given manifest differences in patterns of environmental and biotic change among them; (ii) the Late Ordovician and Late Devonian events were associated with carbon burial and long-term climatic cooling, i.e. changes that are inconsistent with a bolide-impact or LIP-eruption mechanism; and (iii) claims of periodicity in Phanerozoic mass extinctions depended critically on the now-disproven idea that they shared a common extrinsic trigger (i.e. bolide impacts).

Are Impact Craters and Extinction Episodes Periodic? Implications for Planetary Science and Astrobiology

A review of the results of published spectral analyses of the ages of terrestrial impact craters (58 analyses) and biotic extinction events (35 analyses) reveals that about 60% of the crater trials support a statistically significant cycle averaging ∼29.7 million years (My), and about 67% of the trials of extinction episodes found a significant cycle averaging ∼26.5 My. Cross-wavelet transform analysis of the records of craters and extinctions over the past 260 My shows a mutual ∼26 My cycle and a common phase, suggesting a connection. About 50% of the best-dated impact craters seem to occur in approximately nine pairs or clusters in the past 260 My, apparently carrying the signal of an ∼26- to 30-My cycle. It has been suggested that periodic modulation of impacts and extinctions might be related to periodic comet storms that follow the solar system's oscillations in and out of the galactic mid-plane. Problems arise, however, with regard to the compatibility of such periodic pulses of comet flux with the makeup of the steady-state Near Earth Object (NEO) population, the estimated long-term NEO cratering rates on the terrestrial planets, and the predicted small contribution of Oort Cloud-derived comets to the terrestrial cratering record. Asteroid storms may be possible, but at present there are no accepted mechanisms for creating an ∼30-My period in asteroid breakup events and impacts. Astrobiological implications arise if extra-solar habitable planets suffer similar cyclical or episodic catastrophic bombardment episodes affecting long-term biotic evolution on those planets. Other planetary systems might commonly have comet reservoirs, but they are less likely to contain an asteroid belt in the proper orbital position. Further, frequent impacts of ∼1-km diameter comets and asteroids could affect the establishment and longevity of technological civilizations, including our own.

Some Implications of Mass Extinction for the Evolution of Complex Life

Extinction has the destructive effect of eliminating established lineages from an evolutionary system and the constructive effect of vacating ecospace into which new lineages can evolve. Mass extinctions, which are times of unusually intense extinction, have been consistently followed by major radiations of new lineages. Extraterrestrial impacts associated with extinction events and a periodic recurrence of these events implicates an extraterrestrial forcing mechanism as the ultimate cause of mass extinction. This suggests that the extraplanetary environment has played an important, active role in the development of complex life on Earth.

Evidence for a Solar Companion Star

Periodicity seen in both the mass extinctions and large impact cratering on earth can be explained if one postulates that the sun has a companion star, orbiting in a moderately eccentric orbit with a major axis of 2.8 light-years. No other explanations that have been suggested are compatible with known facts of physics and astronomy. If the companion is a red dwarf star, the most common kind in the galaxy, then no previous astronomical observations would have found it. A search for red objects with large parallax is now underway at Berkeley, and has a good chance of identifying the star in the near future.

Dark matter as a trigger for periodic comet impacts

Although statistical evidence is not overwhelming, possible support for an approximately 35×106  yr periodicity in the crater record on Earth could indicate a nonrandom underlying enhancement of meteorite impacts at regular intervals. A proposed explanation in terms of tidal effects on Oort cloud comet perturbations as the Solar System passes through the galactic midplane is hampered by lack of an underlying cause for sufficiently enhanced gravitational effects over a sufficiently short time interval and by the time frame between such possible enhancements. We show that a smooth dark disk in the galactic midplane would address both these issues and create a periodic enhancement of the sort that has potentially been observed. Such a disk is motivated by a novel dark matter component with dissipative cooling that we considered in earlier work. We show how to evaluate the statistical evidence for periodicity by input of appropriate measured priors from the galactic model, justifying or ruling out periodic cratering with more confidence than by evaluating the data without an underlying model. We find that, marginalizing over astrophysical uncertainties, the likelihood ratio for such a model relative to one with a constant cratering rate is 3.0, which moderately favors the dark disk model. Our analysis furthermore yields a posterior distribution that, based on current crater data, singles out a dark matter disk surface density of approximately 10M⊙/pc2. The geological record thereby motivates a particular model of dark matter that will be probed in the near future.

The evolutionary significance of mass extinctions

Local extinctions of populations, species or groups of species in a particular area are commonly observed by biologists. There are also historical records of the total extinction of single species such as the Dodo, the Great Auk and the Tasmanian Wolf. Mass extinctions are on a much larger scale, and their study is based on the fossil record. The aims of this review are to explore the nature of mass extinctions and their evolutionary significance. The key questions are: what is mass extinction, what are the causes of mass extinctions, do mass extinctions follow a regular pattern, and how do mass extinctions affect our understanding of evolutionary processes?

Flood basalt volcanism during the past 250 million years

A chronology of the initiation dates of major continental flood basalt volcanism is established from published potassium-argon (K-Ar) and argon-argon (Ar-Ar) ages of basaltic rocks and related basic intrusions. The dating is therefore independent of the biostratigraphic and paleomagnetic time scales. Estimated errors of the initation dates of the volcanic episodes determined from the distributions of the radiometric ages are, approximately, plus or minus 4 percent. There were 11 distinct episodes during the past 250 million years. Sometimes appearing in pairs, the episodes have occurred quasi-periodically with a mean cycle time of 32 +/- 1 (estimated, error of the mean) million years. The initiation dates of the episodes are close to the estimated dates of mass extinctions of marine organisms. Showers of impacting comets may be the cause.

The impact rate on Earth

Recent data, and modelling of the interaction between asteroids and the atmosphere, has defined a complete size-frequency distribution for terrestrial impactors, from meteorite-sized objects up to kilometre-sized asteroids, for both the upper atmosphere and the Earth's surface. Although there remain significant uncertainties in the incidence of specific size-fractions of impactors, these estimates allow us to constrain the threat posed by impacts to human populations. It is clear that impacts remain a significant natural hazard, but uniquely, they are a threat that we can accurately predict, and take steps to avoid.

Periodicity of mass extinctions without an extraterrestrial cause

We study a lattice model of a multispecies prey-predator system. Numerical results show that for a small mutation rate the model develops irregular long-period oscillatory behavior with sizeable changes in a number of species. The periodicity of extinctions on Earth was suggested by Raup and Sepkoski [Proc. Natl. Acad. Sci. 81, 801 (1984)], but thus far is lacking a satisfactory explanation. Our model indicates that this might be a natural consequence of the ecosystem dynamics and not the result of any extraterrestrial cause.

Isotopic evidence for the Cretaceous-Tertiary impactor and its type

High-precision mass spectrometric analysis of chromium in sediment samples from the Cretaceous-Tertiary (K-T) boundary coincident with the extinction of numerous organisms on Earth confirms the cosmic origin of the K-T phenomenon. The isotopic composition of chromium in K-T boundary samples from Stevns Klint, Denmark, and Caravaca, Spain, is different from that of Earth and indicates its extraterrestrial source. The chromium isotopic signature is consistent with a carbonaceous chondrite-type impactor. The observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with Earth during its history.

Geochemical evidence for a comet shower in the late Eocene

Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began approximately 1 My before and ended approximately 1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters approximately 36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud.

Target Earth: evidence for large-scale impact events

Unlike the Moon, the Earth has retained only a small sample of its population of impact structures. Currently, over 150 impact structures are known and there are 15 instances of impact known from the stratigraphic record, some of which have been correlated with known impact structures. The terrestrial record is biased toward younger and larger structures on the stable cratonic areas of the crust, because of the effects of constant surface renewal on the Earth. The high level of endogenic geologic activity also affects the morphology and morphometry of terrestrial impact structures; although, the same general morphologic forms that occur on the other terrestrial planets can be observed. A terrestrial cratering rate of 5.6 +/- 2.8 x 10(-15) km-1 a-1 for structures > or = 20 km in diameter can be derived, which is equivalent to that estimated from astronomical observations. Although there are claims to the contrary, the overall uncertainties in the ages of structures in the impact record preclude the determination of any periodicity in the record. Small terrestrial impact structures are the result of the impact of iron or stony iron bodies, with weaker stony and icy bodies being crushed on atmospheric passage. At larger structures (>1 km), trace element geochemistry suggests that approximately 50% of the impact flux is from chondritic bodies, but this may be a function of the signal:noise ratio of the meteoritic tracer elements. Evidence for impact in the stratigraphic record is both chemical and physical. Although currently small in number, there are indications that more evidence will be forthcoming with time. Such searches for evidence of impact have been stimulated by the chemical and physical evidence of the involvement of impact at the K/T boundary. There will, however, be problems in differentiating geochemically the signal of even relatively large impact events from the background cosmic flux of every day meteoritic debris. Even with these biases and difficulties, the terrestrial impact record is the dominan source of ground truth information on the details of the impact flux and its known and potential effects on the evolution of the Earth and its biosphere. For although the record is poorly known, what evidence there is represents an integration over considerable geologic time. On the timescales of 10(5)-10(6) a, it is clear that impact represents a major threat to human civilization. Given the stochastic nature of impact, the timing of such an event is unknown.

A unified theory of impact crises and mass extinctions: quantitative tests

Several quantitative tests of a general hypothesis linking impacts of large asteroids and comets with mass extinctions of life are possible based on astronomical data, impact dynamics, and geological information. The waiting times of large-body impacts on the Earth derived from the flux of Earth-crossing asteroids and comets, and the estimated size of impacts capable of causing, large-scale environmental disasters, predict the impacts of objects > or = 5 km in diameter (> or = 10(7) Mt TNT equivalent) could be sufficient to explain the record of approximately 25 extinction pulses in the last 540 Myr, with the 5 recorded major mass extinctions related to impacts of the largest objects of > or = 10 km in diameter (> or = 10(8) Mt events). Smaller impacts (approximately 10(6) Mt), with significant regional environmental effects, could be responsible for the lesser boundaries in the geologic record. Tests of the "kill curve" relationship for impact-induced extinctions based on new data on extinction intensities, and several well-dated large impact craters, also suggest that major mass extinctions require large impacts, and that a step in the kill curve may exist at impacts that produce craters of approximately 100 km diameter, smaller impacts being capable of only relatively weak extinction pulses. Single impact craters less than approximately 60 km in diameter should not be associated with detectable global extinction pulses (although they may explain stage and zone boundaries marked by lesser faunal turnover), but multiple impacts in that size range may produce significant stepped extinction pulses. Statistical tests of the last occurrences of species at mass-extinction boundaries are generally consistent with predictions for abrupt or stepped extinctions, and several boundaries are known to show "catastrophic" signatures of environmental disasters and biomass crash, impoverished postextinction fauna and flora dominated by stress-tolerant and opportunistic species, and gradual ecological recovery and radiation of new taxa. Isotopic and other geochemical signatures are also generally consistent with the expected after-effects of catastrophic impacts. Seven of the recognized extinction pulses seem to be associated with concurrent (in some cases multiple) stratigraphic impact markers (e.g., layers with high iridium, shocked minerals, microtektites), and/or large, dated impact craters. Other less well-studied crisis intervals show elevated iridium, but well below that of the K/T spike, which might be explained by low-Ir impactors, ejecta blowoff, or sedimentary reworking and dilution of impact signatures. The best explanation for a possible periodic component of approximately 30 Myr in mass extinctions and clusters of impacts is the pulselike modulation of the comet flux associated with the solar system's periodic passage through the plane of the Milky Way Galaxy. The quantitative agreement between paleontologic and astronomical data suggests an important underlying unification of the processes involved.

Review

Book review in this article: Introdução histórica à biologia comparada com especial referência à biogeografia, vols. I, II e III (Historical introduction to comparative biology with special reference to biogeography-(Vols. I, II and III) (In Portuguese) by Nelson Papavero. New concepts in global tectonics edited by Sankar Chatterjee and Nicholas Hotton, III.

Periodicity in extinction and the problem of catastrophism in the history of life

The hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera. In the interval from the Permian to Recent, these data encompass some 13,000 generic extinctions, providing a more sensitive indicator of species-level extinctions than previously used familial data. Extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 Ma intervals over the last 270 Ma. Most of these peaks correspond to extinction events recognized in more detailed, if limited, biostratigraphic studies. These new data weaken or negate most arguments against periodicity, which have involved criticisms of the taxonomic data base, sampling intervals, chronometric time scales, and statistical methods used in previous analyses. The criticisms are reviewed in some detail and various new calculations and simulations, including one assessing the effects of paraphyletic taxa, are presented. Although the new data strengthen the case for periodicity, they offer little new insight into the deriving mechanism behind the pattern. However, they do suggest that many of the periodic events may not have been catastrophic, occurring instead over several stratigraphic stages or substages.

Numerical simulations and the problem of periodicity in the cratering record

Ages of craters in the record of impacts on earth may be uniformly period, totally random, or a mixture of the two. These alternatives are studied through numerical simulation wherein time-series analysis is performed on real and simulated sequences to which random noise has been added to represent age-dating uncertainty. We conclude that the real record is most likely to have been generated by a mixture of random and periodic impacts, with the random events constituting the majority.

Accretion Rate of Extraterrestrial Matter: Iridium Deposited 33 to 67 Million Years Ago

Iridium measured in 149 samples of a continuous 9-meter section of Pacific abyssal clay covering the time span 33 to 67 million years ago shows a well-defined peak only at the Cretaceous/Tertiary boundary. In the rest of the section iridium ranges from a minimum concentration near 0.35 nanograms per gram in the Paleocene to a maximum near 1.7 in the Eocene; between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years. Correction for terrestrial iridium leads to an extraterrestrial flux of9 +/- 3 nanograms of iridium per square centimeter per million years, and an estimated annual global influx of 78 billion grams of chondritic matter, consistent with recent estimates of the influx of dust, meteorites, and crater-producing bodies with masses ranging from 10(-13) to 10(18 )grams. Combining the recent flux of objects ranging in mass from 10(6) to 10(7) grams with the flux of 10(14) - to 10(15) -gram objects indicates that the number of objects is equal to 0.54 divided by the radius (in kilometers) to the 2.1 power. Periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years; the predicted iridium maxima (more than 30 times background) are not present in the intervals associated with the Cretaceous/Tertiary boundary or the tektiteproducing late Eocene events.

Biological extinction in earth history

Virtually all plant and animal species that have ever lived on the earth are extinct. For this reason alone, extinction must play an important role in the evolution of life. The five largest mass extinctions of the past 600 million years are of greatest interest, but there is also a spectrum of smaller events, many of which indicate biological systems in profound stress. Extinction may be episodic at all scales, with relatively long periods of stability alternating with short-lived extinction events. Most extinction episodes are biologically selective, and further analysis of the victims and survivors offers the greatest chance of deducing the proximal causes of extinction. A drop in sea level and climatic change are most frequently invoked to explain mass extinctions, but new theories of collisions with extraterrestrial bodies are gaining favor. Extinction may be constructive in a Darwinian sense or it may only perturb the system by eliminating those organisms that happen to be susceptible to geologically rare stresses.

Periodic extinction of families and genera

Eight major episodes of biological extinction of marine families over the past 250 million years stand significantly above local background (P < 0.05). These events are more pronounced when analyzed at the level of genus, and generic data exhibit additional apparent extinction events in the Aptian (Cretaceous) and Pliocene (Tertiary) Stages. Time-series analysis of these records strongly suggests a 26-million-year periodicity. This conclusion is robust even when adjusted for simultaneous testing of many trial periods. When the time series is limited to the four best-dated events (Cenomanian, Maestrichtian, upper Eocene, and middle Miocene), the hypothesis of randomness is also rejected for the 26-million-year period (P < 0.0002).