Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link?
Introduction
In the past two decades, there has been an often acrimonious debate between proponents of the idea that large extraterrestrial asteroid or comet impacts caused many, if not all of the five great Phanerozoic mass extinctions, and proponents who have favored a terrestrial origin for mass extinctions linked to the rifting of continental shields and associated (carbon-rich) kimberlite/carbonatite activity and continental flood basalt eruptions. Since Alvarez et al.’s suggestion that the presence of a large iridium concentration anomaly in sediments at the Cretaceous–Tertiary (K–T) boundary was evidence of a large bolide impact at this time that caused the K–T mass extinction, geologists have searched for and reported many possible ‘impact tracers’ at the times of great mass extinctions, summarized in Table 1. During this debate, impact proponents have become largely convinced that basaltic volcanic processes cannot create the ‘impact signals’ of shocked quartz, microspherules, and fullerenes that presently have been reported at the times of the most recent four of the five great extinctions. What has become somewhat lost in this muddled debate is that there is also increasingly well documented geologic evidence for continental flood basalts and rifting-related kimberlite/carbonatite activity at the four most recent of the past five great Phanerozoic extinctions. Here we discuss that these multiple coincidences of apparent synchronicity of great Phanerozoic mass extinctions with both cratonic continental flood basalts (CFBs) [1], [2], [3], [4], [5], [6]and the geologic ‘traces’ of large bolide impacts [7], [8], [9], [10], [11] is extremely unlikely to arise by chance – strongly arguing for either a causal link between all three or that the reported geologic evidence of ‘impact signals’ is spurious or the byproduct of much smaller non-lethal impacts. (The occurrence and apparent near-synchronicity of great mass extinctions and CFBs is not in doubt.) To begin we briefly review the evidence for CFBs and geologic ‘impact signals’ at the time of each of the four most recent of the five great Phanerozoic mass extinctions.1
As summarized in Fig. 1 and Table 1, Table 2, geologic tracers consistent with bolide impacts and CFBs have now been documented for the most recent four great Phanerozoic mass extinctions: the K–T at ∼66 Ma, the Triassic–Jurassic (Tr–J) at ∼201 Ma, the Permian–Triassic (P–Tr) at ∼251 Ma, and the Late Devonian (including the Frasnian–Famennian (FF)) mass extinction which apparently happened in several sharp pulses between ∼380 Ma and ∼364 Ma. (We will refer to the geologic tracers of bolide impacts as ‘impact signals’, the rarest of which is a large crater which has only been inferred for the K–T.) Cratonic flood basalts are best known as the eruption sites of extremely large volumes (>106 km3) of tholeiitic (mid-ocean-ridge-like) basalts within a time span of a million years or less (documented by paleomagnetic reversal evidence, cf. [2], [15], and Ar/Ar dating [16], [17]). However, within continental cratons, these flood basalts are sometimes associated with the eruption of carbon-rich kimberlites [16] and carbonatites. Kimberlites are carbon- and volatile-rich basaltic magmas that appear to have the most rapid and explosive ascent from their source of any terrestrial magmas, and which are the only known transport vehicle rapid enough to carry metastable diamonds from their source depth to Earth’s surface.
The ∼66 Ma K–T boundary is the time of the synchronous occurrence of one of the largest known terrestrial impact structures, Chicxulub, and a very large CFB, the Deccan Traps event associated with continental rifting above the Reunion plume (see Fig. 2). Other impact geosignals of this event are an iridium-rich sediment stratum [11] (found worldwide, also between two of the lower Deccan Traps massive basalt flows [18]), globally distributed findings of altered microspherule deposits [13], rarer shocked quartz microcrystals and even rarer stishovite (high-pressure quartz) microcrystals [12], and nanodiamonds [19]. Other geosignals of a sudden mass extinction event are sudden excursions in marine δ13C, δ18O, and 87Sr/86Sr, and a spike in the abundance of fungal spores [12] that is interpreted as evidence of an equally sudden ‘killing time’ on land.
The ∼201 Ma Tr–J boundary is the time of near-synchronous Central Atlantic Magmatic Province (CAMP) flood basalt volcanism [17] associated with the initial slow rifting of North America from Africa and South America (see Fig. 2), and ‘impact signals’ of shocked quartz (in Italy [7]), a small Ir sediment excursion (eastern USA [8]), and possible impact-induced slump deposits (seismite) across the UK [20]. Note that these end-Triassic ‘impact signals’ are distinct from those caused 10–15 Myr earlier by the Manicougan impact, which created a large crater as well as widespread shocked quartz and microspherules [21] but which is not associated with a mass extinction (Fig. 1). The Tr–J extinction record is also associated with a sudden excursion in marine δ13C [22], and a spike in the abundance of fern spores [8].
The ∼251 Ma P–Tr boundary has been long known to be the time of both the largest Phanerozoic mass extinction and the largest well documented CFB, the Siberian Traps (Fig. 2). Recently, it has been recognized that the end-Permian extinction is likely to have occurred in two sudden pulses [23], a smaller pulse at ∼257 Ma (end-Guadalupian) believed to be synchronous with the Emeishan CFB now preserved in China (Fig. 2) [23], and a second larger pulse at ∼251 Ma, synchronous with the Siberian Traps CFB [24]. ‘Impact signals’ at the P–Tr boundary are reported as non-atmospheric rare gas ratios trapped in P–Tr C60 and C70 fullerenes [10] (this reported finding has been recently disputed [25]; note also that it occurs in the same bentonite bed of altered volcanic ash that was used to date the P–Tr event by Renne et al. [24]), microspherules and shocked mineral ejecta [14], and a sudden rapid change in δ34S which is interpreted to record the sudden release of mantle sulfur triggered by melting induced by an oceanic bolide impact [9]. The P–Tr extinction is also associated with sudden marine excursions in δ13C [26], δ18O [27], and a spike in the abundance of fungal spores [28]; late Permian (Changxingian) marine fauna disappears at the base of a 5 cm thick smectite white clay (volcanic ash) layer bounded by pyrite lamellae characteristic of anoxic ocean conditions [29].
The geologic record of the ∼380 Ma and ∼364 Ma (Late Devonian) mass extinction events is more complex than the record of the three more recent ones, with evidence for two or three extinction ‘surges’ within a ∼15 Myr period. At ∼380 Ma, there is evidence for both ‘impact signals’ including shocked quartz [30] and plume-influenced [31] cratonic kimberlite and carbonatite emplacement on the Kola Peninsula (Baltic Shield) [32]. It has been proposed that the ∼50 km diameter Siljan crater in Sweden (close to the Kola Peninsula, but considered too small to cause a mass extinction) was formed at this time, and the ∼364 FF boundary appears to also be synchronous with the eruption of the now almost completely buried and hence poorly documented Pripyat–Dniepr–Donets CFB [33] in the Ukraine and southern Russia and recorded kimberlite activity at both the Kola Peninsula [32] and near what is now the southernmost exposure of the Siberian Traps [34]. While microspherule and shocked mineral traces have also been found in some sediment records of this era (see Table 1), δ13C, δ18O, and δ34S excursions appear to be more complex in the Late Devonian events than at the three younger great mass extinction boundaries [12].
Thus, at least three and maybe four synchronous CFBs (with associated kimberlite activity), ‘impact signals’, and mass extinctions have occurred within the past 390 Myr (Fig. 1). How likely is this to arise by coincidence?
Section snippets
Do two (or more) synchronous CFB/bolide impact events imply a causal link?
Bolide impacts that produce a ∼180–300 km multi-ring basin the size of Chicxulub [35] are believed to occur less than once every 109 years. The initial characterization [11] of the impact mass extinction hypothesis proposed that ‘large enough’ bolide impacts could occur every ∼100 Myr [36]. Within the last 250 Myr for which a continuous continental record is reasonably well known, CFB events also occurred infrequently [2], roughly once every ∼30–50 Myr, with the primary volcanic pulse lasting
Can large bolide impacts initiate CFBs and their subsequent long-lived hotspot activity?
The most recent and best preserved K–T record contains strong geologic evidence which implies that large impacts do not initiate either flood basalts or mantle plumes. A sediment pocket containing the iridium ‘impact signal’ is found lying between two of the lower Deccan Trap lava flows [18], documenting that the impact must have post-dated the onset of, and thus cannot have caused, the Deccan CFB. Furthermore, the Deccan–Reunion trace can be followed as a time-progressive lineament of 3He-rich
Do subcratonic mantle plumes cause ‘impact signals’ and CFBs?
Thus we are seemingly left with only two possibilities: either the reported ‘impact signals’ at all except maybe one of the K–T, Tr–J, P–Tr, or Late Devonian mass extinctions must be spurious (since the geologic evidence for extinction-synchronous CFBs is clear, and the odds of even one such large extraterrestrial impact/CFB/extinction coincidence within the past 400 Myr are only 1 in 8); or we must seriously consider the implication that ‘impact signals’ were somehow created by processes
Cratonic lithospheric gas explosions – the great extinctions’ missing terrestrial link?
As noted above, the K–T, Tr–J, and P–Tr boundaries are times of eruption of voluminous plume-related and probably volatile-rich magma through rifting cratonic lithosphere up to ∼250 km in thickness, leading to cratonic eruptions of kimberlite-type diatremic alkaline volcanism [50] in addition to more voluminous basaltic lava flows. Kimberlite activity is a likely indicator of plume material ponding beneath and incubating the base of old, cold, volatile-rich, cratonic lithosphere [51], [52]. We
Carbon-rich plume melts can bring significant mantle carbon into cratonic lithosphere
Carbon’s actual volume fraction within the convecting mantle is poorly known, in large part because any carbon-rich mantle components are likely to have much lower melting (solidus) temperatures within the upper mantle [54], [55] than their host carbon-poor peridotites or pyroxenites, thus are most likely to be efficiently extracted into deep-forming carbonatitic/kimberlitic magmas. Such magmas would be a normal low-volume byproduct of plume volcanism – and their presence is infrequently
Cratonic lithosphere incubation – a possible mechanism for CO2 buildup
Experimental petrologists have long noted that CO2 remains stably ‘dissolved’ within a carbon-rich magma only at pressures greater than ∼2.5–2.7 GPa (∼80 km) [54], [60], [61]. At lower pressures, for almost any plausible upper mantle/deep continental lithosphere temperature (see Fig. 4), silicate magmas tend to exsolve CO2 into an immiscible vapor phase [55] that forms bubbles within the surrounding matrix. As can be seen from the phase diagram in Fig. 4, the exothermic carbonate
Energetics of a large lithospheric gas explosion
The energy release from a large Verneshot may be as much as that from a large bolide impact. During a Verneshot, mechanical energy will be released by the decompression of escaping gases [72] and strain relaxation of the overpressured lithosphere itself. To estimate the energy release from explosive decompression of a compressed lithospheric gas phase, consider isothermal gas expansion following the ideal gas law. (This assumes that any gas expansion cooling during decompression is buffered by
Recognizing a preserved Verneshot pipe
The model outlined above predicts a number of characteristic features of the Verneshot pipe that may be recognizable in the geological record.
- 1.
A subcircular crater/depression containing shattered/brecciated rock.
- 2.
Shocked quartz in the surrounding country rock.
- 3.
Shattercones pointing toward the center of the pipe (possibly generated during the initial explosive gas release, more likely generated during the subsequent snapping shut of the pipe).
- 4.
Pseudotachylite generated by the seismic faulting
Kimberlites – byproducts of ‘micro’-Verneshots?
Kimberlites are ultramafic, volatile- and carbon-rich magmas formed under reducing conditions, which ascend through the lithosphere faster than any other preserved magma type. Furthermore, their association with cratons, with volatiles and with hotspot tracks [51] suggests that they may be the nearest analogue for the type of event we envisage. However, the mechanics of kimberlite emplacement is hotly debated, with evidence for both hot emplacement of magma and cold emplacement of fluidized
Ecological effects of a Verneshot
The ecological effects of a Verneshot are those previously proposed for a CFB-induced mass extinction [2], [6], [14], [87], with the only difference that a Verneshot event would be even more sudden. Multiple Verneshot events could also occur within a single phase of rifting. The massive amounts of carbon and sulfur vapor ‘propellent’ released during the Verneshot considered above would instantaneously poison the atmosphere and the ocean’s thin surface photosynthetic layer (more than doubling
Was the Chicxulub crater caused by the impact of a Verneshot mass jet?
It seems unlikely that the Chicxulub crater itself was formed by the impact of a mass jet from a Deccan/Reunion Verneshot, more likely Chicxulub is just the 1 in 8 ‘bad luck’ coincidence of a cratonic flood basalt and bolide impact that could happen by chance within the past 400 Myr. If indeed due to a Verneshot (intriguingly, the proposed low-azimuth southeast arrival direction of the Chicxulub impactor [89] is also consistent with the Verneshot hypothesis), its location 135° away from the
Unresolved problems of the Verneshot hypothesis
We are well aware that the Verneshot hypothesis is extreme, and that the mechanical arguments marshalled in this study only demonstrate that it appears to be possible that Verneshots have occurred in the past, with many problems remaining to be sorted out. For instance, is it possible, as ‘classical’ craton yield strength envelope arguments imply, for ∼1 GPa stresses to build up within a few Myr at ∼70 km depths within continental lithosphere, or will a more complete model of an elastic–plastic
Summary
Since Alvarez et al.’s proposal in 1980 that the impact of a large extraterrestrial chondritic meteorite was the cause of the K–T mass extinction, postulated ‘impact signals’ have been searched for and reported at the times of the four largest mass extinctions within the past 380 Myr. During the past 20 years, greatly enhanced precision in geologic radiometric and paleomagnetic dating techniques has developed to the point where at least three and most likely all of the most recent four great
Acknowledgements
We thank two anonymous reviewers, Vincent Courtillot, Jay Melosh, Paul Renne, John Vandecar, and Paul Wignall for helpful and critical input during the evolution of this paper.[VC]
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