Tag Archives: flood basalts

Impact debris in Britain

These days reports of geological evidence for asteroid impacts are not regarded with a mixture of disbelief, wonder and foreboding: well, not by geologists anyway. But for such a small area as Britain now to have three of widely different ages and in easily accessible places is pretty good for its brand as the place to visit for practically every aspect of Earth history. The first to be discovered lies at the base of Triassic mudstones near Bristol (see Britain’s own impact) and would need some serious grubbing around at a former construction site. The next to emerge was located in one of the best geological districts in the country at several easily accessed coastal exposures in Northwest Scotland. A glass-rich ejecta layer occurs in the basal Torridonian Stoer Group on Stac Fada, Stoer, Sutherland (UK National Grid Reference 203300, 928400). The most recently found (Drake, S.N. and 8 others 2018. Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland. Geology, v. 46, p. 171-174; doi:10.1130/G39452.1) is on the Inner Hebridean island of Skye at the base of its famous Palaeocene flood basalt sequence (UK National Grid Reference 155371,821112).

View to the northwest across Loch Slapin to the Cuillin Hills of Skye (Central Igneous Complex). The flood basalts beneath which the ejecta layer occurs are just above the trees. (Credit: Wikipedia)

The last is perhaps the most spectacular of the three, as it contains the full gamut of provenance, matched only by material from the drill core into the 66 million year-old Chicxulub crater. The 0.9 m thick debris layer rests directly on mid-Jurassic sandstones beneath Palaeocene basalts of the North Atlantic Igneous Province (NAIP). The layer contains a basalt clast dated at 61.54 Ma, but is dominantly reminiscent of a pyroclastic ignimbrite flow as it contains glass shards. But there the resemblance ends for the bulk of small clasts are of quartz and K-feldspar, sandstone and gneiss. Zircons extracted from the debris show shock lamellae and give Archaean and Proterozoic ages commensurate with the local basement, but also with the bulk of the Scandinavian and Canadian Shields. So the impact could have been anywhere in such widespread terrains, although the enclosed basalt narrows this down to areas where basement is overlain by lavas of the NAIP. The Skye impactite contains unmelted meteorite fragments in the form of titanium nitrides alloyed with vanadium and niobium, metallic iron-silicon alloy containing exsolved carbon, and manganese sulfide.

Although it may be coincidental, the situation of the ejecta layer immediately beneath the Skye lavas, its containing a clast of basalt whose age corresponds to the oldest flows anywhere in the NAIP is fascinating. But the actual impact site is, as yet, unknown. Even so, the layer provokes thoughts about whether an impact may have been more than spatially related to the large NAIP flood basalt pile, preserved on either side of the North Atlantic. If the event was large, then surely the ejecta should be preserved near the base of the flood basalts elsewhere in NW Britain and further afield


Deccan Trap sprung by bolide?

English: Alvarez and K-T Boundary

Luis and Walter Alvarez at the end-Mesozoic Boundary (credit: Wikipedia)

It was 35 years back that father and son team Luis and Walter Alvarez upset a great many geoscientists by suggesting that a very thin layer of iridium-rich mud that contained glass spherules and shocked mineral grains was evidence for a large meteorite having struck Earth. They especially annoyed palaeontologists because of their claim that it occurred at the very top of the youngest Cretaceous and that the mud was spread far and wide in deep- and shallow-marine stratigraphic sequences and also in those of continental rocks. It marked the boundary between the Mesozoic and Cenozoic Eras and, of course, the demise of the dinosaurs and a great many more, less ‘sexy’ beasts. Luis was a physicist, his son a proper geologist and their co-researchers were chemists. It can hardly be said that they stole anyone’s thunder since the issue of mass extinctions was quiescent, yet their discovery ranks with that of Alfred Wegener; another interloper into the closed-shop geoscientific community. They got the same cold-shoulder treatment, but massive popular acclaim as well, even from a minority of geologists who welcomed their having shaken up their colleagues, 15 years after the last ‘big thing’: plate tectonics. And then the actual site of the impact was found by geophysicists in a sedimentary basin in the Gulf of Mexico off the small town of Chicxulub on the Yucatan peninsula.

Chicxulub impact - artist impression

Chicxulub impact – artist impression (credit: Wikipedia)

As they say, ‘the rest is history’ and a great many geoscientists didn’t just jump but pounced on this potential bandwagon. Central to this activity was the fact that, within error, the ages of the impact, the mass extinction and a vast pile of continental lavas in western India, the Deccan Traps, were more or less the same (around 66 Ma). Flood basalt events are just about as dramatic as mega-impacts because of their sheer scale, of the order of a million cubic kilometres; that they were exuded in a mere million years or so, but in only a few tens of stupendous lava flows; and they are far beyond the direct experience of humans, blurting out only every 30 Ma or so. This periodicity roughly tallies with mass extinctions, great and small, through the Mesozoic. There have been two large bands of enthusiasts engaged in the causality of the end-Mesozoic die-off – the extraterrestrials and the parochialists who favoured a more mundane, albeit cataclysmic snuffing-out. Mass extinctions in general have been repeatedly examined, and in recent years it has become clear that most of those since 250 Ma ago seem to be associated with basalt-flood events and are purely terrestrial in origin. As regards the event that ended the Mesozoic, it has proved difficult to resolve whether to point the finger at the Deccan Traps or the Chicxulub impact. Both might have severely damaged the biosphere in perhaps different ways, so a ‘double whammy’ has become a compromise solution.

The Western Ghat hills at Matheran in Maharash...

Deccan flood basalts forming the Western Ghats in Maharashtra, India (credit: Wikipedia)

Unsurprisingly, a lot of effort from different quarters has gone into charting the progress of the Deccan volcanism. Some dating seemed at one stage to place the bulk of the volcanism significantly before the mass extinction and impact, others had them spot on and there were even signs of an hiatus in eruptions at the critical juncture. The problem was geochronological precision of the argon-argon method of radiometric dating that is most used for rocks of basaltic composition: many labs cannot do better than an uncertainty of 1%, which is ±0.7 Ma for ages around the end of the Mesozoic, not far short of the entire duration of these huge events. Some Deccan samples have now been dated to a standard of ±0.1 Ma by the Ar-Ar lab at the Department of Earth and Planetary Sciences, University of California-Berkeley (Renne, P.R. et al. 2010. State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science, v. 350, p. 76-78). The results, between 65.5 to 66.5 Ma, nicely bracket the K/T (now K/Pg) boundary age of 66.04±0.04 Ma. It looks like the double whammy compromise is the hypothesis of choice. But there is more to mere dating.

Renne and colleagues plot the ages against their position in the volcanic stratigraphy of the Deccan Traps in two ways: against the estimated height from base in the pile and against the estimated volume of the erupted materials as it built up – the extent and thickness of successive flows varies quite a lot. The second plot provided a surprise. After the K/Pg event the mean rate of effusion – the limited number of individual flows capped by well-developed soils shows that the build-up was episodic – doubled from 0.4±0.2 to 0.9±0.3 km3 yr-1. Despite the much larger uncertainty in the extent and volume of individual lava Formations than that of their ages, this is clearly significant. Does it imply that the Chicxulub impact somehow affected the magma production from, the mantle plume beneath the Deccan? It had been suggested early in the debate that the antipodean position of the lava field relative to that of Chicxulub may indicate that the huge seismicity from the impact triggered the Deccan magma production. Few accepted that possibility when it first appeared. However, Renne and co. do think it deserves another look, at least at the possibility of some linked effect on the magmatism. Perhaps the magma chamber was somehow enlarged by increased global seismicity; other chambers could have been added; magma might have been ‘pumped’ out more efficiently, or a combination of such effects. The ‘plumbing’ of flood basalt piles is generally hidden, but huge dyke swarms in Precambrian times have been suggested as feeders to long-eroded flood basalts. Seismicity of the scale produced by asteroid impacts can do a lot of damage. The Chicxulub impactor at around 10 km diameter would have carried energy a million times greater than that of the largest thermonuclear bomb, equivalent to an earthquake of Magnitude 12.4 that would have been a thousand times more powerful than the largest recorded earthquake with tectonic causes. Extensional faulting sourced in this fashion in the Deccan area may have increased the pathways along which magma might blurt out.

Duncan, R. 2015. Deadly combination. Nature, v. 527, p. 172-173.

Mass extinctions’ connections with volcanism: more support

Plot the times of peaks in the rates of extinction during the Mesozoic against those of flood basalt outpourings closest in time to the die-offs and a straight line can be plotted through the data. There is sufficiently low deviation between it and the points that any statistician would agree that the degree of fit is very good. Many geoscientists have used this empirical relationship to claim that all Mesozoic mass extinctions, including the three largest (end-Permian, end-Triassic and end-Cretaceous) were caused in some way by massive basaltic volcanism. The fact that the points are almost evenly spaced – roughly every 30 Ma, except for a few gaps – has suggested to some that there is some kind of rhythm connecting the two very different kinds of event.

Major extinctions and flood basalt events during the Mesozoic (credit: S Drury)

Major Mesozoic extinctions and flood basalt events (credit: S Drury)

Leaving aside that beguiling periodicity, the hypothesis of a flood-basalt – extinction link has a major weakness. The only likely intermediary is atmospheric, through its composition and/or climate; flood volcanism was probably not violent. Both probably settle down quickly in geological terms. Moreover, flood basalt volcanism is generally short-lived (a few Ma at most) and seems not to be continuous, unlike that at plate margins which is always going on at one or other place. The great basalt piles of Siberia, around the Central Atlantic margins and in Western India are made up of individual thick and extensive flows separated by fossil soils or boles. This suggests that magma blurted out only occasionally, and was separated by long periods of normality; say between 10 and 100 thousand years. Evidence for the duration of major accelerations, either from stratigraphy and palaeontology or from proxies such as peaks and troughs in the isotopic composition of carbon (e.g. EPN Ni life and mass extinction) is that they too occurred swiftly; in a matter of tens of thousand years. Most of the points on the flood-basalt – extinction plot are too imprecise in the time dimension to satisfy a definite relationship. Opinion has swung behind an instantaneous impact hypothesis for the K-P boundary event rather than one involving the Deccan Traps in India, simply because the best dating of the Deccan suggests extinction seems to have occurred when no flows were being erupted, while the thin impact-related layer in sediments the world over is exactly at the point dividing Cretaceous flora and fauna from those of the succeeding Palaeogene.

Yet no such link to an extraterrestrial factor is known to exist for any other major extinctions, so volcanism seems to be ‘the only game in town’ for the rest. Until basalt dating is universally more precise than it has been up to the present the case is ‘not proven’; but, in the manner of the Scottish criminal law, each is a ‘cold case’ which can be reopened. The previous article  hardens the evidence for a volcanic driver behind the greatest known extinction at the end of the Permian Period. And in short-order, another of the Big Five seems to have been resolved in the same way. A flood basalt province covering a large area of west and north-west Australia (known as the Kalkarindji large igneous province)has long been known to be of roughly Cambrian age but does it tie in with the earliest Phanerozoic mass extinction at the Lower to Middle Cambrian boundary? New age data suggests that it does at the level of a few hundred thousand years (Jourdan, F. et al. 2014. High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction. Geology, v. 42, p. 543-546). The Kalkarindji basalts have high sulfur contents and are also associated with widespread breccias that suggest that some of the volcanism was sufficiently explosive to have blasted sulfur-oxygen gases into the stratosphere; a known means of causing rapid and massive climatic cooling as well as increasing oceanic acidity. The magma also passed through late Precambrian sedimentary basins which contain abundant organic-rich shales that later sourced extensive petroleum fields. Their thermal metamorphism could have vented massive amounts of CO2 and methane to result in climatic warming. It may have been volcanically-driven climatic chaos that resulted in the demise of much of the earliest tangible marine fauna on Earth to create also a sudden fall in the oxygen content of the Cambrian ocean basins.

Nickel, life and the end-Permian extinction

The greatest mass extinction of the Phanerozoic closed the Palaeozoic Era at the end of the Permian, with the loss of perhaps as much as 90% of eukaryote diversity on land and at sea. It was also over very quickly by geological standards, taking a mere 20 thousand years from about 252.18 Ma ago. There is no plausible evidence for an extraterrestrial cause, unlike that for the mass extinction that closed the Mesozoic Era and the age of dinosaurs. Almost all researchers blame one of the largest-ever magmatic events that spilled out the Siberian Traps either through direct means, such as climate change related to CO2, sulfur oxides or atmospheric ash clouds produced by the flood volcanism or indirectly through combustion of coal in strata beneath the thick basalt pile. So far, no proposal has received universal acclaim. The latest proposal relies on two vital and apparently related geochemical observations in rocks around the age of the extinctions (Rothman, D.H. et al. 2014. Methanogenic burst in the end-Permian carbon cycle. Proceedings of the National Academy of the United States, v. 111, p. 5462-5467).

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

In the run-up to the extinction carbon isotopes in marine Permian sediments from Meishan, China suggest a runaway growth in the amount of inorganic carbon (in carbonate) in the oceans. The C-isotope record from Meishan shows episodes of sudden major change (over ~20 ka) in both the inorganic and organic carbon parts of the oceanic carbon cycle. The timing of both ‘excursions’ from the long-term trend immediately follows a ‘spike’ in the concentration of the element nickel in the Meishan sediments. The Ni almost certainly was contributed by the massive outflow of basalt lavas in Siberia. So, what is the connection?

Some modern members of the prokaryote Archaea that decompose organic matter to produce methane have a metabolism that depends on Ni, one genus being Methanosarcina that converts acetate to methane by a process known as acetoclastic methanogenesis. Methanosarcina acquired this highly efficient metabolic pathway probably though a sideways gene transfer from Bacteria of the class Clostridia; a process now acknowledged as playing a major role in the evolution of many aspects of prokaryote biology, including resistance to drugs among pathogens. Molecular-clock studies of the Methanosarcina genome are consistent with this Archaea appearing at about the time of the Late Permian. A burst of nickel ‘fertilisation’ of the oceans may have resulted in huge production of atmospheric methane. Being a greenhouse gas much more powerful than CO2, methane in such volumes would very rapidly have led to global warming. Before the Siberian Traps began to be erupted nickel would only have been sufficiently abundant to support this kind of methanogen around ocean-floor hydrothermal springs. Spread globally by eruption plumes, nickel throughout the oceans would have allowed Methanosarcina or its like to thrive everywhere with disastrous consequences. Other geochemical processes, such as the oxidation of methane in seawater, would have spread the influence of the biosphere-lithosphere ‘conspiracy’. Methane oxidation would have removed oxygen from the oceans to create anoxia that, in turn, would have encouraged other microorganisms that reduce sulfate ions to sulfide and thereby produce toxic hydrogen sulfide. That gas once in the atmosphere would have parlayed an oceanic ‘kill mechanism’’ into one fatal for land animals.

There is one aspect that puzzles me: the Siberian Traps probably involved many huge lava outpourings every 10 to 100 ka while the magma lasted, as did all other flood basalt events. Why then is the nickel from only such eruption preserved in the Meishan sediments, and if others are known from marine sediments is there evidence for other such methanogen ‘blooms’ in the oceans?