Tag: Asteroid impact

News from the Chicxulub drilling project

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Artist’s impression of an asteroid slamming into the shallow sea off the present Yucatán Peninsula about 65 Ma ago (Credit: Donald E. Davis of NASA)

Aimed at resolving the impact versus volcanism debate about the causes of the K-Pg mass extinction, the International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) began drilling into the focus of the Chicxulub impact structure off the Yucatán Peninsula, Mexico in 2016. The project recovered 830 m of rock core, of which  about 140 cm contained the boundary between tsunami deposits and the post-impact marine limestones of Danian Age (basal Palaeogene); as close as one can get to the moment when the asteroid hit the sea floor. That an impact close to the start of the Danian had taken place was first discovered from abnormally high concentrations of the platinum-group metal iridium (Ir), shocked mineral grains and glass spherules, among other anomalous materials, in 350 marine and terrestrial sections across the globe. If the Chicxulub crater contained similar features to these ‘smoking guns’ then the link might seem to be done and dusted. A report on the crucial few centimetres from the Chicxulub drill core shows this to be the case (Goderis, S. and 32 others 2021. Globally distributed iridium layer preserved within the Chicxulub impact structure. Science Advances, v. 9, article eabe3647; DOI: 10.1126/sciadv.abe3647).

Yet the boundary layer at Chicxulub could not have been emplaced at the instant of impact. The gigantic power involved would have flung debris outwards, including seawater as well as the rocks that were once at considerable depth below the seabed. Much in the manner of a stone falling into a pond molten crust would have rebounded from the initial strike to form an axial peak and a ringed basin. Likewise huge tsunamis would have rolled away from the impact, then to return and fill the new basin, perhaps several times. Some of the ejected debris would have reached low orbit in the form of pulverised rock and asteroid to remain there for a while before completely falling back to Earth. The core includes about 130 m of once partly molten debris (suevite) above more-or-less intact granitic basement. Only the top 3.5 m show signs of having been deposited in water; fine-grained, well-sorted and laminated suevite containing clasts of once molten material and even late-Cretaceous foraminifera tests, formed probably by the refilling of the impact basin during the backflow of tusunamis. A mere 3 cm of silt and clay just below marine limestones has yielded the characteristic high Ir and nickel concentrations. This Ir-rich layer also contains the earliest Palaeocene foraminifera.

Grains in the Ir-rich layer were the last to settle, the main question being ‘How long after the impact took place did that happen?’ Being very fine they are estimated to have fallen-out from suspension and circulation in the atmosphere over a period of up to a few decades. Coarser material below them would have taken no longer than a few weeks to years. Yet these estimates are based mainly on Stokes’ law governing particles of different sizes falling through a viscous fluid. Taking an empirical view based on actual rates of clay sedimentation in the ocean (~5 mm per thousand years) the Ir-rich layer may have been deposited over 6000 years. That is hardly the ‘instant of the impact’. But the timing does say something interesting about the return of life to the seas; in geological terms it was swift, if the forams are anything to go by. Since the tsunamis swept onto and drained the surrounding land masses a great deal of nutrient would have ended up in the sea awaiting organisms at the bottom of the food chain. Biomarker chemicals and trace fossils in the Ir-rich layer suggest  thriving bacterial communities, with forams, crustacea and larval fish.

The authors conclude ‘The clear association of the Ir anomaly within the Chicxulub impact structure and the recorded biotic response confirms the direct relationship between the impact event and the K-Pg mass extinction’. Whether that is accepted by those geoscientists with their eyes on the Deccan Trap hypothesis is not so certain …

Subglacial impact structure: trigger for Younger Dryas?

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Radar microwaves are able to penetrate easily through several kilometres of ice. Using the arrival times of radar pulses reflected by the bedrock at glacial floor allows ice depth to be computed. When deployed along a network of flight lines during aerial surveys the radar returns of large areas can be converted to a grid of cells thereby producing an image of depth: the inverse of a digital elevation model. This is the only means of precisely mapping the thickness variations of an icecap, such as those that blanket Antarctica and Greenland. The topography of the subglacial surface gives an idea of how ice moves, the paths taken by liquid water at its base, and whether or not global warming may result in ice surges in parts of the icecap. The data can also reveal topographic and geological features hidden by the ice (see The Grand Greenland Canyon September 2013).

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Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Such a survey over the Hiawatha Glacier of NW Greenland has showed up something most peculiar (Kjaer, K.H. and 21 others 2018. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances, v. 4, eaar8173; DOI: 10.1126/sciadv.aar8173). Part of the ice margin is an arc, which suggests the local bed topography takes the form of a 31km wide, circular depression. The exposed geology shows no sign of a structural control for such a basin, and is complex metamorphic basement of Palaeoproterozoic age. Measurements of ice-flow speeds are also anomalous, with an array of higher speeds suggesting accelerated flow across the depression. The radar image data confirm the presence of a subglacial basin, but one with an elevated rim and a central series of small peaks. These are characteristic of an impact structure that has only been eroded slightly; i.e. a fairly recent one and one of the twenty-five largest impact craters on Earth.. Detailed analysis of raw radar data in the form of profiles through the ice reveals  that the upper part is finely layered and undisturbed. The layering continues into the ice surrounding the basin and is probably of Holocene age (<11.7 ka), based on dating of ice in cores through the surrounding icecap. The lower third is structurally complex and shows evidence for rocky debris. Sediment deposited by subglacial streams where they emerge along the arcuate rim contain grains of shocked quartz and glass, as well as expected minerals from the crystalline basement rocks. Some of the shocked material contains unusually high concentrations of transition-group metals, platinum-group elements and gold; further evidence for impact of extraterrestrial material – probably an iron asteroid that was originally more than 1 km in diameter. The famous Cape York iron meteorite, which weighs 31 t – worked by local Innuit to forge harpoon blades – fell in NW Greenland about 200 km away.

The central issue is not that Hiawatha Glacier conceals a large impact crater, but its age. It certainly predates the start of the Holocene and is no older than the start of Greenland glaciation about 2.6 Ma ago. That only Holocene ice layers are preserved above the disrupted ice that rests immediately on top of the crater raises once again the much-disputed possibility of an asteroid impact having triggered the Younger Dryas cooling event and associated extinctions of large mammals in North America at about 12.9 ka (see Impact cause for Younger Dryas draws flak May 2008). Only radiometric dating of the glassy material found in the glaciofluvial sediments will be able to resolve that particular controversy.

Impact linked to the Palaeocene-Eocene boundary event

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The Palaeocene–Eocene (P-E) boundary at 55.8 Ma marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera.  It was a time of sudden global warming (5-10°C in 10-20 ka) superimposed on the general Cenozoic cooling from the ‘hothouse’ of the Cretaceous Period. It coincided with a decrease in the proportion of 13C in marine carbonates.  Because photosynthesis, the source of organic carbon, favours light 12C, such a negative δ13C “spike” is generally ascribed to an unusually high release of organic carbon to the atmosphere.  The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments. But another process may yield such a signature; massive burning of organic material at the land surface. Since its discovery, the P-E thermal maximum has been likened to the situation that we may face should CO2 emissions from fossil-fuel burning continue to rise without control. Unsurprisingly, funds are more easily available for research on this topic than, say, ‘Snowball Earth’ events.

Climate change during the last 65 million year...
Climate change during the last 65 million years. The Paleocene–Eocene Thermal Maximum is labelled PETM. (Photo credit: Wikipedia)

Three seafloor sediment cores off the east coast of the US that include the P-E boundary have been found to contain evidence for an impact that occurred at the time of the δ13C “spike” (Schaller, M.F. et al. 2016. Impact ejecta at the Paleocene-Eocene boundary. Science, v. 354, p. 225-229). The evidence is dominated by tiny spherules and tear-shaped blobs of glass, some of which contain tiny crystals of shocked and high-temperature forms of silica (SiO2). These form part of the suite of features that have been used to prove the influence of asteroid impacts. Two other onshore sites have yielded iridium anomalies at the boundary, so it does look like there was an impact at the time. The question is, was it large enough either to cause vast amounts of methane to blurt out from shall-water gas hydrates or set the biosphere in fire? Two craters whose age approximates that of the P-E boundary are known, one in Texas the other in Jordan, with diameters of 12 and 5 km respectively; far too small to have had any global effect. So either a suitably substantial crater of the right age is hidden somewhere by younger sediments or the association is coincidental – the impact that created the Texan crater could conceivably have flung glassy ejecta to the area of the three seafloor drilling sites.

Almost coinciding with the spherule-based paper’s publication another stole its potential thunder. Researchers at Southampton University used a mathematical model to investigate how a methane release event might have unfolded (Minshull, T.A. et al. 2016. Mechanistic insights into a hydrate contribution to the Paleocene-Eocene carbon cycle perturbation from coupled thermohydraulic simulations. Geophysical Research Letters, v. 43, p. 8637-8644, DOI: 10.1002/2016GL069676). Their findings challenge the hypothesized role of methane hydrates in causing the sudden warming at the P-E boundary. But that leaves out the biosphere burning, which probably would have neded a truly spectacular impact.

More on mechanisms for ancient climate change

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Deccan Trap sprung by bolide?

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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.

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

Batter your planet

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K/T extinction event theory. An artist's depic...
Artist’s depiction of the asteroid impact 65 million years ago that caused the K-T mass extinction. (Photo credit: Wikipedia)

Just in time for the festive season I have been sent the URL for an on-line impact simulator written by a team from Imperial College London and the University of Arizona (Collins, G.S. et al. 2005. Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth. Meteoritics and Planetary Science, v. 40, p. 817–840), with a web presence designed at Purdue University, Indiana. ImpactEarth (http://www.purdue.edu/impactearth/) has been around for two years and has a scientifically pleasing level of precision, thanks to the authors, Gareth Collins, Jay Melosh and Robert Marcus.

The fact that the target shown by the accompanying animation and other graphics seems to be the Washington-New York megalopolis may be a cause for some concern for US readers, especially the Department of Homeland Security, National Security Agency and CIA. They can rest easy, however, as this seems to be a matter of artistic license: the choice of parameters allows for ocean strikes and targets of sedimentary or crystalline rocks. Others are impactor diameter and density, impact angle and speed, plus distance from ground zero. An element of whimsy allows the casual user to choose inbound humpback whales, school buses and the Empire State Building as well as more astronomically likely scenarios.

There are a number of missing parameters such as direction relative to Earth’s rotation, latitude and the likely affect of an ice-cap strike, and no mention in the results of the electromagnetic burst from atmospheric compression on entry – the Diesel effect. However, the thermal effects on bystanders, buildings and vegetation at the ‘viewpoint’ personalise the experience to some extent. It is the detail about crater dimensions and evolution, lithospheric melting and what might happen to the Earth’s axial tilt and day length that the wealth of computations produce surprises. It is not easy to destroy our planet: using a body with a density of 3000 kg m-3 and the diameter of Asia causes no significant melting or changes in axial tilt at speeds less than 12 km s-1, but does change the length of the day by up to 113 hours. This is because the power of impacts and therefore the work done by them is proportional to the square of the speed. Mind you, nothing is left standing as the seismic effect has a Richter Magnitude of more than 15! Yet, curiously, no atmospheric or thermal radiation effects are noted.

Have fun.

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