Category Archives: Planetary, extraterrestrial geology, and meteoritics

A hint of life on Mars

We can be certain that life was around on Planet Earth around 3.5 billion years ago, if not before, because unmetamorphosed sedimentary rocks of that age from Western Australia in which stromatolites occur contain a black to brownish, structureless material known as kerogen. The material is a hodgepodge of organic compounds that form during the breakdown of proteins and carbohydrates in living matter. It is the source material for petroleum compounds when kerogen-rich rocks are heated during burial. The vast bulk of organic compounds preserved on Earth are in the form of ancient kerogen, whose mass exceeds that of the living biosphere by about 10 thousand times. A good sign that it does represent ancient life lies in sedimentary kerogen’s depletion in ‘heavy’ 13C compared with 12C (negative values of δ13C), because in metabolising carbon dioxide living cells preferentially use the lighter of these two isotopes. Conceivably, 13C can be removed from inorganic carbon by metamorphic processes, so low values of δ13C in metasediments from West Greenland might be organically derived or, equally, they might not.

At the time of writing, geoscientists specialising in Martian matters had become excited by some results from the geochemical system aboard the surviving functional NASA rover. Curiosity has slowly been making its way up Mount Sharp at the centre of Gale Crater near to Mars’s equator. Analysis of high-resolution images taken from orbit suggest that the rocks forming the mountain are sediments. the lowest and oldest strata are suspected to have been deposited in a crater lake when conditions were warmer and wetter on Mars, about 3 billion years ago. Curiosity was equipped with a drill to penetrate and sample sediment unaffected by ultraviolet radiation that long ago would have destroyed any hydrocarbons exposed at the surface. In late 2016, before the rover had reached the lake sediments, the drill’s controller broke down. Since then, Curiosity had moved on to younger, less promising sediments. More than a year later mission engineers fixed the problem and the rover backtracked to try again. Heating the resulting samples to almost 900°C yields any volatile components as a gas to a mass spectrometer, results from which give clues to the molecules released.

‘Selfie’ of Curiosity rover en route to Mount Sharp. (Credit: NASA)

The Sample Analysis at Mars (SAM) team report a range of thiophenic, aromatic and aliphatic molecules of compounds of carbon, hydrogen and sulfur (Eigenbrode, J.L and 21 others 2018. Organic matter preserved in 3-billion-year-old mudtsones at Gale crater, Mars. Science, v. 360, p. 1096-1101; doi:10.1126/science.aas9185). The blend of pyrolysis products closely resembles those which form from heated terrestrial kerogens and coals, but also from pyrolysis of carbonaceous chondrite meteorites. So, the presence of Martian kerogen is not proven. But the results are so promisingly rich in hydrocarbons that another weapon in SAM’s armoury will be deployed, dissolving organic compounds directly from the drill cuttings. This may provide more convincing evidence of collagen. Yet only when samples are returned to labs on Earth will there be a chance to say one way or the other that there was once life on Mars. The results reported in Science’s 8 June issue will surely add weight to the clamour for the Mars 2020 sample-return mission to be funded. Whether or not there is life on Mars demands a great deal more investment still…

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

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

Hadean potentially fertile for life

The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km2) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.

Ice cliffs on Mars

An illustration of what Mars might have looked...

An illustration of what Mars might have looked like during an ice age between 2.1 million and 400,000 years ago, when Mars’s axial tilt is believed to have been much larger than today.  (credit: Wikipedia)

For Mars to support life and for life to have emerged there demand water that is readily accessible from the surface. There is evidence that in the distant past liquid water may have flowed across the Martian surface to erode river-like features, some associated with the vast canyon system of Valles Marineris. That feature is thought to have been initiated by tectonic forces and perhaps flowing magma, but it shows definite signs of water erosion. Water in great volume was released during the Noachian phase of Mars’s evolution possibly by major impacts 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment). Large tracts of the Martian surface that are more muted than Valles Marineris show topographic features reminiscent of huge braided stream systems. Water may have covered vast, low-lying areas in the planet’s Northern Hemisphere to form an early ocean. Yet today the Red Planet seems extremely dry and its thin atmosphere shows only minute traces of water vapour – it is dominated by carbon dioxide. Results from various rovers deployed across its surface and from Mars orbiting satellites have, however, revealed signs of waterlain sediments and minerals that can only have formed by the breakdown of igneous rocks by water. Signs that liquid water continues to flow occasionally down steep slopes, such as rill-like features and ephemeral darkened patches, have been much disputed.

Mars does have an ice cap at its North Pole that waxes and wanes with its seasons, but rather than melting during Martian ‘summers’ the ice sublimates directly to water vapour. Conversely, the polar ices probably form from frost. Yet, astonishingly, there appear to be active glaciers complete with flow lines and moraines, but chances are that some of them are sediment flows ‘lubricated’ by frost binding together mineral particles and boulders that undergoes pressure-induced regelation. Data from orbiting neutron and gamma-ray spectrometers reveal that between 60°N and 60°S the top metre of Martian soil contains between 2 to 18% of ice, making it akin to terrestrial permafrost. So, contrary to its appearance Mars is rich in water, but almost exclusively in solid form. Until very recently, the bulk was thought to be as a matrix binding together sediments, accessible to future crewed mission in useful volumes only by surface mining. That somewhat pessimistic view has now changed dramatically.

Monochrome HiRISE image of a cliff on Mars (the pinkish swath is a simulated natural colour image – see below). beneath the cliff is a zone of jumbled ground formed by cliff collapses. (credit: NASA)

Careful study of fine resolution imagery from the HiRISE instrument on the Mars Reconnaissance Orbiter at latitudes a little less than 60° has centred on cliffs formed by recent erosion (Dundas, C.M and 11 others 2018. Exposed subsurface ice sheets in the Martian mid-latitudes. Science, v. 359, p. 199-201; doi: 10.1126/science.aao1619). Colin Dundas of the US Geological Survey, Flagstaff, Arizona, and US colleagues used the multispectral capacities of HiRISE data to study the composition of sedimentary layers exposed in the cliffs. In eight cases, the cliffs contained layered, almost pure blue ice tens of metres thick and only a few metres below the surface. The cliffs seem to have formed as ice has sublimated where exposed, thereby undermining to sedimentary cover. Below the cliffs are jumbled zones of collapsed material. Being so close to the surface and underlain by apparently ice-free sediments, the layered ice sheets must be geologically quite young.

Simulated natural-colour HiRISE image of a Martian cliff showing nearly pure water ice in blues. Note the layered structure that may represent seasonal variations during the period of ice formation (credit: NASA)

Unlike the Earth, whose axial tilt is stabilised to a large degree by the Moon’s gravity, Mars’s two tiny moons have little effect of this kind. So Mars’s axis wobbles between its current 25° tilt to as much as 45°. This results in large climatic shifts, of which there have been an estimated forty over the last 5 million years. At high tilts solar energy heats up the poles and releases water vapour by accelerated sublimation to be laid down at lower latitudes as frost or snow. Mars’s present tilt suggests that it is experiencing a cold episode so that wind blown dust has covered and preserved mid-latitude ice sheets over tens of thousand years. Nearly pure ice is easier to exploit than permafrost layers. Yet optimism among enthusiasts for a crewed Mars mission and eventual colonisation is tempered by the latitudes of the discoveries. While ready supplies of water from ice and CO2 from the Martian atmosphere give the ingredients for oxygen, methane through catalysis of CO2 and hydrogen, agricultural photosynthesis and all kinds of other useful chemistry, low latitudes offer the most assured solar energy supplies. Latitudes around 55° are frigid and dark during Martian winters; perhaps totally inhospitable. So the remote-sensing search is likely to continue in cliffs closer to the ‘tropics’ of Mars.

Lid tectonics on Earth

Geoscientists have become used to thinking of the Earth as being dominated by plate tectonics in which large, rigid plates of lithosphere move across the surface. They are driven mainly by the sinking of cold, densified lithosphere in slabs at subduction zones. The volume of recycled slabs is replaced by continual supply of mafic magma to form oceanic crust at constructive margins. Such a process has long been considered to have reached far back into the Precambrian past and there are lively debates concerning when this modus operandi first arose and what preceded it. Now that we know more about other rocky planets and moons it appears that Earth is the only one on which plate tectonics has occurred. The other, more common, behaviour is dominated by stagnancy, although some worlds evidence volcanism and resurfacing as a result of giant impacts. Their subdued activity has come to be known as ‘lid tectonics’, in which their highly viscous innards slowly convect beneath a rigid, stagnant lid through which thermal energy is lost by convection: they are ‘one-plate’ systems. Although Earth loses internal heat by conduction through plate interiors, a large amount dissipates by convection associated with constructive margins: the oceanic parts of its plates lose heat laterally, as they grow older. Six papers in an advance, online issue of the free-access journal Geoscience Frontiers are concerned with the issue of terrestrial lid-tectonics and whether or not it dominated the Earth repeatedly in its Precambrian history.

A model is emerging for a hot, early Earth that was dominated by a form of lid tectonics (Bédard, J.H. 2018 Stagnant lids and mantle overturns: implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics. Geoscience Frontiers, v. 9, 19-49; https://doi.org/10.1016/j.gsf.2017.01.005). Bedard’s model centres on lithosphere that was so weak because of its temperature that its subduction was impossible. Density of the lithosphere rarely increased above that of the mantle because the necessary mineralogical changes were not achieved – those involved in plate tectonics require low-temperature, high-pressure metamorphism as oceanic lithosphere is driven down at modern subduction zones. Even if such reactions did happen, the lithosphere would have been too weak to sustain slab-pull force and dense lithosphere would have simply ‘dripped’ back to the mantle. Mantle convection in a hotter Earth would have been in the form of large, long-lived upwelling zones rather than the relatively ephemeral and narrow plumes known today. Low density materials resulting from magma fractionation, the precursors of continental crust, would have been shifted willy-nilly across the face of the planet to collide. accrete and undergo repeated partial melting. In Bedard’s view, plate tectonics arose as Earth’s heat production waned below a threshold that permitted rigid lithosphere, probably in the late Archaean, to dominate after 2.5 Ga.

Bédard’s impression of an early Archaean lid-tectonic scenario. (credit: Jean H Bédard 2018, Figure 3B)

A radically different view is that stagnant-lid episodes alternated with periods of limited subduction and plate tectonics in the Archaean. Some Archaean cratons – the so-called ‘granite-greenstone terrains – seems to provide geological evidence for lid tectonics (Wyman, D. 2018. Do cratons preserve evidence of stagnant lid tectonics? Geoscience Frontiers, v. 9, 19-49; https://dx.doi.org/10.1016/j.gsf.2017.02.001). Others, such as the famous Isua supracrustal belt in West Greenland hint at plate tectonics. John Piper, of Liverpool University in Britain, argues from a series of Archaean palaeomagnetic polar wander curves that in three periods – ~2650 to 2200 Ma, 1550 to 1250 Ma, and 800 to 600 Ma – the poles shifted comparatively slowly with respect to the cratons providing the magnetic data; a feature that Piper ascribes to dominant lid tectonics (Piper, J.D.A., 2018. Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian Times: palaeomagnetism confirms prolonged quasi-integrity and absence of Supercontinent Cycles. Geoscience Frontiers, v. 9, p. 61-89; https://doi.org/10.1016/j.gsf.2017.07.009). Similarly, there is some evidence based on the geochemical variation of basaltic rocks derived from the mantle. Through the Archaean, geochemical changes roughly follow cycles in the abundance of zircon radiometric ages and other geological changes that may reflect plate- and lid-tectonic episodes (Condie, K.C. 2018. A planet in transition: the onset of plate tectonics on Earth between 3 and 2 Ga? Geoscience Frontiers, v. 9, p. 51-60; https://doi.org/10.1016/j.gsf.2016.09.001). Interestingly, the age-frequency plot of almost three thousand Archaean and Hadean zircons recovered from the famous 1.6 Ga old sandstones of the Jack Hills Formation in Western Australia reveals similar cycles that may reflect such tectonic fluctuations in the Hadean (Wang, Q. & Wilde, S.A. 2017. New constraints on the Hadean to Proterozoic history of the Jack Hills belt,Western Australia. Gondwana Research, v. 55, p. 74-91; https://doi.org/10.1016/j.gr.2017.11.008). Since zircons are most likely to crystallize from intermediate and felsic magmas – i.e. precursors of continental material – their abundance in the Jack Hills rocks suggests that their source must have been in the 3.7 to 3.3 Ga gneisses on which the younger sediments rest. That is, part of those Archaean gneisses may well be made up of Hadean continental material that was repeatedly reworked and maybe remelted since such crust first appeared (in the form of surviving zircons) around 4.4 to 4.5 Ga, perhaps during vigorous lid-tectonic regimes.

Possible evolution of magmatic and tectonic styles for large silicate planets. (Credit: Stern et al. 2018, Figure 3)

Based on their reassessment of tectonic activity revealed by 8 rocky planets and moons Robert Stern of the University of Texas (Dallas) and colleagues from ETH-Zurich suggest a possible evolutionary sequence of tectonics and magmatism that Earth-like bodies might go through (Stern, R.J. et al. 2018. Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids. Geoscience Frontiers, v. 9, p. 103-119 ; https://doi.org/10.1016/j.gsf.2017.06.004). In their scheme plate tectonics requires certain conditions of lithospheric density and strength to evolve and suggest that, depending on planetary characteristics, slab-pull driven tectonics is likely to be preceded and followed by stagnant lid tectonics, to give perhaps a cyclical geotectonic history.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Banded iron formations (BIFs) reviewed

This image shows a 2.1 billion years old rock ...

2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)

During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent. But commercial reserves are confined mainly to sedimentary sequences dating from about 3 to 2 billion years ago. They are not the only commercial iron formations, but dominate supplies from estimated reserves of around 105 billion tons. From a non-commercial standpoint they are among the most revealing kinds of sediment as regards the Earth system and its evolution. All scientific aspects of BIFs and similar Fe-rich sediments are reviewed in a recent volume of Earth Science Reviews. (Konhauser, K.O. and 12 others 2017. Iron formations: a global record of Neoarchaean to Palaeoproterozoic environmental history. Earth Science Reviews, v. 172, p. 140-177; doi: 10.1016/j.earscirev.2017.06.012).

The chemical, mineral and isotopic compositions of BIFs form a detailed repository of the changing composition of seawater during a crucial period for the evolution of Earth and life – the transition from an anoxic surface environment to one in which water and air contained a persistent proportion of oxygen, known as the Great Oxidation Event (GOE). Paradoxically, BIFs are highly oxidized rocks, the bulk of which formed when other rocks show evidence for vanishingly small amounts of oxygen in the surface environment. The paradox began to be resolved when it was realized that ocean-ridge basaltic volcanism and sea-floor hydrothermal activity would have released vast amounts of soluble, reduced iron-2 into anoxic seawater, in the upper parts of which the first photosynthetic organisms evolved. Evidence for the presence of such cyanobacteria first appears around 3.5 billion years ago, in the form of carbonates whose structure suggests they accumulated from growth of microbial mats. Oxygen generated by photosynthesis in iron-rich water immediately acts to oxidize soluble iron-2 to iron-3 to yield highly insoluble iron oxides and hydroxides and thus deposits of BIFs. While oceans were iron-rich, formation of ironstones consumed ecologically available oxygen completely.

Other biological processes seem to have been involved in ironstone formation, such as photosynthesis by other bacteria that used dissolved iron-2 instead of water as a reductant for CO2, to release iron-3 instead of oxygen. That would immediately combine with OH­ ions in water to precipitate iron hydroxides. Konhauser and colleagues cogently piece together the complex links in chemistry and biology that emerged in the mid- to late Archaean to form a linkage between carbon- and iron cycles, which themselves influenced the evolution of other, less abundant elements in seawater from top to bottom. The GOE is at the centre. The direct evidence for it lies in the sudden appearance of ancient red soils at about 2.4 billion years, along with the disappearance of grains of sulfides and uranium oxides – both readily oxidized to soluble products – from riverine sandstones, which signifies significant oxygen in the atmosphere. Yet chemical changes in Precambrian marine sediments perhaps indicate that oxygen began to rise in ocean water as early as 3 billion years ago. That suggests that for half a billion years biogenic and abiogenic processes in the oceans were scavenging oxygen as fast as it could be produced so that only tiny amounts, if any, escaped into the atmosphere. Among other possible factors, oceanic methane emissions from methanogen bacteria may have consumed any atmospheric oxygen – today methane lasts only for about 9 years before reaction with oxygen forms CO2. If and when methanogens declined free oxygen would have been more likely to survive in the atmosphere.

The theme running through the review is that of changing and linked interactions between life and the inorganic world, mantle, lithosphere, hydrosphere and atmosphere that involved all available chemical elements. The dominant chemical process, as it is today, was the equilibrium between oxidation and reduction – the loss and gain of electrons among possible chemical reactions and in metabolic processes. Ironstones were formed more commonly between 3 to 2 Ga than at any time before or since, and form a substantial part of that periods sedimentary record. Their net product and that of the protracted organic-inorganic balancing act – oxygenation of the hydrosphere and atmosphere – opened the way for eukaryote organisms, their reproduction by way of the splitting and recombination of nuclear DNA and their evolutionary diversification into the animal and plant life that we know today and of which we are a part. It is possible that even a subtly different set of global processes and interactions set in motion during early evolution of a planet apparently like Earth may have led to different and even unimaginable biological outcomes in later times. The optimism of exobiologists should be tempered by this detailed review.

Mega-impacts and tectonics

Because they are fast as well as weighty, destination-Earth asteroids and comets pack quite a punch. That is because their kinetic energy is proportional to the square of their speed (at least 13 km s-1) as well as half their mass. So, even all one half a kilometre across carries an energy a hundred times the solar energy received by Earth in a year, and a great deal more when compared with geothermal heat production. Much of the focus on the effects of impact events has dwelled on the upper crust, the oceans and atmosphere. Yet they also have huge seismic effects, with a proportion of their shock effect being dissipated throughout the entire planet. One obvious consequence would be a thermal anomaly directly beneath the crater as well as some thinning of the lithosphere and body waves affecting the rest of the solid Earth.

Thermal and mechanical processes lie at the core of tectonics, so a big question has been ‘Could impacts create mantle plumes or set new tectonic processes in motion?’ There has been speculation of diverse kinds since impacts became popular following the link between the Chicxulub crater and the end-Cretaceous mass extinction, discovered in 1980. But ‘educated guesses’ have generated more hot air than clear conclusions. Much as most of us are modelling-averse, a mathematical approach is the only option in the welcome absence of any severe extraterrestrial battering to which scientists have borne witness. With refined algorithms that cover most of the nuances of projectiles and targets – conservation of mass, energy and momentum in the context of the solid Earth behaving as a viscous medium –  Craig O’Neill and colleagues at Macquairie University, Australia, and the Southwest Research Centre in Boulder, CO USA, have simulated possible tectonic outcomes during plausible bombardment scenarios during the Hadean (O’Neil, C. et al. 2017. Impact-driven subduction on the Hadean Earth. Nature Geoscience, v. 10, p. 793-797; DOI: 10.1038/NGEO3029).

It appears that truly gargantuan objects – radius >500 km – are required to stimulate sufficient thermal anomalies that would lead to mantle upwellings whose evolution might lead to subduction at their margins. One at the limit posed by lunar cratering history (~1700 km radius) could have resulted in wholesale subduction of the entire lithosphere present at the time about 4 Ma after the impact. In the Hadean, it is likely that the lithosphere would have had a roughly mantle composition, so that the density excess needed for slab descent would have been merely temperature dependent. Note: after the onset of a basalt-capped lithosphere heat flow would have needed to be below the limit at which basalt converts to eclogite at high pressures, and thus to a density greater than that of the mantle, for continuing subduction. The authors’ Hadean scenario is one of episodic subduction dependent on the projectile flux and magnitude; i.e. with an early Hadean with stop-start subduction waning to tectonic stagnation and then a restart during the Late Heavy Bombardment after 4.1 Ga. Evidence for this is clearly scanty, except for Hadean zircons, whose presence indicates differentiation of early magmas with a peak between 4.0 to 4.2 Ga, in which magnetic intensities are preserved that are roughly as predicted by the scenario.

No impacts preserved in Precambrian to Recent times suggest extraterrestrial objects with the power to induce significant changes to global tectonics.

The winter of dinosaurs’ discontent

Under the auspices of the International Ocean Discovery Program (IODP), during April and May 2016 a large team of scientists and engineers sank a 1.3 km deep drill hole into the offshore, central part of the Chicxulub impact crater, which coincided with the K-Pg mass extinction event. Over the last year work has been underway to analyse the core samples aimed at investigating every aspect of the impact and its effects. Most of the data is yet to emerge, but the team has published the results of advanced modelling of the amount of climate-affecting gases and dusts that may have been ejected (Artemieva, N. et al. 2017. Quantifying the release of climate-active gases by large meteorite imp-acts with a case study of Chicxulub. Geophysical Research Letters, v. 44; DOI: 10.1002/2017GL074879).  . From petroleum exploration in the Gulf of Mexico the impact site is known to have been underlain by about 2.5 to 3.5 km of Mesozoic sediments that include substantial amounts of limestones and evaporitic anhydrite (CaSO4) – thicknesses of each are of the order of a kilometre. The impact would inevitably have yielded huge volumes of carbon- and sulfur dioxide gases, as well as water vapour plus solid and molten ejecta. The first, of course, is a critical greenhouse gas, whereas SO2 would form sulfuric acid aerosols if it entered the stratosphere. They are known to block incoming solar radiation. So both warming and cooling influences would have been initiated by the impact. Dust-sized ejecta that lingered in the atmosphere would also have had climatic cooling effects. The questions that the study aimed to answer concerns the relative masses of each gas that would have reached more than 25 km above the Earth to have long-term, global climatic effects and whether the dominant effect on climate was warming or cooling. Both gases would have added the environmental effects of making seawater more acid.

Chicxulub2

3-D simulation of the Chicxulub crater based on gravity data (credit: Wikipedia)

Such estimates depend on a large number of factors beyond the potential mass of carbonate and sulfate source rocks. For instance: how big the asteroid was; how fast it was travelling and the angle at which it struck the Earth’s surface determine the kinetic energy involved and the impact mechanism. How that energy was distributed between atmosphere, seawater and the sedimentary sequence, together with the pressure-temperature conditions for the dissociation of calcite and anhydrite all need to be accounted for by modelling. Moreover, the computation itself becomes extremely long beyond estimates for the first second or so of the impact. Earlier estimates had been limited by computer speeds to only the first few seconds of the impact and could not allow for other than vertical impacts. The new study, by supercomputers and improved algorithms, used a likely 60° angle of impact, new data on mineral decomposition and simulated the first 15 to 30 seconds. The results suggested that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO2 were ejected, compared with earlier estimates of 40-560 Gt of sulfur and 350-3,500 Gt of CO2.  The greater proportion of sulfur release to the stratosphere pushes the model decisively towards global cooling, probably over a lengthy period – perhaps centuries. Taking dusts into account implies that visible sunlight would also have been blocked, devastating the photosynthetic base of the global food chain, in the sunlit parts of oceans as well as on land.

But we have to remember that these are the results of a theoretical model. In the same manner as this study has thrown earlier modeling into doubt, more data – and there will be a great many from the Chicxulub drill core itself – and more sophisticated computations may change the story significantly. Also, the other candidate for the mass extinction event, the flood basalt volcanism of the Deccan Traps, and its geochemical effects on the climate have yet to be factored in. The next few lines of Shakespeare’s soliloquy for  Richard III may well emerge from future work

… Made glorious summer by this sun of York;
And all the clouds that lour’d upon our house
In the deep bosom of the ocean buried …

See also: BBC News comment on 31 October 201

 

Shock and Er … wait a minute

Chicxulub2

Enhanced gravity map of the Chicxulub crater (credit: Wikipedia)

Michael Rampino has produced a new book (Rampino, M.R. 2017. Cataclysms: A New Geology for the Twenty-First Century. Columbia University Press; New York). As the title subtly hints, Rampino is interested in mass extinctions and impacts; indeed quite a lot more, as he lays out a hypothesis that major terrestrial upheavals may stem from gravitational changes during the Solar System’s progress around the Milky Way galaxy. Astronomers reckon that this 250 Ma orbit involves wobbling through the galactic plane and possibly varying distributions of mass – stars, gas, dust and maybe dark matter – in a 33 Ma cycle. Changing gravitational forces affecting the Solar System may possibly fling small objects such as comets and asteroids towards the Earth on a regular basis. In the 1980s and 90s Rampino and others linked mass extinctions, flood-basalt outpourings and cratering events, with a 27 Ma periodicity. So the books isn’t entirely new, though it reads pretty well.

Such ideas have been around for decades, but it all kicked off in 1980 when Luis and Walter Alvarez and co-workers published their findings of iridium anomalies  at the K-Pg boundary and suggested that this could only have arisen from a major asteroid impact. Since it coincided with the mass extinction of dinosaurs and much else besides at the end of the Cretaceous it could hardly be ignored. Indeed their chance discovery launched quite a bandwagon. The iridium-rich layer also included glass spherules, shocked mineral grains, soot and other carbon molecules –nano-scale diamonds, nanotubes and fullerenes whose structure is akin to a geodesic dome – and other geochemical anomalies. Because the Chicxulub crater off the Yucatán Peninsula of Mexico is exactly the right age and big enough to warrant a role in the K-Pg extinction, these lines of evidence have been widely adopted as the forensic smoking gun for other impacts. In the last 37 years every extinction event horizon has been scrutinized to seek such an extraterrestrial connection, with some success, except for exactly coincident big craters.

The K-Pg event is the only one that shows a clear temporal connection with a small mountain falling out of the sky, most of the others seeming to link with flood basalt events and their roughly cyclical frequency – but hence Rampino’s Shiva hypothesis that impacts may have caused the launch of mantle plumes from the core-mantle boundary. Others have used the ‘smoking gun’ components to link lesser events to a cosmic cause, the most notorious being the 2007 connection to the extinction of the North American Pleistocene megafauna and the start of the Younger Dryas return of glacial conditions. Since 1980 alternative mechanisms for producing most of the impact-connected materials have been demonstrated. It emerged in 2011 that nano-diamonds and fullerenes may form in a candle flame and their global distribution could be due to forest fires. And now it seems that shocked mineral grains can form during a lightning strike (Chen, J. et al. 2017. Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters, v. 44, p. 8757-8768; doi:10.1002/2017GL073843). Shocked or not, quartz and feldspar grains are resistant enough to be redistributed into sediments. Although platinum-group metals, such as iridium, are likely to be mainly locked away in Earth’s core, some volcanic exhalations and many flood basalts – especially those with high titanium contents – significantly are enriched in them. So even the Alvarez’s evidence for a K-Pg impact has an alternative explanation. Rampino is to be credited for acknowledging that in his book.

An awful lot of ideas about rare yet dreadful events in the biosphere depend, like many criminal cases, on the ‘weight of evidence’ and defy absolute proof. The evidence generally permits alternatives, such the cunning Verneshot hypothesis for the extinction-flood basalt connection supported by one of the founders of plate tectonics, W. Jason Morgan. As regards The K-Pg extinction, it is certain that a very large mass did fall on Chicxulub at the time of the mass extinction, whereas the Deccan flood basalts span a million years or so either side. But the jury is out on whether either or both did the deed. For other events of this scale and larger ones the money is on internal origins. As for Rampino’s galactic hypothesis, the statistics are decidedly dodgy, but chasing down more forensics is definitely on the cards.

English: From source; an animation showing the...

Animation showing the Chicxulub Crater impact. ( credit: University of Arizona, Space Imagery Center)

Wildfires and climate at the K-Pg boundary

It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO2 and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO2 emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here