Earliest plate tectonics tied down?

Papers that ponder the question of when plate tectonics first powered the engine of internal geological processes are sure to get read: tectonics lies at the heart of Earth science. Opinion has swung back and forth from ‘sometime in the Proterozoic’ to ‘since the very birth of the Earth’, which is no surprise. There are simply no rocks that formed during the Hadean Eon of any greater extent than 20 km2. Those occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay, which have been grossly mangled by later events. But there are grains of the sturdy mineral zircon ZrSiO4)  that occur in much younger sedimentary rocks, famously from the Jack Hills of Western Australia, whose ages range back to 4.4 Ga, based on uranium-lead radiometric dating. You can buy zircons from Jack Hills on eBay as a result of a cottage industry that sprang up following news of their great antiquity: that is, if you do a lot of mineral separation from the dust and rock chips that are on offer, and they are very small. Given a laser-fuelled SHRIMP mass spectrometer and a lot of other preparation kit, you could date them. Having gone to that expense, you might as well analyse them chemically using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to check out their trace-element contents. Geochemist Simon Turner of Macquarie University in Sydney, Australia, and colleagues from Curtin University in Western Australia and Geowissenschaftliches Zentrum Göttingen in Germany, have done all this for 32 newly extracted Jack Hills zircons, whose ages range from 4.3 to 3.3 Ga (Turner, S. et al. 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the HadeanNature Communications, v. 11, article 1241; DOI: 10.1038/s41467-020-14857-1). Then they applied sophisticated geochemical modelling to tease out what kinds of Hadean rock once hosted these grains that were eventually eroded out and transported to come to rest in a much younger sedimentary rock.

Artist’s impression of the old-style hellish Hadean (Credit : Dan Durday, Southwest Research Institute)

Zircons only form duuring the crystallisation of igneous magmas, at around 700°C, the original magma having formed under somewhat hotter conditions – up to 1200°C for mafic compositions. In the course of their crystallising, minerals take in not only the elements of which they are mainly composed, zirconium, silicon and oxygen in the case of zircon , but many other elements that the magma contains in low concentrations. The relative proportions of these trace elements that are partitioned from the magma into the growing mineral grains are more or less constant and unique to that mineral, depending on the particular composition of the magma itself. Using the proportions of these trace elements in the mineral gives a clue to the original bulk composition of the parent magma. The Jack Hills zircons  mainly  reflect an origin in magmas of andesitic composition, intermediate in composition between high-silica granites and basalts that have lower silica contents. Andesitic magmas only form today by partial melting of more mafic rocks under the influence of water-rich fluid driven upwards from subducting oceanic lithosphere. The proportions of trace elements in the zircons could only have formed in this way, according to the authors.

Interestingly, the 4.2 Ga Nuvvuagittuq greenstone belt contains metamorphosed mafic andesites, though any zircons in them have yet to be analysed in the manner used by Turner et al., although they were used to date those late-Hadean rocks. The deep post-Archaean continental crust, broadly speaking, has an andesitic composition, strongly suggesting its generation above subduction zones. Yet that portion of Archaean age is not andesitic on average, but a mixture of three geochemically different rocks. It is referred to as TTG crust from those three rock types (trondhjemite, tonalite and granodiorite). That TTG nature of the most ancient continental crust has encouraged most geochemists to reject the idea of magmatic activity controlled by plate tectonics during the Archaean and, by extension, during the preceding Hadean. What is truly remarkable is that if mafic andesites – such as those implied by the Jack Hills zircons and found in the Nuvvuagittuq greenstone belt – partially melted under high pressures that formed garnet in them, they would have yielded magmas of TTG composition. This, it seems, puts plate tectonics in the frame for the whole of Earth’s evolution since it stabilised several million years after the catastrophic collision that flung off the Moon and completely melted the outer layers of our planet. Up to now, controversy about what kind of planet-wide processes operated then have swung this way and that, often into quite strange scenarios. Turner and colleagues may have opened a new, hopefully more unified, episode of geochemical studies that revisit the early Earth . It could complement the work described in An Early Archaean Waterworld published on Earth-logs earlier in March 2020.

An Early Archaean Waterworld

In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.

How did the planets form?

Animation of the 3-D shape of planetesimal Arrokoth. (Credit: Roman Tkachenko, NASA)

The latest addition to knowledge of the Solar System looks a bit like a couple of potatoes that have lain together and dried over several years. It also has a name – Arrokoth – that might have been found in a novel by H.P. Lovecraft. In fact Arrokoth meant ‘sky’ in the extinct Powhatan language once spoken by the native people of Chesapeake Bay. The planetesimal was visited by the New Horizons spacecraft two years after it had flown by Pluto (see; Most exotic geology on far-off Pluto, Earth-logs 6 April 2016). It is a small member of the Kuiper Belt of icy bodies. Data collected by a battery of imaging instruments on the spacecraft has now revealed that it has a reddish brown coloration that results from a mixture of frozen methanol mixed with a variety of organic compounds including a class known as tholins – the surface contains no water ice. Arrokoth is made of two flattened elliptical bodies (one 20.6 × 19.9 × 9.4 km the smaller 15.4 × 13.8 × 9.8 km) joined at a ‘waist’. Each of them comprises a mixture of discrete ‘terrains’ with subtly different surface textures and colours, which are likely to be earlier bodies that accreted together. On 13 February 2020 a flurry of three papers about the odd-looking planetesimal appeared in Science.

The smooth surface implies a lack of high-energy collisions when a local cluster of initially pebble sized icy bodies in the sparsely populated Kuiper Belt gradually coalesced under extremely low gravity. The lack of any fractures suggests that the accretions involved relative speeds of, at most, 2 m s-1; slow-walking speed or spacecraft docking (McKinnon, W.B. and a great many more 2020. The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt. Science, article eaay6620; DOI: 10.1126/science.aay6620). The authors regard this quiet, protracted, cool accretion to have characterised at least the early stages of planet formation in the Outer Solar System. The extent to which this can be extrapolated to the formation of the giant gas- and ice worlds, and to the rocky planets and asteroids of the Inner Solar System is less certain, to me at least. It implies cold accretion over a long period that would leave large worlds to heat up only through the decay of radioactive isotopes. Once large planetesimals had accreted, however that had happened, the greater their gravitational pull the faster other objects of any size would encounter them. That scenario implies a succession of increasingly high-energy collisions during planet formation.

This hot-accretion model, to which most planetary scientists adhere, was supported by a paper published by Science a day before those about Arrokoth hit the internet (Schiller, M. et al. 2020. Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth. Science Advances, v. 6, article eaay7604; DOI: 10.1126/sciadv.aay7604). This work hinged on the variation in the proportions of iron isotopes among meteorites, imparted to the local gas and dust cloud after their original nucleosynthesis in several supernovas in the Milky Way galaxy during pre-solar times. Iron found in different parts of the Earth consistently shows isotopic proportions that match just one class of meteorites: the CI carbonaceous chondrites. Yet there are many other silicate-rich meteorite classes with =different iron-isotope proportions. Had the Earth accreted from this mixed bag by random ‘collection’ of material over a protracted period prior to 4.54 billion years ago, its overall iron-isotope composition would have been more like the average of all meteorites than that of just one class. The authors conclude that Earth’s accretion, and probably that of the smaller body that crashed with it to form the Moon at about 4.4 Ga, must have taken place quickly (<5 million years) when CI carbonaceous chondrites dominated the inner part of the protoplanetary disc.

See also: Barbuzano, J. 2020. New Horizons Reveals Full Picture of Arrokoth . . . and How Planets Form. Sky & Telescope

Mineral grains far older than the Solar System

If a geologist with broad interests was asked, ‘what are the oldest materials on Earth?’ she or he would probably say the Acasta Gneiss from Canada’s North West Territories at 4.03 billion years (Ga) (see: At last, 4.0 Ga barrier broken, November 2008. A specialist in the Archaean Eon might say the Nuvvuagittuq Greenstone Belt on the eastern shore of Hudson Bay (see: Archaean continents derived from Hadean oceanic crust, March 2017); arguably 4.28 Ga old. An isotope geochemist would refer to a tiny 4.4 Ga zircon grain that had been washed into the much younger Mount Narryer quartz sandstone in Western Australia (see: Pushing back the “vestige of a beginning”, January 2001). A real smarty pants would cite a 4.5 Ga old sample of feldspar-rich Lunar Highland anorthosite in the Apollo Mission archive in Houston, USA. The last is less than 100 Ma younger than the formation of the Solar System itself at 4.568 Ga. Yet there are meteorites that have fallen to Earth, which contain minute mineral  grains that were incorporated into the initial dust from which the planets formed. Until recently, the best known were white inclusions in a 2 tonne meteorite that fell near Allende in Mexico; the largest carbonaceous chondrite ever found. This class of meteorite represents the most primitive material in orbit around the Sun. Its tiny inclusions contain proportions of isotopes of a variety of elements that are otherwise unknown in any other material from the Solar System and they are older. The conclusion is that these dust-sized, presolar grains originated elsewhere in the galaxy, perhaps from supernovas or red-giant stars.

A presolar grain from the Murchison meteorite made up of silicon carbide crystals (credit: Janaína N. Ávila)

Carbonaceous chondrites, as their name suggests, contain a huge variety of carbon-based compounds and they have been closely examined as possible suppliers of the precursor chemicals for the origin of life. Another large example of this class fell near the town of Murchison in Victoria, Australia in 1969. The first people to locate fragments of the 100 kg body noted a distinct smell of methylated spirits and steam rising from it: when crushed half a century later it still smells like rotting peanut butter. The Murchison meteorite has yielded signs of 14 thousand organic compounds, including 70 amino acids. It has also been a target for extracting possible presolar grains. This entails grinding small fragments and then dissolving out the carbonaceous and silicate material using various reagent to leave the more or less inert silicon carbide grains. The residue contains the most durable grains: despite being described as ‘large’ they are of the order of only 10 micrometres across. Some are made of silicon carbide; the same as the well-known abrasive carborundum. Throughout their lifetime in interstellar space the grains have been bombarded by high-energy protons and helium nuclei which move through space at nearly the speed of light – generally known as ‘cosmic rays’. When interacting with other matter they behave much like the particles in the Large Hadron Collider, being able to transmute natural isotopes into others. Measuring the relative proportions of these isotopes in material that has been bombarded by cosmic rays enables their exposure time to be estimated. In the case of the Murchison presolar grains the isotopes of choice are those of the noble gas neon (Heck, P.R. and 9 others 2020. Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide. Proceedings of the National Academy of Sciences, 201904573; DOI: 10.1073/pnas.1904573117). Analyses of 40 such grains yielded ages from 4.6 to 7.5 Ga, i.e. up to 3 billion years before the Solar System formed. They are, indeed, exotic. The highest age exceeds that of the oldest from such previously measured by 1.5 billion years

Investigations up to now suggest that dusts amount to about 1 % of interstellar matter, the rest being gases, mainly hydrogen and helium. With the formation of the planets and the parent bodies of asteroids a high proportion of presolar grains would have accreted to them to be mixed with other, more common stuff. What Heck and colleagues have discovered puts the Solar System into a broad framework of time and space. The grains must have formed at some stage in the evolution of stars older and larger than the Sun, to be blown out into the interstellar medium of the Milky Way galaxy. One possibility is that about 7 billion years ago there was a burst of star formation in a nearby sector of the galaxy. How the resulting dust made its way to the concentration of interstellar matter that eventually formed the Sun and Solar System is yet to be commented on.

See also: Bennett, J.  2020 Meteorite Grains Are the Oldest Known Solid Material on Earth.  Smithsonian Magazine(online)  13 January 2020.

Active volcanic processes on Venus

Earth’s nearest neighbour, apart from the Moon, is the planet Venus. As regards size and estimated density it could be Earth’s twin. It is a rocky planet, probably with a crust and mantle made of magnesium- and iron-rich silicates, and its bulk density suggests a substantial metallic core. There the resemblance ends. The whole planet is shrouded in highly reflective cloud (possibly of CO2 ‘snow’) at the top of an atmosphere almost a hundred times more massive than ours. It consists of 96% CO2 with 3% nitrogen, the rest being mainly sulfuric acid: the ultimate greenhouse world, and a very corrosive one. Only the four Soviet Venera missions have landed on Venus to provide close-up images of its surface. They functioned only for a couple of hours, after having measured a surface temperature around 500°C – high enough to melt lead. One Venera instrument, an X-ray fluorescence spectrometer – did crudely analyse some surface rock, showing it to be of basaltic composition. The atmosphere is not completely opaque, being transparent to microwave radiation. So both its surface textures and elevation variation have been imaged several times using orbital radar. Unlike the Earth, whose dual-peaked distribution of elevation – high continents and low ocean floors thanks to plate tectonics – Venus has just one and is significantly flatter. No tectonics operate there. There are far fewer impact craters on Venus than on Mars and the Moon, and most are small. This suggests that the present surface of Venus is far younger than are theirs; no more than 500 Ma compared to 3 to 4 billion years.

Volcanic ‘pancake’ domes on the surface of Venus, about 65 km wide and 1 km high, imaged by orbital radar carried by NASA’s Magellan Mission.

Somehow, Venus has been ‘repaved’, most likely by vast volcanic outpourings akin to the Earth’s flood basalt events, but on a global scale. Radar reveals some 1600 circular features that are undoubtedly volcanic in origin and younger than most of the craters. They resemble huge pancakes and are thought to be shield volcanoes similar to those seen on the Ethiopian Plateau but up to 100 times larges. Despite the high surface temperature and a caustic atmosphere, chemical weathering on Venus is likely to be much slower than on Earth because of the dryness of its atmosphere. Also, unlike the hydration reactions that produce terrestrial weathering, on Venus oxidizing processes probably produce iron oxides, sulfides, some anhydrous sulfates and secondary silicates. These would change the reflective properties of originally fresh igneous rocks, a little like the desert varnish that pervades rocky surfaces in arid areas on Earth. A group of US scientists have devised experiments to reproduce the likely conditions at the surface of Venus to see how long it takes for one mineral in basalt to become ‘tarnished’ in this way (Filberto, J. et al. 2020. Present-day volcanism on Venus as evidenced from weathering rates of olivine. Science Advances, v. 6, article eaax7445; DOI: 10.1126/sciadv.aax7445). One might wonder why, seeing as the planet’s atmosphere hides the surface in the visible and short-wavelength infrared part of the spectrum, which underpins most geological remote sensing of other planetary bodies, such as Mars. In fact, that is not strictly true. Carbon dioxide lets radiation pass through in three narrow spectral ‘windows’ (centred on 1.01, 1.10, and 1.18 μm) in which fresh olivine emits more radiation when it is heated than does weathered olivine. So detecting and measuring radiation detected in these ‘windows’ should discriminate between fresh olivine and that which has been weathered Venus-style. Indeed it may help determine the degree of weathering and thus the duration of lava flow’s exposure.

Venus VNIR
Colour-coded image of night-time thermal emissivity over Venus’s southern hemisphere as sensed by VIRTIS on Venus Express (Credit: M. Gilmore 2017, Space Sci. Rev. DOI 10.1007/s11214-017-0370-8; Fig. 3)

The European Space Agency’s Venus Express Mission in 2006 carried a remote sensing instrument (VIRTIS) mainly aimed at the structure of Venus’s clouds and their circulation. But it also covered the three CO2 ‘windows’, so it could detect and image the surface too. The image above shows significant areas of the surface of Venus that strongly emit short-wave infrared at night (yellow to dark red) and may be slightly weathered to fresh. Most of the surface in green to dark blue is probably heavily weathered. So the data may provide a crude map of the age of the surface. However, Filberto et al’s experiments show that olivine weathers extremely quickly under the surface conditions of Venus. In a matter of months signs of the fresh mineral disappeared. So the red areas on the image may well be lavas that have been erupted in the last few years before VIRTIS was collecting data, and perhaps active eruptions. Previous suggestions have been that some lava flows on large volcanoes are younger than 2.5 Ma and possible even younger than 0.25 Ma. Earth’s ‘evil twin’ now seems to be vastly more active, as befits a planet in which mantle-melting temperatures (~1200°C) are far closer to the surface as a result of the blanketing effect of its super-dense atmosphere.

Should you worry about being killed by a meteorite?

In 1994 Clark Chapman of the Planetary Science Institute in Arizona and David Morrison of NASA’s Ames Research Center in California published a paper that examined the statistical hazard of death by unnatural causes in the United States (Chapman, C. & Morrison, D. 1994. Impacts on the Earth by asteroids and comets: assessing the hazard. Nature, v. 367, p. 33–40; DOI:10.1038/367033a0). Specifically, they tried to place the risk of an individual being killed by a large asteroid (~2 km across) hitting the Earth in the context of more familiar unwelcome causes. Based on the then available data about near-Earth objects – those whose orbits around the Sun cross that of the Earth – they assessed the chances as ranging between 1 in 3,000 and 1 in 250,000; a chance of 1 in 20,000 being the most likely. The results from their complex calculations turned out to be pretty scary, though not as bad as dying in a car wreck, being murdered, burnt to death or accidentally shot. Asteroid-risk is about the same as electrocution, at the higher-risk end, but significantly higher than many other causes with which the American public are, unfortunately, familiar: air crash; flood; tornado and snake bite. The lowest asteroid-risk (1 in 250 thousand) is greater than death from fireworks, botulism or trichloroethylene in drinking water; the last being 1 in 10 million.

Chapman and Morrison cautioned against mass panic on a greater scale than Orson Welles’s 1938 CBS radio production of H.G. Wells’s War of the Worlds allegedly resulted in. Asteroid and comet impacts are events likely to kill between 5,000 and several hundred million people each time they happen but they occur infrequently. Catastrophes at the low end, such as the 1908 Tunguska air burst over an uninhabited area in Siberia, are likely to happen once in a thousand years. At the high end, mass extinction impacts may occur once every hundred million years. As might be said by an Australian, ‘No worries, mate’! But you never know…

Michelle Knapp’s Chevrolet Malibu the morning after a stony-iron mmeteorite struck it. Bought for US$ 300, Michelle sold the car for US$ 25,000 and the meteorite fetched US$ 50,000 (credit: John Bortle)

How about ordinary meteorites that come in their thousands, especially when the Earth’s orbit takes it through the former paths taken by disintegrating comets? When I was a kid rumours spread that a motor cyclist had a narrow escape on the flatlands around Kingston-upon-Hull in East Yorkshire, when a meteorite landed in his sidecar: probably apocryphal. But Michelle Knapp of Peeskill, New York, USA had a job for the body shop when a 12 kg extraterrestrial object hit her Chevrolet Malibu, while it was parked in the driveway. In 1954, Ann Hodges of Sylacauga, Alabama was less fortunate during an afternoon nap on her sofa, when a 4 kg chondritic meteorite crashed through her house roof, hit a radiogram and bounced to smash into her upper thigh, badly bruising her. For an object that probably entered the atmosphere at about 15 km s-1, that was indeed a piece of good luck resulting from air’s viscous drag, the roof impact and energy lost to her radiogram. The offending projectile became a doorstop in the Hodge residence, before the family kindly donated it to the Alabama Museum of Natural History. Another fragment of the same meteorite, found in a field a few kilometres away, fetched US$ 728 per gram at Christie’s auction house in 2017. Perhaps the most unlucky man of the 21st century was an Indian bus driver who was killed by debris ejected when a meteorite struck the dirt track on which he was driving in Tamil Nadu in 2016 – three passengers were also injured. Even that is disputed, some claiming that the cause was an explosive device.

Extraterrestrial sugar

The coding schemes for Earth’s life and evolution (DNA and RNA), its major building blocks and basic metabolic processes have various sugars at their hearts. How they arose boils down to two possibilities: either they were produced right here by the most basic, prebiotic processes or they were supplied from interplanetary or interstellar space. All kinds of simple carbon-based compounds turn up in spectral analysis of regions of star formation, or giant molecular clouds: CN, CO, C­2H, H2CO up to 10 or more atoms that make up recognisable compounds such as benzonitrile (C6H5CN). Even a simple amino acid (glycene –CH2NH2COOH) shows up in a few nearby giant molecular clouds. Brought together in close proximity, instead of dispersed through huge volumes of near-vacuum, a riot of abiotic organic chemical reactions could take place. Indeed, complex products of such reactions are abundant in carbonaceous meteorites whose parent asteroids formed within the solar system early in its formation. Some contain a range of amino acids though not, so far, the five bases on which genetics depends: in DNA adenine, cytosine, guanine and thymine (replaced by uracil in RNA). Yet, surprisingly, even simple sugars have remained elusive in both molecular clouds and meteorites.

Artist’s impression of the asteroid belt from which most meteorites are thougtht to originate (Credit: NASA/JPL)

A recent paper has broken through that particular barrier (Furukawa, Y. et al. 2019. Extraterrestrial ribose and other sugars in primitive meteorites. Proceedings of the National Academy of Sciences. Online; DOI: 10.1073/pnas.1907169116). Yoshihiro Furukawa and colleagues analysed three carbonaceous chondrites and discovered traces of 4 types of sugars. It seems that sugar compounds have remained elusive because those now detected are at concentrations thousands of times lower than those of amino acids. Contamination by terrestrial sugars that may have entered the meteorites when they slammed into soil is ruled out by their carbon isotope ratios, which are very different from those in living organisms. One of the sugars is ribose, a building block of RNA (DNA needs deoxyribose). Though a small discovery, it has great significance as regards the possibility that the components needed for living processes formed in the early Solar System. Moon formation by giant impact shortly after accretion of the proto-Earth would almost certainly have  destroyed such organic precursors. So, if the Earth’s surface was chemically ‘seeded’ in this way it is more likely to have occurred at a later time, perhaps during the Late Heavy Bombardment 4.1 to 3.8 billion years ago (see: Did mantle chemistry change after the late heavy bombardment? In Earth-logs September 2009)

Ordovician ice age: an extraterrestrial trigger

The Ordovician Period is notable for three global events; an explosion in biological diversity; an ice age, and a mass extinction. The first, colloquially known as the Great Ordovician Biodiversification Event, occurred in the Middle Ordovician around 470 Ma ago (see The Great Ordovician Diversification, September 2008) when the number of recorded fossil families tripled. In the case of brachiopods, this seems to have happened in no more than a few hundred thousand years. The glacial episode spanned the period from 460 to 440 Ma and left tillites in South America, Arabia and, most extensively, in Africa. Palaeogeographic reconstructions centre a Gondwanan ice cap in the Western Sahara, close to the Ordovician South Pole. It was not a Snowball Earth event, but covered a far larger area than did the maximum extent the Pleistocene ice sheets in the Northern Hemisphere. It is the only case of severe global cooling bracketing one or the ‘Big Five’ mass extinctions of the Phanerozoic Eon. In fact two mass extinctions during the Late Ordovician rudely interrupted the evolutionary promise of the earlier threefold diversification, by each snuffing-out almost 30% of known genera.

ord met
L-chondrite meteorite in iron-stained Ordovician limestone together with a nautiloid (credit: Birger Schmitz)

A lesser-known feature of the Ordovician Period is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in sedimentary rocks, particularly those exposed around the Baltic Sea (Schmitz, B. and 19 others 2019. An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science Advances, v. 5(9), eaax4184; DOI: 10.1126/sciadv.aax4184). Yet there is not a sign of any major impact of that general age, and the meteoritic anomaly occupies a 5 m thick sequence at the best studied site in Sweden, representing about 2 Ma of deposition, rather than the few centimetres at near-instantaneous impact horizons such as the K-Pg boundary. Intact meteorites are almost exclusively L-chondrites dated at around 466 Ma. Schmitz and colleagues reckon that the debris represents the smashing of a 150 km-wide asteroid in orbit between Mars and Jupiter. Interestingly, L-chondrites are more abundant today and in post-Ordovician sediments than they were in pre-Ordovician records, amounting to about a third of all finds. This suggests that the debris is still settling out in the Inner Solar System hundreds of million years later. Not long after the asteroid was smashed a dense debris cloud would have entered the Inner Solar System, much of it in the form of dust.

The nub of Schmitz et al’s hypothesis is that considerably less solar radiation fell on Earth after the event, resulting in a sort of protracted ‘nuclear winter’ that drove the Earth into much colder conditions. Meteoritic iron falling the ocean would also have caused massive phytoplankton blooms that sequestered CO2 from the Ordovician atmosphere to reduce the greenhouse effect. Yet the cooling seems not to have immediately decimated the ‘booming’ faunas of the Middle Ordovician. Perhaps the disruption cleared out some ecological niches, for new species to occupy, which may explain sudden boosts in diversity among groups such as brachiopods. Two sharp jumps in brachiopod species numbers are preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains in one Middle Ordovician sequence. One possibility, suggested in an earlier paper (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53; DOI: 10.1038/ngeo.2007.37)  is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).

See also: Sokol, J. 2019. Dust from asteroid breakup veiled and cooled Earth. Science, v. 365, pp. 1230: DOI: 10.1126/science.365.6459.1230, How the first metazoan mass extinction happened (Earth-logs, May 2014)

Last day of the dinosaurs

As they say, ‘everyone knows’ that the dinosaurs were snuffed out, except, of course, for those that had evolved to become birds and somehow survived. When it happened is known quite precisely – at the end of the Cretaceous (66.043 ± 0.011 Ma) – and there were two possible causal mechanisms: emissions from the Deccan Trap flood basalts and/or the Chicxulub impact crater. But what was the Cretaceous-Palaeogene (K-Pg) boundary event actually like? Many have speculated, but now there is evidence.

In 2016 a deep-sea drilling rig extracted rock core to a depth of 1.35 km beneath the sea floor off Mexico’s Yucatan Peninsula, slightly off the centre of the circular Chicxulub structure (see K-T (K-Pg) boundary impact probed, November 2016). This venture was organised and administered jointly by the International Ocean Discovery Program IODP) and the International Continental Scientific Drilling Program (ICDP) as Mission Specific Platform Expedition no. 364. Results from the analysis of the cored rock sequence have been generating pulses of excitement among palaeontologists, petrologists and planetary scientist on a regular basis. The science has been relatively slow to emerge in peer-reviewed print. Appetites have been whetted and the first substantial paper is about the bottom 130 metres of the core (Gulick, S.P.S. and 29 others 2019. The first day of the Cenozoic. Proceedings of the National Academy of Sciences. 9 September 2019; DOI: 10.1073/pnas.1909479116). It might seem as though the publication schedule has been stage managed to begin with, literally, the ‘bang’ itself.

The deepest 20 m thick layer is mainly silicate glass. It was formed in the seconds after the 12 km-wide impactor arrived to smash through the water and sea-floor sediments of the early Caribbean Sea, at speed of around 20 Km s-1. It vaporised water and rock as well as shoving aside the surrounding sea and blasting debris skyward and outward. In an instant a new hole in the crust was filled with molten rock. The overlying rock is a veritable apple-crumble of shattered debris mixed with and held together by glass, and probably formed as water flowed into the crater to result in explosive reaction with the molten crystalline crust beneath. The fragments lessen in size up the core, probably reflecting ejected material mixed in the displaced seawater. Impact specialists have estimated that this impactite layer formed in little more than ten minutes after collision. The glass-laden breccia is abruptly capped by bedded sediments, considered to have been delivered by the backwash of a huge, initial tsunami. In them are soils and masses of charcoal, from the surrounding land areas, scorched and burnt by the projectile’s entry flash, inundated by the tsunami and then dragged out to sea as it receded. These are the products of the hours following the impact as successive tsunamis swashed to and fro across the proto-Caribbean Basin; hence ‘The first day of the Cenozoic’, of Gulick et al.’s title.

IMGP6086
Artist’s impression of the Chicxulub impact (Credit: Barcroft Productions for the BBC)

Other cores drilled beyond the scope of the Chicxulub crater during offshore oil exploration show a sequence of limestones with thick beds of gypsum (CaSO4.2H2O). Yet the crater debris itself contains no trace of this mineral. Around 325 Gt of sulfur, almost certainly in the form of SO2, entered the atmosphere on that first day, adding to the dust. Ending up in the stratosphere as aerosols it would have diffused solar radiation away from the surface, resulting in an estimated 25°C global cooling that lasted 25 years. The sulfur oxides in the lower atmosphere ended up in acid rain that eventually acidified the upper ocean to devastate shallow-marine life.

See also: Amos, J. 2019. The day the dinosaurs’ world fell apart. (BBC News 10 September 2019); Rocks at asteroid impact site record first day of dinosaur extinction (Phys.org); Wei-Haas, M. 2019. Last day of the dinosaurs’ reign captured in stunning detail.  National Geographic, 9 September 2019.

A major Precambrian impact in Scotland

The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol
Grossly simplified geological map of NW Scotland (credit: British Geological Survey)

Continue reading “A major Precambrian impact in Scotland”