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.
Where did all our water come from? The Earth’s large complement of H2O, at the surface, in its crust and even in the mantle, is what sets it apart in many ways from the rest of the rocky Inner Planets. They are largely dry, tectonically torpid and devoid of signs of life. For a long while the standard answer has been that it was delivered by wave after wave of comet impacts during the Hadean, based on the fact that most volatiles were driven to the outermost Solar System, eventually to accrete as the giant planets and the icy worlds and comets of the Kuiper Belt and Oort Cloud, once the Sun sparked its fusion reactions That left its immediate surroundings depleted in them and enriched in more refractory elements and compounds from which the Inner Planets accreted. But that begs another question: how come an early comet ‘storm’ failed to ‘irrigate’ Mercury, Venus and Mars? New geochemical data offer a different scenario, albeit with a link to the early comet-storms paradigm.
Three geochemists from the Institut für Planetologie, University of Münster, Germany, led by Gerrit Budde have been studying the isotopes of the element molybdenum (Mo) in terrestrial rocks and meteorite collections. Molybdenum is a strongly siderophile (‘iron loving’) metal that, along with other transition-group metals, easily dissolves in molten iron. Consequently, when the Earth’s core began to form very early in Earth’s history, available molybdenum was mostly incorporated into it. Yet Mo is not that uncommon in younger rocks that formed by partial melting of the mantle, which implies that there is still plenty of it mantle peridotites. That surprising abundance may be explained by its addition along with other interplanetary material after the core had formed. Using Mo isotopes to investigate pre- and post-core formation events is similar to the use of isotopes of other transition metals, such as tungsten (see Planetary science, May 2016). Continue reading “Earth’s water and the Moon”
The spacecraft Chang’E-4 landed on the far side of the Moon in January; something of a triumph for the Peoples’ Republic of China as it was a first. It was more than a power gesture at a time of strained relations between the PRC and the US, for it carried a rover (Yutu2) that deploys a panoramic camera, ground penetrating radar, means of assessing interaction of the solar wind with the lunar surface, and a Visible and Near-infrared Imaging Spectrometer (VNIS). The lander module itself bristles with instrumentation, but Yutu2 (meaning Jade Rabbit) has relayed the first scientific breakthrough.
The New Yorker magazine normally features journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. So it is odd that this Condé Nast glossy for the chattering classes snaffled online what may be the geological scoop of the 21st century so far (Preston, D. 2019. The day the dinosaurs died. The New Yorker 8 April 2019 issue). The paper that lies at the centre of the story had not been published and nor had the issue of The New Yorker in which Douglas Preston’s story was scheduled for publication. The very day (29 March 2019) that Britain was thwarted of its Brexit moment the world’s media was frothing with news about the end of another era; the Mesozoic. The paper itself was published online on April Fools’ Day with a title that is superficially arcane (DePalma, R.A. and 11 others 2019. A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Science, early online publication;p DOI: 10.1073/pnas.1817407116). But its contents are the stuff of dreams for any aspiring graduate student of palaeontology; the Indiana Jones opportunity.
An ‘onshore surge deposit’ occurs at many Western Hemisphere sites where the K-Pg boundary outcrops in terrestrial or shallow-marine sediments. The closer to the Chicxulub crater north of Mexico’s Yucatan Peninsula the more obvious they are, for they result from the tsunamis that immediately followed the asteroid impact. Lead author Robert DePalma, now of the University of Kansas, became focussed on the dinosaur-rich, Late Cretaceous Hell Creek Formation of North Dakota as an undergraduate. Accepted for graduate studies he was directed to a project on the fauna of lacustrine sediments close to the K-Pg boundary layer, which is well-known in the area, and that’s what he has been engaged with ever since. In 2012 he was guided to a remarkable locality by a rockhound, disappointed because it exposed extremely fossil-rich sediments but was so soft that none could be extracted intact with a hammer and chisel. It turned out to have resulted from a surge along a sinuous river that had washed debris onto a point-bar deposit at the inside of a meander. The debris includes remains of both marine and terrestrial organisms and shows clear signs of having been swept upriver, i.e. from the sea and possibly the result of a tsunami. Being capped by a thin, iridium-rich layer of impactite, the 1.5 metre surge deposit is part of the K-Pg boundary layer, and probably represented only a few hours before being blanketed by ejecta.
This Event Deposit comprises two graded, fining-upwards units and thus two distinct surges, with a thin mat of vegetation fragments immediately below the Ir-rich clay cap that also contains sparse shocked quartz grains. The Event Deposit contains altered glass spherules throughout, which cgradually become smaller higher in the 1.5 m sequence. Some of the larger spherules produced ‘micro-craters’ in the sediments. Fossils include marine ammonite fragments (some still nacreous) and freshwater fish (paddlefish and sturgeon). The fish are so complete as to suggest an absence of scavengers. The paper itself contains little of the information that dominated Preston’s New Yorker article and the global media coverage. This included clear evidence that the fish ingested spherules, found clogging their gills and possible causing their death. There are examples of spherules embedded in amber formed from plant sap, which suggests sub-aerial fall of ejecta, and among the marine faunal samples are teeth of fish and reptiles (see DePalma et al’s Supplemental Data). The most startling finds reported by Preston are nowhere to be found in DePalma et al’s paper or its supplement. These include possible dinosaur feathers; a fragment of ceratopsian dinosaur skin attached to a hip bone; a burrow containing a mammal jaw that penetrates the K-Pg boundary layer; dinosaur remains, including an egg (complete with embryo) and hatchlings of dinosaurian groups found at deeper levels in the Hell Creek Formation. Previously, palaeontologists had found no dinosaur remains less than 3 m below the K-Pg boundary layer anywhere on Earth, prompting the suggestion that they had become extinct before the near-instantaneous effects of Chicxulub, and were perhaps victims of the general effects of the Deccan Trap volcanism. If verified in later peer-reviewed publications, DePalma et al’s work would help resolve the gradual vs sudden hypotheses for the end-Cretaceous mass extinction.
Preston reports some academic scepticism about DePalma’s work, and emphasises his showmanship at conferences; for instance, he named the site ‘Tanis’ after the ancient city in Egypt featured in the 1981 film Raiders of the Lost Ark. There are geophysical queries too. If the inundation was by the on-shore effects of a tsunami it doesn’t tally with the abundance of ejecta fallout of glass spherules: tsunamis propagate in shallow seawater at speeds less than 50 km h-1 and more slowly still in channels, whereas impact ejecta travel much faster. This is acknowledged in the paper’s supplement, and the paper refers to a seiche wave activated by seismic waves associated with the Chicxulub impact which could have arrived in North Dakota at about the same time as its ejecta blanket. The paper’s authorship includes the imprimatur of other authorities in different geoscientific fields, including Walter Alvarez, jointly famed with his father Luis for the discovery of the K-Pg boundary horizon and its impact connections in 1981. So it carries considerable weight. No doubt further comment and further papers on the Tanis site will emerge: DePalma has yet to complete his PhD. It may become the lagerstätte of the K-Pg extinction; in DePalma’s words, ‘It’s like finding the Holy Grail clutched in the bony fingers of Jimmy Hoffa, sitting on top of the Lost Ark.’ …
The nickel in stainless steel, the platinum in catalytic converters and the gold in jewellery, electronic circuits and Fort Knox should all be much harder to find in the Earth’s crust. Had the early Earth formed only by accretion and then the massive chemical resetting mechanism of the collision that produced the Moon all three would lie far beyond reach. Both formation events would have led to an extremely hot young Earth; indeed the second is believed to have left the outer Earth and Moon completely molten. All three are siderophile metals and have such a strong affinity for metallic iron that they would mostly have been dragged down to each body’s core as it formed in the early few hundred million years of the Earth-Moon system, leaving very much less in the mantle than rock analyses show. This emerged as a central theme at the Origin of Life Conference held in Atlanta GA, USA in October 2018. The idea stemmed from two papers published in 2015 that reported excessive amounts in basaltic material from both Earth and Moon of a tungsten isotope (182W) that forms when a radioactive isotope of hafnium (182Hf), another strongly siderophile metal, decays. Hafnium too must have been strongly depleted in the outer parts of both bodies when their cores formed. The excesses are explained by substantial accretion of material rich in metallic iron to their outer layers shortly after Moon-formation, some being in large metallic asteroids able to penetrate to hundreds of kilometres. Hot iron is capable of removing oxygen from water vapour and other gases containing oxygen, thereby being oxidised. The counterpart would have been the release of massive amounts of hydrogen, carbon and other elements that form gases when combined with oxygen. The Earth’s atmosphere would have become highly reducing.
Had the atmosphere started out as an oxidising environment, as thought for many decades, it would have posed considerable difficulties for the generation at the surface of hydrocarbon compounds that are the sine qua non for the origin of life. That is why theories about abiogenesis (life formed from inorganic matter) hitherto have focussed on highly reducing environments such as deep-sea hydrothermal vents where hydrogen is produced by alteration of mantle minerals. The new idea revitalises Darwin’s original idea of life having originated in ‘a warm little pond’. How it has changed the game as regards the first step in life, the so-called ‘RNA World’ can be found in a detailed summary of the seemingly almost frenzied Origin of Life Conference (Service, R.F. 2019. Seeing the dawn. Science, v. 363, p. 116-119; DOI: 10.1126/science.363.6423.116).
Isotope geochemistry has also entered the mix in other regards, particularly that gleaned from tiny grains of the mineral zircon that survived intact from as little as 70 Ma after the Moon-forming and late-accretion events to end up (3 billion years ago) in the now famous Mount Narryer Quartzite of Western Australia. The oldest of these zircons (4.4 Ga) suggest that granitic rocks had formed the earliest vestiges of continental crust far back in the Hadean Eon: Only silica-rich magmas contain enough zirconium for zircon (ZrSiO4) to crystallise. Oxygen isotope studies of them suggest that at that very early date they had come into contact with liquid water, presumably at the Earth’s surface. That suggests that perhaps there were isolated islands of early continental materials; now vanished from the geological record. A 4.1 Ga zircon population revealed something more surprising: graphite flakes with carbon isotopes enriched in 12C that suggests the zircons may have incorporated carbon from living organisms.
Such a suite of evidence has given organic chemists more environmental leeway to suggest a wealth of complex reactions at the Hadean surface that may have generated the early organic compounds needed as building blocks for RNA, such as aldehydes and sugars (specifically ribose that is part of both RNA and DNA), and the amino acids forming the A-C-G-U ‘letters’ of RNA, some catalysed by the now abundant siderophile metal nickel. One author seems gleefully to have resurrected Darwin’s ‘warm little pond’ by suggesting periodic exposure above sea level of abiogenic precursors to volcanic sulfur dioxide that could hasten some key reactions and create large masses of such precursors which rain would have channelled into ‘puddles and lakes’. The upshot is that the RNA World precursor to the self-replication conferred on subsequent life by DNA is speculated to have been around 4.35 Ga, 50 Ma after the Earth had cooled sufficiently to have surface water dotted with specks of continental material.
There are caveats in Robert Services summary, but the Atlanta conferences seems set to form a turning point in experimental palaeobiology studies.
Because it is so geologically active the Earth progressively erases signs of asteroid and comet impacts, by erosion, burial or even subduction in the case of the oceanic record. As a result, the number of known craters decreases with age. To judge the influence of violent extraterrestrial events in the past geologists therefore rely on secondary outcomes of such collisions, such as the occasional presence in the sedimentary record of shocked quartz grains, glassy spherules and geochemical anomalies of rare elements. The Moon, on the other hand, is so geologically sluggish that its surface preserves many of the large magnitude impacts during its history, except for those wiped out by later such events. For instance, a sizeable proportion of the lunar surface comprises its dark maria, which are flood basalts generated by gigantic impacts around 4 billion years ago. Older impacts can only be detected in its rugged, pale highland terrains, and they have been partially wiped out by later impact craters. The Moon’s surface therefore preserves the most complete record of the flux and sizes of objects that have crossed its orbit shared with the Earth.
The Earth presents a target thirteen times bigger than the cross sectional area of the Moon so it must have received 13 times more impacts in their joint history. Being about 81 times as massive as the Moon its stronger gravitational pull will have attracted yet more and all of them would have taken place at higher speeds. The lunar samples returned by the Apollo Missions have yielded varying ages for impact-glass spherules so that crater counts combined with evidence for their relative ages have been calibrated to some extent to give an idea of the bombardment history for the Earth Moon System. Until recently this was supposed to have tailed off exponentially since the Late Heavy Bombardment between 4.0 to 3.8 billion years ago. But the dating of the lunar record using radiometric ages of the small number of returned samples is inevitably extremely fuzzy. A team of planetary scientists from Canada, the US and Britain has developed a new approach to dating individual crater using image data from NASA’s Lunar Reconnaissance Orbiter (LRO) launched in 2009 (Mazrouei, S. et al. 2019. Earth and Moon impact flux increased at the end of the Paleozoic. Science, v. 363, p. 253-257; DOI: 10.1126/science.aar4058).
The method that they devised is, curiously, based on thermal imagery from the LRO’s Diviner instrument which records the Moon’s surface temperature. Comparison of day- and night-time temperatures produces a measure of surface materials’ ability to retain heat known as thermal inertia. A material with high thermal inertia stays warmer for longer at night. When a crater forms it partly fills with rock fragments excavated by the impact. When fresh these are full of large blocks of rock that were too massive to be blasted away. But these blocks are exposed to bombardment by lesser projectiles for the lifetime of the crater, which steadily reduces them to smaller fragments and eventually dust. Blocks of solid rock retain significantly more solar heat than do gravelly to dust-sized materials: thermal inertia of the crater floor therefore decreases steadily with age.
As well as day- and night thermal data provided by the Diviner instrument, from which thermal inertia values are calculated, the LRO deploys two cameras that capture black and white images of the surface in the visible range, with a resolution of about a metre. They enable the blockiness of crater floors to be estimated. Sara Mazrouei and colleagues measured blockiness and thermal inertia of the floors of 111 craters more than 10 km across, ages of nine of which had been modelled independently using counts of smaller craters that subsequently accumulated on their floors shown by even finer resolution images from the Japanese Kaguya orbiter. Their results are surprising. About 290 Ma ago the rate of large impacts on the Moon increased by a factor of 2.6. This might explain why the Neoproterozoic and Palaeozoic Eras are deficient in terrestrial craters. Another inference from the results is that the number of objects in Earth-crossing orbits suddenly increased at the end of the Carboniferous. Maybe that resulted from an episode of collisions and break-up of large bodies in the Asteroid Belt or, perhaps, some kind of gravitational perturbation by Jupiter. The age-distribution of large craters on Earth is no help because of their ephemeral nature. Moreover, apart from Chicxulub that is bang on the K-Pg boundary, there is little evidence of an increase in impact-driven mass extinctions in the Mesozoic and Cenozoic. Nor for that matter did igneous activity or sediment deposition undergo any sudden changes. There are sediments that seem to have formed as a result of tsunami devastation, but none greater in magnitude than could have been caused by major earthquakes. Or … maybe geologists should have another look at the stratigraphic record.
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