A new twist to Pleistocene climate cycles

The combined gravitational pulls of the sun and moon modulate variations in local tidal range. High spring tides occur when the two bodies are opposed at full moon or in roughly the same direction at new Moon. When the positions of sun and moon are at right angles (1st quarter and 3rd quarter) their gravitational pulls partly cancel each other to give neap tides. Consequently, there are two tidal cycles every lunar month.  In a similar way, the varying gravitational pulls of the planets during their orbital cycles impart a repetitive harmony to Earths astronomical behaviour. But their combined effects are on the order of tens of thousand years. Milutin Milankovich (1879-1958), a Serbian engineer, pondered on the possible causes of Earth’s climatic variations, particularly the repetition of ice ages. He was inspired by 19th century astronomers’ suggestion that maybe the gravitational effects of other planets might be a fruitful line of research. Milankovich focussed on how the shape of Earth’s orbit, the tilt of its rotational axis and the way the axis wobbles like that of a spinning top affect the amount of solar heating at all points on the surface: the effects of varying eccentricity, obliquity and precession, respectively.

 Earlier astronomers had calculated cycles of gravitational effects on Earth of the orbits of Jupiter and Saturn of the three attributes of Earth’s astronomical behaviour and found periods of about 100, 41 and 23 thousand years (ka) respectively. The other 3 inner planets and the much more distant giants Uranus and Neptune also have gravitational effects on Earth, but they are negligible compared with those of the two nearest giant planets, because gravitation force varies with mass and inversely with the square of distance. Sadly, Milankovich was long dead when his hypothesis of astronomical climate forcing was verified in 1976 by frequency analysis of the record of oxygen isotopes in foraminifera found in two ocean sediment core from the Southern Indian Ocean. It revealed that all three periods interfered in complex ways during the Late Pleistocene, to dominate variations in sea-surface temperatures and the fluctuating volume of continental ice sheets for which δ18O is a proxy (see: Odds and ends about Milankovich and climate change; February 2017).

Precession of the axis of a spinning top and that of the Earth. At present the northern end of Earth’s axis points to what we now call the Pole Star. Around 11.5 ka from now it will point to the star Vega

This was as revolutionary for climatology as plate tectonics was for geology. We now know that in the early Pleistocene glacial-interglacial cycles were in lockstep with the 41 ka period of axial obliquity, and since 700 ka followed closely – but not perfectly – the 100 ka orbital eccentricity forcing. The transitional period between 1.25 and 0.7 Ma (the Mid-Pleistocene Transition or MPT) suggested neither one nor the other. Milankovich established that axial tilt variations have the greatest influence on solar heating, so the early 41 ka cycles were no surprise. But the dominance of orbital eccentricity on the last 700 ka certainly presented a puzzle, for it has by far the weakest influence on solar heating: 10 times less than those of axial obliquity and precession. The other oddity concerns the actual effect of axial precession on climate change. There are no obvious 23 ka cycles in the climate record, despite the precession signal being clear in frequency analysis and its effect on solar heating being almost as powerful as obliquity and ten times greater than that of orbital eccentricity. Precessional wobbling of the axis controls the time of year when one hemisphere or the other is closest to the Sun. At one extreme it will be the Northern and 11.5 ka later it will be the Southern. The times of solstices and equinoxes also change relative to the calendar that we use today.

There is an important, if obvious, point about astronomical forcing of climate. It is always there, with much the same complicated interactions between the factors: human activities have absolutely no bearing on them. Climatic ‘surprises’ are likely to continue!

Changes in ice-rafted debris (IRD) since 1.7 Ma in a sediment core from the North Atlantic (orange fill) compared with its oxygen-isotope (δ18O) record of changes in continental ice cover (blue fill). At the top are the modelled variations in 23 ka axial precession (lilac) and 41 ka obliquity (green). The red circles mark major interglacial episodes, blue diamonds show the onset of significant ice rafting and orange diamonds are terminations of ice-rafting (TIR). (Credit: Barker et al., Fig. 2)

Sea temperature and ice-sheet volume are not the only things that changed during the Pleistocene. Another kind of record from oceanic sediments concerns the varying proportion in the muddy layers of abnormally coarse sand grains and even small pebbles that have been carried by icebergs; they are known as ice-rafted debris (IRD). The North Atlantic Ocean floor has plenty of evidence for them appearing and disappearing on a layer-by-layer basis. They were first recognised in 1988 by an oceanographer called Helmut Heinrich, who proposed that six major layers rich in IRD in North Atlantic cores bear witness to iceberg ‘armadas’ launched by collapse, or ablation, at the front of surging ice sheets on Scandinavia, Greenland and eastern Canada. Heinrich events, along with Dansgaard-Oeschger events (rapid climatic warming followed by slower cooling) in the progression to the last glacial maximum have been ascribed to a variety of processes  operating on a ‘millennial’ scale. However, ocean-floor sediment cores are full of lesser fluctuations in IRD, back to at least 1.7 Ma ago. That record offers a better chance of explaining fluctuations in ice-sheet ablation. A joint European-US group has investigated their potential over the last decade or so (Barker, S. et al. 2022. Persistent influence of precession on northern ice sheet variability since the early Pleistocene. Science, v. 376, p. 961-967; DOI: 10.1126/science.abm4033). The authors noted that in each glacial cycle since 1.7 Ma the start of ice rafting consistently occurred during a time of decreasing axial obliquity. Yet the largest ablation events were linked to minima in the precession cycles. In the last 700 ka, such extreme events are associated with the terminations of each ice age.

In the earlier part of the record, the 41 ka obliquity ‘signal’ was sufficient to drive glacial-interglacial cycles, hence their much greater regularity and symmetry than those that followed the Mid-Pleistocene Transition. The earlier ice sheets in the Northern Hemisphere also had consistently smaller extents than those after the MPT. Although the records show a role for precession in pre-MPT times in the form of ice-rafting events, the lesser effect of precession on summer warming at higher latitudes, compared with that of axial obliquity, gave it no decisive influence. After 700 ka the northern ice sheets extended much further south – as far as 40°N in North America – where summer warming would always have been commensurately greater than at high northern latitudes. So they were more susceptible to melting during the increased summer warming driven by the precession cycles. When maximum summer heating induced by axial precession in the Northern Hemisphere coincided with that of obliquity the ice sheets as a whole would have become prone to catastrophic collapse.

It is hard to say whether these revelations have a bearing on future climate. Of course, astronomical forcing will continue relentlessly, irrespective of anthropogenic greenhouse gas emissions. Earth has been in an interglacial for the last 11.5 ka, since the Younger Dryas; i.e. about half a precession cycle ago. The combination of obliquity- and precession-driven influences suggest that climate should be cooling and has been since 6,000 years ago, until the Industrial Revolution intervened. Can the gravitational pull of the giant planets prevent a runaway greenhouse effect, or will human effects defy astronomical forces that continually distort Earth’s astronomical behaviour?

Lower-mantle blobs may reveal relics of event going back to the Hadean

The World-Wide Standardised Seismograph Network (WWSSN) records the arrivals of waves generated by earthquakes that have passed through the Earth’s interior. There are two types of these body waves: S- or shear waves that move matter at right angles to their direction of movement; compressional or P-waves that are a little like sound waves as materials are compressed and expanded along the direction of movement. Like sound, P-waves can travel through solids, liquids and gases. Since liquids and gases are non-rigid they cannot sustain shearing, so S-waves only travel through the solid Earth’s mantle but not its liquid outer core. However, their speed is partly controlled by rock rigidity, which depends on the temperature of the mantle; the hotter the lower the mantle’s rigidity.

Analysis of the S-wave arrival times throughout the WWSSN from many individual earthquakes enables seismologists to make 3-D maps of how S-wave speeds vary throughout the mantle and, by proxy, the variation of mantle rigidity with depth. This is known as seismic tomography, which since the late 1990s has revolutionised our understanding of mantle plumes and subduction zones, and also the overall structure of the deep mantle. In particular, seismic tomography has revealed two huge, blob-like masses above the core-mantle boundary that show anomalously low S-wave speeds, one beneath the Pacific Ocean and another at about the antipode beneath Africa: by far the largest structures in the deep mantle. They are known as ‘large low-shear-wave-velocity provinces’ (LLSVPs) and until recently they have remained the enigmatic focus of much speculation around two broad hypotheses: ‘graveyards’ for plates subducted throughout Earth history; or remnants of the magma ocean thought to have formed when another protoplanet impacted with the early Earth to create the Moon about 4.4 billion years ago.

Three-dimensional rendition of seismic tomography results beneath Africa. Mantle with anomalously low S-wave speeds is show in red, orange and yellow. The faint grey overlay represents the extent of surface continental crust today – Horn of Africa at right and Cape Town at the lower margin – the blue areas near the top are oceanic crust on the floor od the Mediterranean Sea. (Image credit: Mingming Li/ASU)

Qian Yuan and Mingming Li of Arizone State University, USA have tried to improve understanding of the shapes of the two massive blobs (Yuan, Q. & Li, M. 2022. Instability of the African large low-shear-wave-velocity province due to its low intrinsic density. Nature Geoscience, v. 15  DOI: 10.1038/s41561-022-00908-3) using advanced geodynamic modelling of the seismic tomography. Their work reveasl that the Pacific LLSVP extends between 500 to 800 km above the core-mantle boundary. Yet that beneath Africa reaches almost 1000 km higher, at 1300 to 1500 km. Both of them are less rigid and therefore hotter than the surrounding mantle. In order to be stable they must be considerably denser than the rest of the mantle surrounding them. But, because it reaches much higher above the core, the African LLSVP is probably less dense than the Pacific one. A lower density suggests two things: the African blob may be less stable; the two blobs may have different compositions and origins.

Both the Pacific Ocean floor and the African continent are littered with volcanic rocks that formed above mantle plumes. The volcanic geochemistry above the two LLSVPs differs. African samples show signs of a source enriched by material from upper continental crust, whereas those from the Pacific do not. Yuan and Li suggest that the enrichment supports the ‘plate graveyard’ hypothesis for the African blob and a different history beneath the Pacific. The 3-D tomography beneath Africa (see above) shows great complexity, perhaps reflecting the less stable nature of the LLSVP. Interestingly, 80 % of the pipe-like African kimberlite intrusions that have brought diamonds up from mantle depths over that last 320 Ma formed above the blob.

But why are there just two such huge blobs of anomalous material that lie on opposite sides of the Earth rather than a continuous anomaly or lots of smaller ones? The subduction graveyard hypothesis is compatible with the last two distributions. In a 2021 conference presentation the authors suggest from computer simulations that the two blobs may have originated at the time of the Moon’s formation after a planetary collision (Yuan, Q. et al. 2021. Giant impact origin for the large low shear velocity provinces. Abstracts for the 52nd Lunar and Planetary Science Conference: Lunar and Planetary Institute, Houston). Specifically, they suggest that the LLSVPs originated from the mantle of the other planet (Theia) after its near complete destruction and melting, which sank without mixing through the magma ocean formed by the stupendous collision. Yet, so far, no geochemists have been bold enough to suggest that there are volcanic rocks of any age that reveal truly exotic compositions inherited from deep mantle material with such an origin. If Theia’s mantle was dense enough to settle through that of the Earth when both were molten, it would be sufficiently anomalous in its chemistry for signs to show up in any melts derived from it. There again, because of a high density it may never have risen in plumes to source any magma that reached the Earth’s surface …

Note added later: Simon Hamner’s Comment about alternative views on seismic tomography has prompted me to draw attention to something I wrote 19 years ago

‘Smoking gun’ for Younger Dryas trigger refuted

In 2018 airborne ice-penetrating radar over the far northwest of the Greenland revealed an impact crater as large as the extent of Washington DC, USA beneath the Hiawatha Glacier. The ice surrounding it was estimated to be younger than 100 ka. This seemed to offer a measure of support for the controversial hypothesis that an impact may have triggered the start of the millennium-long Younger Dryas episode of frigidity (12.9 to 11.7 ka). This notion had been proposed by a group of scientists who claimed to have found mineralogical and geochemical signs of an asteroid impact at a variety of archaeological sites of roughly this age in North America, Chile and Syria. A new study of the Hiawatha crater by a multinational team, including the original discoverers of the impact structure, has focussed on sediments deposited beyond the edge of the Greenland ice cap by meltwater streams flowing along its base. (Kenny, G.G. et al. 2022. A Late Paleocene age for Greenland’s Hiawatha impact structure. Science Advances, v.8, article eabm2434; DOI: 10.1126/science.eabm2434).

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

Where meltwater emerges from the Hiawatha Glacier downstream of the crater there are glaciofluvial sands and gravels that began to build up after 2010 when rapid summer melting began, probably due to global warming. As luck would have it, the team found quartz grains that contained distinctive planar features that are characteristic of impact shock. They also found pebbles of glassy impact melts that contain clasts of bedrock, further grains of shocked quartz and tiny needles of plagioclase feldspar that crystallised from the melt. Also present were small grains of the mineral zircon (ZrSiO4), both as pristine crystals in the bedrock clasts and porous, grainy-textured grains showing signs of deformation in the feldspathic melt rock. So, two materials that can be radiometrically dated are available: feldspars suitable for the 40Ar/39Ar method and zircons for uranium-lead (U-Pb) dating. The feldspars proved to be about 58 million years old; i.e. of Late Palaeocene age. The pristine zircon grains from bedrock clasts yielded Palaeoproterozoic U-Pb ages (~1915 Ma), which is the general age of the Precambrian metamorphic basement that underpins northern Greenland. The deformed zircon samples have a very precise U-Pb age of 57.99±0.54 Ma. There seems little doubt that the impact structure beneath the Hiawatha Glacier formed towards the beginning of the Cenozoic Era.

During the Palaeocene, Northern Greenland was experiencing warm conditions and sediments of that age show that it was covered with dense forest. The group that since 2007 has been advocating the influence of an impact over the rapid onset of the Younger Dryas acknowledges that the Hiawatha crater cannot support their view. But they have an alternative: an airburst of an incoming projectile. Although scientists know such phenomena do occur, as one did over the Tunguska area in Siberia on the morning of 30 June 1908. Research on the Tunguska Event has discovered  geochemical traces that may implicate an extraterrestrial object, but coincidentally the area affected is underlain by the giant SIberian Traps large igneous province that arguably might account for geochemical anomalies. Airbursts need to have been observed to have irrefutable recognition. Two posts from October 2021 – A Bronze Age catastrophe: the destruction of Sodom and Gomorrah? and Wide criticism of Sodom airburst hypothesis emerges – suggest that some scientists question the data used repeatedly to infer extraterrestrial events by the team that first suggested an impact origin for the Younger Dryas.

See also: Voosen, P, 2022. Controversial impact crater under Greenland’s ice is surprisingly ancient. Science, v. 375, article adb1944;DOI: 10.1126/science.adb1944

End-Cretaceous mass extinction occurred in northern spring

This post’s title seems beyond belief for an event that occurred 66 million years ago: how can geologists possibly say that with any conviction? The claim is based on fossil fishes found in the Late Cretaceous Hell Creek Formation of North Dakota (see: A bad day at the end of the Cretaceous. April, 2019), described in a paper published on 1 April 2019. The horizon that displays all the classic evidence for an impact origin for the K-Pg extinction is a freshwater sediment laid down by a surge into a river system: the upstream result of the mega-tsunami driven by the Chicxulub impact in the Gulf of Mexico. Amongst much else it contains intact marine ammonites – the last of their kind – and freshwater paddlefish and sturgeon. The fishes are preserved exquisitely, with no sign of scavenging. Parts of their gills are clogged with microscopic spherules made of impact glass. They are pretty good ‘smoking guns’ for an impact, and are accompanied by dinosaur remains – an egg with an embryo, hatchlings and even a piece of skin.

A group of scientists from the Netherlands, Sweden, Belgium and the UK examined thin sections of the fishes’ bones (During, M.A.D. et al. 2022. The Mesozoic terminated in boreal springNature online publication, 23 February 2022; DOI: 10.1038/s41586-022-04446-1). These revealed growth layers that show lines of arrested growth (LAGs) separated by thicker layers. Such LAGs in modern paddlefish and sturgeon bones may indicate conditions of low food availability in winter, most growth being during warmer times of year. Each bone that was examined has only a thin outer zone of accelerated growth following its last LAG. So it seems that each specimen died in the Northern Hemisphere spring. This was confirmed by variations within the cyclic zonation of the relative proportions of carbon isotopes 13C and 12C, expressed as δ13C. In the LAGs δ13C is lower than in the thicker zones, which is consistent with decreased prey availability in winter, but see below.

Thin sections of fish bones from the K-Pg boundary layer in the Hell Creek Formation, showing lines of arrested growth marked by red arrowheads. The outermost (top) LAGs are succeeded by only a thin zone of accelerated growth during their last weeks of the fishes’ lives (credit: During et al., Fig. 2)

The paper by During et al. follows one with very similar content from the same deposit that was published about 12 weeks earlier (DePalma, R.A., et al. 2021. Seasonal calibration of the end-cretaceous Chicxulub impact event. Nature Science Reports, v. 11, 23704; DOI: 10.1038/s41598-021-03232-9). Yet During et al. do not refer to it, despite acknowledging DePalma’s guidance in the field and his granting access to his team’s specimens: maybe due to poor communications … or maybe not. DePalma et al. note thatmodern sturgeons are able to spend winters in the sea, which may also explain the low δ13C in the LAGs, as well as decreased prey availability does. They also examined damage by leaf-mining insects in fossil leaves at the site, which supports the springtime extinction hypothesis. Another study in DePalma et al. is the size range of newly hatched fish of three different Families that are founds as fossils in the K-Pg deposit. By comparing them with the growth histories of closely-related modern hatchlings they conclude that perhaps late spring to early summer is implied. Whatever, both papers go on to discuss the implications of their basic conclusions. Spring is a particularly sensitive time for the life cycles of many organisms; i.e. annual reproduction and newborns’ early growth. But some groups of egg-laying animals, such as perhaps dinosaurs, require longer incubation periods than do others, e.g. birds, and may be more vulnerable to rapid environmental change. That may explain the demise of the dinosaurs while their close avian relatives, or at least some of them, survived.  Yet the season in the Southern Hemisphere when the Chicxulub impact occurred would have been autumn. That may go some way towards explaining evidence that ecological recovery from mass extinction in the southern continents seems to have been faster. Almost certainly, the impact would have induced a double climatic whammy: warming in its immediate aftermath followed by global cooling plus a shutdown of photosynthesis as dust clouds enveloped the planet. Then there is the issue of contamination by potentially toxic compounds raised by Chicxulub. The K-Pg boundary seems likely to run and run as a geoscientific story more than four decades since it was first proposed.

See also: Sample, I. 2022. Springtime asteroid ramped up extinction rates, say scientists. The Guardian, 23 September 2022.

A cometary air-burst over South America 12 thousand years ago

Earth-logs has previously covered quite a few hypotheses involving catastrophic astronomical events of the past, often returning to them as new data and ideas emerge. They range from giant impacts, exemplified in the mass extinction at the K-Pg boundary to smaller-scale events that may have coincided with important changes in climate, such as the sudden onset of the Younger Dryas, and a few that have been suggested as agencies affecting local human populations such as the demise of Sodom by a cosmogenic air-burst. Some of the papers that spurred the Earth-pages posts have been widely regarded in the geoscience community. Yet there have been others that many have doubted, and even condemned. For instance, data used by the consortium that suggested an extraterrestrial event triggered the frigid millennium of the Younger Dryas (YD) have been seriously and widely questioned. A sizeable number of the team that were under close scrutiny in 2008 joined others in 2019 to back the YD air-burst hypothesis again, using similarly ‘persuasive’ data from Chile. Members of the original consortium of academics also contributed to the widely disputed notion of a cosmic air-burst having destroyed a Bronze Age urban centre in Jordan that may, or may not, have been the site of the Biblical Sodom. Again, they cited almost the ‘full monty’ of data for high-energy astronomical events, but again no crater or substantial melt glass, apart from tiny spherules. Now another paper on much the same theme, but none of whose authors contributed to those based on possibly ‘dodgy’ data, has appeared in Geology (Schultz, P.H. et al. 2021. Widespread glasses generated by cometary fireballs during the Late Pleistocene in the Atacama Desert, Chile. Geology, published online November 2, 2021; doi: 10.1130/G49426.1).

Peter Schultz of Brown University, USA and colleagues from the US and Chile make no dramatic claims for death and destruction or climate destabilisation, and simply report a fascinating discovery. In 2012 one of the authors, Nicolas Blanco of the Universidad Santo Tomás in Santiago, Chile, found slabs made of glassy material up to half a metre across. They occurred in several 1 to 3 km2 patches over a wide area of the Atacama Desert. Resting on Pleistocene glacio-fluvial sediments, they had been exposed by wind erosion of active sand dunes. The glass is dark green to brown and had been folded while still molten. For the glass slabs to be volcanic bombs presupposes a nearby volcano, but although Chile does have volcanoes none of the active vents are close enough to have flung such large lumps of lava into the glass-strewn area. The glassy material also contains traces of vegetation, and varies a great deal in colour (brown to green). Its bulk chemical composition suggests melting of a wide variety of surface materials: quite unlike volcanic glasses.

Chilean glass occurrence: panorama of large glass fragments in the Atacama Desert; a specimen of the glass; thin section of glass showing bubbles and dusty particles (Credit: Schultz et al. 2021; Figs 1B, 2D and 2C)

Microscopic examination of thin sections of the glasses also reveals nothing resembling lava, except for gas bubbles. The slabs are full of exotic fragments, some of which closely resemble mineral assemblages found in meteorites, including nickel-rich sulfides embedded in ultramafic material. Others are calcium-, aluminium- and titanium-rich inclusions, such as corundum (Al2O3) and perovskite (CaTiO3), thought to have originated as very-high temperature condensates from the pre-solar nebula: like the celebrated ‘white inclusions’ in the Allende meteorite. Some minute grains resemble dust particles recovered by the NASA Stardust mission to Comet 81P/Wild-2 which returned samples to Earth in 2006. Zircon grains in the glasses, presumed to be locally derived, have been decomposed to zirconium oxide (baddeleyite), suggesting melting temperatures greater than 1670°C: far above the highest temperature found in lavas (~1200°C). Interestingly, the green-yellow silica glass strewn over the Sahara Desert around the southern Egypt-Libya border also contains baddeleyite and cometary dusts, together with anomalously high platinum-group elements and nanodiamonds that are not reported from the Chilean glass. Much prized by the elite of pharaonic Egypt and earlier makers of stone tools, the Saharan glass is ascribed to shock heating of the desert surface by a cometary nucleus that exploded over the Sahara. Unsurprisingly, Schultz et al. come to the same conclusion.

Any object entering the Earth’s atmosphere does so at speeds in excess of our planet’s escape velocity (11.2 km s-1). Not only does that result in heating by friction with the air, but much of the kinetic energy of hypersonic entry goes into compressing air through shock waves, especially with objects larger than a few tens of metres. Such adiabatic compression can produce temperatures >>10 thousand °C. Hence the ‘fireballs’ associated with large meteorites. With very large air-bursts the flash of radiant energy would be sufficient to completely melt surface materials in microseconds, though rugged topography could protect areas shadowed from the air-burst by mountains, perhaps explaining the patchy nature of the glass occurrences.  (Note: the aforementioned papers on the YD and Sodom ‘air-bursts’ do not mention large glass fragments, whereas some surface melting would be expected). Some of the Chilean glass contains carbonised remnants of vegetation. Radiocarbon dating of four samples show that the glass formed at some time between 16.3 to 12.1 ka. Yes, that does include the age of the start of the YD (12.9 ka) and human migrants had established themselves in northern Chile and coastal Peru after 14.2 ka. Yet the authors, perhaps wisely, do no more than mention the coincidence, as well as that with the disappearance of South American Pleistocene megafaunas – more severe than on any other continent. With a very distinctive product, probably spanning a far larger area of South America, and attractive to humans as an ornament or a resource for sharp tools, expect follow-up articles in the future.

See also: http://www.sci-news.com/space/atacama-desert-comet-10247.html, Science News, 8 November 2021; Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet, Eureka Alert, 2 November 2021.

Pinpointing the source of Martian meteorites and a stab at magmatism on Mars

Most meteorites found on the Earth’s surface are fragments of small bodies left over from the accretion of the planets around 4.5 billion years ago, thanks largely to collisions among larger, asteroid-sized bodies. A minority have other origins: some as debris from otherwise icy comets and a few that have been flung off other rocky planets or large moons by crater-forming impacts. Meteorites suspected to have originated through impact are ‘rocky’ – i.e.  made of silicates – and have textures and mineral contents suggesting they formed late in planetary evolution. Most are igneous with basaltic or ultramafic composition: respectively lavas and cumulates formed in magma chambers. Some are breccias, hinting at a pyroclastic origin. The radiometric ages of such planetary fragments are generally far younger than the times when the solar system and planets formed.  Almost 300 have been classified as coming from Mars, only two of which are older than 1400 Ma. The most numerous group of Martian meteorites, known as shergottites, crystallised between 575 and 150 Ma ago to form crust of igneous origin. During the journey from their source to Earth meteorites are exposed to high-energy cosmic rays that generate a variety of new isotopes, from whose relative proportions their travel time can be estimated. The shergottites all seem to have been blasted from Mars a mere 1.1 Ma ago, suggesting that a single impact launched them. So, identifying their source crater on Mars would enable the shergottites to be treated in the same way as samples collected by geologists from a small locality on Earth. Their geochemistry should give important clues to processes within Mars over a time period that spans the late-Precambrian to early Cretaceous on Earth.

Kuiper crater on the Moon, with rays and secondary craters. (Credit: NASA/Johns Hopkins University, USA)

There are many craters on Mars, so homing-in on a single source for shergottite meteorites might seem a tall order. A strategy for doing that depends on recognising craters formed by impacts with sufficient energy to eject debris at the escape velocity from Martian gravity: about 5 km s-1 compared with 11 km s-1 for Earth. Calculations suggest that such impacts would produce craters larger than 3 km across. Large ejecta travelling at slower speeds from them would fall back to produce smaller craters arranged radially from the main crater, forming distinctive rays. Anthony Lagain and colleagues from Curtin University, Western Australia and other institutions in Australia, USA, France and Côte d’ Ivoire adapted a detection algorithm to locate craters less than 1 km across that formed in rays around larger craters (Lagain, A. and 10 others 2021. The Tharsis mantle source of depleted shergottites revealed by 90 million impact craters. Nature Communications, v. 12, article6352; DOI: 10.1038/s41467-021-26648-3). They used 100 m resolution images of thermal emission from the Martian surface that most clearly distinguish large craters that have ejecta deposits around them. Then they turned to images with 0.25 m resolution covering the visible spectrum that can spot very small craters. The authors’ analysis compiled around 90 million impact craters smaller than 300 metres across (a quarter the size of the celebrated Meteor Crater in Arizona).

Laser-altimetry data that show two large impact craters and their ejecta aprons on the Tharsis Plateau of Mars and two of its huge volcanoes: grey-brown-red-orange-yellow-green = high-to-low elevations. (Credit: NASA / JPL-Caltech / Arizona State University)

Dust storms on Mars gradually fill and obscure small craters and ejecta rays, so the younger the impact event, the more visible are rays and secondary small craters. Luckily, just two large craters on Mars have well-preserved rays that contain high densities of small secondary craters. Both of them lie on the Tharsis Plateau near the Martian Equator. This is a vast bulge on the planet’s surface – 5000 km across and rising to 7 km – characterised by three enormous shield volcanoes that rise to 18 km above the average elevation of Mars. The authors judge that one or the other crater is the source for shergottite meteorites, and that this meteorite class collectively samples the most recent igneous rocks that form the Tharsis Plateau. So vast is its mass, that the plateau has probably built-up over most of Mars’s history. One hypothesis is that the bulging has progressively developed over a huge thermal anomaly that has supported a mantle superplume for billions of years from which basaltic magma has steadily moved to the surface.

This model of a perpetual hot spot beneath Tharsis implies that the magmas that it has generated in the past have progressively depleted the underlying mantle in the incompatible trace elements that preferentially enter magma rather than remaining in solid minerals during partial melting. Having been able to suggest that the 575 to 150 Ma-old shergottites represent the upper crust of Tharsis that formed at that late stage in its history, Lagain et al. use those meteorites’ well-established trace-element geochemistry to test that hypothesis. They do indeed suggest their derivation by partial melting of mantle rocks that had in earlier times been strongly depleted in incompatible elements. One of the greatest mysteries about Mars’ evolution may have been resolved without the need for a crewed mission.

Multiple impacts set back oxygen build-up in the Archaean

Earth’s present atmosphere contains oxygen because of one form of photosynthesis that processes water and carbon dioxide to make plant carbohydrates, leaving oxygen at a waste product. The photochemical trick that underpins oxygenic photosynthesis seems only to have evolved once. It was incorporated in a simple, single-celled organism or prokaryote, which lacks a cell nucleus but contains the necessary catalyst chlorophyll. Such an organism gave rise to cyanobacteria or blue-green bacteria, which still make a major contribution to replenishing atmospheric oxygen. Chloroplasts that perform the same function in plant cells are so like cyanobacteria that they were almost certainly co-opted during the evolution of a section of nucleus-bearing eukaryotes that became the ancestors of plants. A range of evidence suggests that oxygenic photosynthesis appeared during the Archaean Eon, the most tangible being the presence of stromatolites, which cyanobacteria mats or biofilms form today. These knobbly structures in carbonate sediments extend as far back as 3.5 billion years ago (see: Signs of life in some of the oldest rocks; September 2016). Yet it took a billion years before the first inklings of biogenic oxygen production culminated in the Great Oxygenation Event or GOE (see: Massive event in the Precambrian carbon cycle; January, 2012) at around 2400 Ma. Then, for the first time, oxidised iron in ancient soils turned them red. If oxygen was being produced, albeit in small amounts, in shallow, sunlit Archaean seas, why didn’t it build up in the atmosphere of those times? Geochemical analyses of Archaean sediments do point to trace amounts, with a few ‘whiffs’ of more substantial amounts. But they fall well below those of Meso- and Neoproterozoic and Phanerozoic times. One hypothesis is that Archaean oceans contained dissolved, ferrous iron (Fe2+) – a powerful reducing agent – with which available oxygen reacted to form insoluble ferric iron (Fe3+) oxides and hydroxides that formed banded iron formations (BIFS). The Fe2+ in this hypothesis is attributed to hydrothermal activity in basaltic oceanic crust. There is, however, another possibility for suppression of atmospheric oxygen accumulation in the Archaean and early-Palaeoproterozoic.

Summary of the evolution of atmospheric oxygen and related geological features. The percentage scale is logarithmic with the modern level being100%. Credit Alex Glass, Duke University

Simone Marchi of the Southwest Research Institute of Boulder, CO, USA and colleagues from the US, Austria and Germany suggest that planetary bombardment offers a plausible explanation (Marchi, S. et al 2021. Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth. Nature Geoscience, v. 14 advance publication; DOI: 10.1038/s41561-021-00835-9). Over the last 20 years evidence of extraterrestrial impacts has emerged, in the form of thin spherule-bearing layers in Archaean sedimentary strata, probably formed by impacts of objects around 10 km across. So far 35 such layers have been identified from several locations in South Africa and Western Australia. They span the last billion years of the Archaean and the earliest Palaeoproterozoic, although they are not evenly spaced in time. The spherules represent droplets of mainly crustal but some meteoritic rocks that were vaporised by impacts and then condensed as liquid. Meteorites in particular contain reduced elements and compounds, including iron, whose oxidation by would remove free oxygen.

The evidence from spherule beds is supplemented by the team’s new calculations of the likely flux of impactors during the Archaean. These stem from re-evaluation of the lunar cratering record that is used to estimate the number and size of impacts on Earth up to 2.5 Ga ago. This flux amounts to the ‘leftovers’ of the catastrophic period around 4.1 Ga when the giant planets Jupiter and Saturn ran amok before they settled into their present orbits. Their perturbation of gravitational fields in the solar system injected a long-lived supply of potential impactors into the inner solar system, which is recorded by craters on the post-4.1 Ga lunar maria. The calculations suggest that the known spherule layers underestimate the true number of such collisions on Earth. Modelling by Marchi et al., based on the meteorite flux and the oxidation of vaporised materials produced by impacts, plausibly accounts for the delay in atmospheric oxygen build-up.

It is worth bearing in mind, however, that large impacts and their geochemical aftermath are, in a geological sense, instantaneous events widely spaced in time. They may have chemically ‘sucked’ oxygen out of the Archaean and early-Palaeoproterozoic atmosphere. Yet photosynthesising bacteria would have been generating oxygen continuously between such sudden events. The same goes for the supply of reduced ferrous iron and its circulation in the oceans of those times, capable of scavenging available oxygen through simple chemical reactions. In fact we can still observe that in action around ocean-floor hydrothermal vents where a host of reduced elements and compounds are oxidised by dissolved oxygen. The difference is that oxygen is now produced more efficiently on land and in the upper oceans and a less vigorous mantle is adding less iron-rich basalt magma to the crust: the balance has changed. Another issue is that the Great Oxygenation Event terminated the oxygen-starved conditions of the Archaean and Palaeoproterozoic in about 200 million years, despite the vast production of BIFs before and after it happened. The Wikipedia entry for the GOE provides a number of hypotheses for how that termination came about. Interestingly, one idea looks to a shortage of dissolved nickel that is vital for methane generating bacteria: a nickel ‘famine’. A geochemical setback for methanogens would have been a boost for oxygenic photosynthesisers and especially their waste product oxygen: methane quickly reacts with oxygen in the atmosphere to produce CO2 and water. Anomalously high nickel is a ‘signature element’ for meteorite bombardment, though it can be released by hydrothermal alteration of basalt. Had meteoritic nickel been fertilising methane-generating bacteria in the oceans prior to the GOE?

See also: A new Earth bombardment model. Science Daily, 21 October 2021.

Climate change reducing Earth’s albedo

According to a new study (Goode, P. R.et al. 2021. Earth’s albedo 1998–2017 as measured from earthshine. Geophysical Research Letters, v. 48, article e2021GL094888; DOI: 10.1029/2021GL094888) the ability of the Earth to reflect solar radiation back into space has been decreasing significantly over the last two decades. The conclusion has arisen from measurements of the brightness of the lunar surface. A new Moon is barely visible, apart from a thin sliver illuminated by the Sun. Its overall faint brightness is due to sunlight reflected from the Earth’s surface that faces the Moon: so-called ‘earthshine’. New Moons occurs when it is above the lit side of the Earth, so they appear during daylight hours. Earthshine depends on the ability of the Earth’s surface and cloud cover to reflect solar radiation, or its albedo. Albedo was high during the last ice age because of continental ice sheets and it can also occur when there is an unusually large percentage of cloud cover or a lot of dust and aerosols in the atmosphere, perhaps after a large volcanic eruption. High albedo leads to global cooling. Decreased albedo allows the atmosphere to heat up, and conspires with the greenhouse effect to produce global warming.

Philip Goode and his colleagues measured earthshine on the Moon between 1998 and 2017 to precisely determine daily, monthly, seasonal, yearly and decadal changes in terrestrial albedo. The Earth reflects roughly 30% of the solar energy that falls on it, although it varies with Earth’s rotation, depending on the proportion of land to ocean that is sunlit. Over the two decades earthshine decreased gradually by ~0.5 W m-2, indicating a 0.5% decrease in Earth’s albedo and a corresponding increase in the amount of solar energy received at the land and ocean surfaces. To put this in perspective the estimated warming from anthropogenic greenhouse emissions over the same period increased by just a little more (0.6 W m-2). Albedo decrease is reinforcing the greenhouse effect.

Sea-surface temperature anomalies over the Pacific Ocean during a ‘positive’ phase of the Pacific Decadal Oscillation – reversal to a ‘negative’ phase cools the eastern Pacific and warms the west (Credit: Wikipedia)

Although it might seem that increased seasonal melting of polar sea ice would have the main effect on albedo, this is not borne out by the earthshine data. What is strongly implicated is a decrease over the Eastern Pacific Ocean of highly reflective low-altitude clouds. That might seem counterintuitive, since warming of the sea surface increases evaporation, but the reduced low-cloud cover has been measured from satellites. Many scientists and most climate-change deniers have thought that an increase in cloud cover at low latitudes and thus albedo would moderate surface warming. The opposite seems to be happening. The key may lie in one of the Earth’s largest climate phenomena, the Pacific Decadal Oscillation (PDO). This has a major effect on global climate through long-distance connections (teleconnections) to other climatic processes. The satellite data hint at the changes in albedo of the Western Hemisphere having been related to a long-term reversal in the PDO. The Earth’s climate system increasingly reveals its enormous complexity.

See also: Earth is dimming due to climate change, Science Daily, 30 September 2021.

Tectonics on Venus

The surface of Venus is not easily observed because of the almost opaque nature of its atmosphere. The planet is veiled by a mixture of CO2 (96.5%) and nitrogen (3.5%), with a little sulfur dioxide and noble gases. The atmosphere’s mass is almost 100 times that of the Earth’s, and has a density about 6.5% that of liquid water at the surface. The opacity stems from a turbulent upper layer of mainly sulfuric acid. Venus is the victim of runaway greenhouse conditions. Despite that, radar can penetrate the atmosphere to reveal details of its surface morphology – roughness and elevation – at a spatial resolution of 150 m. Although coarser than that available from radar remote sensing of the Earth from orbit, the Magellan data are still geologically revealing.

Earlier interpretation of Venus radar images revealed the surface to be far simpler than that of the Earth, Mars and all other rocky bodies in the Solar System. Yet it has more volcanoes than does the Earth or Mars. However, despite being subject to very little erosion – Venus is a dry world – only around 1000 impact craters have been found: far short of the number seen on Mars or the Moon. This deficiency of evidence for bombardment suggests that Venus was ‘repaved’ by vast volcanic outpourings in the geologically recent past, estimated to have occurred 300 to 600 Ma ago. This early work concluded that plate tectonics was absent; indeed that for half a billion years the lithosphere on Venus had been barely deformed. It has been suggested that Venus has been involved in megacycles of sudden, planet-wide magmatic activity separated by long periods of quiescence. This could be attributed to the lack of plate tectonics, which is the principal means that Earth continuously rids itself of heat produced at depth by decay of radioactive isotopes in the mantle. Venus has been suggested to build up internal temperatures until they reach a threshold that launches widespread partial melting of its mantle. Planet-wide eruption of magma then reduces internal temperatures.

Polygonal blocks or ‘campuses’ on the lowland surface of Venus. Note the zones of ridges that roughly parallel ‘campus’ margins. Credit: Paul K. Byrne, North Carolina State University and Sean C. Solomon, Lamont-Doherty Earth Observatory

It comes as a surprise that 26 years after Magellan plunged into the Venusian atmosphere new interpretation of its radar images suggests a completely different scenario (it may be that academic attention generally switched to research on Mars because of all the missions to the ‘Red Planet’ since Magellan disappeared). It is based on features of the surface of Venus so large that their having been missed until now may be a planetary-scale example of ‘not seeing the woods for the trees’! Geoscientists from the US, Turkey, the UK and Greece have mapped out features ranging from 100 to 1000 km across that cover the lowland parts of Venus (Byrne, P.K. et al. 2021. A globally fragmented and mobile lithosphere on Venus. Proceedings of the National Academy of Science, v. 118, article e2025919118; DOI: 10.1073/pnas.2025919118). They resemble 1950s ‘crazy paving’ or floes in Arctic pack ice, but on a much larger scale. Extending the ice floe analogy, the polygonal blocks are separated by what resemble pressure ridges that roughly parallel the block margins. Paul Byrne of North Carolina State University, USA, and co-workers also found evidence that the large blocks of lithosphere had rotated and moved laterally relative to one another: they had ‘jostled’. Moreover, some of the movement has disturbed the youngest materials on the surface.

To distinguish what seem to be characteristic of Venus’s tectonics from Earthly tectonic plates, the team hit on the name ‘campus’, meaning ‘field’ in Latin. Rather than having remained a single spherical skin of lithosphere, the surface of at least part of Venus has broken into a series of ‘campuses’. It does display tectonics, but not as we know it on planet Earth. This could be ascribed to an outcome of stress transfer from deep convective motion in the Venerean mantle. Being in the virtually non-magmatic phase of Venus’s thermal cycling, there is neither formation of new lithosphere nor subduction of old, cold plates that characterise terrestrial plate tectonics. ‘Campus’ tectonics seems likely to be another form of planetary energy and matter redistribution, and Byrne et al. have likened it to how the Earth may have functioned during the ‘missing’ 600 Ma of the Hadean Eon on Earth. But perhaps not …

The runaway greenhouse has resulted in surface temperatures on Venus being 450°C higher than on Earth: enough to melt lead. It is not just solar heat that is trapped by the atmosphere, but that from the Venerean interior. This must result in a very different geotherm (the way temperature varies with depth in a planet) from that characterising the Earth. The temperature of the beginning of mantle melting – about 1200°C – must be much shallower on Venus. On Earth that is at depths between 50 and 100 km below active plate margins and within-plate hotspots, and is not reached at all for most of the Earth that lies beneath the tectonic plates. If the mantle of Venus contained a similar complement of heat-producing isotopes to that of Earth wouldn’t we expect continual volcanism on Venus rather than the odd dribble that has been observed by Magellan? Or does the jostling of ‘campuses’ absorb the thermal energy and help direct it slowly to space by radiation through the dense, greenhouse atmosphere. Here’s another poser: If the Earth and Venus are geochemically similar and Hadean Earth went through such a phase of ‘campus tectonics’ – perhaps our world had a CO2-rich atmosphere too – what changed to allow plate tectonics here to replace that system of thermal balance? And, why hasn’t that happened on Venus? Perhaps some light will be thrown on these enigmas once a series of new missions to Venus are launched between now and the 2030s, by NASA and the European Space Agency.

Update: Can a supernova affect the Earth System?

Earth-pages asked this question in August 2020 because it had been suggested that at least one mass extinction – the protracted faunal decline during the Late Devonian – may provide evidence that supernovas can have deadly influence. The authors of the paper that I discussed proposed mass spectrometric analysis of isotopes, such as 146Sm, 235U and  244Pu  in sediments deposited in an extinction event to test the hypothesis. In the 14 May issue of Science a multinational group of geochemists and physicists, led by Anton Wallner of the Australian National University, report detection of alien isotopes in roughly 10 million-year-old sediments sampled from the Pacific Ocean floor (Wallner, A and 12 others 2021. 60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae. Science, v. 372, p. 742-745; DOI: 10.1126/science.aax3972).

Many of the chemical elements whose atomic masses are greater than 56 form by a thermonuclear fusion process known as rapid neutron capture – termed the ‘r-process’ by physicists. This requires such high energy that the likely heavy-element ‘nurseries must be events such as supernovas and/or mergers of neutron stars. The iron and plutonium isotopes  detected at very low concentrations are radioactive, with half-lives of 2.6 Ma for 60Fe and 80.6 Ma for 244Pu. That makes it impossible for them to be terrestrial in origin because, over the lifetime of the Earth, they would decayed away completely. They must be from recent, alien sources either in our galaxy or one of the nearby galaxies. In fact two ‘doses’ were involved. The authors make no comment on any relationship with marine or continental extinctions at that time in the Miocene Epoch