Read about the threat posed by deep-ocean mining of polymetallic nodules at Earth-logs
Read about the threat posed by deep-ocean mining of polymetallic nodules at Earth-logs
The 2017 discovery in Morocco of fossilised, anatomically modern humans (AMH) dated at 286 ka (see: Origin of anatomically modern humans, June 2017) pushed back the origin of our species by at least 100 ka. Indeed, the same site yielded flint tools around 315 ka old. Aside from indicating our antiquity, the Jebel Irhoud discovery expanded the time span during which AMH might have wandered into Eurasia, as a whole variety of earlier hominins had managed since about 1.8 Ma ago. Sure enough, the widely accepted earliest modern human migrants from Skhul and Qafzeh caves in Israel (90 to 120 ka) were superseded in 2018 by AMH fossils at Misliya Cave, also in Israel, in association with 177 ka stone artefacts (see Earliest departure of modern humans from Africa, January 2018). Such early dates helped make more sense of very old ages for unaccompanied stone tools in the Arabian Peninsula as tracers for early migration routes. Unlike today, Arabia was a fertile place during a series of monsoon-related cycles extending back to about 160 ka (see: Arabia : staging post for human migrations? September 2014; Wet spells in Arabia and human migration, March 2015). The ‘record’ has now shifted to Greece.
Fossil human remains unearthed decades ago often undergo revised assessment as more precise dating methods and anatomical ideas become available. Such is the case for two partial human skulls found in the Apidima Cave complex of southern Greece during the late 1970s. Now, using the uranium-series method, one has been dated at 170 ka, the other being at least 210 ka old (Harvati, K. and 11 others 2019. Apidima Cave fossils provide earliest evidence of Homo sapiens in Eurasia. Nature, v. 571 online; DOI: 10.1038/s41586-019-1376-z). These are well within the age range of European Neanderthals. Indeed, the younger one does have the characteristic Neanderthal brow ridges and elongated shape. Albeit damaged, the older skull is more rounded and lacks the Neanderthals’ ‘bun’-like bulge at the back; it is an early member of Homo sapiens. In fact 170 ka older than any other early European AMH, and a clear contemporary of the long-lived Neanderthal population of Eurasia; in fact the age relations could indicate that Neanderthals replaced these early AMH migrants.
Given suitable climatic conditions in the Levant and Arabia, those areas are the closest to Africa to which they are linked by an ‘easy’, overland route. To reach Greece is not only a longer haul from the Red Sea isthmus but involves the significant barrier of the Dardanelles strait, or it requires navigation across the Mediterranean Sea. Such is the ‘specky’ occurrence of hominin fossils in both space and time that a new geographic outlier such as Apidima doesn’t help much in understanding how migration happened. Until – and if – DNA can be extracted it is impossible to tell if AMH-Neanderthal hybridisation occurred at such an early date and if the 210 ka population in Greece vanished without a trace or left a sign in the genomics of living humans. Yet, both time and place being so unexpected, the discovery raises optimism of further discoveries to come
A jawbone discovered in a Tibetan cave turned out to be that of a Denisovan who had lived and died there about 160,000 years ago (see: Denisovan on top of the world; 6 May, 2019). That discovery owed nothing to ancient DNA, because the fossil proved to contain none that could be sequenced. But the dentine in one of two molar teeth embedded in the partial jaw did yield protein. The teeth are extremely large and have three roots, rather than the four more common in modern, non-Asian humans, as are Denisovan teeth from in the Siberian Denisova Cave. Fortunately, those teeth also yielded proteins. In an analogous way to the genomic sequencing of nucleotides (adenine, thymine, guanine and cytosine) in DNA, the sequence of amino acids from which proteins are built can also be analysed. Such a proteomic sequence can be compared with others in a similar manner to genetic sequences in DNA. The Tibetan and Siberian dentine proteins are statistically almost the same.
At present the most ancient human DNA that has been recovered – from an early Neanderthal in the Sima de los Huesos in Spain – is 430,000 years old (see: Mitochondrial DNA from 400 thousand year old humans; December 2013). Yet it is proving difficult to go beyond that time, even in the cool climates that slow down the degradation of DNA. The oldest known genome of any animal is that of mtDNA from a 560–780 thousand year old horse, a leg bone of which was extracted from permafrost in the Yukon Territory, Canada. The technologies on which sequencing of ancient DNA depends may advance, but, until then, tracing the human evolutionary journey back beyond Neanderthals and Denisovans seems dependent on proteomic approaches (Warren, M. 2019. Move over, DNA: ancient proteins are starting to reveal humanity’s history. Nature, v. 570, p. 433-436; DOI: 10.1038/d41586-019-01986-x). Are the earlier Homo heidelbergensis and H. erectuswithin reach?
It seems that they may be, as might even earlier hominins. The 1.8 Ma Dmanisi site in Georgia, now famous for fossils of the earliest humans known to have left Africa, also yielded an extinct rhinoceros (Stephanorhinus). Proteins have been extracted from it, which show that Stephanorhinus was closely related to the later woolly rhinoceros (Coelodonta antiquitatis). Collagen protein sequences from a 3.4 Ma camel preserved in the Arctic and even from a Tanzanian 3.8 Ma ostrich egg shell show the huge potential of ancient proteomics. Most exciting is that last example, not only because it extends the potential age range to that of Australopithecus afarensis but into tropical regions where DNA is at its most fragile. Matthew Warren points out potential difficulties, such as the limit of a few thousand amino acids in protein sequences compared with 3 million variants in DNA, and the fact that the most commonly found fossil proteins – collagens – may have evolved very little. On the positive side, proteins have been detected in a 195 Ma old fossil dinosaur. But some earlier reports of intact diosaur proteins have been questioned recently (Saitta, E.T. et al. 2019. Cretaceous dinosaur bone contains recent organic material and provides an environment conducive to microbial communities. eLife,8:e46205; DOI: 10.7554/eLife.46205)
Steadily, the record of stone tools has progressed further back in time as archaeological surveys have expanded, especially in East Africa (Stone tools go even further back, May 2015). The earliest known tools – now termed Lomekwian – are 3.3 million years old, from deposits in north-western Kenya, as are cut-marked bone fragments from Ethiopia’s Afar region. There is no direct link to their makers, but at least six species ofAustralopithecus occupied Africa during the Middle Pliocene. Similarly, there are various options for who made Oldowan tools in the period between 2.6 and 2.0 Ma, the only known direct association being with Homo habilis in 2.0 Ma old sediments from Tanzania’s Olduvai Gorge; the type locality for the Oldowan.
The shapes of stone tools and the manufacturing techniques required to make them and other artefacts, are among the best, if not the only, means of assessing the cognitive abilities of their makers. A new, detailed study of the shapes of 327 Oldowan tools from a 2.6 Ma old site in Afar, Ethiopia has revealed a major shift in hominin working methods (Braun, D.R. and 17 others 2019. Earliest known Oldowan artifacts at >2.58 Ma from Ledi-Geraru, Ethiopia, highlight early technological diversity. Proceedings of the National Academy, v. 116, p. 11712-11717; DOI: 10.1073/pnas.1820177116). The sharp-edged tools were made by more complex methods than the Lomekwian. Analysis suggests that they were probably made by striking two lumps of rock together, i.e. by a deliberate two-handed technique. On the other hand, Lomekwian tools derived simply by repeatedly bashing one rock against a hard surface, not much different from the way some living primates make rudimentary tools. But the morphology of the Ledi-Geraru tools also falls into several distinct types, each suggesting systematic removal of only 2 or 3 flakes to make a sharp edge. The variations in technique suggest that several different groups with different traditions used the once lake-side site.
Ledi-Geraru lies about 5 km from another site dated about 200 ka earlier than the tools, which yielded a hominin jawbone, likely to be from the earliest known member of the genus Homo. A key feature that suggested a human affinity is the nature of the teeth that differ markedly from those of contemporary and earlier australopithecines. It appears that the tools are of early human manufacture. The ecosystem suggested by bones of other animals, such as antelope and giraffe was probably open grassland – a more difficult environment for hominin subsistence. The time of the Lomekwian tools was one of significantly denser vegetation, with more opportunities for gathering plant foods. Perhaps this environmental shift was instrumental in driving hominins to increased scavenging of meat, the selection pressure acting on culture to demand tools sharp enough to remove meat from the prey of other animals quickly, and on physiology and cognitive power to achieve that.
See also: Solly, M. 2019. Humans may have been crafting stone tools for 2.6 million years (Smithsonian Magazine)
The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19thcentury. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).
The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered.
There are now large amounts of geochemical data from Neoproterozoic sedimentary rocks that bear on processes in the atmosphere, seawater, continental crust and the biosphere of the time. Some are indicative of the reducing/oxidising (redox) potentials of ocean water in which various sediments were deposited. Carbon isotopes chart organic burial and the abundance of CO2 in the oceans and atmosphere. Strontium isotopes give details of the rates of continental erosion. The age statistics of zircon grains in sediments are useful; the proportion of zircons close in age to the time of sediment deposition relative to older grains is a proxy for the rate of continental-arc volcanism and thus for subduction rates. Joshua Williams of Britain’s University of Exeter and colleagues from the universities of Edinburgh and Leeds have used complex modelling to assess the pace at which oxygen was added to the surface environment through the Ediacaran Period (Williams, J.J. et al. 2019. A tectonically driven Ediacaran oxygenation event. Nature Communications, v. 10 (1); DOI: 10.1038/s41467-019-10286-x).
They estimate a 50% increase in atmospheric oxygen during the Ediacaran to about 0.25 % of the present concentration, which would be sufficient to support large, mobile animals. They attribute this primarily to a boost in the supply of CO2 to the atmosphere as a result of increased volcanic activity. This would have warmed the surface environment so that exposed rock on the continents underwent accelerated chemical weathering. By freeing from continental crust increased amounts of nutrients, such as phosphorus and potassium, the boost to photosynthesis would have increased the oceanic biomass, thereby emitting oxygen. Multicelled animals would have been beneficiaries of such a transformation. The trend continued into the Cambrian, thereby unleashing the explosion of animals and their evolution that continued through the Phanerozoic. Ultimately, the trigger was increased Late-Neoproterozoic tectonic activity that drove the massive Pan-African orogeny and the accretion of the Gondwana supercontinent.
Note added, 26 June 2019: Roger Mason has referred me to the carbon-isotope record during the Ediacaran. It shows some of the stratigraphic record’s largest negative δ13C excursions in carbonate rocks (Tahata, M. and 10 others 2013. Carbon and oxygen isotope chemostratigraphies of the Yangtze platform, South China: Decoding temperature and environmental changes through the Ediacaran. Gondwana Research, v.23, p. 333-353; DOI: 10.1016/j.gr.2012.04.005). Such isotopic excursions went on throughout the Ediacaran, along with sudden fossil appearances and disappearances – so-called ‘Strangelove’ oceans – plus fluctuations in sediment types and climate. The Ediacaran was a wild time in most respects.
At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant food chain. The key factor that is missing is dissolved divalent iron that acts as a minor, but vital, nutrient for phytoplankton. Much of the soluble iron that does help stimulate plankton ‘blooms’ emanates from the land surface in wind blown dust (Palaeoclimatology September 2011) or dissolved in river water. A large potential source is from hydrothermal vents on the ocean floor, which emit seawater that has circulated through the basalts of the oceanic crust. Such fluids hydrate the iron-rich mafic minerals olivine and pyroxene, which makes iron available for transport. The fluids originate from water held in the muddy, organic-rich sediments that coat the ocean floor, and have lost any oxygen present in ocean-bottom water. Their chemistry is highly reducing and thereby retains soluble iron liberated by crustal alteration to emanate from hydrothermal vents. Because cold ocean-bottom waters are oxygenated by virtue of having sunk from the surface as part of thermohaline circulation, it does seem that ferrous iron should quickly be oxidised and precipitated as trivalent ferric compounds soon after hydrothermal fluids emerge. However, if some was able to rise to the surface it could fertilise shallow ocean water and participate in phytoplankton blooms, the sinking of dead organic matter then effectively burying carbon beneath the ocean floor; a ‘biological pump’ in the carbon cycle with a direct influence on climate. Until recently this hypothesis had little observational support.
The Southern Ocean surrounding Antarctica is iron-starved for the most part, but it does host huge phytoplankton blooms that are thought to play an important role in sequestration of CO2 from the atmosphere. Oceanographic research now benefits from semi-autonomous buoys set adrift in the deep ocean. The most sophisticated (Argo floats ) are able to dive to 2 km below the surface, measuring variations of physical and chemical conditions with depth for long periods. There are 4,000 of them, owned by several countries. Two of them drifted with surface currents across the line of the Southwest Indian Ridge through waters thought to be depleted in phytoplankton, despite having high nitrate, phosphate and silica contents – major ‘fertilisers’ in water. They showed up ‘spikes’ in chlorophyll concentrations in the upper levels of the Southern Ocean (Ardyna, M. and 11 others 2019. Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nature Communications, 5 June 2019, online; DOI: 10.1038/s41467-019-09973-6). Their location relative to a large cluster of hydrothermal vents on the Southwest Indian Ridge was ‘downstream’ of them in the circum-Antarctic Current, but remote from any known terrestrial source of iron (continental shelves, dust deposition melting sea ice). Earlier oceanographic surveys that detected anomalous helium isotope, typical of emanations from the mantle, show that hydrothermal-vent water moves through the two areas. Although the Argo floats are equipped for neither helium nor iron measurements, it is likely that the blooms benefitted from hydrothermal iron. Modelling of the likely current dispersion of material in the hydrothermal plumes also outlines a large area of ocean where iron fertilisation may encourage regular blooms where they would otherwise be highly unlikely. Unfortunately, the study does not include any direct evidence for elevated soluble iron.
One thing that the study does foster is renewed interest in deliberate iron-fertilisation of the oceans to speed up the ‘biological pump’ as a means of managing global warming (Boyd, P. & Vivian, C. 2019. Should we fertilize oceans or seed clouds? No one knows. Nature, v. 570, p. 155-157; doi: 10.1038/d41586-019-01790-7). Small scale pilots of such ‘geoengineering’ have been tried, but raised outcries from environmental groups. Other than detecting, or hinting at, soluble iron from a deep natural source, scientific research has provided scanty evidence of what iron-seeding at the surface might do. There could be unexpected consequences, such as methane emission from decay of the blooms – a worse greenhouse gas than carbon dioxide.
See also: An iron age for climate engineering? (Palaeoclimatology, July 2007); Dust in the wind: North Pacific Ocean fertilized by iron in Asian dust ( National Science Foundation 2019)
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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.
One such mystery lies close to the base of the vast pile of reddish Torridonian sandstones, the Stac Fada Member of the Stoer Group. Beneath it is a common-or-garden basal breccia full of debris from the underlying Lewisian Complex, then red sandstones and siltstones deposited by a braided river system. The Stac Fada Member is a mere 10 m thick, but stretches more than 50 km along the regional NNE-SSW strike. It comprises greenish to pink sandstones with abundant green, glassy shards and clasts, previously thought to be volcanic in origin, together with what were initially regarded as volcanic spherules – the results of explosive reaction of magma when entering groundwater or shallow ponds. Until 2002, that was how ideas stood. More detailed sedimentological and geochemical examination found quartz grains with multiple lamellae evidencing intense shock, anomalously high platinum-group metal concentrations and chromium isotopes that were not of this world. Indeed, the clasts and the ensemble as a whole look very similar to the ‘suevites’ around the 15 Ma old Ries Impact crater in Germany. The bed is the product of mass ejection from an impact, a designation that has attracted great attention. In 2015 geophysicists suggested that the impact crater itself may coincide with an isolated gravity low about 50 km to the east. A team of 8 geoscientists from the Universities of Oxford and Exeter, UK, have recently reported their findings and ideas from work over the last decade. (Amor, K, et al. 2019. The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry. Journal of the Geological Society, online; DOI: 10.1144/jgs2018-093).
The age of the Stac Fada member is around 1200 Ma, determined by Ar-Ar dating of K-feldspar formed by sedimentary processes. Geochemistry of Lewisian gneiss clasts compared with in situ basement rocks, magnetic data from the matrix of the deposit, and evidence of compressional forces restricted to it suggest that the debris emanated from a site to the WNW of the midpoint of the member’s outcrop. Rather than being a deposit from a distant source, carried in an ejecta curtain, the Stac Fada material is more akin to that transported by a volcanic pyroclastic flow. That is, a dense, incandescent debris cloud moving near to the surface under gravity from the crater as ejected material collapsed back to the surface. On less definite grounds, the authors suggest that a crater some 13 to 14 km across penetrating about 3 km into the crust may have been involved.
Together with evidence that I described in Impact debris in Britain (Magmatism February 2018) and Britain’s own impact (Planetary Science November 2002) it seems that Britain has directly witnessed three impact events. But none of them left a tangible crater.
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About 39 thousand years ago all sign of the presence of Neanderthal bands in their extensive range across western Eurasia disappears.Their demise occurred during a period of relative warmth (Marine-Isotope Stage-3) following a cold period at its worst around 65 ka (MIS-4). They had previously thrived since their first appearance in Eurasia at about 250 ka, surviving at least two full glacial cycles. Their demise occurred around 5 thousand years after they were joined in western Eurasia by anatomically modern humans (AMH). During their long period of habitation they had adapted well to a range of climatic zones from woodland to tundra. During their overlap both groups shared much the same food resources, dominated by large herbivores whose numbers burgeoned during the warm period, with the difference that Neanderthals seemed to have depended on ranges centred on fixed sites of habitation while AMH maintained a nomadic lifestyle. Having shared a common African ancestry about 400 thousand years ago, DNA studies have revealed that the two populations interbred regularly, probably in the earlier period of overlap in west Asia from around 120 thousand years ago and possibly in Europe too after 44 ka. Considering their previous tenacity, how the Neanderthals met their end is something of a mystery. It may have been a result of competition for resources with AMH, which could be countered by the increase in food resources. Maybe physical conflict was involved, or perhaps disease imported with AMH from warmer climes. Genetic absorption through interbreeding of a small population with a larger one of AMH is a possibility, although DNA evidence is lacking. An inability to adapt to climate change contradicts the Neanderthals long record and their disappearance during MIS-3. Previous population estimates of changing Neanderthal populations in the Iberian Peninsula (see Fig. 2 in Roberts, M.F. & Bricher, S.E 2018. Modeling the disappearance of the Neanderthals using principles of population dynamics and ecology. Journal of Archaeological Science, v. 100, p.16-31; DOI: 10.1016/j.jas.2018.09.012) show decline from about 70,000 to 20,000 before MIS-4, then recovery to about 40,000 before the arrival of AMH at 44 ka followed by a decline to extinction thereafter. Roberts and Bricher developed a model for investigating demographics from archaeological evidence that is neutral as regards any particular hypothesis for Neanderthal extinction.
Attempting to take modelling further, another research consortium from France has focussed on the demographic changes needed to draw Neanderthals to extinction (Degioanni, A. et al. 2019. Living on the edge: Was demographic weakness the cause of Neanderthal demise? PLOS One, v. 14(5): e0216742; DOI: 10.1371/journal.pone.0216742). It is based on studies of living hunter-gatherer groups and those from the recent past. Survival of individuals in such groups is strongly age-dependent, i.e. low survival among juveniles, high among individuals in their prime and decreasing among the elderly. Fertility also varies among females, increasing from post-pubescence to ages between 21 to 30 years. In groups that practice sexual pairing between individuals from different communities (exogamy) migration from one to another is necessary to avoid inbreeding. The modellers assumed that only individuals from 16 to 18 years old migrated in this way. They found that a small decrease (~8%) in the fertility rate of younger females (<20 years) having a child for the first time could produce the decreasing trend in Neanderthal populations during the 5,000 year period of sharing resources with AMH populations. This would have culminated in the extinction of the Neanderthals, irrespective of the fertility rates of older, pre-menopausal females. So what could trigger such a change from a primiparous fertility rate that gave stable or growing population to one that ended so badly? The authors make no suggestion, eschewing the ‘why’ for the ‘how’. All they suggest is that the decrease in Neanderthals, which would have benefited AMH settlement in the vacated areas, could have occurred without any need for some catastrophic event, such as disease, slaughter or climate change. Any of these causes would probably have resulted in more rapid extinction. However, the lead author, Anna Degioanni from Aix Marseille Université, when interviewed by The Independentnewspaper said. ‘First-time pregnancies, especially in young females (less than 20 years old), are on average more at risk than second and other pregnancies… a slight decrease in food may explain a reduction in fertility, especially among first-time mothers’.
One of the key features of Neanderthals is that they were probably sedentary with widely spaced communities across their huge range. So exogamy would have been more difficult for them than it would have been for nomadic groups. Genetic evidence from a few Neanderthals suggests that inbreeding was an issue. Had it been widespread among Neanderthals – risky to infer from such scanty information – that may also account for decreased primiparous fertility and also survival of newborns.
Related article: Neanderthals may have died out because of infertility, new model suggests. (The Independent)
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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 (seePlanetary science, May 2016).
Budde and colleagues showed that the 95Mo and 94Mo abundances in water- and carbon-poor meteorites that come from the Asteroid Belt and formed in the inner Solar System differ consistently from those in volatile-rich carbonaceous chondrites that formed much further away from the Sun. The average abundances of the two molybdenum isotopes in the Earth’s silicate rocks, which ultimately had their origin in the mantle, fall between those of the two classes of meteorites (Budde, G. et al. 2019. Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy, v. 3, online ; DOI: 10.1038/s41550-019-0779-y). They must reflect the materials that accreted after core formation. If the 95Mo and 94Mo abundances resembled those in non-carbonaceous, dry meteorites that would suggest late accretion with much the same composition as expected from Earth’s position in the Inner Solar System. Alternatively, some molybdenum from Earth’s original formative materials failed to unite with iron in the core. The Mo ‘signature’ of volatile-rich carbonaceous meteorites in the mantle’s make-up points to a large amount of accreting material from the Outer Solar System. In contrast, lunar rocks show no carbonaceous meteorite component of Mo isotopes, which helps to explain its overall dryness compared with the Earth. Yet, the Moon is strongly believed to have formed from material blasted away by an impact between the proto-Earth and an errant, Mars-sized body (Theia).
The authors suggest a high probability that Theia was a carbon- and volatile-rich body from the outer Solar System flung inwards by gravitational perturbation associated with the then unstable orbits of the giant planets Jupiter and Saturn. In that case Theia could have delivered not only the anomalous molybdenum, but most of Earth’s water and other volatile compounds. If the theory is correct, then the cataclysmic event that formed the Moon laid the basis for Earth’s continual tectonic activity and its eventually sparking up life; without the Moon, there would be no life on Earth. That kind of chance event isn’t a factor considered in either the Drake Equation or the Goldilocks Zone. Life, natural selection and sentient beings that might spring from them may be a great deal more elusive than commonly believed by exobiologists.
See also: Formation of the moon brought water to Earth (Science Daily, 21 May 2019)