Earth-pages is closing soon

Dear Earth-pages readers,

It is almost two decades since I was invited to write a regular series of articles on developments in the geosciences at Earth-pages. The site’s archives comprise more than 1200 of my commentaries, covering over 1500 publications. Since 2011 its annual readership has been between 40,000 to 80,000. Sadly, Earth-pages will close in late July 2019.

Given its wide and loyal readership, I believe that the archives will continue to remain useful, especially for students, teachers and those hoping to begin geoscientific research. So, with the permission of Wiley-Blackwell, they are preserved at a new site called Earth-logs (URL – http://earthlogs.org/).

header2

The format is different: the early posts (2000 to 2018) are logged annually under 12 broad themes: GeohazardsGeomorphologyHuman evolution and migrationsMagmatismMiscellaneous CommentaryPalaeoclimatologyPalaeobioloy; Physical ResourcesPlanetary ScienceRemote SensingSedimentology and Stratigraphy, and Tectonics. Each of these pages indexes the research topics covered during each year, along with links to PDFs of the annual logs.

New posts are added regularly to the Earth-logs Home Page. I intend to continue writing these commentaries in the same style as I have adopted at Earth-pages, for as long as I can. An important addition is direct web access to most of the papers on which the posts and the entries in annual logs are based, so that readers can download them as PDFs for their own use.

Thanks for reading my stuff here. Hopefully you will continue to do so at Earth-logs

Steve Drury

Ancient proteins: keys to early human evolution?

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.

collagen
Triple helix structure of collagen, colour-coded to represent different amino acids (credit: Wikipedia)

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)

Multiple invention of stone tools

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.

oldowan
Various 2.6 Ma old Oldowan stone tools from Ledi-Geraru, Ethiopia (credit: Braun et al., 2019)

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)

Geochemical background to the Ediacaran explosion

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).

ediacaran-1

Artist’s impression of the Ediacaran Fauna (credit: Science)

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 eventNature 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.

See also: https://www.sciencedaily.com/releases/2019/06/190619130315.htm

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 EdiacaranGondwana 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.

3-gorge-c

Geochemical changes recorded in the complete Ediacaran sedimentary sequence of the Three Gorges of the Yangtze River, China (credit: Tahata et al. 2013; Fig. 4)

Soluble iron, black smokers and climate

 

Phytoplankton bloom in the Channel off SW England (Landsat image)

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)

A major Precambrian impact in Scotland

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

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

NW Scotland geol

Simplified geological map of NW Scotland (credit: British Geological Survey)

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.

Neanderthal demographics and their extinction

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

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.

nea-family-1

Artistic reconstruction of Neanderthal family group (credit: Nikola Solic, Reuters)

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)

Earth’s water and the Moon

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

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.

earth-moon

Simulated view of the Earth from lunar orbit: the ‘wet’ and the ‘dry’. (credit: Adobe Stock)

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)

Anthropocene edging closer to being ‘official’

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

The issue of erecting a new stratigraphic Epoch encompassing the time since humans had a global effect on the Earth System has irked me ever since the term emerged for discussion and resolution by the scientific community in 2000. An Epoch in a chronostratigraphic sense is one of several arbitrary units that encompass all the rocks formed during a defined interval of time. The last 541 million years (Ma) of geological time is defined as an Eon – the Phanerozoic. In turn that comprises three Eras – Palaeozoic, Mesozoic and Cenozoic. The third level of division is that of Periods, of which there are 11 that make up the Phanerozoic. In turn the Periods comprise a total of 38 fourth-level Epochs and 85 at the fifth tier of Ages. All of these are of global significance, and there are even finer local divisions that do not appear on the International Chronostratigraphic Chart . If you examine the Chart you will find that no currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even Ages span a range from hundreds of thousands to millions of years (averaged at 6 Ma).

lignite

The Vattenfall lignite mine in Germany; the Anthropocene personified

In the 3rd week of May 2019 the 34-member Anthropocene Working Group (AWG) of the International Commission on Stratigraphy (ICS) sat down to decide on when the Anthropocene actually started. That date would be passed on up the hierarchy of the geoscientific community  eventually to meet the scrutiny of its highest body, the executive committee of the International Union of Geological Sciences, and either be ratified or not. In the meantime the AWG is seeking a site at which the lower boundary of the Anthropocene would be defined by the science’s equivalent of a ‘golden spike’; theGlobal boundary Stratotype Section and Point (GSSP).

Several options were tabled for discussion and decision, summarised by a 2015 paper in Nature. A case against the erection of an official Anthropocene Epoch on stratigraphic grounds appeared in a GSA Todaypaper in 2016. Despite the fact that there is evidence for the start of human geological, geochemical and biological influences as far back as 8 000 years ago (in effect the Holocene is the Epoch of rapid human growth and transformations), the 2015 paper concludes that there are two candidates for the base of the Anthropocene. The earliest is the decline in atmospheric CO2 that began around 1570 CE and its recovery around 1620 CE recorded in Greenland ice cores. This is suggested to mark a fall in the indigenous population of the Americas from ~60 to ~6 million that followed the completion of European conquest, as a result of genocide, disease and famine. Regeneration of the American forest lands (~5 x 107 hectare) that the dead had once occupied drew down CO2.  However this overlaps with the coolest part of the Little Ice Age which may also have resulted in absorption of the greenhouse gas by cooled ocean water. The beginning of the industrial revolution was discounted on the grounds that it was diachronous as well as being difficult to define, having arisen first in Europe at some time in the 18th century. The second candidate was the period when ~500 nuclear weapons were tested above-ground, beginning in 1945 and ending by treaty between the then nuclear powers in 1963. These distributed long-lived plutonium globally, which resides in sediments as a ‘spike’. Around 1963 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. It is this last option upon which the AWG settled, with 29 members for and 5 against, and is to forward up the ‘chain of command’ in the geoscientific bureaucracy. A detailed and sometimes amusing account of the AWG’s deliberations appeared in the online Guardian newspaper on 30 May 2019.

The decision, in my opinion, signifies that the Anthropocene is an Epoch that includes the future, which is somewhat pessimistic as well as being scientific nonsense. Yet, coinciding as it does with rapidly escalating efforts, mainly by young people, to end massive threats to the Earth System, that can only be welcomed. It is an essentially political statement, albeit with a learned cloak thrown over it.  The only way to erase the exponentially growing human buttock print on our home world is for growth-dependent economics to be removed too. That is the only logical basis for the ‘green’ revolt that is unfolding. If that social revolution doesn’t happen, there will be a mass extinction to join the ‘Big Five’, and society in all its personifications will collapse. That is known as barbarism…

 

The effect of surface processes on tectonics

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

The Proterozoic Eon of the Precambrian is subdivided into the Palaeo-, Meso- and Neoproterozoic Eras that are, respectively, 900, 600 and 450 Ma long. The degree to which geoscientists are sufficiently interested in rocks within such time spans is roughly proportional to the number of publications whose title includes their name. Searching the ISI Web of Knowledge using this parameter yields 2000, 840 and 2700 hits in the last two complete decades, that is 2.2, 1.4 and 6.0 hits per million years, respectively. Clearly there is less interest in the early part of the Proterozoic. Perhaps that is due to there being smaller areas over which they are exposed, or maybe simply because what those rocks show is inherently less interesting than those of the Neoproterozoic. The Neoproterozoic is stuffed with fascinating topics: the appearance of large-bodied life forms; three Snowball Earth episodes; and a great deal of tectonic activity, including the Pan-African orogeny. The time that precedes it isn’t so gripping: it is widely known as the ‘boring billion’ – coined by the late Martin Brazier – from about 1.75 to 0.75 Ga. The Palaeoproterozoic draws attention by encompassing the ‘Great Oxygenation Event’ around 2.4 Ga, the massive deposition of banded iron formations up to 1.8 Ga, its own Snowball Earth, emergence of the eukaryotes and several orogenies. The Mesoproterozoic witnesses one orogeny, the formation of a supercontinent (Rodinia) and even has its own petroleum potential (93 billion barrels in place in Australia’s Beetaloo Basin. So it does have its high points, but not a lot. Although data are more scanty than for the Phanerozoic Eon, during the Mesoproterozoic the Earth’s magnetic field was much steadier than in later times. That suggests that motions in the core were in a ‘steady state’, and possibly in the mantle as well. The latter is borne out by the lower pace of tectonics in the Mesoproterozoic.

For decades geologists have pondered on ‘orogenic cycles’ and whether they are roughly equally spaced in time. The ‘boring billion’ refutes any such regularity. Stephan Sobolev and Michael Brown of the universities of Potsdam in Germany, and Maryland, USA, have investigates an hypothesis that may account for the long-term irregularity in tectonic processes (Sobolev, S.V. & Brown, M. 2019. Surface erosion events controlled the evolution of plate tectonics on Earth. Nature, v. 570, p. 52-57; DOI: 10.1038/s41586-019-1258-4). This stems from a suggestion in the late 1980’s that, once they begin to be subducted, unconsolidated sediments have a lubricating effect. If so, in the long term, the rate of accumulation of sediments at continental margins has a lot to do with the pace of tectonics. And that leads back to the rate of continental erosion. The two authors use a proxy for the global rate of subduction based on the variation over time of the cumulative length of mountain belts that show paired high- and low-pressure zones of metamorphism. They chart variations in continental erosion from its geochemical effects on ocean water, recorded by strontium isotopes in limestones, and by changes in the hafnium and oxygen isotopes of detrital zircons through time. Three time intervals show increases in Sr and O isotope parameters while that for Hf decreases. These indicators of greater continental erosion coincide with evidence for increased tectonic activity around the end of the Archaean Eon (centred on 2.5 Ga), in the early Palaeoproterozoic (2.2 Ga) and the early Neoproterozoic (0.75 Ga). The latter two bracket episodes of global glaciation that would certainly have shifted eroded material towards continental margins. Sobolev and Brown make a case for each representing episodes of increased lubrication. Lying between the last two tectonic paroxysms, the ‘boring billion’ delivered little sediment from the continents so any subduction was frictionally slowed.

I have little doubt that this view will have its detractors, not the least because the Earth continually generates heat as a result of its internal radioactivity. Plate tectonics is the main means whereby that heat emerges at the surface and radiates to space, thereby balancing heat production. Another issue is that mountain building elevates Earth’s surface, which provides the gravitational potential to drive products of erosion oceanwards. But it increases frictional resistance

Related article: Behr, W. 2019. Earth’s evolution explored. Nature, v. 570, p. 38-39; DOI: 10.1038/d41586-019-01711-8