Hadean potentially fertile for life

The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km2) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.

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Earliest departure of modern humans from Africa

In June 2017 the likely age of the earliest anatomically modern humans (AMH) was pushed back to almost 300 ka with the dating of their remains found at Jebel Irhoud in Morocco. It seemed only a matter of time before their first departure from Africa would also be shown to be earlier than generally believed at between 90 to 120 ka measured from AMH remains in the Skhul and Qafzeh caves of Israel. Such an exodus may be reflected by dates (80 to 113 ka) from fragmentary and indeterminate human remains in China, but a more definite, far-travelled AMH presence in east Asia is, so far, limited to about 60 ka. Yet there is genetic evidence from Neanderthal DNA from Germany and Siberia for human-Neanderthal interbreeding at some time between 219 and 460 thousand years before present: a very hazy intimation but one that needs accounting for. The main phase of genetic introgression from Neanderthals into Homo sapiens has been estimated to have occurred at between 50 to 60 ka; more easily explained by the known AMH peregrination into Asia in that period.

Misliya Cave on Mount Carmel, Israel has now added to the Levantine AMH record. A partial upper jaw and some teeth provide morphological data that fall within the range of H. sapiens fossils, along with tools ascribed to the Levallois technology. This involved striking flakes from a prepared core – a tortoise-like bulge on the flake that detaches when struck properly to form a pre-sharpened flake, flat on one side and rounded on the other. This method was shared by both AMH and Neanderthals, and examples of the tools extend as far back as 500 ka in Africa and may have been invented by a common ancestor of both human groups. Levallois tools were found with the AMH fossils at Jebel Irhoud and also in the Levant at Tabun, dated at 190 to 260 ka, but with no associated fossil remains of their makers. Those at Mislya Cave yielded a mean age from the use of three different dating methods at least 177 ka ago, making the fossil jaw found with them the earliest direct sign of AMH outside Africa (Hershkovitz, I. and 34 others 2018. The earliest modern humans outside Africa. Science, v. 359, p. 456-459; doi: 10.1126/science.aap8369).

So, Mislya supports the genetic evidence of human-Neanderthal Introgression in Eurasia (see; Stringer, C & Galway-Witham, J. 2018. When did modern humans leave Africa? Science, v. 359, p. 389-390; doi: 10.1126/science.aas8954) and provides a spur to extend work in China and between Arabia and eastern Asia. For decades the anatomically modern human remains in the Levant have been sidelined, that near-Mediterranean area being widely regarded as a ‘boulevard of broken dreams’. That is, until Levalloisian tools dated at up to 125 ka were found in the United Arab Emirates and Arabia as a whole had been shown to have had a monsoonal climate during the glacial period that preceded the last, Eemian interglacial and in several later episodes. Once in the Levant, and provided they continually had a foothold there, AMH had many windows of opportunity to move further east without having to await falls in sea-level to open routes such as that across the Red Sea via Straits of Bab el Mandab.

Ice cliffs on Mars

An illustration of what Mars might have looked...

An illustration of what Mars might have looked like during an ice age between 2.1 million and 400,000 years ago, when Mars’s axial tilt is believed to have been much larger than today.  (credit: Wikipedia)

For Mars to support life and for life to have emerged there demand water that is readily accessible from the surface. There is evidence that in the distant past liquid water may have flowed across the Martian surface to erode river-like features, some associated with the vast canyon system of Valles Marineris. That feature is thought to have been initiated by tectonic forces and perhaps flowing magma, but it shows definite signs of water erosion. Water in great volume was released during the Noachian phase of Mars’s evolution possibly by major impacts 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment). Large tracts of the Martian surface that are more muted than Valles Marineris show topographic features reminiscent of huge braided stream systems. Water may have covered vast, low-lying areas in the planet’s Northern Hemisphere to form an early ocean. Yet today the Red Planet seems extremely dry and its thin atmosphere shows only minute traces of water vapour – it is dominated by carbon dioxide. Results from various rovers deployed across its surface and from Mars orbiting satellites have, however, revealed signs of waterlain sediments and minerals that can only have formed by the breakdown of igneous rocks by water. Signs that liquid water continues to flow occasionally down steep slopes, such as rill-like features and ephemeral darkened patches, have been much disputed.

Mars does have an ice cap at its North Pole that waxes and wanes with its seasons, but rather than melting during Martian ‘summers’ the ice sublimates directly to water vapour. Conversely, the polar ices probably form from frost. Yet, astonishingly, there appear to be active glaciers complete with flow lines and moraines, but chances are that some of them are sediment flows ‘lubricated’ by frost binding together mineral particles and boulders that undergoes pressure-induced regelation. Data from orbiting neutron and gamma-ray spectrometers reveal that between 60°N and 60°S the top metre of Martian soil contains between 2 to 18% of ice, making it akin to terrestrial permafrost. So, contrary to its appearance Mars is rich in water, but almost exclusively in solid form. Until very recently, the bulk was thought to be as a matrix binding together sediments, accessible to future crewed mission in useful volumes only by surface mining. That somewhat pessimistic view has now changed dramatically.

Monochrome HiRISE image of a cliff on Mars (the pinkish swath is a simulated natural colour image – see below). beneath the cliff is a zone of jumbled ground formed by cliff collapses. (credit: NASA)

Careful study of fine resolution imagery from the HiRISE instrument on the Mars Reconnaissance Orbiter at latitudes a little less than 60° has centred on cliffs formed by recent erosion (Dundas, C.M and 11 others 2018. Exposed subsurface ice sheets in the Martian mid-latitudes. Science, v. 359, p. 199-201; doi: 10.1126/science.aao1619). Colin Dundas of the US Geological Survey, Flagstaff, Arizona, and US colleagues used the multispectral capacities of HiRISE data to study the composition of sedimentary layers exposed in the cliffs. In eight cases, the cliffs contained layered, almost pure blue ice tens of metres thick and only a few metres below the surface. The cliffs seem to have formed as ice has sublimated where exposed, thereby undermining to sedimentary cover. Below the cliffs are jumbled zones of collapsed material. Being so close to the surface and underlain by apparently ice-free sediments, the layered ice sheets must be geologically quite young.

Simulated natural-colour HiRISE image of a Martian cliff showing nearly pure water ice in blues. Note the layered structure that may represent seasonal variations during the period of ice formation (credit: NASA)

Unlike the Earth, whose axial tilt is stabilised to a large degree by the Moon’s gravity, Mars’s two tiny moons have little effect of this kind. So Mars’s axis wobbles between its current 25° tilt to as much as 45°. This results in large climatic shifts, of which there have been an estimated forty over the last 5 million years. At high tilts solar energy heats up the poles and releases water vapour by accelerated sublimation to be laid down at lower latitudes as frost or snow. Mars’s present tilt suggests that it is experiencing a cold episode so that wind blown dust has covered and preserved mid-latitude ice sheets over tens of thousand years. Nearly pure ice is easier to exploit than permafrost layers. Yet optimism among enthusiasts for a crewed Mars mission and eventual colonisation is tempered by the latitudes of the discoveries. While ready supplies of water from ice and CO2 from the Martian atmosphere give the ingredients for oxygen, methane through catalysis of CO2 and hydrogen, agricultural photosynthesis and all kinds of other useful chemistry, low latitudes offer the most assured solar energy supplies. Latitudes around 55° are frigid and dark during Martian winters; perhaps totally inhospitable. So the remote-sensing search is likely to continue in cliffs closer to the ‘tropics’ of Mars.

Fish influence mountain ranges

When asked if he would like water in his whisky W.C Fields famously remarked that he didn’t drink water because fish procreate in it (his actual words were somewhat racier). Migratory salmon do so in their millions with a great deal of energy, specifically in the gravel beds of high-energy streams. Before spawning, females lash the stream bed with their tails to create a pit or redd in the gravel, in which they lay their eggs to be fertilised  by males. Then she fills-in the redd with more gravel excavated from upstream. Salmon spawning grounds are thus easily recognised as pale patches of freshly overturned gravel on a stream bed that also contain lower amounts of fine sediment and are thereby loosened. As well as discouraging bibulous old men from diluting their liquor, it occurred to Alexander Fremier of Washington State University and other American colleagues that here was a noteworthy example of an active part of the biosphere physically intervening in the rock cycle. Not that it comes even close to what humans have become capable of since the Industrial Revolution, but it might be an object lesson in the fragility of what are otherwise the robust processes of erosion. Moreover, since salmon emerged at some time in the past, their actions might help demonstrate that evolutionary events – speciation, adaptive radiations, mass extinctions etc – play a role in transforming geological processes.

Pacific salmon are semelparous or "big ba...

Pacific Sock-eye salmon that die shortly after spawning (credit: Wikipedia)

Fremier and colleagues (Fremier, A.K. et al. 2017. Sex that moves mountains: The influence of spawning fish on river profiles over geologic timescales. Geomorphology online publication; doi.org/10.1016/j.geomorph.2017.09.033) modeled the consequences of salmon spawning habits for the critical stress needed to set grains in motion, theoretically and in a flume tank. Based on a significant reduction of the critical stress, models for the evolution on various river profiles in the vicinity of salmon spawning grounds suggest that river beds may cut deeper at rates up to 30% faster than they would in the absence of salmon. Were salmon to be reduced or extirpated through dam construction or overfishing then sedimentation in channels would increase. In some areas of extensive farming of salmon in offshore pens, escape and colonization of rivers would eventually change sedimentation and erosion patterns. The findings vary from species to species, but salmon may have had a significant effect on generally rugged landscapes following their appearance in local ecosystems.

The terrestrial-marine-terrestrial migratory habits of salmon, including the return of adults to their birth rivers to spawn, are uncommon if not unique. Their forbears must have evolved to this behaviour at some time in the geological past, separately in the case of North Atlantic and North Pacific species. The authors suggest that adaptive radiation of salmon may have been favoured by orogenic events in western North America around 100 Ma ago that created the system of fast flowing rivers that salmon favour. In turn, salmon may have significantly influenced Western Cordillera landscapes of Alaska, Canada and the conterminous Unites States. A nice example of the inseparability of cause and effect on the scale of the Earth System.

Sunrise Girl-Child: the first American colonist

Thanks to a variety of archaeological finds of tools and animal bones bearing cut marks, together with precise dating, it now seems clear that the Americas began to be colonised as early as the Last Glacial Maximum from tangible evidence from Bluefish Cave in the Yukon territory of Canada and as early as 15.5 ka close to the southern tip of South America in Chile. Although confirmation remains to be found, there is even a possibility that pre-sapiens people had arrived far earlier. Advances in analysis of ancient genetic material help understand the divergence of early colonisers. Y-chromosome DNA from living indigenous men suggests that all early Americans stemmed from 4 separate colonising populations who may have entered by crossing the Beringia land bridge, exposed as a result of glacial fall in global sea level, to follow different routes, including along the Pacific coast. A possible common ancestor of all native Americans emerged in 2013 from the mitochondrial and Y-chromosome DNA of the skeleton of a young man from near Lake Baikal in Siberia who lived about 24 ka ago. At the very start of 2018 an online paper in Nature took the story even further.

This image was first published in the 1 st (18...

The diversity of Native American people. (credit: Wikipedia from a 19th century Norwegian painting)

The remains of a ~6-week-old girl recovered from a site at Upward Sun River in Alaska – called ‘Xach’itee’aanenh t’eede gay’, or ‘sunrise girl-child’ by indigenous Alaskans – dated at 11.5 ka, has yielded a precise genome (Moreno-Mayar, J. and 17 others 2018. Terminal Pleistocene Alaskan genome reveals first founding population of Native Americans. Nature; doi:10.1038/nature25173).  The baby girl’s DNA shows that the group to which she belonged was ancestral to contemporary and fossilised ancient Native Americans. She was probably a member of a founding population of ‘Beringians’. At the end of the last glacial epoch (11.5 ka) a separate branch of Native Americans was already established in unglaciated North America further south. That group had split into two further groups sometime between 17.5 to 14.6 ka, who became ancestors of most of the indigenous people of the Americas. The ‘Beringian’ people were therefore probably stranded in the far north by the difficulties of crossing the vast North American ice sheet. Probing deeper into time, using demographic modelling, suggests that the founding population of all Native Americans, including the ‘Beringians’, split from East Asians around 36 ka ago. Gene flow among them and with East Asians persisted until about 25 thousand years ago, with some admixture with ancient northern Eurasians up to 20 ka. It seems that the ‘Beringians’, of whom little ‘sunrise girl-child’ was a late member, became isolated genetically between 22-18 ka.

The ancestral mixture of both East Asian and northern Eurasians that led to the founders of the whole panoply of geographically isolated Native Americans is remarkable. It shows just how far human groups moved and mingled during the run-up to the Last Glacial Maximum, which made the far north just about uninhabitable – or so it has been assumed. For a small ethnically mixed group to survive such conditions for so long suggests considerable ingenuity in living off the land.

http://www.bbc.co.uk/news/science-environment-42555577

Early human dispersal through Asia

When first mooted, the Out of Africa model for the spread of anatomically modern humans (AMH) centred on a single exodus from African to Eurasia, which researchers broadly agreed to have occurred about 60 thousand years ago. That was when an advance of continental glaciers and sea level fall narrowed to manageable proportions the obstacle presented by the Red Sea. The only archaeological drawback was that AMH had occupied the Levant at around 110 ka. That was formerly considered to have been a temporary occupation corralled by hyperarid conditions immediately to the east and a mountain barrier to the north, with the Mediterranean Sea to the west. Yet, during humid periods there was every chance that the eastern barrier would occasionally have been permeable. Plumping for the 60 ka exit model was a conservative view stifled by a lack of high-quality dates for scattered suggestions of an Asian AMH presence, such as occurrences of stone tools resembling those of early moderns and even rarer, incomplete and often ambiguous skeletal remains. The ‘modern-looking’ tools that occurred both above and below the 74 ka Toba ash deposit in southern India were disposed of as ‘advanced’ tools of earlier migrants; probably Homo erectus. In retrospect, the established fact of earlier occupation of Eurasia by such ‘primitive’ African migrants, as long ago as ~1.8 Ma in the case of Homo fossils in Georgia, should have encouraged the view that culturally better-endowed AMH would have had less problem in diffusing eastwards once they found an escape route from Africa.

Whatever, the flurry during the last couple of decades of more skeletal and archaeological remains of AMH in Asia, genetic evidence for their interbreeding in the west and east with earlier human groups and, principally, improvements in dating ancient sites suggests a more complex geographic flow. Christopher Bae of the University of Hawaii and colleagues based in the UK, Germany and the US have reviewed this growing wealth of new data to put forward various scenarios for Out of Africa dispersal through Asia (Bae, C.J. et al. 2017. On the origin of modern humans: Asian perspectives. Science, v. 358, p. 1269 (summary); online full paper DOI: 10.1126/science.aai9067). They highlight growing evidence for at least one pre-60 ka dispersal, and probably several, to reach the Levant, Arabia, India, China, Laos, Indonesia, the Philippines and Australia before that date. This tallies with Neanderthal and Denisovan DNA segments within the genomes of living Eurasians that indicate interbreeding before 60 ka.

Bae and colleagues also assemble data that bear on where AMH managed to move out of Africa. They resolve the dispute between routes around the northern shores of the Red Sea and crossing the southern Straits of Bab el Mandab by concluding ‘why not both’. Where the migrations went to is currently suggested by the distribution of sites that reveal either pre- or post-60 ka occupancy. The earlier dispersals may have been dominated by following coastlines along the Mediterranean in North Africa to the Levant and via Bab el Mandab across the Persian Gulf, along the northern Indian Ocean littoral to south-east and east Asia. The later, more ‘adventurous’ movements using both routes led to Europe and deeper into continental Asia and thence to its north east. The review conveniently covers in seven pages much the same geoarchaeological and anthropological ground as Earth-Pages has visited bit-by-bit as it has unfolded since 2000. Clearly, great swathes of Asia have not been explored by palaoanthropologists. As in most geographic sciences there is a tendency to follow up known sites year after year – often decade after decade – to ensure publishable results, and that will consume lots of economic and human resources. It is more risky to try and fill in the gaps, but that basic field work is urgently needed to supply new material.

Lid tectonics on Earth

Geoscientists have become used to thinking of the Earth as being dominated by plate tectonics in which large, rigid plates of lithosphere move across the surface. They are driven mainly by the sinking of cold, densified lithosphere in slabs at subduction zones. The volume of recycled slabs is replaced by continual supply of mafic magma to form oceanic crust at constructive margins. Such a process has long been considered to have reached far back into the Precambrian past and there are lively debates concerning when this modus operandi first arose and what preceded it. Now that we know more about other rocky planets and moons it appears that Earth is the only one on which plate tectonics has occurred. The other, more common, behaviour is dominated by stagnancy, although some worlds evidence volcanism and resurfacing as a result of giant impacts. Their subdued activity has come to be known as ‘lid tectonics’, in which their highly viscous innards slowly convect beneath a rigid, stagnant lid through which thermal energy is lost by convection: they are ‘one-plate’ systems. Although Earth loses internal heat by conduction through plate interiors, a large amount dissipates by convection associated with constructive margins: the oceanic parts of its plates lose heat laterally, as they grow older. Six papers in an advance, online issue of the free-access journal Geoscience Frontiers are concerned with the issue of terrestrial lid-tectonics and whether or not it dominated the Earth repeatedly in its Precambrian history.

A model is emerging for a hot, early Earth that was dominated by a form of lid tectonics (Bédard, J.H. 2018 Stagnant lids and mantle overturns: implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics. Geoscience Frontiers, v. 9, 19-49; https://doi.org/10.1016/j.gsf.2017.01.005). Bedard’s model centres on lithosphere that was so weak because of its temperature that its subduction was impossible. Density of the lithosphere rarely increased above that of the mantle because the necessary mineralogical changes were not achieved – those involved in plate tectonics require low-temperature, high-pressure metamorphism as oceanic lithosphere is driven down at modern subduction zones. Even if such reactions did happen, the lithosphere would have been too weak to sustain slab-pull force and dense lithosphere would have simply ‘dripped’ back to the mantle. Mantle convection in a hotter Earth would have been in the form of large, long-lived upwelling zones rather than the relatively ephemeral and narrow plumes known today. Low density materials resulting from magma fractionation, the precursors of continental crust, would have been shifted willy-nilly across the face of the planet to collide. accrete and undergo repeated partial melting. In Bedard’s view, plate tectonics arose as Earth’s heat production waned below a threshold that permitted rigid lithosphere, probably in the late Archaean, to dominate after 2.5 Ga.

Bédard’s impression of an early Archaean lid-tectonic scenario. (credit: Jean H Bédard 2018, Figure 3B)

A radically different view is that stagnant-lid episodes alternated with periods of limited subduction and plate tectonics in the Archaean. Some Archaean cratons – the so-called ‘granite-greenstone terrains – seems to provide geological evidence for lid tectonics (Wyman, D. 2018. Do cratons preserve evidence of stagnant lid tectonics? Geoscience Frontiers, v. 9, 19-49; https://dx.doi.org/10.1016/j.gsf.2017.02.001). Others, such as the famous Isua supracrustal belt in West Greenland hint at plate tectonics. John Piper, of Liverpool University in Britain, argues from a series of Archaean palaeomagnetic polar wander curves that in three periods – ~2650 to 2200 Ma, 1550 to 1250 Ma, and 800 to 600 Ma – the poles shifted comparatively slowly with respect to the cratons providing the magnetic data; a feature that Piper ascribes to dominant lid tectonics (Piper, J.D.A., 2018. Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian Times: palaeomagnetism confirms prolonged quasi-integrity and absence of Supercontinent Cycles. Geoscience Frontiers, v. 9, p. 61-89; https://doi.org/10.1016/j.gsf.2017.07.009). Similarly, there is some evidence based on the geochemical variation of basaltic rocks derived from the mantle. Through the Archaean, geochemical changes roughly follow cycles in the abundance of zircon radiometric ages and other geological changes that may reflect plate- and lid-tectonic episodes (Condie, K.C. 2018. A planet in transition: the onset of plate tectonics on Earth between 3 and 2 Ga? Geoscience Frontiers, v. 9, p. 51-60; https://doi.org/10.1016/j.gsf.2016.09.001). Interestingly, the age-frequency plot of almost three thousand Archaean and Hadean zircons recovered from the famous 1.6 Ga old sandstones of the Jack Hills Formation in Western Australia reveals similar cycles that may reflect such tectonic fluctuations in the Hadean (Wang, Q. & Wilde, S.A. 2017. New constraints on the Hadean to Proterozoic history of the Jack Hills belt,Western Australia. Gondwana Research, v. 55, p. 74-91; https://doi.org/10.1016/j.gr.2017.11.008). Since zircons are most likely to crystallize from intermediate and felsic magmas – i.e. precursors of continental material – their abundance in the Jack Hills rocks suggests that their source must have been in the 3.7 to 3.3 Ga gneisses on which the younger sediments rest. That is, part of those Archaean gneisses may well be made up of Hadean continental material that was repeatedly reworked and maybe remelted since such crust first appeared (in the form of surviving zircons) around 4.4 to 4.5 Ga, perhaps during vigorous lid-tectonic regimes.

Possible evolution of magmatic and tectonic styles for large silicate planets. (Credit: Stern et al. 2018, Figure 3)

Based on their reassessment of tectonic activity revealed by 8 rocky planets and moons Robert Stern of the University of Texas (Dallas) and colleagues from ETH-Zurich suggest a possible evolutionary sequence of tectonics and magmatism that Earth-like bodies might go through (Stern, R.J. et al. 2018. Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids. Geoscience Frontiers, v. 9, p. 103-119 ; https://doi.org/10.1016/j.gsf.2017.06.004). In their scheme plate tectonics requires certain conditions of lithospheric density and strength to evolve and suggest that, depending on planetary characteristics, slab-pull driven tectonics is likely to be preceded and followed by stagnant lid tectonics, to give perhaps a cyclical geotectonic history.

The rise of the eukaryotes

You and I, and all the living things that we can easily see belong to the most recently evolved of the three great domains of life, the Eukarya. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the Bacteria and the Archaea. Their genetic material floats around in their cell’s fluid, while ours resides mainly in the eukaryote cell’s nucleus with a bit in various organelles known as mitochondria and the chloroplasts of plant cells. Unlike the chicken and egg question, that concerning which came first, prokaryotes or eukaryotes, is answered by DNA. Eukaryote DNA contains a lot from prokaryotes, but the converse does not hold. That contrast posed the question of how eukaryotes arose from the two earlier, simpler forms of life, the answer to which Lynn Margulis suggested to be a whole series of symbiotic relationships among various prokaryotes that shared a host cell; her hypothesis of endosymbiosis. Now, the vast majority of eukaryotes depend on free oxygen for their metabolism, so when the first of them arose boils down to the period of geological history following the Great Oxidation Event around 2.4 billion years ago.

Structure of a typical animal cell

Structure of a typical eukaryote (animal) cell (credit: Wikipedia)

Molecular-clock estimates based on the range of variation in the genomes of a wide range of eukaryotes suggest it took place sometime between 1000 and 2000 Ma. A better means of homing in on a date for the Last Eukaryote Common Ancestor (LECA – as opposed to that of the first organism LUCA) would be that of the earliest fossil to show eukaryote affinities. Grypania from 1.85 Ga, a sort of whorl-like fossil, is a good candidate and is widely thought to be the earliest of our kind but lacks signs of actual cells. More convincing fossils – known generically as acritarchs – from times between 1.5 and 1.0 Ga look like primitive fungi, red algae and slime moulds. A comprehensive review of the microfossils of the Palaeoproterozoic (2.5 to 1.6 Ga) includes both prokaryotes and probable early eukaryotes (Javaux, E.J. & Lepot, K. 2017. The Paleoproterozoic fossil record: Implications for the evolution of the biosphere during Earth’s middle-age. Earth Science Reviews, v. 176, p. 68-86; doi: 10.1016/j.earscirev.2017.10.0001). Yet, despite rapidly accumulating evidence, especially from rocks in China, the picture remains one of monotony; for instance Grypania spans the best part of half a billion years. Bacteria and Archaea cannot be distinguished easily in the absence of preserved DNA. Despite evidence for oxygen in the oceans and atmosphere, apart from a few shallow-water oxygenated examples the chemistry of Palaeoproterozoic marine sediments is dominated by mineralogical outcomes of reducing chemistry. Many chemical isotopic environmental proxies ‘flat-line’ to the extent that the early Proterozoic is sometimes referred to as the ‘boring billion’, yet our ultimate precursors were part of the marine ecosystem. That is, unless one accepts the possibility that that fossils labelled ‘eukaryote’ are colonial prokaryotes – evidence for cell nuclei is sparse. Endosymbiosis, although an attractive model for eukaryote origins, is not proven. The reason for lingering scepticism is that there are only a tiny number of modern examples of prokaryote cells ending up inside those of other prokaryotes.

Whatever, chemical biomarkers in sediments older than about 720 Ma indicate that prokaryotes were the only notable primary producers in the oceans until the Neoproterozoic. Microscopic fossils that are inescapably eukaryotes in the form of amoeba suddenly emerge around that time. This development from the lingering marginality of early eukaryotes to thriving ecosystems that they dominated thereafter is a puzzle seeking a plausible explanation. It coincides with the onset of the Snowball Earth glaciations of the Cryogenian Period (850 to 635 Ma) and a rise in atmospheric and presumably oceanic oxygen. Then macroscopic eukaryotes ‘bloomed’ into distinctively different forms in the Ediacaran Period (635 to 541 Ma) and thereafter. Before the Cryogenian we can perhaps regard eukaryan life and the endosymbiosis that may have given rise to it as a series of ecological experiments repeatedly knocked-back by chemical conditions and competition with the vastly more abundant prokaryotes.

 

Banded iron formations (BIFs) reviewed

This image shows a 2.1 billion years old rock ...

2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)

During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent. But commercial reserves are confined mainly to sedimentary sequences dating from about 3 to 2 billion years ago. They are not the only commercial iron formations, but dominate supplies from estimated reserves of around 105 billion tons. From a non-commercial standpoint they are among the most revealing kinds of sediment as regards the Earth system and its evolution. All scientific aspects of BIFs and similar Fe-rich sediments are reviewed in a recent volume of Earth Science Reviews. (Konhauser, K.O. and 12 others 2017. Iron formations: a global record of Neoarchaean to Palaeoproterozoic environmental history. Earth Science Reviews, v. 172, p. 140-177; doi: 10.1016/j.earscirev.2017.06.012).

The chemical, mineral and isotopic compositions of BIFs form a detailed repository of the changing composition of seawater during a crucial period for the evolution of Earth and life – the transition from an anoxic surface environment to one in which water and air contained a persistent proportion of oxygen, known as the Great Oxidation Event (GOE). Paradoxically, BIFs are highly oxidized rocks, the bulk of which formed when other rocks show evidence for vanishingly small amounts of oxygen in the surface environment. The paradox began to be resolved when it was realized that ocean-ridge basaltic volcanism and sea-floor hydrothermal activity would have released vast amounts of soluble, reduced iron-2 into anoxic seawater, in the upper parts of which the first photosynthetic organisms evolved. Evidence for the presence of such cyanobacteria first appears around 3.5 billion years ago, in the form of carbonates whose structure suggests they accumulated from growth of microbial mats. Oxygen generated by photosynthesis in iron-rich water immediately acts to oxidize soluble iron-2 to iron-3 to yield highly insoluble iron oxides and hydroxides and thus deposits of BIFs. While oceans were iron-rich, formation of ironstones consumed ecologically available oxygen completely.

Other biological processes seem to have been involved in ironstone formation, such as photosynthesis by other bacteria that used dissolved iron-2 instead of water as a reductant for CO2, to release iron-3 instead of oxygen. That would immediately combine with OH­ ions in water to precipitate iron hydroxides. Konhauser and colleagues cogently piece together the complex links in chemistry and biology that emerged in the mid- to late Archaean to form a linkage between carbon- and iron cycles, which themselves influenced the evolution of other, less abundant elements in seawater from top to bottom. The GOE is at the centre. The direct evidence for it lies in the sudden appearance of ancient red soils at about 2.4 billion years, along with the disappearance of grains of sulfides and uranium oxides – both readily oxidized to soluble products – from riverine sandstones, which signifies significant oxygen in the atmosphere. Yet chemical changes in Precambrian marine sediments perhaps indicate that oxygen began to rise in ocean water as early as 3 billion years ago. That suggests that for half a billion years biogenic and abiogenic processes in the oceans were scavenging oxygen as fast as it could be produced so that only tiny amounts, if any, escaped into the atmosphere. Among other possible factors, oceanic methane emissions from methanogen bacteria may have consumed any atmospheric oxygen – today methane lasts only for about 9 years before reaction with oxygen forms CO2. If and when methanogens declined free oxygen would have been more likely to survive in the atmosphere.

The theme running through the review is that of changing and linked interactions between life and the inorganic world, mantle, lithosphere, hydrosphere and atmosphere that involved all available chemical elements. The dominant chemical process, as it is today, was the equilibrium between oxidation and reduction – the loss and gain of electrons among possible chemical reactions and in metabolic processes. Ironstones were formed more commonly between 3 to 2 Ga than at any time before or since, and form a substantial part of that periods sedimentary record. Their net product and that of the protracted organic-inorganic balancing act – oxygenation of the hydrosphere and atmosphere – opened the way for eukaryote organisms, their reproduction by way of the splitting and recombination of nuclear DNA and their evolutionary diversification into the animal and plant life that we know today and of which we are a part. It is possible that even a subtly different set of global processes and interactions set in motion during early evolution of a planet apparently like Earth may have led to different and even unimaginable biological outcomes in later times. The optimism of exobiologists should be tempered by this detailed review.

Mega-impacts and tectonics

Because they are fast as well as weighty, destination-Earth asteroids and comets pack quite a punch. That is because their kinetic energy is proportional to the square of their speed (at least 13 km s-1) as well as half their mass. So, even all one half a kilometre across carries an energy a hundred times the solar energy received by Earth in a year, and a great deal more when compared with geothermal heat production. Much of the focus on the effects of impact events has dwelled on the upper crust, the oceans and atmosphere. Yet they also have huge seismic effects, with a proportion of their shock effect being dissipated throughout the entire planet. One obvious consequence would be a thermal anomaly directly beneath the crater as well as some thinning of the lithosphere and body waves affecting the rest of the solid Earth.

Thermal and mechanical processes lie at the core of tectonics, so a big question has been ‘Could impacts create mantle plumes or set new tectonic processes in motion?’ There has been speculation of diverse kinds since impacts became popular following the link between the Chicxulub crater and the end-Cretaceous mass extinction, discovered in 1980. But ‘educated guesses’ have generated more hot air than clear conclusions. Much as most of us are modelling-averse, a mathematical approach is the only option in the welcome absence of any severe extraterrestrial battering to which scientists have borne witness. With refined algorithms that cover most of the nuances of projectiles and targets – conservation of mass, energy and momentum in the context of the solid Earth behaving as a viscous medium –  Craig O’Neill and colleagues at Macquairie University, Australia, and the Southwest Research Centre in Boulder, CO USA, have simulated possible tectonic outcomes during plausible bombardment scenarios during the Hadean (O’Neil, C. et al. 2017. Impact-driven subduction on the Hadean Earth. Nature Geoscience, v. 10, p. 793-797; DOI: 10.1038/NGEO3029).

It appears that truly gargantuan objects – radius >500 km – are required to stimulate sufficient thermal anomalies that would lead to mantle upwellings whose evolution might lead to subduction at their margins. One at the limit posed by lunar cratering history (~1700 km radius) could have resulted in wholesale subduction of the entire lithosphere present at the time about 4 Ma after the impact. In the Hadean, it is likely that the lithosphere would have had a roughly mantle composition, so that the density excess needed for slab descent would have been merely temperature dependent. Note: after the onset of a basalt-capped lithosphere heat flow would have needed to be below the limit at which basalt converts to eclogite at high pressures, and thus to a density greater than that of the mantle, for continuing subduction. The authors’ Hadean scenario is one of episodic subduction dependent on the projectile flux and magnitude; i.e. with an early Hadean with stop-start subduction waning to tectonic stagnation and then a restart during the Late Heavy Bombardment after 4.1 Ga. Evidence for this is clearly scanty, except for Hadean zircons, whose presence indicates differentiation of early magmas with a peak between 4.0 to 4.2 Ga, in which magnetic intensities are preserved that are roughly as predicted by the scenario.

No impacts preserved in Precambrian to Recent times suggest extraterrestrial objects with the power to induce significant changes to global tectonics.