How rich are deep-sea resources?

My first task as a Lecturer in Earth Sciences at the British Open University, from 1971 onward, was to write teaching materials about the economics, formation and geological setting of metal resources. Much of the content was about the full range of ‘conventional’ metal ores, but something being publicised as having huge potential intrigued me. This concerned manganese-rich nodules (with the aesthetic appeal of unwashed potatoes) and crusts found sitting on top of sediments of the abyssal ocean floor, at depths between 3 to 5 kilometres.  While manganese is by no means a rare element and occurs in vast ore reserves on the continents, the nodules contain unusually high concentrations of other, more valuable metals, such as copper, nickel, zinc, cobalt and lead. Some contained more than 3% of Cu, Ni and Co combined, above the ‘grades’ of economic deposits of ores of the individual metals on land. This was the source of their potential: simple, albeit very deep dredging of the nodules would provide multi-metal ore of very high profitability. Moreover, the nodules are in truly vast tonnages (about 10 kg m-2) and continually grow by precipitation from seawater in the underlying sediments at a few millimetres per million years – they are renewable resources.

Manganese nodules taken from the bottom of the...

Manganese nodules from the Pacific abyssal plains. (credit: Wikipedia)

A variety of reasons, not the least of which was the vexatious question of ownership of sea-floor resources far from land, have meant that commercial operations have yet to begin. However, spiralling prices for metals on the world market together with depletion of on-shore, high-grade reserves are beginning to make the opportunity of nodule mining irresistible. Fifteen companies, with licence areas issued by the intergovernmental  International Seabed Authority of around 75 000 km2 each, are now engaged in economic assessment of one of the most remote swathes of the Pacific abyssal plains (Peacock, T. & Alford M.H. 2018. Is deep-sea mining worth it? Scientific American, v. 318(5) (May 2018 issue), p. 63-67). There are several controversial issues surrounding deep-sea mining. First, dredging, like beam trawling disturbs and destroys ocean-floor ecosystems and turns bottom water turbid, the very fine grain size of sediments resulting in settling being very slow ( about 1 mm s-1). Second, preliminary ore processing on board dredging vessels results in plumes of turbid and metal-rich slurry in the wakes, threatening surface and mid-water ecosystems. Such plumes will rapidly spread far from operational areas in surface current systems, eventually to smother pristine areas of ocean floor. Re-examination of areas of experimental dredging from 30 years ago have revealed that they are still sterile of lifeforms larger than 50 micrometres. Added to these effects, onshore processing will produce large amounts of waste – about 75% of the volume of dredged nodules. Conventional mines eventually backfill their excavations, but with nodule mining disposal would be an environmental nightmare.

Japanese sea-floor mining machine. (credit: Japan Times)

Economically, it seems that nodule dredging is potentially highly profitable. To break even requires lifting about a million metric tons, which would yield of the order of 37 000 t of Ni, 32 000 t of Cu, 6000 t of Co and 750 000 t manganese. If all 15 companies begin extraction, production at these levels will have a downward effect on world metal prices, tending to undercut production from conventional mines. One little-considered issue is that the ‘blend’ of metals from nodules will not match the industrial demand for each of them, further destabilising markets. Added to mining of the abyssal plains, plans are well advanced for multi-metal mining of massive sulfide deposits forming at hydrothermal vents or ‘black smokers’ along mid-ocean ridge systems, in which gold figures strongly. Only a few Pacific island states have resisted the ‘promise’ of such operations. Japanese companies are already mining the seabed off Okinawa within their own offshore waters and seemingly are producing zinc equivalent to the country’s annual consumption as well as gold, copper and lead.

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When dinosaurs roamed the Western Isles

Cuillin Hills, Isle of Skye, Scotland, UK

Cuillin Hills, Isle of Skye, Scotland, UK (credit: Wikipedia)

The Isle of Skye off the northwest coast of Scotland  is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser known Monster, the dread Each Uisge (Water Horse). Yet evidence is emerging for the former presence in the Hebrides of other, more tangible monsters.

Skye’s great volcanic edifice rests on Mesozoic sedimentary rocks including shallow-water muddy limestones of the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age. For dinosaur specialists this is of the time when meat-eating theropods and herbivorous sauropods began growing to colossal sizes. Yet the Bathonian is notable for its global paucity in well exposed terrestrial and near-shore sedimentary sequences. Easily accessible, the Skye Bathonian sequence is much visited and has yielded a rich, though generally fragmentary fauna. A group of recent visiting palaeontologists from the University of Edinburgh, the Chinese Academy of Sciences and Skye’s Staffin Museum have discovered an extensive tract of wave-cut platform on the east shore of the Trotternish Peninsula where lagoonal carbonate muds were trampled by several dinosaurs that left around 50 tracks (dePolo, P.E. et al. 2018. A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, online; doi:10.1144/sjg2017-016).

Dinosaur foot prints from Skye. Left example of a sauropod rear-foot print; right theropod. (credit dePolo, P.E. et al. 2018, modified from Figs 8 and 9a)

Some are of medium-sized sauropods (either Parabrontopodus or Breviparopus – both names for footprints rather than any genus of dinosaur) whose crudely elephant-like footprints are up to 0.5 m across (the largest, from Western Australia, are about 1.7 m across). Although there are fragmentary dinosaur bones from the same strata, assigning the footprint to a known species is not possible. However, foot size can be used to estimate how high the creatures’ hips stood (2 to 2.5 m): hefty beasts but not the true giants of later times A variety of three-toed, clawed, somewhat bird-like, footprints also occur. They are assigned to probably bipedal carnivores or theropods. Variation in foot size suggests a range of hip-height from about 0.9 to 2 metres, so these carnivores would have been pretty formidable.

Volcano heading for the sea

John Murray of The Open University, UK has been studying Europe’s largest active volcano Mount Etna on Sicily for most of his career. With a group of colleagues he installed high-precision GPS receivers at over 100 stations on the flanks of the mountain. This was to monitor any shifts in elevation and geographic position, which might be related to magmatic events within the volcano, such as inflation and contraction of the magma chamber. Measurements of position gathered annually since 2001 reveal a somewhat alarming picture (Murray, J.B. et al. 2018. Gravitational sliding of the Mt. Etna massif along a sloping basement. Bulletin of Volcanology, v. 80 online, open access; doi /10.1007/s00445-018-1209-1). The edifice is moving relentlessly ESE at 14 mm yr-1, on average, towards the Mediterranean Sea. Research by one of Murray’s co-authors, Benjamin van Wyk de Vries of the Université Clermont Auvergne, established that many volcanoes have associated signs of deformation due to their huge masses. Often, this is a matter of radial spreading that produces thrust-like faults at their base and in the basement on which they grew. In the case of Etna all the annual displacements on its flanks are skewed to the ESE. The researchers are able to show that this is not a case of flank instability that ultimately may result in lateral collapse but the entire volcano is slowly slipping sideways.

English: Mount Etna, Sicily, topped in snow It...

Mount Etna, Sicily, topped in snow (credit: Wikipedia)

An experimental mock up of the volcano– a cone and flanking layers of lava and pyroclastic rocks made of sand on a substrate of putty to represent underlying sedimentary strata – began to slide once it was tilted at a shallow angle. This suggests that the base of the volcano and igneous debris that it has emitted dips gently to the ESE. The underlying materials are poorly consolidated Quaternary sediments, which are likely to be rheologically weak. Geophysics shows that the NW side of the volcano rests on an almost horizontal plateau, the cone itself being above a spoon-like depression, probably produced by the cone’s mass, and the base dips seawards  in the SE sector. It is through this basement that magma makes its way to Etna’s summit vent system, probably along fractures.

The authors warn that such sliding volcanoes are prone to devastating sector collapse on the downslope side, although there are no signs that might be imminent. Yet it will almost certainly have an effect on eruptive activity as the magma conduits are continually changing. Future research needs to focus on periods when there is horizontal contraction on the volcano, as happens during lengthy periods of dormancy – the period for which there are data has been one of expansion.

Human evolution and revolution in Africa

Toba eruption: a hindrance or a spur to humans?

English: Erosional dissection of an ash deposi...

Ash-fall blanket at Mount Pinatubo volcano in the Philippines. (credit: Wikipedia)

The eruption that created the 100 by 30 km Toba caldera 74 ka ago was the largest recorded volcanic event during the two million years of the genus Homo’s evolution. It ejected an estimated 800 cubic kilometres of ash to blanket the land surface thousands of kilometres away. By analogy with the known effects of stratospheric ash and sulfate aerosols from  the much smaller 1991 eruption of the Mount Pinatubo in the Philippines, which  reduced mean global temperature by 0.5 °C, Toba might be expected to have had an even larger cooling effect, perhaps by as much as 10° C. Such a scenario has led palaeoanthropologists to suggest that there would have been major effects on humans migrating across Eurasia at the time, such as dramatic population reduction and maybe a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations. Yet, not only are stone tools found below the Toba Ash in Sumatra, but also in South India and immediately above it too. Yet analysis of Toba’s environmental effects recorded by sediments on the bed of Lake Malawi in southern African reveal little if any sign of a global ‘volcanic winter’.

A letter in Nature, published online on 15 March 2018 (Smith, E.J. and 15 others 2018. Humans thrived in South Africa through the Toba eruption about 74,000 years ago. Nature, v. 555; doi:10.1038/nature25967) decisively refutes any retardation of human cultural progress, in southern Africa at least, and suggests the opposite. Smith and colleagues from South Africa, Australia and the US found ash dated at around 74 ka 9,000 km away from Toba in sedimentary sequences that contain anatomically modern human remains and artefacts at the coastal Vleesbaai and Pinnacle Point sites in Cape Province South Africa. To check on the likelihood of a fortuitous coincidence of another eruption having shed the ash, the team compared detailed geochemical analyses of ash samples with those from other volcanoes and bona fide Toba samples, with a clear confirmation of provenance. In the archaeological record, rather than any sign of a cultural setback, the intensity of use of the sites increased after the ash-fall event, accompanied by significant technological innovations. Perhaps the Pinnacle Point community was lucky and also responded in the spirit of the adage ‘necessity is the mother of invention’. Discovery of the Toba ash at other ancient human sites would resolve the issue.

Hominin cultural revolution 320,000 years ago

As regards stone tools, the Olorgesailie Basin in southern Kenya is about as good as it gets; a long-used ‘factory’ that covers a time span from about 1. 2 to 0.03 Ma. In places, large areas of the surface underlain by sedimentary strata, some of which have become major tourist attractions, are liberally strewn with tools and debitage from their manufacture. The area is ideal for stone-tool makers: being within the East African Rift it contains outcrops of many hard, fine-grained volcanic rocks and cherts formed by hot springs interleaved with its dominant fill of lake and riverine sediments. There are two dominant sedimentary units: the Olorgesailie Formation (1.2 to 0.49 Ma) overlain by the Oltulelei  Formation (0.32 to 0.05 Ma), the time gap between the two marking an extended period of regional erosion. Despite the rich tool assemblages, hominin remains have yet to be unearthed from the sediments, although there are plenty of bones from potential prey mammals. Olorgesailie was  good place to live, especially as the Rift would have channelled migrating herds predictably between its steep-sided flanks.

Acheulean biface tools strewn on a bedding surface in the Olorgesailie Basin, Kenya (credit: mmercedes_78 https://www.flickr.com/photos/27325832@N06/3833105587)

The older formation has yielded biface tools of the Acheulean technology from bottom to top. The earliest Acheulean tools in Africa date back to about 1.7 Ma and have been attributed to Homo ergaster/erectus, although examples in Europe are associated with H. antecessor and H. heidelbergensis – the technology was active for about 1.4 Ma. The Acheulean method involved striking flakes from large blocks of rock to result in a symmetrical, pear-shaped core that served as a multipurpose tool.   The oldest strata in the Oltulelei  Formation contain exclusively tools that are very different , having been made by a significantly more complicated procedure and covering a wide variety of designs with different uses. This Levallois technique focused on thin flakes produced from cores after careful preparation, which enabled similar tools to be made repeatedly rather than relying on chance fracturing. Precise dating of the oldest of these assemblages gives an age of 320 ka (Deino, A.L. et al. 2018. Chronology of the Acheulean to Middle Stone Age* transition in eastern Africa. Science, v. 359 online; DOI: 10.1126/science.aao2216). The makers clearly were able to visualize the finished product within the original lump of raw stone, but in a more nuanced way than did the makers of Acheulean biface tools. The first-described Levallois tools were associated with European Neanderthals.

English: Levallois flake obtained by the prefe...

Producing a flake by the Levallois technique (credit: Wikipedia)

Sometime in the 500 to 320 ka interval removed by erosion a major shift in technology and almost certainly cognition took place. Not only was this a technological revolution, but the Levallois tools are found in association with a variety of pigments, such as ochres, which show signs of having been worked, presumably for decoration of some kind. Also, the tool makers seemed to have a clear preference for specific rocks – black, glassy obsidian and cherts of white and green hues from sources 25 to 90 km from the tool-making sites (Brooks, A.S. and 14 others 2018. Long-distance stone transport and pigment use in the earliest Middle Stone Age. Science, v. 359 online; DOI: 10.1126/science.aao2646). The sheer volume of tools at each site and the evidence for long-distance transport of the raw materials have prompted the authors to hazard a guess at some kind of trade, or at least cooperative intergroup interaction. Together with the use of pigment, probably for body ornamentation, this suggests individual and perhaps group identity within a kind of social network.

Of course, the big question is: Who made the leap? That’s a hard one in the absence of human remains associated with the tool-making factories at Olorgesailie . The authors of both papers argue for the earliest modern humans. But, to me, this seems like an assumption based on the age of the transition rather than any convincing evidence. The original Levallois tools from northern France were found in association with skeletal remains of Neanderthals but much later in the Pleistocene. An age of 320 ka does place the Olorgesailie tools in the same ballpark as early AMH fossils from Morocco, later than the genetically derived date of separation of Neanderthals and AMH. However, the 180 ka time gap in which the technological revolution took place gives some room for so-called African ‘archaic modern humans’ (not subdivided as are similar fossils from Europe) are known from Zimbabwe, Tanzania and Ethiopia. If the Neanderthals were using the Levallois technique in Europe there is every reason to suspect that they, or their possible forebears H. heidelbergensis, may have brought it with them from Africa.

The Olorgesailie tools figure in a third paper in the same volume of Science, but one with less shaky grounds (Potts, R. and 14 others 2018. Environmental dynamics during the onset of the Middle Stone Age in eastern Africa. Science, v. 359 online; DOI: 10.1126/science.aao2200). The wet-dry cycle of the Pleistocene, related to global warming and cooling in interglacial and glacial episodes respectively, had become more marked after the 500 to 320 ka period of tectonically induced erosion. In itself, this would have resulted in more marked shifts in the ecosystems of the basin – perhaps a case of necessity being the mother of invention. Yet, the evidence base for changing climate cycles in Africa is not from local lake-sediment stratigraphy, micropalaeontology or geochemistry, but from the modelled variation of insolation based on Milankovich’s hypothesis.

*Note:  The Middle Stone Age in Africa does not correlate with the Mesolithic of Europe, but is a legacy of the development of archaeology in Africa. It corresponds to the European Middle Palaeolithic.

See also:  Gibbons, A. Complex behaviour arose at dawn of humans. Science, v. 359, p. 1201-1202 (with video)

Sophisticated Neanderthal art now established

The first detailed description and analysis of the amazing cave paintings of Western Europe that have been attributed to anatomically modern humans (AMH) were made in the early 20th century by the Jesuit priest Abbé Henri Breuil. As well as that those of Lascaux and Altamira, which have been dated, many works in Spanish caves have not. Art ascribed to AMH includes figurative work depicting a wide range of Late Pleistocene animals, abstract and perhaps symbolic designs, and ‘signatures’ of individual people in the form of direct prints or stencils of hands. The earliest known graphic work made by modern humans is a 100 ka-old baton of ochre with a zig-zag set of sharp incisions found with ochre-filled shells possibly for body painting at Blombos Cave in South Africa.

Evidence for pre-AMH work in Europe is sparse and widely  judged to be ambiguous; for instance 50 ka-old ochre-stained and pierced shells associated with Neanderthal remains in Spain.  Hints at even earlier origins for art lie in the geometrically etched bivalve shells excavated by Eugene Dubois at the site in Java where he discovered Homo erectus crania in 1891. They have recently been dated at around half a million years old.  Occasionally, radiometric dating of drawings has revealed quite meagre red dots that are slightly older than the widely accepted date of first entry of AMH into Europe (~40-45 ka) and may have been made by Neanderthals. Of course, there are many European cave paintings associated with dates earlier than the extinction of Neanderthals (around 30 ka) that may have been made by them, but which are generally ascribed to AMH by assuming that only our species has the wit to make them.  Even the sophisticated Châtelperronian stone tools and rough ornaments associated with undeniable Neanderthal remains are considered by many paleoanthropologists to show skills copied from AMH.

This AMH-centric view of art depends on two outlooks: simple prejudice that any beings markedly different in appearance from us were intellectually inferior – generally condemned as racist if applied to different groups of living humans; lack of incontrovertible and unambiguous evidence to the contrary. Both are set to be rigorously challenged by the growing use of sophisticated radiometric U-Th dating of the thin films of chemically precipitated calcite (flowstone or speleothem) that often coat the walls of caves and are at least as old as the art that they cover. A German-Spanish-British team has applied the technique to artwork and painted stalactites on the walls of three caves in Spain known to have been occupied by hominins over the last 100 ka (Hoffmann, D.L and 13 others 2018. U-Th dating of carbonate crusts reveals Neandertal origin of Iberian cave art. Science, v. 359, p. 912-915; doi: 10.1126/science.aap7778. See also: Appenzeller, T. 2018. Europe’s first artists were Neandertals. Science, v. 359, p.852-853; doi: 10.1126/science.359.6378.852). One cave that was analysed is that at La Pasiega in Cantabria whose art was sketched by Abbé Breuil. The team’s results are dramatic: all the dated samples pre-date 40 Ka, the oldest at 79.66±14.90 ka being from La Pasiega. Precisely dated art includes hand stencils, painted stalactites, geometric patterns and line drawings of animals. Many of the caves’ artworks remain to be dated, including some well-executed animals and strange, possibly symbolic designs.

Symbolic Neanderthal art in La Pasiega cave, Spain – left: recent photograph; right: sketch produced Abbé Breuil in 1913. The red, ladder-like symbol has a minimum age of 64 ka but it is unclear if the animals and other symbols were painted later. (credit: Hoffmann et al. 2018, Supplementary Data Figure S4)

The implications of this work are far-reaching. Handprints and stencils are common throughout the archives of European cave art and seem generally to be the oldest at each site. The dating method is yet to applied to the bulk of cave art, much of which is encased in speleothem, so it is quite possible that ‘dual authorship’ may be discovered in some caves. It now seems clear that Neanderthals invented permanent art independently of AMH, and since art is a form of communication that has implications for the ability to speak as well as to think ‘outside-the-box’. The 177 ka corral-like enclosures made of stalactites and associated hearths deep within Bruniquel Cave seem more likely to have ritual significance, far from the light of day, for the Neanderthals that made them. The finds throw doubt on the implausibility of Neanderthal invention of so-called ‘transitional’ technologies, such as the Châtelperronian. Finally, fully modern humans in Africa and Neanderthals in Europe were doing much the same things over roughly the same time period; genetically and physically they parted company about 450 to 400 ka ago; both were capable of artistic symbolism and fulfilled that potential. That implies that their common ancestor may have passed on the proclivity, as might their predecessor H. erectus who created the etched mollusc shells of Trinil half a million years ago.

More on Neanderthals, Denisovans and AMH genetic relatedness

Editorial from the Guardian Newspaper 26 February 2018.

Impact debris in Britain

These days reports of geological evidence for asteroid impacts are not regarded with a mixture of disbelief, wonder and foreboding: well, not by geologists anyway. But for such a small area as Britain now to have three of widely different ages and in easily accessible places is pretty good for its brand as the place to visit for practically every aspect of Earth history. The first to be discovered lies at the base of Triassic mudstones near Bristol (see Britain’s own impact) and would need some serious grubbing around at a former construction site. The next to emerge was located in one of the best geological districts in the country at several easily accessed coastal exposures in Northwest Scotland. A glass-rich ejecta layer occurs in the basal Torridonian Stoer Group on Stac Fada, Stoer, Sutherland (UK National Grid Reference 203300, 928400). The most recently found (Drake, S.N. and 8 others 2018. Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland. Geology, v. 46, p. 171-174; doi:10.1130/G39452.1) is on the Inner Hebridean island of Skye at the base of its famous Palaeocene flood basalt sequence (UK National Grid Reference 155371,821112).

View to the northwest across Loch Slapin to the Cuillin Hills of Skye (Central Igneous Complex). The flood basalts beneath which the ejecta layer occurs are just above the trees. (Credit: Wikipedia)

The last is perhaps the most spectacular of the three, as it contains the full gamut of provenance, matched only by material from the drill core into the 66 million year-old Chicxulub crater. The 0.9 m thick debris layer rests directly on mid-Jurassic sandstones beneath Palaeocene basalts of the North Atlantic Igneous Province (NAIP). The layer contains a basalt clast dated at 61.54 Ma, but is dominantly reminiscent of a pyroclastic ignimbrite flow as it contains glass shards. But there the resemblance ends for the bulk of small clasts are of quartz and K-feldspar, sandstone and gneiss. Zircons extracted from the debris show shock lamellae and give Archaean and Proterozoic ages commensurate with the local basement, but also with the bulk of the Scandinavian and Canadian Shields. So the impact could have been anywhere in such widespread terrains, although the enclosed basalt narrows this down to areas where basement is overlain by lavas of the NAIP. The Skye impactite contains unmelted meteorite fragments in the form of titanium nitrides alloyed with vanadium and niobium, metallic iron-silicon alloy containing exsolved carbon, and manganese sulfide.

Although it may be coincidental, the situation of the ejecta layer immediately beneath the Skye lavas, its containing a clast of basalt whose age corresponds to the oldest flows anywhere in the NAIP is fascinating. But the actual impact site is, as yet, unknown. Even so, the layer provokes thoughts about whether an impact may have been more than spatially related to the large NAIP flood basalt pile, preserved on either side of the North Atlantic. If the event was large, then surely the ejecta should be preserved near the base of the flood basalts elsewhere in NW Britain and further afield

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.

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.