Younger Dryas impact trigger: evidence from Chile

A sudden collapse of global climate around 12.8 ka and equally brusque warming 11.5 ka ago is called the Younger Dryas. It brought the last ice age to an end. Because significant warming preceded this dramatic event palaeoclimatologists have pondered its cause since it came to their attention in the early 20th century as a stark signal in the pollen content of lake cores – Dyas octopetala, a tundra wild flower, then shed more pollen than before or afterwards; hence the name. A century on, two theories dominate: North Atlantic surface water was freshened by a glacial outburst flood that shut down the Gulf Stream [June 2006]; a large impact event shed sufficient dust to lower global temperatures [July 2007]. An oceanographic event remains the explanation of choice for many, whereas the evidence for an extraterrestrial cause – also suggested to have triggered megafaunal extinctions in North America – has its supporters and detractors. The first general reaction to the idea of an impact cause was the implausibility of the evidence [November 2010], yet the discovery by radar of a major impact crater beneath the Greenland ice cap [November 2018] resurrected the ‘outlandish’ notion. A recent paper in Nature: Scientific Reports further sharpens the focus.

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Temperature fluctuations over the Greenland ice cap during the past 17,000 years, showing the abrupt cooling during the Younger Dryas. (credit: Don Easterbrook)

Since 2007, a team of Chilean and US scientists has been working on a rich haul of late Pleistocene fossil mammals from Patagonian Chile that turned up literally in someone’s suburban back garden in the town of Osorno. The stratigraphy has been systematically dated using the radiocarbon method. A dark layer composed of peat with abundant charcoal gave an age of about 12.8 ka, thereby marking both the local base of the Younger Dryas episode and a cap to the rich mammalian fossil assemblage. Similar beds have been found at more than 50 sites elsewhere in the world at this stratigraphic level, including a site in Arizona carrying Clovis artifacts. Steadily, such ‘black mats’ have been yielding magnetised spherules, elevated concentrations of platinum-group metals, gold, native iron, fullerenes and microscopic diamonds, plus convincing signs of wild fires at some sites; the very evidence that most researchers had panned when first reported. The Chilean example contains much the same pointers to an extraterrestrial cause, attributed to air-burst impacts (Pino, M. and 14 others 2019. Sedimentary record from Patagonia, southern Chile supports cosmic-impact triggering of biomass burning, climate change, and megafaunal extinctions at 12.8 ka. Scientific Reports, v. 9, article 4413; DOI: 10.1038/s41598-018-38089-y)

A larger team of researchers, to which several of the authors of the Chilean paper are affiliated, claim the evidence supports some kind of impact event 12.8 ka ago, possibly several produced by the break-up of a comet. Yet the criticisms persist. For instance, had there been wildfires on the scales suggested, then there ought to be a significant peak in the proportion of charcoal in lake bed sediments from any one region at 12.8 ka. In fact such data from North America show no such standalone peak among many from the age range of the Younger Dryas. The fossil record from the last few millennia of the Pleistocene does not support a sudden extinction, but a decline. The Clovis-point culture, thought by many to have wrought havoc on the North American megafauna, may have come to an end around 12.8 ka, but was quickly succeeded by an equally efficient technology – the Folsom point.  As regards the critical evidence for impacts, shocked mineral grains, none are reported, and some of the reported evidence of microspherules and nanodiamonds is not strongly supported by independent analysis – and nor are they unique to impact events. How about the dating? The evidence from ice cores strongly suggests that the Younger Dryas began with an 8° C temperature decline over less than a decade, and the end was equally as sudden. Is radiocarbon dating capable of that time resolution and accuracy? Certainly not

Related articles: Gramling, C. 2018. Why won’t this debate about an ancient cold snap die? (Science News); Easterbrook, D.L. 2012.The Intriguing Problem Of The Younger Dryas—What Does It Mean And What Caused It? (Watts Up With That); Wolbach, W.S. and 26 others 2018.  Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 1. Ice cores and Glaciers. Journal of Geology, v. 126, p. 165-184; DOI: 10.1086/695703; Wolbach, W.S. and 30 others 2018. Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments. Journal of Geology, v. 126, p. 185-205; DOI: 10.1086/695704.

Frack me nicely?

‘There’s a seaside place they call Blackpool that’s famous for fresh air and fun’. Well, maybe, not any more. If you, dear weekender couples, lie still after the ‘fun’ the Earth may yet move for you. Not much, I’ll admit, for British fracking regulations permit Cuadrilla, who have a drill rig at nearby Preston New Road on the Fylde coastal plain of NW England, only to trigger earthquakes with a magnitude less than 0.5 on the Richter scale. This condition was applied after early drilling by Cuadrilla had stimulated earthquakes up to magnitude 3. To the glee of anti-fracking groups the magnitude 0.5 limit has been regularly exceeded, thereby thwarting Cuadrilla’s ambitions from time to time. Leaving aside the view of professional geologists that the pickings for fracked shale gas in Britain [June 2014] are meagre, the methods deployed in hydraulic fracturing of gas-prone shales do pose seismic risks. Geology, beneath the Fylde is about as simple as it gets in tectonically tortured Britain. There are no active faults, and no significant dormant ones near the surface that have moved since about 250 Ma ago; most of Britain is riven by major fault lines, some of which are occasionally active, especially in prospective shale-gas basins near the Pennines. When petroleum companies are bent on fracking they use a drilling technology that allows one site to sink several wells that bend with depth to travel almost horizontally through the target shale rock. A water-based fluid containing a mix of polymers and surfactants to make it slick, plus fine sand or ceramic particles, are pumped at very high pressures into the rock. Joints and bedding in the shale are thus forced open and maintained in that condition by the sandy material, so that gas and even light oil can accumulate and flow up the drill stems to the surface.

Shale, being dominated by ultra-fine clay minerals, is slippery when wet. Consequently, any elastic strain built-up in the rock, either by active tectonics or from long in the past, is likely to be released by fracking. The fractures that release the gas also facilitate the escape of formation water locked in the shale from when it was originally deposited. Being rich in organic matter, target shales maintain highly reducing chemical conditions. So as well as being salty, such formation water may contain high abundances of heavy metals and arsenic, unlike the groundwater in naturally permeable and oxygenated rocks, such as sandstones and limestones. Fracking carries a pollution risk too. Toxic waste fluid is generally disposed of by pumping into permeable strata beneath the well site. There is no knowing where such noxious water might go, other than to follow lines of least resistance, such as large joints and dormant faults that may well be unsuspected at the depths to which drilling might penetrate. That too poses seismic rick by lubrication of the pathways taken by the fluids.

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The sheer scale of shale-gas fracking in the US is indicated by the light emitted at night by well-lit installations and gas flares in a mature shale-gas basin in Texas targeting the mature, gas-rich Eagle Ford shale. (see: https://geology.com/articles/eagle-ford/)

Britain has barely been touched by fracking or conventional petroleum drilling, unlike large swathes of North America. Fracking began in Kansas, USA in 1947 but got underway in earnest in the 1970s to dominate US natural gas production since the 1990s. The effects of fracking in the long term [July 2013] show up in the active shale-gas basins there. Even in geological settings as quiescent as the Fylde seems to be, the picture is one of repeated earthquakes induced by fracking, which often exceed magnitude 3.0, including one of magnitude 5.6 in Oklahoma that destroyed 14 homes in 2016. A recent paper in Science examines how fluid migration induces dormant structures to move again (Bhattacharya, P. & Viesca, R.C. 2019. Fluid-induced aseismic fault slip outpaces pore-fluid migration. Science, v. 364, p. 464-468; DOI: 10.1126/science.aaw7354). The authors, from Tufts University in the US, used experimental fluid injection in France to indicate that aseismic slip resulting from fluid injection transmits stress far and wide, and more quickly than expected from the outward movement of the injected fluids. This explains why earthquakes produced by deliberate fluid injection into the crust often occur more frequently in active shale-gas basins than they do in areas of naturally high seismic activity

Related article: Fracking: Earthquakes are triggered well beyond fluid injection zones (Science News)

Denisovan on top of the world

Who the Denisovans were is almost completely bound up with their DNA. Until 2019 their only tangible remains were from a single Siberian cave and amounted to a finger bone, a toe bone three molars and fragment of limb bone. Yet they provided DNA from four individuals who lived in Denis the Hermit’s cave from 30 to more than 100 thousand years ago. The analyses revealed that the Denisovans, like the Neanderthals, left their genetic mark in modern people who live outside of Africa, specifically native people of Melanesia and Australia . Remarkably, one of them revealed that a 90 ka female Denisovan was the offspring of a Denisovan father and  a Neanderthal mother whose DNA suggested that she may have come from the far-off Balkans. Living, native Tibetans, whose DNA has been analysed, share a gene (EPAS1) with Denisovans, which regulates the body’s production of haemoglobin and enables Tibetans and Nepalese Sherpas to thrive at extremely high altitudes (see The earliest humans in Tibet).

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Baishya Karst Cave in eastern Tibet with Buddhist prayer flags (Credit: Dongju Zhang, Lanzhou University)

Part of a hominin lower jaw unearthed by a Buddhist monk in 1980 from a cave on the Tibetan Plateau, at a height of 3280 m, found its way by a circuitous route to the Max Planck Institute for Evolutionary Anthropology in Leipzig in 2016. It carries two very large molars comparable in size with those found at the Denisova Cave, and which peculiarly have three roots rather than the four in the jaws of non-Asian, living humans’. East Asians commonly show this trait. This and other aspects of the fossil teeth resemble those of some uncategorised early hominin fossils from China. Dating of speleothem calcium carbonate with which the jaw is encrusted suggests that the fossil dates back to at least 160 thousand years ago, around the oldest date recovered from Denisova Cave; during the glacial period before the last one. So the individual was able to survive winter conditions worse than those experienced today on the Tibetan Plateau. Further excavation in the cave found numerous stone artefacts and cut-marked animal bones (Chen, F. and 18 others 2019. A late Middle Pleistocene Denisovan mandible from the Tibetan Plateau. Nature, v. 569, published online; DOI: 10.1038/s41586-019-1139-x).

Unfortunately the Tibetan Jaw did not yield DNA capable of being sequenced, so the issues of inheritance of the ‘high-altitude’ gene and wider relatedness of the individual could not be checked. However, one of the teeth did contain preserved protein that can be analysed in an analogous way to DNA, but with less revealing detail. The results were sufficient to demonstrate that the mandible was consistent with a hominin population closely related to the Denisovans of the Siberian cave.

No doubt a path has already been beaten to the Tibetan cave, in the hope of further hominin material. To me the resemblance of the Tibetan fossil jaw to other hominin finds in China, including those from Xuchang, summarised here, is exciting. None of them have been subject to modern biological analysis. Perhaps the ‘real Denisovan’ will emerge from them.

See also: Mysterious ancient human found on the ‘roof of the world’ (National Geographic magazine); Major discovery suggests Denisovans lived in Tibet 160,000 years ago (New Scientist) ; Finally, a Denisovan specimen from somewhere beyond Denisova Cave (Ars Technica)

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A role for iron in the origin of life

Experiments aimed at suggesting how RNA and DNA – prerequisites for life, reproduction and evolution – might have formed from a ‘primordial soup’ have made slow progress. Another approach to the origin of life is investigation of the most basic chemical reactions that it engages in. Whatever the life form, prokaryote or eukaryote, its core processes involve reducing carbon dioxide, or other simple carbon-bearing compounds, and water to synthesise organic molecules that make up cell matter. Organisms also engage in metabolising biological compounds to generate energy. At their root, these two processes mirror each other; a creative network of reactions and another that breaks compounds down, known as the Krebs- and the reverse-Krebs cycles. In living organisms both are facilitated by other organic compounds that, of course, are themselves produced by cells. How such networks arose under inorganic conditions remains unknown, but three biochemists at the University of Strasbourg in France (Muchowska, K.B. et al. 2019. Synthesis and breakdown of universal metabolic precursors promoted by iron. Nature, v. 569, p. 104-107;  DOI: 10.1038/s41586-019-1151-1) have designed an inorganic experiment. They aimed to investigate how two simple organic compounds, which conceivably could have formed in a lifeless early environment, might have been encouraged to kick-start basic living processes. These are glyoxylate (HCOCO2) and pyruvate (CH3COCO2).

The most difficult chemical step in building complex organic compounds is inducing carbon atoms to bond together through C-C bonds; a process that thermodynamics tends to thwart but is accomplished in living cells by adenosine tri-phosphate (ATP). Previous workers focussed on interactions between reactive compounds, such as cyanide and formaldehyde, as candidates for the precursors of life, but such chemistry is totally different from what actually goes on in organisms. Joseph Moran, one of the co-authors of the paper, and his research group recently settled on five fundamental linkages of C, H and O as ‘universal hubs’ at the core of the Krebs cycle and its reverse. Kamila Muchowska and co-workers found that glyoxylate and pyruvate introduced into a simulated hydrothermal fluid that contains ions of ferrous iron (reduced Fe2+) were able to combine in producing all five ‘universal hubs. Ferrous iron clearly acted as a catalyst, through being a powerful reducing agent or electron donor, to get around the stringencies of classic thermodynamics. Moran’s team had previously shown that pyruvate itself can form inorganically from CO2 in water laced with iron, cobalt and nickel ions. Formation of glyoxylate in such a manner has yet to be demonstrated. Nevertheless, the two together in a watery soup of transition metal ions seem destined to produce an abundance of exactly the compounds at the root of living processes. In fact the experiment showed that all but two of the eleven components of the Krebs cycle can be synthesised inorganically.

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Metal-rich ‘black smoker’ at a hydrothermal vent on the mid-Atlantic ridge(credit: Kate Larkin, Seascape, Belgium)

Until the rise of free oxygen in the Earth system some 2400 Ma ago, the oceans would have been awash with soluble ferrous iron. This would have been especially the case around hydrothermal vents that result from the interaction between water and hot mafic lavas of the oceanic crust, together with less abundant transition-metal ions, such as those of nickel and cobalt. The ocean-vent hypothesis for the origin of life seems set for a surge forward.

See also: Katsnelson, A. 2019. Iron can catalyse metabolic reactions without enzymes.

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‘Hobbits’ found in the Philippines

The earliest signs that hominins had colonised the island of Luzon in the Philippines took the form of crude stone tools found around half a century ago. Re-excavation of one of the sites uncovered yet more tools buried in a river-channel deposit, along with remains of a butchered rhinoceros dated at around 700 ka by two methods (see Clear signs of a hominin presence on the Philippines at around 700 ka May 2018). The primitive nature of the tools and their age suggested that Asian Homo erectus had managed to reach the Philippine archipelago, despite it being separated from larger islands by deep water.  Even during large falls in sea level (up to 130 m) during glacial periods that exposed Sundaland, which linked the larger islands of Indonesia to mainland Eurasia, at best only a narrow stretch of sea (~20 km) connected the Philippines to the wider world. For most of the time since the earliest known colonisation any hominins on the islands would have been cut off from other populations.

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Topography of the Philippines, showing location of the Kalinga site. Palest blue sea may have been above sea level only during extreme glacial maxima. (credit: Wikipedia)

The first hominin fossil found by archaeologists in 2007 was a 67 ka old toe bone (metatarsal) in cave sediments from Northern Luzon. It was undoubtedly from Homo, but which species was unclear.  More recent excavations added a mere 12 fossil fragments, probably from three individuals; 7 teeth, 4 adult finger- and toe bones and part of the femur of a juvenile (Détroit, F. and 8 others 2019. A new species of Homo from the Late Pleistocene of the Philippines. Nature, v.  568, p. 181–186; DOI: 10.1038/s41586-019-1067-9). The finger bones, being curved, are unlike those of modern humans and H. erectus. The teeth are even more different; for instance the premolars show two or three roots – ours have but one – and their unusually tiny molars only a single root. The combined features are sufficiently distinct to suggest a separate species (H. luzonensis). The small teeth may indicate that the adults may have been even smaller that the ‘Hobbits’ of Flores and anatomically different.

Like H. floresiensis, as a result of isolation the new human species probably evolved to become small, possibly from very low number of H. erectus original colonisers. But an even stranger possibility is suggested by their curved toe and finger bones. They may have been habitual climbers as much as walkers – unlike us and H. erectus. Could that indicate that their ancestors left Africa already distinct from the rest of Late Pleistocene humans? That is also a disputed hypothesis for the origins of H. floresiensis  remains of whom are more complete. Similarly, they pose the issue of how their progenitors managed to get to the archipelago: deliberately by boat or being carried there clinging in desperation to vegetation torn-up by tsunamis and transported seawards by the back-wash.

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A bad day at the end of the Cretaceous

The New Yorker magazine normally features journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. So it is odd that this Condé Nast glossy for the chattering classes snaffled online what may be the geological scoop of the 21st century so far (Preston, D. 2019. The day the dinosaurs died. The New Yorker 8 April 2019 issue). The paper that lies at the centre of the story had not been published and nor had the issue of The New Yorker in which Douglas Preston’s story was scheduled for publication. The very day (29 March 2019) that Britain was thwarted of its Brexit moment the world’s media was frothing with news about the end of another era; the Mesozoic. The paper itself was published online on April Fools’ Day with a title that is superficially arcane (DePalma, R.A. and 11 others 2019. A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Science, early online publication;p DOI: 10.1073/pnas.1817407116). But its contents are the stuff of dreams for any aspiring graduate student of palaeontology; the Indiana Jones opportunity.

An ‘onshore surge deposit’ occurs at many Western Hemisphere sites where the K-Pg boundary outcrops in terrestrial or shallow-marine sediments. The closer to the Chicxulub crater north of Mexico’s Yucatan Peninsula the more obvious they are, for they result from the tsunamis that immediately followed the asteroid impact. Lead author Robert DePalma, now of the University of Kansas, became focussed on the dinosaur-rich, Late Cretaceous Hell Creek Formation of North Dakota as an undergraduate. Accepted for graduate studies he was directed to a project on the fauna of lacustrine sediments close to the K-Pg boundary layer, which is well-known in the area, and that’s what he has been engaged with ever since. In 2012 he was guided to a remarkable locality by a rockhound, disappointed because it exposed extremely fossil-rich sediments but was so soft that none could be extracted intact with a hammer and chisel. It turned out to have resulted from a surge along a sinuous river that had washed debris onto a point-bar deposit at the inside of a meander. The debris includes remains of both marine and terrestrial organisms and shows clear signs of having been swept upriver, i.e. from the sea and possibly the result of a tsunami. Being capped by a thin, iridium-rich layer of impactite, the 1.5 metre surge deposit is part of the K-Pg boundary layer, and probably represented only a few hours before being blanketed by ejecta.

This Event Deposit comprises two graded, fining-upwards units and thus two distinct surges, with a thin mat of vegetation fragments immediately below the Ir-rich clay cap that also contains sparse shocked quartz grains. The Event Deposit contains altered glass spherules throughout, which cgradually become smaller higher in the 1.5 m sequence. Some of the larger spherules produced ‘micro-craters’ in the sediments. Fossils include marine ammonite fragments (some still nacreous) and freshwater fish (paddlefish and sturgeon). The fish are so complete as to suggest an absence of scavengers. The paper itself contains little of the information that dominated Preston’s New Yorker article and the global media coverage. This included clear evidence that the fish ingested spherules, found clogging their gills and possible causing their death. There are examples of spherules embedded in amber formed from plant sap, which suggests sub-aerial fall of ejecta, and among the marine faunal samples are teeth of fish and reptiles (see DePalma et al’s Supplemental Data). The most startling finds reported by Preston are nowhere to be found in DePalma et al’s paper or its supplement. These include possible dinosaur feathers; a fragment of ceratopsian dinosaur skin attached to a hip bone; a burrow containing a mammal jaw that penetrates the K-Pg boundary layer; dinosaur remains, including an egg (complete with embryo) and hatchlings of dinosaurian groups found at deeper levels in the Hell Creek Formation. Previously, palaeontologists had found no dinosaur remains less than 3 m below the K-Pg boundary layer anywhere on Earth, prompting the suggestion that they had become extinct before the near-instantaneous effects of Chicxulub, and were perhaps victims of the general effects of the Deccan Trap volcanism. If verified in later peer-reviewed publications, DePalma et al’s work would help resolve the gradual vs sudden hypotheses for the end-Cretaceous mass extinction.

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X-ray and CT images of impact spherules in the gills of a fossil sturgeon from the Tanis K-Pg site, North Dakota (credit DePalma et al. 2019; Fig. 6)

Preston reports some academic scepticism about DePalma’s work, and emphasises his showmanship at conferences; for instance, he named the site ‘Tanis’ after the ancient city in Egypt featured in the 1981 film Raiders of the Lost Ark. There are geophysical queries too. If the inundation was by the on-shore effects of a tsunami it doesn’t tally with the abundance of ejecta fallout of glass spherules: tsunamis propagate in shallow seawater at speeds less than 50 km h-1  and more slowly still in channels, whereas impact ejecta travel much faster. This is acknowledged in the paper’s supplement, and the paper refers to a seiche wave activated by seismic waves associated with the Chicxulub impact which could have arrived in North Dakota at about the same time as its ejecta blanket. The paper’s authorship includes the imprimatur of other authorities in different geoscientific fields, including Walter Alvarez, jointly famed with his father Luis for the discovery of the K-Pg boundary horizon and its impact connections in 1981. So it carries considerable weight. No doubt further comment and further papers on the Tanis site will emerge: DePalma has yet to complete his PhD. It may become the lagerstätte of the K-Pg extinction; in DePalma’s words ‘It’s like finding the Holy Grail clutched in the bony fingers of Jimmy Hoffa, sitting on top of the Lost Ark.’ …

Read more on Palaeobiology and Impacts

Gravity signals of earthquakes

A sign that an earthquake is taking place is pretty obvious: the ground moves. Seismometers are now so sensitive that they record significant seismic events at the far side of the world. The Richter magnitude scale commonly used to assign the power of an event is logarithmic, and the difference between each unit represents an approximately 32-fold change in the energy released at the source, so that a magnitude 6.0 earthquake is 32 times more powerful than one rated as magnitude 5.0. Because seismic motion affects a mass of rock it also perturbs the gravitational field. So, theoretically, gravimeters should also be able to detect an earthquake. Seismic waves travel at a maximum speed of about 6 to 8 km s-1 about 20 times the speed of sound, yet changes in the gravitational field propagate at the speed of light, i.e. almost instantaneously by comparison. The first ground disturbances of the magnitude 9.0 Tohoku-Oki earthquake of NE Japan on 11 March 2011 hit Tokyo about 2 minutes after the event began offshore. Although that is a quite short time it would be sufficient for people to react and significantly reduce the earthquake’s direct impact on many of them. A seismic gravity signal would give that warning. The full horror of Tohoku-Oki was unleashed by the resulting tsunami waves, whose speed in the deep ocean water off Japan was about 800 km hr-1 (0.22 km s-1). An almost real-time warning would have allowed 40 times more time for evasion.

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Devastation in NE Japan caused by the Tohoku-Oki tsunami in March 2011

Japan is particularly well endowed with advanced geophysical equipment because of its notorious seismic and volcanic hazards. The first data to be analysed after Tohoku-Oki were understandably those from Japan’s large array of seismometers. The records from two super-sensitive gravimeters, between 436 and 515 km from the epicentre, were examined only recently. These instruments measure variations in gravity as small as a trillionth of the average gravitational acceleration of the Earth using a superconducting sphere suspended in a magnetic field, capable of detecting snow being cleared from a roof. Masaya Kimura and colleagues from Tokyo University and other geoscientific institutes in Japan undertook the analyses of both seismic and gravity data (Kimura, M. et al. 2019. Earthquake‑induced prompt gravity signals identified in dense array data in Japan. Earth Planets and Space, v. 71, online publication. DOI: 10.1186/s40623-019-1006-x). The gravimeter record did show a statistically significant perturbation at the actual time of the earthquake, albeit after complex processing of both gravity and seismographic data.

That only 2 superconducting gravimeters detected the event in real-time is quite remarkable, despite the need for a great deal of processing. It amounts to a test of the concept that such instruments or others based on different designs and deployed more widely may eventually be deployed to give prompt warnings of seismic events that could save thousands.

Read more on Geohazards

The Cambrian Explosion: a broader view

The base of the Cambrian has long been defined as the level where abundant shelly fossils and most phyla first occur in the stratigraphic record. That increase in diversity led to the nickname ‘Cambrian Explosion’, despite the fact that sheer numbers and diversity of lesser taxa took a long time to rise to ‘revolutionary’ levels. Yet a great deal of animal evolution was going on during the preceding Proterozoic Era that was revealed once palaeobiological research blossomed in rocks of that age range. Today, the earliest occurrences, or at least hints, of quite a few phyla can be traced to the last 100 Ma of the Precambrian. Clearly, the Cambrian Explosion needs a fresh look now that so many data are in. Any palaeontologist would benefit from reading a Perspective article in the latest issue of Nature Ecology & Evolution (Wood, R. and 8 others 2019. Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology & Evolution, v. 3, online publication; DOI: 10.1038/s41559-019-0821-6)

Rachel Wood of Edinburgh University and co-authors working elsewhere in Britain, Canada, Japan and Finland sift the growing wealth of fossil and trace-fossil evidence that predate the start of the Cambrian. They also consider the geochemical events that stand out in the Ediacaran Period that succeeds the Snowball Earth events of the Cryogenian. Their account recognises that the geochemical changes – principally a series of carbon-isotope (δ13C) excursions – may have resulted from tectonic changes. The carbon-isotope data mark a series of short-lived penetrations of oxygen-rich conditions deep into the ocean water column and longer periods of oxygen-starved deep water. Such perturbations in oceanic redox conditions ‘speed-up’ thorough the late-Ediacaran into the Cambrian: a profound and protracted transition from the Neoproterozoic world to that of the Phanerozoic. Over the same time span there is a ‘progressive addition of biological novelty’ in the form and function of the evolving biota, so that  each successive assemblage builds on the earlier advances.

The fossil evidence suggests that the earliest Ediacaran fauna was metazoan but with no sign of bilaterian affinities (i.e. having ‘heads’ and ‘tails’). The rise of bilaterians of which most animal phyla are members occupied the later Ediacaran , with the first evidence of locomotion – and almost by definition animals with ‘fore’ and ‘aft’ – being around 560 Ma. Each discrete shift from more to less oxic conditions in the oceans seems to have knocked-back animal life, the reverse being accompanied by diversification of survivors. Oxygenation at the very start of the Cambrian marked the beginnings of a diversification clearly manifested by animals capable of biomineralisation and the secretion of hard parts with clear patterns. Such ‘shelly faunas’ are present in the latest Ediacaran sediments but with a multiplicity of seemingly arbitrary forms, although trace fossils suggest soft-bodied animals did have definite morphological pattern.

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Diorama of the Lower Cambrian Qingjiang fauna (Credit: Fu et al. 2019; Fig 4)

Adding yet more information to early metazoan history is the recently discovered Cambrian Qingjiang lagerstätte of Hubei Province in southern China dated at 518 Ma; similar in its exquisite preservation to the Burgess (508 Ma) and Chengjiang (518 Ma) biotas (Fu, D. and 14 others 2019. The Qingjiang biota—A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China. Science, v. 363, p. 1338-1342; DOI: 10.1126/science.aau8800). The two previously discovered Cambrian lagerstättes are notable for their very diverse arthropod and sponge faunas. That at Qingjiang adds an abundance of cnidarians, jellyfish, sea anemones, corals and comb jellies, rare in the other two biotas, plus kinorhynchs or mud dragons – moulting invertebrates known only from Cambrian and modern sediments. The fossils at Qingjiang include only about 8% of the taxa of the same age found at Chengjiang, suggesting different environments

The idea of a sudden, discrete explosive event in the history of life, which coincided with the start of the Cambrian, now seems difficult to support. This should not damage the status of 541 Ma as the start of the Phanerozoic because stratigraphy basically gives form to the passage of time and has done since its emergence in the 19th century, so keeping the names of the divisions is essential to continuity.

Related articles: Daley, A.C. 2019. A treasure trove of Cambrian fossils. Science, v. 363, p. 1284-1285; DOI: 10.1126/science.aaw8644. Switek, B. 2019. Fossil Treasure Trove of Ancient Animals Unearthed in China (Smithsonian.com)

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Tectonics and glacial epochs

Because the configuration of continents inevitably affects the ocean currents that dominate the distribution of heat across the face of the Earth, tectonics has a major influence over climate. So too does the topography of continents, which deflects global wind patterns, and that is also a reflection of tectonic events. For instance, a gap between North and South America allowed exchange of the waters of the Pacific and Atlantic Oceans throughout the Cenozoic Era until about 3 Ma ago, at the end of the Pliocene Epoch, although the seaway had long been shallowing as a result of tectonics and volcanism at the destructive margin of the eastern Pacific. That seemingly minor closure transformed the system of currents in the Atlantic Ocean, particularly the Gulf Stream, whose waxing and waning were instrumental in the glacial-interglacial cycles that have persisted for the last 2.5 Ma. This was partly through its northward transport of saltier water formed by tropical evaporation that cooling at high northern latitudes encouraged to sink to form a major component of the global oceanic heat conveyor system.   Another example is the rise of the Himalaya following India’s collision with Eurasia that gave rise to the monsoonal system  dominating the climate of southern Asia. The four huge climatic shifts to all-pervasive ice-house conditions during the Phanerozoic Eon are not explained so simply: one during the late-Ordovician; another in the late-Devonian; a 150 Ma-long glacial epoch spanning much of the Carboniferous and Permian Periods, and the current Ice Age that has lasted since around 34 Ma. Despite having been at the South Pole since the Cretaceous Antarctica didn’t develop glaciers until 34 Ma. So what may have triggered these four major shifts in global climate?

Five palaeoclimatologists from the University of California and MIT set out to find links, starting with the most basic parameter, how atmospheric greenhouse gases might have varied. In the long term CO2 builds up through its emission by volcanoes. It is drawn down by several geological processes: burial of carbon and carbonates formed by living processes; chemical weathering of silicate minerals by CO2 dissolved in water, which forms solid calcium carbonate in soil and carbonate ions in seawater that can be taken up and buried by shell-producing organisms. Rather than comparing gross climate change with periods of orogeny and mountain building, mainly due to continent-continent collisions, they focused on zones that preserve signs of subduction of oceanic lithosphere – suture zones (Macdonald,F.A. et al. 2019. Arc-continent collisions in the tropics set Earth’s climate state. Science, v. 363 (in press); DOI: 10.1126/science.aav5300 ). Comparing the length of all sutures active at different times in the Phanerozoic with the extent of continental ice sheets there is some correlation between active subduction and glaciations, but some major misfits. Selecting only sutures that were active in the tropics of the time – the zone of most intense chemical weathering – results in a far better tectonic-climate connection. Their explanation for this is not tropical weathering of all kinds of exposed rock but of calcium- and magnesium-rich igneous rocks; basaltic and ultramafic rocks. These dominate oceanic lithosphere, which is exposed to weathering mainly where slabs of lithosphere are forced, or obducted, onto continental crust at convergent plate margins to form ophiolite complexes. The Ca- and Mg-rich silicates in them weather quickly to take up CO2 and form carbonates, especially in the tropics. Through such weathering reactions across millions of square kilometres the main greenhouse gas is rapidly pulled out of the atmosphere to set off global cooling.

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Top – variation in the total length of active, ophiolite-bearing sutures during the Phanerozoic; middle – length of such sutures in the tropics; bottom – extent of Phanerozoic glaciers. (Credit: Macdonald et al. 2019; Fig.2

Rather than the climatic influence of tectonics through global mountain building, the previous paradigm, Macdonald and colleagues show that the main factor is where subduction and ophiolite obduction were taking place. In turn, this very much depended on the configuration of continents on which ophiolites can be preserved. The most active period of tectonics during the Mesozoic, as recorded by the global length of sutures, was at 250 Ma – the beginning of the Triassic Period – but they were mainly outside the tropics, when there is no sign of contemporary glaciation. During the Ordovician, late-Devonian and Permo-Carboniferous ice-houses active sutures were most concentrated in the tropics. The same goes for the build-up to the current glacial epoch.

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The mid-Pleistocene transition

As shown by oxygen-isotope records from marine sediments, before about 1.25 Ma global climate cycled between cold and warm episodes roughly every 41 ka. Between 1.25 to 0.7 Ma these glacial-interglacial pulses lengthened to the ~100 ka periods that have characterised the last seven cycles that were also marked by larger volume of Northern Hemisphere ice-sheet cover during glacial maxima. Both periodicities have been empirically linked to regular changes in the Earth’s astronomical behaviour and their effects on the annual amount of energy received from the Sun, as predicted by Milutin Milankovich. As long ago as 1976 early investigation of changes of oxygen isotopes with depth in deep-sea sediments had revealed that their patterns closely matched Milankovich’s  hypothesis. The 41 ka periodicity matches the rate at which the Earth’s axial tilt changes, while the ~100 ka signal matches that for variation in the eccentricity of Earth’s orbit. 19 and 24 ka cycles were also found in the analysis that reflect those involved in the gyroscope-like precession of the axis of rotation. Surprisingly, the 100 ka cycling follows by far the weakest astronomical effect on solar warming yet the climate fluctuations of the last 700 ka are by far the largest of the last 2.5 million years. In fact the 2 to 8 % changes in solar heat input implicated in the climate cycles are 10 times greater than those predicted even for times when all the astronomical influences act in concert. That and other deviations from Milankovich’s hypothesis suggest that some of Earth’s surface processes act to amplify the astronomical drivers. Moreover, they probably lie behind the mid-Pleistocene transition from 41 to 100 ka cyclicity. What are they? Changes in albedo related to ice- and cloud cover, and shifts in the release and absorption of carbon dioxide and other greenhouse gases are among many suggested factors. As with many geoscientific conundrums, only more and better quality data about changes recorded in sediments that may be proxies for climatic variations are likely to resolve this one.

Adam Hazenfratz of ETH in Zurich and colleagues from several other European countries and the US have compiled details about changing surface- and deep-ocean temperatures and salinity – from δ18O and Mg/Ca ratios in foraminifera shells from a core into Southern Ocean-floor sediments – that go back 1.5 Ma (Hazenfratz, A.P. and 9 others 2019. The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle. Science, v. 363, p. 1080-1084; DOI: 10.1126/science.aat7067). Differences in temperature and salinity (and thus density) gradients show up at different times in this critical sediment record. In turn, they record gross shifts in ocean circulation at high southern latitudes that may have affected the CO2 released from and absorbed by sea water. Specifically, Hazenfratz et al. teased out fluctuations in the rate of  mixing of dense, cold and salty water supplied to the Southern Ocean by deep currents with less dense surface water. Cold, dense water is able to dissolve more CO2 than does warmer surface water so that when it forms near the surface at high latitudes it draws down this greenhouse gas from the atmosphere and carries it into long-term storage in the deep ocean when it sinks. Deep-water formation therefore tends to force down mean global surface temperature, the more so the longer it resides at depth. When deep water wells to the surface and warms up it releases some of its CO2 content to produce an opposite, warming influence on global climate. So, when mixing of deep and surface waters is enhanced the net result is global warming, whereas if mixing is hindered global climate undergoes cooling.

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The Southern Ocean, where most dissolved and gaseous carbon dioxide are emitted and absorbed by seawater (Credit: British Antarctic Survey)

The critical factor in the rate of mixing deep with surface water is the density of that at the surface. When its salinity and density are low the surface water layer acts as a lid on what lies beneath, thereby increasing the residence time of deep water and the CO2 that it contains. This surface ‘freshening’ in the Southern Ocean seems to have begun at around 1.25 Ma and became well established 700 ka ago; that is, during the mid-Pleistocene climate transition. The phenomenon helped to lessen the greenhouse effect after 700 ka so that frigid conditions lasted longer and more glacial ice was able to accumulate, especially on the northern continents. This would have made it more difficult for the 41 ka astronomically paced changes in solar heating to have restored the rate of deep-water mixing to release sufficient CO2 to return the climate to interglacial conditions That would lengthen the glacial-interglacial cycles. The link between the new 100 ka cyclicity and very weak forcing by the varying eccentricity of Earth’s orbit may be fortuitous. So how might anthropogenic global warming affect this process? Increased melting of the Antarctic ice sheet may further freshen surface waters of the Southern Ocean, thereby slowing its mixing with deep, CO2-rich deep water and the release of stored greenhouse gases. As yet, no process leading to the decreased density of surface waters between 1.25 and 0.7 Ma has been suggested, but it seems that something similar may attend global warming.

Related articles: Menviel, L. 2019. The southern amplifier. Science, v. 363, p. 1040-1041; DOI: 10.1126/science.aaw7196; The deep Southern Ocean is key to more intense ice ages (Phys.org)

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