Geobiology, palaeontology, and evolution – Page 5

The launch of modern life on Earth

Published on March 1, 2014 by zooks777Leave a comment

To set against five brief episodes of mass extinction – some would count the present as being the beginning of a sixth – is one short period when animals with hard parts appeared for the first time roughly simultaneously across the Earth. Not only was the Cambrian Explosion sudden and pervasive but almost all phyla, the basic morphological divisions of multicellular life, adopted inner or outer skeletons that could survive as fossils. Such an all-pervading evolutionary step has never been repeated, although there have been many bursts in living diversity. Apart from the origin of life and the emergence of its sexual model, the eukaryotes, nothing could be more important in palaeobiology than the events across the Cambrian-Precambrian boundary.

This eminent event has been marked by most of the latest issue of the journal Gondwana Research (volume 25, Issue 3 for April 2014)in a 20-paper series called Beyond the Cambrian Explosion: from galaxy to genome (summarized by Isozaki, Y., Degan, S.., aruyama,, S.. & Santosh, M. 2014. Beyond the Cambrian Explosion: from galaxy to genome. Gondwana Research, v. 25, p. 881-883). Of course, these phenomenal events have been at issue since the 19^th century when the division of geological time began to be based on the appearance and vanishing of well preserved and easily distinguished fossils in the stratigraphic column. On this basis roughly the last ninth of the Earth’s history was split on palaeontological grounds into the 3 Eras, 11 Periods, and a great many of the briefer Epochs and Ages that constitute the Phanerozoic. Time that preceded the Cambrian explosion was for a long while somewhat murky mainly because of a lack of means of subdivision and the greater structural and metamorphic damage that had been done to the rocks that had accumulated over 4 billion years since the planet accreted. Detail emerged slowly by increasingly concerted study of the Precambrian, helped since the 1930s by the ability to assign numerical ages to rocks. Signs of life in sediments that had originally been termed the Azoic (Greek for ‘without life’) gradually turned up as far back as 3.5 Ga, but much attention focused on the 400 Ma immediately preceding the start of the Cambrian period once abundant trace fossils had been found in the Ediacaran Hills of South Australia that had been preceded by repeated worldwide glacial epochs. The Ediacaran and Cryogenian Periods (635-541 and 850-635 Ma respectively) of the Neoproterozoic figure prominently in 9 of the papers to investigate or review the ‘back story’ from which the crucial event in the history of life emerged. Six have a mainly Cambrian focus on newly discovered fossils, especially from a sedimentary sequence in southern China that preserves delicate fossils in great detail: the Chengjian Lagerstätte. Others cover geochemical evidence for changes in marine conditions from the Cryogenian to Cambrian and reviews of theories for what triggered the great faunal change.

Since the hard parts that allow fossils to linger are based on calcium-rich compounds, mainly carbonates and phosphates that bind the organic materials in bones and shells, it is important to check for some change in the Ca content of ocean water over the time covered by the discourse. In fact there are signs from Ca-isotopes in carbonates that this did change. A team of Japanese and Chinese geochemists drilled through an almost unbroken sequence of Ediacaran to Lower Cambrian sediments near the Three Gorges Dam across the Yangtse River and analysed for ⁴⁴Ca and ⁴²Ca (Sawaki, Y. et al. 2014. The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area, South China. Gondwana Research, v. 25, p. 1070-1089) calibrated to time by U-Pb dating of volcanic ash layers in the sequence (Okada, Y. et al. 2014. New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng’an and Chengjiang areas, South China. Gondwana Research, v. 25, p. 1027-1044). They found that there were significant changes in the ratio between the two isotopes. The isotopic ratio underwent a rapid decrease, an equally abrupt increase then a decrease around the start of the Cambrian, which coincided with a major upward ‘spike’ and then a broad increase in the ⁸⁷Sr/⁸⁶Sr isotope ratio in the Lower Cambrian. The authors ascribe this to an increasing Ca ion concentration in sea water through the Ediacaran and a major perturbation just before the Cambrian Explosion, which happens to coincide with Sr-isotope evidence for a major influx of isotopically old material derived from erosion of the continental crust. As discussed in Origin of the arms race (May 2012) perhaps the appearance of animals’ hard parts did indeed result from initial secretions of calcium compounds outside cells to protect them from excess calcium’s toxic effects and were then commandeered for protective armour or offensive tools of predation.

"SNOWBALL EARTH" - 640 million years ago — Artists impression of a Snowball Earth event 640 Ma ago (credit: guano via Flickr)

Is there is a link between the Cambrian Explosion and the preceding Snowball Earth episodes of the Cryogenian with their associated roller coaster excursions in carbon isotopes? Xingliang Zhang and colleagues at Northwest University in Xian, China (Zhang, X. et al. 2014. Triggers for the Cambrian explosion: Hypotheses and problems. Gondwana Research, v. 25, p. 896-909) propose that fluctuating Cryogenian environmental conditions conspiring with massive nutrient influxes to the oceans and boosts in oxygenation of sea water through the Ediacaran set the scene for early Cambrian biological events. The nutrient boost may have been through increased transfer o f water from mantle to the surface linked to the start of subduction of wet lithosphere and expulsion of fluids from it as a result of the geotherm cooling through a threshold around 600 Ma (Maruyama, S. et al. 2014. Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion. Gondwana Research, v. 25, p. 910-944). Alternatively the nutrient flux may have arisen by increased erosion as a result of plume-driven uplift (Santosh, M. et al. 2014. The Cambrian Explosion: Plume-driven birth of the second ecosystem on Earth. Gondwana Research, v. 25, p. 945-965).

A bolder approach, reflected in the title of the Special Issue, seeks an interstellar trigger (Kataoka, R. et al. 2014. The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution in the late Neoproterozoic and Cambrian periods. Gondwana Research, v. 25, p. 1153-1163). This looks to encounters between the Solar System and dust clouds or supernova remnants as it orbited the galactic centre: a view that surfaces occasionally in several other contexts. Such chance events may have been climatically and biologically catastrophic: a sort of nebular winter, far more pervasive than the once postulated nuclear winter of a 3^rd World War. That is perhaps going a little too far beyond the constraints of evidence, for there should be isotopic and other geochemical signs that such an event took place. It also raises yet the issue that life on Earth is and always has been unique in the galaxy and perhaps the known universe due to a concatenation of diverse chance events, without structure in time or order, which pushed living processes to outcomes whose probabilities of repetition are infinitesimally small.

Massive trove of Canadian fossils gives near-unprecedented glimpse of Cambrian explosion

Plate tectonics and the Cambrian Explosion

Published on January 27, 2014 by zooks777Leave a comment

A rough-and-ready way of assessing the rate at which silicic magmatic activity has varied through time is to separate out grains of zircon that have accumulated in sedimentary rocks of different ages. Zircon is readily datable using the U-Pb method, if you have access to mass spectrometry. While some of the zircons will date from much older continental crust that was exposed while the sediments originated, sometimes there are grains that formed only a few million years before the sediments accumulated. Those are likely to have crystallized from silica-rich volcanic rocks above subduction zones where ocean-floor has been driven beneath continental crust; i.e. at continental volcanic arcs. Such young zircons therefore help assess the tectonic conditions close to sedimentary basins. The potential of detrital zircon geochronology was first suggested to me by Dr M.V.N. Murthy of the Geological Survey of India in 1978, long before anyone could aspire to mass zircon dating. M.V.N. had by then amassed kilograms of zircon grains from every imaginable source in India, and may have been the first geologist to realise their potential. It has become a lot quicker and cheaper in the last two decades, thanks to methods of dating single zircon grains both precisely and accurately and M.V.N.’s prescient suggestion has been borne out globally.

Results for the late Precambrian to early Palaeozoic have recently been compiled (McKenzie, N.R. et al. 2014. Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution. Geology, online publication doi:10.1130/G34962.1). One of the striking correlations is between the abundance of ‘young’ zircons relative to Cambrian sedimentary deposition and the pace of diversification of animal faunas during the Cambrian. During the Cambrian Period there may have been far more continental-margin arc volcanism than in the preceding late Neoproterozoic or later in the early Palaeozoic. That would match with evidence for the Cambrian atmosphere having reached the greatest CO₂ concentration of Phanerozoic times and the fact that the Gondwana supercontinent (comprising the present southern continents plus India) was assembled at that time by collision of several Precambrian continental masses. Global temperatures must have been rising.

The rapid emergence of all the major animal groups by the middle Cambrian – the Cambrian Explosion – took place during and despite climatic warming. Environmental stress, perhaps increased calcium and bicarbonate ions in sea water as a result of acid conditions, may have forced animals to develop means of getting both ions out of their cells to form carbonate skeletons: the Cambrian Explosion really marks the first appearance of shelly faunas and a good chance of fossilisation. Yet at the peak of volcanically-induced warming faunal diversity, especially of reef-building animals, fell-off dramatically to create what some palaeobiologsts have termed the Cambrian ‘dead interval’. Marine life really took-off in a big way during the Ordovician while temperatures were falling globally; so much so that the close of the Ordovician was marked by the first major glaciation focused on Gondwana. The zircon record indicates that continental-arc volcanism also declined during the Ordovician, and maybe the Cambrian silicic volcanics were chemically weathered during that Period to remove carbon-dioxide from the atmosphere, along with renewed reef building to bury carbonate fossils.

Volcanic mayhem drove major burst of evolution

Land almost colonized during the Cambrian Explosion

Published on January 20, 2014 by zooks777Leave a comment

One of the major shale-gas source strata in the eastern USA, the Middle Cambrian Conasauga Shale, formed in a shallow inland sea. Consequently the sedimentology of the lowest Palaeozoic Era of the region and the strange structures affecting it during deformation that formed the Appalachian Mountains have become a focus of intense tectonic and stratigraphic interest – economic potential generally helps fund academic research at a time when money for pure science is short. This has extended into the deepest part of the Cambrian lying unconformably just above the crystalline Precambrian basement. The Lower Cambrian of the Appalachians marks the earliest stage of rifting that flooded former dry land and comprises the multicoloured mudstones, siltstones and sandstones of the Rome Formation. Though only sparsely fossiliferous, the Rome formation contains archetypical trilobites of the genus Olenellus, typical of the Lower Cambrian and used to correlate sedimentary rocks of this age far and wide. They occur far across the North Atlantic in coeval rocks of the Northwest Highlands of Scotland, but not in those a mere couple of hundred kilometres to the south in Wales. This faunal disparity forms a major line of evidence that the olenelid fauna occupied one side of a once major ocean – Iapetus – another different bunch of early trilobites being characteristic of its opposite flank. The almost hemispherical extent of similar faunas was long regarded as an indication that they inhabited open ocean water. In fact, their wide distribution is as much due to juvenile arthropods being planktonic, while adults may have occupied all sorts of marine environments. It now turns out that Olenellus lived in very shallow water (Mángano, M.G. et al. 2014. Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion. Geology, online pre-publication doi:10.1130/G34980.1).

Gabriela Mángano of the University of Saskatchewan and colleagues from Argentina and the US found that the Rome Formation is full of sedimentary structures typical of modern intertidal zones. Tidal-flat strata are full of suncracks but are also criss-crossed by tracks made by substantial arthropods, only fossil olenellid trilobites being big enough to have made them while feeding , maybe on microbial mats formed on the mudflats or on worms that burrowed the muds. Clearly these animals were literally only a few steps away from colonising the land very shortly after abundant, sturdy animal life appeared in the Cambrian Explosion. Currently the dominant hypothesis for permanent entry of animals onto land is that the colonizers first adapted to fresh- or brackish water habitats. Yet, apparently, there was little to stop a direct invasion from the sea.

Greening the Earth, Devonian forest fires and a mass extinction

Published on November 28, 2013 by zooks7772 Comments

Land plants begin to appear in the fossil record as early as the late Ordovician (~450 Ma), show signs of diversification during the Silurian and by the end of the Devonian Period most of the basic features of plants are apparent. During the Carboniferous Period terrestrial biomass became so high as to cause a fall in atmospheric carbon dioxide, triggering the longest period of glaciation of the Phanerozoic, and such a boost to oxygen in the air (to over 30%) that insects, huge by modern standards, were able to thrive and the risk of conflagration was perhaps at its highest in Earth’s history. Yet surprisingly, the first signs of massive forest fires appear in the Devonian when vegetation was nowhere near so widespread and luxuriant as it became in the Carboniferous (Kaiho, K. et al. 2013. A forest fire and soil erosion event during the Late Devonian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 272-280). Moreover, Devonian oxygen levels were well below those of the present atmosphere and CO₂ was more than 10 times even the post-industrial concentration (387 parts per million in 2013). Such atmospheric chemistry would probably have suppressed burning.

Kunio Kaiho of Tohoku University in Japan and colleagues from Japan, the US and Belgium analysed organic molecules in Belgian marine sediments from the time of the late-Devonian mass extinction (around the Frasnian-Famennian boundary at 372 Ma). A range of compounds produced by hydrocarbon combustion show marked ‘spikes’ at the F-F boundary. The thin bed that marks the extinction boundary also shows sudden increase then decrease in δ¹³C and total organic carbon, indicative of increase burial of organic material and a likely increase in atmospheric oxygen levels. Another biomarker that is a proxy for soil erosion follows the other biogeochemical markers, perhaps signifying less of a binding effect on soil by plant colonisation: a likely consequence of large widlfires. Unlike the biomarkers, magnetic susceptibility of the boundary sediments is lower than in earlier and later sediments. This is ascribed to a decreased supply of detrital sediment to the Belgian marine Devonian basin, probably as a result of markedly decreased rainfall around the time of the late-Devonian mass extinction. But the magnetic data from 3 metres either side of the boundary also reveal the influence of the 20, 40, 100 and 405 ka Milankovich cycles.

This set of environmentally-related data encourages the authors to suggest a novel, if not entirely plausible, mechanism for mass extinction related to astronomically modulated dry-moist climate changes that repeatedly killed off vegetation so that dry woody matter could accumulate en masse during the Frasnian while atmospheric oxygen levels were too low for combustion. A mass burial of organic carbon at the end of that Age then boosted oxygen levels above the burning threshold to create widespread conflagration once the wood pile was set ablaze. Makes a change from continental flood basalts and extraterrestrial impacts… Yet it was about this time that vertebrates took it upon themselves to avail themselves of the new ecological niche provided by vegetation to haul themselves onto land.

Earth’s first major glacial epochs

Published on November 27, 2013 by zooks7771 Comment

The global glaciations of the Neoproterozoic that reached low latitudes – the so-called ‘Snowball Earth’ events have dominated accounts of ancient glaciations since the start of the 21^st century. Yet they are not the oldest examples of large-scale effects of continental ice sheets. Distinctive tillites or diamictites that contain large clasts of diverse, exotic rocks occur in sedimentary sequences of Archaean and Palaeoproterozoic age.

Diamictite from the Palaoproterozoic Gowganda Formation in Ontario Canada (credit: Candian Sedimentology Research Group) — Diamictite from the Palaeoproterozoic Gowganda Formation in Ontario Canada (credit: Canadian Sedimentology Research Group)

This item can be read in full at Earth-logs in the Palaeoclimatology archive for 2013

Could volcanism have spread organisms?

Published on November 7, 2013 by zooks777Leave a comment

Recently there have been worrying accounts about pathogens, for instance the viruses that cause foot and mouth disease in livestock, flu in humans and other animals and the sheep disease bluetongue carried by tiny midges, being transported for thousands of kilometres in dust storms. They raise the question of whether or not in the past organisms small enough to be carried by winds in aerosol suspension might have helped colonise regions distant from where they evolved.

Studies of volcanic ash thought to have been transported at high latitudes in the Southern Hemisphere from a 25 thousand-year old major volcanic eruption on the North Island of New Zealand add volcanic activity to violent meteorological phenomena as a possible means of transport (Van Eaton, A.R. et al. 2013. High-flying diatoms: Widespread dispersal of microorganisms in an explosive volcanic eruption. Geology, v. 41, p. 1187-1190). Ash from as far as 850 km from the volcano turns out to incorporate abundant remains of diatoms – species of algae that secrete distinctively intricate skeletons made from silica. The volcano, Taupo, erupted from beneath a lake bed, explaining the diatoms’ origin from lake muds and the water column itself. Even details of the organisms’ soft parts and pigmentation are preserved in the ash, suggesting that at least some of them might have been transported alive. Astonishingly, the New Zealand authors’ counts of organic material in the ash suggest that as much as 0.6 km³ of diatom remains were dispersed during the eruption.

English: Circle of diatoms on a slide — Assorted species of diatoms on a microscope slide (credit: Wikipedia)

Violent sub-aqueous eruptions can entrain liquid water as spray as well as water vapour and glassy magma shards, carrying the mixture into the stratosphere, far above wind belts in the lower atmosphere. At such altitudes transport can spread fine aerosols through an entire hemisphere because they remain in suspension for long periods.

Different species of diatom live in subtly different environments, so that their relative proportions and presence or absence in ash provide a ‘fingerprint’ for the volcano responsible. So the discovery by the team from the Victoria University of Wellington (a ‘first’) presents a new tool for identifying the source of ash layers in the volcanic record that came from other volcanoes associated with caldera lakes – common for those capable of launching huge volumes of material aloft, such as Toba that erupted in Sumatra at around 74 ka and may have influenced the first modern human migrants from Africa. But could minute organisms survive both the volcanic heat and blast and a traverse through the dry stratosphere to result in colonisation? If that were possible it would have significant implications for the spread of early life forms during the far more volcanically active Hadean and Archaean Eons of Earth’s history.

Commenting on the article, Jennifer Pike of Cardiff University, UK (Pike, J. 2013. Of volcanoes and diatoms. Geology, v. 41, p. 1199-2000) surmises that diatoms might survive drying out in the stratosphere, provided they were in the form of spores encased in silica. Such spores were not found in the Taupo ash, but who is to say that they will not be discovered in other ancient volcanic ash layers? Spores are extremely durable and other micro-organisms than diatoms produce them and have done in the past.

Super-Eruption Launched Algae Army Into the Sky (livescience.com)

An early oxygenated atmosphere

Published on October 9, 2013 by zooks777Leave a comment

The Earth’s earliest atmosphere undoubtedly had a chemistry dominated by carbon dioxide and nitrogen, together with transient water vapour, outgassed from volcanoes giving pervasive reducing conditions at the surface and in the oceans. Until the last couple of decades the only clear evidence of a switch to oxidising conditions and presumably significant atmospheric oxygen was direct, mineralogical evidence. The most obvious signs are ancient, reddened soils formed when soluble Fe²⁺ lost electrons to molecular oxygen to form the distinct red, orange and brown oxides and hydroxides of insoluble Fe³⁺that impart a deep staining in even small quantities. Others include the disappearance from river-transported sediments of clearly transported grains of metal sulfides and uranium oxide that remain stable under reducing conditions but quickly break down in the presence of oxygen.

Widespread observations in Precambrian sediments, eventually linked with reliable radiometric ages, strongly suggested a fundamental environmental change at around 2.3 billion years ago: the Great Oxidation Event. A few such signs emerge from somewhat older rocks back to 2.7 Ga, but only the 2.3 Ga event created a permanent feature of our home world; at first toxic to many of the prokaryote life forms of earlier times but eventually a prime condition for the rise of the Eukarya and eventually metazoan animals. Isotopic analysis of sulfur from Precambrian sediments also gave hints of a more complex but much debated transition because of the way S-isotopes fractionate under different environmental conditions. Now other indirect, isotopic approaches to redox conditions have become feasible, with a surprising result: powerful evidence that about 3 billion years ago there was appreciable atmospheric oxygen (Crowe, S.A. et al. 2013. Atmospheric oxygenation three billion years ago. Nature, v. 501, p. 535-538).

The Danish-South African-German-Canadian group relied on a fractionation process among the isotopes of chromium, which can exist in several oxidation states. When minerals that contain Cr³⁺ are weathered under oxidising conditions to release soluble Cr⁶⁺ the loss in solution preferentially removes the ⁵³Cr isotope from residual soil. If the isotope enters groundwater with reducing conditions to precipitate some Cr³⁺-rich material yet more ⁵³Cr remains in solution. Eventually such enriched water may enter the oceans, where along with iron and other transition-group metal ions chromium can end up in banded iron formations (BIFs) to preserve isotopic evidence for oxidising conditions along it route from land to sea.

The team analysed both a palaeosol and a BIF unit from a stratigraphic sequence in the Achaean of NE South Africa that is between 2980 and 2924 Ma old. A substantial proportion of the palaeosol is depleted in ⁵³Cr whereas the lower part of the slightly younger BIF is significantly enriched. Changes in the concentration of redox sensitive elements, such as chromium itself, uranium and iron, in the two lithologies helps confirm the isotopic evidence for a major ~3 Ga oxidation event. It is possible to use the data to estimate what the atmospheric oxygen content might have been at that time: not enough to breathe, but significant at between 6 x 10^-5 to 3 x 10^-3 the atmospheric level at present. Oxygen can be produced abiogenically through irradiation of water vapour in the atmosphere as well as by organic photosynthesis. However, the first route seems incapable of yield more than a billionth of present atmospheric concentrations, so the spotlight inevitably falls on a ‘much deep history’ of the action of blue-green bacteria (cyanobacteria) than hitherto suspected.

Ancient soils provide early whiff of oxygen – BBC News (topbreakingnews.info)
Signs of oxygen in 3-billion-year-old soil (arstechnica.com)

Pushing back DNA sequencing: a Spanish cave bear

Published on September 25, 2013 by zooks777Leave a comment

At the time, only 3 years ago, publication of the first full Neanderthal genome seemed miraculous. Yet the apparent magic proved repeatable, including for an obscure but distinct group of extinct humans – the Denisovans – known only from their DNA in a single pinkie bone. These advances astonished the world by showing that anatomically modern humans were capable of interbreeding with both groups; and did so that many people now living outside of Africa carry the genetic evidence. But the samples analysed for DNA were little more than 40 thousand years old. Older fossils of extinct animals have given up their genetic features, such as the wooly mammoth and a horse about 700 ka old, but only from samples frozen into permafrost at high northern latitudes.

The degradation of DNA over time seemed destined to limit palaeo-genetics, even when slowed down by natural freezing. The degradation breaks down any surviving genetic material into shorter and shorter fragments of the DNA molecule, ultimately to its atoms being recombined in new molecules of totally unrelated compounds through the chemical processes of fossilisation. Reassembling the fragments correctly becomes increasingly difficult the smaller they are. Few outside of a highly skilled specialists were optimistic of breaking the 100 ka barrier, even using frozen fossils. Unsurprisingly, having had such dramatic successes, the specialists continue to ride their luck and their ingenuity.

Excavations at the site of Gran Dolina, in Ata... — Excavations at Gran Dolina, in Atapuerca, Spain. (Photo credit: Wikipedia)

The cave complex of the Atapuerca Mountains in northern Spain, whose sediments range in age from almost a million years ago to recent times, contain rich accumulations of human remains, including the pre-Neanderthal Homo heidelbergensis and H. antecessor dating back to more than 800 ka. If ever there was a magnet for archaeo-geneticists Atapuerca is definitely one. Moreover, physical anthropologists seem never to stop disputing their interpretations. Jesse Dabney of the now famous Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and co-workers from Britain, New Zealand, Spain and Australia are now beginning to report results. The first are from a cave bear (probably Ursos deningeri) known to be older than 300 ka (Dabney, J. and 10 others 2013. Complete mitochondrial genome sequence of a Middle Pleistocene cave bear reconstructed from ultrashort DNA fragments in one of its foreleg bones. Proceedings of the National Academy of Sciences, v. 110, doi/10.1073/pnas.1314445110). The bear’s mitochondrial DNA was pieced together from fragments as small as 50 base pairs, and shows its ancestry to bears (U. spelaeus) from the later Pleistocene that became extinct at about 28 ka.

It may be only a matter of time before human DNA emerges from the rich Atapuerca fossil hoard; indeed the authors strongly hint that they are working on that now.

Researchers reconstruct mitochondrial genome of Middle Pleistocene cave bear (phys.org)
It’s Barely There – 300,000 Year Old DNA (genedork.wordpress.com)

Africa-Europe exchange of faunas in the Late Miocene

Published on July 15, 2013 by zooks777Leave a comment

The extremely hazardous seaway through the Straits of Gibraltar and the waterless deserts of the Levant presented considerable barriers to natural exchange of animal groups between Africa and Eurasia throughout the period of hominin evolution known from the African Pliocene and Pleistocene record. These barriers were breached by hominins only occasionally. Through most of the Miocene and back to the Mesozoic Era Iberia and what is now Morocco were separated by a wide seaway preventing faunal exchange. That Betic Seaway eventually closed with the tectonic collision of the two sides to form the modern Betic Cordillera in southern Spain towards the end of the Miocene. This left parts of the Mediterranean to evaporate during what is known as the Messinian Salinity Crisis, which reached completion at 5.59 Ma. Yet this Europe-Africa connection was short-lived, being breached by what is regarded as one of the most dramatic events in Cenozoic history: the Zanclean Flood. At 5.33 Ma the Atlantic burst through what is now the Straits of Gibraltar to refill the Mediterranean Basin within a period between a few months and two years. The flooding began as a vast system of rapids some 1 km high with an estimated flow a thousand times that of the modern Amazon.

During the existence of the Europe-Africa land bridge it was possible for animals to move between north-west Africa and western Europe. Evidence that such an exchange did take place comes from a number of Late Miocene localities in southern Spain and North Africa. The first recorded migrants into Spain were African gerbils, then evidence mounted for larger animals, including hippos and early camels moving into Europe and a reverse migration of rabbits and mice. One of the Spanish sites (Gibert, L. et al. 2013. Evidence for an African-Iberian mammal dispersal during the pre-evaporitic Messinian. Geology, v. 41, p. 691-694) has allowed precise magnetostratigraphic dates to be put on the migrations. The Spanish-US team suggests conditions ripe for migration were in three distinct phases: around 6.3 Ma when hippos managed to swim to Europe; around 6.2 Ma which saw European small mammals making the journey south and camels moving to Europe; in a 300 ka window of opportunity from 5.6 to 5.3 Ma for African mice to make the journey into Europe. Several distinct episodes probably reflect some ups and downs of sea level related to glacial retreats and advances in Antarctica.

One implication of the short-lived Messinian land bridge is that it may have been followed by primates, though evidence has yet to be found. A particularly interesting genus, suggested by some as a possible common ancestor for hominins and chimpanzees, is Oreopithecus a bipedal ape recorded from the Miocene of Italy

End-Triassic mass extinction link to CAMP: It’s official?

Published on June 10, 2013 by zooks7771 Comment

Mass extinctions and smaller but significant die-offs in the marine and terrestrial domains have been linked in the geoscientific imagination with many things: asteroid impacts; gamma-ray bursts from distant supernovae; belches of methane from the sea floor; emissions of hydrogen sulfide gas from seawater itself during ocean anoxia events; sea-level changes and more. The most intriguing, since it suggests a causal link between the core-mantle boundary and the biosphere, is the influence of flood basalt events and the gases, both greenhouse and toxic, that they undoubtedly released.

The famous K-T extinction (now K-Pg since the Palaeogene became the Period following the Cretaceous rather than the Tertiary) has swayed back and forth between the Chicxulub impact in Mexico’s Yucatán Peninsula and the flood basalts of the Deccan Traps in western India as likely mechanisms, Chicxulub currently being in pole position. The equally devastating event at the close of the Triassic (at 201 Ma) that presaged the rise of the dinosaurs has had a similar external versus internal causality controversy, both the Rochechouarte crater and the Central Atlantic Magmatic Province being candidates.

Rochechouarte, however, was nowhere near as energetic an event as the Chicxulub impact. The problem is, as with all events for which the weight of evidence points to very short time scales – of the order of tens to hundreds of thousand years, is the dating of candidate causes. Rochechouarte happened at 201±2 Ma: it may or may not have coincided with faunal change. Yet timing of the CAMP flood basalts has hitherto been even more coarsely tagged. This imprecision is not unconnected with the choice of radiometric dating methods, the ⁴⁰Ar/³⁹Ar approach being ‘easy’ and hence popular, but limited in its precision and accuracy. The ‘gold standard’ is zircon U-Pb geochronology that depends on the far greater reluctance of the host mineral to lose either parent or daughter isotopes compared with the feldspars, micas and amphiboles used in many other methods. Zircon still in its igneous parent is crucial: it is so durable that vastly older zircons are often found in sediments. Yet basalts contain few zircons.

Zircon geochronology has now emerged from the CAMP flood basalts of eastern Canada, the Atlantic seaboard of the US and that of Morocco, which has a precision of around 30 ka, one to two orders of magnitude better than other methods (Blackburn, T.J. and 8 others 2013. Zircon U-Pb geochronology links the end-Triassic extinction with the Central Atlantic Magmatic Province. Science, v. 340, p. 941-945). The extinction is defined most readily by a sudden change in fossil pollen and spores, possibly within less than 10 ka, as well as extinction of Triassic marine fauna and large numbers of terrestrial reptile and amphibian taxa followed by diversification of early Jurassic dinosaurs. The oldest CAMP basalts are from Morocco immediately above spores of clearly Triassic age; i.e. before the extinction, whereas the basalt flows in Canada and the eastern US (a mere 3 to 13 ka younger)are above the turnover. So, the start of the CAMP flood volcanism brackets the extinction.

But did CAMP cause, indeed could it have caused the extinction? Blackburn and colleagues cannot be certain. A negative carbon-isotope spike associated with the extinction is estimated to have required almost a million km³ of magma to have been erupted almost instantaneously to inject excess CO₂ into the atmosphere. The dating suggests four major pulses of eruption in the areas studied spread over around 600 ka, the last three being associated with biological diversification and recovery in the earliest Jurassic. In fact the research seems merely to suggest strongly that flood volcanism accompanied the extinction, but leaves its causing the death toll still an open question.

The CAMP events marked the beginning of Pangaea’s break-up and the formation of Tethys separating Eurasia and North America from the old Gondwana continental mass. That tropical seaway became the site of massive production of marine carbonates, presumably to draw down any carbon dioxide excess in the atmosphere.

The Top 5 Extinctions (danniteboul.wordpress.com)
What really happens in a flood basalt eruption
(Geological Society of London blog)

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