From small beginnings

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Camarasaurus, Brachiosaurus, Giraffatitan, Euh...
Some really cool sauropods. Image via Wikipedia

The great vegetarian sauropod dinosaurs, such as Brachiosaurus, were the biggest animals to walk the Earth, weighing up to 100 tonnes, as long as 60 m from snout to the end of their tails and more than 10 m tall. So big, indeed, that even the largest contemporary predators would have been unable to get sufficient purchase with their jaws to do them much damage. This vast bulk, unlike even bigger modern whales, was unsupported by water and would have posed major problems had the sauropods not evolved very porous, low-density neck and tail bones and kept their heads small relative to the rest of their bodies. Such small heads needed to take in up to a tonne of vegetation each day to keep the monsters alive and  ambling. Their teeth are not those of a chewer, being peg- or spoon-like and pointed forwards; specialised for raking in leaves and twigs, swallowed unchewed in great gulps. Once that style of eating developed in their precursors, with no need for massive chewing muscles it became possible to evolve necks up to 15 m long with increasingly diminutive heads. Studies of large numbers of some species of sauropod precursors indicate that juveniles grew astonishingly quickly, essential if their initial vulnerability was to be outpaced; newly hatched they would have weighed little more than 10 kg. At the growth rates of modern reptiles, the largest sauropods would only have reached full size in about a century. The estimated growth rates suggest warm bloodedness, research suggesting that they maintained body temperatures up to 12°C higher than do alligators. Clearly, sauropod dinosaurs were highly specialised, and their evolution is now known to have been lengthy.

A major news feature in Nature (Heeren, F. 201. Rise of the titans. Nature, v. 475, p. 159-161) traces that evolution through several surprising stages. The earliest likely ancestors, which appear in the Late Triassic (~230 Ma), were about the size of a turkey and had teeth adapted for shredding fibrous plant material; other early dinosaurs show clear signs of a predatory lifestyle. There is a limit to the size of predators bound up with the energy balance between flesh consumption and the energy expended in casing down prey and killing them. The limits on the size of plant eaters are mechanical: how much they can stuff in and the strength of their bodies, especially legs. In a world dominated in numbers by predatory dinosaurs, the selection pressure for herbivores to outgrow them and become too big to bite would have been substantial.

Little Triassic Panphagia (‘eater of everything’) was also bipedal, but the fossil record of sauropod precursors clearly shows their growth to the order of 10 m by the Early Jurassic, but not yet a four-legged gait though they had evolved relatively short but sturdy legs, signs of mass-saving porous neck and tail bones, and jaws with a large gape suited to gulping rather than chewing. By the mid-Jurassic Period sauropods were big, strong and four-legged, and by the Cretaceous they reached unmatched dimensions with the titanosaurs. This evolutionary path was not the only one adopted for dinosaurian herbivory. The famous Iguanodon discovered in 1822 by Gideon Mantell in the Early Cretaceous of Sussex was a member of a bipedal group of herbivores, including the duck-billed dinosaurs, that spanned more or less the same time range as sauropods. Fredric Heeren’s article is accompanied by an on-line ‘tour’ of sauropod evolution (go.nature.com/c7zlct), while the American Museum of Natural History has a website for a major exhibition of sauropods (www.amnh.org/exhibitions/wld/ and http://www.youtube.com/AMNHorg ) that includes footage of  a full-scale animatronic Mamenchisaurus from China which breathes and moves, (Switek, B. 2011. Living it large: review of The World’s Largest Dinosaurs exhibition. Nature, v. 475, p. 172).

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Core’s comfort blanket and stable magnetic fields

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Pangea animation
Pangaea and its break-up. Image via Wikipedia

The record of the Earth’s magnetic field for the most part bears more than a passing resemblance to a bar-code mark, by convention black representing normal polarity, i.e. like that at the present, and white signifies reversed polarity. The bar-code resemblance stems from long periods when the geomagnetic poles flipped on a regular, short-term basis, by geological standards. The black and white divisions subdivide time as represented by geomagnetic into chrons of the order of a million-years and subchrons that are somewhat shorter intervals. Stemming from changes in the Earth’s core, magnetostratigraphic divisions potentially occur in any sequence of sedimentary or volcanic igneous rocks anywhere on the planet and so can be used as reliable time markers; that is, if they can be defined by measurements of the remanent magnetism preserved in rock, which is not universally achievable. Yet this method of chronometry is extremely useful, for most of the Phanerozoic. However, there were periods when the geomagnetic field became unusually stable for tens of million years so the method is not so good. These have become known as superchrons, of which three occur during Phanerozoic times: the Cretaceous Normal Superchron when the field remained as it is nowadays from 120 to 83 Ma; a 50 Ma long period of stable reversed polarity (Kiaman Reverse Superchron) from 312 to 262 Ma in the Late Carboniferous and Early Permian; the Ordovician Moyero Reverse Superchron from 485 to 463 Ma.

Because the geomagnetic field is almost certainly generated by a self-exciting dynamo in the convecting  liquid metallic outer core, polarity flips mark sudden changes in how heat is transferred through the outer core to pass into the lower mantle. It follows that if there are no magnetic reversals then the outer core continued in a stable form of convection; the likely condition during superchrons. But why the shifts from repeated instability to long periods of quiescence? That is one of geoscience’s ‘hard’ questions, since no-one really knows how the core works at any one time, let alone over hundreds of million years. There is however a crude correlation with events much closer to the surface. The Kiaman superchron spans a time when Alfred Wegener’s supercontinent Pangaea had finished assembling so that all continental material was in one vast chunk. The Cretaceous superchron was at a time when sea-floor spreading and the break-up of Pangaea reached a maximum. The Ordovician, Moyero superchron coincides with the unification of what are now the northern continents into Laurasia and the continued existence of the southern continents lumped in Gondwana, so that the Earth had two supercontinents. Those empirical observations may have been due to chance, but at least they provide a possible clue to linkage between lithosphere and core, despite their separation by 2800 km of convecting mantle that transfers the core heat as well as that produced by the mantle itself to dissipate at the surface. Enter the modellers.

How part of the Earth transfers heat is, not unexpectedly, very complex, depending not only on what is happening at that point but on heat-transfer processes and heat inputs both above and below it. The surface heat flow is complex in its own right ranging from less than 20 to as much as 350 mW m-2, the largest amount being through zones of sea-floor spreading and the least  through continental lithosphere. Wherever heat is released in the core and mantle, willy-nilly the bulk of it leaves the solid Earth along what is today a complex series of lines; active oceanic ridge and rift systems such as the mid-Atlantic Ridge.  These lines weave between six drifting continental masses and many more sites of additional heat loss – hot spots and mantle plumes. The many heat escape routes today complicate the deeper convective processes and there are many possibilities for the core to shed heat, yet they continually change pace and position. When, inevitably, all continental lithosphere unites in a supercontinent, almost by definition, the sites of heat loss simplify too, the supercontinent acting like an efficient insulating blanket. In a qualitative sense, this kind of evolving scenario is what modellers try to mimic by putting in reasonable parameters for all the dynamic aspects involved.  Two physicists at the University of Colorado in Boulder, USA, Nan Zhang and Shije Zhong, have formulated 3-D spherical models of mantle convection with plate tectonics as a basis for whole Earth thermal evolution over that last 350 Ma (Zhang, N & Zhong, S. 2011.  Heat fluxes at the Earth’s surface and core–mantle boundary since Pangea formation. Earth and Planetary Science Letters, v. 306, p. 205-216). The acid test is whether the model can end with a close approximation to modern variations in heat flow and distribution of ages on the sea floor; it does. A probable key to stability in the means of transfer of heat from core to lower mantle – itself a key to a constant outer-core dynamo and geomagnetic polarity – is reduced heat flow at equatorial latitudes; a sort of equatorial downflow of convection with upflows in both northern and southern hemispheres. Zhang and Zhong’s model produced minimal core-to-mantle heat flow at  the Equator at 270 and 100 Ma, both within geomagnetic-field superchrons. Well, that is a good start. Superchrons seem also to have occurred from time to time during the Precambrian, one being documented at the Mesoproterozoic-Neoperoterozoic boundary about 1000 Ma ago. At that time, all continental lithosphere was assembled in a supercontinent dubbed Rodinia (‘homeland’ or ‘birthplace’ in Russian).

Search on for past tsunamis

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Wandoor is a small village and beach near the ...
Relics of the 2004 tsunami on the coast of South Andaman Island. Image via Wikipedia

Spurred by the horrific scenes and death toll wrought by tsunamis following  the 26 December 2004 Sumatran and 11 March 2011 Sendai giant earthquakes, environmental geologists are beginning to look for signs that can reveal past tsunamis in order to evaluate risk from region to region. Before the 11 March disaster Japanese scientists had in fact traced signs of a tsunami in 869 CE and showed that it had reached almost as far inland as that following the Sendai earthquake. There are a number of geological features that mark the wake of a tsunami: dislodgement of huge boulders on rocky shores; signs of powerful scouring of sallow marine sediments as water recedes from the land; chaotic sediments made up of a jumble of clasts; sediments associated with high-energy flow interleaved with those that mark long periods of low energy deposition; marine faunas unexpectedly found in otherwise terrestrial sediments.

Shortly after the 2004 Indian Ocean tsunamis Indian and Japanese scientists visited the Andaman Islands, which were at the northern end of the megathrust deformation, to seek onshore signs of previous catastrophes (Malik, J.N. et al. 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India. Geology, v. 39, p. 559-562). They discovered a layer of ripped-up lumps of mud set in a sandy matrix dumped on a low-energy black mud, the sandy unit showing inclined stratification that dips inland. All the evidence pointed to deposition by a tsunami. An earlier event reveals swamping of older non-marine sediments by the black mud unit that contains brackish-marine diatoms; a probable result of sudden subsidence linked to an earthquake affecting the Andamans in much the same was as did that of December 2004. The mud had also been intruded by a body of structureless sand , probably resulting from liquefaction as a result of the seismicity. Dating the events using radiocarbon methods proved difficult. Although dating of the earlier event suggested an event around 1670 CE, carbon from the later one gave much older ages, suggesting that the tsunami had ripped up older sediments and redeposited them. However it may be correlated with the major Arakan earthquake of 2 April 1762 close to the coast of Myanmar.

Evidence of this kind can easily be overlooked, and rather less research centres on recent coastal-zone sediments than on sedimentary rocks of the distant past. Areas where such signs of neotectonics have been sought assiduously are those surrounding coastal nuclear installations, but largely to check for evidence of recent faulting that may indicate potential seismic threat but not tsunamis. Clearly it was that kind of threat that decisively put the Japanese Fukushima Daiichi nuclear power station out of action and almost resulted in complete melt-down in March 2011, and severely set back construction of an advanced fast-breeder reactor on the eastern coat of India at Kalpakkam, near Chennai in 2004.

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Earliest animals from continental environments

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Skolithos trace fossil. Scale bar is 10 mm.
Skolithus burrows. Image via Wikipedia

Following closely on discovery in 1 Ga old sediments of the earliest evidence for eukaryote life in continental environments (see Eukaryote conquest of the continents posted June 11, 2011) it seems that metazoan animals colonised non-marine environments earlier than had previously been thought. Up to now most palaeontologists believed that there was a lag of at least 80 Ma between the emergence of marine bilaterian metazoans and their expansion into freshwater, due to a number of physiological hurdles that had to be overcome, such as regulation of trace element chemistry within their cells and bodily fluids. It has been know for more than a century that the first signs of sturdy animals in the marine realm are burrows in tidal sediments that formed more or less at the Cambrian-Precambrian boundary; the earlier sac-like Ediacaran fauna seemed ill-suited to a burrowing or infaunal habitat. A considerable thickness of clastic sediments occur in the Cambrian of eastern California, USA. The earliest are clearly shallow-marine and contain abundant evidence of burrowing. Succeeding them are intensively studied fluviatile sands and silts that have been used a model for sedimentation in the absence of the stabilising influence of land plants. What has been overlooked until recently is evidence for colonisation of the river-laid deposits by burrowing animals (Kennedy, M.J. & Droser, M.L. 2011. Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation. Geology, v. 39, p. 583-586).

The burrows include the vertical U-shaped forms given the name Arenicolites, which is the most common trace fossil, simple vertical tubes (Skolithus) and horizontal, meandering tubes with furrowed sides (Psammichnites). Anyone who has seen the Early Cambrian Pipe Rock of NW Scotland will also have seen these trace fossils, yet the Pipe Rock shows evidence of tidal deposition and is shallow marine. Their non-marine equivalents in California are coeval with the earliest known trilobites in the Cambrian marine sequence. It seems that whatever the burrowing animals were, they easily overcame any physiological or environmental barriers to adopting a life in freshwater, encouraged by the ready sustenance that terrestrially adapted acritarchs and cyanobacteria had provided for half a billion years previously.

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Coast-to-coast seismic section of Canada

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Geological Map of Canada
Geological map of Canada. Image via Wikipedia

In the last few decades there have been several massive programmes aimed at imaging the lithospheric structure beneath continents, often linked with a re-assessment of the various tectonic provinces thought to be present. One of the first was a joint Indian-Soviet project managed by the National Geophysical research Institute in Hyderabad to investigate the crust of South India in the 1970s, which still graces my office wall as a memento of my own contribution to unravelling the underpinnings of this ravishing area. This was followed-up by one from the Himalaya southwards, and others have focused on Britain, the Baltic Shield and the USA by the Consortium for Continental Reflection Profiling (COCORP); the last revealing in detail large-scale, low-angle thrust faulting in the Appalachians and crustal-scale detachment faults in the eastern Basin and Range. These experiments must have been great fun, as they involved detonating large amounts of high explosive to produce sufficient energy to get returns from 100 r more km below, with all the planning needed to avoid fear and loathing among the populace, let alone frightening the horses. Nowadays, most seismic profiling onshore is done using Vibroseis, best imagined as large trucks jacked up on pads on which they bounce up and down, in manner of an LA ‘lowrider’. By comparison, marine surveys are far easier, although marine mammals have seemingly had major setbacks as a result of endless closely spaced seismic lines needed for 3-D subsurface analysis. Onshore, you only get one chance and need to pick your route with great care. Now a Canadian consortium has gone one better by using state-of-the-art seismic refraction and reflection techniques (Hammer, P.T.C. et al. 2011. The big picture: A lithospheric cross section of the North American continent. GSA Today, v. 21 (June 2011 issue), p. 4-9). Uniquely, the Canadian Lithoprobe project  coordinated a full spectrum of geological, geochemical, and geophysical research,  covering 20 years of deep-crustal research by hundreds of contributors.

A large-format profile in a supplement to the paper shows the deep relationships in the Mesozoic Cordilleran Orogen in the west, through the plexus of Precambrian Provinces of the Canadian Shield to the Palaeozoic Orogen in the east: a tract some 6000 km from west to east. The general picture is repeated stacking of orogens, with a remarkable repetition of very similar gross tectonic styles. Clearly, large-scale compressional processes have remained largely unchanged since the middle of the Archaean, and several upper parts of long-dead subduction zones and accretionary duplexes spring from the profile. The surface picture of much of the crust crossed by the stitched-together traverses gives the impression of both complex tectonics and many plutons of different ages, yet on the grand scale of the crust and lithosphere it is the tectonics that dominates: the passage of voluminous melts towards the surface has left the plethora of gently dipping deep shear zones and faults largely unmodified. Indeed, the seismic data reveal astonishingly well-preserved subducted or delaminated crust associated with collisions that occurred 2-3 billion years ago. Despite repeated accretionary tectonics spanning 3 Ga, and the Phanerozoic erosion of the Shield to reveal its innermost and deepest secrets, the crust-mantle boundary, the Moho, is astonishingly flat, ranging from 33-43 km deep. Nor is there much sign of ‘roots’ beneath orogens in the underlying lithospheric mantle; a long standing concept that appears not to be generally supportable over this stretch of the North American continent. The synthesis raises questions as to whether the Moho has always been that shallow or whether it can, in some situations, be a dynamic ‘boundary’. For that to be the case requires that the geologic crust-mantle boundary may not always correspond to the seismic discontinuity with which the Moho has previously been correlated.

PDFs of the profile can be downloaded from ftp://rock.geosociety.org/pub/GSAToday/1106insert-hammer/

Snowball Earth melting hypothesis weakened

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"SNOWBALL EARTH" - 640 million years ago
Artist's impression of the Neoproterozoic Earth during a Snowball episode. Image by guano via Flickr

The combination of glaciogenic sediments with palaeomagnetic evidence for their formation at low-latitudes, together with dates that show glacial events were coeval in just two or three Neoproterozoic episodes are the linchpins for the Snowball Earth hypothesis.  There is little doubt that the latest Precambrian Era did witness such extraordinary climatic events. Evidence is also accumulating that, in some way, they were instrumental in that stage of biological evolution from which metazoan eukaryotes emerged: the spectacular Ediacaran fossil assemblages follow on the heels of the last such event (see Bigging-up the Ediacaran in Earth Pages for March 2011). One of the difficulties with the ‘hard’ Snowball Earth hypothesis is how the middle-aged planet was able to emerge from a condition of pole-to-pole ice cover; hugely increased reflectivity of that surface should have driven mean global temperature down and down. Clearly the Earth did warm up on each occasion, and the leading model for how that was possible is massive release of greenhouse gases from sea-floor sediments or deep-ocean waters to increase the heat-retaining powers of the atmosphere; sufficiently voluminous release from volcanic action seems less likely as there is little evidence of upsurges in magmatism coinciding with the events. Almost all glaciogenic units from the Neoproterozoic have an overlying cap of carbonate rocks, indicating that hydrogen carbonate (formerly bicarbonate) ions together with those of calcium and magnesium suddenly exceeded their solubilities in the oceans.

White flocculent mats in and around the extrem...
Modern sea-floor hydrothermal vent. Image via Wikipedia

To seek out a possible source for sufficient carbon release in gaseous form geochemists have turned to C-isotopes in the cap carbonates. Early studies revealed large deficits in the heavier stable isotope of carbon (13C) that seemed to suggest that the releases were from large reservoirs of carbon formed by burial of dead organisms: photosynthesis and other kinds of autotrophy at the base of the trophic pyramid selectively take up lighter 12C in forming organic tissues compared with inorganic chemical processes). As in the case of the sharp warming event at the Palaeocene-Eocene boundary around 55.8 Ma ago (See The gas-hydrate ‘gun’ in June 2003 Earth Pages), these negative d13C spikes have been interpreted as due to destabilisation of gas hydrates in sea-floor sediments to release organically formed methane gas. This powerful greenhouse gas would have quickly oxidised to CO2 thus acidifying the oceans by jacking up hydrogen carbonate ion concentrations.  Detailed carbon-, oxygen- and strontium-isotope work in conjunction with petrographic textures in a Chinese cap carbonate (Bristow, T.F. et al. 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. Nature, v. 274, p. 68-71) suggests an alternative mechanism to produce the isotopically light carbon signature at the end of Snowball events. The greatest 13C depletion occurs in carbonate veins that cut through the cap rock and formed at temperatures up to 378°C and even the early-formed fine grained carbonate sediment records anomalously high temperatures. So, it seems as if the cap-rock was thoroughly permeated by hydrothermal fluids, more than 1.6 Ma after it formed on the sea floor. This triggered oxidation of methane within the sediments themselves, with little if any need for an atmospheric origin through massive methane release from destabilised gas hydrates elsewhere.

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Hominin round-up

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The skull of Australopithecus africanus so-cal...
Australopithecus africanus from Sterkfontein cave, South Africa. Image via Wikipedia

Strontium isotopes and australopithecine habits

Viewers of Channel 4’s Time Team will be used to seeing eating habits and places of habitation being derived from strontium isotopic analyses of the teeth of modern humans found by archaeologists. The methods enabled scientists to work out where ‘Ötzi the Iceman’, whose mummified remains were found on the alpine border of Austria and Italy, hailed from: it was most likely to have been the South Tyrol province of Italy. Other isotopes (nitrogen and carbon) shows that he was predominantly vegetarian; i.e. he was neither a hunter, nor an especially privileged member of Tyrolean Chalcolithic society.

The same methods offer insights into the life styles of far earlier hominins and has recently been used on teeth of australopithecines (Australopithecus africanus and Paranthropus robustus) found in the famous Sterkfontein and Swartkrans caves South Africa (Copeland, S.R. et al. 2011. Strontium isotope evidence for landscape use by early hominins. Nature, v. 474, p. 76-78). The caves formed in Precambrian dolomites and it was expected that all the teeth would show signs that the individuals from whose jaws they were collected lived their entire lives in a small tract of dolomites (~30 km2) surrounding the caves. For large individuals that was indeed the case, but teeth from smaller fossils show 87Sr/86Sr ratios that are significantly different from those characteristic of local rocks and soils. That suggests the smaller individuals came from further afield than the restricted tract of carbonate strata. Although pelvic remains are normally the best guide to the sex of primate fossils, they are less frequently found than those of crania and dentition. Size variations of adults in a primate species, however, may indicate sexual dimorphism – larger males than females – and this is well-accepted for australopithecines. The implication is that for both species males had small home ranges on the dolomites, or that they preferred that tract. Yet females had dispersed from their parental groups and moved into the area.

Most living primates do not show this kind of sexual dispersion pattern, termed male philopatry,  it being common among modern humans, chimpanzees and bonobos. In the case of the australopithecines that were being studied, both were diminutive creatures living in open savannah with risks of predation from a range of large carnivores. Perhaps the bands living in the dolomite area had better refuges in caves than those elsewhere, and therefore able to attract females.

Arctic Neanderthals

Mousterian Point
A Mousterian stone point, possibly for a spear. Image via Wikipedia

The last Neanderthals known to have been alive were close to the southernmost limit of Europe, in caves on the Rock of Gibraltar at about 24 ka, shortly before the last glacial maximum. Their remains have been found in a >6000 km west-east zone at temperate latitudes, south of 50°N, which extended from western Europe to the Denisova cave in the Altai republic of Russia (50°N, 87°E). This suggests that they subsisted in deciduous woodland and temperate steppe, diffusing southwards as conditions cooled during 2 or 3 past glacial periods. Consequently, sites at higher northern latitudes that preserve only cultural remains – Palaeolithic tools – have hitherto been regarded as signs of fully modern human occupation; it takes considerable skill to distinguish Neanderthal from early modern human artefacts, which are very similar during the time of overlapping occupation (~40-30 ka). A site in northern Siberia at Byzovaya  in the Polar Urals, close to the Arctic circle, is a case in point. A French, Norwegian and Russian team of archaeologists re-examined the site (Slimak, L. et al. 2011. Late Mousterian persistence near the Arctic Circle. Science, v. 332, p. 841-845) and dated it to between 31-34 ka. They also analysed a suite of stone tools, finding that they are directly comparable with Mousterian (Middle Palaeolithic) implements from western Europe rather than products of modern human’s industry of similar antiquity. At that time high-latitude climate was well on its way to frigid, dry conditions (there were no substantial continental ice sheets in northern Russia). The animal remains found at the site were dominated by those of mammoth, with minor proportions of other cold-steppe large mammals, such as woolly rhino, musk ox, horse and bear.

A notable feature of the results is that they suggest that Neanderthals, or others people with a Mousterian culture, were occupying this bleak terrain at roughly the same time as modern humans, who left considerably richer suites of artefacts, including tools, ornaments and figurines carved from bone and ivory, but were after more or less the same prey species. Both groups clearly were able to cope with and thrive on the harsh conditions, until recently only within the scope of highly specialised cultures such as the Innuit and original Siberian peoples. The dating shows that whoever produced and used the Mousterian tools not only shared the terrane with modern humans, but lingered until well after the previously accepted time (~37 ka) of the Neanderthals’ demise except for a few refuges in the Iberian Peninsula and Balkans. Despite the occupation of northern Siberia by different cultural groups, until their bones are found who they were is not certain. Denisova Cave showed that Neanderthals and the genetically different Denisovans co-occupied temperate central Siberia (see Other rich hominin pickings in the May 2010 issue of EPN) so there are currently two options.

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Eukaryote conquest of the continents

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NW end of a classic example of a mesa form of ...
Suilven, a spectacular outlier of Torridonian terrestrial sandstones resting on a buried landscape of Archaean gneisses near Lochinver, Sutherland. Image via Wikipedia

Geologists often assume that the continents were first colonised by plants, insects then vertebrates beginning in the Ordovician Period with preservation of spores very like those of the liverworts, which incidentally can only be removed from gravel driveways by the use of acetic acid, glyphosate, pycloram and flamethrowers having no lasting effect. The most intractable of all organisms found on the land surface today are prokaryotic (nucleus-free cells) cyanobacteria whose biofilms cement desert varnish (see Desert varnish, May 2008 in Subjects: GIS and Remote Sensing). Cyanobacteria have long been suspected to have been the first life forms to adopt a terrestrial habit, and their cells have been discovered in the now-famous Neoproterozoic lagerstätten in the Doushantuo Formation of China (see The earliest lichens, May 2005 in Subjects: Geobiology, palaeontology, and evolution) The oldest un-metamorphosed sediments in Britain, the Torridonian redbeds that form the magnificent scenery of north-western Scotland, now push back the date of the earliest eukaryotic (cells with nuclei) terrestrial life, of which we are one form, half a billion years before the Doushanto cyanobacteria (Strother, P.K. et al. 2011. Earth’s earliest non-marine eukaryotes. Nature, v. 473, p. 505-509). The Torridonian is one of the thickest (~12 km) terrestrial sequences on the planet, and spans a time range of around 200 Ma (1.2 to 1 Ga). It is a repository of almost the entire range of humid continental sedimentary environments: colluvial fan; bajada; alluvial; deltaic and lacustrine build-ups. Grey lake-bed mudstones and phosphate nodules in the Torridonian yield small organic fossils lumped in the sack-term acritarchs. Similar bodies, whose affinities are diverse and generally obscure, have been reported from marine sediments as old as 3.2 Ga. The fascination of those from the Torridonian, other than their terrestrial association, is that some include aggregates of spherical cells with tantalising suggestions of central nuclei and, as a whole assemblage, exhibit a range of morphologies far beyond that of nucleus-free prokaryotes and the signature of cytoskeletal filaments that form a ‘scaffold’ for eukaryote cells. Worth noting is that one of the authors is Martin Brasier of Oxford University, whose meticulous bio-morphological skills in microscopy has made him one of the foremost critics of speculation on Precambrian  microfossils (see Doubt cast on earliest bacterial fossils April 2003 in Subjects: Geobiology, palaeontology, and evolution). The authors opine that the ecological diversity of freshwater and land systems, and the physico-chemical stress associated with repeated wetting and desiccation compared with the marine domain may have been instrumental in origination of the Eucarya, which should give the Torridonian a scientific reputation that extends beyond these shores.

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Wide-eyed dinosaurs

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Dinosaur Exhibition Beijing
Image by Ivan Walsh via Flickr

One of the surprises concerning the dinosaurs was that some species were able to live at near-polar latitudes. The surprise is not about their ability to survive a cold climate for the Cretaceous world was one characterised by greenhouse conditions and ice-free polar regions swathed in forests. On top of that, evidence is accumulating that some dinosaurs at least were able to regulate their body temperature; they may have been warm-blooded. The oddity is that they were able to survive the winter darkness of latitudes above those of the Arctic and Antarctic Circles. It now seems that some groups of dinosaurs evolved excellent night-time vision (Schmitz, L. & Motani, R. 2011. Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. Science, v. 332, p. 705-708). Not only did some have large eyes, but preservation of the fibrous outer ring of the eye or sclera – the ‘whites’ in our case – in some large-eyed dinosaurs shows a reduction in width that is characteristic of good scotopic or night vision. Since much of the polar ‘night’ is more like twilight than perpetually full darkness, enhanced night vision would have allowed high-latitude dinosaurs to survive winter by crepuscular feeding habits. This more or less extinguishes the notional day-night duality of terrestrial vertebrate life during the Mesozoic; dinosaurs by day and early mammals by night that allowed mammalian ancestors to escape the clutches of dinosaur predators. Indeed many Mesozoic mammals show signs of diurnality.

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A sign of the times; the ‘Anthropocene’

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Alternative version of image:Wooden hourglass ...
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On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposed to mark the impact on the Earth of our species. The Society, and no doubt others internationally, is interested in gathering thoughts, reflections and observations about the Anthropocene. There is indeed a a powerful and vocal, though not necessarily large, lobby directed at the International Commission on Stratigraphy (ICS) to enshrine this new division. That lobby  has been active since 2000 (see: No escape from global warming;  Changing the world; Epoch, Age, Zone or Nonsense in EPN issues of November 2000, April 2005 and March 2008 respectively)

We currently live in the Holocene (‘entirely recent’), an Epoch with ICS imprimatur. Yet the last 11.7 ka has been but one of very many interglacials since about 2.6 Ma ago; the start of the Pleistocene Epoch and the Quaternary Period – Arduino’s last surviving division of geological time, and lately resurrected from an untimely demise! The ‘golden spike’ for the Pleistocene/Holocene boundary is at the agreed combination of signals – ‘deuterium excess values, accompanied by more gradual changes in 18O, dust concentration, a range of chemical species, and annual layer thickness’ – of the end of the Younger Dryas/Greenland Stadial 1 in a single Greenland ice core (NGRIP) held in a cold store in Copenhagen.

The Holocene itself was based on anthropocentric grounds; i.e. it roughly coincides with the transition from human foraging to sedentary life, agriculture, the relentless development of exploitation of the majority of humans and the commodification of the physical and organic environments following the Younger Dryas stadial. I guess that paraphrases how the ‘Anthropocene’ is proposed to be defined – a signal of the beginning of irreversible global change due to human activities whose future we cannot predict.

Even if it was possible to agree on some definitive signal of the onset of human-induced global change in the geological record there remains the formal difficulty for the ICS of agreeing on the location as well as the age and likely durability of the GSSP that would mark the beginning of the ‘Anthropocene’. The originator of the idea, Nobel Prize winning chemist Paul Crutzen, argued vaguely for the ‘start of the Industrial Revolution’. Recently it has been proposed by some to be 6 August 1945 marked by long-lived radioactive fallout from the atomic massacres of civilians at Hiroshima and Nagasaki.  Quaternary researchers decided some time back that the ‘present’ (as in ‘before present’ or b.p.) should be the year 1950 when atmospheric testing of thermonuclear weapons created excess 14C that will make radiocarbon dating of the next 50 ka somewhat more uncertain than it otherwise would have been. The ICS may well have a lengthy debate on its hands if the proposal ever reaches its deliberations.

Furthermore, the advocates are concerned that we are living in the transition into their ‘Anthropocene’ and that it will be so rapid and biologically disastrous as to manifest itself in stratigraphic sections of the future as a mass-extinction event. No previous mass extinction event has been allocated epochal status, being so brief, though never so brief (~10 ka) as the Holocene or any other interglacial of the past 2.6 Ma.

All that I can conclude is that should there still be geologists in, say, a million years time, who will be living in conditions and possessing intellects about which we would be ill advised to guess, they will still be in awe of the vast tracts of geological time and their stratigraphic and tectonic records over the last 4.55 Ga. Consequently, it is possible that they may well regard the then ancient proposal for an ‘Anthropocene’ as premature, hubristic and not a little reminiscent of the fable of Chicken Little; a humorous legacy of their somewhat startled predecessors. By all means let us be concerned  about and take action to halt adverse human influences on the planet, but sloganeering to climb aboard a bandwagon does neither. At the Geological Society meeting, Paul Crutzen observed  “… it will probably take another 20 years before it is formally accepted.” Thank goodness for a sense of reality: we may all be extinct by then…

Added 12 August 2011: Between 11.5 and 3.5 ka the greatest event in the evolution of modern humans took place on all continents except Australia and Antarctica; a foraging lifestyle gave way to settlement and the domestication of both plants and animals – the Neolithic Agricultural Revolution. The production of surplus value, stored in the form of livestock herds and grain, marked by this transition set humanity on the road to its current social, ecological and economic crisis. Interestingly, William Ruddiman of the University of Virginia in 2005 noted a shift in the CO2 content of glacial ice around 8 ka, which he ascribed to intense farming and suggested that if there were to be an Anthropocene Epoch it should coincide with the start of agriculture. Combining geological and societal factors points unerringly to the start of the Holocene, so there is little need for a new Epoch. That sensible view receives support from a palaeo-demographic survey of 133 burial sites in the Northern Hemisphere: some before the local transition to agriculture, others following it (Boquet-Appel, J.-P. 2011. When the world’s population took off: the springboard of the Neolithic demographic transition. Science, v. 333, p. 560-561). The proportion of 5 to 19 year-old remains in the cemeteries shows a marked rise in the thousand years after the first local signs of agriculture thereafter to stabilise at a new higher level. This indicates a significant increase in female fertility, perhaps by as much as two births per woman. That would set in train the relentless, 1200-fold rise in world population from the estimated 6 million at the start of the Holocene to 7 billion at present.

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