Relative age sequences in sequences of fossiliferous sediments are frequently intricate, thanks to animal groups that evolved quickly to leave easily identifiable fossil species. Yet converting that one-after-the-other dating to absolute values of past time has been difficult and generally debateable. Up to now it has relied on fossil-based correlation with localities where parts of the sequence of interest interleave with volcanic ashes or lavas that can be dated radiometrically. Igneous rocks can provide reference points in time, so that age estimates of intervening sedimentary layers emerge by assuming constant rates of sedimentation and of faunal speciation. However, neither rate can safely be assumed constant, and those of evolutionary processes are of great biological interest.
Sunset at St Hilda’s Abbey, Whitby NE England; fabled haunt of Count Dracula (credit: epicnom)
If only we could date the fossils a wealth of information would be accessible. In the case of organisms that apparently evolved quickly, such as the ammonites of the Mesozoic, time resolution might be extremely fine. Isotopic analysis methods have become sufficiently precise to exploit the radioactive decay of uranium isotopes, for instance, at the very low concentrations found in sedimentary minerals such as calcium carbonate. So this prospect of direct calibration might seem imminent. Geochemists and palaeontologists at Royal Holloway University of London, Leicester University and the British Geological Survey have used the U-Pb method to date Jurassic ammonites (Li, Q. et al. 2014. U–Pb dating of cements in Mesozoic ammonites. Chemical Geology, v. 376, p. 76-83). The species they chose are members of the genus Hildoceras, familiar to junior collectors on the foreshore below the ruined Abbey of St Hilda at the small port of Whitby, in NE England. The abundance and coiled shape of Hildoceras was once cited as evidence for the eponymous founder of the Abbey ridding this choice locality of a plague of venomous serpents using the simple expedient of divine lithification.
Hildoceras from the Toarcian shales of Whitby (credit: Wikipedia)
The target uranium-containing mineral is the calcite formed on the walls of the ammonites’ flotation chambers either while they were alive or shortly after death. This early cement is found in all well-preserved ammonites. The Hildoceras genus is found in one of the many faunal Zones of the Toarcian Age of the Lower Jurassic; the bifrons Zone (after Hildoceras bifrons). After careful selection of bifrons Zone specimens, the earliest calcite cement to have formed in the chambers was found to yield dates of around 165 Ma with precisions as low as ±3.3 Ma. Another species from the Middle Jurassic Bajocian Age came out at 158.8±4.3 Ma. Unfortunately, these precise ages were between 10-20 Ma younger than the accepted ranges of 174-183 and 168-170 Ma for the Toarcian and Bajocian. The authors ascribe this disappointing discrepancy to the breakdown of the calcium carbonate (aragonite) forming the animals’ shells from which uranium migrated to contaminate the after-death calcite cement.
Geologists who study turbidites assume that the distinctive graded beds from which they are constructed and a range of other textures represent flows of slurry down unstable steep slopes when submarine sediment deposits are displaced. Such turbidity currents were famously recorded by the severing of 12 transatlantic telecommunication cables off Newfoundland in 1929. This happened soon after an earthquake triggered 100 km hr-1 flows down the continental slope, which swept some 600 km eastwards.
Typical structures in Upper Carboniferous turbidites near Bude, Cornwall, UK (credit: Flickr, Earthwatcher)
Sea beds at destructive margins provide the right conditions for repeated turbidity currents and it is reasonable to suppose that patterns should emerge from the resulting turbidite beds that in some way record the seismic history of the area. British and Indonesian geoscientists set out to test that hypothesis at the now infamous plate margin off Sumatra that hosted the great Acheh Earthquake and tsunamis of 26 December 2004 to kill 250 thousand people around the rim of the Indian Ocean (Sumner, E.J. et al. 2013. Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatra margin? Geology, v. 41, p.763-766).
Animation of Indonesian tsunami of 26 December 2004 (credit: Wikipedia)
Cores through turbidite sequences along a 500 km stretch of the margin formed the basis for this important attempt to test the possibility of recording long-term seismic statistics. To avoid false signals from turbidity currents stirred up by storms, floods and slope failure from rapid sediment build-up 17 sites were cored in deep water away from major terrestrial sediment supplies, which only flows triggered by major earthquakes would be likely to reach. To calibrate core depth to time involved a variety of radiometric and stratigraphic methods
Disappointingly, few of the sites on the submarine slopes recorded turbidites that match events during the 150-year period of seismic records in the area, none being correlatable with the 2004 and 2005 great earthquakes. Indeed very little correlation of distinctive textures from site to site emerged from the study. Some sites on slopes revealed no turbidites at all from the last 150 years, whereas turbidites in others that could be accurately dated occurred when there were no large earthquakes. Only cores from the deep submarine trench consistently preserved near-surface turbidites that might record the 2004 and 2005 great earthquakes.
These are surprising as well as depressing results, but perhaps further coring will discover what kind of bathymetric features might yield useful and consistent seismic records from sediments.
In its 125th year the Geological Society of America is publishing invited reviews of central geoscience topics in its Bulletin. They seem potentially useful for both undergraduate students and researchers as accounts of the ‘state-of-the-art’ and compendia of references. The latest focuses on major controls on past sea-level changes by processes that operate in the solid Earth (Conrad, C.P. 2013. The solid Earth’s influence on sea level. Geological Society of America Bulletin, v. 125, p. 1027-1052), a retrospective look at how geoscientists have understood large igneous provinces (Bryan, S. E. & Ferrari, L. 2013. Large igneous provinces and silicic large igneous provinces: Progress in our understanding over the last 25 years. Geological Society of America Bulletin, v. 125, p. 1053-1078) and the perennial topic of how granites form and end up in intrusions (Brown, M. 2013. Granite: From genesis to emplacement Geological Society of America Bulletin, v. 125, p. 1079-1113).
Sea level change
Conrad covers sea-level changes on the short- (1 to 100 years), medium- (1 to 100 ka) and long term (1 to 100 Ma). The first two mainly result from local deformation of different kinds associated with glacial loading and unloading. These result in changes in the land surface, the sea surface nearby and on thousand year to 100 ka timescales to ups and downs of the sea-bed. Global sea-level changes due to melting of continental glaciers at the present day amount to about half the estimated 2 to 3 mm of rise each year. But increasingly sensitive measures show it is more complex as the rapid shifts of mass involved in melting ice also result in effects on the solid Earth. At present solid mass is being transferred polewards, but at rates that differ in Northern and Southern hemispheres and which are changing with anthropogenic influences on glacial melting. Viscous movement of the solid Earth is so slow that effects from previous glacial-interglacial episodes continue today. As a result rapid elastic movements are tending to produce relative sea-level falls in polar regions of up to 20 mm per year with rising sea level focusing on areas between 30°N and 30°S. The influence of the slower viscous mass transfer has an opposite sense: sea-level rise at high latitudes. Understanding the short- and medium-term controls is vital in predicting issues arising in the near future from natural and anthropogenic change.
Comparison of two sea level reconstructions during the last 500 Ma. (credit: Wikipedia)
Most geologists are concerned in practice with explanations for major sea-level changes in the distant past, which have a great deal to do with changes in the volumes of the ocean basins. If the global sea-floor rises on average water is displaced onto former land to produce transgressions, and subsidence of the sea floor draws water down from the land. Conrad gives a detailed account of what has been going on since the start of the Cretaceous Period, based on the rate of sea-floor spreading, marine volcanism and sedimentation, changes in the area of the ocean basins and the effects of thermally-induced uplift and subsidence of the continents, showing how each contribution acted cumulatively to give the vast transgressions and regressions that affected the late Phanerozoic. On the even longer timescale of opening and closing of oceans and the building and disintegration of supercontinents the entire mantle becomes involved in controls on sea level and a significant amount of water is chemically exchanged with the mantle.
Large igneous provinces
The Web of Science database marks the first appearance in print of “large igneous province” in 1993, so here is a topic that is indeed new, although the single-most important attribute of LIPs, ‘flood basalt’ pops up three decades earlier and the term ‘trap’ that describes their stepped topography is more than a century old. Bryan and Ferrari are therefore charting progress in an exciting new field, yet one that no human – or hominin for that matter – has ever witnessed in action. One develops, on average, every 20 Ma and since they are of geologically short duration long periods pass with little sign of one of the worst things that our planet can do to the biosphere. In the last quarter century it has emerged that they blurt out the products of energy and matter transported as rising plumes from the depths of the mantle; they, but not all, have played roles in mass extinctions; unsuspected reserves of precious metals occur in them; they play some role in the formation of sedimentary basins and maturation of petroleum and it seems other planets have them – a recipe for attention in the early 21st century. Whatever, Bryan and Ferrari provide a mine of geological entertainment.
In comparison, granites have always been part of the geologist’s canon, a perennial source of controversy and celebrated by major works every decade, or so it seems, with twenty thousand ‘hits’ on Web of Science since 1900 (WoS only goes back that far). Since the resolution of the plutonist-neptunist wrangling over granite’s origin one topic that has been returned to again and again is how and where did the melting to form granitic magma take place? If indeed granites did form by melting and not as a result of ‘granitisation. Lions of the science worried at these issues up to the mid 20th century: Bowen, Tuttle, Read, Buddington, Barth and many others are largely forgotten actors, except for the credit in such works as that of Michael Brown. Experimental melting under changing pressure and temperature, partial pressures of water, CO2 and oxygen still go on, using different parent rocks. One long-considered possibility has more or less disappeared: fractional crystallisation from more mafic magma might apply to other silicic plutonic rocks helpfully described as ‘granitic’ or called ‘granitoids’, but granite (sensu stricto) has a specific geochemical and mineralogical niche to which Brown largely adheres. For a while in the last 40 years classification got somewhat out of hand, moving from a mineralogical base to one oriented geochemically: what Brown refers to as the period of ‘Alphabet Granites’ with I-, S- A- and other-type granites. Evidence for the dominance of partial melting of pre-existing continental crust has won-out, and branched into the style, conditions and heat-source of melting.
Typical granite tor near Kisumu, Kenya (credit: Wikipedia)
All agree that magmas of granitic composition are extremely sticky. The chemical underpinnings for that and basalt magma’s relatively high fluidity were established by physical chemist Bernhardt Patrick John O’Mara Bockris (1923-2013) but barely referred to, even by Michael Brown. Yet that high viscosity has always posed a problem for the coalescence of small percentages of melt into the vast blobs of low density liquid able to rise from the deep crust to the upper crust. Here are four revealing pages and ten more on how substantial granite bodies are able to ascend, signs that the puzzle is steadily being resolved. Partial melting implies changes in the ability of the continental crust to deform when stressed, and this is one of the topics on which Brown closes his discussion, ending, of course, on a ‘work in progress’ note that has been there since the days of Hutton and Playfair.
Time Lord, possibly outside the offices of the International Commission on Stratigraphy (credit: Sorcyress via Flickr)
Because it is the ultimate historical discipline, the essence of geology centres on time, measuring its passage and establishing correlations in time on a global scale so that an interlinked story of Earth evolution can be told. In fact geology is not just about a record of what happened in the four dimensions of place and time; it is a great deal more multidimensional, involving temperature, strain, chemistry, erosion, deposition, sea-level , the course of life and much more besides. Ever more multifaceted and, sadly, divided into subdisciplines and interfaces with other aspects of natural science that few if any individuals can grasp, an almost legally enforceable set of rules is needed to keep the order orderly. Unlike history and more akin to archaeology geological time is of two kinds, its precisely quantitative measure being a relative newcomer.
Since it emerged in the Enlightenment that began in the late 17th century geology has been dominated by a relative sense of timing: Steno’s Law of Superposition, and those relating to deformation, igneous eructations, erosion and deposition, first addressed systematically by James Hutton, being the most familiar. The notion of an absolute time scale into which events separated relative to one another could be fitted with confidence is a real latecomer. Although first attempted between 1650 and 1654 by Archbishop of Armagh James Ussher – he reckoned from the Old Testament that everything began at dusk on Saturday 22 October 4004 BCE – the only useful and broadly believable approach to absolute time has been based on the decay of radioactive isotopes incorporated into minerals once they had formed within a rock. But that is no panacea for the simple reason that most of them form through igneous or metamorphic processes and only rarely in the course of sedimentation. It also has only become reliable and precise in the last two or three decades.
Tying together global records of all the kinds of process that have made, shaped and changed the Earth has therefore become an increasingly complex blend between local relative dating, burgeoning regional to global means of correlation and the odd point in absolute time. What has arisen is a dual system that, if truth were told, is often used in a cavalier fashion. Equally to the point, the rules have of late become unfit for purpose and are in need of revision, which is a task for the Time Lords, properly known as the International Commission on Stratigraphy (ICS). The trouble is, the rules have themselves evolved somewhat episodically while their subject is appropriately in continual motion and change, if not anarchic. To the outsider things can seem very odd indeed. Most reasonably well-read souls will have heard of the Cambrian and the Jurassic, largely because of the popularity of trilobites that blossomed in the one and dinosaurs that strutted the land in the other. What is less well known is that the two names have different usages as adjectives: one to signify an interval of time called a Period, the other a System of essentially piled-up sedimentary rocks.
There are greater dualisms that group the Period/System divisions: the largest Eon/Eonothem groupings of Archaean, Proterozoic and Phanerozoic; the Era/Erathem signifiers such as Palaeoproterozoic, Mesozoic and Cenozoic. Incidentally, the time between the formation of the Earth and the first palpable rocks, from about 4550 to 4000 Ma, has been called the Hadean but has no designated status, possibly because it has no rock record whatsoever. Divisions of Periods/Systems apply only to the time since fossils became abundant 541 Ma ago, and in order of fineness of division are Epoch/Series and Age/Stage. Example of the first can be Lower, Middle and Upper – to spice things up, Middle maybe omitted from some Periods/Systems – or they might be given names derived from type areas, such as the ever popular Llandovery at the base of the Silurian Period/System. Helpfully, the Cambrian contains Terreneuvian, Series 2, Series 3 and Furongian from early to late/bottom to top. The final global division has always floored undergraduates and shows little sign of relief – there are a great many Ages/Stages, in fact a round 100 (I may have miscounted), 98 with names, 2 currently unnamed and 4 in the Cambrian called Stages 2 to 5: confusing, that… has anyone spoken of the Stage 3 Stage or the Stage 5 Age of the Cambrian?
Worryingly, in my hasty overview of the ICS International Stratigraphic Chart above I have reversed the official designation of chronstratigraphic/geochronological nomenclature: is this likely to have me committed to the geoscientific equivalent of Guantanamo Bay, or merely limbo?
I have by no means exhausted officialise. Readers may not be surprised to learn that the Time Lords have bent Heaven and Earth literally to concretise the double entendres of geology. The base of almost every Age/Stage in the Phanerozoic Eonothem/Eon is defined at a suitably agreed point on the ground by, in a few cases, a real golden spike (I may be mistaken on this, as the only one I tried to visit was at the base of a Welsh cliff suitable only to be visited by – in the timeless phrase – ‘a strong party’). More prosaically there are monuments of various ethically appealing designs that go by the sonorous name Global Boundary Stratotype Section and Point. I have it on reasonably good authority that ICS delegates have, on occasion, needed to be physically restrained from fist fights over which nation shall host a particular GSSP (the ‘B’ in the acronym is aspirated).
This is the point that all readers will have been waiting for: it has been suggested to ICS that the whole edifice is looked at very closely and perhaps revised (Zalasiewicz, J, et al. 2013. Chronostratigraphy and geochronology: A proposed realignment. GSA Today, v. 23 (March 2013), p. 4-8). For professionals this is an obligatory read, for others optional: there is no excuse as it is downloadable for free – click on the title. While you are about it, you can also download from GSA Today the famous proposal for an entirely new series/epoch called the Anthropocene (see also A sign of the times: the ‘Anthropocene’ in EPN issue of May 2011)
Inca dry stone wall in Sacsayhuamán fortress, Cusco, Peru (credit: Håkan Svensson via Wikipedia)
These days it is a rare thing for an entirely novel surface process to be discovered; two centuries of geomorphological and sedimentological studies seem to have exhausted all the basic possibilities with only a few bits and pieces to be filled in.
Go to the foot of any steep slope topped by hard rock in an arid or semi-arid area and you are sure to find a boulder field formed by a variety of mass-wasting processes, such as rockfalls. As often as not such boulders are rounded, the usual explanation being that the rounding has resulted either from chemical weathering in the up-slope colluvium or exfoliation (‘onion-skin’ formation) through physical weathering in situ. Boulders are simply too big to have been moved other than by toppling or glacial transport at high latitudes, so rounding by abrasion seems unlikely. Aeolian sandblasting tends to favour just one side of boulders and ‘scallops’ their surface.
The driest place on Earth, Chile’s Atacama Desert, has plenty of boulder fields next to areas of high relief, and sure enough they are beautifully rounded, even though it has barely rained there for around 10 million years. Jay Quade of the University of Arizona, USA, with US, Australian and Israeli colleagues noticed that many of the boulders are surrounded by moat-like depressions and their sides, but not their tops, are nicely smoothed. These features suggested that some process had caused the boulders to move around and to rub one another, but whatever that was it had not caused even quite tall boulders to topple over (Quade, J. et al. 2012. Seismicity and the strange rubbing boulders of the Atacama Desert, northern Chile. Geology, 40, 851-854). An explanation was clearly something to puzzle over, until, that is, two of the authors returned to the area to make further observations. They were caught on the exposure by a magnitude 5.2 earthquake – a not uncommon experience in the foothills of the Andes – when the ton-sized boulders began to sway, rotate and jostle together with a great deal of noise. Here was the novel mechanism of erosion and ‘granulation’: seismic rubbing.
By dating the age of the exposed surfaces using cosmic-ray generated isotopes of beryllium and aluminium, the authors have been able to estimate that over the past 1.3 Ma the boulders have experienced between 40 to 70 thousand hours of rubbing. Indeed, it is quite likely that the whole boulder field, the upslope mass wasting and the sediment in which the boulders are embedded are products of seismicity. Oddly, just such jostling and rubbing of boulders and cobbles is characteristic of Inca architecture in the Andes, whose stonework used no cement but has minimal gaps between the blocks. Who is to deny that the Incas learned their unique building method from observing seismic rubbing.
It is hard to resist curiosity when a phrase includes a superlative. Dickens knew this when he opened A Tale of Two Cities with the words, ‘It was the best of times, it was the worst of times…’. So impacted into post-Victorian English language are they that the Daily Mirror of 13 May 2012 used them to celebrate ‘The most scintillating finish in Premier League history’: referring of course to the footballing tales of the city of Manchester (UK, that is). So it was with some gaiety that I turned to a paper in the May 2012 issue of Geology (Løseth. H. et al. 2012. World’s largest extrusive body of sand? Geology, v. 40, p. 467-470). Now, that is a title to conjure with, and I would advise any academic author to add a superlative adjective of some kind to their next manuscript title, to ensure more than 5 readers and at least one citation to add to her/his CV. Conversely, I caution against seemingly ultra-high impact, exclamatory single-word titles such as ‘Coelocanth!’, Porphyroblast!’, ‘Ignimbrite!’ or ‘Sphenochasm!’: they summon untoward visions of geoscientists much given to ‘snorting and pawing the air in salivating lust and groveling need’, in the manner of Hungry Joe’s reaction to a pornographic cameo brooch (Heller, J. 1961. Catch 22: Simon & Schuster).
The sand body in question lies in the Pleistocene subsurface of the Norwegian sector of the North Sea above the Snorre oilfield, and came to light through a 3-D seismic survey with extraordinarily good resolution that allowed the reconstruction of its base and top structure contours (in two-way time) and thus its overall volume and shape. At 10 km3, were it to have formed yesterday to cover Manhattan the paper’s abstract suggests that it would have reached the 37th floor of the Empire State Building. More parochially, had it engulfed London’s old financial quarter centred on London Bridge (Post Codes EC1 to 4 and SE1) 30 St Mary Axe (‘The Gherkin’) and ‘The Shard’ would be buried in their entirety leaving one of capitalism’s iconic heartlands a curiously gnarled sandy plain.
Small mud volcano, Romania (Photo credit: Wikipedia)
That the sand is extrusive rather than being simply a sedimentary stratum is revealed by its extraordinary shape. Its thickest part is in a depression surrounded by mounds of the underlying unit – the former seabed – above which the body is absent. These mounds show marginal signs on the seismic sections of dykes that could have acted as feeders from stratiform sands deeper in the sequence, the dykes coinciding with the base of ‘ditches’ in the body’s upper surface. In turn, the ditches have flanking ridges as if the ditches and the dykes below were feeders for the sand extrusion. Such an extrusive sand body is currently forming at the accidentally triggered Lusi sand volcano in Indonesia where a single vent exudes about 50 thousand m3 each day; a rate that would take 550 years to produce the Snorre field body. Pleistocene stratigraphy surrounding the vast North Sea ‘boil’ suggests that it formed during a period of rapid sedimentation from the huge North Sea ice shelf supplied by the Scandinavian ice sheet.
Helge Løseth and colleagues from Statoil and the University of Rennes ran a series of dry sandbox experiments to mimic the process of sand injection. By pumping air through interbedded sand, glass ballotini and silica powder, to represent two types of non cohesive sands and cohesive mudrocks, they found that increasing the overall air pressure in the box eventually fluidized the ‘sands’ which blurted through the ‘clays’ to form ‘volcanoes’ with plumes of sand that enlarged the area of deposition at the surface. Cutting into the sediments after the experiments revealed a remarkably real-looking system of intrusive sand bodies (dykes, sills and laccoliths) as well as the extrusive mass of sand. Chances are that such bodies may form more commonly in marine sequences, given encouraging over-pressuring through sudden increases in normal sedimentation. If so, the very open grain structure of the vented sands might provide superb petroleum reservoir characteristics.
The last of five written papers in my 1967 final-year exams was, as always, set by the ‘Prof’. One question was ‘Rock and rhythm: discuss’ – it was the 60s. Cyclicity has been central to observational geology, especially to stratigraphy, the difference from that era being that rhythms have been quantified and the rock sequences they repeat have been linked to processes, in many cases global ones. The most familiar cyclicity to geologists brought up in Carboniferous coalfields, or indeed any area that preserves Carboniferous marine and terrestrial rocks, is the cyclothem of, roughly, seat-earth – coal – marine shale – fluviatile sandstone – seat-earth and so on. Matched to the duration of Carboniferous to Permian glaciations of the then southern hemisphere, and with the relatively new realisation that global sea level goes down and up as ice caps wax and wane, the likeliest explanation is eustatic regression and transgression of marine conditions in coastal areas in response to global climate change. Statistical analysis of cyclothemic sequences unearths frequency patterns that match well those of astronomical climate forcing proved for Pleistocene glacial-interglacial cycles.
The Milankovich signals of the Carboniferous are now part of the geological canon, but rocks of that age more finely layered than sediments of the tropical continental margins do occur. Among them are rhythmic sequences interpreted as lake deposits from high latitudes, akin to varves formed in such environments nowadays. Those from south-western Brazil present spectacular evidence of climate change in the Late Carboniferous and Early Permian (Franco, D.R. et al. 2012. Millennial-scale climate cycles in Permian-Carboniferous rhythmites: Permanent feature throughout geological time. Geology, v. 40, p. 19-22). They comprise couplets of fine-grained grey quartz sandstones from 1-10 cm thick interleaved with black mudstones on a scale of millimetres, which together build up around 45 m of sediment. Their remanent magnetism and magnetic susceptibility vary systematically with the two components. Frequency analysis of plots of both against depth in the sequence show clear signs of regular repetitions. Low-frequency peaks reveal the now well-known influence of astronomical forcing of Upper Palaeozoic climate, but it is in the lower amplitude, higher frequency part of the magnetic spectrum that surprises emerge from a variety of peaks. They are reminiscent of the Dansgaard-Oeschger events of the last Pleistocene glacial, marked by sudden warming and slow cooling while world climate cooled towards the last glacial maximum (~1.5 ka cyclicity) and Heinrich events, the ‘iceberg armadas’ that occurred on a less regular 3 to 8 ka basis. There are also signs of the 2.4 ka solar cycle. The relatively brief cycles would have been due to events in a very different continental configuration from today’s – that of the supercontinent Pangaea – and their very presence suggests a more general global influence over short-term climate shifts that has been around for 300 Ma or more.
El Niño effect on sea -surface temperatures in the eastern Pacific Ocean. Image via Wikipedia
Closer to us in time, and on a much finer time scale are almost 100 m of finely laminated shales from the marine Late Cretaceous of California’s Great Valley (Davies, A. et al. 2012. El Niño-Southern Oscillation variability from the late Cretaceous Marca Shale of California. Geology, v. 40, p. 15-18). The laminations contain fossil diatoms: organisms that are highly sensitive to environmental conditions and whose species are easily distinguished from each other. It emerges from studies of the diatoms in each lamination set that they record an annual cycle of seasonal change related to marine upwellings and their varying strengths, with repeated evidence for influx of fine sediment derived from land above sea level and for varying degrees of bioturbation that suggests periods of oxygenation. Spectral analysis of the intensity of bioturbation, which assumes the lamina are annual, and other fluctuating features reveals peaks that are remarkably close to those of the ENSO cyclicity that operates at present, at 2.1-2.8 and 4.1-6.3 a, as well as repetitions with a decadal frequency.
The annual cycles bear similar hallmarks to those imposed by the monsoonal conditions familiar from modern California, which fluctuated in the Late Cretaceous in much the same way as it does now – roughly speaking, alternating El Niño and La Niña conditions. That is not so surprising, as the relationship between California and the Pacific Ocean in the Cretaceous would not have been dissimilar from that now. The real importance of the study is that it concerns a period in Earth’s climate history characterised by greenhouse conditions, that some predict would create a permanent El Niño – an abnormal warming of surface ocean waters in the eastern tropical Pacific that prevents the cold Humboldt Current along the Andean coast of South America from supplying nutrient to tropical waters. The very cyclicity recorded by the Marca Shale strongly suggests that the ENSO is a stable feature of the western Americas. Recent clear implications of ENSO having teleconnections that affect global climate, on this evidence, may not break down with anthropogenic global warming. This confirms similar studies from the Palaeogene and Neogene Periods.
Altyn Tagh range at top - click for detail. Image via Wikipedia
The transport of sediment by wind action is generally visualised as sand dunes of all kind of shapes. Yet shifting sand particles arm strong wind in the manner of a sand blaster so that it can act as an agent of erosion to form peculiar landforms known as yardangs, which often parallel the prevailing wind as linear ridges. Yardangs very rarely form from crystalline rocks, but poorly cemented sedimentary rocks are particularly prone to wind erosion. In a few areas that are very arid it is the dominant sculpting process. One such area is the Qaidam Basin (<50 mm of rain per year) at the northern edge of the Tibetan Plateau. The basin is flanked to the north by the Altyn Tagh mountains, and major passes in that range funnel powerful winds across the basin floor. The yardangs of Qaidam are enormous, reaching up to 50 m high and show clearly on satellite images and often camouflage the trend of bedding in the sedimentary rocks from which they are carved. Formerly thought to be a basin in which sediment was accumulating and being actively folded by tectonic forces related to the India-Asia collision zone, recent work reveals several very surprising aspects of local wind action (Kapp, P. et al. 2011. Wind erosion in the Qaidam basin, central Asia: implications for tectonics, palaeoclimate, and the source of the Loess Plateau. GSA Today, v. 21 (April/May 2011) p. 4-10). Since the Late Pliocene the rate of wind erosion has reached as much as 1 mm per year, so that it is a source of sediment not a repository, to the extent that at least a third of the basin is occupied by exposed folded sediments that wind erosion has exhumed. Yet this is not an area noted for large dust storms.
Yardangs in Quaidam. Image by Joe Zhou via Flickr
The folded sediments are early Pleistocene lacustrine silts and fine sands, which sand blasting has easily sculpted, but many of the yardangs are encrusted with a crust of salt. Indeed several generations of such crusts mark wind-eroded surfaces of different relative ages. It seems that the erosion has occurred in episodes, most likely during cold-dry glacial and stadial periods when the northern jet stream probably shifted south from its present local position around 48°N to the latitude of Qaidam (around 40°N) when the Altyn Tagh’s funnelling effect would have been maximised by prevailing north westerly winds that parallels the yardangs. Such episodes can be shown to have eroded hundreds to thousands of metres of the slowly deforming sediments since about 2.8 Ma. It was at that time that folding began in earnest, and quite possibly the unloading effect of the wind erosion may have assisted the deformation. Where did such vast volumes of sediment end up? Downwind to the south east are the famous loess deposits in the headwaters of the Huang He (Yellow River), whose transport of eroded loess accounts for the great fertility of much of China’s soils and thereby its great carrying capacity for human population. Interestingly, the loess deposits show intricate alternations that match the ups and downs of climate through the late Pleistocene. The link with the Qaidam yardang fields seems convincing
The Geologic Time Spiral: A Path to the Past. Designed by Joseph Graham, William Newman, and John Stacy. Get it from http://pubs.usgs.gov/gip/2008/58/
The Système International d’Unités (SI) is the agreed arbiter that defines the units in which phenomena are measured. There are 7 SI base units (length, mass, time, electric current, temperature, intensity of radiation and amount of substance) from which others are derived as they become necessary. Geoscientists have striven to comply, though not always happily. For instance the doubly-derived SI unit for pressure, the pascal (Pa) is a newton (derived unit of force) per square metre (N m-2), and in base units 1 kg m-1 s-2. The pascal replaced the long employed arbitrary unit, the kilobar (1 kb = 1000 x surface atmospheric or barometric pressure) one of which represents about 3.5 km depth in the earth. The reluctance to shift units is probably innate conservatism, for 1 kb = 100 MPa: simples!
Another problem has arisen as regards the SI base unit for time – the second. This is unwieldy for geological time, the Earth having formed approximately 1.435 x 1017 seconds ago. It’s not so handy for history either, about 3 x 1010 seconds having elapsed since William of Normandy won the Battle of Hastings.
The year is what we remember, but even that in a historical sense has its problems, for instance the BC/AD division where some scholars even dare to suggest that Christ was born in 4 BC. The more politically correct Common Era (CE) and Before the Common Era (BCE) of course don’t fool anyone. Interestingly, Wikipedia (en.wikipedia.org/wiki/Year) indicates, there are over ten current versions of a ‘year’ depending on context (for instance, astronomers favour the Julian year). Historical and thus geological time has the unnerving habit of continually getting longer, and it is a major problem to measure historical time precisely, either from increasingly vague records as one delves back in historical documents or because of the inherent imprecision in measuring radioactive isotopes and their daughter products that underpins archaeological and geological time. Archaeologists have a very hard time of it, for their workhorse is radiocarbon dating that depends on the production of radioactive 14C in the atmosphere by cosmic ray’s interaction with nitrogen. The rate of 14C production varies over time with the cosmic ray flux from extra-solar sources, and even worse, a very large amount was produced by testing nuclear weapons in the atmosphere in the mid 20th century. Abandoning the BC/AD division that lurks still with historians and archaeologists, geoscientists speak of time ‘before present’ (bp), which doesn’t matter a damn for geological Periods, Eras and Eons which are immensely long whatever the unit. But it does for the Holocene, mainly calibrated by radiocarbon methods: bomb-test production of 14C , which will linger about 50 thousand years before near-complete decay, has forced the ‘present’ to be set at 1950 AD!
So the year is here to stay, even though it is arbitrary and changes all the time, along with kilo, mega and giga prefixes for thousands, millions and billions of years. Yet teeth are now being ground over what the unit’s symbol should be (Biever, C. 2011. Push to define year sparks time war. New Scientist, v. 210 (30 April 2011), p. 10). A task group of geoscientists and chemists set up by the International Union of Pure and Applied Chemistry, IUPAC, and the International Union of Geological Sciences, IUGS in 2006 have now defined the year – why chemists, you might wonder; they measure the radioactive decay constants of isotopes used in radiometric dating. The link to the SI system through the base unit of one atomic-standard second is to be standardised by the solar year; the time in seconds between one solstice and the next at the equator for year 2000: i.e. 3.1556925445 × 107 s (Holden, N.E. et al. 2011. IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011). Pure and Applied Chemistry, v. 83, p. 1159-1162). It is to be called the annus (a), applied in ka, Ma or Ga to two usages of time, the time difference between ‘now’ and an event in the past, and the time difference between two events in the past. This dual usage of the same symbol is the source of the gnashing. Whereas Ma, for instance, was quite acceptably used for the measured age of a rock relative to the present, there are at least three schools of thought for other uses of time. Some have been quite happy to use Ma for measured age, a fixed time datum in the past such as the Precambrian-Cambrian boundary, and a time duration such as that of a geological Period or some major event such as an orogeny (that has been used in Earth Pages News since its outset). Others would distinguish between the first and the other two, as for instance Ma for the first and Myr for the other two. But there are variants, the symbol mya having been used for ‘million years ago’, and the international science journal Nature currently uses Myr for the first but now takes the safe path of using ‘million years’ for the other two. Nicholas Christie-Blick of Columbia University in New York is reported as having opined that the rationalisation to one-symbol-fits-all is a huge step backwards, and he is not alone; Science editorial staff will continue to demand of their authors a distinction between age and time span, since a switch would ‘confuse its readers’, long accustomed to that usage.
Also it is so easy to write, ‘the rock has an Ar-Ar age of 25 Ma’, ‘it took 25 Ma for this trilobite to disappear from the geological record’, and ‘about 25 Ma ago, there is a gap in the fossil record of primates’. I personally welcome the simplification, especially as it will encourage authors to write more nicely.
At the end of the Miocene tectonic uplift in the region of the present Straits of Gibraltar cut the Mediterranean Sea off from the Atlantic. The only water able to flow into the isolated marine basin was that carried by the major rivers: the Rhône, Danube, Dneiper and Nile. Their volume was exceeded by evaporation, so the Mediterranean became more and more salty, eventually almost drying out completely to leave thick evaporite deposits that still underlie its deepest parts. 5.33 Ma ago, the tectonic barrier was breached so that Atlantic water flooded the whole Mediterranean basin. The Zanclean flood at the start of the Pliocene has been rated as the greatest catastrophic event in the Phanerozoic history of the oceans, but just how dramatic it was has previously only been guessed at. Seismic profiles across and along the line of flooding reveal channels several kilometres across, about 200 km long and up to 250 m deep, now filled with debris (Garcia-Castellanos, D. et al. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, v. 462, p. 778-781). Using a well-established model of river incision in mountain rivers, the authors have suggested how the flooding proceeded. From an initial trickle when the original barrier subsided below Atlantic sea level, flow grew exponentially over a few thousand years to about three times that of the modern Amazon discharge (~108 m s-1), at which rate incision reached more than 0.4 m per day. Around 90% of the Mediterranean basin’s entire volume was flooded in a matter of a few months to two years, sea level rising at up to 10 m per day.
Formation of BIFs halted by Sudbury impact
The peculiar story of banded iron formations (BIFs) is one that ‘runs and runs’, as journalists say. Most of the steel on which North American capital was built comes from gigantic BIF deposits around Lake Superior that formed during the Palaeoproterozoic. Apart from a brief return in the Neoproterozoic, associated with conditions peculiar to ‘Snowball Earth’ conditions, the Superior Province BIFs are the last of any consequence. Most geologists look to a gradual shift in the oxygen content of ocean water as photosynthetic life grew to dominate the Earth after about 2.4 Ga, but the BIFs around Lake Superior turn out to be capped by a blanket of ejecta from a massive extraterrestrial impact that formed the Sudbury Complex (Slack, J.F. & Cannon, W.F. 2009. Extraterrestrial demise of banded iron formations 1.85 billion years ago. Geology, v. 37, p. 1011-1014). But how could even a monstrous bolide have changed ocean chemistry so decisively? John Slack and William Cannon of the US Geological Survey believe that the impact was so violent that it resulted in wholesale mixing of oxygen-bearing surface waters with those of the deep ocean. The evidence they cite is a coincident change in the nature of deep-water hydrothermal deposits from sulfide-bearing to those dominated by iron-oxides.
The Sudbury impact produced a crater around 150 to 270 km across (one of the three largest known on Earth), and it is dominated by remelted basaltic rocks so almost certainly struck the Palaeoproterozoic ocean floor. Its ejected debris probably covered almost 2 million km2 and is found up to 800 km from Sudbury, Ontario. Yet, even with impact cavitation and massive tsunamis it seems barely feasible that an impact of a size dwarfed by those of the Lunar surface could completely remix the oceans. However, it is likely that in the Palaeoproterozoic continental crust was gathered together in a supercontinent so that tsunamis could scour much of the surrounding ocean. A plume of vaporised seawater may also have scavenged oxygen from the atmosphere. The evidence seems compelling, and another possibility is that Sudbury was not the only impact site…
And another oddity…
That a major climatic warming occurred at the end of the Palaeocene (55 Ma) is now undoubted, as is its probable cause by emission from the ocean floor of vast amounts of methane. Yet oddly the Palaeocene-Eocene Thermal Maximum (PETM) coincides with a brief geomagnetic reversal 53 ka long (Lee, Y.S. & Kodama, K. 2009. A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum. Geology, v. 37, p. 1047-1050). Both events were short, so a coincidence seems unlikely, in the authors’ opinion. They suggest a connection through the massive power imparted to climatic processes by the PETM (at least a terawatt and perhaps orders of magnitude more), including the deep thermohaline circulation of the oceans that did shift during the event. Had they exceeded a threshold power for circulation of the liquid outer core they may have triggered the brief reversal, which quickly reverted to its previous magnetic polarity. Ths association is not unique, detailed magnetic studies of the K-T boundary event at 65 Ma has revealed a similar short reversal spanning the duration of the iridium peak ascribed to the Chicxulub impact. However, Chicxulub delivered a power of the order a year’s solar radiation in about one second: vastly larger than the climate perturbation of the PETM. Are we seeing here a hidden signal of an extraterrestrial impact behind the methane release? Impacts are no longer as popular as they once were…
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