Month: February 2012

Very persistent cycles

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Carboniferous shale
Carboniferous shale (Photo credit: tehsma)

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.

OSTM/Jason-2's predecessor TOPEX/Poseidon caug...
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.

Time wars flare up again

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English: A diagram of the geological time scal...
Time's spiral Image via Wikipedia

Last year Earth Pages News reported a rationalisation of the way in which geological time is signified (Rationalising geological time 7 May 2011). A working group set up by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences (IUGS) defined the year as the base unit, standardizing it to the time in seconds between one solstice and the next at the equator for year 2000 (3.1556925445 × 107 s) thereby linking it to the Système international d’unités or SI base unit of the second, itself defined in terms of behaviour of the caesium atom. It is to be signified by ‘a’ for annus (year in Latin) and preceded by ‘k’, ‘M’ and ‘G’  for thousands, millions and billion years, complying with the SI progression in steps of 103 for units.

The sticking point for some, mainly in the US (e.g. Science magazine and many geoscientists there) is that the ka, Ma, Ga symbols are to apply not only to times before the present but also to spans of geological time. Since the agreed convention is incorporated into SI it has almost the force of law for scientists , so that the Cretaceous Period will be said to have begun at 145.5±4.0 Ma ago, ended at 65.5±0.3 Ma ago and was 80 Ma long, instead of the latter being in m.y., m.yr., mya  or Myr according to what seem to have been personal quirks or those of scientific journals.

Somewhat florid reaction against the rationalisation (Christie-Blick, N. 2012 Geological time conventions and symbols. GSA Today, v. 22 (February 2012 issue), p. 28) seems to have flowed from a deliberation on the IUPAC-IUGS proposal (in Prague, Spring 2010) by a lesser world body: the International Commission on Stratigraphy’s  (ICS) International Subcommission on Stratigraphic Classification (ISSC). The meeting voted 16 to 2 to reject the proposal – a substantial number of voting members abstained – claiming that it violated SI ‘rules’ regarding base- and derived units. The issue, on reaching the ICS meeting, as the same Prague workshop, seems to have been greeted by a 50:50 split. A closed meeting of the ICS Bureau (now we can begin to see the kind of thinking involved here…) on the workshop’s last day unanimously adopted the motion ‘We neither accept nor reject the IUGS-IUPAC Task Group’s recommendation to apply Ma, generally, as the unit of deep time. We accept the argument for Ma as a single unit for time but would recommend flexibility, allowing for the retention of Ma as specific notation for points in time (i.e., dates) and myr as a unit of time denoting duration. We agree with the spirit of this statement’ [my italics]. ‘Neither accepting nor rejecting’ is something familiar from minutes of the Central Committee of the former USSR, being rumoured to have been Joseph Vissarionovich Stalin’s favoured formulation in moments of uncertainty: a little like the old ‘Belfast Question’, ‘Are you for us or against us’ from someone whose politics is not entirely clear.

An argument proffered by Christie-Blick is, ‘No one objects to the storming of the Bastille on 14 July 1789 (a date) or to the construction of Stonehenge from 2600–1600 BC (an interval specified by two dates). In the case of the latter, we say that the job took 1000 years, not 1000 BC.’ This forgets something quite practical: geochronologist rarely if ever, ‘neither accept nor reject’ AD, BC BCE, or CE but express time in years before present, with the odd convention that ‘the present’ was 1950, before atmospheric testing of thermonuclear devices. What is wrong with the answer to the question, ‘When did the Cretaceous begin?’ being 145.5 Ma ago, or ‘80 Ma’ in answer to, ‘How long did it last?’ Who would prefer the alternative to the second question –  80 (choose your preferred symbol from the following: m.y., m.yr., mya.  Myr., million years or millions of years)?

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Geophysics reveals secrets of the beaver

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Beaver Hut
Beaver lodge and dam (Photo credit: Bemep)

One of the interesting things about the beaver is that its obsession with civil engineering may have a profound effect upon landscape. Before Europeans set foot in North America, it is estimated that up to 400 million of them inhabited the continent. The ponds that they create by building the dams in which they live securely, encourage sedimentation. It is quite possible that this creates recognisable stratigraphic formations; but no-one really knows as active and wet beaver habitats hide what lies beneath them. It is clearly urgent to obtain this intelligence: the Geological Society of America’s monthly Geology contained in its first issue for 2012 a paper that indeed probes the legacy of large rodents long gone (Kramer, N. et al. 2012. Using ground penetrating radar to ‘unearth’ buried beaver dams. Geology, v. 40, p. 43-46).

The target for surveillance was the eponymous Beaver Meadows in Colorado, USA, and not only did the researchers from Colorado State University deploy ground-penetrating radar, but used the seismic reflection method as well, to quantify volumes of beaver-induced sedimentation. Fortunately, despite their past presence in some strength, beavers no longer frequent Beaver Meadows and no ethical lines in the sand were crossed. Beaver and elk seemingly have long competed for the meagre resources of Beaver Meadows, the rodent having finally succumbed locally to determined efforts by the elk to consume the beavers’ victuals. As disconcerted and no doubt sulking beavers failed to maintain their dams and lodges, the water table fell, further encouraging the elk. Eventually, at some time after the Beaver Survey of 1947, the last of them moved to new meadows. Their ravages (see http://animal.discovery.com/videos/fooled-by-nature-beaver-dams.html) of what would otherwise be dense woodland have, however, made it possible for geophysicists to try out their sophisticated kit on a new and thorny issue: they ran 6 km of GPR and seismic profiles.

I came across this handsome animal (Castor can...
A beaver. Image via Wikipedia

In much the same way as larger scale geophysical data are interpreted for petroleum traps, signs of hydrocarbons, mighty listric faults and zones of tectonic inversion, the beaver-oriented sections potentially yield considerable insight to the trained eye. There are indeed beaverine sedimentary aggradations of Holocene age above the local glacial tills. Beneath Beaver Meadow they amount to as much as 50% of post-glacial sediment. Apparently, the deposits have a linear element that follows the local drainages.

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Petrologists probe Minoan collapse

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Partial panorama of Santorini and Thera caldera
Modern Santorini and the drowned Thera caldera. Image via Wikipedia

A burning topic for Bronze Age archaeologists, such as the delightful  Bettany Hughes – biographer of Helen of Troy, is the explosive collapse of the volcano Thera (modern Santorini) whose distant effects (ash and tsunamis)wiped out the Minoan civilisation of Crete around 1600 BCE, giving rise to Plato’s legend of Atlantis. It was a big one alright, hurling of the order of 60 km3 of pulverised magma skywards, though not the largest historic eruption: that involved 160 km3 from the Tambora volcano on Indonesia’s island of Sumbawa in 1815. The inhabitants of Santorini simply disappeared, after evacuating their homes during precursor earthquakes and small eruptions, which were then buried beneath many metres of tephra when Thera literally ‘blew its top’. Little ash fell on Crete, yet its northern coast shows clear signs of a major tsunami. The reason for such an engulfing wave is revealed by the nature of Thera’s eruption: after evacuating magma, the edifice collapsed to form a caldera clearly revealed by the elliptical bay around which the remnants stand as the various islands of Santorini.  Caldera formation would have displaced vast amounts of sea water.

Santorini has been well studied by volcanologists, still being an astonishingly awesome spectacle as well as preserving the full record of the eruption and the archaeology that it buried (http://santorini-eruption.org.uk/). Empirical research reveals four distinct eruptive phases probably over a period of a few months. The explosive force of the final catastrophe probably resulted from seawater reaching the sub-volcanic magma chamber: not a difficult feat of imagination. What has not been known is how the magma evolved over times leading up to the cataclysm, and that is a knotty issue for all volcanoes that pose a major threat because of evidence for repeated and perhaps cyclic activity. A new technique is now capable of lifting the veil on such purely magmatic evolution, and is based on the changes that took place in minerals that crystallised over lengthy periods while the magma cooled slowly at depth but was periodically added to (Druitt, T.H et al. 2012. Decadal to monthly timescale of magma transfer and reservoir growth at a caldera volcano. Nature, v. 482, p. 77-80).

Such phenocrysts are commonly found in fragments of pumice that make up Theran tephra, and they are commonly zoned in a concentric fashion, especially those of the mineral feldspar, each zone marking a phase of growth that occasionally traps samples of magma in the form of now glassy inclusions. The zones mark chemical changes in the magma as new pulses are added in the sub-volcanic chamber, and sometimes temperature changes and loss of gas. Although the zone boundaries a are expected to be sharp in terms of chemical differences, in practice they are blurred as a result of element diffusion at high temperatures. Diffusion is a predictable process and so the degree of blurring indicates the time at which a new zone formed relative to that of eruption and cooling, when diffusion would have stopped abruptly. Rates of high-temperature diffusion depend on the element concerned. So using a suite of trace elements in feldspar zones gives a variety of chronometers. A fast-diffusing element such as Mg can chart changes of the order of decades to months, while a more sluggish trace element – for instance titanium – can examine changes on longer timescales.

The results obtained by the authors present a surprise: although Thera had last erupted catastrophically 18 ka previously, additional magma recharged the volcano only in the last few decades before it extinguished life on Santorini and set the Minoan civilisation on a downward spiral. Indeed, magma continued to be added even in the last few months. Calderas, such as that at Yellowstone in the western US, to which are linked ancient ash layers covering areas hundreds and thousands of kilometres away, pose threats as large and even bigger than Thera. If Thera is anything to go by, they lie in repose long after an eruption and signs of recharge may herald eruption in the near future. The Yellowstone caldera, that has lain dormant for 640 ka is indeed showing signs of magmatic ‘stoking’, as the Earth’s surface there is slowly bulging. It produced ‘supereruptions’ that dwarfed Thera at 2.1 Ma (2500 km3), 1.3 Ma (280 km3) and 0.6 ka (1000 km3). For each of these and several other calderas there are abundant tuffs that carry phenocrysts, whose zonation is yet to be checked for signs of past behaviour by their local magma chambers.

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