Author Archives: linarite

Record of rising Holocene sea-level in the tropics

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I.et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene.Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6 ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

 

Sabotage in Science

Scientists are supposedly objective but a recent case in Michigan USA sheds a worrying light on a dark reality of research. A former post-doctoral researcher at the Ann Arbor campus of the University of Michigan has been found guilty of changing the experimental results of a PhD student who worked in the same lab; the charge was malicious destruction of personal property, which in the USA usually means vandalism. The postdoc claims his otherwise inexplicable actions stemmed from internal pressures and that he intended to slow down the student’s work (Maher, B. 2010. Sabotage. Nature, v. 467, p. 516-518). At first the student believed that she was making mistakes herself, but then realised some unknown person had swapped labels on her samples. When she aired her suspicions she was told she was being paranoid and going through a bad patch in her studies. She persisted despite such resistance, until her supervisor alerted the university’s security officers. They launched an investigation into the student herself! After two interrogations and a lie-detector test, the university police installed cameras in the lab, which led to the culprit being caught red-handed.

Research misconduct is notoriously difficult to apprehend, institutional authorities often balk at clear evidence and end up in what amounts to a whitewash to protect the institution’s integrity. Daniele Fanelli of the University of Edinburgh UK has made a study of malpractice in science, ranging from this kind of willful derailing of a research project to withholding information and vindictive reviews that are rarely considered misconduct. She has found that up to 30% of scientists admit (anonymously) to lesser but still baleful issues, and a staggering 70% say they have witnessed deliberate damage to fellow researchers. This malice that dare not speak its name, even were it to be rarer than Famelli has discovered, is a blight that should be recognised by institutional authorities rather than ignored or actually turned against the complainants.

Crowd Science

Malice and/or mendacity are not the sole ways to get on unfairly. A mild form is somehow to join a team, preferably with a role that involves little actual work. ‘Brownie-points’ in the promotion stakes are guaranteed nowadays by authorship in peer-reviewed journals: senior or sole author is best; next being in a small authors list in a journal that demands an account of the role of each; but even being an also-ran or last of a great many can go nicely on your CV. Does one have to have some je ne sais quoi to be accepted by a team? Well it depends on what the quois might be. Some might say seniority or prestige as that helps the paper to be accepted; others that having the only accessible scientific machine for the topic more or less guarantees a place; but is it possible merely to lurk in the corridor and still get on board?

The vast majority of author lists are surely completely honest, but there is a definite tendency for them to get longer as time goes by. During the days when analysis of lunar rocks from the Apollo Missions was booming a team of geochemists – the Lunatic Asylum – was formed at the California Institute of Technology (incidentally, in 1920 Caltech changed its name from Throop University – after Amos Gager Throop, former Mayor of Pasadena). Its founder and leader was and remains Gerry Wasserburg, and occasionally papers were published under the anonymity of the group, so it is hard to tell just how many of them were involved. The Atlas experiment at the CERN Large Hadron Collider has given rise to a paper authored by 230 individuals from 169 institutions (The ATLAS Collaboration et al. 2008. The ATLAS Experiment at the CERN Large Hadron Collider. Journal of Instrumentation, v. 3, doi: 10.1088/1748-0221/3/08/S08003), but that consortium does not hold the record. As far as I know, the biscuit is taken, for the moment, by Members of the Genetic Investigation of ANthropocentric Traits (GIANT) consortium (Allen, H.L et al. 2010. Hundreds of variants clustered in genomic loci and biological pathways affect human height. Nature, v. 467, p. 832-838) whose title is self-explanatory. Of its 7 pages, 3 are taken up by the names of its 287 authors, their 203 institutions and a not inconsiderable number of funding agencies. At just under 3000 words (not including the names and affiliations of the authors), each author on average has just over 10 words to their name. Interestingly, 10 of the authors (the first 6 and last 4 ) ‘contributed equally to this work’ – how is not specified, and 4 authors are each affiliated with 5 institutions. By comparison, geosciences is definitely little league as regards collaborative ventures, but opportunities there surely are.

 

Low-angle extensional detachments at ocean ridges

The discovery in the 1970s that some low-angled faults have an extensional or normal sense of displacement stemmed from extensional systems in the continental crust, exemplified by the Basin and Range Province of western North America. Yet the largest extensional systems on Earth are those associated with mid-ocean ridges, and in the 1980s some of those were shown to involve low-angled detachments too. Michael Cheadle and Craig Grimes (University of Wyoming and Mississippi State University, USA) review the latest word on oceanic extensional complexes revealed at the AGO Chapman Conference in May 2010 (Cheadle, M. & Grimes, C. 2010. To fault or not to fault. Nature Geoscience, v. 3, p.454-456). As in continental extension, this kind of deformation at divergent margins may produce core complexes uplifted as a result of tectonic unroofing by low-angled detachments, thereby revealing oceanic mantle lithosphere on the ocean floor. Such peculiarities seem to be absent from fast spreading ridges such as the East Pacific Rise and occur where spreading is slow. They are best developed where spreading is starved of magma injection to produce the classic sheeted-dyke complexes of the middle oceanic crust, and with unusually thick oceanic lithosphere. Yet the ocean floor must spread at these localities, and that is achieved by extensional tectonics that accommodates up to 125 km of spreading with next to no magmatism: 4 Ma-worth of spreading.

For extensional faults to develop into low-angled detachments rocks must be weak, otherwise simple steep, domino-style faults would form. Penetration of seawater down faults weakens oceanic lithosphere through hydration reactions that produce clays and serpentines, which encourage the formation of ductile shear zones. Interestingly, some of the largest hydrothermal systems on the mid-Atlantic Ridge coincide with core complexes, and exude hydrogen – a product of serpentinisation – as well as methane and metal-rich brines.

Plate theory moves on

The last 40 to 50 years have seen the theory of plate tectonics supported by more and more empirical evidence from sea-floor magnetism, seismicity, bathymetry and a growing number of other features that relate to Earth’s dynamism. Yet the original concepts of rigid plates and their dislocation from one another and the underlying mantle have been undermined to a degree by the wealth of data now available. Increasing resolution of seismic tomography is revealing what is happening in the depths of the mantle on which growing confidence can be placed. Matching these increasingly revealing sources of data has been the computing power to try to blend them all with rheological theory and thereby model the way the world works. The latest of these modelling ventures does seem to move plate theory onto a significantly higher plane (Stadler, G. et al. 2010. The dynamics of plate tectonics and mantle flow: from local to global scales. Science, v. 329, p. 1033-1038). The keys to this step are: increasingly sophisticated software that encompasses the contributory factors, akin to models used by mechanical and hydraulic engineers; faster computing that allows a decrease in the size of the 3-D cells used in assessing all the interactions as realistically as possible, and a great deal of graphic creativity so that we can visualise the results. At its centre is varying rock strength, the principal ‘engineering’ input derived from seismic tomography, blended with the gravitational and thermal forces that drive Earth’s ‘engine’.

Stadler et al.’s development divides up the planet into a 3-D mesh whose resolution varies according to the likely complexity of motions within and upon the Earth. For instance there is not much call for detail for what lies below abyssal plains of the ocean floor, so available computing power can be focused on the more intricate parts of the tectonic set-up, especially subduction zones that are both the most spectacular features of the Earth’s behaviour and the source of the main force that drives its surface parts – slab pull. Already the approach is producing more questions than answers. For instance, building in the data that show something of convection in the deep mantle makes the model’s output for the more shallow-seated and better known processes deviate more than expected from what is observed – less comprehensive and more coarse approaches previously seemed to be match deep and shallow processes quite well. This is a difficult topic to express merely in words, but fortunately the paper has been made freely available at http://users.ices.utexas.edu/~carsten/papers/StadlerGurnisBursteddeEtAl10.pdf

See also: Becker, T. 2010. Fine-scale modelling of global plate tectonics. Science, v. 329, p. 1020-1021.

Carbonates on Mars

Ancient valley systems, huge water-carved gorges and sedimentary deposits signify with little room for doubt that early in its history Mars was wet; it must therefore have been warm. A thick CO2-rich atmosphere seems obligatory to give the kind of greenhouse warming that prevented Earth from freezing over when the young Sun was weaker than now. The question is, where did the CO2 go so that the planet became chilled? Gravity on Mars is sufficient to have retained the gas, unlike water vapour that dissociates to hydrogen and oxygen, of which hydrogen easily escapes even a much stronger gravitational field. A consensus is developing that it resides in carbonate minerals. The other likely greenhouse gas is sulfur dioxide, for whose drawdown there is ample evidence in the form of sulfates detected from orbit and by surface rovers. Carbonates have a relatively simple, and unique spectrum of reflected solar radiation, with an absorption feature at a wavelength around 2.3 micrometres. Carbonates have been detected on Mars using orbital hyperspectral imaging, but only in patches. The NASA rovers rely on serendipity for any discovery, yet Spirit did stumble on a large carbonate-rich outcrop identified by its on-board Mössbauer spectrometer (Morris, R.V. and 12 others 2010. Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science, v. 329, p. 421-424). It appears to be a Fe-Mg variety in association with olivines, and carbonate makes up to 34 % of part of the outcrop. The texture is granular, yet the area abounds with evidence for hydrothermal activity in the form of sulfates and silica-rich materials, implying that some kind of circulation system deposited the carbonates. The associated olivine is odd, as that mineral is prone to rapid breakdown to serpentines in the presence of water.

The discovery of carbonate rock does help the CO2 early greenhouse theory and the fate of the warming gas, but aside from the fact the identification has been done at vast distance does it rank with geoscience that can be accomplished on Earth? It is a small piece in the jigsaw of Mars’s climatic evolution, but cannot resolve the issue of drawdown of greenhouse gas. The real drama there lay in the finding of abundant signs of water erosion on many scales set against today’s surface hyperaridity; evidence for glaciation and subsurface water ice in apparently large volumes. Earth had to have had a thick CO2-rich atmosphere at the same time as that of Mars, but we are still not sure where all that carbon ended up in the early Precambrian, despite limestones and carbon-rich mudstones dating back to 3.4 Ga: as we cannot quantify that aspect of Earth’s history, neither can we expect an early answer for Mars. Indeed, what is the benefit set against the cost?

See also: Harvey, P. 2010. Carbonates and Martian climate. Science, v. 329, p. 400-401.

Wet Moon, dry Moon

Regular readers will remember my remarkable though very reluctant conversion to the notion that there may be water on Mars. My stubborn reaction had been against the background that shrouded the hypothesis with a certain desperation; the need of any future crewed mission to Mars for a water supply and thereby one of hydrogen fuel, plus the determination of the whole Mars-oriented community to justify such a mission by hyping ‘xenobiology’ on the ‘Red Planet’. A similar desperation claoked the search for surface water on the Moon, although one more dominated by the ‘Everest’ syndrome: since the boot prints and flags appeared, everyone wants to go. The Moons internal water is an entirely different kettle of fish. The hypothesis of the Moon’s formation by condensation from an incandescent mass flung into orbit after a planet – planet collision involving the Earth has the corollary that the lunar mantle ought to be bone dry: and so it seemed to be from bulk analyses of rocks brought back by the Apollo missions. In fact, there are a number of possibilities to explain vanishingly small amounts of internal water: the Moon is made of impactor that happened to be dry rather than terrestrial material; Earth and Moon are a mix of both and both Earth and impactor started out dry, but the Earth later received its water from comets; low pressure condensation of the Moon ruled out water entering itss silicate minerals and so on. Then water was found in apatite grains from lunar maria basalts (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). Within a couple of months we are back to the dry-as-an-alco’s-throat view (Sharp, Z.D. et al. 2010. The chlorine isotope composition of the Moon and implications for an anhydrous mantle. Science, v. 329, p. 1050-1053). Both terrestrial and meteoritic chlorine isotopes are in remarkably consistent proportions, but lunar rocks show an 25 times greater spread by comparison. To cut a long and complicated discussion short, such a range could only have formed if chlorides of a variety of metals were vaporised from lunar magmas each having its own effect on fractionation of Cl isotopes. In turn, combination of chlorine with metal ions requires virtually no hydrogen ions and therefore vanishingly little water in the moon, otherwise chlorine would have been combined in HCl and not subject to any fractionation when that volatilised on eruption. So that seems settled, then…

Catastrophic canyon formation

Huge canyons, such as the Grand Canyon and the Gorge of the Blue Nile, have generally been supposed to have resulted from steady-state erosion through resistant rocks, accelerating during annual floods. There are exceptions that produced spectacular gorges during emptying of proglacial lakes in North America and on a lesser scale in northern Britain. Just how efficient at erosion individual floods may be was demonstrated by release of reservoir water through a spillway in Texas for about 3 days in 2002 (Lamb M.P. & Fonstad, M.A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, v. 3, p. 477-481). The peak discharge was ~1500 m3s-1, which is not especially huge, yet up to 12 m of erosion occurred through bedrock to produce a sizeable canyon in what was previously a typical small stream valley. Although some erosion was by plucking of joint blocks a considerable amount occurred by potholes scoured by boulders swirling in the rapid currents. Small islands, resembling those preserved in glacial lake outburst floods, were sculpted mainly by suspended sediment rather than by boulder impacts. Another feature that forces a rethink of erosional processes is that waterfalls show no sign of headward retreat by undercutting, but seem to have formed as slabs were plucked by the hydraulic force and slid down stream to form tabular boulders. The implication is that canyons may form episodically during flood events, when the shear stress of the flow on its bed is sufficient to lift and slide joint-bounded slabs.

The vestige of a beginning

Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (3He) to that released by radioactive decay (4He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of 4He, so the more reworked a mantle source for lavas is the lower its 3He/ 4He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.

The anatomy of a small landslip

At the centre of the Peak District National Park in England is a small mountain called Mam Tor, at the summit of which is a large Iron Age fort complete with defensive ramparts and ditches. Complete, that is, except for its southern parts, which are chopped through by a large arcuate cliff. Below that is hummocky ground typical of landslips, but such disturbed ground is common over large tracts in the Peak District that lie below hills, especially those underlain by Lower Namurian shales of the region. Mam Tor is the only one of these that has an active landslip. Since my early childhood the local authority has tried to keep trafficable a once major road linking the cities of Sheffield and Manchester, but to no avail; most winters it was buckled and cracked by continued motion. The road was abandoned in 1979 and is now a magnificent laboratory for judging the kind of motion involved in the Mam Tor slip. The Iron Age people had much the same problem, as the slip began around 1500 BC long before the fort was built. Clearly, they were not engineering geologists, though the unclimbable scar was maybe a defensive bonus, provided the old, the bewildered and the very young were kept well away from it, as they are today.

Records of the movement have been kept since the road was constructed in 1820, and one milestone has moved 50 m in 190 years at a constant annual rate, but just how it moves has only become clear since Manchester University geologists installed tilt and creep meters, and 50 survey stations in 2004-5. Their preliminary results are just in (Green, S. et al. 2010. The effects of groundwater level and vegetaion on creep of the Mam Tor landslip. Geology Today, v. 26, p. 134-139). The creep rate is clearly governed by groundwater level beneath the slip, and has risen as high as 19.5 mm per day. From the logarithmic plot between the two variables it is possible to estimate the creep rate with completely saturated ground, which would be an ominous 0.6 m per day. Thankfully, drainage through the slip is good, as beneath lie highly unstable mudstones; but things could change. The team has also monitored local rainfall, and precipitation underwent a marked increase from 2000 onward (1.64 m per year) compared with the average since 1930 of 1.3 m per year. Fortunately, spring and summer rains are quickly returned to the atmosphere by vigorous evapotranspiration by the lush grasses and ferns on the slipped mass. The greatest creep takes place in the winter when vegetation has died back.  Mam Tor is indeed highly instructive, but at present poses no great hazard, yet it might become less predictable should annual rainfall increase. It is unlikely to attain the awesome pace of that in Calabria, southern Itaaly on 15 February 2010 at Maierato near Vibo Valentia (view www.stumbleupon.com/su/9LP6H7/sorisomail.com/email/42722/ja-viram-desmoronar-uma-montanha.html).