Category: Sedimentology and stratigraphy

The ups and downs of sea level through geological time constitute a ‘beat’ to which sedimentation responds by inundation of and withdrawal from the land.  The ‘big picture’ is one forced by changes in the volume of the ocean basins as plate tectonics waxed and waned, together with long periods when land ice locked sea water away.  A closer focus has stemmed from the changes of oxygen isotopes in benthonic (bottom-dwelling) plankton remains that record details about advances and retreats of polar land ice, most spectacularly from the record of the Pliocene and Pleistocene. These ongoing, higher frequency fluctuations in sea level formed the key to verifying Milankovich’s theory of astronomical controls over climate.  There are also fluctuations of the order of thousands to tens of thousand years that seem terrestrial in origin, such as the Bond and Dansgaard-Oeschger cycles.  Shorter cycles still have had various causes ascribe to them.  For inhabitants of near-sea level cities and flat ocean islands, rising sea level is a realistic concern. It is rising just now at about 3 mm per year (in the 1950s the annual rise was half that), mainly because surface sea water is expanding as a result of anthropogenic warning and polar ice is melting.

November 2005 was valuable for geoscientists interested in fluctuating sea level, and most sedimentologists are in that category because the stratigraphic record is primarily governed by this eustatic (world-wide) rhythm. The earliest information on long-term sea-level change came from studies of continental transgressions and regressions that are preserved as onlap and offlap features between strata. That approach was greatly aided by detailed seismic sections gathered by petroleum explorationists, in which such features show up a great deal more readily than they do in limited exposures on land. The results of many different methods of charting eustasy are wonderfully summarised by a large team of US geoscientists (Miller, K.G. and 9 others 2005. The Phanerozoic record of global sea-level change. Science, v. 310, p. 1293-1298). Their review covers the last 543 Ma, and reveals several novel aspects. It has been known for over 30 years that the higher frequency sea-level changes correlate well with oxygen isotope records, because of the preferential evaporation of water that contains light ¹⁷O. When evaporated ocean water ends up in long-term storage as land ice, the proportion of heavier ¹⁸O rises in seawater and in carbonates extracted from it by organisms. The broad view also shows a sea-level – d¹⁸O correlation though, and that probably reflects expansion and contraction of the volume of ocean water as mean global temperature rose and fell on the scale of tens of million years. That the Cretaceous was the period during which sea level reached an all time high during the Phanerozoic has been well known for over a century, and manifested itself in the production of giant ‘carbonate factories’ on shallow shelves of inundated continental lowlands. Famously, that was ascribed to vast production of new oceanic crust, both by accelerated sea-floor spreading and outpouring of huge submarine flood basalts, such as the Ontong Java Plateau of the west Pacific floor. Putting together all the pertinent data, however, suggests that Cretaceous tectonics was not nearly as vigorous as once suspected.

Unsurprisingly, sea level studies are ‘hot’ and researchers have a better than even chance of getting publications into press in the most august of journals, and a readership to boot. There is a great deal of information on past and current sea level fluctuations, and a great deal of thought has gone into acquiring data.  Dotted around the world’s coast lines are tide gauges of the most exquisite precision; so precise in fact that the outermost ripples of the Boxing Day tsunamis were detected at the antipode of the earthquake that caused them. Whether or not watching these gauges continuously is a fulfilling task, the long-term records have revealed a surprise (Church, J,A, et al. 2005. Significant decadal-scale impact of volcanic eruptions on sea level and ocean heat content. Nature, v. 438, p. 74-77). Since 1960, global sea level has been up and down like a yo-yo, deviating by ± 2-3 mm from the longer-term mean at a rate measured in decades.  This correlates well with five major volcanic eruptions during the last 45 years, such as El Chichon and Pinatubo. The first effect is a rapid fall (6 mm in a year, after Pinatubo erupted), probably resulting from global cooling and reduced rainfall caused by sulfate aerosols injected into the stratosphere, followed by slow recovery.  It seems odd that volcanoes have a bigger effect on sea level than overall global warming, yet other records show their profound global effects. The fall in sea level must be dominated by shrinkage of cooled surface water. Interesting, and quite possibly a boost for those in denial over global warming. However, my main concern, living at 250 m above mean sea level, is that my bathroom cistern is always overflowing because of a water level rise of 1 mm in a matter of a few minutes.

The last remaining division of geological time that Giovanni Arduino erected in the mid- to late 18^th century, has been under threat for some time (see EPN of September 2004).  For over seven years, the ‘Time Lords’ of the International Commission of Stratigraphy have striven to resolve, at least for a while, al the fundamental divisions of stratigraphic nomenclature.  To the horror of researchers concerned with the last 2 million years or so, publication of the new time scale in 2004 seemed to have allowed the Neogene to swallow the Quaternary Period whole.  Muttering broke into a storm of angry e-mails demanding its restoration.

The reason behind the annoyance is simple.  The Quaternary is unique for two reasons: it includes the Great Ice Age, and it is the time of humanity – the first stone tools appear in the geological record between 2.4 and 2.6 Ma ago.  But those who demand the resurrection of the old name are not entirely in agreement among themselves, particularly about when it started.  The problem arose from the manner in which systematisation of both relative and radiometric time evolved.  Arduino recognised four divisions only, Primary, Secondary, Tertiary and Quaternary based on decreasing compactness and complexity of rocks that he had seen in Italy.  The Quaternary was defined as unconsolidated material that sat upon the other three.  As fossils became the main tools of establishing relative time and wide correlation, Primary and Secondary were soon dropped.  But Tertiary and Quaternary remained as broad divisions until the late 20^th century.  Tertiary strata became divided into 5 lesser palaeontological divisions, and Quaternary into two: Pleistocene and Holocene.  Radiometric dating demonstrated the brevity of the Tertiary compared with major stratigraphic divisions further back in time, so it was designated as a Period, subdivided into 5 epochs.  Tertiary itself then became elevated to Era status as the Cenozoic, despite its short time span, and its first three and last two epochs were bracketed by two new periods: Palaeogene and Neogene.  Development of geosciences was clearly marginalizing the Quaternary Period to which many devotees cling tenaciously.

The furore burst at the 32^nd International Geological Congress in Florence in August 2004, and the ICS was duly chastened and apologetic.  It set up a task force to reunite the warring forces, or at least to draw plans for a truce. The task force voted in early June 2005 to retain the name Quaternary and to set its beginning at 2.6 Ma, thereby defining it as both the Great Ice Age and that of humankind.  Ironically, 2.6 Ma also marks the start of the Late Pliocene, defined by a Global Boundary Stratotype Sections and Point (the midpoint of sapropelic Nicola Bed (“A5”), Monte San Nicola, Gela, Sicily, Italy). You see, there has to be somewhere that you can visit and ‘put your finger on the proper boundary’.  This particular GSSP is defined as a stage in the fluctuation of oxygen isotopes in deep-sea sediments, at the start of the Matuyama geomagnetic reversal, and just below the points of extinction of two echinoid species…..  Incidentally, the ICS is by far the largest of the bodies within the International Union of Geological Sciences, the ‘UN’ of the geoscience community.  Acquiring the prestige of a GSSP ranks with many countries’ geoscientists at least as high as hosting an Olympic Games. Italy hosts 9 of the 22 Cenozoic GSSPs (5 are not yet placed), so clearly Arduino’s influence has been long lasting in some respects.  Several features of the New Timescale as a whole may confuse far into the future (should it stand the test of time).  The Stage names, learned by generations of stratigraphers, often through cunning mnemonics, are mainly taken from places or regions.  Most of the GSSPs at their bases are somewhere else (browse http://www.stratigraphy.org/).

Source: Giles, J. 2005.  Geologists call time on dating dispute.  Nature, v. 435, p. 865.

Getting the stratigraphic column properly calibrated from relative to absolute time is all the rage these days (New benchmarks for geological time in EPN June 2004).  On the recent stratigraphic chart published in late 2003 by the International Commission on Stratigraphy, the Devonian-Carboniferous boundary has a “golden spike” global standard section and point (GSSP) dated at 359.2 ± 2.5 Ma.  Already, that is disputed because of new radiometric dating from an “auxiliary” global stratotype section (Trapp, E. et al. 2004.  Numerical calibration of the Devonian-Carboniferous boundary: Two new U-Pb isotope dilution-thermal ionization mass spectrometry single-zircon ages from Hasselbachtal (Sauerland, Germany).  Geology, v. 32, p. 857-860).  As well as holding the record for length of any publication title yet covered by EPN, the paper contains some intriguing points.  That a carefully determined age for the strata at Hasselbachtal has been possible is thanks to about six, centimetre-thick ash beds in richly fossiliferous sediments just above the faunally determined boundary.  Twenty-three single-zircon ages from the two ashes just above the accepted faunal boundary give ages of 360.5 ± 0.8 and 360.2 ± 0.7 Ma.  Now, to you and I and many less pernickety geochronologists, that spells out the well-known phrase or saying, “within error”, as indeed is that of the GSSP.  And, for a convoluted reason based on plotting an age from another tuff with these ages against the palaeontological data, the age presented for D-C itself is 360.7 ± 0.7 Ma.  This may be a better age than that of the GSSP.  But, so what?  The D-C boundary is not associated with any family-crushing catastrophe like the P-T or K-T boundaries, nor even that within the Late Devonian itself.  Are “they” going to move the GSSP from its present location in southern France, ratified in 1990, along with the vast pyramid of precious and intricately carved crystal, which no doubts marks its spot?  An altogether more serious threat to the established order is the stealthy attempt to abolish the last remnant of the great stratigraphic divisions inspired by Giovanni Arduino’s work in the 18^th century; the Quaternary is besieged!  One of my spies, not unconnected with this episode of our own emergence on the planet, attended a stormy meeting at the 32^nd International Geological Congress in Florence in August 2004, which seemed likely to expunge the Quaternary from the minds of all future geologists.  He gleefully reported that a mighty rearguard action had put off that evil day, at least for a while.  Sadly, the writing is already on the great IUGS/ICS stratigraphic wall chart – its is no longer there!  The last relic in officialdom is in the latest definitive publication (Gradstein, F.M. et al. 2004.  A new geologic time scale with special reference to Precambrian and Neogene.  Episodes, v. 27, p. 83-100).  On page 86, at the very top of the table conferring status on GSSPs, it is written “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”.  So there you have it; the issue is getting things into proportion.

A record of the Palaeoproterozoic lunar cycle

One of the many natural processes that produce rhythmic sediments is the ebb and flow of the tide, twice a day and with an amplitude that peaks and falls twice each lunar month (today a 28-day cycle) to produce spring (new and full moon) and neap tides (the two half moons).  Tidal rythmites consist of thin laminae whose thicknesses vary regularly for many cycles.  Their occurrence dates back to 3.2 Ga, and along with other sedimentary structures formed by tidal action, such as “herring-bone” cross stratification formed by reversals in tidal currents, prove the presence of the Moon in orbit around the Earth.  Fine rythmites can be analysed to work out the length of the lunar month in the past, and help refine ideas on the evolution of the Earth-Moon system.  Rajat Mazumder of Asutosh College, Kolkata, India has analysed the earliest known tidal rythmites from the Palaeoproterozoic of NE India (Mazumder, R. 2004.  Implications of lunar orbital periodicity from the Chaibasa tidal rhythmite (India) of late Paleoproterozoic age.  Geology, v. 32, p. 841-844).  His work shows that between 2.1 to 1.6 Ga the lunar month was 32-days long.  Remarkably, the record in these sediments is as detailed as found in modern ones from estuarine silts.  As well as rhythms, they record occasional perturbations due to storms.  Using the changes in the lunar month during the last 450 Ma erroneously suggests that the system emerged from a period around 1.5 to 2.0 Ga following a major collision – that of course is ruled out by a total lack of evidence of such a catastrophe.  The new datum suggests instead a steady decrease in the lunar month, that corresponds with the Moon’s gradually receding from the Earth.  Energy apparently lost by tidal action is conserved by an increase in the angular momentum of the Earth-Moon system, and that forces the Moon ever further from us – its orbital velocity increases.

In the December 2003 issue of EPN, I mentioned a programme aimed at sorting out the calibration of the stratigraphic column to an absolute or radiometric timescale (Recalibrating the stratigraphic column).  The other side of this task is deciding on where to place the “golden spikes”, otherwise known as global standard stratotype-section and points (GSSPs).  They are locations where the best exposures of world-wide events can be found.  The first, defining the disappearance of graptolites at the Silurian-Devonian boundary (no-one knows why that happened), was placed in 1972 near the wonderfully named town of Klonk in the Czech Republic.  GSSPs are essential in defining events, no matter if their ages change as dating methods and results advance.  Until 1999 the problem was that only 15 of the 91 stage boundaries of the Phanerozoic had been defined agreeably by such “golden spikes”.  That year the International Union of Geological Sciences (IUGS) spurred a crash programme of GSSP definition, but there have been political as well as geological disagreements.  The most important “spike” is at the Permian-Triassic boundary – the end of the Palaeozoic Era, and the time of the largest ever mass extinction – and there have been heated discussions over whether to have it in Iran, Kashmir or China.  Zhejiang Province in China won, and it now has a 6 metre high monument at the boundary!  This and Klonk should be on every geologists’ future tourist itineraries.  There are now 50 stage-boundary GSSPs, and together with a revision of currently accepted dates, the revised stratigraphic column can be downloaded as a (rather large) PDF from http://www.stratigraphy.org/.  All is not so well with Precambrian time, for the obvious reason that it contains no tangible fossils, and it is still arbitrarily split by round-number dates.  But there is some hope for a similar system of “golden spikes” that use probably global events such as glacial epochs, and perhaps shifts in the d¹³C of carbonate sediments that should record global changes in ocean composition.

Source:  Whitfield, J. 2004.  Time lords.  Nature, v. 429, p. 124-125

The Earth’s magnetic field is changing all the time, in its intensity, direction and, now and again, its polarity.  It’s the last that proved the key to sea-floor spreading and plate tectonics, though ocean-floor magnetic “stripes”, and which has become a key stratigraphic tool for correlation and approximate dating.  Along with palaeomagnetic pole determinations, that are vital to continental reconstructions, the whole field still remains largely empirical.  Although widely agreed to be connected to changes in motions in the core, exactly what happens during reversals of geomagnetic polarity remains enigmatic, despite 40 years having passed since they were first recognised.  There is no doubt that they are quick events, but to judge their pace and what happens to field strength and direction during a “flip” requires high quality data that is well-calibrated to time.  Most early work focussed on magnetisation in igneous rocks, where the signal is strong.  Minerals such as igneous magnetite acquire a permanent magnetisation once they cool below their Curie temperature, but since accurate radiometric dating gives an age, not a range of ages, it might seem that all that is possible with lavas and intrusions is to obtain a series of points.  Fine for a time series, but useless for the details of reversals.  However, by modelling the cooling history of an igneous body, it is possible to calibrate different levels within it to time.  With careful choice, it has proved possible to find flows in flood basalt sequences that include the brief progress of a reversal.  The results seem very odd, the pole itself seeming to migrate rather than jump from north to south, and gross changes in intensity over a short time.  Improved instrumentation allows a shift from strongly magnetic basalts, to sediments that preserve much weaker signals.  These are due to the alignment with the field of magnetic grains as they slowly settle.  Marine sediment cores can now be magnetically characterised – the principle behind magneto-stratigraphy.  For geomagnetists the most recent reversals have proved especially instructive, when the sedimentary record is analysed (Clement, B.M. 2004.  Dependence of the duration of geomagnetic polarity reversals on site latitude.  Nature, v. 428, p. 637-640).  On average, the last four “flips” took about 7000 years to complete by migration of the magnetic poles.  Yet there is an oddity in the detail.  Sites at low latitude show significantly shorter periods (down to 2000 years) than those at high latitude (as much as 10000 years).  Clement’s explanation for the difference is the persistence of the lower intensity non-dipole field, which might suggest different core processes or a single process with several components that evolve at different rates.

Sulphur cycling and sea-level change

Sulphur is one the major prerequisites for life after carbon, hydrogen, oxygen and nitrogen, and the bulk of it is supplied by sulphate ions.  After chlorine, the SO₄^2- ion is the most abundant anion in the oceans.  Not very much is added annually by river drainage, and although anaerobic bacteria remove some by reducing it to hydrogen sulphide so that it is removed from solution as a result of precipitation of insoluble iron sulphide, the sulphur cycle has been considered to be the most sluggish of all the major geochemical rhythms at the Earth’s surface.  Because iron sulphide is highly reactive in oxidising conditions, should marine sulphide-rich sediments become exposed at the surface their oxidation to sulphuric acid and iron hydroxide would rapidly add sulphate ions to seawater.  Studies of sulphur isotopes seem to suggest that this is not very important however.  Through sulphate-sulphide reducing bacteria, sulphur is implicated in the carbon cycle because of its sheer abundance, not so much from the encouragement and burial of the bacteria, but because they induce the highly reducing conditions that help a larger proportion of dead organic matter to remain unoxidised and become buried.  In a roundabout way, sulphur has a role in climate controls.  In fact, two roles.  Sulphate ions affect the alkalinity of seawater, and on that depends the oceans’ ability to dissolve CO­₂ from the atmosphere.  The big question is, “Does the sulphate content of seawater ever change fast enough to have some impact on climate in the short term?”.  Most studies of the S-cycle have focused on sulphur isotopes, so a new twist is bound to be interesting.  Alexandra Turchyn and Daniel Schrag of Harvard University looked instead at the isotopes of oxygen within barium sulphate contained within seafloor sediments since the Late Miocene (about 10 Ma ago) (Turchyn, A.V. & Schrag, D.P. 2004.  Oxygen isotope constraints on the sulfur cycle over the past 10 million years.  Science, v. 303, p. 2004-2007).  Up until 6 Ma, the barite d¹⁸O (measured against mean ocean water values) stayed constant at about 9.5‰, and then rose to around 12.5‰ by 3.5 Ma.  Through the Late Pliocene and Pleistocene, the period of repeated glacial-interglacial cycles, it fell dramatically to its present level of 7.9‰.  In that later period, the average d¹⁶O of deep water foraminifera rose significantly.  The decline in “heavy” oxygen in marine sulphates can be linked to increased exposure of pyrite-bearing marine sediments during glacial sea-level falls when “light” atmospheric oxygen enters the sulphate ions that are produced.  Modelling suggests sulphate ions in seawater increased by as much as 20% during the Great Ice Age.  Whether that had an influence on the oceans’ take-up of carbon dioxide from the atmosphere in the last 3 Ma is yet to be evaluated.  However, Turchyn and Schrag’s detection of a short term shift in the sulphur cycle, and attributing it to falling sea level, may allow a new approach to global sea-level change, which has mainly been deduced from features in stratigraphy.

See also:  Derry, L.A. & Murray, R.W. 2004.  Continental margins and the sulfur cycle.  Science, v. 303, p. 1981-1982

Finds of Late Cretaceous dinosaur remains and substantial coal deposits at near-polar latitudes in both hemispheres seemed to confirm that the end of the Mesozoic experienced hothouse conditions.  Even so, both are very odd because of the darkness of polar winters; how could plants photosynthesise and supposedly cold-blooded reptiles stay warm?  To add to these oddities, it has now been suggested that periodically there were Antarctic ice sheets substantial enough to draw down sea-level (Miller, K.G. et al. 2004. Upper Cretaceous sequences and sea-level history, New Jersey Coastal Plain.  Geological Society of America Bulletin, v. 116, p. 368-393).  The possibility comes from a detailed stratigraphic and palaeontological analysis of Late Cretaceous sequences on and off the eastern seaboard of the US.  There are 11 to 14 sequences that show shallowing-upwards changes in the near-shore environment, somewhat similar to the cyclicity of Carboniferous times.  Calibrating the section with strontium isotopes and fossil changes suggests that sea-level ups and downs greater than 25 metres occurred swiftly (much less than 1 Ma).  This is considerably faster than changes due to variations in the volume of the ocean basins that result from fluctuations in sea-floor spreading rates, but if localised in eastern North America might have resulted from local tectonics, such as episodic deepening related to extensional tectonics.  The surprise is that the changes correlate well with those in western Europe and on the stable Russian platform, pointing to global, eustatic changes in sea level.  There is some correlation with oxygen-isotope records from foraminifera, so there is a strong possibility of a glacial cause.  The degree of fluctuation matches the effect on sea level of ice volumes of the order of 10⁶ to 10⁷ km³.  This is considerably more than the volume of the present Greenland ice cap, but on Antarctica it would have occupied only a small part of the surface.  There is another alternative; that eustatic changes are not well understood and there is a bias because of the Pleistocene correspondence between them and changes in continental Arctic ice sheets.  The amplitudes of the three different records do not match well, although their timing does.

Biology and iron minerals

The principal colouring agents in rocks, especially those of sedimentary origin, are iron minerals, foremost of which are oxides and hydroxides (e.g. hematite and goethite).  It doesn’t take much of either in a sedimentary grain coating to impart the vivid colour variations seen in some sedimentary formations.  It is easy to suppose that such veneers formed while the sediments were at the surface in an unconsolidated state, but there is much evidence that at least some, if not all, formed in buried sediments saturated with groundwater.  But the problem is getting the iron into pore spaces as well as precipitating its oxides and hydroxides.  Iron in its divalent state (Fe-2) is soluble, but exists only under reducing conditions, so it does not easily enter surface waters that supply groundwater.  In its trivalent state (Fe-3) iron is highly insoluble, and that is how it occurs in oxides and hydroxides.  Yet groundwater tends to lose its oxidising potential because dissolved oxygen is consumed by aerobic bacteria, and oxidation is required to convert soluble Fe-2 to insoluble Fe-3, so that hematite and goethite skins can form around sediment grains.  A clue to the precipitation method comes from a study of slime-encrusted surfaces in old mine workings (Chan, C.S. et al. 2004.  Microbial polysaccharides template assembly of nanocrystal fibers.  Science, v. 303, p. 1656-1658).  Although oriented towards the possibility of bacteria creating materials useful in nanotechnology, this non-geological paper might ring a few bells.  It shows how filaments (of the order of a few nm) that make up bacterial slime are associated with similarly thin and long filaments of one of the precursors to goethite.  The bacteria involved use the oxidation (electron removal) of Fe-2 to Fe-3 as a source of metabolic energy.  They colonise highly reducing waters, so there is a ready source of dissolved Fe-2 for them to exploit, especially in old mine workings, but also in groundwater cut off from the air  There is a snag for the bacteria, because Fe-3 is highly insoluble and could easily snuff out processes in the cells and cause their death.  So in evolving this chemo-autotrophic metabolism they would also have to evolve a means of disposing of its by-product.  The filaments are chains of polysaccharides grown outside the cell wall that act as templates for the precipitation of Fe-3 minerals.  The techniques used to show this include very-high resolution electron microscopy.  It would be interesting to see if very high resolution images of iron-stained mineral grains reveal  relics of these intricate structures.  Less powerful methods have already shown tiny spheres of magnetite in sediments above petroleum fields that formed biogenically through another metabolic process.

How old is the Dalradian?

Half the Scottish Highlands, from the Great Glen to the Highland Boundary Fault, and their equivalent in Ireland, is occupied by a convoluted orogen that is dominated by an almost exclusively sedimentary sequence of Neoproterozoic age – the Dalradian Supergroup.  Its importance is historical, for this is where many of the fundamental tenets used in unravelling complex terrains were developed and tested.  This still goes on, building on over a century of research in an easily accessible area.  Briefly, the Dalradian orogen evolved from a series of extensional basins, in a shelf area, that imposed considerable variations in thickness of the Dalradian sequence.  Protracted deformation in the Late Cambrian to Early Ordovician developed the structural complexity of the orogen, partly controlled by the original variations in sedimentary thicknesses.  We know the youngest age of the Dalradian, because its upper parts contain Cambrian fossils, estimated to be about 509 Ma old.  The earliest age for sedimentation has so far only been guessed, and must be younger than the 800 Ma of migmatites on which its lowest members rest .  The problem is that only one series of dateable volcanic rocks occur in the pile, and they are towards the top (601 Ma old).  At most the whole sedimentary sequence spans 300 Ma, and that in itself is most peculiar.  Most geologists have assumed continuous sedimentation under a great range of environments, but only because they have never found evidence for erosion in the sequence; hardly surprising from the complexity, and not-so-good exposure.  Yet nowhere on the planet is there a sedimentary sequence spanning such a time period that does not contain several unconformities; things have never been that quiet for so long.  Probably the only feasible way to get a handle on the duration of the Dalradian sedimentation is by matching geochemistry of the numerous marine limestones in the sequence with the global record for the Neoproterozoic, that is by seeking signs of the secular variations in the composition of seawater during that Era.  Scottish geoscientists have applied that technique, using 47 samples of Dalradian limestones (Thomas, C.W. et al. 2004. ⁸⁷Sr/⁸⁶Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales.  Journal of the Geological Society of London, v. 161, p. 229-242).  Unsurprisingly, the results do not show a smooth curve that can be matched directly with various estimates of secular change in seawater strontium isotopes; the limestones occur haphazardly through the sequence.  The effort is not helped by considerable differences between global seawater strontium isotope curves compiled by several authors, so Thomas and colleagues’ interpretation is limited.  Yes, the Dalradian is younger than 800 Ma, but by how much cannot be said with confidence.  Its base is an unconformity that represents erosion of an older 800 Ma orogen, and how long that took is anyone’s guess.  The lowest Dalradian limestone falls in a strontium-isotope span that matches that for about 700 Ma, which fits with recent evidence for continued thermal activity in the underlying complex at 730 Ma.  Around the middle of the Dalradian deposition there occurs one of the most spectacular examples of possible glaciogenic rocks in the Precambrian, the Port Askaig Formation, which has been widely regarded as a product of one of the “Snowball” Earth events of the late Precambrian.  If the Dalradian deposition did begin around 700 Ma, then this unit cannot have formed in the earliest and best documented Sturtian glacial episode at 730 Ma, but perhaps in the younger Marinoan-Varangerian one (640 to 560 Ma).  The paper concludes with the time-honoured phrase “…await the application of alternative dating techniques”.   It may be a long wait, and perhaps the most important unresolved aspects of the Dalradian are whether or not its 30 km maximum thickness represents several distinct depositional basins, and if it contains numerous breaks in deposition.

Managers of isotopic dating labs may be rubbing their hands with glee.  The absolute dating on which proper correlation of events in Earth’s history depends, is “officially” a dog’s breakfast.  This is partly due to the slowly improving precision and accuracy of radiometric dating applied to ever smaller samples, but also to the high cost of getting the age data.  Many important geological boundaries were dated long ago by methods that would not pass muster today, yet those earlier dates are all that palaeontologists, sedimentologists and palaeoclimate specialists have to go on when estimating rates and correlating events.  Many important stratigraphic and more complex igneous and metamorphic events remain undated, no matter how much their discoverers plead with the isotopic community.  The trend has been to eschew mundane dating in favour of isotopic approaches to petrogenesis, now that really precise data can be had.  Sponsored by a number of geochronological labs in the United States, a meeting in Washington, DC during October 2003 set in motion means to redress the balance.  A proposal is being developed to obtain US$6 million to found three new labs devoted to dating in the USA (plus $2 to 3 million annual operating costs).  The idea is to link similar labs internationally, which would use the same methodology and perform multiple analyses to set standardised dates for important events.  Attendees from other countries will be busy formulating their own proposals at around the same levels, you can be sure.

Source:  Clarke, T. 2003.  Geologists seek to put an end to blind dates.  Nature, v. 425, p. 550-551.

At the end of the Miocene (from 6 to 5.3 Ma) the connection between the Atlantic Ocean and the Mediterranean Sea was blocked somehow.  Over 700 thousand years evaporation deposited a thick layer of salt that now lies beneath much of the Mediterranean basin.  This is known as the Messinian salt crisis.  Equally dramatic, the straits reopened suddenly to allow seawater to flood back in the early Pliocene, in a hydrological catastrophe.  How the Mediterranean basin became cut off has been ascribed to a 60 m sea-level fall, crustal shortening associated with nappe formation in the Betic Cordillera of Spain and the Atlas mountains, or by some kind of tectonic uplift.  Timing of the Messinian crisis rules out the first two options, but sedimentation in the former “gateway”, a shallow seaway through what is now southern Spain, shows evidence of rapid shallowing that would have resulted from regional uplift.  The question is, what drove this regional upwarping?  A team from the GEOMAR Research Centre for Marine Geosciences in Kiel, Germany has discovered evidence from the changing geochemistry of Miocene to Pliocene volcanic rocks in the western part of the Mediterranean (Duggen, S. et al, 2003.  Deep roots of the Messinian salinity crisis.  Nature, v. 422, p. 602-606).  Send Duggan and his co-workers found that the lavas underwent a geochemical shift  from affinities with subduction-zone processes to those typical of intra-plate magmatism around 6.3 Ma ago, volcanism largely ending about 4.8 Ma.  The ending in the late Miocene of eastward subduction of Tethyan sea floor beneath the Mediterranean, which had initiated volcanism around 12 Ma, led to foundering of part of the lithosphere and uprise of asthenosphere.  This is marked by a change from high-silica, early magmas to alkaline, more basaltic varieties during the period of the Messinian salinity crisis.  Uplift resulting from this delamination would have pushed the formed connections between the Atlantic and the Mediterranean as much as 800 m above sea level.  Duggan et al. Suggest that the axis of uplift gradually migrated westwards, so that by the end of the Messinian crisis the area now centred on the Straits of Gibraltar would have been bulged up.  Massive gravitational sliding from this edge of the continental lithosphere into the Atlantic may then have opened the narrow passage through which Atlantic water once again flooded.

Biomineralization is a growing topic that ranges from life’s influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer’s syndrome.  The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago.  Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones.  Many a child, including my young self, consigned a cuddly toy to “petrifying” springs, such as Mother Shipton’s Well in Knaresborough, Yorkshire.  Few retrieved them, which is why there aren’t many rock-like Teddies around..  Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk.  It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002.  Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere.  Geology, v. 30, p. 599-602).

Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite.  It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning).  It seems that Euglena, a common “animalcule” in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent.  It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats.  Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites.  Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism.  The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena.  Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria.  Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma.  If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.

Much of the world’s oil resources formed by maturation and migration of hydrocarbons from organic-rich, marine mudrocks, which seem to have formed episodically during Earth history.  A widely accepted view is that such source rocks’ content of organic matter fell to the ocean floor as the remains of tiny organism.  That they were not oxidized by bacterial action seems to suggest that the periods when source rocks accumulated were characterized by low oxygen levels in bottom waters.  Each major source rock has been linked to such ocean-anoxia events, and to periods when deep-ocean circulation effectively stopped, so cutting off oxygen supplies to deep levels.  However, studies of modern deposition of organic matter in marine sediments at continental margins reveals that discrete particles of organic matter are far outweighed by biological molecules that coat the surfaces of minerals, particularly those of clay minerals.  The amount of organic carbon in a modern sediment depends largely on its content of clay minerals derived from intense chemical weathering of continental rocks.  Such coatings are protected from normal processes of decay, so that the adsorbed organic carbon compounds can be buried, more or less intact

It should be possible to check whether ancient source rocks are similar to modern carbon-rich sediments by seeking a strong correlation between clay content and organic content – mudrocks also contain fine silt particles made of non absorbent quartz. It seems that in at least one Cretaceous source rock in the US Mid-West such a correlation is clear (Kennedy, M.J., Pevear, D.R. and Hill, R.J. 2002.  Mineral surface control of organic carbon in black shale.  Science, v. 295, p. 657-660).  This suggests that oil-shale deposition is as much related to the intensity of continental weathering of silicates as it is to ocean-water chemistry.  Since clays, especially the sponge-like smectites, adsorb organic molecules from solution in seawater, they draw on a vast pool of material, so that enhanced biological productivity need not be linked to oil-shale formation either.  The fact that most organic material in such rocks is structureless kerogen, rather than identifiable particles, also supports this alternative hypothesis.

Both petroleum geologists and palaeoclimatologists have assumed a source rock – ocean anoxia connection in both exploration strategies and assessment of past climate shifts.  So Martin Kennedy et al.’s painstaking findings are sure to cause a major stir.  However, what cannot be avoided is that increased chemical weathering of the continents is likely to accompany globally warm conditions, and they in turn sponsor growth in planktonic productivity.  Likewise, global warmth does not favour the formation of dense, cold and therefore oxygenated sea-surface water, which sinks to aerate deep oceans when the planet is cool.

Measuring erosion rates.

So many landscapes show evidence of changes in the rate of erosion, such as terraces, waterfalls and signs of changing rates of sediment deposition, that a means of accurately measuring rates opens up an important new phase in geomorphological research.  Precise dating of modern surfaces is not possible using stratigraphic or radio-carbon methods, and this has hidden much of landform history.  Once a surface is exhumed, it becomes exposed to cosmic ray bombardment.  These particles travel at near-relativistic speeds, and so have sufficient energy to transmute common element nuclei to unstable isotopes.  The longer the exposure of a surface material, the more radioactive it becomes, albeit very weakly so  Since erosion and sedimentary processes move and quickly bury particles dislodged from a surface, material has a finite time during which it can be irradiated.  The particles themselves carry the isotopic signature of their surface residence time, the slower erosion is the more radioactive are particles derived from the surface.

Cosmogenic dating uses sedimentary grains from sands deposited in a drainage basin, particularly those of quartz that are common and stable.  Oxygen and silicon in silica can become ¹⁰Be and ²⁶Al when struck by cosmic rays.  Although sampling is fraught with pitfalls, essentially it amounts to scooping up a handful of sand that represents the past erosion of the entire catchment above a sample point.  Measuring the minute concentrations of new isotopes  costs of the order of $1000 per sample, using a high-energy accelerator mass spectrometer.  Since dozens of samples provide sufficient data for meaningful interpretation, this is not a method that will spread widely to places that come anywhere near fully reflecting the intricacies of erosional shifts over the large age range that cosmogenic dating can address.  Nonetheless, its early results are astonishing.  Work in Idaho suggests that through the period of the last glacial maximum into the early Holocene the average rate of erosion was 17 times faster than it is at present.  That possibly signifies either continual high erosion, that has petered out, or, more likely, that erosion has had episodic, catastrophic pulses.  As might be expected, anthropogenic disturbance of the surface enhances erosion rates, but a cosmogenic study of river sediments in Sri Lanka indicates that 200 years of intensive farming in rugged highland areas have resulted in a 20- to 100-fold acceleration.  Most awkward of all, another study of long-term erosion in California’s Sierra Nevada showed no relation between weathering and erosion rates and climate change.  Geochemists contributing to the debate over climate controls by weathering take note.  It seems that the primary control of erosion rates in western California was purely tectonic, which could tally with the notion that newly rising mountains have a major influence over sequestering of CO₂ by silicate breakdown. 

The obvious next step is blending cosmogenic sediment dating with that of crustal exhumation from Ar-Ar and U-Th/He dating of cooling due to uplift and erosion.

Source:  Greensfelder, L. 2002.  Subtleties of sand reveal how mountains crumble.  Science, v. 295, p. 256-258.