Category Archives: Sedimentology and stratigraphy

Gas hydrates: a warning from the past

Detailed acoustic imaging above the Troll gas field in the northern North Sea off western Norway has revealed  tens of thousands of elliptical pits on the seabed. At around 10 to 20 per square kilometre over an area of about 15,000 km2 there are probably between 150 to 300 thousand of them. They range between 10 to 100 m across and are about 6 m deep on average, although some are as deep as 20 m. They are pretty much randomly distributed but show alignment roughly parallel to regional N-S sea-floor currents. Many of the world’s continental shelves display such pockmark fields, but the Troll example is among the most extensive. Almost certainly the pockmarks formed by seepage of gas or water to the surface. However, detailed observations suggest they are inactive structures with no sign of bubbles or fluid seepage. Yet the pits cut though glacial diamictites deposited by the most recent Norwegian Channel Ice Stream through which icebergs once ploughed and which is overlain by thin Holocene marine sediments. One possibility is that they record gas loss from the Troll field, another being destabilisation of shallow gas hydrate deposits.

Troll pockmarks

Parts of the Troll pockmark field off Norway. A: density of pockmarks in an area of 169 square km. B: details of a cluster of pockmarks. (Credit: Adriano Mazzini, Centre for Earth Evolution and Dynamics (CEED) University of Oslo)

Norwegian geoscientists have studied part of the field in considerable detail, analysing carbonate-rich blocks and foraminifera in the pits (Mazzini, A. and 8 others 2017. A climatic trigger for the giant Troll pockmark field in the northern North Sea. Earth and Planetary Science Letters, v. 464, p. 24-34; http://dx.doi.org/10.1016/j.epsl.2017.02.014). The carbonates show very negative δ13C values that suggest the carbon in them came from methane, which could indicate either of the two possible means of formation. However, U-Th dating of the carbonates and radiocarbon ages of forams in the marine sediment infill suggest that they formed at around 10 ka ago; 1500 years after the end of the Younger Dryas cold episode and the beginning of the Holocene interglacial. Most likely they represent destabilisation of a once-extensive, shallow layer of methane hydrates in the underlying sediments, conditions during the Younger Dryas having been well within the stability field of gas hydrates. Sporadic leaks from the deeper Troll gas field hosted by Jurassic sandstones is unlikely to have created such a uniform distribution of gas-release pockmarks. Adriano Mazzini and colleagues conclude that rapid early Holocene warming led to sea-floor temperatures and pressures outside the stability field of gas hydrates. There are few signs that hydrates linger in the area, explaining the present inactivity of the pockmarks – all the methane and CO2 escaped before 10 ka.

Gas hydrates are thought to be present beneath shallow seas today, wherever sea-floor sediments have a significant organic carbon content and within the pressure-temperature window of stability of these strange ice-like materials. Mazzini et al.’s analysis of the Troll pockmark field clearly has profound implications for the possible behaviour of gas hydrates at a time of global climatic warming. As well as their destabilisation adding to methane release from onshore peat deposits currently locked by permafrost and a surge in global warming, there is an even more catastrophic possibility. The whole of the seaboard of the southern North Sea was swept by a huge tsunami about 8000 years ago, which possibly wiped out Mesolithic human occupancy of lowland Britain, the former land mass of Doggerland, and the ‘Low Countries’ of western Europe. This was created by a massive submarine landslide – the Storegga Slide just to the north of the Troll field – which may have been triggered by destabilisation of submarine gas hydrates during early Holocene warming of the oceans.

Advertisements

Salt and Earth’s atmosphere

It is widely known that glacial ice contains a record of Earth’s changing atmospheric composition in the form of bubbles trapped when the ice formed. That is fine for investigations going back about a million years, in particular those that deal with past climate change. Obviously going back to the composition of air tens or hundreds of million years ago cannot use such a handy, direct source of data, but has relied on a range of indirect proxies. These include the number of pores or stomata on fossil plant leaves for CO2, variations in sulfur isotopes for oxygen content and so on. Variation over time of the atmosphere’s content of oxygen has vexed geoscientists a great deal, partly because it has probably been tied to biological evolution: forming by some kind of oxygenic photosynthesis and being essential for the rise to dominance of eukaryotic animals such as ourselves. Its presence or absence also has had a large bearing on weathering and the associated dissolution or precipitation of a variety of elements, predominantly iron. Despite progressively more clever proxies to indicate the presence of oxygen, and intricate geochemical theory through which its former concentration can be modelled, the lack of an opportunity to calibrate any of the models has been a source of deep frustration and acrimony among researchers.

Yet as is often said, there are more ways of getting rid of cats than drowning them in butter. The search has been on for materials that trap air in much the same way as does ice, and one popular, if elusive target has been the bubbles in crystals of evaporite minerals. The trouble is that most halite deposits formed by precipitation of NaCl from highly concentrated brines in evaporating lakes or restricted marine inlets. As a result the bubbles contain liquids that do a grand job of preserving aqueous geochemistry but leave a lot of doubt as regards the provenance of gases trapped within them. For that to be a sample of air rather than gases once dissolved in trapped liquid, the salt needs to have crystallized above the water surface. That may be possible if salt forms from brines so dense that crystals are able to float, or perhaps where minerals such as gypsum form as soil moisture is drawn upwards by capillary action to form ‘desert roses’. A multinational team, led by Nigel Blamey of Brock University in Canada, has published results from Neoproterozoic halite whose chevron-like crystals suggest subaerial formation (Blamey, N.J.F. and 7 others, 2016. Paradigm shift in determining Neoproterozoic atmospheric oxygen. Geology, v. 44, p. 651-654). Multiple analyses of five halite samples from an ~815 Ma-old horizon in a drill core from the Neoproterozoic Canning Basin of Western Australia contained about 11% by volume of oxygen, compared with 25% from Cretaceous salt from China, 20% of late-Miocene age from Italy, and 19 to 22% from samples modern salt of the same type.

Salar de Atacama salt flat in the Chilean puna

Evaporite salts in the Salar de Atacama Chile (credit: Wikipedia)

Although the Neoproterozoic result is only about half that present in modern air, it contradicts results that stem from proxy approaches, which suggest a significant rise in atmospheric oxygenation from 2 to about 18% during the younger Cryogenian and Ediacaran Periods of the Neoproterozoic, when marine animal life made explosive developments at the time of repeated Snowball Earth events. Whether or not this approach can be extended back to the Great Oxygenation Event at around 2.3 Ga ago and before depends on finding evaporite minerals that fit stringent criteria for having formed at the surface: older deposits are known even from the Archaean.

A ‘proper’ stratigraphic view of the ‘Anthropocene’

Readers may recall my occasional rants over the years against the growing bandwagoning for an  ‘Anthropocene‘ epoch at the top of the stratigraphic column. I , for one, was delighted to find in the latest issue of GSA Today a more sober assessment of the campaign by two stratigraphers who are well placed to have a real say in whether or not the ‘Anthropocene’ is acceptable, one serving on the International Commission on Stratigraphy, the other on the North American Commission on Stratigraphic Nomenclature (Finney, S.C. & Edwards, L.E. 2016. The “Anthropocene” epoch: Scientific decision or political statement? GSA Today, v. 26 (3–4).

Some cunning radiometric dating

At the end of the 1970’s I was invited by the Deputy Director of the Geological Survey of India (Southern Region) to participate in the Great Postal Symposium on the Cuddapah Basin: a sort of harbinger of the Internet and Skype, but using snail-mail. Feeling pretty honoured and most intrigued I accepted; not that I knew the first thing about the subject. A regular stream of foolscap mimeographed contributions kept me nipping out of my office to check my pigeon hole for about 6 months. I learned a lot, but felt unable to comment. Four years on I was taken across the Cuddapahs by my first research student – a budding moto-cross driver with a morbid fear of bullock carts – en route from the Archaean low-grade greenstone-granite terrains of Karnataka for a peek at the fabled charnockites near Chennai (then Madras). A bit of a round-about route but spurred by my memories of the Great Postal Symposium. Sadly, the detour was marred for me by a severe case of sciatica brought on by manic driving, the state of the trans-Cuddapah highway and a misplaced gamma-globulin shot to ward off several varieties of hepatitis: I mainly blamed the nurse who demanded that I drop my drawers and bravely take the huge needle in a buttock – they do these things more humanely these days. Anyhow, apart from seeing many dusty villages build of slates perfect enough to make a full-size snooker table, my mind was elsewhere and I have long regretted that.

Landsat image mosaic showing part of the Cuddapah Basin.

Landsat image mosaic showing part of the Cuddapah Basin.

Hosting possibly the world’s only diamondiferous Precambrian conglomerate, the Cuddapah Basin contains a 5 km thickness of diverse sedimentary strata, but no tangible fossils. It rests unconformably on the Archaean greenstone-granite terrain of the Dharwar Craton and so is Proterozoic in age; an Eon that spans 2 billion years. The middle of the lowest sedimentary formations (the Papaghni and Chitravati Groups) contains volcanic rocks dated at ~1.9 Ga; another group is cut by a ~1.5 Ga granite, and hitherto the youngest dateable event is the emplacement of 1.1 Ga kimberlites that sourced the diamonds in the conglomerate. Until recently the stratigraphy has been known in some detail, but how to partition it in Proterozoic time is barely conceivable with just three dates in the middle parts that span 800 Ma. All that can be said about the base of the Cuddapah sediments is that they are younger than the 3.1 to 2.6 Ga Archaean rocks beneath. Since the uppermost beds are truncated by a huge thrust system that shoved deep crustal granulites over them their minimum age is equally vague.

Structurally, the Basin began to form on a stable continent underpinned by the Dharwar Craton, but when that collided with Enderbyland in Antarctica, as part of the accretion of the Gondwana supercontinent, sedimentation may have been in an entirely different setting. Indeed, some of the sediments have been carried over the undisturbed part of the basin by a major thrust system. To explore both sedimentary and tectonic evolution Australian, Indian and Canadian geoscientists combined to sample and radiometrically date the entire pile (Collins, A.S. and 13 others 2015. Detrital mineral age, radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin, India. Gondwana Research, v. 28, p. 1294-1309). By precisely dating detrital micas and zircons from the sediments the team was able to check the source region of sedimentary grains as well as to establish a maximum age for each major stratigraphic unit. This helped establish a 3-part sedimentary and tectonic history. The earliest sediments came from the cratonic area to the west, but there are signs that collisional orogeny between 1590 and 1659 Ma produced a new sedimentary source in metamorphic rocks forming to the east. A return to westward provenance marked the youngest sedimentary setting. This enabled the team to suggest a dual evolution of the Basin, first as an extensional rift opening at the east of what is now the Dharwar craton followed by collisional orogeny that transformed the setting to that of a foreland basin, analogous to the Molasse basin in front of the Alps during Cenozoic times, ending with tectonic inversion when extension changed to compression and thrusting.

But to what extent did the work improve the age subdivision of the Cuddapah Basin? Apparently very little, which may be down to a problem with dating detrital minerals. If magmatic and metamorphic evolution was continuous in the areas from which sediments moved, then the youngest grain is a good guide to the maximum age of the sediment being analysed. The more strata are analysed in this way the better the detail of sedimentary timing. But two tectonic terrains are unlikely to produce zircons time and time again during a period approaching a billion years. The data indicate only 3 or 4 episodes of ‘zirconogenesis’ in the sedimentary hinterlands, between about 900 to 1940 Ma. Apart from helping correlate sedimentary formations that were previously deemed stratigraphically different – which did help in tectonically unravelling this complex major feature – several hundred isotopic analyses of zircons and micas have give much the same timing as was known already in more precise terms from stratigraphy assisted by a few dozen conventional radiometric dates.

Fascinating glacial feature found on Mars

Many of the vast wastes of northern Canada and Scandinavia that were ground to a paste by ice sheets during the last glacial cycle show peculiar features that buck the general glacial striation of the Shield rocks. They are round-topped ridges that wind apparently aimlessly across the tundra. In what is now a gigantic morass, the ridges form well-drained migration routes for caribou and became favourite hunting spots for the native hunter gatherers: in Canada they are dotted with crude simulations of the human form, or inugoks, that the Innuit erected to corral game to killing grounds. Where eroded they prove to be made of sand and gravel, which has proved an economic resource in some areas lacking in building aggregate, good but small examples being found in the Scottish Midland Valley that have served development of Glasgow and Edinburgh. They were given the Gaelic name eiscir meaning ‘ridge of gravel’ (now esker) from a few examples in Ireland.

Eskers form from glacial meltwater that makes its way from surface chasms known as moulins to the very bottom of an ice sheet where water flows much in the manner of a river, except in tubes rather than channels. Where the ice base is more or less flat the tubes meander as do normal sluggish rivers, and like them the tubes deposit a proportion of the abundant sediment derived by melting glacial ice. Once the ice sheet melts and ablates away, the sediments lose the support of the tube walls and flop down to form the eponymous low ridges: the reverse of the sediment filled channels of streams that have either dried up or migrated. Eskers are one of the features that shout ‘glacial action’ with little room for prevarication.

The classic form of eskers in the Phlegra Montes  of Mars. (credit:  Figure 6 in Gallagher and Balme, 2015)

The classic form of eskers in the Phlegra Montes of Mars. (credit: Figure 6 in Gallagher and Balme, 2015)

Glacial terrains on Mars have been proposed for some odd looking surfaces, but other processes such as debris flows are equally attractive. To the astonishment of many, Martian eskers have now been spotted during systematic interpretation of the monumental archives of high-resolution orbital images of the planetary surface (Gallagher, C. & Balme, M. 2015. Eskers in a complete, wet-based glacial system in the Phlegra Montes region, Mars. Earth and Planetary Science Letters, v. 431, p. 96-109). The discovery is in a suspected glacial terrain that exhibits signs of something viscous having flowed on low ground around higher topographic features, bombardment stratigraphy suggests a remarkable young age for the terrain or about 150 Ma ago: the Amazonian. Ice and its effects are not too strange to suggest for Mars which today is pretty much frigid, except for a few suggestions of active flow of small watery streams. Eskers demand meltwater in abundance, and Gallagher and Balme attribute some of the other features in the Phlegra Montes to wet conditions. However, the eskers are a one-off, so far as they know. Consequently, rather than appealing to some climatic warm up to explain the evidence for wetness, they suggest that the flowing water tubes resulted from melting deep in the ice as a result of locally high heat flow through the Martian crust, which is a lot more plausible.

A new explanation for banded iron formations (BIFs)

The main source for iron and steel has for more than half a century been Precambrian rock characterised by intricate interlayering of silica- and iron oxide-rich sediments known as banded iron formations or BIFs. They always appear in what were shallow-water parts of Precambrian sedimentary basins. Although much the same kind of material turns up in sequences from 3.8 to 0.6 Ga, by far the largest accumulations date from 2.6 to 1.8 Ga, epitomised by the vast BIFs of the Palaeoproterozoic Hamersley Basin in Western Australia. This peak of iron-ore deposition brackets the time (~2.4 Ga) when world-wide evidence suggests that the Earth’s atmosphere first acquired tangible amounts of free oxygen: the so-called ‘Great Oxidation Event’. Yet the preservation of such enormous amounts of oxidised iron compounds in BIFs is paradoxical for two reasons: the amount of freely available atmospheric oxygen at their acme was far lower than today; had the oceans contained much oxygen, dissolved ions of reduced Fe-2 would not have been able to pervade seawater as they had to for BIFs to have accumulated in shallow water. Iron-rich ocean water demands that its chemical state was highly reducing.

Oblique view of an open pit mine in banded iron formation at Mount Tom Price, Hamersley region Western Australia (Credit Google earth)

Oblique view of an open pit mine in banded iron formation at Mount Tom Price, Hamersley region Western Australia (Credit Google earth)

The paradox of highly oxidised sediments being deposited when oceans were highly reduced was resolved, or seemed to have been, in the late 20th century. It involved a hypothesis that reduced, Fe-rich water entered shallow, restricted basins where photosynthetic organisms – probably cyanobacteria – produced localised enrichments in dissolved oxygen so that the iron precipitated to form BIFs. Later work revealed oddities that seemed to suggest some direct role for the organisms themselves, a contradictory role for the co-dominant silica-rich cherty layers and even that another kind of bacteria that does not produce oxygen directly may have deposited oxidised iron minerals. Much of the research focussed on the Hamersley BIF deposits, and it comes as no surprise that another twist in the BIF saga has recently emerged from the same, enormous repository of evidence (Rasmussen, B. et al. 2015. Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age. Geology, v. 43, p. 303-306).

The cherty laminations have received a great deal less attention than the iron oxides. It turns out that they are heaving with minute particles of iron silicate. These are mainly the minerals stilpnomelane [K(Fe,Mg)8(Si, Al)12(O, OH)27] and greenalite [(Fe)2–3Si2O5(OH)4] that account for up to 10% of the chert. They suggest that ferruginous, silica-enriched seawater continually precipitated a mixture of iron silicate and silica, with cyclical increases in the amount of iron-silicate. Being such a tiny size the nanoparticles would have had a very high surface area relative to their mass and would therefore have been highly reactive. The authors suggest that the present mineralogy of BIFs, which includes iron carbonates and, in some cases, sulfides as well as oxides may have resulted from post-depositional mineral reactions. Much the same features occur in 3.46 Ga Archaean BIFs at Marble Bar in Western Australia that are almost a billion years older that the Hamersley deposits, suggesting that a direct biological role in BIF formation may not have been necessary.

More on BIFs and the Great Oxidation Event

Anthropocene: what (or who) is it for?

The made-up word chrononymy could be applied to the study of the names of geological divisions and their places on the International Stratigraphic Chart. Until 2008 that was something of a slow-burner, as careers go. It all began with Giovanni Arduino and Johann Gotlob Lehman in the mid- to late 18th century, during the informal historic episode known as the Enlightenment. To them we owe the first statements of stratigraphic principles and the beginning of stratigraphic divisions: rocks divided into the major segments of Primitive, Secondary, Tertiary and Quaternary (Arduino). Thus stratigraphy seeks to set up a fundamental scale or chart for expressing Earth’s history as revealed by rocks. The first two divisions bit the dust long ago; Tertiary is now an informal synonym for the Cenozoic Era; only Quaternary clings on as the embattled Period at the end of the Cenozoic.  All 11 Systems/Periods of the Phanerozoic, their 37 Series/Epochs and 85 Stages/Ages in the latest version of the International Stratigraphic Chart have been thrashed out since then, much being accomplished in the late 19th and early 20th centuries. Curiously, the world body responsible for sharpening up the definition of this system of ‘chrononymy’, the International Commission on Stratigraphy (ICS), seems not to have seen fit to record the history of stratigraphy: a great mystery. Without it geologists would be unable to converse with one another and the world at large.

Yet now an increasing number of scientists are seriously proposing a new entry at the 4th level of division after Eon, Era and Period: a new Epoch that acknowledges the huge global impact of human activity on atmosphere, hydrosphere, biosphere and even lithosphere. They want it to be called the Anthropocene, and for some its eventual acceptance ought to relegate the current Holocene Epoch, in which humans invented agriculture, a form of economic intercourse and exchange known as capital and all the trappings of modern industry, to the 5th division or Stage. Earth-pages has been muttering about the Anthropocene for the past decade, as charted in a number of the links above, so if you want to know which way its author is leaning and how he came to find the proposal an unnecessary irritation, have a look at them. Last week things became sufficiently serious for another comment. Simon Lewis and Mark Maslin of the Department of Geography at University College London have summarised the scientific grounds alleged to justify an Anthropocene Epoch and its strict definition in a Nature Perspective (Lewis, S.J. & Maslin, M.A. 2015. Defining the Anthropocene. Nature, v. 519, p. 171-180).-=, which is interestingly discussed in the same Issue by Richard Monastersky.

Lewis and Maslin present two dates that their arguments and accepted stratigraphic protocols suggest as candidates for the start of the Anthropocene: 1610 and 1964 CE, both of which relate to features that are expressed by geological records that should last indefinitely. The first is a decline and eventual recovery in the atmospheric CO2 level recorded in high-resolution Antarctic ice core records between 1570 and 1620 CE that can be ascribed to the decline in the population of the Americas’ native peoples from an estimated 60 to 6 million. This result of the impact of European first colonisation – disease, slaughter, enslavement and famine – reduced agriculture and fire use and saw the regeneration of 5 x 107 hectares of forest, which drew down CO2 globally. It also coincides with the coolest part of the Little Ice Age from 1594-1677 CE. They caution against the start of the Industrial Revolution as an alternative for a ‘Golden Spike’ since it was a diachronous event, beginning in Europe. Instead, they show that the second proposal for a start in 1964 has a good basis in the record of global anthropogenic effects on the Earth marked by the peak fallout of radioactive isotopes generated by atomic weapons tests during the Cold War, principally 14C with a 5730 year half life, together with others more long-lived. The year 1964 is also roughly when growth in all aspects of human activity really took off, which some dub in a slightly Tolkienesque manner the ‘Great Acceleration’. [There is a growing taste for this kind of hyperbole, e.g. the ‘Great Oxygenation Event’ around 2.4 Ga and the ‘Great Dying’ for the end-Permian mass extinction]. Yet they neglect to note that the geochronological origin point for times past has been defined as 1950 CE when nucleogenic 14C contaminated later materials as regards radiocarbon dating, which had just become feasible.   Lewis and Maslin conclude their Perspective as follows:

To a large extent the future of the only place where life is known to exist is being determined by the actions of humans. Yet, the power that humans wield is unlike any other force of nature, because it is reflexive and therefore can be used, withdrawn or modified. More widespread recognition that human actions are driving far-reaching changes to the life-supporting infrastructure of Earth may well have increasing philosophical, social, economic and political implications over the coming decades.

So the Anthropocene adds the future to the stratigraphic column, which seems more than slightly odd. As Richard Monastersky notes, it is in fact a political entity: part of some kind of agenda or manifesto; a sort of environmental agitprop from the ‘geos’. As if there were not dozens of rational reasons to change human impacts to haul society back from catastrophe, which many people outside the scientific community have good reason to see as  hot air on which there is never any concrete action by ‘the great and the good’. Monastersky also notes that the present Anthropocene record in naturally deposited geological materials accounts for less than a millimetre at the top of ocean-floor sediments. How long might the proposed Epoch last? If action to halt anthropogenic environmental change does eventually work, the Anthropocene will be  very short in historic terms let alone those which form the currency of geology. If it doesn’t, there will be nobody around able to document, let alone understand, the epochal events recorded in rocks. At its worst, for some alien, visiting planetary scientists, far in the future, an Anthropocene Epoch will almost certainly be far shorter than the 104 to 105 years represented by the hugely more important Palaeozoic-Mesozoic and Mesozoic-Cenozoic boundary sequences; but with no Wikipedia entry.

Not everybody gets a vote on these kinds of thing, such is the way that science is administered, but all is not lost. The final arbiter is the Executive Committee of the International Union of Geological Sciences (IUGS), but first the Anthropocene’s status as a new Epoch has to be approved by 60% of the ICS Subcommission on Quaternary Stratigraphy, if put to a vote. Then such a ‘supermajority’ would be needed from the chairs of all 16 of the ICS subcommissions that study Earth’s major time divisions. But first, the 37 members of the Subcommission on Quaternary Stratigraphy’s ‘Anthropocene’ working group have to decide whether or not to submit a proposal: things may drag on at an appropriately stratigraphic pace. Yet the real point is that the effect of human activity on Earth-system processes has been documented and discussed at length. I’ll give Marx the last word in this ‘The philosophers have only interpreted the world, in various ways. The point, however, is to change it’. A new stratigraphic Epoch doesn’t really seem to measure up to that…

January 2015 photo of the month

Angular unconformity on the coast of Portugal at Telheiro Beach (credit: Gabriela Bruno)

Angular unconformity at Telheiro Beach, Portugal (credit: Gabriela Bruno)

This image posted at Earth Science Picture of the Day would be hard to beat as the definitive angular unconformity. It shows Upper Carboniferous  marine metagreywackes folded during the Variscan orogeny overlain by Triassic redbeds. Structurally it is uncannily similar to Hutton‘s famous unconformity at Siccar Point on the coast of SE Scotland, although the tight folding there is Caledonian in age and the unconformable redbeds are Devonian in age.

Reconstructing the structure of ancient vegetation canopies

One of the central measures used to describe modern ecosystems is the ratio of foliage area to that of the ground surface – the leaf area index (LAI) – which expresses the openness of vegetation canopies. A high LAI helps to retain moisture in the soil, partly by shading and cooling the surface to reduce evaporation and partly by stopping surface soil from being battered to a concrete-like consistency by heavy rain, which reduces the amount of water that can infiltrate. It is possible to estimate LAI across today’s entire land area using satellite image data but a proxy for palaeoecological LAI has remained hard to find.

English: Creative Commons attribution "ph...

Hemispherical photograph used to calculate modern canopy cover. (credit: Wikipedia; photo by S.B. Weiss)

The outer coating of leaves in well-shaded (high LAI) areas tends to have protective or pavement cells that are larger and have more complicated shapes than does that of leaves in more open canopies. The framework of leaf cells is silica-based and made up of structures known as phytoliths whose morphologies vary in much the same way as the cells that they support. So theoretically it is possible to use fossil phytoliths in terrestrial sediments to estimate LAI variations through time in local canopies, but first the approach needs a means of calibration from living ecosystems. The vegetation of Central American Costa Rica varies through the entire range of possible LAI values, which leads to varying amounts of sunlight available to the leaves of cover plants. Measuring the area and the degree of shape-complexity of phytoliths in modern soils there shows that each is positively correlated with LAI.

Lowland Paca near Las Horquetas, Costa Rica. F...

A modern herbivorous mammal (lowland paca) from dense forest in Costa Rica. (Photo credit: Wikipedia)

Putting this approach to use in the Cenozoic terrestrial sediments of Patagonia, US and Argentinean palaeoecologists aimed to examine how the evolution of the teeth of herbivorous mammals – a major feature in their speciation – linked to changes in vegetation structure (Dunn, R.E. et al. 2015. Linked canopy, climate and faunal change in the Cenozoic of Patagonia. Science, v. 347, p. 258-261). Using phytoliths they were able to show that in the Eocene the area was covered by dense, closed forest canopies that gradually became more open towards the end of the Eocene to be replaced by open forest and shrubland habitats in the Oligocene and Miocene, with a brief period of regreening. It was during the period of more open vegetation that tooth structure underwent the most change. Chances are that the vegetation shifts began in response to the onset of Antarctic glaciation at the beginning of the Oligocene Epoch and related climate change at the northern margin of the Southern Ocean. Changes in the herbivore teeth may have been in response to the increasing amount of dust adhering to leaves as canopies became more open and soil increasingly dried out.

Age calibration of Mesozoic sedimentary sequences: can it be improved?

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.

Setting Sun at Whitby Abbey

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.

English: Hildoceras bifrons (Bruguière 1789) L...

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.

Enhanced by Zemanta