Category Archives: Sedimentology and stratigraphy

A major Precambrian impact in Scotland

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The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol

Simplified geological map of NW Scotland (credit: British Geological Survey)

One such mystery lies close to the base of the vast pile of reddish Torridonian sandstones, the Stac Fada Member of the Stoer Group. Beneath it is a common-or-garden basal breccia full of debris from the underlying Lewisian Complex, then red sandstones and siltstones deposited by a braided river system. The Stac Fada Member is a mere 10 m thick, but stretches more than 50 km along the regional NNE-SSW  strike. It comprises greenish to pink sandstones with abundant green, glassy shards and clasts, previously thought to be volcanic in origin, together with what were initially regarded as volcanic spherules – the results of explosive reaction of magma when entering groundwater or shallow ponds. Until 2002, that was how ideas stood. More detailed sedimentological and geochemical examination found quartz grains with multiple lamellae evidencing intense shock, anomalously high platinum-group metal concentrations and chromium isotopes that were not of this world. Indeed, the clasts and the ensemble as a whole look very similar to the ‘suevites’ around the 15 Ma old Ries Impact crater in Germany. The bed is the product of mass ejection from an impact, a designation that has attracted great attention. In 2015 geophysicists suggested that the impact crater itself may coincide with an isolated gravity low about 50 km to the east. A team of 8 geoscientists from the Universities of Oxford and Exeter, UK, have recently reported their findings and ideas from work over the last decade. (Amor, K, et al. 2019. The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry. Journal of the Geological Society, online; DOI: 10.1144/jgs2018-093).

The age of the Stac Fada member is around 1200 Ma, determined by Ar-Ar dating of K-feldspar formed by sedimentary processes. Geochemistry of Lewisian gneiss clasts compared with in situ basement rocks, magnetic data from the matrix of the deposit, and evidence of compressional forces restricted to it suggest that the debris emanated from a site to the WNW of the midpoint of the member’s outcrop. Rather than being a deposit from a distant source, carried in an ejecta curtain, the Stac Fada material is more akin to that transported by a volcanic pyroclastic flow. That is, a dense, incandescent debris cloud moving near to the surface under gravity from the crater as ejected material collapsed back to the surface. On less definite grounds, the authors suggest that a crater some 13 to 14 km across penetrating about 3 km into the crust may have been involved.

Together with evidence that I described in Impact debris in Britain (Magmatism February 2018) and Britain’s own impact  (Planetary Science November 2002) it seems that Britain has directly witnessed three impact events. But none of them left a tangible crater.

The effect of surface processes on tectonics

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The Proterozoic Eon of the Precambrian is subdivided into the Palaeo-, Meso- and Neoproterozoic Eras that are, respectively, 900, 600 and 450 Ma long. The degree to which geoscientists are sufficiently interested in rocks within such time spans is roughly proportional to the number of publications whose title includes their name. Searching the ISI Web of Knowledge using this parameter yields 2000, 840 and 2700 hits in the last two complete decades, that is 2.2, 1.4 and 6.0 hits per million years, respectively. Clearly there is less interest in the early part of the Proterozoic. Perhaps that is due to there being smaller areas over which they are exposed, or maybe simply because what those rocks show is inherently less interesting than those of the Neoproterozoic. The Neoproterozoic is stuffed with fascinating topics: the appearance of large-bodied life forms; three Snowball Earth episodes; and a great deal of tectonic activity, including the Pan-African orogeny. The time that precedes it isn’t so gripping: it is widely known as the ‘boring billion’ – coined by the late Martin Brazier – from about 1.75 to 0.75 Ga. The Palaeoproterozoic draws attention by encompassing the ‘Great Oxygenation Event’ around 2.4 Ga, the massive deposition of banded iron formations up to 1.8 Ga, its own Snowball Earth, emergence of the eukaryotes and several orogenies. The Mesoproterozoic witnesses one orogeny, the formation of a supercontinent (Rodinia) and even has its own petroleum potential (93 billion barrels in place in Australia’s Beetaloo Basin. So it does have its high points, but not a lot. Although data are more scanty than for the Phanerozoic Eon, during the Mesoproterozoic the Earth’s magnetic field was much steadier than in later times. That suggests that motions in the core were in a ‘steady state’, and possibly in the mantle as well. The latter is borne out by the lower pace of tectonics in the Mesoproterozoic.

For decades geologists have pondered on ‘orogenic cycles’ and whether they are roughly equally spaced in time. The ‘boring billion’ refutes any such regularity. Stephan Sobolev and Michael Brown of the universities of Potsdam in Germany, and Maryland, USA, have investigates an hypothesis that may account for the long-term irregularity in tectonic processes (Sobolev, S.V. & Brown, M. 2019. Surface erosion events controlled the evolution of plate tectonics on Earth. Nature, v. 570, p. 52-57; DOI: 10.1038/s41586-019-1258-4). This stems from a suggestion in the late 1980’s that, once they begin to be subducted, unconsolidated sediments have a lubricating effect. If so, in the long term, the rate of accumulation of sediments at continental margins has a lot to do with the pace of tectonics. And that leads back to the rate of continental erosion. The two authors use a proxy for the global rate of subduction based on the variation over time of the cumulative length of mountain belts that show paired high- and low-pressure zones of metamorphism. They chart variations in continental erosion from its geochemical effects on ocean water, recorded by strontium isotopes in limestones, and by changes in the hafnium and oxygen isotopes of detrital zircons through time. Three time intervals show increases in Sr and O isotope parameters while that for Hf decreases. These indicators of greater continental erosion coincide with evidence for increased tectonic activity around the end of the Archaean Eon (centred on 2.5 Ga), in the early Palaeoproterozoic (2.2 Ga) and the early Neoproterozoic (0.75 Ga). The latter two bracket episodes of global glaciation that would certainly have shifted eroded material towards continental margins. Sobolev and Brown make a case for each representing episodes of increased lubrication. Lying between the last two tectonic paroxysms, the ‘boring billion’ delivered little sediment from the continents so any subduction was frictionally slowed.

I have little doubt that this view will have its detractors, not the least because the Earth continually generates heat as a result of its internal radioactivity. Plate tectonics is the main means whereby that heat emerges at the surface and radiates to space, thereby balancing heat production. Another issue is that mountain building elevates Earth’s surface, which provides the gravitational potential to drive products of erosion oceanwards. But it increases frictional resistance

Related article: Behr, W. 2019. Earth’s evolution explored. Nature, v. 570, p. 38-39; DOI: 10.1038/d41586-019-01711-8

Plants first to succumb to the end-Permian event

We have become accustomed to thinking that up to 90% of organisms were snuffed out by the catastrophe at the Permian-Triassic boundary 252 Ma ago. Those are the figures for marine organisms, whose record in sediments is the most complete. It has also been estimated to have lasted a mere 60 ka, and the recovery in the Early Triassic to have taken as long as 10 Ma. There are hints of three separate pulses of extinction related to: initial gas emission from the Siberian Traps; coal fires; and release of methane from sea-floor gas hydrates at the peak of global warming. Various terrestrial sequences record the collapse of dense woodlands, so that the Early Triassic is devoid of coals that are widespread in the preceding Late Permian. A new detailed study of terrestrial sediments in the Sydney Basin of eastern Australia reveals something new (Fielding, C.R. and 10 others 2019. Age and pattern of the southern high-latitude continental end-Permian extinction constrained by multiproxy analysis. Nature Communications, v. 10, online publications: DOI: 10.1038/s41467-018-07934-z).

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The distinctive, tongue-like form of Glossopteris leaves that dominate the coal-bearing Permian strata of the southern coninents. Their occurrence in South America, Africa, India, Australia, New Zealand, and Antarctica prompted Alfred Wegener to suggest that these modern continents had been united in Pangaea by Permian times: a key to continental drift. (Credit: Getty Images)

Christopher Fielding or the University of Nebraska-Lincoln and colleagues focused on pollens, geochemistry and detailed dating of the sedimentary succession across the P-Tr boundary exposed on the New South Wales coast. The stratigraphy is intricately documented by a 1 km deep well core that penetrates a more or less unbroken fluviatile and deltaic sequence that contains eleven beds of volcanic ash. The igneous layers are key to calibrating age throughout the sequence (259.10 ± 0.17 to 247.87 ± 0.11 Ma using zircon U-Pb methods). The pollens change abruptly from those of a Permian flora, dominated by tongue-like glossopterid plants, to a different association that includes conifers. The change coincides with a geochemical ‘spike’ in the abundance of nickel and a brief change in the degree of alteration of detrital fledspars to clay minerals. The first implicates the delivery of massive amounts of nickel to the atmosphere, probably by the eruption of the Siberian Traps , which contain major economic nickel deposits. The second feature suggests a brief period of warmer and more humid climatic conditions. A third geochemical change is the onset of oscillations in the abundance of 13C that are thought to record major changes in plant life across the planet. These features would have been an easily predicted association with the 252 Ma mass extinction were it not for the fact that the radiometric dating places them about 400 thousand years before the well-known changes in global animal life. Detailed dating of the Siberian Traps links the collapse of Glossopteris and coal formation to the earliest extrusion of flood basalts, which suggests that the animal extinctions were driven by cumulative effects of the later outpourings

Related article: Chris Fielding comments on the paper at Nature Research/Ecology and Evolution

Read more on Palaeobiology and Stratigraphy

Calibrating 14C dating

Radiocarbon dating is the most popular tool for assessing the ages of archaeological remains and producing climatic time series, as in lake- and sea-floor cores, provided that organic material can be recovered. Its precision has steadily improved, especially with the development of accelerator mass spectrometry, although it is still limited to the last 50 thousand years or so because of the short half-life of 14C (about 5,730 years,). The problem with dating based on radioactive 14C is its accuracy; i.e. does it always give a true date. This stems from the way in which 14C is produced – by cosmic rays interacting with nitrogen in the atmosphere. Cosmic irradiation varies with time and, consequently, so does the proportion of 14C in the atmosphere. It is the isotope’s proportion in atmospheric CO2 gas at any one time in the past, which is converted by photosynthesis to dateable organic materials, that determines the proportion remaining in a sample after decay through the time since the organism died and became fossilised. Various approaches have been used to allow for variations in 14C production, such as calibration to the time preserved in ancient timber by tree rings which can be independently radiocarbon dated. But that depends on timber from many different species of tree from different climatic zones, and that is affected by fractionation between the various isotopes of carbon in CO2, which varies between species of plant. But there is a better means of calibration.

The carbonate speleothem that forms stalactites and stalagmites by steady precipitation from rainwater, sometimes to produce visible layering, not only locks in 14C dissolved from the atmosphere by rainwater but also environmental radioactive isotopes of uranium and thorium. So, layers in speleothem may be dated by both methods for the period of time over which a stalagmite, for instance, has grown. This seems an ideal means of calibration, although there are snags; one being that the proportion of carbon in carbonates is dominated by that from ancient limestone that has been dissolved by slightly acid rainwater, which dilutes the amount of 14C in samples with so called ‘dead carbon’. Stalagmites in the Hulu Cave near Nanjing in China have particularly low dead-carbon fractions and have been used for the best calibrations so far, going back the current limit for radiocarbon dating of 54 ka (Cheng, H. and 14 others 2018. Atmospheric 14C/12C during the last glacial period from Hulku Cave. Science, v. 362, p. 1293-1297; DOI: 10.1126/science.aau0747). Precision steadily falls off with age because of the progressive reduction to very low amounts of 14C in the samples. Nevertheless, this study resolves fine detail not only of cosmic ray variation, but also of pulses of carbon dioxide release from the oceans which would also affect the availability of 14C for incorporation in organic materials because deep ocean water contains ‘old’ CO2.

Read more on Stratigraphy

The great Cambrian unconformity

My first field trip from the Geology Department at the University of Birmingham in autumn 1964 was located within hooter distance of the giant British Leyland car plant at Longbridge. It involved a rubbish-filled linear quarry behind a row of shops on the main road through south Birmingham. Not very prepossessing but it clearly exposed a white quartzite, which we were told was a beach deposit laid down by a massive marine transgression at the start of the Cambrian. An hour later we were shown an equally grim exposure of weathered volcanic rocks in the Lickey Hills; they were a sort of purple brown, and said to be Precambrian in age. Not an excellent beginning to a career, but from time to time other Cambrian quartzites sitting unconformably on Precambrian rocks entered our field curriculum: in the West Midlands, Welsh Borders and much further afield in NW Scotland, as it transpired on what had been two separate continental masses of Avalonia and Laurentia. This had possibly been a global marine transgression.

In North America, then the Laurentian continent, what John Wesley Powell dubbed the Great Unconformity in the Grand Canyon has as its counterpart to the Lickey Quartzite the thrillingly named Tonto Group of the Lower Cambrian resting on the Vishnu Schists that are more than a billion years older. Part of the Sauk Sequence, the Tonto Group is, sadly, not accompanied by the Lone Ranger Group, but the Cambrian marine transgression crops out across the continent. In fact it was a phenomenon common to all the modern continents. Global sea level rose relative to the freeboard of the continents then existing. A recent study has established the timing for the Great Unconformity in the Grand Canyon by dating detrital zircons above and below the unconformity (Karlstrom, K, et al. 2018. Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience, v. 11, p. 438-443; doi:10.1038/s41561-018-0131-7). Rather than starting at the outset of the Cambria at 542 Ma, the marine transgression was a protracted affair that began around 527 Ma with flooding reaching a maximum at the end of the Cambrian.

Extensive flooding of the continents at the end of the Cambrian (credit: Ron Blakey , Colorado Plateau Geosystems)

It seems most likely that the associated global rise in sea level relative to the continents was a response to the break-up of the Rodinia supercontinent by considerable sea-floor spreading. The young ocean floor, having yet to cool to an equilibrium temperature, would have had reduced density so that the average depth of the ocean basins decreased, thereby flooding the continents. The creation of vast shallow seas across the continents has been suggested to have been a major factor in the explosive evolution of Cambrian shelly faunas, partly by expanding the range of ecological niches and partly due to increased release of calcium ions to to seawater as a result of chemical weathering.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Late Palaeozoic glacial features in Chad

The longest and most extreme glacial epoch during the Phanerozoic took place between 360 and 260 Ma ago, when it dominated the Carboniferous and Permian sedimentary sequences across the planet. On continents that lay athwart the Equator during these times, sedimentation was characterised by cycles between shallow marine and terrestrial conditions. These are epitomised by the recurring ‘Coal-Measure’ cyclothem of, from bottom to top: open-sea limestone; near-shore marine mudstone; riverine sandstone; coal formed in swamps. This sequence represents a rapid rise in sea level as ice sheets melted, sustained during an interglacial episode and then falling sea level as ice once again accumulated on land to culminate in a glacial maximum when coal formed in coastal mires. During the Late Palaeozoic Era a single supercontinent extended from pole to pole. The break-up of Pangaea was charted by Alfred Wegener in 1912, partly by his using glacial deposits and ice-gouged striations on the southern continents. With the present widely separated configuration of major landmasses glacial sediments and the directions of inferred ice movements could only be reconciled by reassembling Africa, India, South America, Antarctica and Australia in the form of a single, congruent southern continent that he called Gondwanaland. In Wegener’s reconstruction the glacial features massed together on Gondwanaland with the striations radiating outwards from what would then have been the centre of a huge ice cap.

There are many localities on the present southern continents where such striations can be seen on the surface of peneplains etched into older rocks that underlie Carboniferous to Permian tillites, but later erosion has removed the continuity of the original glacial landscape. There are, however, some parts of central Africa where it is preserved. By using the high-resolution satellite images (with pixels as small as 1 m square) that are mosaiced together in Google Earth, Daniel Paul Le Heron of Royal Holloway, University of London has revealed a series of 1 to 12 km wide sinuous belts in a 6000 km2 area of eastern Chad that are superimposed unconformably on pre-Carboniferous strata (Le Heron, D.P. 2018. An exhumed Paleozoic glacial landscape in Chad. Geology, v.46(1), p. 91-94; doi:10.1130/G39510.1). They comprise irregular tracts of sandstone to the south of a major Carboniferous sedimentary basin. Zooming in to them (try using 17.5° N 22.25°E as a search term in Google Earth) reveals surfaces dominated by wavy, roughly parallel lines. Le Heron interprets these as mega-scale glacial lineations, formed by ice flow across underlying soft Carboniferous glacial sediments as seen in modern glacial till landforms in Canada. In places they rest unconformably on older rocks, sometimes standing above the level of the sandstone plateaux as relics of what may have been nunataks. There are even signs of elliptical drumlins.

An oblique Google Earth view looking to the south-east shows mega-scale glacial lineations from a glacial flow way in eastern Chad. The lower-right quadrant shows the unconformity atop older bedded strata that are dipping to the west. Click on the image to see a full resolution view. (Credit: Google Earth)

Glacial tillites and glaciofluvial sediments of Late Palaeozoic age are common across the Sahara and in the Sahelian belt, but in areas as remote as those in eastern Chad. So a systematic survey using the resolving power of Google Earth may well yield yet more examples. It is tedious work in such vast areas, unless, of course, one bears in mind Alfred Wegener, the founder of the hypothesis of continental drift and ‘Big’ Earth Science as a whole, who would have been gleeful at the opportunity.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Banded iron formations (BIFs) reviewed

This image shows a 2.1 billion years old rock ...

2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)

During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent. But commercial reserves are confined mainly to sedimentary sequences dating from about 3 to 2 billion years ago. They are not the only commercial iron formations, but dominate supplies from estimated reserves of around 105 billion tons. From a non-commercial standpoint they are among the most revealing kinds of sediment as regards the Earth system and its evolution. All scientific aspects of BIFs and similar Fe-rich sediments are reviewed in a recent volume of Earth Science Reviews. (Konhauser, K.O. and 12 others 2017. Iron formations: a global record of Neoarchaean to Palaeoproterozoic environmental history. Earth Science Reviews, v. 172, p. 140-177; doi: 10.1016/j.earscirev.2017.06.012).

The chemical, mineral and isotopic compositions of BIFs form a detailed repository of the changing composition of seawater during a crucial period for the evolution of Earth and life – the transition from an anoxic surface environment to one in which water and air contained a persistent proportion of oxygen, known as the Great Oxidation Event (GOE). Paradoxically, BIFs are highly oxidized rocks, the bulk of which formed when other rocks show evidence for vanishingly small amounts of oxygen in the surface environment. The paradox began to be resolved when it was realized that ocean-ridge basaltic volcanism and sea-floor hydrothermal activity would have released vast amounts of soluble, reduced iron-2 into anoxic seawater, in the upper parts of which the first photosynthetic organisms evolved. Evidence for the presence of such cyanobacteria first appears around 3.5 billion years ago, in the form of carbonates whose structure suggests they accumulated from growth of microbial mats. Oxygen generated by photosynthesis in iron-rich water immediately acts to oxidize soluble iron-2 to iron-3 to yield highly insoluble iron oxides and hydroxides and thus deposits of BIFs. While oceans were iron-rich, formation of ironstones consumed ecologically available oxygen completely.

Other biological processes seem to have been involved in ironstone formation, such as photosynthesis by other bacteria that used dissolved iron-2 instead of water as a reductant for CO2, to release iron-3 instead of oxygen. That would immediately combine with OH­ ions in water to precipitate iron hydroxides. Konhauser and colleagues cogently piece together the complex links in chemistry and biology that emerged in the mid- to late Archaean to form a linkage between carbon- and iron cycles, which themselves influenced the evolution of other, less abundant elements in seawater from top to bottom. The GOE is at the centre. The direct evidence for it lies in the sudden appearance of ancient red soils at about 2.4 billion years, along with the disappearance of grains of sulfides and uranium oxides – both readily oxidized to soluble products – from riverine sandstones, which signifies significant oxygen in the atmosphere. Yet chemical changes in Precambrian marine sediments perhaps indicate that oxygen began to rise in ocean water as early as 3 billion years ago. That suggests that for half a billion years biogenic and abiogenic processes in the oceans were scavenging oxygen as fast as it could be produced so that only tiny amounts, if any, escaped into the atmosphere. Among other possible factors, oceanic methane emissions from methanogen bacteria may have consumed any atmospheric oxygen – today methane lasts only for about 9 years before reaction with oxygen forms CO2. If and when methanogens declined free oxygen would have been more likely to survive in the atmosphere.

The theme running through the review is that of changing and linked interactions between life and the inorganic world, mantle, lithosphere, hydrosphere and atmosphere that involved all available chemical elements. The dominant chemical process, as it is today, was the equilibrium between oxidation and reduction – the loss and gain of electrons among possible chemical reactions and in metabolic processes. Ironstones were formed more commonly between 3 to 2 Ga than at any time before or since, and form a substantial part of that periods sedimentary record. Their net product and that of the protracted organic-inorganic balancing act – oxygenation of the hydrosphere and atmosphere – opened the way for eukaryote organisms, their reproduction by way of the splitting and recombination of nuclear DNA and their evolutionary diversification into the animal and plant life that we know today and of which we are a part. It is possible that even a subtly different set of global processes and interactions set in motion during early evolution of a planet apparently like Earth may have led to different and even unimaginable biological outcomes in later times. The optimism of exobiologists should be tempered by this detailed review.

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.

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).