The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19th century. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).
The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered. Continue reading “Geochemical background to the Ediacaran explosion”→
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.
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. Continue reading “The effect of surface processes on tectonics”→
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).
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
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.