Did reefs exist in the Proterozoic?

Analysis of the growth pattern of stromatolites and relationships of their thick carbonate bodies with host rocks are used to substantiate the lack of reefs sensu stricto, i.e., buildups notably towering above the sea bottom, in the Proterozoic.     

Did plate tectonics shutdown in the Palaeoproterozoic? A view from the Siderian geologic record

The early Proterozoic Era between 2.45 and 2.2 Ga is well known for a distinct minima in juvenile magmatism and detrital zircon abundance, an intriguing observation given its coincidence with many fundamental changes in Earth processes. A recent hypothesis seeks to explain this Siderian ‘Quiet Interval’ as the result of a plate tectonic shutdown in which extended tectonic quiescence is due to widespread lithospheric stagnation in an episodic mantle overturn regime. The model suggests that this period characterizes a ‘pre-modern’ geodynamic style and has profound implications for many geodynamic processes.We use spatially-linked chronostratigraphic and paleomagnetic databases to assess the major predictions of the model and find six of its key predictions are not supported by current data. The quiet interval includes a greater extent of contractional orogenesis and a broader range of paleopressures than previously known and is not characterized by LP–HT metamorphism proposed to have been related to higher upper mantle temperatures from decreased upper mantle cooling. Glacial conditions do not appear to have been triggered by the coincidence of the onset of magmatic shutdown with the end of mass-independent sulphur isotope fractionation and oxygenation of the atmosphere, as the initial glacial episodes predate this time. The glacial record, moreover, requires four episodes of climatic amelioration during the proposed shutdown, for which a mechanism appears lacking. A purported gap in Large-Igneous-Province formation, related to decreased mantle vigour, is not apparent. Quiet interval magmatism includes juvenile, arc-type and TTG magmatism, supporting significant crustal additions on a number of cratons. The prediction of negligible plate velocities during shutdown is not borne out by the well-constrained Superior Province paleomagnetic record. We suggest that plate tectonics did not shut down but that the Siderian Quiet Interval represents overall diminished tectonic activity during peripheral orogenesis, as is known for other relatively quiet periods following supercontinent or supercraton amalgamation.     

Did the 7/9/1999 M5.9 Athens Earthquake Come with a Warning?

Prior to the 7/9/1999 M_S = 5.9 Athens earthquake, regional seismicity has exhibited a power-law increase, of the form = K+A(t_c - t)ⁿ, where is estimated using an expression log = cM + d and t_c is the time of the culminating event. Such changes appeared after the 17/8/1999 M7.4 Izmit event. We quantified the performance of the power law vs. the null hypothesis of constant seismic release rates, by defining the curvature C as the ratio of the power law fit RMS/linear fit RMS, so that the smaller C is, the better the power law behaviour. By mapping C, we have established a critical radius of 110 km and observed that the region of correlated accelerating seismic release extended from the N. Aegean, through Euboea and Attica to the SW Peloponnese. A few days prior to the Athens event, min(C) was centred at the epicentral area and numerical simulation yielded t_c = 1999.676 and predicted M_S = 5.77. Seismicity rates returned to normal (quasi-constant) after the Athens event. We interpret this effect as critical point behaviour, following remote excitation of a broad area by stress redistribution due to the Izmit event which, at Athens, has triggered `premature' failure of a fault nearing its load bearing capacity. If this is correct, we have documented a case of remote earthquake triggering by another earthquake, as well as insight into the mechanisms producing it. As a corollary, we note that a large event may beget another large event in its broader region of interaction, which may be preceded by characteristic precursory seismicity changes.     

Did the transition to plate tectonics cause Neoproterozoic Snowball Earth?

When Earth's tectonic style transitioned from stagnant lid (single plate) to the modern episode of plate tectonics is important but unresolved, and all lines of evidence should be considered, including the climate record. The transition should have disturbed the oceans and atmosphere by redistributing continents, increasing explosive arc volcanism, stimulating mantle plumes and disrupting climate equilibrium established by the previous balance of silicate‐weathering greenhouse gas feedbacks. Formation of subduction zones would redistribute mass sufficiently to cause true polar wander if the subducted slabs were added in the upper mantle at intermediate to high latitudes. The Neoproterozoic Snowball Earth climate crisis may reflect this transition. The transition to plate tectonics is compatible with nearly all proposed geodynamic and oceanographic triggers for Neoproterozoic Snowball Earth events, and could also have contributed to biological triggers. Only extraterrestrial triggers cannot be reconciled with the hypothesis that the Neoproterozoic climate crisis was caused by a prolonged (200–250 m.y.) transition to plate tectonics.     

Did the circum-Rodinia subduction trigger the Neoproterozoic rifting along the Congo–Kalahari Craton margin?

International Journal of Earth Sciences - Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the...     

West African provenance for Saxo-Thuringia (Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana? – U/Pb-SHRIMP zircon evidence and the Nd-isotopic record

Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data.     

Did William Smith (1769–1839), the father of biostratigraphy,discover a Jurassic pterosaur tooth?

A partial, isolated tooth from the Middle Jurassic (Bathonian) Stonesfield ‘slate’ of the Taynton Limestone Formation of Oxfordshire is identified as likely coming from a ctenochasmatid pterosaur. Referral to Ctenochasmatidae is based on its very slender, slightly curved crown with near circular cross-section and subparallel margins, slightly inflated root and its stratigraphic age. The tooth is part of the William Smith fossil collection held at the Natural History Museum, London. The collection was assembled as part of William Smith's attempt to identify and map strata around England and Wales in the last decade of the 18th century and the first 15 years of the 19th century. Smith's extensive fossil collection of more than 2500 specimens was purchased by the British Museum in 1816 (Wigley et al., 2018), and thus the specimen is the first pterosaur to be accessioned to the national collection.     

A thousand years of loneliness? Globalization from the perspective of a city in a European periphery

Most cities face the challenge of increasing global and local change. Much of what has been said about cities in a globalized world has been concerned with large metropolitan cites. It has been postulated that increased competitiveness is the relevant response, and that urban governance has to change from managerialism to entrepeneurialism in order to cope with this challenge. The first part of the paper discusses some aspects of this body of theory, and the relevance for sub-national regional capitals. It also discusses how the scope of strategies depend on changing national and regional policies. The second part uses the case of Trondheim to discuss how these cities perceive and deal with globalization. Four policy options are discussed; the clientist strategy, the competitive strategy, the isolationist strategy and finally the option of doing nothing at all. The article concludes that global challenges will force local government in small cities to forge new strategies, but the preferred option is a clientist strategy rather than an entrepreneurial one, and the scope of strategy is national rather than global. Thus, when dealing with small peripheral cities and globalization, the range of perspectives must be extended beyond entrepreneurialism and competitiveness.     

Is the Bayan Obo ore deposit a micrite mound? A comparison with the Sailinhudong micrite mound

The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been debated and various hypotheses have been proposed. Among them is that the Bayan Obo ore deposit is correlated with and has the same origin as the Sailinhudong micrite mound in the southern limb of the Bayan Obo synclinorium. To test this model, the Bayan Obo ore deposit and the Sailinhudong micrite mound are systematically compared for their geological features, elemental geochemistry, and C, O, and Mg isotopic geochemistry. We show that the Bayan Obo ore deposit and the Sailinhudong micrite mound are both calcareous, lens-like in shape, lack bedding features, and are both hosted in a sedimentary formation that consists of clastic sediments and carbonates, unconformably overlying the Archaean–Palaeoproterozoic crystalline basement. However, their geochemical characteristics differ markedly. Compared with the Sailinhudong micrite carbonates, the Bayan Obo ore-hosting dolomite marbles are strongly enriched in LREEs, Ba, Th, Nb, Pb, and Sr, and have very different (PAAS)-normalized REE patterns. Sailinhudong micrite carbonates have higher δ¹³C_PDB and δ¹⁸O_SMOW values, falling into the typical sedimentary field, but the Bayan Obo ore-hosting dolomites are isotopically intermediate between primary igneous carbonatite and typical sedimentary limestone. The δ²⁶ Mg values of the Sailinhudong micrite carbonates are lighter than those of normal Mesoproterozoic sedimentary dolostone, while those of the Bayan Obo ore-hosting dolomite marble are isotopically heavier, similar to δ²⁶ Mg of mantle xenoliths and Bayan Obo intrusive carbonatite. We conclude that the Bayan Obo ore deposit is not correlated with the Sailinhudong micrite mound; it is neither a micrite mound nor an altered micrite mound.     

Did glaciers exist during Pleistocene in the Midyan region, northwest corner of the Arabian Peninsula?

Spectacular erratic boulders, polymictic conglomerates, and sedimentary structures are discovered in many localities at the Midyan Peninsula, east of the Gulf of Aqaba. These deposits are discontinuous, and they unconformably overlie older lithological units of Proterozoic, Cretaceous, Paleogene, and Neogene ages. The erratic boulders are composed of basement rocks, sandstone, and limestone. Some of the boulders exceed 2?m (6?ft) in diameter. The boulders are sub-rounded to rounded, striated, polished, grooved, and faceted. The polymictics are composed of heterogeneous coarse clastics (ranging from boulders to conglomeratic sandstones) of various rock types and fill incised paleovalleys. The sedimentary structures include polished, faceted, and striated boulders, pavements, U-shaped valleys, slumps, and fan-shaped structures. These deposits are herein informally named the Midyan Formation. A Pleistocene age is assigned to the formation based on its stratigraphic position. The polymictic conglomerates are interpreted as glaciogenic tillite, and the erratic boulders and sedimentary structures are considered to have been deposited by glacial erosion, deposition, and possibly tectonics. This interpretation raises the question; did glaciers exist during Pleistocene in the Midyan region? If this interpretation is valid, then recognition of Pleistocene glaciation in Arabia fills a blank spot in the global map of the ??Ice Age??. Pleistocene glacial features were reported in Yemen and regions in north Iraq.     

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Steady decline in the percentage of 235U in terrestrial uranium made natural fission impossible after about 1.8 Ga.Fission before 1.8 Ga disturbed the lead isotope system at various places worldwide,su...     

Did satellite imagery supersede aerial imagery? A perspective from 3D geopositioning accuracy

In this study, the geometric accuracy comparison of aerial photos and WorldView-2 satellite stereo image data is evaluated with the different number and the distribution of the ground control points (GCPs) on the basis of large scale map production. Also, the current situation of rivalry between airborne and satelliteborne imagery was mentioned. The geometric accuracy of Microsoft UltraCam X 45 cm ground sampling distance (GSD) aerial imagery and WorldView-2 data both with and without GCPs are also separately analyzed. The aerial photos without any GCP by only using global navigation satellite system (GNSS) and inertial measurement unit (IMU) data with tie points give an accuracy of ±1.17 m in planimetry and ±0.71 m in vertical that means nearly two times better accuracy than the rational polynomial coefficient (RPC) of stereo WorldView-2. Using one GCP affects the accuracies of aerial photos and WorldView-2 in different ways. While this situation distorts the aerial photo block, it corrects the shift effect of RPC in WorldView-2 and increases the accuracy. By using four or more GCPs, ½?pixel (～0.23 m) accuracy in aerial photos and 1 pixel (～0.50 m) accuracy in WorldView-2 can be achieved in horizontal. In vertical, aerial photos have 1 pixel (～0.55 m) and WorldView-2 has 1.5 pixels (～0.85 m) accuracy. These results show that Worldview-2 imagery can be used in the production of class I 1:5000 scale maps according to the ASPRS Accuracy Standards for Digital Geospatial Data in terms of geometric accuracy. It is concluded that the rivalry between aerial and satellite imagery will continue for some time in the future.     

Layered amphibolite sequence in NE Sardinia,Italy: remnant of a pre-Variscan mafic silicic layered intrusion?

A banded amphibolite sequence of alternating ultramafic, mafic (amphibolite) and silicic layers, tectonically enclosed within Variscan migmatites, outcrops at Monte Plebi (NE Sardinia) and shows similarities with leptyno-amphibolite complexes. The ultramafic layers consist of amphibole (75–98%), garnet (0–20%), opaque minerals (1–5%) and biotite (0–3%). The mafic rocks are made up of amphibole (65–80%), plagioclase (15–30%), quartz (0–15%), opaque minerals (2–3%) and biotite (0–2%). The silicic layers consist of plagioclase (60–75%), amphibole (15–30%) and quartz (10–15%). Alteration, metasomatic, metamorphic and hydrothermal processes did not significantly modify the original protolith chemistry, as proved by a lack of K₂O-enrichment, Rb-enrichment, CaO-depletion, MgO-depletion and by no shift in the rare earth element (REE) patterns. Field, geochemical and isotopic data suggest that ultramafic, mafic and silicic layers represent repeated sequences of cumulates, basic and acidic rocks similar to macrorhythmic units of mafic silicic layered intrusions. The ultramafic layers recall the evolved cumulates of Skaergaard and Pleasant Bay mafic silicic layered intrusions. Mafic layers resemble Thingmuli tholeiites and chilled Pleasant Bay mafic rocks. Silicic layers with Na₂O: 4–6 wt%, SiO₂: 67–71 wt% were likely oligoclase-rich adcumulates common in many mafic silicic layered intrusions. Some amphibolite showing a strong Ti-, P-depletion and REE-depletion are interpreted as early cumulates nearly devoid of ilmenite and phosphates. All Monte Plebi rocks have extremely low Nb, Ta, Zr, Hf content and high LILE/HFSE ratios, a feature inherited from the original mantle sources. The mafic and ultramafic layers show slight and strong LREE enrichment respectively. Most mafic layer samples plot in the field of continental tholeiites in the TiO₂–K₂O–P₂O₅ diagram and are completely different from N-MORB, E-MORB and T-MORB as regards REE patterns and Nd, Sr isotope ratios but show analogies with Siberian, Deccan and proto-Atlantic rift tholeiites. Comparisons with Thingmuli, Skaergaard and Kiglapait rocks and with experimental data suggest that the Monte Plebi intrusion was an open-to-oxygen system with fO₂ FMQ. Mafic and ultramafic samples yielded _Nd(460)=+0.79 /+3.06 and ⁸⁷Sr/⁸⁶Sr=0.702934–0.703426, and four silicic samples _Nd(460)=–0.53/–1.13; ⁸⁷Sr/⁸⁶Sr=0.703239–0.703653. Significant differences in Nd isotope ratios between mafic and silicic rocks prove that both groups evolved separately in deeper magma chambers, from different mantle sources, with negligible interaction with crustal material, and were later repeatedly injected within a shallower magma chamber. The spectrum of Sr and Nd isotope data is consistent with a slightly enriched mantle metasomatized during an event earlier than 460 Ma. The metasomatising component was represented by alkali-Th-rich fluids of crustal origin rather than by sedimentary materials, able to modify alkali and Sr–Nd isotope systematics. Monte Plebi layered amphibolites might represent the first example of a strongly metamorphosed fragment of an early Paleozoic mafic silicic layered intrusion emplaced in a thinning continental crust and then tectonically dismembered by Variscan orogeny.     

A pre-outburst of the classical symbiotic star BF Cyg in 1987–1989

We analyze previously unpublished spectrophotometric data for the classical symbiotic star BF Cyg obtained in 1982–1989. These data show that BF Cyg already started a new period of activity in 1987–1989, earlier believed to correspond to quiescence. An optical spectrum taken on September 29, 1987 displays several features characteristic of activity in classical symbiotic stars. During the same time interval, excess near-UV radiation was detected, possibly from the accretion disk, and evidence for Rayleigh scattering of the hot component’s radiation on atomic hydrogen was detected in the far-UV, at phases previously believed not to be characteristic of this effect. This latter finding may testify to an enhanced (compared to 1979–1986) rate of mass loss from the cool component of BF Cyg. We discuss the interrelation between an increased mass-loss rate of the cool component and the hot component’s activity in the context of classical symbiotic stars. Original Russian Text ￠ A.A. Tatarnikova, E.A. Kolotilov, A.M. Tatarnikov, 2008, published in Astronomicheskiĭ Zhurnal, 2008, Vol. 85, No. 2, pp. 151–162.     

Did the Harappan settlement of Dholavira (India) collapse during the onset of Meghalayan stage drought?

Radiocarbon dating of archaeological carbonates from seven cultural stages of Dholavira, Great Rann of Kachchh (GRK), the largest excavated Harappan settlement in India, suggests the beginning of occupation at ~5500 years BP (pre-Harappan), and continuation until ~3800 years BP (early part of the Late Harappan period). The settlement rapidly expanded under favourable monsoonal climate conditions when architectural elements such as the Citadel, Bailey, Lower and Middle Town were added between the Early and mid-Mature Harappan periods. Abundant local mangroves grew around the GRK sustaining prolific populations of the edible gastropod Terebralia palustris. Oxygen isotope (δ¹⁸O) sclerochronology of Early Harappan gastropod shell suggests seasonal mixing of some depleted (δ¹⁸O ~ −12‰) river water in summer/monsoon months (through ancient Saraswati and/or Indus distributary channels) with seawater that periodically inundated the GRK. Evaporation from this semi-enclosed water body during the non-monsoon months enriched the δ¹⁸O of water/shell carbonates. The humid fluvial landscape possibly changed due to a catastrophic drought driving the final collapse of the settlement of Dholavira exactly at the onset of the Meghalayan (Late Holocene) stage (~4300–4100 years BP ). Indeed, Dholavira presents a classic case for understanding how climate change can increase future drought risk as predicted by the IPCC working group. Copyright © 2019 John Wiley & Sons, Ltd.     

A brief review of isotopically light Li – a feature of the enriched mantle?

Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.     

Did humans and natural forests coexist in close proximity? A case study of the Lateglacial and Holocene Mollusca in the Moravian Karst,Czech Republic

Prehistoric settlements are usually perceived as being in opposition to the natural development of the landscape. Indeed, for woodland snail assemblages in anthropogenic landscapes in central Europe, considerable impoverishment is typical. However, it remains unclear whether this has been caused by humans only or also by climate effects. From an archaeological point of view, the Moravian Karst is one of the classic prehistorical locations in central Europe, but with a more humid climate than the previously studied anthropogenic areas. To learn more about coexistence of humans and natural forests during the Lateglacial and Holocene, we analysed 11 mollusc successions covering this entire area, a unique data set for such a relatively small area. These mollusc successions show several specific features compared to the standard development known from other mid‐European areas. One is that although the Moravian Karst is not far from the Western Carpathians, Carpathian species appeared relatively late, only during the second half of the Holocene climatic optimum. Similarly, some western European and Alpine elements appeared later than expected. In contrast to this, however, a number of forest species with central European range appeared relatively early during the Lateglacial or Early Holocene. Two even survived the Last Glacial Maximum in the Moravian Karst. This would suggest an early occurrence of forest patches in a mosaic landscape. Humans have apparently inhabited this area since the Lateglacial amongst islands of forests, which later changed during the Boreal and then the climatic optimum into humid canopy forests. Thus, a mosaic of anthropogenic and natural habitats persisting in close vicinity was possible in rugged and humid landscapes practically until the Industrial Revolution.     

Did prolonged two-stage fragmentation of the supercontinent Kenorland lead to arrested orogenesis on the southern margin of the Superior province?

Recent geochronological investigations reinforce the early suggestion that the upper part of the Paleoproterozoic Huronian Supergroup of Ontario,Canada is present in the Animikie Basin on the south shore of Lake Superior.These rocks,beginning with the glaciogenic Gowganda Formation,are interpreted as passive margin deposits.The absence of the lower Huronian(rift succession) from the Animikie Basin may be explained by attributing the oldest Paleoroterozoic rocks in the Animikie Basin(Chocolay Group)to deposition on the upper plate of a north-dipping detachment fault,which lacks sediments of the rift phase.Following thermal uplift that led to opening of the Huronian Ocean on the south side of what is now the Superior province,renewed uplift(plume activity) caused large-scale gravitational folding of the Huronian Supergroup accompanied by intrusion of the Nipissing diabase suite and Senneterre dikes at about 2.2 Ga.Termination of passive margin sedimentation is normally followed by ocean closure but in the Huronian and Animikie basins there was a long hiatus- the Great Stratigraphic Gap- which lasted for about 350 Ma.This hiatus is attributed to a second prolonged thermal uplift of part of Kenorland that culminated in complete dismemberment of the supercontinent shortly before 2.0 Ga by opening of the Circum-Superior Ocean.These events caused regional uplift(the Great Stratigraphic Gap) and delayed completion of the Huronian Wilson Cycle until a regional compressional tectonic episode,including the Penokean orogeny,belatedly flooded the southern margin of the Superior province with foreland basin deposits,established the limits of the Superior structural province and played an important role in constructing Laurentia.