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[en] As species accumulate in a community, competition for available ecospace is expected to prevent the addition of new species and to facilitate species extinction, thus producing a dynamic equilibrium of diversity. This mechanism remains under debate since its empirical support comes mainly from indirect or partial evidence, with very few direct tests at the species level. Here a new method is described to detect the presence of selfregulation feedbacks between species richness and turnover rates. It consists of Monte Carlo simulations which randomize the distribution of species ranges among stratigraphic intervals, providing predictions which allow the detection of genuine self-regulation feedbacks in the real data. Since the simulations include any potential bias due to preservation, sampling, or change in depositional environment, and these biases would also affect the real dataset, they are thus ruled out as explanations for any difference found. This method is applied to one of the best known fossiliferous sequences worldwide, the Rambla de Valdemiedes in Murero (RV1 section, middle Cambrian, Spain), a classic locality that has been studied for more than 150 years and which stands out due to its excellent sampling density, continuous deposition, and homogeneous fossil preservation. The results show that trilobite species richness was self-regulated due to positive feedback with extinction rate, which implies that compensatory extinction regulated this fauna in spite of the on-going Cambrian radiation. The lack of evidence of any origination feedback suggests ecological opportunities were not limiting for new species to colonize this Cambrian community. (Author)
[en] The pre-Variscan rocks of the Pyrenees exhibit a polyphase deformation linked to the Variscan crustal shortening and a low-pressure–high-temperature metamorphism. However, there is scarce chronostratigraphic evidence of this Variscan deformation. In the Pyrenean low-grade metamorphic domains, maximum ages have been provided by the synorogenic Carboniferous Culm deposits. In medium- to high-grade metamorphic areas, the Variscan regional metamorphism or intrusive magmatic bodies constrain the age of the main Variscan deformation structures. However, these data usually provide a minimum age. Here, we present new palaeobotanical records that assign a Namurian age to the base of the Culm deposits of la Cerdanya in the eastern Pyrenees. This dating is based on the co-occurrence of the sphenopsids Archaeocalamites radiatus, Mesocalamites cistiiformis and the seed of Cardiocarpus sp. The plant remains were found in sandstone facies produced by high-density turbidity flows of a deep-sea fan system. The new biostratigraphic information constrains the age of the Carboniferous Culm succession in the eastern Pyrenees.
[en] The Ediacaran-Lower Ordovician successions exposed in the Eastern Pyrenees are updated and revised based on recent U-Pb zircon radiometric ages, intertonguing relationships of carbonate-dominated strata, and onlapping patterns marking the top of volcano-sedimentary complexes. A stratigraphic comparison with neighbouring pre-Variscan outcrops from the Montagne Noire (southern French Massif Central) and Sardinia is related to i) the absence of Cadomian deformation close to the Ediacaran-Cambrian boundary interval; ii) the presence of an episodic, Cadomian-related, acidic-dominant volcanism related to carbonate production punctuating the Ediacaran-Cambrian transition, similar to that recorded in the northern Montagne Noire; and iii) the lack of Guzhangian (Late Cambrian Epoch 3) regressive shoal complexes present in the Montagne Noire and probably in Sardinia.
[en] Excellent dolomite exposures are observed in the eastern Salt Range (Pakistan), where the Cambrian Jutana Formation consists of two distinct units (i.e. oolitic – pisolitic unit and massive dolomite unit). Field observations revealed that the lower, oolitic-pisolitic unit mostly comprises medium to thick bedded, interlayered brown yellowish dolostone containing ooids/pisoids and faunal assemblages, and grey whitish sandstone with distinct depositional sedimentary features (i.e. trough-, herringbone- and hhummocky crossbedding). The upper massive dolostone unit consists of thick bedded to massive dolostone. These two units are separated by shale. Petrographic studies identified three dolomite types, which include: fine crystalline dolomite (Dol. I), medium-coarse crystalline dolomite (Dol. II) and fracture associated, coarse crystalline dolomite (Dol. III). Stable isotope studies indicate less depleted δ18O values for Dol. I (-6.44 to -3.76‰V-PDB), slightly depleted δ18O values for Dol. II (-7.73 to -5.24‰V-PDB) and more depleted δ18O values for Dol. III (-7.29 to -7.20‰V-PDB). The δ13C values of the three dolomite phases are well within the range of Cambrian sea-water signatures. Furthermore, δ26Mg-δ25Mg signatures (Dol. I; δ26Mg=-1.19 to -1.67, δ25Mg=-0.61 to -0.86 and Dol. II; δ26Mg=-1.34 to -1.59, δ25Mg=-0.70 to -0.83) indicate three phases of dolomitization in different diagenetic settings. First, an initial stage of dolomitization during the early Cambrian resulted from altered marine, Mg-rich fluids associated with the mixing zone mechanism. Second, a late stage of dolomitization was associated with burial during late Permian. A third dolomitization phase was related to post-Eocene times.
[en] Semipalatinsk test site territory is situated at the turn of Chengiz-Torbagatay (Caledonian) and Irtysh-Zajsan (Hercynian) structural-formation zones, distinguishing by geological structure and metallogeny, and they are divided by Kalba-Chengiz deep-seated break. In the geological structure of the Upper Pre-Cambrian formation and by the all systems of Paleozoic era. The Upper Pre-Cambrian is presented with three rock masses of porphyrites and porphyritoides, peach stones, micro-quartzite developed in the Chengiz sub-zone. The Cambrian system is presented in the Chengiz sub-zone with volcano-gen-terrigenous rock mass. Analysis of formation precipitation types by elementary content and assumed condition of formation, separated formation series by all structure-formation sub-zones is carried out
[en] The biogeographic distribution of foraminifers and their belonging to either the southern or northern margins of the Paleo-Tethys are used here for paleogeographic reconstructions of Iran during the Carboniferous. Lower Carboniferous foraminiferal assemblages from northern and central Iran show a cosmopolitan character and affinities to both the southern and northern borders of the Paleo-Tethys. Hence, in the Early Carboniferous Iran occupied an intermediate southern latitude position, forming part of Gondwana. This conclusion is consistent with the Late Ordovician to Early Carboniferous drift history of Iran based on paleomagnetic data. In the Late Carboniferous, the foraminiferal affinities of northern and central Iran with the northern part of Paleo-Tethys suggest that Iran separated from Gondwana and moved northwards to a lower latitude. This separation is also evidenced by the Upper Carboniferous coal-bearing sandstones of the Sardar Formation and sandstones with high degree of chemical weathering, which would indicate warm and humid conditions. Considering the composition of foraminiferal fauna along with the evidence of magmatic activities in northwest Iran, it can be inferred that the commencement of the Neo-Tethys opening and continental break-up in Iran occurred sometime in the Late Carboniferous, which contradicts the previous claims that the separation of Iran from Gondwana occurred in Permian and/or Triassic times.
[en] Carbon-isotope stratigraphy of the Furongian (stage 10; Upper Cambrian) and Tremadocian (lowermost Ordovician) reveals distinct variations from the carbonates of the Berry Head and Watts Bight formations of the East Isthmus Bay section that accumulated in a shallow-marine setting on the eastern Laurentian platform in a passive margin setting in western Newfoundland, Canada. The East Isthmus Bay δ13C values show insignificant correlation with their Sr (R2 = 0.04), Mn (R2 = 0.001) and Fe (R2 = 0.02) counterparts, implying preservation of at least near-primary C-isotope compositions. The investigated section is largely fossil poor, but the δ13C profile shows a pattern with distinct variations that can be matched with those of the western Laurentian Lawson Cove Auxiliary Boundary Stratigraphic Section and Point (ASSP) section, Utah, USA. Therefore, it was possible to reconstruct a conodont biozonal scheme by matching the δ13C profile with its counterpart from the Lawson Cove ASSP section. At the base of the East Isthmus Bay section, the δ13C profile exhibits a broad excursion (the top of the Herllnmaria - Red Tops Boundary), which can be matched with the base of the Eoconodontus Zone (mid-Furongian), followed by an enrichment trend through the Cordylodus intermedius Zone (top Furongian). A positive excursion (Hirsutodontus simplex spike) is recorded in the Cordylodus intermedius Zone (top Cambrian), and a prominent positive peak characteristic for the Cordylodus lindstromi Zone is recorded from the top of the investigated section. The δ13C values of the Newfoundland carbonates are generally ∼1‰ Vienna Pee Dee Belemnite lower than those of Lawson Cove, which is likely attributable to a relatively higher productivity and (or) organic burial in the Utah region. (author)