20 research outputs found

    (Re)proposal of Three Cambrian Subsystems and Their Geochronology

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    The Cambrian is anomalous among geological systems as many reports divide it into three divisions of indeterminate rank. This use of “lower”, “middle”, and “upper” has been a convenient way to subdivide the Cambrian despite agreement it consists of four global series. Traditional divisions of the system into regional series (Lower, Middle, Upper) reflected local biotic developments not interprovincially correlatable with any precision. However, use of “lower”, “middle”, and “upper” is unsatisfactory. These adjectives lack standard definition, evoke the regional series, and are misused. Notably, there is an almost 50 year use of three Cambrian subsystems and a 1997 proposal to divide the Avalonian and global Cambrian into four series and three subsystems. The global series allow proposal of three formal subsystems: a ca. 32.6 Ma Lower Cambrian Subsystem (Terreneuvian and Series 2/proposed Lenaldanian Series), a ca. 9.8 Ma Middle, and a ca. 10 Ma Upper Cambrian Subsystem (=Furongian Series). Designations as “Lower Cambrian Subsystem” or “global Lower Cambrian” distinguish the new units from such earlier units as “Lower Cambrian Series” and substitute for the de facto subsystem terms “lower”, “middle”, and “upper”. Cambrian subsystems are comparable to the Carboniferous’ Lower (Mississippian) and Upper (Pennsylvanian) Subsystems

    Chemostratigraphic Correlations Across the First Major Trilobite Extinction and Faunal Turnovers Between Laurentia and South China

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    During Cambrian Stage 4 (~514 Ma) the oceans were widely populated with endemic trilobites and three major faunas can be distinguished: olenellids, redlichiids, and paradoxidids. The lower–middle Cambrian boundary in Laurentia was based on the first major trilobite extinction event that is known as the Olenellid Biomere boundary. However, international correlation across this boundary (the Cambrian Series 2–Series 3 boundary) has been a challenge since the formal proposal of a four-series subdivision of the Cambrian System in 2005. Recently, the base of the international Cambrian Series 3 and of Stage 5 has been named as the base of the Miaolingian Series and Wuliuan Stage. This study provides detailed chemostratigraphy coupled with biostratigraphy and sequence stratigraphy across this critical boundary interval based on eight sections in North America and South China. Our results show robust isotopic evidence associated with major faunal turnovers across the Cambrian Series 2–Series 3 boundary in both Laurentia and South China. While the olenellid extinction event in Laurentia and the gradual extinction of redlichiids in South China are linked by an abrupt negative carbonate carbon excursion, the first appearance datum of Oryctocephalus indicus is currently the best horizon to achieve correlation between the two regions

    Questioning a widespread euxinia for the Furongian (Late Cambrian) SPICE event: indications from delta C-13, delta O-18, delta S-34 and biostratigraphic constraints

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    Results from a high-resolution study of delta C-13(carb), delta O-18(carb), delta S-34(CAS), delta S-34(CRS) and elemental concentrations (Ca, Fe, Mg, Mn and Sr) in the Furongian Kyrshabakty section, southern Kazakhstan, are reported here. The investigated interval covers the Drumian to Jiangshanian stages of the Cambrian Period, respectively the regional Ptychagnostus atavus to Ivshinagnostus ivshini-Irvingella major trilobite zones. delta C-13(carb) data include the Steptoean positive carbon isotope excursion (SPICE) with a local peak value of + 5 parts per thousand. The onset of SPICE corresponds to the Kormagnostus simplex Glyptagnostus stolidotus zones and pre-dates the base of the Paibian Stage/Furongian Series. delta S-34(CAS) data already increase during the Lejopyge armata biozone culminating in three positive excursions prior and after the SPICE maximum. Differences in onset, peak values and shape of the delta C-13(carb) and/or delta S-34(CAS) excursions at Kyrshabakty, but also in almost all sections characterized by the SPICE, are pointing towards Furongian seawater that was low in sulphate concentration and heterogeneous in its carbonate carbon and sulphate sulphur isotopic composition. The occurrence of benthic faunal elements in almost all SPICE-related sections strongly supports oxygenated conditions at the seafloor, therefore excluding widespread anoxia or euxinia. Regional anoxic conditions are most probable. A positive delta O-18(carb) excursion parallel to the SPICE could probably be explained by a decline in seawater pH associated with a sea-level rise. Again, no euxinic conditions would be mandatory for explaining the SPICE event

    Unexpectedly curved spines in a Cambrian trilobite: considerations on the spinosity in Kingaspidoides spinirecurvatus sp. nov. from the Anti-Atlas, Morocco, and related Cambrian ellipsocephaloids

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    The new ellipsocephaloid trilobite species Kingaspidoides spinirecurvatus has a spectacular morphology because of a unique set of two long and anteriorly recurved spines on the occipital ring and the axial ring of thoracic segment 8. Together with the long genal spines this whimsical dorsally directed spine arrangement is thought to act as a non-standard protective device against predators. This is illustrated by the body posture during different stages of enrolment, contrasting with the more sophisticated spinosities seen in later trilobites, which are discussed in brief. Kingaspidoides spinirecurvatus from the lower–middle Cambrian boundary interval of the eastern Anti-Atlas in Morocco has been known for about two decades, with specimens handled as precious objects on the fossil market. Similar, but far less spectacular, spine arrangements on the thoracic axial rings are known from other ellipsocephaloid trilobites from the Anti-Atlas of Morocco and the Franconian Forest region of Germany. This suggests that an experimental phase of spine development took place within the Kingaspi-doides clade during the early–middle Cambrian boundary interval

    Isotopic evidence for temperate oceans during the Cambrian Explosion

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    The Cambrian Explosion was a key event in the evolution of life on Earth. This event took place at a time when sea surface temperatures have been proposed to reach about 60 °C. Such high temperatures are clearly above the upper thermal limit of 38 °C for modern marine invertebrates and preclude a major biological revolution. To address this dichotomy, we performed in situ ÎŽ18O analyses of Cambrian phosphatic brachiopods via secondary ion mass spectrometry (SIMS). The ÎŽ18Ophosphate data, which are considered to represent the most primary ÎŽ18Oseawater signature, were identified by evaluating the diagenetic alteration of the analyzed shells. Assuming ice-free conditions for the Cambrian ocean and no change in ÎŽ18Oseawater (-1.4‰ to -1‰; V-SMOW) through time, our temperatures vary between 35 °C ± 12 °C and 41 °C ± 12 °C. They are thus clearly above (1) recent subequatorial sea surface temperatures of 27 °C–35 °C and (2) the upper lethal limit of 38 °C of marine organisms. Our new data can therefore be used to infer a minimal depletion in early Cambrian ÎŽ18Oseawater relative to today of about -3‰. With this presumption, our most pristine ÎŽ18Ophosphate values translate into sea surface temperatures of about 30 °C indicating habitable temperatures for subequatorial oceans during the Cambrian Explosion

    Carbonate-associated sulfate: Experimental comparisons of common extraction methods and recommendations toward a standard analytical protocol

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    The aim of this study was to establish a protocol for the extraction of carbonate-associated sulfate (CAS) for the purpose of tracing the sulfur isotope composition of seawater. Existing CAS extraction methods were evaluated for their efficacy in eliminating non-CAS sulfur from the final CAS isotopic analysis. Five leaching methods were tested on three carbonate samples: (1) 10% NaCl (aq); (2) 10% NaCl (aq) followed by 10% NaOCl (aq); (3) 10% NaOCl (aq); (4) 10% NaCl (aq) followed by 10% H2O2 (aq); and (5) pure water only. All leaching steps were performed until no dissolved sulfate was seen to precipitate on addition of BaCl2 (aq). CAS was then liberated from the carbonate lattice by adding HCl from a dropping filter. All leachates, CAS fractions, and insoluble residues after CAS extraction (chromium-reducible sulfur or CRS) were analyzed for their isotopic composition. These experiments demonstrate that the leachable non-CAS sulfate fraction in carbonates can be proportionately far greater than, and isotopically distinct from the lattice-bound carbonate sulfate fraction. Here we show that some form of pre-leaching, other than with pure water, is necessary to isolate the CAS fraction in carbonates. However, even in cases of repeated pre-leaching and testing for non-CAS sulfate, measured delta S-34(CAS) values may still be significantly influenced by the non-CAS sulfate fraction if delta S-34(NaCl) and delta S-34(CAS) values are sufficiently different. Pre-leaching once or twice with NaOCl and/or H2O2 is shown to be insufficient to ensure elimination of reduced sulfur, e.g. in the form of pyrite, while partial oxidation of reduced sulfur during pre-leaching with these powerful oxidants extends pre-leaching times, and can thus contaminate the final CAS value. Both of these leaching methods are shown to alter final delta S-34(CRS) values by partial oxidation of reduced sulfur, and so need to be applied with care. For a secure CAS extraction from carbonate rocks we recommend repeated leaching with NaCl solution as a standard protocol in future studies, with complementary analyses of pre-leach sulfate concentrations and delta S-34(NaCl), and CRS concentrations and delta S-34(CRS) as routine checks on possible contamination as well as tools for interpretation. Analyzing delta C-13(carb). delta O-18(carb), and elemental concentrations (Ca, Fe, Mg, Mn, Sr) of the carbonate host rock may help to constrain diagenetic alteration of the measured delta S-34(CAS). Published interpretations of rapidly changing seawater delta S-34 and sulfate concentrations need to be reconsidered in the light of these data. (C) 2012 Elsevier B.V. All rights reserved

    Revised Cambrian stratigraphy in the Franconian Forest (Frankenwald), Germany, reveals typical West Gondwanan succession in the Saxothuringian Belt

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    New investigations of the Cambrian in the Franconian Forest region lead to a revision of the lithostratigraphic succession into seven units: Rauschbach Unit, Tiefenbach Formation, Tannenknock Formation (with Galgenberg and Wildenstein members), Triebenreuth Formation, Lippertsgrun Formation and Berglesh of Formation, with partly revised stratigraphical ranges and lithological characteristics. The succession indicates a fairly complete succession from Cambrian Series 2 (late early Cambrian) through the end of the Miaolingian, with gaps being a result of incomplete exposure and structural complexity rather than distinct hiatuses. New finds expand the known fossil record and provide additional data for reconstructing biogeographical relationships and depositional environments. These features indicate shallow marine conditions throughout the Cambrian with characteristics typical for West Gondwanan shelf areas. In particular, strong similarities to the Moroccan Atlas regions are indicated. Additional information on volcanic activity from middle Cambrian through Tremadocian times provides clues for crustal extension that affected the depositional setting in this region. Stratigraphy, depositional environments and facies distribution all suggest reinterpretation of earlier geotectonical models used to interpret the history of the Franconian Forest area in the Saxothuringian Belt

    C-, O- and Sr-isotope stratigraphy across the Lower–Middle Cambrian transition of the Cantabrian Zone (Spain) and the Montagne Noire (France), West Gondwana

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    C-, O- and Sr-isotopes have been analysed for three sections of the Cantabrian Zone (northern Spain) and the southern Montagne Noire (France) that cross the Lower–Middle Cambrian transition, or Bilbilian–Leonian transition in its Iberian sense. The base of the Leonian (Middle Cambrian) is regionally defined by the first appearance of the trilobite Acadoparadoxides mureroensis, which is well constrained in the Cantabrian Zone but absent in the Montagne Noire. In the Bilbilian parts of the three sections, nearshore to peritidal strata are capped by erosive unconformities. The succeeding Leonian relative sea-level rise is recorded in similar fashion in both environments, leading to the deposition of aggradational bioclastic limestones and/or transgressive limestone/shale couplets (the so-called griotte facies) overlain by offshore shales, which relate to platform drowning and coastal smothering during the Leonian–Caesaraugustan transition.\ud \ud Bilbilian–Leonian carbonates show a long-term trend to higher ÎŽ13C values that was punctuated by a succession of positive excursions. In CrĂ©menes–ValdorĂ© (Cantabrian Zone) and Ferrals-les-Montagnes (Montagne Noire), two Bilbilian ÎŽ13C shifts (an older uncertain peak at + 0.1‰/+ 0.01‰ and a younger certain shift at + 0.6‰/+ 1.7‰), and two Leonian shifts (peaking at + 1.1‰/+ 1.2‰ and + 1.3‰, respectively) are apparent; at Genestosa (Cantabrian Zone) a fifth early-middle Caesaraugustan shift is identified (peaking at + 2.1‰). These positive excursions and increasing trend are similar to published trends from the Lower–Middle Cambrian transition in Siberia and Australia, although in those areas only two ÎŽ13C excursions have been reported so far. The marked negative ÎŽ13C excursion reported from the Lower–Middle Cambrian boundary of Laurentia and China is not recorded in our dataset. Oxygen isotope and trace element (Ca, Mn and Sr) data indicate that our ÎŽ13C trends are unlikely to be diagenetic features, while the lack of correlation with relative sea-level fluctuations, facies changes, or benthic community dynamics means that they could be of potential for global stratigraphic correlation.\ud \ud Sr isotope compositions for the Cantabrian Zone succession exhibit consistent values upsection, which suggests that seawater 87Sr/86Sr remained around 0.7089–0.7090. Our more limited dataset for the Montagne Noire region confirms this value, and both are consistent with published constraints from Laurentia for seawater 87Sr/86Sr across the Lower–Middle Cambrian boundary. The only other 87Sr/86Sr study to cover the boundary interval reported significantly lower 87Sr/86Sr values not, vert, similar0.7087–0.7088, indicating perhaps that the Lower–Middle Cambrian boundary in Siberia lies stratigraphically below its equivalent levels in Europe and Laurentia. However, we consider it more likely that these Siberian samples have been diagenetically altered to less radiogenic values
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