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The excellent preservation of Lower Pleistocene Glycymeris
No full textSpecies belonging to the genus Glycymeris Da Costa, 1778, commonly named ‘dog-cockles’ or ‘bittersweets’, appear in the Lower Cretaceous and have a rich fossil record; they are free-living seafloor inhabitants (up to 100 m of depth), widespread and very common in fossil faunas and are also very abundant in present seas and oceans, being absent from polar, deep-sea regions and brackish water.
Species of the genus Glycymeris have an excellent potential as palaeoenvironmental archives and are thus important tools for palaeoclimatic reconstructions and sclerochronological analyses. The study of the shell ultrastructure under scanning electron microscope (SEM) is a fundamental step before doing geochemical and isotopic analyses; it is very important for fossil specimens, but also for recent ones, to test the preservation of the shell ultrastructure in order to exclude diagenetic alteration which could affect the results of the geochemical and isotopic analyses.
In the present analysis new data about the shell ultrastructure of species of the genus Glycymeris are obtained through a comparison between the fabric of recent specimens from Brittany (France) and fossil specimens collected from the Lower Pleistocene Castell’Arquato Formation cropping out along the Arda River in Western Emilia (Italy). This comparison, made using Scanning Electron Microscope (SEM), results in a strong similarity between the two fabrics, highlighting the great preservation of fossil one. Both fossil and recent specimens show a well preserved outer simple crossed lamellar layer and an inner irregular and cone complex crossed lamellar layer; the inner and outer layers are separated by an irregular simple prismatic pallial myostracum; these mineralized layers are penetrated by parallel, not ramificated and not bifurcated cylindrical tubules, which represent a peculiar character of the Arcoida shells. This analysis, besides revealing new data on the pattern and origin of the tubules, shows that the fossil species of Glycymeris are very well preserved and can be used for subsequent geochemical analysis. It also shows that Glycymeris shell fabric has not changed for at least the last 2 million years, underlining the conservative character of the genus not only in its morphology but also in its ultrastructure
Guadalupian (Permian) brachiopods from the Ruteh Limestone, North Iran
No full textThirty-three brachiopod species from the Guadalupian Ruteh Limestone of North Iran are here systematically described and illustrated. Brachiopods have been collected bed-by-bed along five stratigraphic sections and in one fossiliferous locality in the region between Dorud and Shirinibad in the Alborz Mountains. Four new species and one new genus are erected in the present paper: Haydenella eminens n. sp. Perigeyerella rutehiana n. sp.; Martinia bassa n. sp. and Bisolcatelasma iraniana n. gen. n. sp. Quantitative biostratigraphic analysis of the brachiopod data based on the Unitary Association method (Guex, 1991) has lead to the construction of a local sequence of three discrete biozones: The Squamularia sp. B-M. bassa Biozone at the base of the formation, the H. kiangsiensis-N. (N.) asseretoi Biozone in its middle part and the R. exile-R. gemmellaroi Biozone at its top. The latter however has been recognized only in the Shirinabad section. As already envisaged for the Carboniferous and Lower Permian brachiopod faunas from North Iran, the Guadalupian fauna is comprised mostly of cosmopolitan taxa, confirming the role of the Iranian microplate as a staging-post for most of the late Palaeozoic. When compared to the younger Lopingian faunas collected in the same regions of North Iran, the Ruteh brachiopods appear significantly different, indicating a marked biotic change in the brachiopod communities across the end- Guadalupian biotic crisis
THE SHELL ULTRASTRUCTURE OF THE GENUS <em> GLYCYMERIS </em> DA COSTA, 1778: A COMPARISON BETWEEN FOSSIL AND RECENT SPECIMENS
No full textNew data about the shell ultrastructure of species of the genus Glycymeris are obtained through a comparison between the fabric of recent specimens from Brittany (France) and fossil specimens collected from the Lower Pleistocene Castell’Arquato Formation cropping out along the Arda River in Western Emilia (Italy). This comparison, made using Scanning Electron Microscope (SEM), results in a strong similarity between the two fabrics, highlighting the good preservation of fossil ones. Both fossil and recent specimens show a well preserved outer simple crossed lamellar layer and an inner irregular and cone complex crossed lamellar layer. The inner and outer layers are separated by an irregular simple prismatic pallial myostracum. These mineralized layers are penetrated by parallel, not ramified and not bifurcated cylindrical tubules, which represent a peculiar character of the Arcoida shells. This analysis provides a more complete picture of Glycymeris shell ultrastructure. It shows that Glycymeris shell fabric has not changed for the last 2 million years and that the fossil specimens are pristine. Furthermore new data on the pattern and origin of tubules are reported, allowing to conclude that it is unlikely that they have a deterrence function for boring organisms. They may instead function to increase the volume of the organic content of the shell at lower metabolic cost without increasing the shell surface
GEOCHEMICAL AND SCLEROCHRONOLOGICAL ANALYSES OF THE LOWER PLEISTOCENE MACROFAUNA OF WESTERN EMILIA (NORTHERN ITALY): PALAEOENVIRONMENTAL AND PALAEOCLIMATIC IMPLICATIONS
No full textThe Early Pleistocene is a time interval characterized by several climatic oscillations which has its lower and upper boundaries coinciding respectively with the beginning of the Northern Hemisphere Glaciation and the Middle Pleistocene Transition. The Mediterranean area was strongly affected by the Early Pleistocene climatic changes. One of the most important biotic event is here represented by the appearance of the boreal guests (e.g. the bivalve Arctica islandica and the foraminifer Hyalinea balthica) at the beginning of the Calabrian Stage, suggesting significant cooling of the Mediterranean Sea, which is also confirmed by a change in the pollen flora indicating cold climatic conditions.
The Arda River marine succession, cropping out in Western Emilia, Northern Italy, is very rich in macrofossils and it covers without significant gaps the Early Pleistocene. It thus represents an ideal setting where to study the climatic oscillations of this time interval.
This study, combining sedimentology, taxonomy, palaeoecology, biostratigraphy and geochemistry and sclerochemistry of bivalve shells, provides an integrated investigation of the palaeoenvironmental and palaeoclimatic conditions accompanying these major climatic changes in the Arda section.
The Arda marine succession is 237.40 m-thick and consists of sandstones, siltstones and mudstones deposited in a tectonically active setting during phases of advance of fan deltas; it is bounded at the top by continental conglomerates indicating a major sea level drop and the establishment of a continental environment with vertebrate faunas and fresh water mollusks.
The taxonomic analysis of the macrofauna allows to identify 159 taxa, of which bivalves are dominant with 105 taxa, followed by gastropods (44 taxa) and a few corals (3 taxa) and serpulids (2 taxa); brachiopods, echinoids, barnacles, bryozoans and scaphopods do also occur. The comparative sedimentological and palaeoecological analysis shows that the Arda marine succession deposited in an infralittoral to a shallow circalittoral environment, where the maximum depth of the succession should not have exceeded 40-50 m. This comparative analysis confirms also the general regressive trend of the studied marine succession, punctuated by eight lower order transgressive and regressive cycles; however, no evidence of subaerial exposure or shift to water depths exceeding 50 m has been recorded, framing the depositional depth through the transgressive and regressive cycles between 5 m and 50 m of depth.
According to mollusk and nannofossil biostratigraphy the Arda River section has a late Gelasian-Calabrian (Early Pleistocene) age. The main mollusk bioevents comprise the last occurrences of Chama placentina, Glycymeris inflata and Aequipecten scabrella in the basal part of the section, the first occurrence of Arctica islandica at 103.70 m from the base and the last occurrence of the gastropods Nassarius prysmaticus and Turritella tricarinata in the upper part of the section. The identified nannofossil zones range from Zone CNPL7 to the lower part of Zone CNPL9.
The palaeoclimatic significance of the Arda biota has been investigated at two different levels: first the analyses of its mollusk fauna and of its pollen content and then a more detailed research involving geochemistry and sclerochemistry of the bivalve shells.
The occurrence of boreal guests, such as Arctica islandica, Pseudamussium septemradiatum and possibly also Mytilus edulis, from 103 m upward, suggests that a climatic change occurred in the Arda marine succession with a shift to colder seawater temperatures. However, aside from the occurrence of boreal guests, the fauna is mainly dominated by eurythermal species having a cosmopolitan distribution, lacking the strictly arctic or tropical ones. Preliminary data from pollen analysis suggest that a climatic change occurs also in the Arda flora; in particular pollen data suggest the presence between 91 and 110 m of an interval characterized by a high seasonality with taxa preferring colder winters, which is followed by an interval in which taxa preferring milder winters are dominant, thus possibly indicating a glacial/interglacial shift.
To examine in detail the palaeoclimatic evolution of the Arda section, 249 fossil specimens from 141 stratigraphic beds belonging to Glycymeris glycymeris, Glycymeris insubrica, Glycymeris inflata, Glycymeris sp., Aequipecten opercularis, Aequipecten scabrella and Arctica islandica have been analyzed for carbon and oxygen isotope analyses. In addition, ten shells collected in seven distinct stratigraphic beds and belonging to the species Glycymeris insubrica, Glycymeris inflata and Arctica islandica have been analyzed for sclerochemistry in order to examine if and how seasonality varies along the section.
The geochemical and isotopic composition of calcareous skeletons has long been recognized to record past and present environmental conditions. As bivalves record in their calcium carbonate shells the primary seawater isotope composition, with little or no vital effect, the geochemical signature registered in bivalve shells can be thus used as an archive of global change in seawater composition and temperature. The equation used here to obtain palaeotemperatures from oxygen data from Arctica and Glycymeris aragonitic shells is that of Grossman & Ku (1986); the equation for the Aequipecten calcitic shell is that of O'Neil et al. (1969).
As diagenetic processes may alter fossil bivalve shell geochemical composition, I have undertaken four different screening tests (SEM microstructural examination, Cathodoluminescence, X-ray Powder Diffraction and Feigl’s solution) to check if the shell is pristine, and thus effectively suitable to a correct interpretation of isotope analyses. All these tests indicate that bivalve shells of the Arda River marine succession are very well preserved.
The oxygen isotope bulk data obtained from species of the genera Glycymeris, Aequipecten and Arctica have been used to construct a mean oxygen curve, which was then compared to the Mediterranean and Atlantic stacks, in order to make a correlation with the Marine Isotope Stages.
The main outcome of the bulk isotope analysis is that seasurface palaeotemperatures seem to remain rather constant during the deposition of the Arda marine succession and they do not suggest any significant cooling during Gelasian and Calabrian (Early Pleistocene). Sclerochemical analyses give further detail on the palaeoclimatic evolution of the section. In the stratigraphic interval from the base of the section to 103.70 m, bivalve shells record a low seasonality (5.7°C), suggesting that probably the Northern Hemisphere Glaciation exerted a minor control on the Mediterranean climate in this moment. In the interval between 103.70 m and 110 m, which corresponds to the arrival of the boreal guests, the shells record a high seasonality (16.0° of winter-summer difference) and low palaeotemperatures (winter palaeotemperatures of 0.8-1.7°C), as also supported by other proxies in the Mediterranean region. This may be due to cold currents from the North Atlantic entering the Mediterranean Sea with annual-interannual variability causing a progressively cooling of the seawater, particularly during winter; a cooling threshold was reached and crossed, acting as a trigger factor for the arrival and successful recruitment of the boreal guests. This cold episode is followed by an interval (110 m - top of the section) in which bivalve shells record a clear increase in seasonality toward the top of the Arda section (from 9.2 to 19.0°C of seasonal change). However, they registered higher palaeotemperatures compared to the interval of the arrival of boreal guests described above, and in the case of Arctica islandica also a lower, but still increasing upward, seasonality. At this time the Apennine chain was rapidly uplifting, as suggested by a change in the pollen content, increase in sedimentation rates and terrigenous input, causing a perturbation in the Mediterranean climate system. The Apennine uplift may have acted as a shield for cold and dry continental air masses coming from the Northern Hemisphere, locally obscuring the impact of the climatic variations linked to the Northern Hemisphere Glaciation dynamics.
The complex palaeoclimatic and palaeoenvironmental evolution of the Arda River succession points out that the Mediterranean region was affected by the Northern Hemisphere Glaciation dynamics, which exerted a strong control on the Mediterranean climate during most of the Early Pleistocene. The significantly cold event recorded at 103.70 m, indicates that the arrival of the boreal guests in the Mediterranean Sea was accompanied by a high seasonality and by particularly cold winters, as suggested by both sclerochemistry and pollen analyses.
Seasonality increases approaching the Middle Pleistocene Transition and the beginning of the continental glaciations in the Northern Hemisphere, representing thus a clear signal of climatic deterioration in the Mediterranean Sea during the Early Pleistocene. However, no cooling trend in the average seawater temperatures has been observed both in sclerochemical and bulk shells oxygen isotope data during this interval.
The changes in seasonality may well have amplified and propagated the signal of climate change toward the Middle Pleistocene Transition, representing thus a very important component in abrupt climate switches
Intersymbol Interference in Binary and Quaternary PSK and DCPSK Systems - Part II
No full textThe expressions obtained in Part I, giving in general conditions the error rate in binary and quaternary, coherent and differentially coherent phase-shift-keyed systems due to thermal noise in presence of intersymbol interference, are applied to the case of a real channel with a gaussian amplitude characteristic. The effects of a bandwidth limitation and of a nonconstant group delay in the four modulation systems above are evaluated and the results are reported in diagrams
Performance Analysis of Quaternary CP-FSK Systems with Modulation Pulse Shaping and Limiter-Discriminator Detection
No full textThe multilevel CP-FSK transmission systems with limiter-discriminator detection in which the modulation pulses are rectangular with a length τ admitting of any value not higher than symbol time T, are here considered. First, a procedure is presented in order to determine the optimum values of the modulation levels and of the decision thresholds, which are strictly correlated with τ. Next, the optimization of four-level CP-FSK systems with reference to different types of transmission channels is carried out by determining for each the value of τ, the modulation and threshold levels and the equivalent noise bandwidth, which guarantee the error probability Pe = 10-6 with the minimum transmitting power. For comparison, the same evaluation is made for the conventional system, for which τ = T and the distribution of the modulation and threshold levels is uniform. The results obtained, experimentally verified, show that: -the optimum choice of the modulation and thresholds levels is important even if τ = T;-the optimum system performance takes place, in all the considered cases, for non-critical value τ = 0.2 T, for which the improvement compared to the conventional case is always very great.The experiments, carried out at the Telettra laboratories, have also shown that the considered modulation method allows the use of highly non linear power amplifiers in the transmitter, without any practical effect on system performance
Bivalve shells as high resolution biomineral archives of early Pleistocene seasonality
No full textBivalves are among the best tools for palaeoclimatic and palaeoenvironmental reconstructions because they are known to precipitate their shells in isotopic equilibrium with the seawater in which they live. Also, the analysis of their shell microstructure shows that these organisms modify their shell fabric in response to environmental variations forming growth lines. However, diagenetic processes may alter fossil bivalve shell microstructure and shell isotopic composition; for this reason it is important to perform screening tests to check if the shell is pristine and thus confidently identify biogenic calcium carbonate as a reliable proxy of primary seawater chemistry. Here, we performed a detailed study of the microstructure of ten aragonite bivalves shells belonging to Glycymeris insubrica,
Glycymeris inflata and Arctica islandica, collected from the lower Pleistocene Arda River marine succession (northern Italy), to check their preservation for subsequent sclerochemical stable isotope analyses (18O, 13C). To assess shell preservation 4 different screening techniques were used: Scanning Electron Microscopy (SEM), Catholuminescence (CL), X-Ray Powder Diffraction (XRD) and Feigl’s solution. Shells of species of Glycymeris, under SEM, show an outer crossed lamellar layer, an inner irregular and cone complex crossed lamellar layer and an irregular simple prismatic pallial myostracum; all the layers are penetrated by cylindrical tubules. Arctica islandica has an outer homogenous/crossed lamellar/crossed acicular layer, an inner fine complex crossed lamellar layer and an irregular simple prismatic pallial myostracum. The comparative analysis with recent fabrics shows a consistent pattern for species of both Glycymeris and Arctica genera, the recent and fossils shell microstructures being almost identical. Analyses at CL, XRD and Feigl’s solution support that these fossil species have a non luminescent shells composed of pure aragonite. The excellent preservation and the distinct growth lines of the Arda taxa makes them a powerful archive to study, through sclerochemistry, the change in seasonality during the early Pleistocene, a time interval characterized by several climatic oscillations linked to glacial/interglacial cycles, in the Mediterranean area
Shell microstructure and mineralogy of the mollusc species Anadara uropigimelana (Bory de Saint-Vincent, 1827), Tivela stefaninii (Nardini, 1933) and Oliva bulbosa (Röding, 1798)
No full textMollusc shells are composite structures build of crystals of calcium carbonate (calcite, aragonite, or both) and biopolymers, arranged in a great variety of microstructures. Shell formation is affected by environmental and physiological factors and differences in shell microstructures are believed to be of phylogenetic and adaptive biomechanical significance. Here, we performed a detailed characterization and illustration, through SEM and XRD analyses, of the shell microstructure and mineralogy of specimens belonging to the bivalves Anadara uropigimelana (Bory de Saint-Vincent, 1827) and Tivela stefaninii (Nardini, 1933), and to the gastropod Oliva bulbosa (Röding, 1798), collected in the Upper Holocene (Meghalayan) HAS1 settlement and in a shell midden in the Khor Rori Archaeological Park (Dhofar, Oman). Specimens of Anadara uropigimelana show an aragonitic shell with an outer crossed lamellar layer, an inner complex crossed lamellar layer and an irregular simple prismatic pallial myostracum; furthermore, periodic bands of dendritic nondenticular composite prisms occur in the outer part of the outer layer, reflecting seasonal changes in water temperatures. Specimens of Tivela stefaninii show an aragonitic shell with an outer composite prismatic layer, a middle crossed lamellar layer and an inner complex crossed lamellar layer, whereas specimens of Oliva bulbosa have an aragonitic shell characterised by an irregular alternation of crossed lamellar layers. With this investigation we aim to provide novel data on the shell fabric of these poorly known mollusc species, increasing the availability of useful characters for phylogenetic, evolutionary, palaeoenvironmental and crystallographic studies
Selecting the best brachiopod biomineral archive of the Wuchiapingian climate change
Full text linkThe Late Permian was a time interval characterized by extreme environmental perturbations, culminating with the Siberian Traps-related gas emissions and the subsequent global warming and ocean acidification, which produced the most severe mass extinction of the Phanerozoic (Dal Corso et al., 2022). Evidence of these perturbations are recorded in fossil archives, as pristine brachiopod shells (Garbelli et al., 2017). Here, we show shell microstructural variations and stable isotopes profiles recorded by specimens of Araxilevis intermedius (Abich, 1878), a large-sized and thick-shelled brachiopod species abundant in the Upper Permian of Iran. Nine specimens of A. intermedius were selected from several Wuchiapingian beds of the Hambast Formation in the Abadeh Section and of the Julfa Formation in the Ali Bashi Mountains Main Valley section (Iran), following the correlation by Viaretti et al. (2021). Prior to the isotope analysis different screening tests were performed on the brachiopod shells from both sections: Scanning Electron Microscope microstructural analysis, cathodoluminescence (CL) and trace elements analyses. Specimens of A. intermedius are characterized by a three-layered shell, comprising a secondary layer of cross-bladed laminae and a tertiary columnar layer; the primary layer is not preserved. The specimens from the Hambast Formation of Abadeh show a partially altered shell, whereas those from the Julfa Formation of the Ali Bashi Mountains show a well-preserved microstructure, despite CL analysis indicating that all the specimens were non-luminescent, both the microstructurally well-preserved and the altered ones. After having checked the shell preservation, 12 to 29 powder samples were collected from the longitudinal shell section of each pristine specimen of A. intermedius using a sclerochronological approach. This method allowed to investigate the seasonal environmental changes recorded by the brachiopod shells of A. intermedius from Iran and to test if this species of the Class Strophomenata, abundant in this time interval also outside Iran, can be considered a good archive for paleoclimatic and paleoenvironmental reconstructions in the Late Permian
Some Researches on PSK Relay Systems
No full textIn the last few years the authors of this paper have focused their attention on PSK radio relay systems with the purpose of giving design criteria for them. On the results achieved has been based the design of a 13 GHz PCM system. The essentials of these investigation are here reported and, as an application example, a contribution to the optimization of PSK radio links, is given
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