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  • 1
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Hiati of various duration in carbonates are commonly expressed as discontinuity surfaces. The understanding of processes that form and affect these surfaces leads to an improved sequence-stratigraphic interpretation, a reliable outcrop correlation, and better models for reservoir compartmentalization. Various intraformational discontinuities were analysed and interpreted in a well-exposed study window, 2·5 km in lateral length and 60 m in height comprising the Barremian-Aptian Qishn Formation (Haushi-Huqf area, central Oman). This study focuses on the lateral extent and morphology of the surfaces, the petrography of the underlying rocks, and the facies changes and geochemical trends across these discontinuities. Furthermore, the lateral variability of discontinuity surfaces was documented. Three genetic types of discontinuities are differentiated: (i) erosion surfaces; (ii) omission surfaces (hard- and firmgrounds); and (iii) composite surfaces with evidence for both subaerial exposure and submarine boring. Field observations, combined with petrographic and geochemical data, suggest that 17 surfaces are laterally extensive for at least 20 km and record relative sea-level fluctuations of regional scale. In contrast, a large number of laterally limited surfaces (〈1 km) are related to locally active processes such as waves and current erosion. The lateral variability along extensive surfaces is the result of the depositional environment below the discontinuity, the sea-floor topography, waves and currents and differential erosion. The most pronounced lateral variability is present along six laterally extensive composite surfaces that record terrestrial exposure and subsequent flooding of a tidal flat environment. This variability is caused by spatial variability in the tidal flat environment, meteoric alteration and differential erosion. This study emphasizes the spatial and temporal complexity of processes that form and modify discontinuity surfaces. This variability must be kept in mind when interpretations and correlations are based on one-dimensional sections or cores.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Two regionally significant microbial-foraminiferal episodes (∼150 kyr each) occur within the Early Aptian shallow marine platform in Oman and throughout eastern Arabia. The stratigraphically lower of these two intervals is characterized by isolated or coalescent domes that share similarities with modern, open-marine stromatolites from the Exuma Cays, Bahamas. The upper interval is predominantly built by a problematic Lithocodium/Bacinella consortium in buildup and massive boundstone facies. Based on high-resolution chemostratigraphy, these shoalwater intervals are coeval with oceanic anoxic event 1a (OAE1a; Livello Selli). Field evidence demonstrates that the buildup episodes alternate with stratigraphic intervals dominated by rudist bivalves. This biotic pattern is also recognized in other coeval Tethyan sections and is perhaps a characteristic shoalwater expression of the OAE1a. The short-lived regional expansions of this microbial-foraminiferal out-of-balance facies cannot be explained by local environmental factors (salinity and oxygen level) alone and the buildup consortia do not occupy stressed refugia in the absence of grazing metazoans. Judging from recent analogues, the main fossil groups, i.e. microbial assemblages, macroalgae, larger sessile foraminifera, and rudist bivalves, all favoured elevated trophic levels but with different tolerance limits. The implication of this is that the influence of palaeofertility events, possibly related to OAE1a, on carbonate platform community structures must be investigated. The observations made in these coastal sections are a significant first step for the improved understanding of the Early Aptian period of biotic, oceanic and climatic change.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford UK : Blackwell Science Ltd
    Sedimentology 48 (2001), S. 0 
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A high-energy Aptian–Albian platform margin in northern Oman fronted onto an open oceanic basin, making the area a valuable analogue for coeval guyot margins. Most similar aged carbonate margins described in the literature faced either intracratonic or minor oceanic basins. The studied margin is characterized by a stabilized outer rim, which, although it did not rise discernibly above the adjacent lagoonal deposits, flanked a steep upper slope (32–40°) basinwards with a relief of at least 30 m. Two main facies provided the rigidity of the outer margin: Lithocodium boundstones that constituted up to 50% of the rock volume; and marine fibrous cements that occluded up to 35% of primary pore space. In contrast, coral–rudist patches and other shelly sessile benthos were distributed irregularly, and the rudist bioherms of the outer margin were often disrupted, with shells being transported and redeposited. The inner margin is characterized by wedge-shaped storm layers that radiate from the platform top lagoonwards, where they interdigitate with carbonate sands and small rudist bioherms. Polygenetic discontinuity surfaces that bear evidence of both marine hardground and subaerial exposure stages are prominent features of the margin. Throughout the latest Aptian to Middle Albian, the platform succession recorded some 30 relative sea-level falls, of which seven reached amplitudes of many tens of metres. These seven high-amplitude falls in sea level are recorded across the entire south-eastern portion of the Arabian craton and are probably of eustatic origin.
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  • 5
    Publication Date: 2018-06-05
    Description: Fossil carbonate skeletons of marine organisms are archives for understanding the development and evolution of palaeo-environments. However, the correct assessment of past environment dynamics is only possible when pristine skeletons and their biogenic characteristics are unequivocally distinguishable from diagenetically-alteredskeletal elements and non-biogenic features. In this study, we extend our work on diagenesis of biogenic aragonite (Casella et al. 2017) to the investigation of biogenic low-Mg calcite using brachiopod shells. We examined and compared microstructural characteristics inducedby laboratory-based alteration to structural features derived from diagenetic alteration in natural environments. We used four screening methods: cathodoluminescence (CL), cryogenic and conventional field emission-scanning electronmicroscopy (FE-SEM), atomic force microscopy (AFM) and electron backscatter diffraction (EBSD).We base our assessments of diagenetic alteration and overprint on measurements of, a) images of optical overprint signals, b) changes in calcite crystal orientation patterns, and c) crystal co-orientation statistics. According to the screening process, altered and overprinted samples define two groups. In Group 1 the entire shell is diagenetically overprinted, whereas in Group 2 the shell contains pristine as well as overprinted parts. In the case of Group 2 shells, alteration occurred either along the periphery of the shell including the primary layer or at the interior-facing surface of the fibrous/columnar layer. In addition, we observed an important mode of the overprinting process, namely the migration of diagenetic fluids through the endopunctae corroborated by mineral formation and overprinting in their immediate vicinity, while leaving shell parts between endopunctae in pristine condition. Luminescence (CL) and microstructural imaging (FE-SEM) screening give first-order observations of the degree of overprint as they cover macro-to micron scale alteration features. For a comprehensive assessment of diagenetic overprint these screening methods should be complemented by screening techniques such as EBSD and AFM. They visualise diagenetic changes at submicron and nanoscale levels depicting the replacement of pristine nanocomposite mesocrystal biocarbonate (NMB) by inorganic rhombohedral calcite (IRC). The integration of screening methods allows for the unequivocal identification of highly-detailed alteration features as well as an assessment of the degree of diagenetic alteration.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 6
    Publication Date: 2018-07-19
    Description: The depositional slope of the Sierra del Cuera, a Carboniferous (Bashkirian–Moscovian), high, steep-margined carbonate platform, provides excellent cross-sections of lithofacies zonations and associated stratal geometries. The steep (30–40°) and nearly planar upper slope is dominated by massive sheet-like layers of microbial, cement-rich boundstone, which alternate with intercalations of red-stained bryozoan cementstone with crinoids and brachiopods. The slightly gentler (20–26°) and concave-upward lower slope is characterized by clast-supported resedimented deposits. The upper slope extended from platform break to ca. 300 m water depth, whereas lower slope sediments were deposited in water depths up to 600–700 m, at which level the slope beds flatten to a few degrees (toe-of-slope) and interfinger with spiculitic and argillaceous basinal sediments. Between 250 and 450 m water depth, boundstones and breccias alternated in a transitional zone. The lower slope sediments include clast-supported breccias with radiaxial fibrous cement in interparticle space, mud- to clast-supported breccias with red-stained carbonate mud matrix and packstone to grainstone and rudstone beds. Most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived. A pervasive submarine cementation occurs along the upper two-thirds of the flank and this stabilized the slope. Slope deposition is interpreted as follows. During active boundstone accretion, microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe-of-slope. Rock falls and avalanches were generated whenever the shear strength of the substrate of loose (or partly lithified) sediment was exceeded. Upper-slope boundstone accretion and shedding, independent of the depth of light penetration, controlled most of the depositional processes on the slope. Cement-dominated intervals are considered to be related to early highstand (and/or flooding) phases. Relative sea-level fluctuations and/or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation. Whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is, as yet, unclear.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2012-11-06
    Description: Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island. Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2019-02-01
    Description: Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.
    Type: Article , PeerReviewed
    Format: text
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  • 9
    Publication Date: 2019-03-26
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 10
    Publication Date: 2019-01-22
    Description: The occurrence of aragonite in speleothems has commonly been related to high dripwater Mg/Ca ratios, because Mg is known to be a growth inhibitor for calcite. Laboratory aragonite precipitation experiments, however, suggested a more complex array of controlling factors. Here, we present data from Pleistocene to Holocene speleothems collected from both a dolostone and a limestone cave in northern Morocco. These stalagmites exhibit both lateral and stratigraphic calcite-to-aragonite transitions. Aragonite fabrics are well-preserved and represent primary features. In order to shed light on the factors that control alternating calcite and aragonite precipitation, elemental (Mg, Sr, Ba, U, P, Y, Pb, Al, Ti and Th) abundances were measured using LA-ICP-MS, and analysed with Principal Component Analysis. Samples were analyzed at 100–200 μm resolution across stratigraphic and lateral transitions. Carbon and oxygen isotope ratios were analysed at 100 μm resolution covering stratigraphic calcite-to-aragonite transitions. Results show that the precipitation of aragonite was driven by a decrease in effective rainfall, which enhanced prior calcite precipitation. Different geochemical patterns are observed between calcite and aragonite when comparing data from the Grotte de Piste and Grotte Prison de Chien. This may be explained by the increased dripwater Mg/Ca ratio and enhanced prior aragonite precipitation in the dolostone cave versus lower dripwater Mg/Ca ratio and prior calcite precipitation in the limestone cave. A full understanding for the presence of lateral calcite-to-aragonite transitions is not reached. Trace elemental analysis, however, does suggest that different crystallographic parameters (ionic radius, amount of crystal defect sites, adsorption potential) may have a direct effect on the incorporation of Sr, Mg, Ba, Al, Ti, Th and possibly Y and P.
    Type: Article , PeerReviewed
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