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  • 1
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    Inside a sediment‐stressed Middle Devonian carpet reef: Cave exposes details of three‐dimensional facies architecture and palaeoecology (2023)
    Details
    Publication Date: 2024-05-22
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉ABSTRACT〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉During the Middle Devonian, reef growth reached an acme, and corals and stromatoporoids colonized depositional niches commonly considered unfavourable for reefal organisms. This paper documents the detailed facies architecture and palaeoecology of a stratigraphically thin (〈italic toggle="no"〉ca〈/italic〉 12 m, ‘carpet reef’), lower Givetian reefal body exposed along the walls and ceilings of the labyrinthine passages in the Klutert Cave in western Germany. The cave exposures (〈italic toggle="no"〉ca〈/italic〉 26 000 m〈sup〉2〈/sup〉 of rock surface) and data from short cores, neighbouring caves and outcrops document the growth and demise of an autoparabiostrome. The reef forms part of a parasequence with a lower carbonate and an upper clastic unit, bounded by flooding surfaces. Despite the comparatively small study area (〈italic toggle="no"〉ca〈/italic〉 1 km〈sup〉2〈/sup〉), the exceptional exposure quality reveals facies changes over relatively short distances both vertical and lateral. The sedimentary matrix of the reefal build‐up contains between 20 to 95 wt.‐% of clay and quartz of silt to sand fraction. Based on this observation, the corals and stromatoporoids thrived in murky waters and under sediment‐stressed conditions. Stromatoporoids, for example, display irregular ragged flanks, a feature that is in agreement with a sediment‐stressed environment. No evidence of reduced growth rates, decreased calcification rates, or lower numbers of species is found. In fact, coral diversity and density are highest within one of the two biostromal units that show peak clastic matrix values, indicating a remarkable adaptation of reef builders to sediment‐stressed conditions. The initial settlement of rugose phaceloid corals took place on a mixed clastic–carbonate substrate (the basal flooding surface). Up‐section, a succession of coral–stromatoporoids is present that is here described in great detail. Reef collapse occurred when much of the accommodation space was filled, and argillaceous sediments suffocated stromatoporoids and corals in a protected, low‐energy environment.〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:560 ; Biostrome ; corals ; Givetian ; mixed clastic–carbonate system ; reef model ; stromatoporoids
    Language: English
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  • 2
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    Anaerobic microbial activity affects earliest diagenetic pathways of bivalve shells (2018)
    International Association of Sedimentologists
    In:  Sedimentology, 65 (4). pp. 1390-1411.
    Details
    Publication Date: 2021-02-08
    Description: The earliest diagenetic post-mortem exposure of biogenic carbonates at the sea floor and in the uppermost sediment column results in the colonization of hard-part surfaces by bacterial communities. Some of the metabolic redox processes related to these communities have the potential to alter carbonate shell properties, and hence affect earliest diagenetic pathways with significant consequences for archive data. During a three-month in vitro study, shell subsamples of the ocean quahog Arctica islandica (Linnaeus, 1767) were incubated in natural anoxic sediment slurries and bacterial culture medium of the heterotrophic Shewanella sediminisHAW-EB3. Bulk analyses of the liquid media from the Shewanella sediminis incubation revealed an over ten-fold increase in total alkalinity, dissolved inorganic carbon and ΩAragonite, and the alteration of the Mg/Ca, Mg/Sr and Sr/Ca ratios relative to control incubations without cultures. Ion ratios were most affected in the incubation with anoxic sediment, depicting a 25% decrease in Mg/Ca relative to the control. Shell sample surfaces that were exposed to both incubations displayed visible surface dissolution features, and an 8 wt% loss in calcium content. No such alteration features were detected in control shells. Apparently, alteration of shell carbonate properties was induced by microbially driven decomposition of shell intercrystalline organic constituents and subsequent opening of pathways for pore fluid-crystal exchange. This study illustrates the potential influence of benthic bacterial metabolism on biogenic carbonate archives during the initial stages of diagenetic alteration within a relatively short experimental duration of only three months. These results suggest that foremost the biological effect of bacterial cation adsorption on divalent cation ratios has the potential to complicate proxy interpretation. Results shown here highlight the necessity to consider bacterial metabolic activities in marine sediments for the interpretation of palaeo-environmental proxies from shell carbonate archives.
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  • 3
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    Climate and cave control on Pleistocene/Holocene calcite-to-aragonite transitions in speleothems from Morocco: Elemental and isotopic evidence (2012)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 92 . pp. 23-47.
    Details
    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.
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  • 4
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    Impact of diagenetic alteration on brachiopod shell magnesium isotope (δ26Mg) signatures: Experimental versus field data (2016)
    Elsevier
    In:  Chemical Geology, 440 . pp. 191-206.
    Details
    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
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  • 5
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    Lithofacies and depositional processes on a high, steep-margined Carboniferous (Bashkirian-Moscovian) carbonate platform slope, Sierra del Cuera, NW Spain (2004)
    Elsevier
    In:  Sedimentary Geology, 166 . pp. 145-156.
    Details
    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.
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  • 6
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    Neogene and Quaternary reef terraces of Mangaia, Rarotonga and Aitutaki (southern Cook Islands, South Pacific) revisited: gauges of sea-level change and island uplift? (2022)
    Wiley
    Details
    Publication Date: 2022-12-19
    Description: New sedimentological data of facies and diagenesis as well as chronological data including strontium (87Sr/86Sr)-isotope ratios and uranium (U)-series dating, radiocarbon (14C) accelerator mass spectrometry (AMS) dating and biostratigraphy from elevated reef terraces (makatea) in the southern Cook Islands of Mangaia, Rarotonga and Aitutaki contribute to controversial discussions regarding age and sea-level relationships of these occurrences during the Neogene and Quaternary. The oldest limestones of the uplifted makatea island of Mangaia include reef-related facies which are mid-Miocene in age, based on new Sr-isotope and biostratigraphical data. In between these older deposits and the lowest coastal reef terrace of marine isotope stage (MIS) 5e, various older Pleistocene reef-related facies were identified. Based on Sr-isotope ratios, these were deposited during earlier Pleistocene highstands (as old as 2.28 Ma). Rare reef terraces on Rarotonga belong to the Plio-Pleistocene and the late Miocene, according to 87Sr/86Sr ratios. The late Miocene age is enigmatic as it exceeds the age of subaerially exposed volcanic rocks of Rarotonga island. The fossil reef could have formed on an older submarine volcanic high that was later displaced by younger volcanism to its present position, or the Sr-age could be too old due to diagenetic resetting. The Plio-Pleistocene Rarotonga reef terraces are overlain irregularly by Holocene reef deposits that are interpreted as storm rubble. Reef terraces on Aitutaki represent evidence of a higher-than-present (up to 1 m) sea-level during the late Holocene, based on 14C AMS age data. They are very similar to elevated late Holocene reefs of adjacent French Polynesia with regard to composition, elevation and age.
    Type: Book chapter , NonPeerReviewed
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  • 7
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    Reorganization of the North Atlantic Oscillation during early Holocene deglaciation (2016)
    Nature Research
    Details
    Publication Date: 2023-08-01
    Description: The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia1. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic, freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.
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  • 8
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    Microbial activity affects sulphur in biogenic aragonite (2021)
    Wiley
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    Publication Date: 2024-02-07
    Description: Carbonates that exhibit obvious diagenetic alteration are usually excluded as archives in palaeoenvironmental studies. However, the potential impact of microbial alteration during early diagenesis is still poorly explored. To investigate the sensitivity of sulphur concentration, distribution, oxidation state and isotopic composition in marine aragonite to microbial alteration, Arctica islandica bivalves and Porites sp. corals were experimentally exposed to anaerobic microbial activity. The anoxic incubation media included a benthic bacterial strain Shewanella sediminis and a natural anoxic sediment slurry with a natural microbial community of unknown species. Combined fluorescence microscopy and synchrotron‐based analysis of the sulphur distribution and oxidation state enabled a comparison of organic matter and sulphur content in the two materials. Results revealed a higher proportion of reduced sulphur species and locally stronger fluorescence within the pristine bivalve shell compared to the pristine coral skeleton. Within the pristine bivalve specimen, reduced sulphur was enriched in layers along the inner shell margin. After incubation in the anoxic sediment slurry, this region revealed rust‐brown staining and a patchy S2‐ distribution pattern rather than S2‐‐layers. Another effect on sulphur distribution was rust‐brown coloured fibres along one growth line, revealing a locally higher proportion of sulphur. The δ34S value of carbonate‐associated sulphate remained largely unaffected by both incubation media, but a lower δ34S value of water‐soluble sulphate reflected the degradation of insoluble organic matter by microbes in both experiments. No significant alteration was detected in the coral samples exposed to microbial alteration. The data clearly identified a distinct sensitivity of organically bound sulphur in biogenic aragonite to microbial alteration even when “traditional” geochemical proxies such as δ18OCARB or δ13CCARB in the carbonate didn’t show any effect. Differences in the intensity of microbial alteration documented are likely due to inherent variations in the concentration and nature of original organic compositions in the samples.
    Type: Article , PeerReviewed
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  • 9
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    Testing the preservation potential of early diagenetic dolomites as geochemical archives (2021)
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    Publication Date: 2021-07-05
    Description: Early marine diagenetic dolomite is a rather thermodynamically‐stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post‐depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U‐Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike‐slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric‐preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto‐Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high‐grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late‐stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically‐complex dolomite archives must be considered with caution.
    Description: DFG http://dx.doi.org/10.13039/501100001659
    Keywords: 551.9 ; Diagenesis ; dolomite ; dolomitization ; fluid inclusions ; geochemistry ; petrography ; tectonic overprint ; U‐Pb dating
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  • 10
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    Evaluating the role of coastal hypoxia on the transient expansion of microencruster intervals during the early Aptian (2021)
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    Publication Date: 2021-07-01
    Description: Worldwide, a growing number of modern coastal marine ecosystems are increasingly exposed to suboxic‐ or even anoxic conditions. Low seawater oxygen levels trigger significant ecosystem changes and may result in mass mortality of oxygen‐sensitive biota. The applicability of observations from recent (anthropogenically influenced) suboxic coastal settings to fossil anoxic shallow‐marine environments is, however, as yet poorly explored. The test case documented here are upper Barremian to lower Aptian strata in the Lusitanian Basin (Ericeira section, Portugal). These are characterized by the transient demise of rudist–coral communities and the rapid establishment of microencruster facies in the vacant ecological niches. The hypothesis is tested that the temporal expansion of the microencrusting organism Lithocodium aggregatum took place in response to platform‐top seawater oxygen depletion. We critically discuss the outcome of a multi‐proxy palaeoseawater redox approach (e.g. Rare Earth Elements (REEs), U isotopes and palaeoecology) and put the robustness of the proxies applied here to the test. This is done by considering issues with these methods in general but also emphasizing the significance of terrigenous contamination and fractionation effects. Data shown here document that evidence for coastal seawater oxygen depletion in the prelude of Oceanic Anoxic Event (OAE) 1a is lacking, and hence, anoxia was not the driving mechanism for the demise of rudist–coral ecosystems in the proto‐North Atlantic platform setting studied here. In contrast, well‐oxygenated early Aptian platform‐top water masses are proposed for this site. Geologically short (decades to millennia) fluctuations in seawater oxygen levels cannot be excluded, however. But even if these took place, they offer no explanation for the Kyr to Myr‐scale patterns discussed here. The present paper is relevant as it sheds light on the complexity of mechanisms that drive punctuated Early Cretaceous coral–rudist ecosystem turnover, and assess strengths and weaknesses of redox proxies applied to ancient shallow‐marine platform carbonates.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: 560.45 ; Anoxia ; cerium anomalies ; Cretaceous ; Oceanic Anoxic Event 1a ; redox proxies ; uranium isotopes
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