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| Titel |
Experimental approaches to marine and meteoric dissolution-to-repreciptiation cycles of fine-grained marine carbonate sediments |
| VerfasserIn |
Adrian Immenhauser, Dieter Buhl, Sylvia Riechelmann, Ola Kwiecien, Stephen Lokier, Rolf Neuser |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250124864
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| Publikation (Nr.) |
 EGU/EGU2016-4361.pdf |
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| Zusammenfassung |
| Fine-grained carbonate (carbonate ooze), or microcrystalline carbonate (micrite), its lithified counterpart, forms a main constituent of limestones throughout much of Earth's history. Fine-grained carbonates are deposited below the permanent fair-weather wave base in neritic lagoonal environments or below the storm-wave base in basinal settings. The origin of components forming these fine-grained carbonates often remains poorly understood and represents a major challenge in carbonate sedimentology, particularly when these materials are used as carbonate archives (bulk micrite geochemistry). Here we present a novel experimental approach exposing natural, fine-grained carbonate sediments to dissolution-reprecipitation cycles under non-sterile conditions that mimick earth-surface conditions. In a first stage, the experiment simulated subaerial exposure of an ooid (aragonite) shoal and leaching and carbonate dissolution under meteoric phreatic conditions. In a second stage, CO2 was added to the experimental fluid (natural rainwater) representing soil-zone activity. In a third stage, partly dissolved (micro-karstified) sediments were exposed to marine phreatic conditions simulating renewed flooding of the shoal carbonates. During the third stage, precipitation was induced by degassing the CO2 in the fluid with N2. Degassing induced nucleation and growth of a diagenetic inorganic aragonite (and subordinate calcite) phase upon the surface of carbonate particles. The outcome of these first experiments is promising. The CO2 concentration of the fluid and the air are low under atmospheric conditions and increase as expected due to adding CO2 to the experiment resulting in a lower pH. Carbonate dissolution increases conductivity, alkalinity, and calcium concentration reaching a plateau at the end of the first experimental phase. Small surficial damages to ooids represent zones of weakness and form the preferred sites of dissolution leading to a deepening and widening of these features and a general smoothening of ooidal surfaces. Following N2 addition, CO2 concentrations of water and air, conductivity, alkalinity, and calcium concentrations decrease, whilst the pH increased. Scanning electron microscope images reveal the re-precipitation of dissolved calcium carbonate in the form of newly formed diagenetic needle-like aragonite crystal on the surface of carbonate particles. Oxygen isotope analyses of altered sediments document a ca 0.3 permil shift to lower bulk carbonate ratios whereas carbon isotope data are lower by up to 1 permil in the leached and cemented samples. |
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