Zaffani, Mariachiara (2013) Analisi geochimiche su materia organica (Toc e Î´15N) ed indagini biostratigrafiche dell'intervallo Norico-Retico della sezione di Pignola-Abriola, Bacino di Lagonegro, Potenza. [Magistrali biennali]
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The stratigraphic succession of Pignola-Abriola (Southern Apennines, Italy), belonging to the Lagonegro Basin (Western Tethys), was investigated by means of biostratigraphical and geochemical analyses. On the base of conodont and radiolarian associations, this section is referred to the upper Norian-Rhaetian interval. The new and integrated conodont biostratigraphy improves the previous one: Misikella hernsteini has been found in a lower position, in sample PR 13 (24,90 m), Misikella buseri has been found in sample PIG 40 (56,60 m) and different specimens of Zieglericonus genus have been found in the uppermost part of the section (from sample GNC 100, 55,93 m). Two new species ascribable to Misikella genus have been found in the uppermost part of the section and are named Misikella sp. A and Misikella sp. B. Specimens belonging to Misikella sp. A show expanded spear-shaped to drop-shaped basal cavity, posterior cusp, at least five-denticle blade made up of compressed, slightly inclined, equal-size small denticles. They first occur in sample GNC 100 (55,93 m). Forms here ascribed to Misikella sp. B show strongly expanded triangular to heart-shaped basal cavity, posterior cusp, very deep V-shaped incision at the posterior end of the blade, four-denticle blade made up of compressed, slightly inclined, equal-size denticles. They first occur in sample PIG 38 (54,20 m). Radiolarians, tiny glass spheres and fangs have also been found along the studied section. Geochemical analyses on organic matter (i.e., TOC and δ15N) may indicate anoxic conditions in the Lagonegro Basin at the Norian-Rhaetian boundary. In fact, the TOC and δ15N profiles show the typical signature of an oceanic anoxic event, that is, for instance, relatively high TOC contents. In detail, these proxies are characterized by low values in the lower part of the section, followed by a rising to a maximum, which ended with an interval characterized by lower values but still distinctly above the background conditions. This distinctive trend might be interpreted as an oxygen minimum zone, which leads to a slowdown of the organic matter oxidation, eventually increasing the burial rate of the organic matter. Within the oxygen minimum zone, nitrogen-fixing cyanobacteria prevail over the planktonic population, leading to the denitrification of marine waters. This mechanism explains the variation observed in the nitrogen isotope profile. These two profiles, TOC and δ15N, have been compared with δ13C data available from the Pignola-Abriola section, showing a stepped negative excursion, here tentatively attributed to a massive influx of isotopically light carbon into the ocean-atmosphere system, followed by a positive excursion that may be related to an enhanced burial of marine organic matter (likely on a global scale). However, the extent to which the geochemistry of this event reflected the composition of the global ocean is not still completely understood. For this reason, analysis and comparison with other sections from different basins are necessary.
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