Phosphogenesis at the base of the Bonarelli Level from the Calabianca stratigraphic section (northwestern Sicily) was investigated using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, and stable-isotope mass spectrometry. The anoxic event onset is marked by high P2O5tot concentrations related to the presence of authigenic carbonate-fluorapatite. This mineral is poorly crystallized and occurs in three different forms: (i) fish debris locally showing evidence of re-crystallization,(ii) phosphatically cemented layers, and (iii) dense aggregates of microcrystals. Petrographic features are indicative of a microbial genesis for the carbonate-fluorapatite. Conversely, this evidence islacking in phosphate from the upper part of the section, where lower P2O5tot concentrations are linked to the presence of sparse unaltered hydroxy-apatite fish debris. Distribution patterns of rare earth elements (REE) throughout the Calabianca section, the Ce anomaly and V/(V þ Ni) values suggest fluctuating redox conditions during phosphogenesis. These conditions triggered the iron redox cycle that operated as phosphate sink-switching mechanism. Depth profiles of P mass accumulation rate and C-isotope compositions highlight the decoupling of P and C-org cycles, whose global implication is confirmed by correlation with the same proxies from other well known stratigraphic sections, which embody sedimentary expressions of the Oceanic Anoxic Event 2 (OAE2).
|Numero di pagine||12|
|Stato di pubblicazione||Published - 2010|
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