Carbon Accumulation and Organic Matter Preservation in the Pesciara Section (Bolca, Italy).

Exceptional fossilization in Fossil-Lagerstätten has been often explained with anoxic conditions allowing the preservation of organic matter. A long-lasting debate involves the genesis of sediments rich in organic matter, such as black shales and sapropels. Two main mechanisms have been proposed: 1) an increase in primary productivity; 2) an improved organic matter preservation in stagnant/anoxic basins. The geochemical investigation of the Early Eocene Fossil-Lagerstätte of Bolca (Italy) could help in the attempt to solve this problem. In fact, reversing the problem, if we guarantee taphonomic conditions of exceptional fossilization in anoxic settings, should we expect high contents of organic matter? Bolca is renowned worldwide for its extraordinarily preserved fish fauna. Most of the spectacular biota comes from fine-grained and evenly laminated limestones, up to 1 m thick. These “productive” layers are intercalated with storm-induced coarse-grained limestones rich in mollusks and foraminifera.Papazzoni & Trevisani (2006) recently dated the classic Pesciara Section of Bolca to the Early Eocene by means of Alveolina biozones and proposed a depositional model referring to a basin with restricted circulation and anoxic conditions on the bottom. To improve the model, some geochemical analyses were performed. The preliminary results are reported below.Total organic carbon concentration (TOC) has been measured for 13 samples from either the laminated (LL) or the coarse-grained limestones (CL). The organic matter content is usually lower than 0.5%, ranging between 0.16% and 0.49%, with 0.33% as mean value. An isolated maximum TOC content of 8.6% has been measured only in one sample from the lowermost fish-level, probably reflecting an episode of enhanced concentration of organic matter. The LL have TOC values only slightly higher than the intercalated CL. The TOC values measured in the Pesciara Section are comparable with those reported from other Fossil-Lagerstätten, such as Solnhofen (with TOC 0.2-0.9%; Hückel, 1974). These data demonstrate that exceptional fossil preservation and anoxic conditions are not necessarily coupled with high organic matter contents. Anoxia per se does not appear, therefore, to have had any direct effect on carbon accumulation or to have guaranteed high organic matter contents in the Fossil-Lagerstätte of Bolca.The δ13C isotopic composition of the Pesciara samples ranges between -23.98‰ and -26.66‰. Values of CL (mean -26.12‰) are significantly lighter than those of LL (mean -25.22‰). Organic matter C/N ratios vary between 33 and 48, with a mean value of 42.8 for LL and a mean value of 37.2 for CL. The general lack of variation in δ13C and C/N values between LL and CL suggests no significant changes in the source of the organic matter preserved in the Pesciara Section. Furthermore, these data would indicate a terrestrial origin for the organic carbon. References: Hückel, U., 1974, Vergleich des Mineralbestandes der Plattenkalke Solnhofens und des Libanon mit anderen Kalken. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 145: 153-182.Papazzoni, C. A. & Trevisani, E., 2006, Facies analysis, palaeoenvironmental reconstruction, and biostratigraphy of the “Pesciara di Bolca” (Verona, northern Italy): An early Eocene Fossil-Lagerstätte. Palaeogeography, Palaeoclimatology, Palaeoecology, 242: 21-35.