Analisi geochimiche (delta13Corg) attorno al limite Norico-Retico del Bacino Lombardo (Alpi Bergamasche)

The stratigraphic successions of Brumano-Italcementi and Malanotte Formation (Bergamasc Alps, northern Italy) were investigated with the aid of chemostratigraphycal analyses of δ13Corg. The successions belong to the Lombard Basin with a time-scale ranging between the Norian-Rhaetian (NRB) and the Triassic-Jurassic boundaries (TJB). The outcoming isotopic curve of δ13Corg shows a prominent isotopic excursion around the Norian-Rhaetian boundary (at the base of the Zu2 Member, minimum of δ13Corg = -27,86‰). In the early Rhaetian the isotopic curve returns to background values (δ13Corg = ca. -25,75‰), but in the middle Rhaetian a strong negative shift within the Zu3a Member (minimum of δ13Corg = -28,13‰) can be noted. Eventually, around the TJB another drop is showed, reaching the minimum value at the Hettangian base (within the base of the Malanotte Formation, δ13Corg = -28,75‰). These three major δ13Corg shifts were correlated with those of other Tethyan sections (Pignola-Abriola section, Lagonegro Basin, southern Italy) and extra- Tethyan sections (Queen Charlotte Islands, Bristish Columbia, Canada). On the basis of these bio-chemostratigraphyc correlations, the global occurrence of the three major δ13Corg shifts was demonstrated, therefore a common global cause can be thought. Negative δ13Corg global excursions are caused by a 12C enrichment in the oceanatmosphere system, likely related to: primary productivity drops, decrease in organic carbon burial rate, gas hydrates dissociations from ocean bottoms, and volcanogenic CO2 giant ejections linked to the emplacement of magmatic provinces. We propose the probable emplacement of a huge magmatic province as the cause of these shifts, likely ascribed to the activity of the Central Atlantic Magmatic Province (CAMP).