MIDDLE AND LATE JURASSIC CARBON STABLE-ISOTOPE STRATIGRAPHY AND RADIOLARITE SEDIMENTATION OF THE UMBRIA-MARCHE BASIN (CENTRAL ITALY)

A continuous carbon isotope curve from Middle-Upper Jurassic pelagic c arbonate rocks was acquired from two sections in the southern part of the Umbria-Marche Apennines in central Italy. At the Colle Bertone sec tion (Terni) and the Terminilletto section (Rieti), the Upper Toarcian to Bajocian Calcari e Marne a Posidonia Formation and the Aalenian to Kimmeridgian Calcari e Marne a Posidonia and Calcari Diasprigni forma tions were sampled, respectively. Biostratigraphy in both sections is based on rich assemblages of calcareous nannofossils and radiolarians, as well as some ammonites found in the upper Toarcian-Bajocian interv al. Both sections revealed a relative minimum of delta(13)C(PDB) close to + 2 parts per thousand in the Aalenian and a maximum around 3.5 pa rts per thousand in early Bajocian, associated with an increase in vis ible chert. In basinal sections in Umbria-Marche, this interval includ es the very cherry base of the Calcari Diasprigni Formation (e.g. at V aldorbia) or the chert-rich uppermost portion of the Calcari a Posidon ia (e.g at Bosso). In the Terminilletto section, the Bajocian-early Ba rthonian interval shows a gradual decrease in delta(13)C(PDB) values a nd a low around 2.3 parts per thousand. This part of the section is ch aracterised by more than 40 m of almost chart-free limestones and corr elates with a recurrence of limestone-rich facies in basinal sections at Valdorbia. A double peak with values of delta(13)C(PDB) around + 3 parts per thousand was observed in the Callovian and Oxfordian, constr ained by well preserved radiolarian faunas. The maxima lie in the Call ovian and the middle Oxfordian, and the minimum between the two peaks should be near the Callovian/Oxfordian boundary. In the Terminilletto section, visible chert increases together with delta(13)C(PDB) values from the middle Bathonian and reaches peak values in the Callovian-Oxf ordian. In basinal sections in Umbria-Marche, a sharp increase in visi ble chert is observed at this level within the Calcari Diasprigni. A d rop of delta(13)C values towards + 2 parts per thousand occurs in the Kimmeridgian and coincides with a decrease of visible chert in outcrop . The observed delta(13)C positive anomalies during the early Bajocian and the Callovian-Oxfordian may record changes in global climate towa rds warmer, more humid periods characterised by increased nutrient mob ilisation and increased carbon burial. High biosiliceous (radiolarians , siliceous sponges) productivity and preservation appear to coincide with the delta(13)C positive anomalies, when the production of platfor m carbonates was subdued and ceased in many areas, with a drastic redu ction of periplatform ooze input in many Tethyan basins. The carbon an d silica cycles appear to be linked through global warming and increas ed continental weathering. Hydrothermal events related to extensive ri fting and/or accelerated oceanic spreading may be the endogenic drivin g force that created a perturbation of the exogenic system (excess CO2 into the atmosphere and greenhouse conditions) reflected by the posit ive delta(13)C shifts and biosiliceous episodes.