A continuous carbon isotope curve from Middle-Upper Jurassic pelagic
carbonate rocks was acquired from two sections in the southern part of
the Umbria-Marche Apennines in central Italy. At the Colle Bertone
section (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 formations 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 interval. 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 parts per thousand in early Bajocian, associated with
an increase in visible chert. In basinal sections in Umbria-Marche, this
interval includes the very cherry base of the Calcari Diasprigni
Formation (e.g. at Valdorbia) or the chert-rich uppermost portion of the
Calcari a Posidonia (e.g at Bosso). In the Terminilletto section, the
Bajocian-early Barthonian interval shows a gradual decrease in
delta(13)C(PDB) values and a low around 2.3 parts per thousand. This
part of the section is characterised by more than 40 m of almost
chart-free limestones and correlates 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, constrained by well preserved radiolarian
faunas. The maxima lie in the Callovian 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-Oxfordian. In basinal sections in Umbria-Marche,
a sharp increase in visible chert is observed at this level within the
Calcari Diasprigni. A drop 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 towards
warmer, more humid periods characterised by increased nutrient
mobilisation 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 platform carbonates was subdued and ceased in many areas, with a
drastic reduction of periplatform ooze input in many Tethyan basins.
The carbon and silica cycles appear to be linked through global warming
and increased continental weathering. Hydrothermal events related to
extensive rifting and/or accelerated oceanic spreading may be the
endogenic driving force that created a perturbation of the exogenic
system (excess CO2 into the atmosphere and greenhouse conditions)
reflected by the positive delta(13)C shifts and biosiliceous episodes.