The carbon isotopic signature of carbonates depends on secular
variations of organic carbon and carbonate carbon production/burial
rates. A decrease in carbonate productivity makes the organic/carbonate
carbon ratio unstable up to the point that even minor variations in the
organic carbon reservoirs can provoke carbon isotopic shifts. The
delta(13)C positive shifts of the middle Carixian (early Pliensbachian)
and the early Bajocian recorded in the Umbria-Marche-Sabina domain
represent a good example of this mechanism. Both sedimentology and
lithostratigraphy of pelagic platform-basin carbonate systems in this
area show that important changes in the source of carbonates correspond
to the observed isotopic shifts. The middle Carixian event is in fact
well correlatable to the drastic reduction of benthic carbonate
production on rift-related intrabasinal highs, which then became pelagic
carbonate platforms. The early Bajocian event is concomitant with the
beginning of a long hiatus on the pelagic carbonate platforms and with a
drop of the biodiversity of calcareous organisms followed by the onset
of biosiliceous sedimentation in basins. (C) 2002 Elsevier Science B.V.
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