Variations in the stable carbon-isotope ratio of marine and continental
sediments can reflect changes in sink and flux modifications of the
palaeocarbon cycle. Here we report carbon-isotope compositions of Middle
Jurassic marine carbonates from the Betic Cordillera (southern Spain),
which represents an ideal region to link the stable carbon-isotope
curves directly to ammonite zones and subzones, and thereby for the
first time achieve an accurate chronostratigraphic calibration. The five
sections studied represent basin and high swell deposits of the Southern
Iberian palaeomargin. We find a similar delta C-13 of carbonates between
different oceanic areas, suggesting a homogeneous carbon-isotope oceanic
reservoir through the Middle Jurassic. The Aalenian-Bajocian transition
is a critical period in ammonite evolution; hence the Early Jurassic
fauna are replaced by new ammonite families which become dominant
throughout the Middle and Late Jurassic. For this reason, we compared
the delta C-13 values of carbonates with ammonite diversity and
extinction rates at different taxonomical levels in order to explore the
possible relationship between the carbon cycle and ammonite evolution.
The carbon-isotope values of carbonates are not exactly linearly
correlated with the extinction rate and ammonite diversity, but the main
faunal turnovers follow minimum delta C-13 values, where extinct taxa
are replaced by new ones. Likewise, radiation episodes are associated
with increasing delta C-13 values and with transgressive sea-level rise.
All these data support the idea that perturbations in the global carbon
cycle reflect rapid palaeoenvironmental changes. We made detailed
analyses of these faunal turnovers, using them as a proxy to identify
major palaeoenvironmental crises in their ecosystems forced by
modification in the carbon cycle. (c) 2006 Elsevier B.V All rights
reserved.