Carbon–isotope stratigraphy and ammonite faunal turnover for the Middle Jurassic in the Southern Iberian palaeomargin

Abstract

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 δ 13C 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 δ 13C 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 δ 13C values, where extinct taxa are replaced by new ones. Likewise, radiation episodes are associated with increasing δ 13C 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.