MAGNETOSTRATIGRAPHIC CALIBRATION OF THE LATE VALANGINIAN CARBON-ISOTOPE EVENT IN PELAGIC LIMESTONES FROM NORTHERN ITALY AND SWITZERLAND

Magnetostratigraphic, biostratigraphic and carbon isotope data are presented from five Lower Cretaceous pelagic limestone sections in the Southern Alps. A positive carbon isotope event is recorded in bulk carbonate from the Upper Valanginian of all five sections, and the magnetostratigraphy and biostratigraphy provide the age model. The record of the isotope event in absolute time is closely replicated in all five sections. As the sections are located in two paleogeographic basins and are separated by up to 300 km, the synchronous isotope event cannot be interpreted as an artifact of diagenesis. A reduction in magnetization intensity coincident with the deltaC-13 peak may be due to diagenetic magnetite dissolution enhanced by increased organic carbon burial. According to the age model, which uses GTS89 timescale [1], background deltaC-13 values of 1.5 parts per thousand are recorded from 145 to 137 Ma. Between 137 and 136 Ma, deltaC-13 increases to a maximum value of about 3 parts per thousand. Values begin to decrease from 136 Ma and reach background values at about 132 Ma. The rate of change of deltaC-13 for the onset of the event (about 1.5 parts per thousand/m.y.) is greater than for the decay (about 0.5 parts per thousand/m.y.), suggesting an abrupt perturbation of the carbon system from equilibrium, followed by a more gradual return to background values.