The stratigraphic depth in the Cismon APTICORE borehole and the age of the average age model sampled by the Monte Carlo tuning algorithm

Orbital tuning, the process of fitting sedimentary cycles to orbital periodicities, can estimate with high resolution the timing and duration of key events in the geological record. We formulate here orbital tuning as the inverse problem of finding the variation in sedimentation rate that matches sediment cycles with orbital periodicities. Instead of obtaining a single best estimate, we apply a Bayesian formulation and define a probability distribution of sedimentation rate variations that result in powerful orbital periodicities. By sampling this distribution with a Monte Carlo method, we quantify the uncertainty in the inferred sedimentation rates due to uncertainties in the tuning periods and to components of the sedimentary signal that are unrelated to orbital cycles. The method is applied to the chronology of a 30 m interval in the Cismon APTICORE borehole (Southern Alps, Italy) that includes the early Aptian oceanic anoxic event 1a (Selli Level), estimated to last 1.11 ± 0.11 Ma (95% interval). The d13C record shows a sudden negative shift of about -1 per mill at the base of the Selli Level (22-47 ka) followed by a recovery to preshift values in approx. 240 ka, consistent with a scenario where light carbon is quickly added and then flushed out of the oceanatmosphere system.