High-resolution cyclochronology of the lowermost Ypresian Arnakatxa section (Basque-Cantabrian basin, western Pyrenees)

Abstract

The early Eocene paleoclimate record provides one of the best analogues for today's global warming. In order to reconstruct the evolution of the early Eocene paleoclimate, and understand how environmental feedback mechanisms acted on it, an accurate time framework is necessary. In this regard, the astronomically calibrated time scale (ATS) provides the highest possible resolution, but models beyond 40-50 Ma are not fully resolved and actual geological data are incomplete. With the aim of filling this gap, the expanded lower Ypresian Arnakatxa section studied herein offered a potentially valuable orbitally paced geological record. This outcrop displays a well-defined arrangement of strata in couplets and bundles. The spectral analyses of colour data series showed the dominance of two main periodicities, which were related to orbital forcing on sedimentation by precession (20 ky) and short (100 ky) eccentricity cycles. Despite not being represented in the spectrograms, the influence of long (405 ky) eccentricity on sedimentation was also deduced. Moreover, the disruption of the orbital signal in the upper half of the Arnakatxa section correlates with a very long (2.4 Myr) eccentricity node centred at similar to 54.6 Ma, which could also have caused the amplification of the orbital signal related to obliquity (41 ky). Taking everything into account, the cyclostratigraphic analyses carried out in Arnakatxa resulted in a precessional scale orbital chronology for the time interval between 55.805 and 54.435 Ma (duration of 1.37 Myr). Thus, the Arnakatxa succession could be reliably correlated with Atlantic ODP records, which are the main reference for Ypresian astrochronology, at precessional scale. Furthermore, the results from Arnakatxa also help to identify the astronomical solutions that better match actual geological data, contributing to the construction of a definitive Ypresian ATS. In this regard, the Arnakatxa results are not a good fit for solutions La10a, La10d, La11 and ZB18a, but match well with the previously thought to be less reliable solutions La10b and La10c.